ASEG Extended Abstracts - ASEG2007 - 19th Geophysical Conference, 2007

This paper describes four different methods to automatically select the optimal set of array configurations that give the maximum resolution of the subsurface with 2D electrical imaging surveys for a limited of measurements. The first method (CR method) directly calculates the improvement in the model resolution for each new array configuration added to the set of measurements. It gives the best results but is the slowest. The second (BGS) and third (ETH) methods use linear approximations (using the second and first power of the sensitivity values respectively) to estimate the change in the model resolution matrix. Both methods are about an order of magnitude faster than the first method. The BGS method produces array configurations with model resolution values that approaches the results obtained by the CR method, while the ETH method gives significantly poorer results. The fourth method uses a combination of the BGS and CR methods. It produces results that are almost identical to the CR method but is several times faster. The different methods were tested using data from synthetic models and field surveys. The results show that the optimised array configuration data sets significantly improve the subsurface resolution achieved by the survey compared to conventional arrays such as the Wenner, Schlumberger and dipole-dipole arrays.

The frequency domain high-resolution (HR) Radon transform can provide improved resolution in the curvature direction compared with the frequency domain least squares transform. The time domain HR transform also provides improved resolution in the time direction, and, paradoxically, in many cases also a further improvement of the resolution in the curvature direction. In this paper the effect of the improved resolution is studied on synthetic and field data, and it is shown that particularly for more complex data, the time domain HR Radon transform can provide improved demultiple efficiency, aliasing protection and primary preservation, compared with the frequency domain HR Radon transform.

Activities within a hydrocarbon reservoir, such as producing oil or injecting fluid, change insitu stresses which consequently cause micro-earthquakes. The induced microseismic events are small earthquakes producing high frequency waves which can be used to give a better understanding of the hydrocarbon reservoir. However, induced microseismic events are too small in magnitude to be detected on the surface due to seismic wave attenuation through the overburden. Therefore, in order to make use of such induced microseismic waves for monitoring, characterizing and/or imaging of the hydrocarbon reservoir, one should use buried sensors within monitoring wells. The microseismic events generated within a hydrocarbon reservoir as a result of the production activities are recorded. Then, the recorded first arrival times are used in inversion process to arrive at a detailed velocity model in the vicinity of the reservoir. The inversion process is based on a fast 3D finite-difference code using the eikonal equation to model the travel times of first arriving seismic events and; therefore, making the inversion of large 3D model practical. The methodology could lead to enhanced understanding and hence efficient management of the hydrocarbon reservoir. This in turn would enhance the understanding of fluid movements resulting in improved petroleum recovery from the reservoir.

Technical Area: Seismic Modelling and Inversion, Reservoir Characterisation

JOGMEC completed construction of the SQUITEM, a three-channel TEM data acquisition system based on the HTS SQUID magnetometer, in 2006. A HTS SQUID is a high-sensitivity magnetic sensor that has wide bandwidth, and offers high field sensitivity, even at low frequencies. The SQUID magnetometer offers advantages over an induction coil receiver in depth of investigation and detecting conductive targets especially in the presence of conductive overburden.

The magnetic field resolution of the SQUITEM is 300 fT/√Hz at 1 kHz as measured in the field. A Slew rate (6.8 mT/sec in maximum) is sufficient to respond to square-wave primary-field variation in metal exploration. Frequency bandwidth from DC to 100 kHz provides minimal distortion of the recorded signal.

JOGMEC applied the SQUITEM to metal exploration at the White Roo Prospect in the Border Project, west of Broken Hill, a JOGMEC/Minotaur Joint Venture. The joint venture was targeting Broken Hill Type Pb-Zn-Ag mineralisation. Initial drilling had intersected pyrrhotite /chalcopyrite mineralization hosted within a mafic gneiss. A SQUITEM survey was conducted in October 2006 to investigate bedrock conductor highlighted by a conventional TEM survey. The SQUITEM data was superior to the conventional TEM system with respect to depth of investigation. The SQUITEM data profiles and inverted sections highlighted a conductive zone at depth that was not apparent in the conventional TEM data. Drilling of this bedrock conductor resulted in the intersection of pyrrhotite-chalcopyrite mineralisation.

EM receiver systems which measure magnetic field (Bfield detectors), not time rate of change of magnetic field, are in frequent use in mineral exploration, and are subject of a current development for detection of unexploded ordnance (UXO). The advantages of B-field detectors stem from both logistical and target discrimination capabilities. This paper quantifies the advantages of the B-field measurement using simple mathematical models for the response of highly-conductive mineral targets in a conducting earth, and for unexploded large-calibre steel munitions in the presence of scrap metal.

The small size of B-field detectors greatly facilitates the use of arrays of sensors. A series of model studies was conducted to establish what improvement in target orientation/shape information could be achieved with single or multiple B-field sensors compared with conventional moving-loop surveys conducted using single-component single-sensor in-loop systems.

The model studies show that when the target size is known (as may be the case in munitions search), the use of a single vector sensor delivers an order of magnitude greater accuracy in location in 3D space and orientation of the target, compared with the use of verticalcomponent only data in conventional metal detectors. When the target size is unknown, a single vector sensor can produce accurate location, but is likely to produce unstable orientation/shape estimates. We conclude that an array of receiver sensors is preferable.

Comparison of a single vector sensor versus an array of three vector sensors in model studies shows that the array of three vector sensors has the advantage of delivering accurate and stable estimates of both location and orientation/shape of the target.

These finding are currently being implemented in a new design of metal detector designed for location and discrimination of unexploded munitions. The results are equally applicable to the design of multiple-receiver arrays for moving-loop EM surveys in mineral exploration.

The SkyTEM system is specially designed for hydrogeophysical surveys with gate times from about 10 microseconds to 5 - 10 milliseconds. SkyTEM data does not need any sort of bias correction. To obtain data with a repeatability better than 5%, the data processing and inversion need to take into account a number of parameters.

The SkyTEM system monitors at all times the movement of the transmitter frame in the airspace by measuring GPS position, the altitude and the tilt. In the subsequent data processing the altitude is filtered using recursive filters to remove reflections from tree tops. The tilt is entered to calculate the altitude perpendicular to the ground at the centre of the frame. Also the exact altitude of the receiver coil is calculated along with an area reduction factor compensating for the reduced horizontal area of the coils when they are tilted. Data are averaged using trapezoid formed filters allowing for a small average of the early time gates and a larger average of the late time gates obtaining as small a lateral average as possible Data are inverted using the Laterally Constrained Inversion (LCI) algorithm with a parameterized and/or a smooth model. The forward response include modelling of the full transmitter waveform, low-pass filters in the instrument and a front gate preventing the primary field from the current turn-off to saturate the amplifiers when the current is turned off. Furthermore, the altitude is entered in the inversion as a constrained inversion parameter.

Numerical modelling shows that the resolution of the subsurface resistivity structures is significantly enhanced if transient electromagnetic (TEM) x- and z-component data are inverted jointly. The enhancement is most pronounced in the upper part of the model as the sensitivity kernel for the x-component is more “condensed” compared to the corresponding kernel for the z-component. Best results are obtained if the ground is relatively conductive and therefore the method is best fitted for surveys where the average resistivities of the ground are below approximately 100 ohmm, e.g. mapping of saline layers and the layers above.

In the field x-component data can be measured with only a few extra costs. However, whereas the method is simple in theory it is complicated in practice where a number of problems have to be addressed. Tilt of the x- and zcomponent receiver coils must be measured with an accuracy of better than 1. degree as even a small tilt adds a significantly amount of z-signal to the x-signal. This “contamination” must be modelled in the forward response of the inversion algorithm and the tilt of the coils must be added as extra (constrained) inversion parameters. It is necessary to low-pass filter the xcomponent data in order to efficiently suppress the high frequency background noise. Finally the timing of the instrument must be better than 200 nanoseconds to be able to model the first time gate at 11 micro seconds. In the presentation we discuss the method, present a parameter sensitivity study and a field example using SkyTEM data collected at the Toolibin test line.

Large parts of Australia are blanketed with a thick regolith that masks mineral deposits. Time Domain Electromagnetic (TEM) technique is a proven method for imaging structures below or within the weathered overburden. Four NW-SE SIROTEM transect lines (three 1.5 km long and one 2.5 km long) were conducted over a magnetic anomaly in the Cu-Au Kalkaroo prospect in the Curnamona Province (S.A.). The TEM survey geometry used 100 metre square transmitting loops with an in-loop central receiver coil and 100 metre station spacing to maximise penetration into the electrically conductive overburden. Initial results of the survey, following a “STEMINV” smooth model inversion process, suggest the TEM soundings penetrate to a depth of 100-150 m and reveal the presence of conductive regolith overlying a resistive zone. Furthermore, the 2 D depth-resistivity image obtained for all the TEM transects clearly indicates the presence of a highly conductive layer within the regolith at a depth of about 20-50 m. Through correlation with nearby drillhole data and potential field data, this conductive zone is interpreted to be the Namba Formation. The TEM results also indicate some of the structural features of the basement.

An efficient algorithm is presented that estimates the apparent source of a microseismic event from the first arrival clock-times at four receiver locations in a 3D volume. Wavefronts are assumed to be locally spherical in a constant velocity medium. Applications for identifying the apparent source range from monitoring hazardous geological sites, estimating the distribution of well fraccing material, the monitoring of sequestered CO2, or global positioning from satellite data.

The clock-time of the source may also be estimated, extending applications to Kirchhoff depth migrations in which traveltimes on a grid may be computed directly, or may be estimated from traveltimes computed along raypaths. The traveltimes at additional grid points can be computed from the apparent source. In heterogeneous media, wavefronts may have an arbitrary shape, but can be considered to be circular over a small region in the neighbourhood of the known points. The velocity in this region is assumed to be constant and may be extended, without error, to enclose the apparent source point.

The escalating cost of drilling wells has meant that first class seismic data quality is essential for delineating reservoirs and identifying new well locations in complex geology. This paper describes an integrated workflow which insured that a new 3D seismic dataset acquired off the North West coast of Australia was designed, acquired, processed and simultaneously inverted to produce enhanced sub-surface image quality over a producing field.

A desire for enhanced production required the geological understanding of the oil field to be improved. The poor quality of existing seismic data made it impossible to accurately identify the top and base of the prime volume carrying reservoir. A new seismic survey was designed with the specific requirement of improving the interpretability of the reservoir away from the wells. Both legacy seismic data and well data were used to aid in the design of the optimal seismic acquisition parameters. This along with single sensor technology produced excellent field data which were further enhanced by a high-end processing flow and inversion. In particular, advanced noise and multiple attenuation techniques have revealed the top reservoir and other previously unseen geological structures. Simultaneous AVO inversion was used to produce rock property volumes of Acoustic Impedance and Poisson’s Ratio which demonstrated an extremely high correlation with the well logs further proving the quality of the seismic.

Following the 9.2 magnitude earthquake offshore Sumatra and the devastating associated Tsunami of 26th December 2004, the SAGER research consortium was formed. As part of a series of planned geophysical experiments, 950 kilometres of 2D deep crustal seismic reflection data was acquired in July 2006 by the vessel M/V Geco Searcher in the vicinity of the earthquake epicentre. The survey design, data acquisition, processing and resultant images of this survey are discussed.

The data were recorded with a reconfigured, modern commercial seismic recording system. Several modifications were made to the overall data acquisition system in contrast to its normal use for oil and gas seismic exploration. Pre-survey modeling and design studies led to a final acquisition configuration including very large volume, deep towed sources, deep towed single sensor cables, with up to twelve kilometres offset and twenty second recording time. Separation and preservation of very low frequency signal from noise was critical to the success of creating the deep structural image. Multiple reflections and velocity determination also proved to be challenges in this data.

The processed images show that the subduction zone can be observed to a depth of over 40 kilometres, along with the associated shallower faults. The interpretation of these data and integration with other geophysical measurements is continuing.

This is a rare example of a reconfigured, modern, large scale, commercial seismic reflection system being employed for successful academic deep crustal research.

Gamma ray emissions at 1120 keV and 1764 keV produced from ²¹⁴Bi (uranium-238 decay series daughter product) are emitted during the same decay reaction with the same probability of emission during decay. Thus, as uranium concentration varies, the ratio of 1120 keV to 1764 keV should remain stable. However, the lower 1120 keV energy is more susceptible to backscattering and normal Compton scatter than the stronger 1764 keV energy, where the probability for scatter to occur is correlated to the density and thickness of the absorber. In natural settings, soil and/or bedrock acts as an absorber. Consequently, as density or thickness of the soil and/or bedrock increases, the probability of scatter increases. Thus changes in the 1120 keV to 1764 keV ratio may indicate changes in soil thickness and/or density. By processing standard 256-channel radiometric data with multispectral processing techniques, ²¹⁴Bi 1120 keV gamma rays can be isolated in addition to standard ²¹⁴Bi 1764 keV. This case study illustrates how the spatial variability of 1120 keV to 1764 keV ratios highlight and map changes in soil thickness and/or density.

Through-out the world, millions of acres of potentially productive land are contaminated with unexploded ordnance due to either past-conflicts or to military training. Low-level helicopter magnetometry (HeliMag) is currently being used to rapidly cover large areas and identify regions that are potentially clear of hazardous munitions. The configuration we currently use comprises seven cesium vapour magnetometers, horizontally spaced 1.5 meters apart on a boom several meters in front-of a Bell 206L helicopter. Magnetometer data are collected at 400 Hz at altitudes as low as 1.5 m above the ground along transects spaced 7 meters apart. From this dense, high-resolution data, potential metallic targets as small as an 81 mm mortar are identified using manual and/or automatic picking methods. The target picks are then used to estimate densities of potential contamination. 100% detection is generally not feasible, so that HeliMag is usually applied in a characterization rather than in a clearance mode. We describe a HeliMag survey collected over a UXO contaminated site at Yekau Lake, near Edmonton, Canada. The objective was to identify the location and extent of an 11.5 pound bomb target area. The target density estimates derived from manual picks were strongly influenced by geology and clutter and did not reflect the underlying density of ordnance and ordnance related clutter. By fitting a dipole model to each target pick, and comparing it to the expected response of the target item, we could estimate the density of objects with similar size/shape to an 11.5 pound bomb. This analysis clearly identified an area of elevated contamination in the same region where 11.5 pound bombs were found during ground reconnaissance. In summary, the new methodology significantly improves the interpretability of HeliMag data when used for UXO site assessment.

In many areas, so-called, Fault Shadows is a serious hindrance to successful seismic imaging. The major part of this problem is caused by large velocity variations in fault zones.

Fault zones contain different types of geological and imaging velocity anomalies that cause seismic image distortions and non-hyperbolic moveout. Pre-stack depth migration with the proper velocity model is the only method that can solve this problem and improve the seismic image below fault zones.

We have developed a special and novel technique, Fault Constrained Tomography, to build high-resolution interval velocity models that are required for fault areas.

Fault Constrained Tomography takes into account the nature of velocity variation within fault zones. Combined with Pre-Stack Depth Migration it can successfully remove fault shadow distortions from seismic images.

A new helicopter time domain AEM system (SeaTEM) is under development for bathymetric surveying in shallow waters. Commercial helicopter AEM systems are often operated in areas of rugged terrain and may not be fitted with bird attitude sensors, and altimetry sensors that operate over seawater. SeaTEM is being designed for surveying at lower altitudes than that typical of AEM systems used for mineral exploration and will incorporate inertial navigation and marine-altimetry sensors. SeaTEM is being developed over three years. The first phase involved a series of static (i.e. non-airborne) investigations over highly resistive ground followed by a series of static trials over seawater - using a stable non-metallic floating platform ~ 20 m diameter that permits full scale AEM transmitter-receiver loop systems to be deployed at simulated flight heights of about 20 m above sea level. The static ground investigations involved experimentation to determine the system self response (with and without navigation and altimetry sensors) using different loop conductors and transmitterreceiver coil configurations. AEM hardware was then suspended 20 m above sea level using the floating platform located in calm waters. We present results of the static seawater tests. This approach provides a full-scale AEM system to be “flight” tested in selected areas of known water depth and sediment type (without the expense of helicopter hire and fuel costs) avoiding electronic interference with helicopter instrumentation and noise caused by bird motion. We believe that this thorough approach will determine an AEM system optimized for bathymetric surveying in shallow coastal waters.

Between November 2005 and October 2006, a series of geophysical surveys were conducted over newly released industrial lots in the Australian Marine Complex (AMC) in Henderson. The objective of the surveys was to characterise the subsurface geology as preliminary geotechnical investigations of the site showed this to be highly variable.

Primarily through the use of GPR, with some additional ERI and MASW testing, we were able to successfully delineate the depth to limestone rock head, location of limestone pinnacles and thickness of gravelly fill layers where present over the site. This information was used in the geotechnical assessment of the sites and subsequent planning of earthworks to ensure the sites were developed to the required geotechnical standards.

GPR using shielded 250MHz antennas was found to be the most effective method to characterise the subsurface (at 50MHz the GPR had insufficient resolution), MASW effectively imaged the depth to rockhead and variations within the limestone and ERI imaged hydrogeological variations.

A consistent definition of the linear relationship between the electric field true vector E and magnetic field pseudovector H yields a true magnetotelluric impedance tensor T. The true tensor nature of T admits diagonalisation in terms of classical eigenstate decomposition. Eigenstate analysis of T is in agreement with biorthogonal methods. Physical properties of the electromagnetic energy at the surface of the earth define hermitian forms that can be viewed as surface functions of the field polarization parameters. Geometrical and physical properties of these surfaces establish the result that the eigenvectors of all the hermitian forms have the same principal directions. This property links the vertical magnetic field with T, reduces the degrees of freedom of T from eight to six, provides a single principal direction that facilitates the interpretation of magnetotelluric data and shows that when T is defined, defining the tipper is unnecessarily restrictive. Synthetic data from a 3D model is used to illustrate the main result.

A borehole hazard index is the integration of interpreted risk indexes with an existing geological and structural 3D mine or exploration model. Individual risk indexes are produced and combined with the purpose of providing a clear visual and quantitative method for determining varying degrees of risk associated with development through a particular geological rock volume.

Disparate data sets are used to characterize separate risk indexes established from user defined criteria. Input data sets include geological and structural logs and mine layouts, complemented by a borehole geophysical suite including borehole radar, optical & acoustic televiewers, density, neutron, flowmeter and full wave form sonic.

The user defined criteria are established for individual project requirements and can include factors such as the intersection of structures, the presence of water ingress, proximity to structures with specific orientations and the presence of lithological units prone to failure.

The requirements of the integration environment vary; certain criteria can be adequately assessed in a 2D environment while other hazard indexes require data to exist in a true topological 3D environment where spatial queries can be performed.

The applications of a borehole hazard index include shaft site evaluations, and shaft sinking development planning. Additionally a hazard index can serve as a mine production tool, evaluating hazards in front of the face which will affect both safety and production rates. Successful deployment requires regular and timely update of the local structural model which can be achieved by automating the hazard index generation once the starting model has been defined.

The presence in Launceston of the ancient Tamar valley, filled with soft Tertiary and Quaternary sediments, is thought to induce a 2Dv seismic resonance pattern, amplifying the surface motion over the valley. We combine the use of spatially averaged coherency (SPAC), frequency-wavenumber (FK) and horizontal to vertical spectrum ratio (HVSR) microtremor (passive seismic) survey methods to identify and characterize possible 2D effects over the Tamar valley.

Array observations are presented at two selected sites; GUNN Company (site GUN) located over an assumed 1D geology and King’s Park (site KPK) inside the valley. We obtain, by interpretation of SPAC data, a precise estimation of the shear wave velocity (or slowness) profile with depth. We calculate the dispersion curves at both sites with SPAC and FK to compare the methods. Both dispersion curves present similar features; however SPAC allows obtaining data over a wider range of frequencies.

HVSR curves are compared with modelled Rayleigh wave ellipticity computed using the slowness profiles obtained by SPAC. At GUN, the peaks on HVSR and ellipticity coincide closely. However, we observe a shift to higher frequency on HVSR peak when compared to ellipticity peak at KPK. We postulate possible 2D effects might explain that shift.

A probabilistic method of matching for dipoles in magnetic line data has shown successes in searching for known ironstone deposits of Tennant Creek. More modest successes have been achieved in providing depths to the basalts which underlie part of the same area.

A significant stage in mineral reconnaissance exploration can be performed on the Internet, using publicly available geophysical images overlaid with known occurrences of the commodity being sought.

Using the Mineral Occurrences Database of the Northern Territory on various images available on the NTGS' Geophysical Image Web Server allows useful patterns to emerge. Similarly, patterns predicted by the user's previous stage of reconnaissance are tested against these public domain images. Lessons arising from this exercise improve the quality of the user's predictions for the next stage of exploration.

Patterns emerge when occurrences are compared with the images of the radiometrics soil map, vertical derivative, surface relief and so on. Associations are demonstrated with examples from the Kombolgie, Kalkarindji and Bitter Springs formations.

It is now more than six years since the initial tests of pole-dipole IP that led to the development of the offset pole-dipole array were carried out at Copper Hill in NSW. In this period, the authors have been involved in more than 50 similar surveys world wide including a complete resurvey of the Copper Hill prospect using a more advanced ‘production’ style configuration.

Numerous practical lessons have been learnt through the progress of these surveys. Different methods of electrode emplacement, array geometry and electrode types have been evaluated both in the field and through modelling.

A study of safety considerations led to the recognition of the importance for extremely good ground contact for remote electrodes in pole-dipole surveys, not only to boost the transmitter current but also to lower the electrical potential of the remote wire to avoid fire and shock hazards.

The pitfalls of 3D configurations where the receiver dipole lies close to an equipotential were not widely appreciated in early surveys but it has been found that these can result in spurious anomalies that may be very difficult to evaluate unless the problems are recognised early in the processing.

The VTEM system developed and operated by Geotech Limited and Geotech Airborne Limited is a central loop configuration system lending itself perfectly to many traditional ground interpretation strategies. One of these is the S-layer (thin, conductive layer) differential transform which is used to generate resistivity-depth sections. An empirical study indicated that delineating conductors in a conductive half space necessitates the implementation of a scale factor in order to obtain the correct depths and conductivity values when applying the S-layer differential transform.

Based on an empirical approach, there was found to be an infinite number of depth correction factors that will still yield acceptable conductivity values and the need arose to explain the origin of this discrepancy and to find the correct depth correction factor. Three possible correction strategies were investigated based on comparison with synthetic data from models which have all shown that depths are overestimated by the S-layer differential transform. The most likely conclusion was that the physical assumptions regarding current distributions made in the S-layer transform lead to poor approximations of the conductors in a conductive half space. Assuming that the equivalent filament for the Slayer behaviour, as with the equivalent filament for the half space behaviour, does not coincide with the electric field maxima in the subsurface led to a plausible depth correction factor which was validated on various synthetic models.

The refinement and implementation of the Radon-on- Activated-Charcoal technique (ROAC), developed in the 1970’s by the South African Atomic Energy Board (Hambleton-Jones and Smit, 1980), is discussed. The objectives were to implement a radon (²²²Rn) detection technique for uranium exploration which is sensitive, cost effective, and relatively rapid in its application. The refinement of the technique, referred to here as RadonX, has primarily involved improvement of counting statistics and sensitivity. Radon, emanating from buried uranium mineralization, is adsorbed onto activated charcoal contained within a cartridge, fitted into the base of an inverted cup, and buried in the ground. The technique differs from alpha-sensitive radon detection systems in that it measures the gamma radiation arising from the daughter products of the adsorbed radon, namely ²¹⁴Bi and ²¹⁴Pb.

Two case study datasets are presented. The first is from an exploration area potentially hosting uraniferous granites, where the sand cover varies in thickness from 10 to 100m. Time domain electromagnetic (TDEM) soundings provided cover thickness and conductivity data as a guideline to porosities. The second is from an orientation survey over a known buried palaeo-channel of duricrust-hosted uranium. The results from the latter area, after a 10-day burial period, showed an improved sensitivity compared to a past alpha-detection survey, conducted with a burial period of 30 days. The technique has proven to be highly effective through both residual and transported surficial cover, with good repeatability. A depth of penetration of 80m or more under favourable porosity conditions has been achieved.

In many cases, the areal extent of closure is one of the biggest uncertainties in determining the in-place hydrocarbon volumes of a field. In this paper, I use the technique of cross-validation to rank depth maps over one particular field. The depth maps are generated by massive numbers of samples from a wide range of possible velocity models. Cross-validation drops out each well in turn, and predicts the top reservoir depth from the missing wells. The RMS prediction error is a measure of the quality of the velocity model. The best 100 velocity models (those with the lowest RMS) give 100 best depth maps. I assume that the best depth maps are equally probable because each has a low crossvalidation RMS value. An automatic spillpoint-finding algorithm is then used to find the area of closure in each of the best maps. Based on these maps, statistics are computed about the likely size of a field, and the uncertainty thereof. I conclude that there is a chance that the field size is significantly different to that given by the initial deterministic “best technical” depth maps.

Hydrogeological models rely on accurate conceptualisations of groundwater flow in the subsurface. For this we require an accurate interpretation of the sub-surface, regolith, architecture and definition of preferential lines, and obstacles to, movement of water. Traditionally, this information is acquired through judicious use of groundwater bores, combined with expert knowledge and assumptions based on the understanding of the regions hydrogeology. Flow nets are created from water level data and flow parameters determined from point determinations through pump tests.

Increasingly, geophysics is being used to help define the sub-surface architecture, identify preferential flow lines and constrain the extents of groundwater models. In particular, airborne geophysics (AG) can provide a contiguous image of subsurface features, defined by the technology being used. Thus, airborne magnetics can define pre-existing, buried river channels from the relict iron oxides on some river gravels; airborne electromagnetics (AEM) can define the preferential flowlines from the higher conductivity of water saturated sediments.

Field mapping and careful calibration of signals is imperative, though this is often an iterative process requiring additional information from new bore holes or cross-comparisons with other technologies.

Examples of where AG technologies have greatly aided the development of groundwater models will be shown from regions in South Australia. Both simple (FLOWTUBE), and complex (MODFLOW), models have been enhanced by using AG data.

By using a closed, accurately laid out and surveyed multiturn, insulated ground loop of known inductance L and resistance R, we can analytically calculate and predict the response of any time domain AEM system. By measuring the current induced in the ground loop, we have tested a two-stage calibration process whereby a system check is made on the transmitter-ground loop coupling and another is made on the ground loopreceiver coupling. Furthermore, in resistive terrain, the ground loop response provides an excellent way to directly measure the dB/dt field of the transmitter.

Using this method we analyse the predicted and measured responses of several AEM systems. In every case, the predicted and measured responses differ. Agreement between measured currents and the prediction can be achieved by solving for errors in: a) the altitude of the system, b) the lateral position along the line compared to the GPS reference, c) system tilts. The final but necessary step to achieve a fit to received data required a prediction of the averaging effects of proprietary noisereduction filters on the predicted response. The method provides a cost-effective way to calibrate time domain AEM systems, and to highlight problems such as transmitter current, receiver window timing and gain.

Measurement of the current waveform of time domain AEM systems which possess coincident-loop receivers can be problematic due to the high moment of the transmitter loop during the on-time. We present a simple method of measuring the transmitter current waveform by measuring the current induced in a closed, accurately laid out and surveyed multi-turn, insulated ground loop of known inductance L and resistance R. The current induced in the ground loop, measured by a 24-bit A/D converter capable of sampling at 96 kHz, is the convolution of the time derivative of the transmitter current waveform with an exponential decay of time constant equal to the L/R ratio of the wire loop. With an understanding of the A/D converter measuring the ground loop response, the transmitter waveform can be deconvolved from the ground loop decay.

The use of equivalent source processing on magnetic datasets is important for the regular gridding and denoising of data before any other processing can occur. The processing technique is setup as an inverse problem and solved for susceptibilities to reproduce the observed data. The drawback to the inverse problem is computation cost and overall speed for large-scale problems. Since aeromagnetics has become common in exploration, it is rare that the datasets acquired are small in data volume or space, and can be handled rapidly on a single workstation. One way to minimize the computational cost is to reduce the number of model parameters. We present an equivalent source processing technique that minimizes the number of cells in the model domain via an adaptive quadtree mesh discretization. The mesh remains coarse where no significant anomalies are present, yet fines on the edges of observed anomalies. The transition from the fine to coarse mesh grid is based on the total-gradient of the dataset, placing smaller cells on the edges of the anomaly where the susceptibilities have the greatest variation spatially. We show that the algorithm can perform over four times as fast as traditional equivalent source processing with a regular cell mesh yet preserves the same accuracy. In this paper, we present a synthetic example for proof of concept.

Knowing the physical properties of rock, enhances the knowledge of engineers to predict more accurately e.g. the flow of water / gas or oil, the production rate and/or capacity of reservoirs and the composition of resources beneficial and important to geo- and petrophysicists. Through this study, being conducted at the newly established South African National Center for Radiography And Tomography (SANCRAT) at Necsa in South Africa, it is experienced that neutrons, being extracted from the SAFARI-1 Nuclear Research reactor, have the ability to penetrate more easily certain geological laboratory samples than X-rays and provide important imbedded information on an 2D or 3D scale to geologist and scientists on a non-destructive basis. This paper focuses on neutron radiography and - tomography as a radiation based analytical imaging technique that complements X-ray radiography and tomography to determine or validate existing data of some important physical properties of rock.

This paper proposes new conceptual models of groundwater flow systems, depending on the elevation ranges of south-west catchments. We now recognise that ANCIENT landscapes comprising Tertiary sediments have persisted further west within a largely REJUVENATED and forested south-west Western Australia. Widespread inset-valley sedimentary profiles, rather than eroding completely, remain variously dissected and show some common hydrostratigraphic attributes. Their lithology, age and elevation are also factors in these landscapes responding differently to key salinity and catchment management actions. For example, reforestation in the REJUVENATED landscape can raise stream salinity by reducing streamflow that is diluting large salt loads from these ANCIENT landscapes.

Although shales comprise a large proportion of the sedimentary pile in many hydrocarbon-rich regions, their behaviour is not well understood from basin scales right down to the microscopic physics of particle interactions. Shale properties impact significantly on exploration, development and production costs through the effect of seismic anisotropy on imaging and depth conversion, the role of shales in 4D seismic response, in addition to associated issues such as pore pressure prediction and prediction of dynamic Poisson’s ratio.

Tests were performed on shales from the North Sea and Officer Basins with a view to measuring their petrophysical and ultrasonic properties. Ultrasonic tests were carried out to evaluate the full elastic tensor and its variation with stress. Variability in dielectric properties could be explained from fabric studies using both SEM and CT scanning. Ultrasonic tests evaluating the full elastic tensor on single shale core plugs show smooth responses in terms of velocity, elastic coefficients and anisotropy over a large stress range and are interpretable in terms of composition and the orientation of microfabric anisotropy with respect to stress anisotropy.

Gravity gradiometry has been heralded as one of the top five developments in advancing airborne geophysics in the past decade. There are presently nine deployed gradiometer systems operating in various configurations (partial tensor and full tensor) on numerous platforms in support of global exploration activities. There are also numerous development programs underway with an aim of producing lower noise gradient measurements. We will review the broad scope of developments in gravity gradient instrumentation, with a view toward how the projected improved performance will require greater attention to other error sources. It is easy to see how improved gradient data will benefit the explorationist, yet lower noise sensors alone do not provide the answer. Improved operational capability will need to come from lower sensor and system noise, as well as addressing the external error sources associated with terrain and geology. This paper discusses a wide range of technologies and operational scenarios under development to achieve a robust gravity gradient measurement. The significant challenges associated with improved gravity gradiometer operational capability including vehicle dynamic noise, terrain noise, geologic noise and other noise sources will be a key focus of this paper.

The Australian Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC) has developed a demonstration pilot project for all aspects of geosequestration of CO2. In preparation for acquiring a time lapse 3DVSP for this project we carried out an extensive series of vertical seismic profiles to assess source performance, operational efficiency, target imaging using a range of zero offset, walkaway, offset source and shear seismic VSPs. We were able to image the target reservoir at 2 km with resolution bandwidth of greater than 140 Hz. Analysis of shear energy in both zero and offset surveys showed the main stress direction consistent with other data. We will review these surveys and the subsequent baseline 3DVSP.We will also review the design of an integrated sampling and geophysical completion for the monitoring well. This completion is configured to provide geochemical sampling at 3 distinct levels, as well as three types of geophysical monitoring activities; an array of geophones centred at 1470 m will be used to acquire walkaway VSP data during injection, a set of three triaxial geophones just above the reservoir will be set to monitor microseismic events, and a set of hydrophones and geophones within the reservoir will be deployed for high resolution travel time measurements.

Total field magnetometer sensors, such as those of the optically-pumped cesium vapour variety, are not conventionally used in electrical geophysics. Exceptions to this are Sub- Audio Magnetics (SAM) surveys, carried out with such a sensor typically moving and measuring magnetic fields continuously at sample rates of 1 - 4 kHz. At the very low base frequencies (often below 1 Hz) used in EM surveys for highly conductive targets in very conductive terrain, cesium vapour magnetometers have an instrument noise level which is superior to almost all sensor types. This is an important issue for the detection and discrimination of highly conductive targets and can accelerate data acquisition.

At the higher frequencies (say 100 Hz and greater) collected during the survey, coil sensors are generally better performers. However, signal-to-noise ratios for a total field survey at these frequencies can be supplemented by modifying the transmitter current waveform to increase signal.

Total field sensors do not need protection from motion during a reading. In some cases data can be collected with the sensor traversing, potentially resulting in a final data set with high spatial resolution. Interpretation of total field EM data is no more difficult than working with vector EM data. In most cases the advantages of superior data quality and logistical simplicity of the total field survey outweigh the loss of magnetic field vector information.

Examples of total field EM data acquisition and processing will be presented, with particular reference to the detection and modeling of highly conductive targets

In the early 1970s, conventional two dimensional (2D) seismic methods were used to understand subsurface geological structure. Three dimensional (3D) surveying had not yet arrived as a method for delineating oil fields. Gulf Oil Research Laboratories was working on the problem of how to convince geologists that to delineate complex structure, seismic data should be recorded more closely spaced than was accepted practice.

A physical modelling system was constructed utilizing a metre square water tank. Scaled models of geologic structures were suspended in the tank and an ultrasonic source and receiver pair was moved over the models to mimic both conventional 2D and experimental 3D seismic reflection surveys. 3D seismic migration algorithms were developed using the digitally recorded model data. The results clearly demonstrated the pitfalls of using widely spaced 2D seismic lines in the interpretation process – 3D acquisition and processing was required for accurate imaging.

Exxon was next to build a physical modelling system. When Gulf was taken over by Chevron, the Gulf modelling system was donated to and installed at the University of Houston, where a new laboratory housed a larger tank. Subsequently, other physical modelling systems were built in China, Japan, Australia, Saudi Arabia and Holland.

State-of-the-art recording has changed from single shot to single receiver, to multi-shots into 48 receivers, with further channel expansion soon. From simple impulsive shot recording simulating explosives, the technology has moved to simulation of any form of vibroseis sweep or frequency required. From simple plastic models, the technology has moved into the realm of injecting fluids into real sand reservoirs in pressure vessels. This paper will discuss the changing face of seismic physical modelling.

When CO2 is injected into a saline aquifer, its flow is controlled by the rock permeability, porosity, chemical composition and fluids, and its state-of-phase which is controlled by the reservoir pressure and temperature conditions.

During the injection procedure, CO2 dissolves in the formation water and with time this increasing dissolution enhances trapping mechanism. While monitoring CO2 as a liquid or gas, it becomes equally important to quantify it both in its dissolved and gaseous states.

We dissolved CO2 in pure water and passed it through a variable low pressure cell at room temperature. During this time, ultrasonic transmission tests were conducted to monitor the seismic response with CO2 in its dissolved and its gaseous phases. It was found that the signal amplitude was far more sensitive to the amount of dissolved CO2 than velocity, and it was observed that the transmission amplitudes were a function of the density of the dissolved CO2 in brine.

These empirical relationships were based on seismic data recorded in a pressure cell containing a CO2:water mixture without a matrix, and a matrix consisting of glass beads saturated with a CO2:water mixture.

These data provide reason to believe that while seismic reflectivity may be used successfully for fluid monitoring, the use of transmission seismic such as is found in vertical seismic profiling, may be useful for quantifying fluid in-place, for verification purposes.

When CO2 is injected into a formation in its supercritical form, it acts as a gas having the ability to be compressed to a much lower volume than in its liquid form. During the Nagaoka site injection in Japan, CO2 was injected in its supercritical form into a saline aquifer and a cross-well tomography experiment was performed in which a seismic source was placed in one well and receivers in another. The objective of the cross-well tomography was to image the CO2 during injection, in order to track the progress of the CO2.

In order to simulate this field experiment, we used a large synthetic sandstone core representing a physical model of the reservoir. The water-filled core was subjected to a confining pressure of 8.5 MPa, with a pore pressure of 8.2 MPa, and had ultrasonic transducers placed down opposite sides. As the supercritical CO2 passed through the core, the seismic system recorded transmission data, to allow the later production of a seismic tomogram, and the seismic transmission response as the supercritical CO2/water interface moved through the core.

This paper presents an experiment to simulate the field response, and the results so far of the injection process. There was a velocity change of some 5% when supercritical gas replaced water, but there was also a major amplitude change with some 25% reduction in transmission amplitude when supercritical CO2 replaced water. The velocity tomogram is an important indicator of how the supercritical CO2 migrated while amplitudes provide an indication of state-of-mix. This has consequences for monitoring the state of phase of CO2 during injection, using seismic data.

For each observation of the potential field full tensor gradient (FTG), a rotation from world coordinates to principal components space is made. The two independent gradient magnitudes together with a succinct ‘quaternion’ representation of the rotation is termed the amplitude / phase representation of the signal.

This separation of concern isolates aspects that are invariant to rotation from the coordinate system. Benefits that flow from this approach are many and include: (1) Full tensor gridding from observed profiles is achieved using SLERP techniques (spherical linear extrapolation). Model studies show this method to correctly estimate intermediate tensors. (2) Applications of the technique to the Frequency Method also show required abilities to deliver low pass filtered data. (3) Just 3 independent power spectra are derived using this approach. This contrasts with the previous ‘best’ practice of 5 power spectra and 15 cross spectra. Obviously this previous practice mixed the rotational and signal strengths aspects.

Practical applications allow the treatment of the full tensor gradient as the signal, maintaining all the correct properties. Visualisation and interpretation aspects of this signal are illustrated.

The pumping and disposal of saline groundwater from the margins of the River Murray in South Australia is an integral part of the State Government’s salinity management strategy. It is specifically aimed at reducing ground water levels and salt accession to the River Murray. Large volumes of saline water are typically disposed at the land surface in what are referred to as “saline-disposal basins”. Although these disposal basins are now common, surprisingly little is known about their long-term efficacy or environmental effects. This study focuses on the analysis and interpretation of RESOLVE frequency domain helicopter electromagnetic data acquired over the Stockyard Plains saline-water disposal basins located southwest of Waikerie, South Australia, with a view to determining the extent of saline plume migration and informing our current understanding of the hydrodynamics of saline groundwater disposal in the area. The airborne EM data was calibrated using conductivity borehole data and statistical methods prior to modelling. Two sets of conductivity models were generated using smooth layered inversion and constrained layered earth inversion. The constrained inversion model provided information on the depth, thickness and presence or absence of aquitards, specifically the Blanchetown Clay, and map variations in groundwater conductivity in the region around the existing natural disposal basins. The smoothed inversion model defined the extent and condition of the groundwater mound beneath the existing disposal basin. In addition these data can be used to investigate the potential for extending disposal options in the vicinity of the existing basin by identifying areas where aquitards (the Blanchetown Clay) are present or absent.

Relationships between magnetic field anomalies, inversion models and geology are investigated, using examples from the Bendigo 1:250,000 map sheet. The objective of this study is to develop a methodology for encapsulating these relationships in databases which can then be used in interpretation elsewhere. A magnetic anomaly database distils the magnetic field variations from which quantitative estimates of source bodies can be recovered. A related database of inversion models provides those source parameters and supplies a more direct linkage between geology and magnetic field anomalies than can be achieved simply by spatial correlation of the anomalies and geological mapping.

Sensitivity tests are important in evaluating the role of inversion in the process of recovering geological information from magnetic field data. Such tests suggest that to recover depth to a buried source it is generally preferable to treat magnetization as a free variable during inversion rather than risk biasing depth estimates by imposing an incorrect fixed value.

The following study examines methods used to reduce depth uncertainty and improve image quality for the reservoir section of the Ichthys field, Browse Basin. The methods implemented were detailed tomographic velocity modelling combined with isotropic Kirchhoff PSDM and subsequent well calibration.

Initial stacking velocity analysis for 3D prestack time migration (PSTM) revealed geologically implausible and undulating RMS velocity patterns at reservoir depths across the main section of the field, reducing image quality and reservoir depth accuracy. Further investigation with forward modelling revealed that the velocity distortions may be primarily induced by shallow Tertiary sequences, containing highly contrasting, narrow, elongate velocity anomalies.

Layered/blocky modelling combined with dense residual moveout picking and 3D finite-offset tomography enabled the construction of a complex velocity model in the shallow section. Subsequent grid-based global tomography with constraints was then used for updating the entire velocity field.

The final derived velocity field was more systematically correlated with that observed at the wells and the corresponding depth structure produced from PSDM appeared to contain less distortion and to be more geologically realistic. The resultant velocity model is currently being incorporated into the structural evaluation for the Ichthys Field.

The Down-hole Magnetometric Resistivity (DHMMR) technique uses a grounded dipole to inject current into the ground and a downhole sensor to measure the resultant magnetic fields. Until recently, the sensor was usually a single component induction coil probe. Recently a 12-hole DHMMR program conducted on the North Mine, Broken Hill, used a 3-component B-field ‘Atlantis’ probe. The results from this survey were spectacular, particularly when considering the target and location: The target was the narrow discontinuous ribbons of low-conductivity Zinc Lodes, located 20-50m above the highly-conductive main North Mine orebody, and underneath/next to the North Mine infrastructure and development. One risk facing the survey was that the main orebody would act as a short-circuit, causing the impressed current to avoid the Zinc Lodes entirely. To mitigate this, one transmitting dipole electrode was placed down a deep drill hole in a Zinc Lodes intersection and the other was dug into a surface expression of the Zinc Lodes, ~1.5km south of the drill hole electrode. This layout very effectively isolated and energized the Zinc Lodes mineralisation.

The 3-component B-field probe has a noise level, at the frequencies used in the Broken Hill survey, which is significantly below that of sensors used previously for DHMMR. This resulted in better data and faster acquisition times. Additionally, there is relatively little processing required after the survey to present data in a meaningful manner for interpretation. One final product of the survey was vector geomagnetic data (not utilized to date). The success and accuracy of this survey using new equipment in difficult conditions is expected to lead to a wider and better appreciation of DHMMR’s capabilities.

The Elk trend gas field is situated in a frontal location of the Papua New Guinea Fold Belt. The Elk reservoir is Miocene carbonate ramp to deepwater basinal Puri Limestone and Eocene to Oligocene shelfal Mendi Limestone. FMI analysis from the Elk-1 well has constrained the in-situ orientation and distribution of fractures, and provides some indication of fracture aperture, but information is constrained to the wellbore. Offset data comprising FMI logs, structural core analysis, and outcrop studies provide information on fracture morphology, generating mechanisms and spacing. These have been used to generate models for the Elk reservoir. The Elk-1 well from DST-1 and DST-2 demonstrated extreme permeability and can only be explained by a significant well connected natural fracture network in the relatively tight section penetrated. Within the Elk reservoir, fractures were divided into producing and non-producing from wireline logging based upon detailed analysis of wellbore temperature, fracture location, orientation and morphology. A limited number of the fractures flowed gas during logging but these have facilitated interpretation of the geometry and interconnectivity of the fracture network outside the wellbore.

Gravitec Instruments is continuing work on a novel direct magnetic gradiometer for use in mobile and static mineral surveying. The compact, robust design lends itself to airborne and borehole deployment and could be used in other applications where space and power are strictly limited. Results from recent testing will be discussed.

Gravitec Downhole Instruments Ltd. is in the final stages of adapting its gravity gradiometer technology for deployment in the borehole environment for use in exploration as well as for time-lapse reservoir monitoring. The project, named Scorpius, is in cooperation with partners Kenda Capital and QinetiQ Ltd.

Laboratory testing of the sensor has produced gravity gradient measurements. Work has also been done to forward model the effects of gravity gradient measurements in the borehole environment.

Geoscience Australia regularly undertakes crustal thickness measurements and it is the only National Agency that maintains a database of such measurements in Australia and its offshore territories, including the Australian Antarctic Territory. The most reliable estimates of crustal thickness come from refraction and wide-angle reflection seismic experiments, as well as from receiver function analyses. As of September 2007, there are only 474 crustal thickness measurements stored in Geoscience Australia’s database derived from such experiments. Crustal thickness data are essential for development of crustal scale tectonic models, assessment of geohazards, constraining of potential field inversions, estimation of crustal stretching rate in sedimentary basin modelling. Estimates of relative significance of depth of subsidence, basement heat production, and thickness of the crust for positioning of hydrocarbon maturation windows show that effects of crustal thickness can outweigh those of basement heat production.

Over the last decade, spectral decomposition techniques have greatly improved seismic interpretation in the exploration of hydrocarbons. Significant efforts have been directed to the development of such methods for tuning hydrocarbon reservoirs, thin beds and channel estimation.

Geological and geophysical changes of the earth formation manifest as discontinuities in a seismic trace. These discontinuities also called singularities thus carry most of the information in the trace. Statistical techniques are required to analyze these discontinuities in order to ascertain the variation in the fluid flow.

This paper discusses a technique for detecting reservoir homogeneity/heterogeneity based on singularity spectrum attributes. Singularity spectrum determined from multifractal analysis, can globally describe the singularity content of a signal. Obtaining the singularity spectrum of signals also helps in the identification and classification of different state changes in a signal.

The developed technique is applied on synthesized seismograms and two attributes of the singularity spectrum namely correlation dimension and width are determined. Results show that by using these attributes the heterogeneity or homogeneity of the reservoir can be evaluated. Such attributes can increase the reliability of suspected hydrocarbon zones; hence unravelling drilling uncertainties and is potentially useful for reservoir fluid flow modelling.

Deep ocean controlled source electromagnetic (CSEM) methods in oil and gas exploration have undergone rapid development in recent years. The CSEM transmitter generates coupled vector fields at low frequencies (less than 0.1 Hz) for a large number of transmitter receiver offsets on or close to the Ocean floor. Representation of the distribution and propagation of the transient electromagnetic fields within and below the ocean is of considerably importance to those people engaged in both pure research and the practical applications of electromagnetic methods.

Typically, representation of deep ocean CSEM field data is via simple plots of amplitude and phase versus transmitter offset from each fixed receiver position. However, the reality of EM field propagation in 4D is more fully represented by distributions of rotating vectors for the total and scattering electric and magnetic fields. Methods for 4D representation in stereo projection are provided. The colocation of acquired and model data in the same virtual space imposes additional requirements on the visualisation methods and several examples are provided. The 4D representations are developed in Open Source Mayavi software.

Curtin University Department of Exploration Geophysics in collaboration with Water Corporation of Western Australia have undertaken a comprehensive geophysical program with the overall objective of resolving hydraulic properties and boundaries below the Gnangara mound. Initial results have been highly encouraging. Radar transects, more than 15 km in length, reveal up to three shallow potentially “water retentive” layers above the regional water table for the superficial aquifer. Both shallow sedimentation such as buried dunes and post deposition layering can be readily interpreted over large areas from the radar data. Interpretation of the radar data is assisting in developing a large scale infiltration distribution models for the Gnangara Mound. The high resolution seismic reflection surveys aim to resolve hydrostratigraphy from surface to 1500m depth. Of particular interest is large scale hydraulic separation between Perth’s major aquifers, which include the Superficial, Leederville and Yarragadee. Previous seismic surveys in this area produced low-resolution discontinuous events that could not be used for such purposes. Recently we acquired highresolution seismic profiles using our telemetric system and a high-power impact source. High CMP fold combined with careful target oriented data processing produced quality images. These enable interpretation of up to 20 layers including several unconformities in the top 1500m. Further advance in acquisition, particularly application of high-power, highfrequency seismic sources and data processing are hoped to enable routine application of reflection seismic methods in hydrogeology. Current results demonstrate significant advances have been achieved with respect to the past trials even at these early stages of the Gnangara Mound research program. Results from the seismic reflection and radar surveys will feed into the PRAMS hydraulic flow model for the Perth Basin.

In recent years the mineral industry in Western Australia has begun to utilize seismic methodologies to explore for new prospects. Processing of hard rock seismic data is cumbersome, time consuming and has to be linked to a most like geological model even in erly processing stages. Crooked seismic lines recorded over complex 3D geology, combined with highly heterogeneous regolith and significant energy scattering, present challenges to applications of seismic methodologies. Lack of deep boreholes and sonic longs makes interpretations of seismic data a daunting task.

With the end stages of data analysis in mind, each of the above issues has a systematic solution. Inversion and future considerations for multi-attribute analysis require accurate structural images and consistent amplitude and phase information from the seismic response. Classifying relationships between seismic attributes and various rock types that are likely to host specific minerals is a key to accurate data analysis. For that purpose, an extensive “seismic response database” needs to be derived from log measurements, core sample tests, and in situ geological knowledge.

Early stage diamond exploration commences with the careful selection of geophysical anomalies that are characteristic of kimberlites. However, it is also essential to map the structural controls on kimberlites and the background geology. When used in conjunction with the expected age of emplacement an understanding of the geology helps determine the degree of kimberlite preservation. This may impact on the economics of a discovery but also provides boundaries on the physical property contrast between the target body and host rock.

Historical mapping of the geology of the area around Jwaneng Diamond Mine in Botswana was accomplished utilizing a combination of magnetic data and drilling as the region is covered by Kalahari sediments with thicknesses of up to 60m. A high-resolution airborne gravity gradiometer survey was flown during 2006 utilising the Bell Geospace Air-FTG™ instrument mounted in an airship platform adding important information to the mapping of the complex geology of this area.

A major intrusive with a diameter of 25 km dominates the gravity and magnetic response of the area with a dense and highly magnetic inner core. The southern edge of this body is truncated by a transcontinental structure that has been correlated with the Thabazimbi-Murchison Lineament.

A number of tools have been developed to help understand the processes of salinisation at work along the Murray River in Southern Australia. Four techniques that have been used to help investigators either directly measure the salt load entering the river, or to image the distribution of conductivities under the river are examined here. They include Run-of-River surveys (ROR), in-stream towed NanoTEM, in-stream towed Resistivity, and Helicopter EM (specifically using the RESOLVE FDHEM system). Each technique has strengths and weaknesses related to its mode of operation and the approach adopted in field data collection. Runof-River samples the water salinity directly and then attempts to estimate river salt load and source location. It provides a direct measure of the salt entering the river but a) only provides salt load information and b) generally only provides information on a kilometre scale. The other three techniques are all geophysical techniques that do not directly inform the investigator about salt loads in the river, but provide information about conductivity distributions in the sediments under the river, which then may be related to salt loads. Each of the geophysical techniques sample the instream environment at three to 20 metre intervals, and provide information from near the river surface to depths of between 10 and 40 metres below the surface. Data may be displayed as depth sections, or as contoured depth slices prepared to examine different levels beneath the river bottom.

This paper describes a new seismic processing method that uses dip-steered filtering to improve the performance of subsequent seismic whitening and relative impedance inversion. Given favourable acquisition and processing parameters, the availability of high frequency signals is ultimately limited by poor Signal/Noise ratio (S/N) at high frequencies. Managing this noise is key to accessing the high frequency information.

We show that although spatial filters applied to broadband seismic data mainly affect the central passband, a frequency splitting approach can specifically benefit the targeted high frequency parts of the spectrum. The dip-steered filters trade spatial resolution for increased S/N, but only in the frequency bands that are boosted during the whitening process.

The S/N of the low frequencies can be improved using a similar process prior to acoustic impedance inversion. In a relative impedance inversion workflow, de-noising the low frequencies allows accurate inversion of thicker beds. Applying the technique prior to full-bandwidth inversion allows the use of lower seismic frequencies, and reduces reliance on a pre-existing background model.

We can apply frequency split dip-steered filtering to prestack common offset volumes or partial stacks in a similar fashion. This delivers frequency balanced partial stacks with lower noise and increased resolution, ultimately leading to a reduction of offset dependent tuning and higher resolution elastic inversion products.

Logs are not required for the process but independent well ties demonstrate the validity of the technique and are used to QC the products.

The value of increased resolution in the seismic data volumes is illustrated with several case histories from different surveys and reservoirs.

Over the last 30 years, the quality and usefulness of information derived from both prestack and poststack seismic amplitudes has led to increases in exploration success rates as well as to more effective use of amplitudes to predict reservoir properties away from well control. However the temporal and spatial resolution and the signal to noise ratio of various attributes even for relatively shallow target depths is often inadequate to address subsurface issues of interest. An ongoing challenge to geophysicists is that of acquiring, processing and analyzing 2D, 3D and 4D surface seismic amplitudes so as to improve resolution and further quantify geologic properties of interest including subsurface porosity, lithology and fluid types.

Seismic acquisition technologies continue to advance with increases in land and marine channel counts, longer offsets, wide azimuths, higher fidelity sources and receivers, more ancillary field measurements and model driven survey designs. Seismic processing industry has advanced with model driven noise suppression, signal distortion corrections, tomographic velocity modeling, wave equation prestack depth imaging and amplitude recovery technologies. Seismic analysis techniques continue to evolve with model-driven, Bayesian based statistical analyses of seismic attributes including the estimation of amplitude attribute uncertainties.

However, in the midst of these seismic technology advances, very little has been done to adequately compensate for the effects of wave propagation from recording surface to subsurface targets of interest. In fact processing models are typically devoid of the earth property detail required to compensate for scattering, illumination and attenuation effects. Analysis of various amplitude attributes taken from walkaway VSP’s and modeling studies strongly indicate that the vertical and angle dependent transmission effects of an overburden on land or marine seismic data imposes a dominant imprint on reflected amplitudes. In many cases this imprint completely obscures subsurface amplitude variations that might otherwise be useful in determining subsurface properties of interest.

Modeling studies have led to a deeper understanding and quantification of the magnitude of earth transmission effects and noise on seismic data. Finite difference models of transmitted and reflected pulses in a layered earth make clear that weak earth lenses and wide angle scattering can have first order effects on the relative phase, time and scale of angle dependent reflections which can lead to very large errors in inverted amplitude attributes. Even when compensated in a "true amplitude" seismic processing sequence, it is unclear the degree to which these effects eventually introduce uncertainty into inversion results.

Post-imaging, pre-inversion amplitude processing sequences that apply time, offset and frequency dependent residual event alignment, scale and phase corrections to CRP gather data can improve inverted amplitude attribute quality. Such software relies on statistical information drawn from well log and borehole data to constrain processed amplitude behavior to conform to that expected of bandlimited earth reflectivity.

Archaean lode gold mineralisation is known to occur near major crustal breaks manifesting as large-scale shear zones; which act as conduits for mineralising fluids. Mineralisation occurs in regions of structural complexity adjacent to the shear zones. This paper reports on-going image processing research that aims to automatically detect such geologically significant characteristics from aeromagnetic data for the prospectivity analysis. The current system finds the regions of magnetic discontinuity that are related to shear zones and lithological boundaries, and then identifies prospective regions nearby. The regions of magnetic discontinuity are detected using a combination of texture analysis and bilateral symmetric feature detection, where line-like features representing high local magnetic variations are identified. Then the areas adjacent to the regions of magnetic discontinuity are examined using a 2D fractal surface analysis technique to search for areas with complex magentic expressions that are related to structural complexity. A preliminary experiment was conducted using aeromagnetic data from the Yilgarn Craton in Western Australia and the regions selected by the proposed system contained over 76% of all known deposit locations and 82% of the greater than 1 tonnedeposit locations. Our on-going research also includes identifying the major shear zones from the regions of discontinuity to examine their spatial and geometrical characteristics.

The world needs more energy. The demand for energy is growing rapidly, with long-term supply having to come from both conventional and unconventional resources. Challenges to deliver supply focus on the substantial future investments requirement and on the technology needed to unlock potentially difficult-to-access hydrocarbons. The industry is already making or gearing up for vast investments in the Asia Pacific region, and in Australia in particular. Shell views technology, and geophysical technology prominently included, as a key enabler to realize the potential of the region. With a long history at the cutting edge of geoscience technology application, Shell is aiming to help shape the forefront of hydrocarbon exploration, development and production for decades to come.

The Athabasca Basin, Saskatchewan Canada is the world’s premier location of high grade uranium deposits. Most occurrences show a close spatial association with graphite shear-fault zones in the basement rocks (overlain by resistive sandstone) and EM techniques have been used for over 30 years to help map these conductive features. While exploration initially focused on shallower parts of the Basin, current exploration is requiring investigation through thicknesses of sandstone well in excess of 500 m depth. With drilling costs typically now approaching CAN$0.5 million per hole in the deeper parts of the Basin, considerable efforts are being expended to define basement targets with as much spatial resolution as possible. Consequently, most companies are employing some form of ground geophysical surveys to try and sharpen the target focus prior to drilling. We have had the opportunity to compare airborne and ground surveys in a number of locations and have found that there can be considerable disagreement between the conductivity models derived from airborne surveys and those produced from ground EM (active and natural field) or DC resistivity surveys. A number of these examples are presented and discussed so as to better understand what are the likely sources of error and how best to manage the risk of multiple but non-conforming outcomes.

The Simpson field in the Barrow Sub-basin (Carnarvon Basin) is nearing depletion. Most of the producing wells are showing relatively high water cuts. Based on volumetric mapping and the drilling results from nearby analogous fields, some unproduced reserves are potentially remaining in the field within compartments separated by low permeability shale barriers. The challenge is to establish a methodology for identifying these compartments and to quantify unproduced oil.

The reservoir of the Simpson field is the Early Cretaceous Flag Sandstone. The reservoir zone has three distinct lithotypes: oil-saturated sandstone, water-saturated sandstone and shale. The shales encountered in the wells have a typical thickness of less than 3 m, significantly below seismic resolution. However, these lithotypes show good statistical separation of elastic properties (e.g. PImpedance and Vp/Vs), so a properly-constrained geostatisitcal inversion can be used to predict the relatively thin shale barriers.

The geostatisitcal inversion is based on Bayesian Inference and uses a Markov Chain Monte Carlo method to randomly sample the probabilistic distribution arising from all input beliefs. The end result is multiple elastic property and lithotype realizations. The analysis of the realizations output from the geostatistical inversion led to improved reservoir volumetric calculations and the identification of highly probable shale barriers.

In November 2004, EOG Resources contracted WesternGeco to acquire an ocean-bottom cable (OBC) 4C swath survey across the Pamberi-1 well in the Lower Reverse L block of the Columbus basin, eastern offshore Trinidad. The purpose of the 4C survey was to evaluate the potential of long-offset multicomponent technology for resolving lithology and stratigraphic detail in an area perturbed by shallow gas, overpressure and illumination shadows from normal regional faults and major anticlinal ridge trends acting as pressure seals. A key motivation for EOG was the realization that a conventional 3D towed-streamer survey acquired the previous year failed to adequately image the target reflectors comprising the reservoir under the main fault.

Details of the P- and PSv-wave processing of this dataset through anisotropic prestack time migration were previously described (Johns et al., 2006) in which it was demonstrated that there existed a qualitative correlation between derived parameters and attributes from P and Sv anisotropic migration velocities and known regional geology. This observation was quite remarkable considering only a limited effort to constrain or validate parameters (in this case, velocities to the Pamberi-1 well checkshots) was performed. Under the “Future work” section of the previous publication, it was suggested that further data quality enhancement in preparation for more quantitative rock property classification could only be achieved after prestack depth imaging.

In this paper, we present precisely that next phase in the 4C processing, advancing the P- and PSv-wave data through anisotropic prestack depth migration, using cellbased tomography with a top-down approach. The Pamberi-1 well was used to constrain the anisotropy in the shallow section, with the deeper, spatial trend away from the proximity of the well determined from the anisotropy derived previously in the time processing.

Prior to proceeding with the anisotropic depth imaging, the magnitude of shear splitting from the presence of azimuthal anisotropy (HTI) was first examined to assess its potential impact on the radial rotated P-S signal. The shear wave splitting analysis revealed a principal angle of polarization that was closely aligned with the regional stress direction delineated by the normal major faults blocks acting as pressure seals.

To compensate for lateral or temporal divergence of converted-wave (PS) imaging as a function of azimuth, a two-tiered workflow for applying a non rigid matching (NRM) process is utilized. The resultant product is a final enhanced 3D PS volume with superior stack response and continuity from the optimum combination of two distinct azimuth sectors of a PS prestack time migration (PrSTM) dataset. The method allows for the crossline artifacts from the effect of azimuthal anisotropy on the convertedwave moveout to be almost completely removed. Furthermore, the severe acquisition footprint, from insufficient crossline aperture as a result of a suboptimum survey design, is effectively mitigated. Seismic data examples of 3D inlines, crosslines and time slices taken from a proprietary 3D/4C OBC survey from the Gulf of Mexico acquired and processed in 2006, demonstrate the compelling benefits of the NRM application and the robustness of the developed workflow. Finally, the output NRM displacement attributes (time-shift values) are found to possess a qualitative value that are not only powerful indicators of azimuthal anisotropy, but also through calibration, may yield valuable information on the magnitude of shear splitting and principal directions of polarization.

Palaeochannels are typical geomorphic features representing drainage streams or rivers, which were flowing either during the past time and now stand buried or shifted due to tectonic or geomorphological processes. They often contain considerable thicknesses of sediment with elements which have been dated from the Mesozoic through to the late Cainozoic. These sediments are known to host, or act as pathfinders, to economic mineralization, and are an important source of potable groundwater, particularly in remote areas. Located about 600km northwest of Alice Springs, the Tanami region of the Northern Territory is an emerging gold province in Australia. Although not genetically related, some of the gold deposits are located on the margins of palaeochannels. Outcrop in these areas is sparse and bedrock is generally covered by in situ and transported regolith materials, that at places may reach more than 300m. Interest from industry and extensive drilling activity in the region has provided us with a natural laboratory to apply a multi-disciplinary approach for studying regolith properties and processes. As part of the research activities of Co-operative Research Centre for Landscape Environments and Mineral Exploration (CRC-LEME), we have carried out ground transient electromagnetic (TEM) and down-hole EM measurements as well as hydrogeochemical sampling of the open drill holes at the Titania mineral prospect. Apart from identifying possible locations of mineralisation, these studies have clearly delineated the structure of the palaeochannel, the character of the fill and the properties of the groundwater. A detailed discussion on the results will be presented.

Major difficulties associated with airborne geophysical surveys are rapidly disappearing with the development of precise position systems. These techniques could be useful in observing the natural source EM inductive field from a moving airborne platform. Recent studies show that fluxgate magnetometers with high sensitivity along with real-time precise positioning techniques could be used in making such airborne measurements. The concept of airborne measurement of natural source EM induction is similar to the ground based geomagnetic depth sounding (GDS). The only difference is that instead of simultaneously observing the magnetic field with an array of ground stations, airborne system is flown over an area of interest and measures a range of high frequency signals at a pre-defined interval. The 3-component fluxgate magnetometer data thus collected could be processed using the GDS method to study the lateral conductivity variations within the subsurface. The depth from which the information is returned depends on the frequency (or periodicity) selected and subsurface conductivity situations. This method could therefore be successful applied to exploration in cratonic areas (e.g. Canadian Plateau) where there are lots of fresh rock exposures. However it is uncertain how successful this method could be applied in areas of thick regolith cover such as in Australian conditions. Numerical modeling could possibly test the likely success of this method under those conditions. In this paper I shall discuss how one can utilize the 3-D Finite Difference forward modelling approach to compute the airborne responses of shallow subsurface lateral conductivity anomalies under various surface/subsurface conditions.

A new seismic monitoring network installed in northern Western Australia in October 2005 seeks to record low-level seismicity in the region, with a view to understanding event frequency, magnitude, location, geological controls on seismicity and deep-crustal structure that may be contributing to crustal weakness. Data from a network of eight semi-permanent stations, located at localities between Shark Bay and the Dampier area, complements data from an additional 80 stations that were deployed for shorter periods as part of other research projects at the ANU between 2000 and 2006. This dense network of seismometers provides the opportunity for a detailed analysis of seismicity and shallow- and deep-crustal structure in the northwest of WA.

For events such as the M5.3 event in Shark Bay in February 2007, our network provided the detailed data required for accurate location and potential fault plane solutions. Potential fault plane orientations have significant implications for our understanding of the neotectonic evolution of this part of Australia, and mechanical contrasts in the crust that may be predisposing areas to failure. Despite the region hosting large earthquakes (Geraldton, 1885, M6.5; offshore WA, M6.2, 1920), including Australia’s largest known event (Meeberrie, M7.3, 1941), little is known about the frequency, magnitude or causes of seismicity in this region, which is far less known than the SW Seismic Zone.

This project is part of an Australian Research Council Linkage project hosted at The University of Western Australia and involving collaboration with The Australian National University and Geoscience Australia, sponsored by Woodside Energy.

There are three principal sources of risk in mining and exploration: the external market, process and mining risk and geological risk. Only the external risk is truly beyond the control of the owner/operator. Underlying all other risk is the quality of the understanding of the geological risk that underpins mining and processing decisions.

A number of case studies are discussed which highlight areas where Anglo and affiliated companies have employed geophysics to reduce the geological risk in exploration, delineation and production.

Geological risk is intrinsically 3-dimensional in nature. Despite the fact that the parameters that describe a geological entity can generally be measured with some precision, e.g. density, grade, chemical composition and hardness, lack of access in the third dimension (depth) combined with cost, means that we invariably deal with incomplete data sets.

Yet it is precisely the geological model that informs the type and size of mine and processing options. The chances are that the resulting mine/process options will be sub-optimal and in the worst cases completely wrong.

Acquiring the necessary information costs money. Not having the information inculcates risk. There must be an optimum position where the overall cost of the information and the reduced risk is at a minimum.

Traditional methods of exploration/delineation no longer suffice. A judicious mixture of traditional and geophysical methods offers an opportunity to gain an adequate understanding of the critical geological parameters and thereby make better decisions on mine/process design.

Spectrem Air Limited is jointly owned by Anglo American and De Beers. The Spectrem2000 system was upgraded to its current configuration in 2000, with significantly improved transmitter and acquisition capabilities. Since then there has been a steady progression of improvements to the acquisition and receiver components.

The Spectrem design philosophy was to build a broadband system with the following capabilities: high resolution mapping; deep penetration; ability to operate in conductive terrain; direct detection and safe operation.

To date over 1,000,000km of surveys have been flown in Africa, South America, North America and Europe. Due to Spectrem’s unique broadband configuration, it has been successful in both direct detection and mapping roles. Recent developments have been aimed at improving the ability to conduct AEM surveys in conductive areas. This includes building a low frequency EM bird and processing developments to obtain better discrimination for more conductive targets.

A number of case studies from various terrains are presented demonstrating the multi-faceted nature of the Spectrem system.

A combined interpretation of gravity, magnetic and seismic data has been used to investigate some key structural elements of the southern part of the Northland Basin which lies off the west coast of North Island, New Zealand. 2-D seismic reflection data reveal a thick sedimentary sequence, a number of significant structural highs and large Miocene volcanic complexes. The basement reflector is generally poorly imaged, especially below locations where volcanics are evident at shallower levels in the seismic sections. Gravity and magnetic maps clearly show the locations of the major volcanic centres which have strong anomalies (up to 50 mGal and 800 nT). In addition, a belt of positive magnetic anomalies (up to ~200 nT) trends N-S in the south and NW-SE in the north. This follows the strike of the main structural trends in the region and suggests the occurrence of a highly magnetic basement terrane. Preliminary 2.75D gravity and magnetic modelling shows that the Miocene volcanic bodies are broader than previously interpreted and extend to basement depths. Modelling also shows that the top of the basement is deeper (up to 9 km) than is evident in the seismic sections. The gravity and magnetic data therefore are invaluable in resolving some of the ambiguities in the seismic interpretation.

In this paper, we discuss the influence of the self-demagnetization effect on magnetic data and present an alternative means of quantitatively interpreting such data in highly magnetic environments. In particular, we present two important results based on simulation which one might consider in their interpretation of magnetic data when self-demagnetization is present. First, current methods for estimating total magnetization, which are typically applied to the problem of remanent magnetization, do not reliably recover this parameter when the anomalous source bodies have high magnetic susceptibilities. And second, a single value estimation of total magnetization does not provide adequate information to properly resolve subsurface geology through inversion. Numerical experiments demonstrate that directly inverting amplitude data, calculated from magnetic data yet weakly-dependent on magnetization direction, produces superior results when interpreting data generated in terrain with high magnetic susceptibilities.

A combined tuning/AVO model was used to predict the oil column height for the Tui, Amokura and Pateke fields in the offshore Taranaki Basin, New Zealand. The Palaeocene aged Kapuni F10 reservoir sands occur at a depth of approximately 3700 mSS. The closure height and areal extents of these fields were initially mapped by converting time structure maps to depth using average velocities derived from stacking velocities. However, the traps at Tui are broad low relief features, extending up to 4 km laterally but with only limited vertical relief. This geometry means that even very small velocity variations have a significant impact on the location and extent of closure.

Weak amplitude anomalies had been noted on the fields and a study was undertaken to review the tuning and AVO behaviour of the rocks in an attempt to understand the anomalies. Vp, Vs and density logs from the Tui-1 and Amokura-1 exploration wells showed that it was possible to map oil column heights using amplitudes extracted from the F10 sand seismic horizon. This modelling predicted an oil column height of 20 m at the crest of the Tui structure prior to the drilling of development well Tui-2H. In the same area a column height of about 10 m was predicted using the average velocity based depth maps. Drilling results showed a maximum column height of 21 m, confirming the validity of the modelling. This result led to the change in the direction of the next well Tui-3H. Four horizontal development wells have been successfully drilled with total estimated gross 2P reserves of 32 MMBO.

Gravity surveying has been available since the early 1900’s in the form of analogue gravity meters, barometric levelling and conventional optical line of sight surveying methods. The gravity method was more generally used for applications in the oil industry.

With the advent of GPS technology in the form of centimetre accuracy from GPS surveying methods, commercial gravity operations developed. Line of sight surveying methods were no longer required and gravity meters became digital, easier and more reliable to use.

Commercial gravity surveys are now undertaken on a routine basis from projects spanning hundreds of kilometers for mapping large sedimentary basins to very small localised engineering projects covering merely hundreds of meters.

This paper outlines the development of the GPS surveying technology and the digital gravity meter. The integration of the two technologies are discussed with respect to the development of the commercial gravity survey method. Several applications and case studies are described with specific examples given from various international gravity contractors.

A regional scale 3D geological map of the upper crustal sequence in the West Arnhem Land region, Northern Territory, was compiled from surface mapping, limited drilling information, and liberal amounts of geological inference. Modelling of the gravity and magnetic field response of this map was proposed as a means of evaluating the viability of this geological hypothesis. A relatively large number of mass density and magnetic property measurements were available to constrain the transformation of 3D geological maps into property models in preparation for potential field modelling. The presence of numerous magnetic dykes, sills, and stratigraphic horizons provided many challenges for producing geologically-realistic magnetic property models at a regional scale. Modelling of the gravity field at this scale was far more straight forward and successful. After completing various forward modelling experiments, a stochastic procedure will be used to derive a large number of geological maps by making small changes to the highly uncertain interpretive parts of the original 3D geological map. It is expected that a subset of these derived geological maps will have associated mass density models that can adequately reproduce the gravity field observations. The common characteristics of the geological models in this subset will be isolated using statistical techniques and used to refine our representation of the regional scale 3D geological features.

Seismic amplitudes and AVO have been used successfully in exploration worldwide. We present a general geologically-based methodology to characterize the expected amplitude and AVO behaviours of reservoirs and seal facies in a basin. We produce a rock physics model based on well data and consistent with the geological model for the basin. The result is summarized in a series of depth trend curves for rock properties and anticipated seismic responses, illustrated here with data from the West of Shetlands, UK, to predict amplitude and AVO responses throughout the basin. A by-product of the technique is the verification of the validity of the amplitude changes and flat-spots which can result from physical property changes across fluid interfaces. For example, seismic flat-spots cross-cutting dipping stratigraphy are commonly observed within the Triassic Mungaroo Formation of the Exmouth Plateau area of the North Carnarvon Basin. We show techniques for quantifying the consistency of flat-spots in 3D, assessing amplitude conformance with structure in map view and automatically determining fluid contact levels.

Shallow anomalous high speed events, evident on the shot gathers, are observed to be associated with gas chimneys in the Central Perth Basin. Shot gathers in the vicinity of gas chimneys display these events, which have apparent speeds of around 3000 m/s in the uppermost 300 ms, and appear to localised around the gas chimneys.

These high speed events have been investigated using existing seismic data from the region, with these high speed events modelled using wide aperture seismic (WAS) techniques.

Understanding the effect of these shallow high speed layers is important for the following reasons -

1. they may provide support for a petroleum prospect,

2. they may represent a possible geohazard during drilling,

3. these high speed layers can have a profound effect on the seismic imaging of the underlying strata.

Dissolved salt in aquifers is a potential threat to fresh water resources and the environment. Interpolated grid maps at aquifer depths, derived from borehole EC measurements on water samples, are combined with more detailed bulk electrical conductivities from airborne electromagnetics to provide detailed estimates of total dissolved salt load and subsurface porosity values. A resistive host matrix assumption implies that our calculated porosity and salt load values are maximum values. This mapping technique is a large area coverage remote senor method which has been tested in different areas of the salt-threatened Murray-Darling basin. This technique provides extensive information on the hydraulic properties of aquifers, important for quantitative hydrology.

Geophysical data from the southern Coromandel Volcanic Zone provide information critical to the investigation of epithermal mineral deposits and their regional structural setting. Regional gravity data show steep linear gradients that coincide with major faults bounding the Hauraki Rift to the west and the Waihi caldera to the east. Regional magnetic data are dominated by a large bi-polar anomaly coincident with the Waihi caldera, which may result from a sub-caldera intrusion, and otherwise by high-amplitude shorter wavelength magnetic anomalies associated with the volcanic rocks.

High-resolution aeromagnetic and radiometric data reveal distinct signatures associated with epithermal deposits. Extensive magnetically quiet areas clearly delineate the location and extent of hydrothermal alteration zones around the deposits, whereas more localised zones of high K/Th ratios in these magnetically quiet areas delineate areas of intense potassium metasomatism. Epithermal deposits exhibit gravity signatures with two contrasting modalities: i) small negative anomalies (e.g. ≤30 gu at Golden Cross and Scotia) and ii) small positive anomalies (e.g. 40-50 gu at Karangahake and Waihi-Favona). Near-surface, low-density clay-altered andesite and/or enhanced fracture volume can account for small negative anomalies. However, positive gravity signatures show that significant mass anomalies must occur at greater depths; these may be either dense intrusions and/or zones of concentrated sulphide mineralisation.

Reverse time pre-stack depth migration, which uses the two-way acoustic wave equation, is not a new concept. Conventionally the method has been very computationally intensive and, therefore, has been considered impractical for production 3D depth imaging projects. Here we describe an efficient algorithm that can be used on large scale 3D seismic data. To make it practical and efficient we employ explicit 2^nd order in time and high order in space domain finite differences. We also use threading and domain decomposition methods to split the image cube amongst multiple CPU’s, when necessary. Only a few finite differencing layers are communicated between related CPU’s, by message passing during domain decomposition. High order spatial finite differences handle numerical dispersion and allow larger time steps than those possible with the, more conventional, pseudo-spectral method. We will show an overview of the method along with 2D and 3D synthetic and real data examples.

The number of oil and gas wells drilled through and near salt structures continues to increase. These wells are some of the most expensive wells to drill and are prone to numerous drilling problems that drastically increase their associated costs. Better understanding of the geomechanical setting in proximity to salt results in more successful drilling and completion of these wells. One promising area of study for investigating the perturbation of the stress field in and around salt bodies is numerical modelling. However, in order to apply the results of the simulated stress field from a numerical model to a well drilled near a real salt body with confidence, it is necessary to compare the modelled results to data observations. Therefore, it is important to have independent observations of the stress state near the salt body. One technique for determining principal stress directions uses cross-dipole shear wave velocity anisotropy data. However, shear wave velocity anisotropy can be induced by structural mechanisms as well as stress-related mechanisms. In this study, we investigate a technique to identify structure-induced velocity anisotropy and to characterize possible stressinduced velocity anisotropy. The investigation uses cross-dipole sonic data from three deep water, sub-salt wells in the Gulf of Mexico. First, we determine the parameters necessary to ensure the quality of the fast azimuth data used in our analysis. We then characterize the quality controlled measured fast directions as either structure-induced or stress-induced by predicting the apparent structure-induced fast direction the dipole sonic tool should measure for known bedding planes. We find that this technique supplements the use of dispersion curve analysis for characterizing anisotropy mechanisms. We also find that this technique has the potential to provide information on the stresses that can be used to validate a numerical model of the salt-related stress perturbations.

The deep gold mines in the Witwatersrand Basin of South Africa are some of the deepest underground mines in the world. For over a century, numerous, often deadly, mining induced earthquakes have been observed. In this work, we develop and test a new technique for determining the virgin state of stress near the TauTona gold mine. This work was completed as part of the Natural Earthquake Laboratory in South African Mines (NELSAM) project. The technique we use to constrain the far-field stress state follows an iterative forward modelling approach that combines observations of drilling induced borehole failures in borehole images, boundary element modelling of the mining-induced stress perturbations, and forward modelling of borehole failures based on the results of the boundary element modelling. The final result is a well constrained stress state consistent with all the observed stress indicators. We find that the stress state is a normal faulting regime with principal stress orientations that are slightly deviated from vertical and horizontal (denoted with a *). The maximum principal stress, Sv*, is deviated 0-20º from vertical towards the NNW and has a magnitude gradient of 27 ± 0.3 MPa/km. The intermediate principal stress, SHmax*, is inclined 0-20º from horizontal with an azimuth of 145º to 168º and has a magnitude gradient of 21.5 to 26 MPa/km. The least principal stress, Shmin*, is inclined 0-10º from horizontal with an azimuth of 235º to 258º and has a magnitude gradient of 13 to 15.5 MPa/km. This stress state indicates that the crust is in a state of frictional faulting equilibrium, such that normal faulting is likely to occur on pre-existing fault planes that are optimally oriented to the stress field. The stress concentrations caused by the mining activities can dramatically alter the range of fault orientations upon which fault slip could occur.

A wave propagation based method was investigated to improve the quality of seismic P-wave fracture prediction. The improvement is achieved through reducing the overburden influence superimposed onto a target layer. Rather than relying on evaluations of layer’s properties across an interface, our method predicts fractures using two “fracture functions” estimated from the top and bottom reflections of a target layer. The two functions are firstly served as a measure of overburden influence, and then incorporated in inversion to minimize the overburden effect that results in improved fracture prediction for a target layer.

Tests on physical model data suggest the viability of the method that operates on pre-stack data, and is applicable to a wide range of seismic surveys including 2D, 3D and VSP.

Led by Google Earth, globe viewers provide a compelling approach to the visual integration of geospatial data in resource exploration. However, finding and evaluating data from both internal data stores and the vast quantities of data available on the Internet has remained a challenge. Finding spatial data requires building a catalog of data services and associated metadata, from which high-speed spatial and text searches can be performed to discover data of interest. Once discovered, evaluation of that data demands responsive visualization and metadata review. This paper discusses the development of a server-based spatial data crawler that builds a catalog of spatial data services on the internet. The challenges of scaling large catalogs, query performance optimization and the delivery of metadata are addressed. We demonstrate how this service has been integrated into an open-source and standards-based globe viewer for use in exploration today.

AEM systems are essential tools for a wide range of mineral exploration and geological or environmental mapping applications. The product of peak dipole moment and the Liu waveform factor provides a quantitative estimate of the effective signal strength of a TEM system at a specific base frequency, and can be used to “compare” systems. Given also the noise levels of an AEM system and its frequency or time sampling, it is easy to predict the capabilities and resolution of the system.

The most challenging development required of AEM is the development of systems operating at 5 Hz or less to penetrate conductive cover and assist in the discrimination of very conductive copper/nickel sulphide deposits. Altimeter errors provide the main limitations in depth resolution of shallow environmental targets. 2D and 3D imaging and inversion strategies are not yet reliable or fast enough for routine application.

Earthquake a phenomenon,thought to be out of control of human is no longer an uncontrolled menace. This paper deals with the cause and effects of earthquakes and its subsequent negation. The shakes and tremors were thought to have been caused by plate movements and related occurrences.

We have presented an alternate hypothesis for these phenomena. Our hypothesis is competitive enough to convincingly deal with the anomalies which are present in the plate tectonic and subordinate theories.

Success with heavy oil production depends as much on knowing the geology of reservoir as it does on understanding the fluid properties of the reservoir. Heavy oils are defined as having high densities and extremely high viscosities. Due to their viscoelastic behaviour the traditional rock physics based on Gassmann theory becomes inapplicable in principle. In this paper, we use effective-medium approach known as coherent potential approximation or CPA as an alternative fluid substitution scheme for rocks saturated with viscoelastic fluids. Such rocks are modelled as solids with elliptical fluid inclusions when fluid concentration is small, and as suspensions of solid particles in the fluid when the solid concentration is small. This approach is consistent with the concepts of percolation and critical porosity, and allows one to model both sandstones and unconsolidated sand.

To test the approach against the known solutions, first, we apply CPA to a rock with fluid pore fill and compare the obtained estimates with Gassmann results. Second, we compare CPA predictions for solid-solid mixtures with numerical simulations. Good match between the results confirms the usefulness of the proposed fluid substitution scheme. We extend the CPA scheme to predict the effective frequency- and temperaturedependent properties of heavy oil rock. We also use viscoelastic extension of Hashin-Shtrikman (H-S) bounds to obtain alternative estimates. The proposed fluid substitution scheme provides realistic estimates of the properties of a rock saturated with heavy oil which lie between H-S bounds.

Fracture-induced anisotropy is of moderate to large magnitude in sand reservoirs and can be examined in order to further advance our knowledge of fractures in the reservoirs. Production and increasing recovery rate of the existing oil and gas fields have shown to attract arrested attention and new emphasis. Fractured reservoirs contain an enormous amount of oil and natural gas. Fractures control in situ permeability; fluid storage and mobility; and rock strength; and thus the reservoir itself. Therefore, a clear insight into the subsurface fracture network will definitely promote optimization of well planning and production. Rock properties in a fractured reservoir indicate the amount of hydrocarbons and the convenience or inconvenience of production. Ignoring open fractures may lead to fluid flux, loss of circulation and other unsuccessful exploration and production efforts.

Hyperspectral drill core scanning undertaken using the CSIRO HyLogger^™ between 2002 and 2005 focused on accumulating spectral data from a series of signature holes from across the State of South Australia. One component of the software used to process the data provides a summary for each hole indicating the amount of each detected mineral as a percentage of the scanned hole. By converting this percentage into the number of metres of detected mineral present in the drill hole this information can be presented in a GIS.

Four mineral suites are coming to the fore in alteration mineral mapping using HyLogger^™: white micas, chlorites, carbonates and iron oxides. Each of these suites can relate to Eh-pH conditions in a mineralising system. For white mica, the transition from muscovite through phengite, as measured by the progressive change in wavelength position of the ~2200 nm absorption feature to longer wavelengths, corresponds with increasing replacement of aluminium in the crystal structure by iron or magnesium. Empirical studies show a correlation between concentration of economic metals and the presence of phengite that may also reflect local fluid pressure conditions. The ratio of Fe/Mg in chlorite has also been shown to vary in proximity to mineralisation. Calcium, iron and magnesium carbonates are a third component. Spectral studies have distinguished between hematite, Fe²⁺ goethite and Fe³⁺ goethite. HyLogger detects in wavelengths appropriate to these suites and software interprets relative abundances. Plotting the number of downhole metres of core containing these assemblages in their relative geographic locations permits interpretation of regional patterns of alteration. With some 600 holes (including 300 on open file) and 61,000 metres of core scanned across South Australia, regional patterns are starting to appear.

The Airborne Geophysical survey of the PNG Highlands and the Papuan Peninsula involves the collection of geophysical data using helicopter-borne magnetic and gamma-ray spectrometric instruments over 2 areas of Papua New Guinea (PNG), where little or no prior geophysical coverage is available. The availability of these data in both digital and paper-map formats is expected to encourage and facilitate exploration in PNG by international mining companies.

All of Area 1, comprising the PNG Highlands portion of the survey, is anticipated to be completed by the end of the 2007 field season in late October. Flight operations will recommence in March 2008 in the Papuan Peninsula. Preliminary interpretation of the magnetic and radiometric data collected to date is providing a much higher degree of resolution and accuracy than has been available to date; with more detail in some areas, and new features being identified in others.

As part of an appraisal program by InterOil of the Bwata gas resource and prior to undertaking further 2D seismic surveying, a 3D geology model of the project was rapidly built using the 3D GeoModeller software.

The software implements a methodology developed in the BRGM to jointly interpolate geological contact data and dips of geology formations. The method uses the chronostratigraphic order of geological formations, and their rock-relationships. The model is calculated using an implicit 3D potential function as the interpolator for each component part of the geological history. The order and relationships recorded in the stratigraphic column are used to automatically resolve the intersections between component parts, and produce volume reconstructions. The methodology allows the geologist to focus on geological issues and consider alternative interpretations.

The 3D structural geology model was built using a single 2D seismic line, well data from the Bwata-1 and Triceratops-1 wells, surface geological data and airborne gravity data. Eight cross sections across the Bwata Anticline were created from surface geology, seismic and well data in 2D Move. These sections were imported into 3D GeoModeller. A 3D model was then created and the forward gravity response computed. Density variations from general background of 2.24 t/m³ are provided by the Cretaceous Ieru Formation at 2.40 ± 0.05 t/m³ and the Puri and Mendi Limestones at 2.70 ± 0.05 t/m³. The computed response was compared to observed data derived from an airborne gravity survey. On the basis of such comparisons several iterations of geologic revision were proposed to improve the fit between the computed and observed data.

The outcome of this study was the prediction of the geological setting and the extent and thickness of the limestone beds. The model incorporated c. 30 degree dipping thrusts and a steeper backthrust and introduced two NE/SW near vertical faults which exhibit a sinistral strike slip and east-side-up displacement. A substantial increase in the size of the field was interpreted at its western end. Using the 3D model will enable InterOil to design a follow-up 2D seismic survey with greater confidence that the survey will meet program objectives.

The objective of this paper is to seek a generalized understanding for the influence of incorporating the gradient of the model’s physical properties (e.g., conductivity, velocity) in the forward modelling numerical algorithm. In order to take a step towards that, we examine an example from Geophysics for solving 3-D Maxwell’s equations using finite difference (FD) methods. The 3-D FD methods to obtain discrete solutions of Maxwell’s equations include the staggeredgrid and balance methods. The balance method 3-D algorithm exploits the conductivity gradient in order to make the FD formulation a seven-point scheme and the resulting matrix a banded septa block diagonal but not symmetric. The staggered grid algorithm is free of conductivity gradient and results in a symmetric 13-diagonal banded matrix. The objective now is to examine and understand better the influence of the conductivity gradient incorporated in the FD equations on the accuracy of the electromagnetic (EM) modelling for two 3-D benchmark models. We use three various discretizations (fine, mildly coarse, and coarse) for each model. The modelling results of each discretization have been computed separately by the balance method and staggered grid method. We have found that the staggered grid method produces accurate results for all the three discretizations investigated. However, the balance method encountered some inaccuracies for the mildly coarse and coarse discretizations. This appears to be due to the presence of the conductivity gradient in the 3-D modelling algorithm. The model studies also suggest that the thicknesses of the horizontal and vertical discretizations at the conductivity boundaries should be about 1/25 and 1/100 skin depth to maintain accurate modelling results when the conductivity derivatives exist in the 3-D modelling algorithm.

We have developed a new inversion algorithm to successively determine the depth (z), polarization angle, and the electric dipole moment of a buried structure from the self-potential (SP) data measured along profile. By utilizing the entire values of the SP profile, the problem of depth determination is formulated into the problem of solving a non-linear equation of the form f (z) = 0. Using the estimated depth and by applying the leastsquares method, the polarization angle is then determined. Finally, having known the depth and polarization angle, the electric dipole moment is determined in a least-squares sense. The proposed SP inverse algorithm has been derived for semi-infinite vertical cylinder, infinitely long horizontal cylinder, and sphere anomalous bodies. The method is tested on synthetic examples with and without random errors, and applied to a field example from Germany for mineral exploration. The estimated depths and other SP model parameters are found in good agreement with the known actual values. The results obtained will be presented and discussed in the conference.

The Tanami 3D crustal-scale model integrates geological mapping with seismic interpretation and gravity modelling in order to enhance our understanding of the 3D architecture of the region.

The Tanami Region is a multiply deformed Palaeoproterozoic terrain consisting of the predominantly turbiditic Tanami Group, the volcanically derived sedimentary and extrusive rocks of the Ware Group and the Mount Charles Formation. Thought to be underlain by Archaean basement, the region was intruded by numerous granitoid bodies during the period 1820-1790 Ma and hosts the Callie world-class and several smaller gold deposits.

The 2005 Tanami seismic survey consisted of four regional traverses. Interpreted sections show the presence of a series of crustal-penetrating structures, that appear to be fundamental to the evolution of the Tanami Region. A number of these interpreted structures appear to link the mid-crust to ‘thin-skinned’ structures within the uppermost crust. The seismic also shows the presence of a southeast-dipping suture and associated pop-up structure, separating the Tanami crust from the Arunta crust. Known ore deposits are all located in close proximity to these interpreted through-going thrust faults and the associated ramp anticlines. Gravity modelling of the southeast-trending backbone seismic traverse (05GAT1) supports the seismic interpretation.

The current 3D model was constructed using 3D GeoModeller incorporating seismic data, modelled gravity, interpreted magnetic and gravity data and geological mapping. The 3D model forms the basis of the Tanami X3D web viewing tool, which allows the model and associated data to be viewed and manipulated over the web.

Converted-wave refraction statics is an algorithm that incorporates both compressional (P-) and shear (S-) wave refraction events to correct S-wave static errors in multicomponent seismic data. Converted refraction (PPS) events are picked on an inline component shot record in the same way as first-breaks are picked on a normal vertical component record. These picks are then analysed using the reciprocal method to create a near-surface model from which S-wave receiver statics are derived.

The derived PPS refraction statics have a similar shortwavelength character to the S-statics obtained via statistical analysis of converted-wave reflections. Based on standard P-wave practice, we believe that an optimal production approach will include converted-refraction analysis, followed by converted-wave residual statics.

Although the thrust of this work has been towards derivation of S-wave statics, an interesting auxiliary output is also available. Based on theoretical modelling, the S-to-P time-depth ratios can be tuned to provide P-to-S velocity ratios (and hence dynamic Poisson’s ratios) for the near-surface. This has interesting implications for lithological and rock-strength analysis in the mining, environmental and engineering contexts.

Tar or bitumen deposits in the Canning Basin, Western Australia, indicate possible paths of petroleum migration. Neutron and X-ray tomography is applied to rocks containing biodegraded tar in order to investigate the distribution of petroleum within the pore system of the samples. Neutron tomography allows the imaging of hydrogen-rich components, such as tar and bitumen, within a sample, whereas X-ray tomography assists density analysis of the grains and pore space.

The results indicate that the methodology discriminates excellently between tar and the other rock matrix and open void components. The images permit analysis of the connectivity of pore paths and enable conclusions to be drawn about petroleum migration.

In the case history presented, the images demonstrate the injection of petroleum into a dilated fault zone, and the interaction of the petroleum with the fault gouge material. The images also demonstrate the nature of petroleum movement into sandstone beds, which were in communication with this fault zone.

This paper presents a novel approach to identifying/tracking interlocking Palaeocene sand and shale-filled channels (based on forward modelling, AVO and inversion-of-inversion techniques) in an appraised subtle Palaeocene deep-marine turbidite, Central North Sea. The Brenda Field has been an excellent example of where rock physics can act as the bridge between petrophysics and seismic in the world of interpretation.

Geophysical, particularly electrical, methods have the potential to provide detailed spatio-temporal information on the distribution of salinity in soils and groundwater that characterise the floodplains of the Murray River in southeastern Australia, thereby assisting our understanding of floodplain processes. This knowledge can help manage the ecology of these settings, particularly in a time of severe drought when floodplain salinisation, vegetation dieback or health decline is a growing problem.

In this paper we examine the relative merits of high resolution helicopter EM technologies for elucidating floodplain processes. Specifically we consider the relative performance of the RESOLVE frequency domain helicopter EM (FDHEM) and the SkyTEM time domain helicopter EM (TDHEM) systems for defining variations in near surface conductivity and sediment salt load across the Bookpurnong Floodplain in the Riverland of South Australia. Results from coincident surveys are reviewed as are strategies for the inversion of these data. Data are examined against available borehole information including sediment chloride content and groundwater conductivity.

FTG Gravity data acquired on airborne and marine platforms measure 5 independent Tensor components that collectively describe a total gravity field. The components capture unique signature patterns related to specific attributes of target geology that when collectively interpreted enable detailed imagery of the target itself in terms of geometry, composition and depth of burial.

The horizontal tensor components Txx, Tyy, Txy, Txz & Tyz are commonly used to identify and map lineaments associated with structural and/or stratigraphic changes or target geometry in a survey area. The vertical tensor component, Tzz, is used to estimate depth and predict compositional information related to target geology. However, these components have traditionally been interpreted separately from one another and often run the risk of missing out on key information.

This paper describes application of a semi-automated approach that combines the individual components into singular representations to best extract the signature pattern common to all components as revealed by the underlying geology. The examples presented are taken from an Air-FTG^® survey onshore Brazil to image the structural framework and identify target geology ahead of a seismic programme, and a Marine-FTG^® survey offshore Norway to resolve salt body geometries imaging areas of overhang development.

The resultant interpretation enables the end-user to fasttrack the exploration initiative by quickly evaluating target geology for detailed follow-up.

Completion of a 1:100 000 geological mapping program over the Koonenberry region of north-western New South Wales has allowed the Geological Survey of New South Wales to develop techniques for potential field modelling of serial, sheet-by-sheet cross-sections, with the ultimate aim of constructing a 3D interpretative tectonic model for this Delamerian margin. Modelling has been aided by comparison of magnetic and gravity structural trends, revealed by edge analysis of upwardly continued fields (“worming”), made practical by improved data density and quality resulting from the Exploration NSW initiative. Worms aid modelling in two senses: indirectly, through qualitative assessment of tectonic styles; and directly, by fixing the position and trend of otherwise poorly imaged deep sources to constrain inversion. We exploit the different rate of decrease during upward continuation of anomalies due to dipole (magnetic) and monopole (gravity) sources, to distinguish structural differences between relatively shallow (<2–5 km) and deeper rocks. This has been particularly useful for examining the fold-and-thrust tectonics of the region. Comparison of synthetic worms derived from the model anomalies with worms derived from the gridded data allows a powerful test of the validity of the model.

Resulting profiles have been combined in GOCAD to produce a 3D model of the belt. Definition of surfaces linking the profiles is guided by an inverted image of the potential field worms. The final 3D model reveals a series of features related to repeated late Neoproterozoic to Cambrian rifting, amalgamation of the Delamerian and Thomson orogens, and subsequent transpression.

For the past decades nuclear magnetic resonance (NMR) technology has gained acceptance as petrophysical tool for evaluating reservoir quality. Comprehensive formation evaluation requires determination of irreducible fluids, movable fluids, and permeability. The presence of clay, their occurrences and distributions, however, in some reservoir rocks tends to introduce complexity in any formation evaluation activities. This can also cause problem for NMR log interpretation. In the presence of clays the most commonly used T2 cut off values, a constant value throughout a formation, seem to eventually yield inaccurate permeability estimates. Therefore, NMR measurements should be integrated with other measurements from conventional cores for a comprehensive formation evaluation, in which T2 cut off may vary for reservoir with different reservoir qualities. This paper presents results of a study that focuses on NMR measurements on Tirrawarra shaly sands taken from 3 wells situated in Cooper Basin, South Australia. The study suggests that the T2 cut off values for the samples vary significantly in order for NMR-derived irreducible water to match core-derived irreducible water. This is also true for NMR-derived permeability estimates when compared to measured permeability values. Comparisons between estimates produced using the normally used ‘constant T2 cut off’ and the suggested ‘varied T2 cut off’, as well as their effect on formation evaluation, are also discussed. In general, the results highlight the need to study T2 cut off values more directly for specific reservoir rocks before their practical uses in the field.

This paper aims to estimate elastic parameters such as Pwave and S-wave velocity, Thomsen’s anisotropy parameters (, ε density, and layer thickness of a layered medium using pre-stack seismic data and some prior information.

Three different algorithms have been developed in an object oriented environment: Travel time inversion, waveform inversion, and joint traveltime and reflectivity inversion. These algorithms can use compressional or joint compressional and converted shear waves data respectively. We have implemented exact parametric travel time equations in a VTI medium for a stack of either anisotropic or isotropic homogeneous layers. Reflection and transmission coefficients for the whole ray path have been calculated. A convolution model is used to calculate synthetic seismograms.

To minimize an objective function a non-linear conjugate gradient (CG) algorithm has been implemented. All of the partial derivatives have been calculated analytically which results in a fast and robust differentiation of traveltimes and reflection and transmission coefficients.

A numerical example of a stack of 10 layers has been tested for different elastic parameters and thicknesses. Compressional and shear waves velocities and thicknesses have been varied 25 percent from true values while  and ε have been varied 100 percent from their true values. Joint traveltime inversion of P  P & P  Sv has successfully estimated the true values of the parameters. Waveform inversion is highly non-linear; hence only anisotropic parameters and density while varying up to 100 percent are estimated. Waveform inversion results in a full estimation of the true values of  and ε and a fairly good estimate of density.

Subcritical crack growth is one of the main causes of time-dependent fracturing in rock. In this study, subcritical crack growth in rock was investigated in distilled water (pH=6) and aqueous solution of sodium hydroxide (NaOHaq, pH=12). Especially, comparing the results in water to those in air, the effect of water was investigated. Additionally, the effect of pH in aqueous environment was also investigated. Rock sample were andesite and granite. The relation between the crack velocity and the stress intensity factor was determined by Double-Torsion method. All experiments were conducted with controlling temperature. It was shown that the crack velocities in water were higher than those in air. These results are in accordance with the results by many researchers who reported that the crack velocity increased in water. Comparing the results in NaOHaq to those in water, however, it was shown that the crack velocity at the same stress intensity factor didn’t change even though pH of surrounding environment changed. This result doesn’t agree with conventional concept that the hydroxyl ion accelerates subcritical crack growth in silicate materials. It is concluded that water accelerates the crack velocity and pH has little effect within the environmental conditions in this study.