ASEG Extended Abstracts - ASEG2004 - 17th Geophysical Conference, 2004

This paper presents a novel way of treating seismic reflection amplitudes for transmission effects due to the overburden. The process has benefits for structural interpretation due to the improved visibility of events. Further processing of the data also benefits from a reduction in overburden-induced amplitude changes that can otherwise cause processing artefacts such as migration smiles.

A wavelet transform decomposition of the data is used to derive local frequency, space and time varying gain factors from windows of seismic data. These are then constrained using a frequency-constant residual Q model in order to stabilise them against noise or other sources of estimation error. The resulting gain factors are applied to each wavelet voice and the data is reconstructed by an inverse wavelet transform. This is referred to as a Q-guided amplitude correction, as shorthand for the process of constraining the data-derived scalars by frequency-constant residual Q.

A by-product of the process is that local estimates of relative Q are generated at the density of the original data volume, and can help with the overall interpretation of the data. Furthermore, once local amplitude variations have been removed, a coarsely sampled estimate of the background Q model can be used to compensate for any slowly varying amplitude and phase changes that exist throughout the survey area.

A time domain airborne electromagnetic (AEM) survey was flown with the HoistEM system over the Woodie Woodie manganese mine corridor in the east Pilbara of Western Australia. Conductivity depth image (CDI) processing and decay analysis helped to discriminate shallow, regolith related responses from conductive ore. EM conductivity mapping using CDIs correlated with about half of the known manganese ore zones, and recent drilling of several new EM targets in areas of regolith and bedrock cover discovered over 6 large tonnage manganese ore bodies to date. EMFlow has made it practical to process data gathered by HoistEM to separate the ore body response from conductive paleochannels and other conductive features related to bedrock geology or the regolith, for data editing to remove noise, and for experimentation with EMFlow parameters to produce more reliable results. The CDI results were further refined by calibration to borehole conductivity results. The survey testes and processing results demonstrate that HEM is a cost-effective method for exploration of podiform, high grade manganese ore bodies, as along as the host rocks are comparatively resistive and flight line spacing of 80m or less is used.

Geophysical logging is routinely undertaken as part of coal mine exploration programs. With minimal analysis, coal seam depth can be determined and estimates made of coal quality, lithology and rock strength. Quantitative log interpretation will add to this information. We discuss log responses in terms of the mineralogy of the clastic sedimentary rocks frequently found in the black coal mining areas of the Sydney and Bowen Basins. We find that the log responses can be tied to the mineralogy with reasonable confidence. If a full suite of logs is run, ambiguities in the interpretation may be resolved. A key driver in this work is geotechnical characterisation. With this additional geophysical input, it should be possible to develop improved rock mass classification schemes.

The HyMap airborne spectrometer is an airborne remote sensing instrument collecting data in 126 spectral channels from the visible (VIS) to the shortwave infrared (SWIR) wavelength regions. Applications are wide spanned ranging from mineral exploration to environmental monitoring. Offshore seepage mapping has always been a potential application, but not many open file reports can be found in freely accessible literature though oil&gas exploration companies are known to use the technology.

As part of a 2003 airborne survey campaign in the USA, HyVista Corporation collected airborne HyMap data over one of the most active natural oil seeps in the world offshore Santa Barbara, California. These seeps are just off Coal Oil Point and spread over an area of 10’s of sqkm. UCSB has been studying this seepage for many years and slick distribution and flow rates are monitored regularly. A production platform called ‘Holly’ is also located in the seep area and can be clearly identified on the imagery.

Airborne hyperspectral seep mapping offers a new possibility of characterising seeps in an exploration area of interest. It offers the advantage of allowing spectral discrimination of seep components which may not be separable with conventional techniques like radar. HyMap’s capability to sense in the SWIR offers the additional possibility of properly separating an effect called ’sunglint’ from any data over open water, since water will absorb radiation almost 100 % at these wavelengths. Due to the same effect it is also possible to discriminate seepage with a strong surface expression from others, which is not possible with sensors having no SWIR spectral region.

The Santa Barbara oils seeps were surveyed with 4 parallel HyMap lines covering the major seepage and migration path of the slicks at the time. Different spectral components in the open water as well as the seep area are identified and mapped. Seamless data products are produced using advanced mapping techniques and utilising the hyperspectral nature of the data. Results of these different processing steps will be presented.

Despite the considerable successes of multi-component seismology in the petroleum industry, there has been relatively little effort devoted to shallow, high-resolution converted-wave imaging in the coal sector. By analogy to petroleum-scale applications, converted-wave imaging in the coal environment offers interesting possibilities for independent validation of mapped structures, clearer imaging in the shallow sub-surface, and detection of gas, sandstone channels and/or fracture swarms.

Over the past two years Velseis Pty Ltd has conducted the first experiments in Australia to utilise shallow, high-resolution multi-component data and converted-wave technology to image coal seams. Three 2D multi-component coal-seismic datasets have been acquired in the Bowen Basin, Australia. Only minimal changes to conventional recording equipment and procedures have been required. A conventional dynamite source has been used, with a single, multi-component geophone replacing the conventional array of vertical geophones at each receiver.

Converted-wave processing algorithms developed within the petroleum sector have been successfully applied to the three trial datasets. This has involved specialised approaches to S-wave receiver statics, PS normal moveout and common conversion-point binning. Considerable experimentation to fine tune processing parameters and the converted-wave processing sequence has been necessary for optimum handling of the shallow, high-resolution data. Overall the processing of the PS data has been significantly more challenging, and has required more geological input, than conventional P-wave processing.

This research has demonstrated that converted-wave imagery is viable in the shallow environment. The derived PS images can extend the interpretation achieved with conventional P-wave images.

The regolith obscures much of Australia’s bedrock geology, posing problems in mineral exploration under cover. Gravity and magnetic tensor data may provide significant improvements over conventional geophysical exploration, by producing maps showing subtle variations in the field data which relate to the subsurface geology, but hidden to standard total or vertical field measurements. This paper examines forward potential field responses of three-dimensional regolith models containing targets like palaeochannels and land mines. A finite element approach is used, summing the field responses from many small elementary cubes, to build up complex structures to yield the full field response at a specified height above the ground. The gravity tensor data ranges over values from -0.08 to 0.1 Eotvos, and magnetic data range from -40 to 40 nanoTeslas per metre (when measured at the surface). When a flight height of 80 m is used, the responses diminish significantly, and only regional features are detectable. These values are compared to the ranges of measurable values from existing systems, and it is shown that the magnetic case is most suited to regolith studies. The resolution required for the gravity tensor appears to be less than is possible with current systems.

Compressional (P) and shear (S) waves respond differently to the Earth’s geology. Hence an integrated interpretation of multi-component seismic data should provide greater information about the sub-surface than is available from P-wave data alone. Conventional multi-component seismic analysis uses scalar component selection to provide P- and S-wave images. This approach has proven successful in many situations. However, where P energy contaminates the horizontal components, and S energy contaminates the vertical component, there is potential to achieve purer P- and S-wave records by more fully exploiting the true vector nature of multi-component seismic data.

One elegant vector-processing technique, here referred to as elastic wavefield decomposition (EWD), takes advantage of the P- and S-wave separation properties of the divergence and curl operators. Practical implementation of EWD requires information about the seismic wavefield at depth. This is achieved via downward continuation of the elastic data in the time domain via a finite-difference approach.

Synthetic and real onshore multi-component seismic data are used to evaluate the practical viability of EWD for real-data applications. The robustness of the wavefield separation is dependent on the accuracy and smoothness of the velocity model used during the downward continuation stage of the algorithm. Velocity errors of up to 10% can be tolerated, after which significant artefacts appear in the separated records. A smooth velocity model will avoid contamination by spurious reflection events. P/S separation is still effective where a constant velocity model is used for data suffering from statics associated with lateral inhomogeneities in the near surface. Moderate noise contamination does not seem to significantly impact on the wavefield separation results. In fact, the downward continuation process appears to suppress random noise. Application of EWD to a real two-component record appears to enhance the relative strength and coherency of the P- and S-wave reflection events in the extracted P and S records.

Hot dry rock (HDR) geothermal energy is obtained by circulating water between injection and production wells through hot subsurface rocks. The recovered hot water should be around 250°C for efficient electricity generation. South Australia has become a focus for HDR developments due to its exceptionally hot subsurface rocks. Previous HDR projects have focused on areas of known high geothermal gradient, based, for example, on experience from petroleum wells, e.g. the European Soultz-sous-Forêts site and Geodynamics’ Habanero-1 well in the Cooper Basin. An alternative strategy is to explore for the highest geothermal gradients closest to electricity markets. Petratherm Ltd. holds geothermal exploration licences within the uniquely hot South Australian Heat Flow Anomaly and will target buried thermally anomalous granites and radiogenic iron oxides therein. Thermal modelling indicates that temperatures of 250°C may be attained at depths <4 km within the licences. The thermal conductivity of the cover rocks is as important a factor as the heat-generating potential of the basement in generating exceptionally high geothermal gradients.

The first of the diamondiferous Murowa kimberlites was discovered in December 1997 through the follow-up of anomalous indicator stream sample results. From March 1997 until the year 2000 a program of drilling and shaft sinking was carried out to evaluate the resource. Borehole logging formed an integral part of the evaluation process used to define lithologies, measure densities and calculate sample volumes.

The five hypabyssal bodies at Murowa are a complex mix of various kimberlite lithologies which make accurate geological logging, especially of reverse circulation holes, difficult. Dual neutron, gamma gamma density and magnetic susceptibility logs were used to estimate kimberlite and host lithology proportions in the reverse circulation holes as a routine part of the geological logging process.

Analysis of the physical properties of the various lithologies indicated that any brecciation and mixing of the hypabyssal macrocrystic kimberlite with country rock inclusions is reflected by a reduction in magnetic susceptibility. The lower apparent resistivities of the kimberlite lithologies within the resistive granite host explain why the pipes respond as conductors to surface geophysics.

LogTrans, a program developed by the Centre for Mining Technology and Equipment, Brisbane, was used to perform automated interpretation of the geophysical borehole logs. The software calculates the median values and ranges for each physical property for each lithology in a suite of training holes, and then applies these statistics to interpret data from other holes. Data from the diamond drillholes were used for training, and interpretation was carried out on the reverse circulation holes. LogTrans was also used to estimate kimberlite content based on a linear relationship at Murowa between percentage of kimberlite and apparent neutron porosity.

True formation temperature (TFT) is a key parameter in geothermal resource evaluation. Measurements of TFT require deployment of a thermometer to depth within the crust, most commonly down a small diameter borehole. Unfortunately, the act of drilling removes heat from the region where temperature is to be recorded. This induced thermal anomaly may persist for months or even years depending upon both in situ conditions and the style and depth of the drilling. In the absence of sufficient time for full down hole thermal re-equilibration, measured bottom of hole temperature (BHT) may be significantly less than TFT. Numerous models have been proposed to correct disturbed BHT measurements. To date, very few of these models have been adequately tested.

A unique opportunity to examine models of BHT recovery exists in data from the Cooper Basin, South Australia. Extensively drilled for both petroleum exploration and production, a large database of post-drilling BHT measurements is available for wells in this region. Included within these data is a subset of 335 wells for which temperature data are available from post-suspension completion logs. These data, collected several weeks to months after the end of drilling, are a close approximation to TFT for the bottom of hole.

This combination of short and long-term temperature measurements has allowed an assessment of the performance of BHT recovery models for a subset of 61Cooper Basin wells. Four models were tested: the Horner plot (as derived from the line source model of Bullard, 1947), the zero-circulation model of Cooper & Jones (1959), the empirical exponential model of Perrier & Raiga-Clemenceau (1984) and the empirical semi-log plot of Pitt (1986). In all cases model predictions were found to be biased when compared with the TFT. The magnitude and direction of this bias is found to be dependent upon assumptions implicit in each model.

Santos has provided Primary Industries and Resources South Australia (PIRSA) with a copy of its workstation files of seismic and well data covering the Cooper and Eromanga Basins of South Australia, The seismic dataset includes workstation-loaded data for over 5000 seismic lines while the well dataset covers 1300 wells in the former PELs 5 and 6 licence areas. These data are now available to other explorers to facilitate successful exploration in the area.

The 2D seismic stacked section data were provided in GeoFrame archive format, while a separate GeoFrame project file exists for well data. The seismic dataset provided by Santos contains a comprehensive set of seismic lines from 1975 to 2000, with several versions of many lines. Bulk shifts/timing issues have been resolved between data vintages and coordinates are included with the trace data.

Whilst the data are reasonably easy to load into GeoFrame workstations, substantial effort has been undertaken to enable loading into other interpretative platforms. This paper describes a number of issues raised in the transcription and the outcomes of the process.

The result of these processes is the ability of PIRSA to now provide to all parties interested in the Cooper and Eromanga Basins, a consistent and comprehensive set of 2D seismic data and well data that is readily loadable to a range of workstation platforms.

Seismic datasets are often spatially undersampled in 3D exploration. Trace interpolation, a well-known solution to this sampling deficiency, is often used to generate unrecorded traces from a spatially undersampled dataset. One interpolation method used routinely for this task is the so-called f-x domain prediction filter interpolation method. This method operates on 2D seismic data to interpolate spatially aliased events. For 3D data, it is possible to extend the method to the f-x-y domain.

F-x-y prediction filters operate in the frequency space domain where for each frequency plane a two-dimensional prediction filter is computed. The 2-D filter can be computed by either 1) solving for a quadrant filter and then placing its conjugate flipped version opposite itself, this is called a pseudo-noncausal filter; or 2) solving for all the prediction coefficients in a single operation, this is called a non-causal filter.

While pseudo-noncausal filters are commonly used in trace interpolation methods, their non-causal counterparts can offer some significant advantages, namely, they are more centre-loaded, less sensitive to the size of window used in their derivation and better in handling amplitude variation.

In this paper we show how the technique of 2-D trace interpolation can be extended to 3-D trace interpolation. In addition, we demonstrate the benefits of using noncausal prediction filters over their pseudo non-causal counterparts through their applications on synthetic and field data.

To correctly invert and interpret Surface Nuclear Magnetic Resonance (SNMR) data collected in conductive terrains, an accurate estimate of subsurface conductivity structure is required. Given such an estimate, it would be useful to determine, before conducting an SNMR sounding, whether or not the conductivity structure would prevent groundwater being detected. Here we use synthetic data to find the maximum depth at which water can be detected and the depth range from which most of the SNMR signal originates in conductive halfspaces.

Using these parameters it is shown that, for coincident loop SNMR soundings, increasing loop dimensions does not significantly improve depth penetration. It is also shown that the results can be used with halfspace approximations of more complicated conductivity structures to give a reasonable estimate of the depth range over which signal is obtainable in conductive terrains.

The Petroleum Group of Primary Industries and Resources of South Australia has a regulatory responsibility to ensure seismic operations are carried out with minimal impact to the environment. As part of this role, continual improvements are made to techniques that are employed in monitoring compliance of seismic field operations with environmental requirements.

An innovative airborne video monitoring system has been developed to improve the effectiveness of assessing seismic field outcomes on the environment. This system increases the efficiency and effectiveness of monitoring extensive seismic lines in often environmentally sensitive or logistically challenging areas.

This system complements traditional ground-based methods of inspecting seismic operations. The aim of the system is to ensure that correct procedures are employed during seismic line preparation and that appropriate restorative work is carried out, to facilitate natural recovery of seismic lines.

The primary characteristics of the surveillance system are that it enables a stable and continuous video recording of large amounts of seismic lines in a short time, observations are environmentally non-invasive and observations can be made over terrain that may otherwise be difficult to access by ground-based systems.

The airborne system has been developed using low-cost, compact, readily available and proven technologies and equipment. It has proven to be economic and versatile for a variety of airborne monitoring operations.

In near mine exploration, gravity surveys are generally detailed and mine infrastructure such as waste dumps, tailings dams and open pits, if not taken into account, can often mask the gravity response from the bedrock geological sources.

This was the case at the Sons of Gwalia Mine near Leonora, Western Australia, where a gravity survey on a nominal 100x100m spacing was undertaken to assist in improving the geological framework. Large scale open pit and underground mining activities over the previous 20 years had resulted in significant mine infrastructure. Standard reductions of the gravity data showed a number of anomalous responses that correlated with the waste dumps and tailings dams. Hence there was a requirement to remove the gravity response of the mine infrastructure in order to maximise the response from bedrock sources and thus improve the interpretation.

The methodology used to minimise the effect of mine infrastructure on the gravity data involved three-dimensional forward modelling and removal of the gravity response of the waste dumps and tailing dams prior to conventional terrain correction. For the estimated densities adopted, the maximum infrastructure response was 3.6 mgal.

The results indicate that the bulk of the effects from mine infrastructure have been removed, allowing a clearer picture of the gravity response from bedrock geological sources. Some residual gravity response from the infrastructure, particularly the southwestern tailings dam, is apparent. Its removal would require a refinement of the forward modelling of the mine infrastructure.

The Early Cretaceous Flag Sandstone within the Barrow Sub-basin is a proven hydrocarbon reservoir. The main trapping mechanism is four-way dip closed anticlines, formed by drape compaction. An integrated, multi-parameter seismic AVO inversion technique was utilised that improved the mapping of the Flag Sandstone and identified the presence of trapped hydrocarbons.

The identification of the Flag Sandstone on seismic sections is complicated due to a low acoustic impedance contrast between the Flag Sandstone and the overlying shales. The same interface gives a good AVO response. Previous interpretation efforts have used the seismic AVO response to map the top of the Flag Sandstone on far-angle stacks. These efforts were very successful for mapping structure but were influenced by wavelet tuning effects and were limited for high-grading the structures based on hydrocarbon potential. To aid in characterising the fluid distribution in the reservoir rocks a simultaneous AVO inversion algorithm was used based on three partial angle stacks and conditioned well logs, inverting for acoustic and shear impedance.

The results of this multi-parameter impedance inversion produced a variety of attributes. The Vp/Vs attribute was aimed at improving the TWT structural pick whereas P-impedance became the basis for discriminating water and oil saturated sands.

The 2003 Gawler Seismic Survey acquired 250 km of 60-fold, deep seismic reflection data along two intersecting lines in order to investigate the crustal structure of the eastern Gawler Craton, South Australia. Seismic reflection data processing has produced images of the crust down to the Moho, through the overlying Neoproterozoic successions, the Mesoproterozoic and Palaeoproterozoic Gawler Craton basement, and the Palaeoproterozoic to Archaean middle crust. Of particular interest is the crustal structure imaged in basement in the vicinity of the Olympic Dam Cu-U-Au deposit.

The Gawler Craton seismic data show that the Olympic Dam deposit lies between two regions of different crustal structural character. The southern half of the north-south traverse is dominated by a series of northward dipping reflectors, some of which correlate spatially with interpreted faults. The northern half of the traverse contains little evidence of this style of faulting. The seismic data show the presence of a major unconformity that extends across this part of the Gawler Craton. A cover sequence with maximum thickness in excess of 5 km lies above this unconformity.

Numerical simulations of fluid flow through 3D pore space can provide accurate estimations for permeability. A digital volume required for these numerical experiments may be obtained directly by microtomography or statistically reconstructed from 2D thin sections. Such a digital pore volume has to be statistically representative of the original rock. However, only small rock fragments, such as drill cuttings, and only 2D images of those may be available in the field. To address this practical constraint, we investigate how permeability can be estimated from small 2D images. We select a number of natural and artificial medium-to-high porosity well-sorted sandstones.            3D microtomography volumes are obtained from each of these physical samples. Then, analogous to making thin sections of drill cuttings, we select a large number of small 2D slices from a 3D scan. As a result, a single physical sample is used to produce hundreds of virtual-drill-cutting 2D images. Corresponding 3D pore space realizations are statistically generated from these 2D images, fluid flow is simulated in 3D, and the absolute permeability is computed. As expected, this permeability does not match the measured permeability of a physical sample, which is due to inherent variations of pore-space geometry among the small images. However, for all the physical samples, a single and clear trend is formed by cross-plotting the simulated permeability versus porosity. This trend is typical for clean sandstone. The simulated permeability of under-representative sandstone fragments does not match the physically measured data. Instead it provides a valid permeability-porosity transform which can be used to estimate permeability if porosity is independently known from well log or seismic measurement.

Water supply exploration in rural regions of eastern Malawi (Africa) and northeastern Brazil (South America) is complicated by the highly irregular nature of aquifers in weathered regolith and crystalline bedrock. Drilling success can be as low as 30%. Improvements in the success rate can be achieved by using simple geophysical investigations to target the thickest zones of weathering. Sophisticated geophysical surveys that image the full range in aquifer types can further improve the success rate to 70%. In Malawi, a total of 250 hand pumps were sited using a combination of borehole geophysics (natural gamma, conductivity and magnetic susceptibility), ground-based frequency-domain electromagnetics (Geonics EM34 and Apex Max-Min), and two-dimensional electrical resistivity imaging. In a similar terrain in northeastern Brazil, resistivity profiling and Very Low Frequency electromagnetic surveys are the mainstay of water exploration. A recent project introduced ground-based and airborne frequency-domain electromagnetics (Geonics EM34 and Aerodat helicopter electromagnetics) to improve the imaging of complex fracture zones and the detection of saline aquifers. Helicopter-borne surveys are a valuable tool in groundwater exploration because they cover large areas at high resolution. In both case studies, inversion of geophysical data integrated with remote sensing images and hydrogeology has increased the chances of drilling successful wells and finding potable water for rural communities.