Exploration Geophysics - Volume 20, Issue 1-2, 1989

Twenty-two digitally recording fluxgate magnetometers measuring short-period natural time variations of the geomagnetic field were deployed during 1988 at 54 sites to define the northward extension of the Eyre Peninsula Conductivity Anomaly (EPCA). The southern portion of this zone of high electrical conductivity was initially mapped during the early 1980s; it trends inland just to the east of north from the southern extremity of the peninsula, at nearly 90° to the continental shelf edge.

The 1988 data show that the axis of the anomaly in the north initially swings to the east, and then turns NNW, following the major structural trends in the Precambrian basement of the Gawler Block.

Short-period geomagnetic variations observed in stacked magnetograms display a significant amplitude and phase change in the vertical (Z) and total field (F) components across the anomaly axis, while the horizontal variation fields (D and H) are enhanced along the axis. These features are consistent with a concentration of current in the crust.

Transfer-functions relating the anomalous vertical to normal horizontal geomagnetic variation fields are expressed in the form of vectors and hypothetical event contours, and these clearly delineate the surface position of the anomaly axis.

The EPCA is coincident with a broad band of seismic activity, and a continuous conductive zone such as this must be formed along a significant structure within the Precambrian crust. The zone probably delineates the boundary between the essentially non-conductive Archean crystalline basement to the west and the multiply deformed Proterozoic metasediments of the Hutchison Group to the east. Its presence is important in understanding the tectonic development of this potentially economic zone.

Zones such as the EPCA can also introduce errors of several nT into aeromagnetic surveys; data from closely-spaced magnetometer arrays may be used to accurately predict and eliminate such errors.

Reflection seismic datasets are obtained in both the exploration of oil and mineral resources and the probing of the deep crust and the upper mantle. To interpret the datasets, considerable effort has been spent on the understanding of seismic wave propagation phenomena by simulating seismic wave propagations in some a priori physical models. A rather simple and efficient modelling technique has been developed to study elastic wave reflections with full inclusion of diffractions.

This modelling technique employs an integral representation of reflections from a surface or a scatterer. High frequency asymptotic approximations are used for the propagation between the seismic source or receiver and a surface or a scatterer. At a scatterer, first order scattering is assumed. At a surface, reflection and transmission effects are estimated using the assumption of a locally plane interface and plane incident wave. With these approximations, the reflected seismograms are calculated by convolving the time derivative of a source function with a model weight function for a particular source-receiver pair. The weight function at a particular time is evaluated by a line integral along a contour of equal total travel time from source to receiver via the scattering surface (an isochron). The kernel of this integral at a reflecting point is the local reflection coefficient which which represents the effects of the amplitude of material parameter contrasts at the reflecting point, the angles between the incoming and outgoing waves and the local surface normal and the local speed of advance of the isochron on the surface, and the geometrical spreading factors from the source and receiver to the reflecting point.

This modelling technique is used to investigate the validity of some of the interpretations of a deep crustal reflection profile collected in central Australia. The modelling results confirm that even with a relatively short (4 km) field spread it would be possible to pick up the reflected energy from faults with dips of about 40°. The largest fault, the Redbank Zone, has significant displacement of the crust-mantle boundary and within the fault zone, it is conceivable to have considerable variability in physical properties.

The deep seismic section shows this boundary as a thick (0.5s) band of complex reflections and diffractions at the reflection time appropriate to the crust-mantle transition. Two possible structures for the crust-mantle boundary were investigated, one where the crustal faults have displaced this interface and created a ‘block-faulted' geometry and the other where the crustal faults are listric near the boundary and appear to sole out on the crust-mantle interface, giving rise to an undulation of the Moho. The modelling results (Figure 1) for an undulating boundary show a band of reflections which strongly resemble the observed seismic reflection data.

A prominent positive gravity anomaly overlies the Macdonald trough in the Sydney basin. Allowing for isostatic compensation and the effect of sedimentary rocks, the anomaly is determined to have an amplitude of 440 GU (μtms^-2) and a width of 60 km. The anomaly is smoothed using cubic splines, FFT and IFFT. It is interpreted by a large mafic body of density 2.9 g cm^-3 underlying the basin to a depth of 13.5 km. A 12 km wide zone with a small positive density contrast underlies the body within the lower crust.

The steep western boundary of the body represents a major basement fault underlying the Lapstone monocline and Kurrajong Fault System.

The anomaly is a member of the Meandarra Gravity Ridge which marks a zone of crustal extension within which dominant nature of intrusion is mafic in character.

In the Albany 1:1M sheet, the 10-50 km wavelength gravity and aeromagnetic anomalies define major boundaries and subdivisions of the Precambrian blocks/provinces and large bodies of granite, while the short wavelength magnetic anomalies define lithological banding and lineaments. The Yilgarn Block in the sheet area is readily subdivided into two major north-northwest to north trending zones of low magnetization separated by a 30 km wide zone of high magnetization. The eastern zone is considered to be due to granite-greenstone terrane, the western boundary of which is located 100 km west of that currently recognised from outcrop geology. The western zone is considered to be due to granite-geiss terrane while the 30 km wide zone between coincides with strongly magnetised granulites. The Albany Province is composed of two structurally distinct east-west trending zones. The southern zone of relatively low magnetization and density coincides with acid gneiss and granites, whereas the highly magnetised, relatively dense zone to the north and west, correlates with highly metamorphosed acid and mafic granulites. Thrusting of the Albany Province during the Mid-Proterozoic has demagnetised and or deformed the margin of the southern Yilgarn Block to at least 50 km north of the block boundary. Throughout the region, significant mineral deposits of Au, Ni, Sn, Ti, and Fe are located within greenstone and high grade metamorphic belts. These belts have characteristic signatures which contrast with extensive areas of relatively homogeneous, low economic mineral potential, granite-gneiss terrane.

Modelling of magnetic terrain and comparison with actual data is an efficient method for assessing large sets when residual anomalies are important. The technique of Blakely (1981) which utilises a rapidly converging series of Fast Fourier Transforms is an efficient and sufficiently accurate method for this assessment.

The technique has been applied to a data set at Kilkivan, south eastern Queensland. Here the magnetic sources are near horizontal Triassic volcanic flows unconformably overlying a non- magnetic Palaeozoic basement.

Geological control is good so that it is possible to model the bottom of the flow. It is postulated that the difference between the calculated and actual data represents paleochannels in the basement. Similar techniques applied to gravity data have not been as successful.

Analysis of refracted first breaks has traditionally assumed that the data have been collected using special geometries that enhance the results, e.g. reversed profiles with regularly spaced geophones in-line between two shots. The geometries used to collect most 3D reflection data are quite different. Thus refraction analysis to obtain statics using traditional methods requires both approximations and selection of a subset of the data. The surface-consistent method allows use of nearly all the data, thus providing the high redundancy required for statistical robustness. A large survey incorporating both a new 3D survey and older 2D lines yielded refraction statics that greatly improved the final results.

The surface-consistent method assumes that the refractor can be approximated by a horizontal plane under each station. When the refracting surface is steeply dipping, this assumption may break down. The Generalized Reciprocal Method is a traditional refraction analysis technique that gives improved results for steeply-dipping refracting surfaces. The surface-consistent method can also be made less sensitive to dip by a generalization, i.e. assume a dipping plane under each station. The refracting surface between the stations can be approximated by interpolation of these planes using the ‘linear projection’ technique. A comparison of results using the two surface-consistent methods on synthetic data generated from a model of a buried, steep-walled valley shows the superior results obtainable.

Instantaneous-phase is the optimum attribute to use in horizontal seismic time slices for rapid, accurate 3-D structural mapping, because of its enhancement of event continuity. Instantaneous-phase slices also provide the interpreter with the most precise display for directly interpreting spatial relationships between seismic sequences, by reasoning analagous to that used to interpret geological maps. In contrast, amplitude time slices lack precision because weak events are lost by scaling, and wavelet phase points can only be crudely estimated.

Three dimensional (3D) marine seismic surveying is expensive and often a lengthy and technically difficult survey to perform. It is therefore only executed when an economically viable discovery is made. An alternative technique is offered which may be used when a marginally economic discovery is made. The technique is inexpensive compared to the conventional full 3D marine survey; it is cheaper than reconnaissance surveying and two boat operations, and provides a 3D migrated annular volume just over 3 kilometres in diameter for the approximate price of a single offset vertical seismic profile (VSP).

The technique uses the exploration drilling rig as the energy source platform, the rig supply vessel as the receiver, and the site location system as the 3D navigation network. In using equipment conventionally mobilized with each drilling rig relocation, costs are substantially reduced and a larger portion of the 3D seismic exploration budget may be transferred to the engineering/drilling budget.

Failure of the technique to be trialled is due to the conservatism found within the industry rather than technical considerations.

The standard approach to stacking and velocity estimation for 3-D seismic reflection surveys is to organise the data by CMP bins and then apply techniques originally developed for 2-D surveys. The benefit of conventional binning is primarily in the display of the 3-D data volume.

Velocity estimation can be carried out directly with 3-D data provided the geometry of the survey is readily available. To minimise the effects of dip it is still desirable to restrict the location of the CMP’s to lie in a restricted region without the requirement of bins of fixed size. Since the stacking velocities are azimuthally dependent, the trace gathers for velocity estimation over a narrow azimuth window should be chosen for that purpose rather than be based on stacking bins. Once the elliptic variation of stacking velocity with azimuth has been estimated, the seismic traces can be simultaneously stacked and interpolated onto a regular grid. The interpolation procedure is of most significance for short reflection times. The regular array of traces is particularly beneficial for the development of 3-D self-consistent statics procedures exploiting recent developments in large scale inverse problems.

As part of an industry funded research project into the application of the technique of LOFOLD3D land seismic surveying, a four fold three dimensional seismic survey was performed in the Perth Basin at Moora, Western Australia in July 1987. The volume covered an area of four kilometres by just under two kilometres, producing a total of 23,000 common midpoint traces. The objective was to collect and process the data in such a manner that a three dimensional structural interpretation would result, which would be the same as that resulting from a conventional three dimensional survey. A cost comparison indicates that a commercial LOFOLD3D survey would reduce the cost of performing a land 3D survey to an estimated 20% of the full fold equivalent, and the technique therefore offers potential for substantial savings if it is adopted on a commercial basis.

In the past, rifts have mainly been identified in terms of sediment troughs. They account for many of the elongate gravity lows distributed in a coherent rectilinear manner over the continent. Other gravity lows can be attributed to granites intruding rift compartments, and some gravity highs can be attributed to basic volcanics in compartments. The total number of rifts which can be thus inferred from gravity and magnetics is very large, and suggests rifting is pervasive over the whole continent and controlled by a systematically distributed “Cardinal” system of ancient vertical crustal fractures.

The extensional concept of rifting is based on a finite number of rifts, all of which have “failed” to split the continent. When a far greater number of rifts is recognised, it becomes difficult to accept that all these rifts have “failed” to reach full opening by extensional processes. In view of the known horizontal compressive forces acting in the crust, it is more probable that rifting is caused by compression. The compartmentali-zation of rifts, clearly observed in gravity data, also implies compression.

Closely spaced rectilinear dyke systems in shield areas may also represent the pervasive “Cardinal” fracture system. In general, this system of orthogonal fractures poses problems for the detatchment rifting concept which assumes that transfer faults are formed at the time a rift forms, whereas they in all probability predate the rift, and owe their existence to a fundamental process operating when continental crust first formed.

Two types of compressive rift models are discussed. One is associated with shear couples between widely spaced parallel fractures. The other is based on the concept of a crust cut by closely spaced fractures in which compression is propagated along a network of linked blocks. In both cases the development of basement ridges is a key issue.

Seismic reflection events beneath marked lateral velocity variations are distorted by complex ray paths. This can result in a stacked section with events that show poor continuity and are affected by ‘pull up’ or ‘push down’. Where the velocity anomaly is not near the surface, conventional statics often fail to produce an adequate result.

A pre-stack solution based on ray tracing is presented, which applies dynamic time corrections that vary with offset and travel time.

The method was applied to a grid of data in the Gippsland Basin, affected by deep erosional canyons on the sea floor. The resulting sections generally showed significant improvement to the continuity of events thus enabling depth maps of greater accuracy to be constructed.

We conclude that the method is more suitable in the study area than other pre-stack techniques given the absence of steep dips beneath the canyons and the exploration objectives. Other applications of the method are also mentioned.

The A-Er-Shan Field, in the Inner Mongolia Region of PRC is a buried hill type structure. The oilfield's nature is such that a detailed grid of 2-D seismic could not adequately map the producing horizons. A 3-D seismic survey of the oilfield was required to provide the proper spatial sampling of the productive horizons to both map the numerous faults in the field and to migrate dipping horizons to their proper spatial position. Thevenard Island in Western Australia is an environmentally sensitive surface feature 6.0 km long by 1.5 km wide within the hydrocarbon rich and environmentally sensitive NW shelf environment.

After extensive pre-survey modelling and subsurface fold simulations optimum field geometry was established for each survey using a multi-line Swathe configuration. Infield experimentation was used to determine relevant source, receiver arrays and modelled Varisweep^TM parameters utilising the Vibroseis seismic source. Following final processing, objectives of vertical and spatial resolution were attained along with significant improvements in S/N over previously recorded data.

The historical interpretation approach is based upon time and amplitude. Using the modern interpretive workstation, the full range of the seismic attributes can be examined in varying color and spatial distribution. Examples of traditional seismic data displayed using conventional methods are shown before and after workstation manipulation, with striking results.

The paper will also address procedures for the economical collection of additional data which will reinforce older available data, as well as planning cost effective acquisition of new data.

The historical interpretation approach is based upon time and amplitude. Using the modern interpretive workstation, the full range of the seismic attributes can be examined in varying color and spatial distribution. Examples of traditional seismic data displayed using conventional methods are shown before and after workstation manipulation, with striking results.

The paper will also address procedures for the economical collection of additional data which will reinforce older available data, as well as planning cost effective acquisition of new data.

Cultus Petroleum N.L. began exploration in petroleum permit EPP 23 of the offshore Otway Basin in December 1987. The permit was sparsely explored, containing only 2 wells and poor quality seismic data. A regional study was made taking into account the shape of the basin and the characteristics of the major seismic sequences. A prospective trend was recognised, running roughly parallel to the present shelf edge of South Australia.

A new seismic survey was orientated over this prospective trend. The parameters were designed to investigate the structural control of the prospects in the basin. To improve productivity during the survey, north-south lines had to be repositioned due to excessive swell noise on the cable. The new line locations were kept in accordance with the structural model. Field displays of the raw 240 channel data gave encouraging results.

Processing results showed this survey to be the best quality in the area. An FK filter was designed on the full 240 channel records. Prior to wavelet processing, an instrument dephase was used to remove any influence of the recording system on the phase of the data. Close liaison was kept with the processing centre over the selection of stacking velocities and their relevance to the geological model. DMO was found to greatly improve the resolution of steeply dipping events and is now considered to be part of the standard processing sequence for Otway Basin data.

Seismic data of a high enough quality for structural and stratigraphic interpretation can be obtained from this basin.

Key words:.

Conventional algorithms for the inversion of seismic data generally produce inverted traces which are limited in bandwidth to that of the input seismic data. This necessitates that a low-frequency model be added to the inverted traces in order to produce full bandwidth seismic logs. A problem with this method is that there is usually a gap in frequency between the low end of the seismic and the high end of the model spectra. A further problem with conventional inversion methods is that it is difficult to incorporate well or geologic information into the inversion process.

To overcome these problems a new broadband constrained inversion method which combines seismic, well, and geologic information is used. This method simultaneously satisfies constraints imposed by the well and geologic information, whilst inverting the seismic data. The output is an optimized broadband acoustic impedance model.

In order to combine the diverse data types of seismic amplitudes, well logs, geologic models, and horizon interpretations, an interactive workstation is used. The interactive environment is ideal for this work as it provides the flexibility required to manipulate and display both the varying incoming data types and the output data model.

Key words:

Three-component (3-C) amplitude versus offset (AVO) inversion is the AVO analysis of the three major energies in the seismic data, P-waves, S-waves and converted waves. For each type of energy the reflection coefficients at the boundary are a function of the contrast across the boundary in velocity, density and Poisson’s ratio, and of the angle of incidence of the incoming wave. 3-C AVO analysis exploits these relationships to analyse the AVO changes in the P, S, and converted waves.

3-C AVO analysis is generally done on P, S, and converted wave data collected from a single source on 3-C geophones. Since most seismic sources generate both P and S-waves, it follows that most 3-C seismic data may be used in 3-C AVO inversion.

Processing of the P-wave, S-wave and converted wave gathers is nearly the same as for single-component P-wave gathers. In split-spread shooting, the P-wave and S-wave energy on the radial component is one polarity on the forward shot and the opposite polarity on the back shot. Therefore to use both sides of the shot, the back shot must be rotated 180 degrees before it can be stacked with the forward shot. The amplitude of the returning energy is a function of all three components, not just the vertical or radial, so all three components must be stacked for P-waves, then for S-waves, and finally for converted waves.

After the gathers are processed, reflectors are picked and the amplitudes are corrected for free-surface effects, spherical divergence and the shot and geophone array geometries. Next the P and S-wave interval velocities are calculated from the P and S-wave moveouts. Then the amplitude response of the P and S-wave reflections are analysed to give Poisson’s ratio. The two solutions are then compared and adjusted until they match each other and the data.

Three-component AVO inversion not only yields information about the lithologies and pore-fluids at a specific location; it also provides the interpreter with good correlations between the P-waves and the S-waves, and between the P and converted waves, thus greatly expanding the value of 3-C seismic data.