Exploration Geophysics - Volume 22, Issue 1, 1991

The transition to fully digital data acquisition systems has been the major advance in virtually all areas of geophysics. In 1985, GEOTEM*, the first fully digital airborne EM system, was introduced into commercial operation. Geoterrex commissioned an Australian based GEOTEM system in 1989. Over 250,000 line kilometres of survey have now been flown with the system allowing a greater appreciation of the advantages of a fully digital operation.

Data acquired over the Freddie Well massive sulphide deposit, near Mt Magnet in Western Australia, provide a good example of the advantages of new technology applied to the detection of difficult targets. The deposit was undetected by the early versions of the Mark V and Mark VI INPUT^* receivers. The enhanced Mark V receiver detected the deposit though the response never dominated the high amplitude response of the adjacent overburden. The GEOTEM survey, which employed dual transmitter frequencies of 75 Hz and 125 Hz, clearly detected the deposit with late channel response exceeding the amplitude of the surrounding conductive cover.

The Durroon Basin containing up to 7 km of predominantly Cretaceous sedimentary infill is recognised on the basis of a different structural history to that of the adjacent Bass Basin. Structural development and sedimentation patterns of the Durroon Basin are the result of two distinct Cretaceous rifting episodes related to major regional tectonic events.

The first episode (Otway Rift; 125-100 Ma) was a pronounced Late Jurassic (?)— Early Cretaceous rift phase during which volcaniclastic sediments were deposited in an elongate series of troughs formed by the initial separation of the Australian and Antarctic Plates. This stage was succeeded by a further rift phase which was responsible for much of the structural character of the Durroon Basin.

The second extensional episode (Tasman Rift; 90-80 Ma), was caused by the opening and subsequent rapid development of the Tasman Sea. This episode created a series of post-depositional, detached, roll-over structures along margins formed by listric faults and formed updip culminations to tilted blocks.

Post-Tasman Rift sedimentation was influenced by both Otway and Tasman drift. The effect was to cause subsidence to move to central parts of the Bass Basin, and by Early Tertiary times the Durroon Basin was buried beneath Cainozoic sediments of the Bass Basin.

This paper investigates the amplitude response of ground-penetrating radar to typical lithofacies bounding surfaces in a sand barrier sequence. Ground-penetrating radar reflection coefficients of typical bounding surfaces in a sand barrier sequence are computed using time-average and Hanai-Bruggeman-Sen effective-medium mixing laws for the dielectric properties. The effects of lithological contrasts due to packing, porosity, grain shape and orientation, mineralogy and fluid content are calculated. Reflection coefficients are typically 0.05 to 0.1 but may range up to 0.45 for large dielectric permittivity contrasts.

For a radar example recorded over a beach strandline, the relative reflection amplitude alone is generally insufficient to uniquely determine a specific type of bounding surface between two lithofacies. Other attributes of the section, such as apparent dip magnitude and direction and the external form, help to refine the interpretation.

This interpretation approach is formalised by defining radar facies units as ‘groups of radar reflections whose parameters (configuration, amplitude, continuity, frequency, interval velocity, attenuation, dispersion) differ from adjacent groups’. Radar facies are distinguished by types of reflection boundaries, configuration of the reflection pattern within the unit, and the external form or shape of the unit.

A series of two dimensional interpolation models was constructed using wells from the Harriet Field. These models were designed to assess the effects of: (a) shale interbeds within the Flag Sand and (b) the fluid content (i.e. gas vs. oil vs. water) on the seismic response. The model responses closely resemble the seismic data.

The results indicate that both of these factors have an impact on the seismic expression of the top Flag Sand interface and the associated amplitude anomaly which occurs immediately below it. In addition, apparent velocity variations which have been observed over the field are probably related, at least in part, to tuning effects resulting from shale and fluid content variations.

In the models constructed, the presence of hydrocarbons correlates with low frequency zones in the seismic data which occur in association with high amplitude in the trough beneath the top Flag Sand reflector.

The application of prestack partial migration or dip moveout to seismic data requires a very fast algorithm to provide an economical product. The time domain method offers the greatest speed, but must address the problems of aliasing, interpolation, and amplitude-phase distortions. The amplitude and phase corrections usually require some form of special processing such as Weiner-Levinson trace matching, or the trajectory method developed by Hale.

The proposed algorithm computes the amplitude in the time domain by a simple method that is based on the trajectory algorithm. When care is taken to accurately treat interpolation, aliasing, and phase shift, results may be obtained that match the quality of transform methods, and without the need of matching filters.

In early 1990, the EP 342 and TP/9 joint venture conducted a seismic survey on and around the Muiron Islands in the Carnarvon Basin. Its objective was to infill the gap in existing marine seismic data coverage resulting from the presence of the islands, which are believed to overlie a prospective trend. Work on the Muiron Islands was done in conjunction with a shallow-water marine seismic survey conducted over the remainder of the permit areas.

Because the Muiron Islands are a nature reserve and a significant nesting site for Wedge-tailed Shearwaters and turtles, environmental assessment of the project was required. Approval to conduct the survey was subject to stringent conditions, including: the avoidance of Shearwater nesting areas, prohibition of the use of vehicles and the clearing of vegetation, prohibition of camping on the islands, and a requirement to use environmental consultants to monitor the survey.

A fully portable operation was conducted on the islands. A recording spread was laid across the islands and extended into the surrounding waters to record shots fired both onshore in drilled or augered shotholes and offshore from the seismic vessel. On the more protected eastern side of the islands a bay cable transition zone spread was extended from the islands to provide a tie with the marine data. However the exposed western side generally proved too rough for the bay cable operation, and a shallow-draft seismic vessel was used in ‘rollout mode’ to obtain a tie between the onshore and offshore data.

The geophysical objectives of the survey were achieved with negligible impact on the environment. There was no known mortality to wildlife and only minor and impermanent disturbance to vegetation.

Seismic data acquired in areas with irregular topography are usually corrected to a flat datum before migration. A time-honored technique for handling elevation changes is to time shift the data before application of migration. This simple time shift, or elevation-static correction, cannot properly represent wide-angle or dipping reflections as they would have been recorded at the datum. As a result, when elevation varies significantly, accuracy in event positioning may be compromised for migration and other wave-equation processes, such as DMO. Although computationally intensive, wave-equation datuming can be used to accurately extrapolate the data recorded over an irregular surface to a flat datum.

Here, we propose an efficient technique for doing migration from irregular surfaces using almost any migration algorithm. As in elevation-static corrections, surface-recorded data are time-shifted to a horizontal datum; for our process, however, we choose that datum elevation to be at or above the highest elevation in the survey. The choice of this datum elevation does not compromise final results because the datum elevation can always be adjusted to any other level after migration. In the migration step, the velocity is set to zero in the layer between the surface and the datum; below the original surface, the interval velocity represents the best estimate of the subsurface geology. By adding a zero-velocity layer, the migration algorithm is applied to the data from the flat datum and no lateral propagation is allowed until nonzero velocity is encountered at the recording surface.

Synthetic field data examples demonstrate that use of the ‘zero-velocity layer’ significantly improves imaging accuracy relative to conventional migration from a flat datum. Moreover, the geologically derived migration-velocity field need not be adjusted to compensate for shortcomings in the datum-static procedure.

Furthermore, the scheme can employ the same efficient finite-difference migration algorithms used in conventional processing. The technique can be extended to prestack processes such as DMO, shot- and receiver-gather downward extrapolation, and migration and thus suggests a unified approach to processing data from irregular surfaces.

Wave-equation datuming is a simple concept which can be used to correct seismic data recorded in areas where there are strong near-surface lateral velocity contrasts. In particular, the idea lends itself to correcting marine data recorded over near-surface limestone reefs which give rise to rapid changes in the sea-floor profile.

A post-stack two-dimensional (2D) Kirchhoff implementation has been designed to run efficiently on an 80386-based microcomputer with or without an attached array processor (AP). It can be generalized readily to pre-stack application or to three-dimensional (3D) application and serves to demonstrate many of the features of these more numerically intensive implementations. The data flow is based on the semi-circle superposition rather than the hyperbola-sum approach to Kirchhoff summation (Claerbout, 1985). This is a single trace input, multiple trace output design which has the main advantages: (1) relaxed requirements on input data organization; (2) streamlined trace interpolation for time-shifting.

In this 2D implementation, a spatially variant convolution in time serves to perform the off-line (y-direction) summation under the assumption of an earth model which is constant in y (Berryhill, 1979). The length of this convolver is the most critical parameter affecting performance. For reef structures which are not strictly 2D, convolver lengths can be set so that the resulting accuracy is commensurate with the limitation of the 2D assumption.

Seismic data acquired in areas in the Timor Sea where reefs are a problem are ideal for demonstrating the applicability of this 2D post-stack datuming algorithm. A model consisting of point diffractors under a typical reef profile can be processed with wave-equation datuming so that migrated images of these diffractors are equally well resolved at different horizontal positions along the profile.

Large lateral velocity changes, particularly those associated with sea-floor topography, can severely distort seismic stack sections, for example, under the submarine canyons in the Gippsland Basin in Bass Strait. Conventional replacement statics partially correct these problems, but are usually inadequate in situations where the sea-floor variation is severe or the water is deep. Dynamic corrections calculated by ray-tracing through the region of strong lateral velocity variation give improved results.

Dynamic corrections derived by ray-tracing can be evaluated by analyzing t² -y² plots where t is time and y is offset within CMP gathers. This facilitates making the following comparisons: (1) the zero-offset time before and after dynamic correction; (2) the RMS velocities (calculated using zero-offset rays traced in the model) with normalized stacking velocities after dynamics; (3) hyperbolic moveout before and after dynamics. A similar evaluation of conventional replacement statics provides a reference for evaluating the ray-tracing derived ‘replacement dynamics’.

Ray-tracing requires a subsurface model which can be difficult to construct in areas of structural complexity. This is often the case near submarine canyons. A simple model using water depth and a single marker horizon or datum when ray-traced to derive replacement dynamics gives significant improvement with respect to the conventional statics solution in submarine canyon areas. Errors in velocity can be reduced by as much as 50% and errors in hyperbolic moveout reduced by about 70%.

This paper investigates a simple model for the evolution of the central Australian (Amadeus, Officer and Ngalia) basins and the adjacent Canning and Bonaparte Gulf basins at the time of the Alice Springs orogeny. Supported by evidence from surface geology, stratigraphic correlations, structural geology, recent deep seismic and gravity anomaly data, the proposed model suggests a genetic link between the two groups of basins. The model calls for a crustal scale shear zone, which has been named by the authors, the Lasseter Shear Zone, running through the Halls Creek Province and continuing south between the Canning and Amadeus basins and beneath the Western Officer Basin. This shear zone would have been active during the Middle Devonian to early Carboniferous, separating crustal blocks in compression to the east from crustal blocks in extension to the west.

Several driving mechanisms acting along the margins of the Australian plate can be called upon to account for the contrasting, yet contemporary, types of deformation on either side of the shear zone.

This paper is concerned with fundamental properties of seismic imaging (forward modelling) and migration (inverse modelling). The precise mathematical nature of the transformations connecting model space and data space are analyzed and found to belong to a family of mappings called contact or symplectic transformations. The transformations are found explicitly for the case of constant velocity media, so that their structure and properties can be analytically developed. Both geometrical and dynamical aspects of the transformations are discussed. The analysis provides a powerful insight into the nature of seismic imaging and migration.

Over the past five years, the use of the transient electromagnetic (TEM) method for both lateral and vertical profiling of polluted groundwater and saline soil has been investigated, and the results have been compared with dc sounding and frequency domain methods. In the groundwater pollution studies, it has been found that with either two-layer or three-layer inversions of the TEM data a subtle low-resistivity feature which correlates with the presence of a pollution plume in the aquifer near a landfill site can be detected. Data obtained since 1985 at approximately six-month intervals have shown a continuing development of the resistivity low.

In the soil salinity studies, investigations have concentrated on the use of TEM and dc sounding methods to determine the thickness and resistivity of the surface leached layer and of salt accumulations. Modelling of the response obtained with a ground-based TEM system shows that measurements at delay times earlier than 1 ms are required to resolve the relevant parameters of a layered-earth model used to approximate a saline soil environment.

Four economic gold deposits occur in the Junction Reefs area near Orange, N.S.W. Three of these are metasomatic replacement deposits within calcareous sediments. The fourth is a structurally controlled stockwork zone adjacent to a late phase intrusion. The shallow dipping replacement deposits are associated with semi-massive pyrrhotite skarn and are both magnetic and electrically conductive. The stockwork body, which contains disseminated pyrite/arsenopyrite, is recognisable as a coincident magnetic low and radiometric high and is responsive to induced polarisation (IP) techniques.

Early geophysical exploration concentrated on ground magnetic and IP surveys. These surveys were generally unsuccessful in defining either the known mineralisation at the Sheahan-Grants deposit or new deposits, due to the geometry of the mineralisation and strong magnetic and IP responses from surrounding country rock.

An airborne magnetic and radiometric survey was flown over the Junction Reefs area. This survey helped to identify and map a large diorite intrusion which lies approximately three kilometres north of the mine. Within the magnetic response due to this intrusion is the magnetic low and radiometric high associated with the Glendale (stockwork) deposit.

Recent geophysical work in the area has concentrated on detailed ground magnetic and electromagnetic surveys, searching for further gold reserves of the Sheahan-Grants style. The use of electromagnetic techniques has only been possible following the development of software for stripping the responses due to the many fences which cross the prospective area.

X-ray computed tomography (CT) is a non-destructive means by which the internal structure of an object may be viewed. It produces a digital image that is a map of the X-ray attenuation in a slice through the object. At Curtin University, this technique has been successfully applied in the analysis of various rock materials, including four inch diameter cores from the petroleum exploration industry. Two scanners, an EMI CT5005 (second generation machine) and a Siemens SOMOTOM DR/H (third generation), both in routine medical use at local Perth hospitals, have been used in this work. Interpretation of X-ray CT results is facilitated by high resolution colour graphic display on a Landmark seismic workstation. CT images have revealed sedimentological and structural features in cores that are, in some cases, quite unexpected, and would not be apparent utilizing conventional (i.e. destructive) core analysis techniques. X-ray CT has much potential for core characterisation and reservoir modelling; it can measure mineral distributions, depositional textures, porosity variability, fracture frequency and orientation, and fluid saturations. Core quality parameters such as the degree of drilling mud invasion can also be assessed. Importantly, X-ray CT can image the internal structure of sealed (i.e. preserved at the rig site) cores.

In seismic data processing the problems associated with rapidly varying sea floor topography are well documented. Numerous two-dimensional procedures are available for solving these problems, ranging in complexity from simple replacement statics to full wave equation datuming. In view of the size of 3D data sets it is imperative that these procedures be cost effective. Our search for such a procedure has resulted in the development of Modelled Offset Dependent Statics (MODS), which gives good results at relatively low cost.

Simple replacement statics based on vertical raypaths are theoretically only correct for flat reflectors at infinite depth. In the MODS method water replacement static corrections are calculated for every individual offset trace by ray tracing to one target horizon only, using a three dimensional depth model of the sea floor and the horizon of interest.

MODS has been successfully applied to a continental shelf edge 3D seismic survey. The data quality improvement demonstrates the appropriateness and the value of the technique.

Regional aeromagnetic interpretation includes an important mapping component as well as location of possible economic targets. Given these varied requirements, objective interpretation techniques based on spectral analysis and application of specialised filters play a major role. Recent improvements in computer hardware, particularly microcomputers, have made it possible to analyse more data and it is now feasible to perform quantitative analysis on complete blocks of aeromagnetic data. Quantitative analysis may be applied to profile or grid data or both, depending on the interpretation objectives and the data quality. Results are presented for two analytical interpretation techniques, location of the maxima of the pseudo-gravity gradient and 3D Euler deconvolution. These results together with apparent susceptibility mapping provide objective information on geological boundaries, depths and lithologies.

The South Australian Department of Mines and Energy, with Delhi Petroleum Pty Ltd and SANTOS Ltd, the operators of PELs 5 and 6, adopted a Joint Operations Management Group (JOMG) approach to the consideration of seismic operations in the Cooper Basin as part of a jointly managed self regulatory environmental control process.

The JOMG, comprising representatives from the Department and the Operators, has developed codes of environmental practice that have been field tested, modified and updated to suit exploration techniques and environmental conditions in PELs 5 and 6. The JOMG has also reviewed programs and monitored effectiveness of these codes.

Recent changes to the Regulations under the Petroleum Act in SA have introduced a requirement for a code of environmental practice to be submitted before an operation is commenced under a licence, and for a declaration of environmental factors to be submitted for approval before the commencement of a geophysical program (Appendix 1). The joint code of environmental practice developed by JOMG over the years will form the basis of a new code produced solely by SANTOS which will now be legally enforceable in PELs 5 and 6. A recent review of exploration activities and their management in PELs 5 and 6 by the CSIRO Division of Wildlife and Rangelands Research, utilizing Landsat Thematic Mapper (TM) imagery combined with field inspections, indicated that the JOMG has been singularly successful in minimising environmental impact of seismic operations in the Cooper Basin.

Standard band-averaging techniques used for representing magnetotelluric data are subject to bias from spurious estimates unrelated to Gaussian noise. Consequently complex curve fitting techniques have been suggested to provide an alternative representation. Suspect bands can be eliminated and representative data can be obtained by interpolation using complex polynomials. The Levy algorithm adopted here has the additional benefit that it can be used for archiving to approximate band averages obtained using existing software.

Conventional stacking techniques are not generally suitable for airborne TEM surveys conducted at speeds exceeding 150 km/hr. For late channels (2-50 ms) linear stacking can be tolerated for up to 8 cycles representing lateral translations of 30 to 40 m accompanied by variations in coupling. The Nth root stack developed to reduce the effect of spikes in geophysical data can result in considerable distortion complicating the effects of sferics activity. Consequently other procedures are required including bandpass filters to compensate for bird motion, decay curve analysis, and spike rejection based on observations at a base station or predictive filtering.