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

Electromagnetic depth sounding (EMDS) data are computed for simulated multi-layer earth models at different frequencies and geometries using digital linear filters. Detectability of sub-surface layers is computed as point-to-point difference of data instances between threelayer - homogeneous layer and two-layer - homogeneous layer media for similar range of frequencies. H and K-type earth models are also considered in the computations. Response curves, computed for two different layer earth media models are superimposed, thus, the separation between curves is the direct indication of the involved “detectability effect”. The degree of separation among response curves between two geometrically or parametrically changing data properties, called resolution, has direct impact on detectability effect.

Though detectability does not provide any qualitative or quantitative interpretation of data attributes, computed detectability effect significantly changes EMDS response resolution at varying layer-earth data attributes. Knowledge on strength of detectability and scalable properties among layer-earth media are interpreted based on resolution and coherency between two model response curves. Because of change in layered earth properties, varying horizontal and vertical resolution and coherency attributes between response curves provide considerable detectability effect. This process, termed as data-mining, facilitates extraction of knowledge of layer properties within multi-layer earth media. This detectability effect provides knowledge of n-layer-earth simulation, which can effectively respond to and aid the interpretation of actual geological models deduced from experimental data. These studies could prove to be useful for investigating shallow petroleum oil and gas seeps and their associated sediment alterations in the basin margin areas.

Gravity meters are commonly used to map spatial changes in the earth’s gravity, achieving resolution of parts per billion on microgravity surveys. The application of gravity surveys to monitor change and recent improvements in gravity instrumentation will be summarized.

The use of gravity to track the progress of a waterflood in Prudhoe Bay, Alaska, provides a 4D Gravity case history. The surface gravity change caused by the injection of water into the gas cap is measured annually and compared to the expected change calculated from the reservoir model. 4D Gravity provides an effective “early warning system” for injection and sequestration projects.

A recent borehole gravity survey in Hanford, Washington, demonstrates the ability to measure bulk densities using a borehole gravimeter. The applications include geotechnical studies at waste disposal sites, bridges and structures, locating and monitoring thief zones in reservoirs, and grade control in iron mines. A new borehole gravity meter will be introduced during 2008 that can be used in smaller, inclined boreholes.

Earth tides and other long period movements of the earth can be recorded by suitably configured gravity meters. The ground shaking in Luxembourg before and after the January 13, 2007, earthquake in Japan was recorded on a long period seismometer, a superconducting gravity meter and a new portable gPhone gravity meter. The match between the seismometer data and the gPhone data during the earthquake is excellent. The relatively inexpensive portable gPhone gravity meter provides the means to record earth movements along active fault zones and other critical locations. gPhone data recorded in Denver, Colorado on August 15, 2007, indicates preshock and post-shock ground motion as well as the 8.0 earthquake in Peru.

It is now possible to invert frequency and time domain data to recover 3D conductivity models. The San Nicolás deposit makes an ideal test site because much is known about the deposit and also many different types of survey data have been collected there. In particular DC resistivity, 3D controlled source frequency domain data, and UTEM data have been acquired. We have inverted each of these data sets. In this talk we describe the data sets and the methodologies for inversion and compare the resultant models. The DC resistivity data were not able to see the deposit because of the conductive overburden, but both the frequency and time domain inversions produced a good image of the primary mineralized zone.

We analyse seismic polarisation of direct and reflected PS arrivals on multi-component ocean bottom (OB) data, by mapping vector fidelity as a function of azimuth and angle of incidence at the seabed. We compare data from a cable system with data from nodes, as well as buried and unburied cable sensors.

As expected, nodes show overall best vector fidelity. For OB cable data, on the other hand, buried sensors show significantly better vector fidelity than unburied sensors, both for down-going direct P waves and up-going PS reflected waves. While both buried and unburied cable sensors exhibit vector infidelity to some degree in the direction along the cable, unburied sensors show in addition poor fidelity in the crossline direction. Vector infidelity of the unburied sensors may be misinterpreted as azimuthal anisotropy with a symmetry axis parallel to the cable.

Most current near-surface seismic refraction operations are under-capitalized, they employ inefficient field procedures, and they deliver an outdated low resolution 2D product. In order to generate growth and more widespread use for environmental and geotechnical applications, it will be necessary to modernize the great majority of refraction operations.

The most significant step will be the adoption of 3D methods, which emphasize the superior lateral resolution of geophysical methods, and which generate more detailed geotechnical models of the subsurface. Furthermore, greater use of amplitudes, the other 50% of the data, is recommended to resolve many ambiguities with model-based inversion, to generate density models, and to recognize inhomogeneities such as voids. Implementing these advances will require more efficient approaches to data processing with full trace methods, such as the RCS, if the greatly increased volumes of 3D data and out-of-plane events are to be handled efficiently. Also, more efficient methods of data acquisition using roll-through methods with greatly increased channel counts are necessary for both 2D and 3D methods. These advances will require many geotechnical organizations to decide whether their core business is data acquisition and processing or alternatively, the provision of specialist geotechnical services through the interpretation and integration of results.

Most of the technology required to modernize refraction methods already exists. Its adoption is being hindered by an “engineering culture” which emphasizes risk minimization, the strict adherence to standard procedures, the adoption of codes of practice and the use of “proven” products, all of which encourage minimum standards rather than innovation and excellence.

The Mt Bulga ore body, which is located near Orange in south-eastern Australia, consists of a narrow (5 – 10 m) syngenetic fine grained banded massive pyrite-galenasphalerite- chalcopyrite in steeply dipping unfolded Silurian altered metasediments. Several major shear zones are associated with the ore body.

A 2D seismic refraction profile was recorded with a station separation of 2.5 m, across a small ridge, which also marks the approximate location of the massive sulphide ore body. The data were processed with wave eikonal traveltime (WET) tomography using a starting model generated with the 1D tau-p inversion algorithm and with the GRM SSM. The 1D tau-p refraction tomogram showed that the ore body has a higher seismic velocity than the adjacent regions, whereas the GRM SSM showed a low seismic velocity in the ore body. A density model, which was generated with the head wave amplitudes and the refractor velocities, showed a marked increase over the ore body, suggesting that the lower seismic velocity in the ore body is more likely. These results demonstrate that seismic refraction profiles can provide useful information on depths of weathering and density contrasts for joint inversion with either airborne or ground gravity data.

These results also demonstrate that the application of seismic reflection methods to the imagining of sulphide ore bodies will require detailed mapping of the nearsurface layers, in order compute useful statics corrections. The rapid changes in depths, as well as the large lateral variations in the seismic velocities in the unweathered material suggest that wave equation redatuming, rather than simple time shifts would be required.

Statics are the corrections to seismic reflection data for the weathered layer and variable topography. Traditional statics routines estimate an initial model of the weathering and then iteratively refine it with NMO velocity analyses and residual statics routines. However, this approach can often be inefficient, especially with the very large volumes of data characteristic of single sensor data, and it has not proven to be especially efficacious with shear wave statics where the low seismic velocities result in quite large traveltimes.

By contrast, a new GRM-RCS approach obtains an accurate time model of the weathering which does not require substantial improvement with residual statics. Accuracies of ±1 millisecond can be routinely achieved with good quality data. As a result, the new GRM-RCS approach can improve the efficiency of normal data processing through eliminating at least one iteration of velocity analyses and residual statics. Furthermore, the improved accuracy is necessary where higher frequencies are recorded with single sensor data, if the field statics are to be within half a cycle of the higher dominant frequencies for residual statics routines to be most effective.

There can be up to five stages with the GRM-RCS approach. Starting with a new 1D QC algorithm, each stage provides greater resolution of the time model than that obtained in the previous stage. In the final stage, the surface consistent time delays in the surface soil layers are separated from the non-surface consistent time delays originating at the base of the weathering.

Because the GRM algorithms are exact and do not require optimization or tomography for accuracy, they can be readily modified to accommodate field data of varying quality, such as with data in which cycle skipping occurs. Furthermore, they can be readily extended to full trace processing with the RCS.

Elastic properties of shales are important for quantitative interpretation of seismic response, detecting high pore pressure before drilling and understanding overburden response during production. Modelling shale properties is difficult however as elastic properties of individual clay minerals are hard to measure directly. We use a transversely isotropic differential effective medium approach to calculate elastic constants of shales with different silt fractions. Increasing silt fraction from 0 to 25% results in an increase of elastic constants C11, C33, C13, C44 and C66 by 15%, 25%, -0.5%, 33% and 75%, respectively, whereas a 50% increase of silt fraction increases the elastic constants by 36%, 70%, -7%, 83% and 220%, respectively, in comparison with the constants of pure clay. These non-uniform alterations of elastic constants might lead to changes in elastic anisotropy. Thomsen’s anisotropy parameters ε, γ, and δ which describe the variation of P- and SH-wave velocities as a function of polar angle with respect to symmetry axis are also calculated. ε decreases by 18% and 43% with increasing silt fraction from 0 to 25% and 0 to 50%, respectively. γ decreases by 15% and 36% by adding 25% and 50% of silt, respectively. δ decreases by 60% with increasing silt fraction from 0 to 25% and changes sign when silt fraction reaches 42%. The results show that the presence of silt in shales cannot be neglected as it substantially increases compressional and shear velocities and reduces anisotropy. Elastic moduli of shales are predicted using known silt fractions and elastic moduli of clays and compared with the results of laboratory measurements.

Geophysics can play an enhanced role in exploration programs when used in conjunction with geology and physical properties. Understanding how geology relates to geophysics is important both for supporting constrained geophysical modelling, and for extracting meaningful information from geophysics. Physical properties, and how geologic processes control physical properties, play a key role to link geology to geophysics and are an important focus of our research. In addition, methods of describing geology in a manner that can be incorporated into geophysical inversions provide another important link between geology and geophysics to aid in the integration process. This information is brought together and applied to deterministic inversion methods that have been developed at the University of British Columbia. When done in the context of a specific exploration goal, earth models can be produced that capture, and are consistent with, available geoscientific information, resulting in a clearer view of the earth.

While constrains on source and receiver sampling for convolution SRME are easily fulfilled in the 2D case, the problem become significantly more complex in 3D. By requiring ideally equal and coincident source and receiver sampling for performing the surface consistent data autoconvolution required by the method, it constrains the acquisition geometry toward the denser possible designs.

For allowing the acquisition effort to be focused in illumination considerations related to imaging purposes instead of anti-multiples constrains, a series of processing solutions are now available, that made possible the use of 3D SRME methods for any kind of 3D acquisition geometries, including OBS (Ocean Bottom Surveys) and WATS (Wide Azimuth Towed Streamer) geometries.

Latest improvements on efficient interpolation methods, allied to larger storage and computing capacities, allow for the regularization of irregularly sampled and aliased data toward regularly sampled grids suitable for the convolution based 3D SRME. It is a purely data based approach, free of any previous knowledge of the propagation velocity fields.

The alternative approach is the partial or full model based approach where wave equation modeling techniques are used for predicting 3D multiple models. The particularity of such approach is that its flexibility allow for handling any extreme acquisition geometry, as it can even apply to OBC geometries when no surface data is made available.

Although natural higher folds related to WATS geometries allow for better stacked or migrated sections even when not any anti-multiple is applied, 3D de-multiples are still needed for improving the data quality pre-stack. In this context, we can show that the state of the art of data-based and model-based 3D multiple modeling techniques allow for an efficient and accurate de-multiple processing.

The regional geology of parts of the state of New South Wales is very poorly known because of a thick regolith cover that makes field mapping difficult. Geologists often rely on the interpretation of geophysical data to help with the regional mapping.

The use of potential field data in the interpretation of geological maps is very common. However, due to the non-uniqueness of potential field modelling, there can be no certain or unambiguous differentiation between different possible source rock geometries without reference to a priori information such as independent geological data. In order to overcome this problem, we use a wavelet analysis of potential field data, by looking at the horizontal gradient at different continuation levels. This method, also called multiscale edge detection (or worming), has proven to be a valuable additional tool for current regional mapping in NSW.

We have applied the edge detection method in several areas of NSW. In this paper, we present results from the Cobar Basin, and also the Koonenberry region in the north-west of NSW. In the Cobar area, the method has helped define the edges of the basin, the major faults as well as their dips, and the internal structure/architecture of the basin. In the Koonenberry region the worms were found to be related to major structures and intrusive margins, and were considered when defining the shape and dips of buried bodies in 2D modelling of cross sections. The method has added an additional degree of confidence in understanding the deep seated tectonic fabrics of these regions.

A glass of Chardonnay, eucalyptus oil and a loaf of bread all begin life with the plant interacting with its environment. For example, sensory characteristics of a glass of Chardonnay may be unique to the region from which the grape vine was grown in. A certain combination of climate, landscape, geology and regolith factors make up this ‘terroir’ characteristic in wines. This may also be the case for wine produced from different vineyards in a specific region and within a vineyard, as such combinations of factors change. Specific regolith features, such as soil texture and rooting depth can change in short distances, so that the yield or vigor of grape vines, wheat crops and eucalyptus species can subsequently be affected. This paper reports on the efficiency of geophysical methods, specifically radiometrics and ground penetrating radar, in identifying regolith characteristic which effect plant growth and their production endpoints.

Over the past 27 years, the AMIRA P223 project series has produced an extensive body of EM modelling and inversion programs used by the minerals, environmental and defence industries for planning and interpreting EM surveys, and for the development of new EM exploration instruments. This project series is now into its final overtime. Historically, the software generated by these projects has been available only to the project sponsors and their designated contractors. All programs are now commercially available through EMIT’s Maxwell graphical user interface. From 2010, the Fortran 90 source code for all programs will be open source. The purpose of this paper is to make the wider EM modelling community aware of the capabilities offered by this extensive, commercially-available software suite. The models for both modelling and inversion include a general 3D full domain finite elements (Loki class), 3D compact finite-elements (Samaya class), 2.5D fulldomain finite-elements (Arjuna class), multiple 3D plates embedded in a multi-layered host (Leroi class) and a 1D layered earth (Airbeo and Beowulf). The programs can be used for any frequency or time-domain airborne, ground or downhole EM system. Sources can include multi-vertex closed loops, grounded wires, magnetic dipoles and plane waves. Receiver types can include multi-vertex loops, grounded wires, and magnetic and electric dipoles. Survey types include the variety of airborne configurations, fixed sources with independent surface or downhole receiver lines, moving sources with multiple fixed-offset receivers and magnetotellurics. Inversion for all model classes is based on an iterative, damped SVD method which concentrates on those parameters that most affect the data whilst ignoring those that are irrelevant.

Airborne magnetic and electromagnetic surveys together with kimberlite indicator mineral geochemistry have been standard practice in the search for kimberlites. The recent advent of the airborne gravity gradiometer (AGG) showed that airborne gravity gradiometry could also be a successful tool in kimberlite exploration. The installation of a digital AGG system on a helicopter led to the first airborne gravity gradient cum magnetic cum electromagnetic survey. The survey was flown over the central part of the Ekati tenement within the Lac de Gras kimberlite province.

Each of these three geophysical methods relies on a different physical property contrast for its success. A selected sub-area shows that no one method would have identified all known pipes. But all known pipes would have been discovered by integrating all three data sets. The pipes in the selected area are associated with conductivity and/or gravity gradient anomalies; a few with magnetic anomalies.

New data alone are not sufficient to guarantee success in a mature exploration environment. The geophysical data were individually inverted to create 3D density, magnetic susceptibility and electrical conductivity models. Integrating and applying classification techniques to the three 3D models was used in the generation of new targets.

Drill testing of the targets has begun leading to the discovery of a new pipe.

This paper is basically a compilation of various anomalies and loop-holes in plate tectonic theory. The tectonic theory explains that earthquakes take place due to tectonic plate movement. Rhetorical questions are raised in opposition to the very basic principles of the tectonic theory and evidences are provided as to why the thin plate theory is implausible. Further evidences opposing crucial concepts such as continental drift theory, seafloor spreading and oceanic floor aging, subduction and emphasising contradiction between data and tectonic model predictions are presented as a part of a preliminary study on Plate Tectonic theory.

Technical Area:

The paper is a study of the concepts of Plate Tectonics and its ability to effectively deal with earthquake occurrences. There has been organized opposition about the theory called plate tectonics throughout the world.

This paper tries to bring out the comparative study between pros and cons of plate tectonics and contains original calculations as well. The paper tries to bring out the facts and figures about plates, its occurrence and leads to another research where the earthquake can actually be negated.

A passive seismic survey to investigate variations in crustal structure across the Yilgarn craton has shown significant contrasts in seismic velocity models between neighbouring terranes/superterranes. The Eastern Goldfields showed a unique variability in crustal structure in agreement with a recent reinterpretation of terrane boundaries within the Yilgarn craton. We further investigated the Eastern Goldfields region using a 3-way approach which combined conventional passive seismic analysis with innovative seismic noise-correlation methods and constraints from active source data. Conventional passive seismic analysis enables the Receiver Function S-velocity structure, and hence composition, of the lower crust to be constrained. Noisecorrelation analysis allows seismic velocity models in the 5-15 km depth range to be determined and provides medium resolution coverage across regions not previously explored using active seismic methods. Where active source data have been acquired, shallow structure and deeper seismic velocity determinations are added, providing an unprecedented combination of seismic constraints on the structure of this complex and economically important region. We find that, although some individual terrane boundaries within the new Eastern Goldfields reinterpretation are open to question, the concept of the multi-terrane amalgamation is substantially justified by the exceptional variability of the lower crustal structure. Upper crustal structure is often characterised by seismic discontinuities which may represent detachment surfaces or layered structure that varies between terranes over a sub-100 km length scale. The accretionary history of the superterrane and associated regional tectonic setting of numerous formations of economic significance would now appear to be beyond question.

The SkyTEM airborne EM system has been deployed in Australia since late 2006, and has been flown for a variety of applications including salinity mapping, palaeochannel detection, geological mapping and base metals exploration. Economic geological applications of the system have included gold and uranium exploration, as well as direct detection of massive sulphides. The SkyTEM instrument was designed to produce airborne electromagnetic data of a quality comparable to that which can be obtained from existing ground TEM systems, and is unique in that it can alternately transmit in low-moment, early-time sampling, and high-moment, late-time sampling modes, thus providing a combination of high-resolution shallow information with a maximum depth of exploration comparable to that of other contemporary EM systems. The instrument directly measures parameters crucial to quantitative interpretation of the electromagnetic data, including pitch, roll and altitude of the transmitter and receiver as well as transmitted current.

We demonstrate application of the SkyTEM system to palaeochannel mapping for uranium exploration at Pells Range, Western Australia. The SkyTEM data is shown to provide results in very good agreement with geological mapping and regional-scale drilling. The SkyTEM survey successfully mapped a paleochannel system within the Moogooloo Sandstone which is host to the uranium mineralisation.

A trial SkyTEM helicopter EM survey was flown over Toolibin Lake in southwestern Australia in October 2006, with the intent of establishing the suitability of this system for the detailed mapping of landscape variability in Australian settings. Survey results are in close agreement with those from a TEMPEST airborne electromagnetic survey flown in 1998, as well as an earlier AEM survey using the SALTMAP system. The Toolibin Lake SkyTEM data were interpreted using the laterally-constrained inversion (LCI) approach. Both fewlayer and smooth-model LCI inversion have been applied to the Toolibin Lake SkyTEM dataset, and examples of both styles of inversion presented here demonstrate the high near-surface resolution of the SkyTEM system. Repeatability of the SkyTEM data is shown via analysis of repeat measurements on a test survey line. LCI is shown to provide a high-quality image of subsurface conductivity delimiting a known palaeochannel system. It also provides much-improved parameter resolution over more conventional layered-earth inversion indicating the applicability of the SkyTEM system for providing baseline information on landscape characteristics that are relevant for salinity management in dryland catchments.

Estimation of reservoir parameters has always been a challenge for shale gas reservoirs. This study has concentrated on neural network technique and multiple regression analysis to predict reservoir properties including porosity, permeability, fluid saturation and total organic carbon content from conventional wireline log data for a large North American shale gas reservoir. More than 262 core analysis data from 3 wells were used as “target” and “response” for neural network and multiple regression analysis. Common log data available in three wells including GR, SP, RHOB, NPHI, DT and deep resistivity were used as “input” and “predictor”.

This study shows that reservoir parameters could be better estimated using the neural network technique than through multiple regression. The neural network method had a correlation coefficient greater than 80% for most of the parameters. Although providing a set of algorithms, multiple regression analysis was less successful for predicting reservoir parameters.