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

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.