ASEG Extended Abstracts - ASEG2010 - 21st Geophysical Conference, 2010

A cosine transform technique has been developed to calculate the complete gravity gradient tensor from preexisting vertical gravity data which provides an alternative determination of the gravity gradient tensor components. Gravity gradient tensor components are computed for a three-dimensional buried Y-type dyke model. A comparison between the discrete cosine transform(DCT) results and forward(or theoretical) calculating gradient components from the 3D model shows that the root-mean-square error for each component, between the two results, is at most 3.94E. In addition, according to another comparison between this DCT technique and FFT method, there is a relatively larger error between the FFT derived and model derived, but the results of DCT derived coincide well with the model derived except for several data of the boundary, indicating that our technique is more efficient than traditional FFT method.

WA is poised to embark on several major new energy developments. These include $250+ Billion AUD investment in new gas reserves that will begin production in the Carnarvon, Browse and Bonaparte basins. Developing these new gas reserves will require handling many Millions of tons per year in CO2 released as a natural byproduct of the LNG process. To avoid venting this natural CO2 into the atmosphere, which may be bad for both the environment and business, the CO2 will have to be disposed of safely. One of the best available options is to accelerate nature's course by re-injecting and storing CO2 into deep rock formations, termed "geo-sequestration".

Geophysical monitoring of producing gas reservoirs will thus play an important role in two ways;

(1) enhancing the gas recovery factor of these projects by improving the reservoir model and understanding geologic flow complexity, and

(2) monitoring any required CO2 injection to ensure it is being safely stored in the subsurface for the longterm.

In addition to petroleum applications, there is a strong interest in developing clean-coal initiatives by capturing the CO2 generated at coal-fired power plants and injecting it into the subsurface. Geophysical techniques will therefore also play a key role in monitoring and verification strategies for clean-coal CO2 sequestration projects.

An inversion algorithm is developed to estimate the depth and associated model parameters of the anomalous body from the gravity or self-potential (SP) whole measured data. The problem of the depth (z) estimation from the observed data has been transformed into a nonlinear equation of the form F(z) = 0. This equation is then solved for z by minimizing an objective functional in the least-squares sense. Using the estimated depth and applying the least-squares method, the polarization angle and the dipole moment or the amplitude coefficient are computed from the measured SP or gravity data, respectively. The proposed approach is applicable for a class of geometrically simple anomalous bodies such as the semi-infinite vertical cylinder, the dike, the horizontal cylinder and the sphere, and it is tested and verified on synthetic examples with and without noise. This technique is also successfully applied to real data sets for mineral exploration, and it is found that the estimated depths and the associated model parameters are in good agreement with the actual values.

Full wave elastic forward modeling from full 3D depth models to produce 3D – 3C shot records is extremely compute intensive. A methodology has been developed that enables geophysicists to utilize an extremely complex geologic model in the X direction, but that is consistent in the Y direction to produce 3D – 3C full wave elastic forward modeling results. This methodology can be successfully used to test the viability of recording parameters in thrust belt environments that exhibit extremely complex structures in the dip direction but relatively stable structures in the strike direction. Many other geologic situations involving fracture systems can be effectively modelled using this technology. The method is capable of taking into account the quasianisotropic and velocity dispersion effects of thin beds. Also, for the case of TTI anisotropy (or tilted fracturing, or some combination of tilted fracture systems) 2.5 D modeling allows us to simulate both “fast” and ‘slow” shear waves and also simulates the effects of wave coupled refraction. In addition, Q parameters for P and S waves can be defined to accurately simulate polarization effects for both surface waves and volume waves. The accuracy of the method is such that geophysicists can predict the azimuthal AVO and velocity effects of fracture systems and test the effectiveness of full elastic inversions and various seismic methods that are available to measure azimuthal variations of seismic parameters. The mathematical derivation of the process will be discussed and examples of the usage of 2.5 D forward modeling in exploration will be illustrated.

We present a case study leading to the 3D inversion of transient electromagnetic (EM) data for delineating reservoir extent at the Alvheim field in the Norwegian sector of the North Sea. The survey was conducted in July and August 2008 using a two ship operation and ocean bottom cables. One ship laid a receiver cable with 30 receiver nodes on the sea floor. The second ship placed a source cable used to generate a coded transient signal on the sea floor. The configuration of the source and receiver spread was analogous to 2D seismic acquisition, as the system was rolled along to obtain multi-fold coverage of the subsurface. The survey spanned 20 km, resulting in measurements of 1270 source-receiver locations. The electric fields for each source-receiver pair were measured and deconvolved with the source current to determine the impulse response function. Preliminary inversions were made for each source-receiver pair using a 1D model, and the results were stitched to a 2D image. Having defined a background model, all data were then simultaneously inverted in 3D with focusing regularization. This revealed high resistivity volumes corresponding to the known hydrocarbon-bearing reservoirs of the Alvheim field.

The southern Ashanti Greenstone Belt, in Ghana, is an area of major economic importance for West Africa, where many companies are actively mining and exploring for gold. As a consequence, a significant geological data set has been collected over the years, but it has not always been subjected to an integrated interpretation, especially away from the main Ashanti fault system and the Tarkwaian portion of the belt, such as around the Wassa Mine. Using geophysical data and field observations, we have revised the geological and structural map of the south of Ashanti Belt, and have produced a new map around the Wassa mine. Crosssections built against potential field data give insights into the third dimension.

This new information, combined with previous studies in the area, suggest the presence of 5 deformation events, corresponding to the Eburnean orogeny and associated with plutonism between 2200 and 2000 Ma. The first phase of shortening (D1), prior to the deposition of the Tarkwaian sediments, is followed by the main tectonic sequence (D2-D3), at around 2.1 Ga, characterized by large folds oriented NE-SW in the Birimian and in the Tarkwaian. After D3, two other deformation events occurred: D4 with sub-horizontal cleavage and recumbent folds and then, D5 with a NE-SW shortening. Gold mineralization and associated sulphids could be correlated with D1, D2 and D3 deformations.

Results of some seismic-based methods of natural fracturing forecast have been studied and compared. Matching of seismic forecast to the well data served as a main criterion for credibility. For a number of fracture zones detected reliably by diverse well data, the accuracy of seismic-based fracture identification is shown for various methods: coherency volumes, various modifications of geometric attributes, azimuthal anisotropy, and duplex wave migration. The results of comparing the effectiveness of these methods to predict fracture permeability are given and analyzed.

Potential field and tomography data have been used to investigate major lithospheric structures associated with the margin of the West Australian craton. This study provides results on the location and geometry of major structural discontinuities at a lithospheric scale, using gravity and magnetic data. These results form part of a wider study that aims to understand the structural architecture of craton margins in terms of strain partitioning and accretionary processes. These data are used as inputs into automated edge detection, potential field inversions, and more traditional filtering techniques in order to understand the structural architecture across the craton margin.

Automated edge detection was performed on the total magnetic intensity and Bouguer gravity data over the Fraser orogen Yilgarn craton transition. Four major orogen parallel features were identified. These features are interpreted as major faults and/or shear zones that extend to significant crustal depths. They are interpreted to be related to more ‘primary’ craton margin structures at depth. Moho inversions of the gravity data were performed across the craton margin. They aimed of identifying steps in the topography of the Moho caused by crustal penetrating shear zones. These initial models were unable to resolve the base of the crust, but did however provide information related to domains and boundaries within the deep crust. Surface wave tomography data was used to investigate possible structures within the sub-continental lithospheric mantle. A large linear gradient in Vs was imaged approximately parallel to the Fraser orogen at 75km depth when filtered for the total horizontal gradient. This gradient was overprinted by a major east-west trending features at 100km, a trend that is also apparent within seismic depth estimates of the Moho.

The necessity to satisfy the world need for oil & gas pushes oil companies to drill in conditions that are getting harder and harder in terms of Temperature and Pressure. The drillers have to know the conditions as precisely as possible before drilling a new borehole. Generally, in complex and deep areas the more traditional relationships linking interval velocities from seismic time processing with the effective stress (and consequently the pore/over pressure), are no longer enough to make a correct prediction of the pore pressure. For this reason several methods have been developed to try to obtain more appropriate velocity fields, and in this paper we compare some of them, mainly CVA (Continuous Velocity Analysis) in the time domain and Grid Tomography, in the depth domain. We intend also to show pressure prediction results that, instead of being obtained directly from velocities, are derived from different seismic attributes. In particular we present some pressure predictions obtained by seismic inversion products such as Ip (Impedence of wave P) and Vp/Vs Ratio.

The paper shows some results from a prospect where the deep targets present very difficult drilling conditions in terms of High Pressure and High Temperature.

A ZTEM (Z-Tipper Axis Electromagnetic) airborne AFMAG survey was conducted over the Reese River Test Block, situated in central Nevada in August, 2009. The Reese River prospect is a “blind” geothermal resource that lacks surface characteristics such as hot springs. A number of geophysical surveys have been conducted at the prospect, including seismic, gravity, magnetotelluric (MT) and radiometric surveys. Resistivity methods, such as MT, are typically used to map structure, lithology and alteration, particularly the smectite-zeolite zones which form a low resistivity cap over the outer margins of the higher resistive reservoir at depth. The ZTEM survey at Reese River has overflown lines previously surveyed using MT for comparison purposes.

The ZTEM results appear to correlate very well with previous magnetotelluric results and the known geology, in particular the presence of both major and secondary fault structures and geologic contacts. 2D inversions of the airborne ZTEM appear to agree very well with the inversions obtained from ground MT, except in areas of pronounced 3D behaviour, where the 2D assumption is no longer valid.

Two airborne EM surveys were conducted over the Drybones Kimberlite by Geotech Ltd. A VTEM helicopter time domain EM survey flown in 2005 is compared to a ZTEM helicopter tipper AFMAG EM survey flown in 2009. Both surveys detect anomalous responses over the pipe, however according to conductivity-depth transforms and skin-depth estimates, supported by 3D modeling, it seems unlikely that VTEM response is representative of the actual kimberlite, buried more than 100 meters underneath the conductive sediments. It appears that the consolidated kimberlite might be too resistive to be differentiated from granodiorites using conventional TEM methods. On the other hand the ZTEM response appears to be able to differentiate between the diatreme (consolidated kimberlite) and the host rock, lying below the conductive blanket.

The helicopter borne time-domain VTEM system has been in constant development since 2002 through complex and multicomponent technical improvements in an effort to satisfy exploration and mining industry requirements.

Progress in the geoelectrical informational level of data obtained with different VTEM systems is illustrated with the results of test surveys over the Caber VMS deposit. Forward plate modeling with different systems for the Caber deposit geometry target with changing conductance, as well as resistivity-depth transforms of the real data, both serve to illustrate significant broadening of the conductance aperture and increasing depth of investigation over time. Most of all the increasing sensitivity of the VTEM system is highlighted.

The Capel and Faust basins lie at water depths of 1500-3000 m, 800 km east of Brisbane. Geoscience Australia began a petroleum prospectivity study of these remote frontier basins with acquisition of reflection and refraction seismic, gravity, magnetic and multi-beam bathymetry data during 2006/07. The approach mapped a complex distribution of sub-basins via traditional 2D reflection seismic interpretation techniques and 3D mapping and gravity modelling. Gravity models were used to inform the seismic interpretation of basement. Gravity models had three sediment layers with average densities inferred from refraction modelling of 1.85, 2.13, 2.31 t/m³ overlying a pre-rift basement of density 2.54 t/m³, itself consisting in part of intruded older basin material. Gravity modelling of the simple density model arising from the initial interpretation of reflection seismic data indicated a first order agreement between observed and calculated data. Second order misfits were accounted for by adjustments to the sediment density values, localised adjustments to basement depths, and heterogeneity in the basement densities. The study concluded that sediment of average velocity 3500 m/s exceeds 6000 m thickness in the northwest of the area, which is sufficient for potential petroleum generation.

Helicopter time-domain AEM is being investigated as a technique for bathymetric mapping in shallow coastal waters. Previous studies in Port Lincoln, South Australia, used a floating AEM system to provide an upper limit to the expected bathymetric accuracy based on current technology for AEM systems. The same survey lines were flown with an airborne system (SeaTEM) on two separate occasions. A comparison of the interpreted water depths obtained from the airborne and floating systems is presented. An empirical data correction method based on modelled and observed EM responses over deep seawater at varying survey altitudes can lead to improvements in interpreted water depths. A comparison between results of the two surveys in Port Lincoln shows that uncorrected data also gives good agreement with known water depths. The results of a full survey of selected areas in Broken Bay (NSW) undertaken with the SeaTEM system are presented. A marine seismic reflection survey together with vibrocore samples of sediment in Broken Bay were used to provide an estimate of the sediment thickness and resistivity of shallow sediments to assist in the interpretation of the EM response. Interpreted water depths and depths to bedrock from SeaTEM data in Broken Bay are generally in good agreement with known bathymetry and seismic estimates of bedrock depths shallower than ~ 90 m in water depths shallower than ~ 25 – 30 m.

In some of Australia’s underground coal mining areas, near-surface basalt layers create problems for seismic reflection surveying. Affected mines have less confidence in their geological conditions and mine safety and productivity may be compromised. In order to gain insights and obtain potential solutions to this important problem, we have investigated seismic reflection and borehole vertical seismic profiling data from surveys at the North Goonyella Mine and the Moranbah South coal mine lease, which are both located in the Bowen Basin of Queensland.

At North Goonyella, seismic velocities determined from the VSP survey indicate that the basalt is not fresh and up-going reflections from layers below the basalt are clearly seen. For the seismic reflection survey, advanced seismic data processing techniques such as pre-stack depth migration were able to improve the continuity of the coal seam reflections across the zone affected by the basalt. No special acquisition parameters were required. At Moranbah South, the problem with the basalts proved to be intractable. The basalts are hard and fresh, their total thickness is up 40 m, their width reaches 360 m and unconsolidated sediments lie between individual flows. The target coal seams are at depths less than 310 m and neither long-offset data acquisition nor prestack depth migration were able to produce satisfactory results.

In general, the main issues for seismic surveying in basalt covered areas are (i) the generation of complex downgoing and up-going wavefields which are due to the strong impedance contrasts between the basalt and the surrounding strata, and (ii) the generation of incoherent scattered waves from inhomogeneities within the basalts and their rough margins. We have studied and confirmed these seismic phenomena through computer modelling.

Analysis of interval seismic velocities from Geoscience Australia’s 2008/09 seismic survey 310 in conjunction with seismic reflection interpretation provides new insights into the geology of the Wallaby Plateau. Seismically distinctive basement core packages and divergent dipping reflector (DDR) packages have been identified. The seismic character of the DDR packages is similar to seaward dipping reflector (SDR) packages of inferred volcanic composition. Initial analysis of seismic velocity profiles indicated affinities between the DDR packages and known sedimentary strata in the Houtman Sub-basin.

However, subsequent water depth adjustment of seismic velocities reduces this distinction between SDR, DDR and sedimentary strata such that discrimination between volcanic and sedimentary strata in DDR or SDR packages is equivocal. Despite ambiguities, velocity analysis provides important insights into the seismic characteristics of the Wallaby Plateau basement. The well constrained decrease in interval velocity from a DDR package into an underlying stratified basement is interpreted to indicate localised continental blocks within the Plateau. Although seismically derived velocities may be equivocal for certain lithologic interpretation, they still provide useful insights into understanding the geology of a rank frontier area such as the Wallaby Plateau.

To understand environmental responses to climate change and to human activity, we need to constrain the rates of processes that shape the Earth’s surface like the production of soils by physical and chemical weathering of rocks and the transport of sediments. These processes are controlled either directly or indirectly by climate change. Studying the history of the sediments deposited throughout the river and its palaeochannels can give insight to the history of climate change occurred. A new approach using uranium series isotopes has been devised and being used recently to address the questions presented above. In silt-size soil material, a (²³⁴U/²³⁸U) activity <1 can be observed because when ²³⁸U decays near the surface, its daughter nuclide ²³⁴Th can be ejected from soil grains, and subsequently decays into ²³⁴U. The measurement of this radioactive disequilibrium can be used to determine how much time has elapsed since the soil material was produced from the bedrock, i.e. a soil residence time. We have measured U-series isotope disequilibrium in soils from four different profiles at Frogs Hollow, a catchment area of the Murrumbidgee River in western New South Wales, which gave a top soil residence time of over a million year and a saprolite residence time of over a hundred thousand years. These ages are significantly different than the ages reported earlier at the same location using cosmogenic radio nuclide measurement. This difference may be due to the need of more developments in the new technique or the assumption of steady state thickness of the soil profile required by the cosmogenic radio nuclide measurement technique, which may not stand if the soil thickness is not steady.

Extremely abundant oil and gas resources in the sedimentary basins of marine carbonate which are of vast areas exist in South China. However, it is very difficult to make a wonderful geophone coupling with the outcrop of hard limestone. And then the qualities of the seismic data are reduced. Now, geophone coupling test is of great blindness due to lack of appropriate theoretical guidance. In this paper, in order to understand fully the transmission mechanism of the geophone-limestone coupling system, considering that the coupling mediums such as gesso, clay and so on are used in field seismic data acquisition, I bring forward the theory of geophonelimestone 3-DOF (degree of freedom) coupling system based on vibration dynamic concept. The geophonelimestone coupling system responses are computed when the parameters of coupling medium, geophone spikes and damping are changed. And we find that the geophonelimestone coupling system is a resonance system which has three resonance frequencies. The results show that the resonance frequency can be increased through increasing the elastic modulus of the coupling medium, decreasing the area of the underside and the height of the coupling medium, using spikes with lower material density and reducing the height of the spike. The results also show that the impact of the narrow frequency bands “band pass filtering” can be decreased by increasing the damping of the coupling system properly. Finally, we preliminary validate the theoretical results of the geophone-limestone 3-DOF coupling system using shake table experiments.

The Gawler Province in South Australia, is a Mesoarchaean to early Mesoproterozoic crystalline basement block underlying nearly half of South Australia. However, large portions do not outcrop, particularly in the west, where specific basement information below Phanerozoic cover is derived mainly from drill hole material. In such environments, geophysical data is of paramount importance in understanding basement geology for the purposes of mineral exploration. Two magnetic surveys in the Tallaringa-Ooldea area of the western Gawler Province were flown for Primary Industries and Resources South Australia (PIRSA) and Geoscience Australia (GA) during 2005-2006. A solid geology and structural interpretation of the areas was undertaken, incorporating the new data with existing data and additional geological and geophysical datasets. This new interpretation illustrates an underlying geology comprising Proterozoic and Archaean lithologies, consistent with the regional setting. Areas of interest in this area include parts of the Tallaringa Trough, the southernmost extent of the Nawa Ridge, and the One Tree Anomaly.