Exploration Geophysics - Volume 22, Issue 1, 1991

Gravity gradiometers have the potential to provide gravity measurements from a moving vehicle, thus allowing airborne gravity surveys. In order to achieve this potential, exacting sensitivity and bandwidth requirements must be met by the instrument and accuracy demands in navigation and terrain surveying must be satisfied.

A gravity gradiometer for geophysical prospecting has been under development at the University of Western Australia for ten years. It is an advanced instrument using cryogenic technology and superconducting electronics and is the only gradiometer developed explicitly for prospecting use, although others elsewhere are being developed for geodetic and space applications. We have already demonstrated appropriate sensitivity and bandwidth in tests of a laboratory prototype and some recent results are presented here.

This paper briefly compares gradiometry with gravimetry and shows that gravity gradiometers are better suited than gravimeters to the measurement scales of interest in geophysical prospecting. This is illustrated with the results of a modelling study of the Teutonic Bore orebody.

The Fitzroy Trough is a northwest/southeast trending series of half graben which formed along the northeast margin of the Canning Basin. New deep seismic reflection data are used to revise the tectonic model of the trough. The trough formed by periods of crustal extension in the latest Middle to Late Devonian, the Carboniferous and the Permian. Estimates of crustal extension based on a structural analysis of the sedimentary strata are 20-25 km, 10 and 5 km in each of these periods, respectively. This represents an extension of approximately 50%, broadly consistent with the amount of crustal thinning beneath the trough. The crust may also have extended during the Ordovician. Where the deep seismic line crossed the trough, extension occurred on two half graben — one under the anticline at Mt Wynne and the other on the Fenton Fault. A third half graben began to form by backstepping of the Fenton Fault, but failed to evolve as a major structure before extension ceased. The half graben are bounded on their southwest sides by listric normal faults which sole onto a sub-horizontal detachment surface at about 7 s two-way time (15-17 km). The Pinnacle Fault, previously thought to be a major bounding fault on the northeast margin of the trough, now appears to be antithetic to the major listric normal faults bounding the southwest sides of the half graben. It detached into the top of the Ordovician/Silurian sequence. Transpression during the Jurassic warped the sediments in the trough into a series of broad synclines and anticlines. The anticlines were focussed on and accentuated pre-existing local highs. Mounding of the basal sediments in the trough, possibly accentuated by slip along salt layers, deformed younger overlying sediments along the northeast side of the trough. It produced movement of the basal sediments up dip out of the trough towards the northeast margin, and possibly also into the core of the Mt Wynne Anticline.

Within the Eromanga Basin, stratigraphic variations in the Jurassic Birkhead Formation are recognizable on seismic sections as lateral character changes. Understanding these character changes and developing representative depositional and structural models results in lower risk and hence more favourable economics when exploring for Birkhead Formation oil.

Authority to Prospect (ATP) 299P is located in the Eromanga Basin, on the southeastern flank of the Cooper Basin. It is 120 km northwest of the Jackson Oil Field. Over 1.5 million barrels of oil have been produced from the permit, of which 500,000 barrels have been produced from the Birkhead Formation, highlighting it as an important exploration target.

By late July 1989 Cranstoun-1, a Birkhead Formation oil well, had produced more than the mapped proved and probable reserves and was still producing 203 barrels of clean oil per day. To investigate this anomaly the existing seismic data was reprocessed with additional 80 m uphole control and a seismic stratigraphic interpretation was carried out. The mapped data identified a Devonian wrench system overlain by a large low relief structure known as the Greater Cranstoun Structure (GCS). Endeavour-1 was drilled on this structure 3 km north of Cranstoun-1 and was completed as a Birkhead oil producer. Post drill analysis revealed that the interpreted top intra-Birkhead seal seismic marker crossed a geologically correlated boundary — from the top Birkhead ('A' sand) seal at the Endeavour wells switching down to the top 'B' sand seal at the Cranstoun wells. A further three wells were drilled based on the mapping of this seismic marker resulting in limited success. Detailed 2D seismic modelling, using the sonic logs from the five wells on the GCS as input, indicated that an increase in the frequency bandwidth of the seismic data was needed to resolve those geologically correlated boundaries within the Birkhead Formation which have an acoustic impedance contrast.

Future seismic acquisition and processing should use good statics together with optimum recording parameters (defined from the 2D modelling results) to produce the required seismic resolution for accurate stratigraphic interpretation and mapping.

Quantitative understanding of the limitations and possible resolution of seismic data is crucial to accurate seismic stratigraphy, which, combined with a valid tectonic model are keys to exploration success in the Birkhead Formation in ATP 299P and throughout the Eromanga Basin.

The horizontal stress field within plate interiors is largely the result of interactions at plate boundaries. There is considerable geological evidence and theoretical support for the hypothesis that large horizontal stresses can propagate for thousands of km into plate interiors, and that changes in plate geometry or boundary configurations therefore lead to significant variations in stress in plate interiors. Because there is a global interdependence of all plate motions, a major change in the nature of one plate boundary (e.g., the India-Asia collision) may have global implications and observable geological consequences many thousands of km from the source. There are two important consequences of large horizontal intraplate stresses and stress variations for petroleum exploration. First, flexural distortions of the plate will be localized by variations in strength and/or thickness of the plate, such as at sedimentary basin boundaries. These distortions may give rise to transgressions/regressions and unconformities that may be confused with eustatic sea-level effects. Second, an increase in stress or change in stress orientation may be relieved by reactivation of a pre-existing structure in the plate interior. Reactivation of basin-forming faults is a particularly widespread consequence of intraplate stresses. Analysis of the structural petroleum traps in Australia's main producing basins shows that a large proportion of the traps were generated by reactivation of underlying, usually basin-forming faults. We discuss examples from the Carnarvon, Bonaparte and Gippsland Basins, and relate them to the global and regional plate tectonic history in the Tertiary and Mesozoic.

Although the Bowen Basin is considered to provide good quality seismic exploration records, the limits of seismic resolution are tested when attempting to interpret sandstone pinch-outs and drapes. Exploration is hindered by seismic resolution constraints in both the vertical and lateral directions. Exploration in the Bowen Basin frequently concentrates on the Rewan Formation and Showgrounds Sandstone, one objective being to determine if a sandstone pinch-out, drape or fault has disrupted hydrocarbon migration paths.

This paper stresses that to assist the determination of whether a potential target is a drape over, or a pinch-out against a basement high, it is desirable to initially build a geophysical model to simulate the geological boundaries. After running a suite of simulated seismic surveys over a number of models, results indicate that the top of the Showgrounds Sandstone could be interpreted where its thickness was greater than 10 metres. It was also apparent that a 10 metre drape over the top of the basement high could not be detected with reflection data of frequency content less than 70 Hz

The application of the discrete Radon transform for plane wave decomposition by the method of slant-stacking or tau-p transformation, is well researched for in-line two dimensional (2D) seismic data recording, and is frequently used to filter events which mask useful seismic data.

In three dimensional (3D) land seismic recording, the swath recording technique causes direct arrivals to be recorded at similar, if not identical, times to the desired reflections. If such arrivals are of identical frequency, the recorded data cannot be readily separated, which results in data which cannot be used in further processing.

This paper introduces a transformation approach to 3D data. The transform is an extension of the 2D slant-stack approach and is the correct application for stacking three-dimensionally recorded data since it sums along radial trajectories. The transform allows further processing of seismic data in the conventional tau-p domain, as well as offset dependent filtering of individual swath records. Offset dependent filtering offers a new dimension in tau-p domain filtering.

The Latrobe stratigraphic section subcropping the Top Coarse Clastics Unconformity' in the eastern Gippsland Basin consists of three depositional sequences of Maastrichtian to Palaeocene age. A Lower Palaeocene sequence illustrates the sedimentological and seismic relationships typical of these sequences. It has been further subdivided into three seismically mappable depositional units.

Unit C is a 40 m basal sandstone section characterised seismically by high frequency sigmoidal clinoforms. It is interpreted to represent lowstand fan deposition prior to a marine transgression.

Unit B consists of a 50 m argillaceous siltstone overlain by a 70 m coarsening upward sandstone section, characterised seismically by high frequency and high amplitude sigmoidal reflections. It is the product of marine transgression and subsequent highstand progradation.

Unit A consists of a thin argillaceous siltstone overlain by a coarsening upwards sandstone deposited during a minor transgression and continued progradation. Unit A also consists of interbedded sands, shales and coals producing parallel, often high amplitude reflections. Deposition of this facies occurred in a coastal plain environment behind the prograding shoreline.

The mapping of these seismic facies allow prediction of stratigraphy and reservoir and seal quality in an area of little well control, the deep water part of the Gippsland Basin. Stratigraphic pinchout plays of Unit B shoreface sandstones prograding into offshore siltstones and claystones and Unit C lowstand fan sandstones into Unit B offshore siltstones and claystones are also identified.

In inverse scattering one attempts to reconstruct the material composition of a domain whose interior is inaccessible to direct measurement by probing it from the outside. To this end the domain is considered as a contrasting domain in a known background configuration. The probing is carried out by exciting the object with a number of sources, while the resulting wavefield is detected at a number of receiver positions. In the corresponding mathematical description of the experiment the wavefield quantities are subject to a spatial-temporal differential operator, and to the boundary conditions, such as, for example, source conditions and the radiation conditions.

In general terms inversion can be formulated as a non-linear expression where the measurements are related to the contrast-function in the medium. This representation is equivalent to a volume integral over the contrasting domain where the contrast function, together with the actual field, acts as weights of the kernel function.

This kernel function depends on the position of two points in the contrasting domain and is known as the Green's function. The Green's function represents the inverse of the differential operator. In the usual formulation of the inverse problem the wave-theoretical character of the inverse operator is predetermined; only the constitutive parameters are allowed to vary. In this sense inversion is equal to inverse forward modelling. However, this approach leads to a restriction on the inversion process. The data to be inverted are harnessed due to this assumption. The parameters do not have enough flexibility to compensate for the discrepancies between observed and calculated data when the observed data cannot be attributed to such a wave problem. In the hierarchical approach it is proven that any wave problem can be decomposed into a set of subproblems. By arranging this set of subproblems in increasing order of complexity the associated inverse process is divided into two steps. The first step consists of determining the contribution of the sub-set members to the whole data set. In the second step a linear inversion is performed to each sub-set member. In this process the influence of less complex and previously determined members is taken into account. This procedure is not equal to inverse forward modelling.

A new set of minimum environmental standards to be met by all companies undertaking onshore seismic operations in Western Australia has been issued. These ‘Guidelines for Onshore Petroleum Geophysical Surveying’:

1. recognise the experience gained over 45 years of operations in Western Australian lands;
2. meet current community expectations using today’s methods and technology; and
3. allow for change to acknowledge new information, priorities and technology. Meeting the Guidelines is a condition of undertaking a seismic survey in the state.

While the majority of survey practices to date have been appropriate in environmentally robust areas the Guidelines seek consistently high standards, particularly in more fragile environments. They address the issues of line construction and rehabilitation techniques, and the planning and consultation necessary to enable environmentally sound exploration to be undertaken.

The Department of Mines will implement the Guidelines by educating relevant parties of their existence, by providing survey planning information and by measuring success through discussion and inspection of sites.

Optimizing the temporal resolution of seismic data in both the acquisition and processing stages increases the detection rate of subtle, multiple stratigraphic traps in complex lithologic sequences. Field sizes ranging from 1 MMBO to 40 MMBO are found in the coastal facies of eolian sandstone reservoirs of the Permian Minnelusa Formation in northeast Wyoming, U.S.A. Generally, three main trap styles exist: (1) an erosional unconformity providing the top and lateral seals with a shoreline marine carbonate acting as the bottom seal; (2) marine carbonates providing both the top and bottom seals; and (3) topographic relief at the top of dune complexes that lack a bottom seal but are overlain by marine carbonates. Usable frequencies in excess of 100 Hz from depths greater than 3300 m allow resolution of 10 m sandstone reservoirs with 15-20% porosity. With this frequency it is possible to distinguish not only porous sandstone reservoir signatures, but also the trapping mechanisms involved. High resolution seismic data acquired prior to a recent oil discovery is shown here to demonstrate a successful methodology used to discover and develop Minnelusa oil in subtle stratigraphic traps.

Successful exploration in coastal deposits such as the Minnelusa Formation requires an integrated approach to exploration. This includes broad band, high frequency seismic data acquisition and processing with a combined geological and geophysical interpretation. Using these techniques, discoveries are possible even in mature basins at exploratory drilling success rates well above the U.S. onshore industry average of 8-12%.

Laboratory studies of the temperature dependence of the electrical conductivity of rock materials can be used to relate electrical conductivity to temperature in the Earth. Electrical conductivity models advanced for central and southeastern Australia are compared with profiles predicted from such laboratory measurements, and are then used to generate representative magnetotelluric response curves. The response curves differ sufficiently to give confidence in their use to detect gross conductivity changes at depth. Observed data from Broken Hill and Ivanhoe, NSW, are interpreted to indicate a temperature contrast in the upper mantle of some 200°C between these two sites (hotter to the east beneath Ivanhoe). The temperature at these large depths is a vital factor in constraining models of crustal underplating and intrusion.

In deep water areas, the interference at important target levels due to water layer multiples can be extremely troublesome. Conventional deconvolution approaches are ineffective, due to variation of the multiple period with time (non-stationarity) and poor statistical estimation due to the limited number of multiple contributions within the autocorrelation window.

Weighted stacking improves the attenuation of long period multiples, and in recent times the demultiple process based on the f-k transform has been routinely applied to supplement the response of CMP stack.

A serious weakness of the f-k technique is that the removal of multiple energy from the record is strongly dependent on offset, the slopes of primary and multiple events converging towards the near offsets, where the process is thus relatively ineffective. More recently, there has been increasing interest in applications of the general class of decompositions known as the Radon transform, which model the data as a set of projections on the zero offset axis along a number of possible geometrical trajectories. A well known example is the linear Radon transform, commonly known as slant-stack or the tau-p transform. Although, like the f-k transform, slant-stack results in a plane wave decomposition of the record, the representation in terms of zero offset intercept versus ray parameter is such that each trace in the transformed record is associated with a constant angle of emergence. Water layer multiples are then represented with constant reverberation periods within each trace, and may then be treated by a conventional statistical deconvolution.

A further example of the general class is the parabolic Radon transform, which models the record as a set of parabolic alignments. With prior application of approximate NMO correction, this transformation can provide a well focused mapping of multiple reflections, enabling them to be identified and isolated for inverse transformation. The predicted multiple is then removed by subtraction from the original record. This method is not dependent on the accuracy of a statistical model and provides effective demultiple on near as well as far offsets.

Merits and disadvantages of these methods are discussed and performances are compared on data from a typical deep water area. Results show that the parabolic Radon transform enables a far more accurate modelling of long period multiples than the other methods considered, thus achieving a more effective demultiple process.

The theoretical attractiveness of utilising both P- and S-reflection information is offset in the shallow environment by the practical difficulty of isolating high resolution reflections from other non-reflection energy. Source design is a critical component in a range of mandatory acquisition and processing optimisations.

Four near-surface P-wave sources (high explosive, shotgun, weight drop, and hammer) have been evaluated in terms of their suitability for shallow reflection at a test site in the Ipswich Basin, southeast Queensland. Cross power spectral analyses of trace segments in the reflection window provide an estimate of the 'coherent bandwidth' of the different sources. Although the high explosive has superior signal levels, consideration of spectral symmetry, autocorrelation shape, and logistic aspects indicates that the shotgun source is a viable alternative.

A pneumatic powered S-wave generator has been constructed and tested. Integrated spectral energies have been used to monitor piston velocity limitations and hence define optimum operating parameters. The pneumatic source is more controllable and of higher energy than a sledgehammer-powered equivalent.

Seafloor magnetotelluric observations of natural fluctuations of the Earth’s magnetic and electric fields provide information about the electrical structure of the suboceanic lithosphere and upper mantle. However, such measurements may be distorted by three-dimensional induction effects in the overlying ocean, associated with the geomagnetic coast-effect and changes in bathymetry.

In the present paper electromagnetic three-dimensional modelling is applied to the Tasman Sea. A thin-sheet algorithm is used, and calculated magnetotelluric parameters are compared to observations made during the Tasman Project of Seafloor Magnetotelluric Exploration (TPSME). The model calculations suggest that, away from coastlines, three-dimensional induction in the ocean distorts the electromagnetic signature of the sub-oceanic lithosphere and upper mantle in a frequency-dependent but smooth manner. The model calculations thus permit the observed TPSME responses to be corrected for distortion, to give improved estimates of the electromagnetic response of the solid-Earth underlying the observation sites. Inversion of such (corrected) response estimates for one-dimensional electric structure reveals that the structures beneath 55 and 70 Ma seafloor sites are very similar. Furthermore, constraints imposed by the thin-sheet modelling indicate that to replicate the TPSME observations, a poorly conductive (less than 10 "³ S/m) upper lithosphere is required. This result implies that electric currents which are induced and ‘trapped’ in the ocean may contaminate magnetotelluric observations far inland.

An in-seam seismic survey was conducted by the Australian Coal Industry Research Laboratories (ACIRL) at the German Creek Central Colliery, Queensland, to detect and delineate dykes obstructing mining operations within the coal seam. A transmission analysis of the Love and Rayleigh waves has been carried out to test for absorption of the seismic energy by the dykes and hence to determine the viability of identifying the dykes using this technique. The seismograms and amplitude spectra for these wave types indicated significant attenuation across the survey area which included the three dykes. Finite difference and finite element modelling of the dykes was used to predict the absorption properties of the dykes. A study of the transmission losses in both the field and model data revealed that in-seam seismic data required coherent signals at frequencies greater than 250 Hz for detecting dykes in a coal seam of 2.5 m thickness.

The early phase of a new mantle plume may result in areas of basaltic volcanism and crustal reworking 1-2 × 10³ km across. Uplift of pre-existing continental crust by up to 800 metres may be sufficient to initiate large-scale tectonic processes, particularly extension. Conduction of heat from a hot plume layer emplaced into the uppermost mantle can result in considerable anatectic reworking of pre-existing continental crust, and in the formation of a granitic upper crust; the Late Archaean reworking of the eastern Yilgarn Block of Western Australia, most of which occurred over a relatively brief time interval 2660-2690 Ma ago, provides a possible example. The magmatic development, and time and length scales inherent in plume-initiated continental reworking are similar for postulated ancient and modern examples, and it is inferred that plume-initiated magmatism and tectonics may play an important role in the development of internally differentiated continental crust.

Boreholes drilled in the search for oil in the Vulcan Sub-basin (Timor Sea, North West Shelf, Australia) commonly exhibit an elliptical cross-section believed to be the result of wellbore failure known as borehole breakout. Breakouts form by compressional shear failure in response to stress concentration around the borehole due to the prevailing in situ stress. The bore wall becomes elongated in the direction of least horizontal compressive stress. The orientation and shape of the breakouts are measured by the four-arm dipmeter tool. The azimuths of the long axes of breakouts in the Vulcan Sub-basin show a reasonably consistent 130-170°N trend implying that maximum horizontal compressive stress (SHmax) is oriented 040-080°N.

This NE-ENE SHmax orientation in the Vulcan Sub-basin is not controlled by compression transmitted from the nearby Australia/Banda Arc collision zone. However, it is consistent with theoretical models of stress distribution in the Indo-Australian plate based on the plate-driving forces at all of its boundaries. The present SHmax orientation is consistent with either strike-slip or normal movement on the pre-existing NE-trending faults in the basin.

Numerical experiments show that the hot thermal plumes in a convecting mantle have a complex 3-dimensional stucture that is approximately axisymmetric near the top of the convecting layer, but followed down towards the base of the layer evolves into a branched structure formed by the bifurcation of a hot sheet. The uplift above a thermal plume is thus characterised by a dominant axisymmetric component, to which a small, irregular, but characteristically triple-armed, perturbation is added. An explanation for the characteristic triple-junction geometry of continental rifts has long been sought in terms of the mechanical properties of the lithosphere, but these experiments suggest that the rift may inherit its basic geometry from the underlying thermal plume.

Seismic, gravity and geothermal data were interpreted to examine the structure and the evolution of the southern Perth Basin. Seismic data show that the principal fault trend is north-south and that strike-slip faulting is of major importance in the formation of the basin. Gravity modelling shows that: (a) crustal thickness of the basin is approximately 30 km; (b) greater crustal thickness beneath the basin than on either side implies isostatic imbalance; (c) the Darling and Busselton Faults are steeply dipping faults which extend into the lower crust and control the structure of the southern Perth Basin.

Analysis of these data shows that the evolution of the basin has had three major periods of tectonism which reactivated major faults: (a) a right-lateral strike-slip motion along the Darling Fault in the Late Permian-Early Triassic; (b) a phase of tectonism with associated left-lateral motion along the Dunsborough Fault in the Jurassic; (c) the separation of Australia from India in the Early Cretaceous which produced a predominantly tensional, and some oblique-transcurrent, style of faulting.