Exploration Geophysics - Volume 28, Issue 1-2, 1997

A wide variety of numerical procedures in potential field geophysics require data modelled on a regular grid. However, airborne data tend to be highly sampled along the flight line and sparsely sampled in the perpendicular direction.

A gridding method commonly called ‘bi-cubic spline’ is widely used in potential field geophysics. Standard bi-cubic spline methods used on aeromagnetic data produce artefacts when a geological feature’s ‘line of strike’ is not perpen-dicular to the direction of the acquisition line. This method has a tendency to break up thin elongated magnetic anomalies, at an oblique angle, into a series of bulls eye artefacts. A method of finding local anomalies and their strike along lines based upon minimum variance principles reduces these effects. This technique has significant impact on the quality of output grids.

In association with the Magnetic Image Project (MAGMAGE) developed by Gunn and collaborators, that involved work on complex attributes of aeromagnetic anomalies, the gridding of phase posed some unique problems. Raw phase is a spiralling function which is periodic and cyclic. Unwrapping of the phase, therefore, is necessary to give a spatially coherent grid for interpretation.

By focussing on two developments in gridding — trending in a bi-cubic spline method and unwrapping of cyclic data — these methods are shown to increase the accuracy of representation of actual data being interpolated. Case studies of these solutions are presented using the INTREPID geophysical processing and visualisation system.

The acquisition and release of airborne magnetic and radiometric data and digital elevation data acquired over the Republic of Vanuatu during 1994 has significantly increased mineral exploration in that island nation. Vanuatu has a recognised potential for significant copper and gold deposits. The geophysical database will also provide a basis for long term studies of geology, groundwater resources, geohazards, geothermal and hydrocarbon potential and landuse capabilities.

The Bass Basin, a failed Mesozoic rift, located offshore between Tasmania and Victoria, was formed by extension associated with the separation of the Australian and Antarctic continents. Integration of the first available complete aeromagnetic coverage of the area, newly available gravity data, existing seismic data, drillhole information and outcrop geology mapping, indicates that northeast-southwest tension ruptured and separated fragments of the upper crust to create a depocentre up to 60 km wide. Fracturing along pre-existing basement lithological contacts and structures allowed transfer fault movements to accommodate this extension. Three main compartments developed in the basin, each of which underwent different degrees of extension. These compartments overlie accumulations of dense magnetic mafic material, evident on 14 s seismic reflection data, which were apparently produced by a mantle decompression process associated with crustal thinning. The largest of these mafic bodies displays the characteristics of a preserved, embryonic, oceanic spreading centre.

The quadrature of a total magnetic intensity field (TMI) exhibits detail not evident in the TMI, the instantaneous phase of the TMI which maps continuity and which appears to have superior resolution to vertical gradient operators, the instantaneous frequency operator which maps character changes in the TMI. Fractional vertical derivatives have enhancement properties intermediate between the classical first and second vertical derivatives. Stabilised downward continuation allows continuation below shallow magnetic layers to enhance the magnetic effects of deep layers. Coherence filtering maps fault zones and anomaly axes. The magnetic mineral accumulations in the regolith may be mapped using appropriate non-linear filtering techniques.

A new compilation of magnetic data over onshore and offshore Tasmania plus existing geological mapping and gravity data have provided the basis for the production of a map illustrating the geometric distribution of the main basement units of the state. Significant aspects of the new interpretation map include: the delineation of the wide-spread sub-surface granite occurrences in Tasmania; identification of more extensive occurrences of Proterozoic rocks than previously known; mapping of the extent of the Dundas Trough and extensive northwestern and southeastern continuations of the Beaconsfield ultramafic rocks; definition of the areal distribution of Mesozoic sedimentary basins; and the control of Proterozoic lineaments, in particular the Arthur Lineament, on the development of the Mesozoic basins.

Ground magnetic surveys for gold exploration in the Archaean Yilgarn Craton of Western Australia are rapidly being replaced by low-level, high-resolution, fixed-wing and helicopter aeromagnetic surveys for detailed geological mapping of large areas. However, for exploration of areas that are either small, or where the buried magnetic sources are located near the surface, or where the magnetic response of the regolith is of interest, ground magnetic surveys are required in order to measure the high-frequency magnetic responses associated with these environments. Surface and subsurface geological features are often of interest when exploring for near-surface gold mineralisation.

Earlier work, by S. Mudge, has shown that bipole plots of second horizontal derivatives of aeromagnetic data are effective in resolving detail in aeromagnetic data. These have been applied to ungridded ground magnetic data and are effective in resolving high-frequency detail in the magnetic responses of subsurface rocks and the regolith. The large high-frequency component of ground magnetic data acquired from maghemite-rich regolith areas presents different and often difficult data processing and presentation problems compared with data acquired from higher-level aeromagnetic surveys. Despite this, second horizontal derivatives of ground magnetic data resolve important detail of magnetic subsurface rocks and the regolith that would otherwise be lost in images and contours of the gridded data.

For an area in the Kalgoorlie district of Western Australia, data from a ground magnetic survey and a low-level aeromagnetic survey were transformed with the second horizontal derivative filter to reveal different degrees of resolution of magnetic sources located in the regolith and the subsurface. Line profiles, and images and contours of the gridded data, from both surveys resolved subsurface sources which have assisted with the identification of drill targets. Importantly, however, magnetic features in the regolith were only resolved in the enhanced ground survey data which has assisted with geological mapping of the regolith.

New high-resolution airborne geophysical data acquired in the Broken Hill Region are being used to assess the deformational history and define the regional structural regime. The interpretation of these data may enable new structural and stratigraphic models to be postulated that will enhance and revitalise the exploration thrust within the Broken Hill Region.

Geophysics has been used in iron ore exploration in the Hamersley Basin for more than twenty years. The use of the magnetic technique for structural and lithological mapping has been well documented. While direct detection of hematite mineralisation through this method has been reported, this is not believed to be a reliable means of targeting iron ore. The application of other techniques, including the radiometric, gravity and electrical methods is not well covered in literature.

Geophysical methods have been applied to aid mapping of the Giles Mini martite-hematite deposit. Regional lithomagnetic units and late stage faults are identified in magnetic data. However, layer-parallel faulting cannot be identified by this technique. Ground electrical methods, on the other hand, have proved to be a useful tool in tracking not only outcropping lithologies to depth, but also detecting and mapping the fault system truncating the orebody to the north.

The Ravva oil and gas field is located offshore eastern India in the Krishna-Godavari Basin. Command Petroleum (India) Pty. Limited was granted operatorship of the field in October 1994 by the government of India, on behalf of a Joint Venture consortium that includes Oil and Natural Gas Corporation (ONGC) Ltd, the Indian national oil company; Videocon Petroleum Ltd; and Ravva Oil Ltd (Marubeni).

One of the first geophysical projects to be initiated in the development plan was reprocessing of the Ravva 3-D seismic dataset, acquired in early 1990 using a single streamer/single source configuration. The geography of the coastline and the shallow water depths in the area (<20 m) required a shooting direction that was predominantly in the strike direction but somewhat oblique to the main structural elements of the field.

This reprocessing project was initiated to achieve an optimum dataset for siting of the development/exploration wells over the next two to three years. Recent developments in computer hardware and the applications to 3-D seismic data processing have allowed the implementation of 3-D prestack migration to develop into a usable process for the practising explorationist.

The reprocessing project is described, with particular emphasis on the testing and analyses that led to use of 3-D prestack time migration to image the data volume. Examples show the improvements in stacking velocity definition and seismic imaging that have been achieved using 3-D seismic processing techniques that were not practicably available when the dataset was initially processed in 1990.

The generally accepted model of a 95 Ma breakup event in the Otway Basin has several unresolved inconsistencies, including evidence of significant post-95 Ma aged crustal extension. An alternative model is proposed: oceanic breakup took place at the end of the Late Cretaceous. A phase of extension during the pre-Aptian was followed by thermal subsidence before renewal of rifting (Cenomanian). Rifting of the already attenuated crust allowed the region to subside below sea level, resulting in marginal to nearshore marine conditions throughout most of the Late Cretaceous.

Continental breakup in the Late Maastrichtian (approximately 67 Ma) was marked by regional uplift along the northern basin margin, and towards the incipient continent-ocean boundary. This uplift is parallel to the present-day shelf. The age of this event is derived from palynological and seismic interpretations, and is consistent with previous thermochronological work. Broad anticlinal features on the basin margin may be associated with ridge-push associated with early oceanic crust development.

The first post-rift depositional cycle was during the latest Maastrichtian. A significant transgression flooded a peneplain (67 Ma unconformity surface). A maximum flooding surface was developed near the Cretaceous/Tertiary boundary, with subsequent highstand deposition developed through most of the basin. A second unconformity marks a depositional hiatus at the end of the deposition of this sequence.

Later cycles of post-rift sedimentation began in the Late Paleocene (55 Ma) with deposition of the ‘Wangerrip Megasequence’. Several eustatic sequences were deposited in a deltaic environment during the subsequent 8 million years.

Kinematic modelling differs from traditional modelling techniques in that model geometries are obtained by subjecting an initial horizontal stratigraphy to specified structural deformations rather than being input as a pre-specified shape. The magnetic field over the southern Joseph Bonaparte Gulf of northwestern Australia is ideally suited to a kinematic modelling approach because the magnetic effect (anomalies) of the area is almost entirely due to an extensive sheet of Proterozoic Carson Volcanics, which apparently originally covered a large proportion of the area. That unit has subsequently been fractured and warped during Palaeozoic rifting. A three-dimensional kinematic model has been produced for the present geometry of the Carson Volcanics such that the computed magnetic field for the model closely duplicates the observed magnetic field. The modelling largely confirmed earlier interpretations of the data based on image analysis and modelling of profile data. Refinements indicated by the three-dimensional modelling were a magnetic feeder pipe to explain a large localised magnetic high and a salt accumulation with a diamagnetic susceptibility giving a negative magnetic susceptibility contrast relative to adjacent sedimentary rocks to explain a magnetic low previously interpreted as being due to a localised absence of the magnetic sheet. The interpretation of the salt body is consistent with drill and seismic data, thus demonstrating the effectiveness of kinematic modelling of magnetic fields.

The Ravva oil and gas field is located offshore eastern India in the Krishna-Godavari Basin. Command Petroleum (India) Pty. Limited was granted operatorship of the field in October 1994 by the government of India, on behalf of a Joint Venture consortium that includes Oil and Natural Gas Corporation (ONGC) Ltd, the Indian national oil company; Videocon Petroleum Ltd; and Ravva Oil Ltd (Marubeni).

One of the first geophysical projects to be initiated in the development plan was reprocessing of the Ravva 3-D seismic dataset, acquired in early 1990 using a single streamer/single source configuration. The geography of the coastline and the shallow water depths in the area (<20 m) required a shooting direction that was predominantly in the strike direction but somewhat oblique to the main structural elements of the field.

This reprocessing project was initiated to achieve an optimum dataset for siting of the development/exploration wells over the next two to three years. Recent developments in computer hardware and the applications to 3-D seismic data processing have allowed the implementation of 3-D prestack migration to develop into a usable process for the practising explorationist.

The reprocessing project is described, with particular emphasis on the testing and analyses that led to use of 3-D prestack time migration to image the data volume. Examples show the improvements in stacking velocity definition and seismic imaging that have been achieved using 3-D seismic processing techniques that were not practicably available when the dataset was initially processed in 1990.

The aeromagnetic surveys flown under the Victorian Initiative for Minerals and Petroleum and by others give a major regional dataset over western Victoria. This paper presents a new interpretation of the structural and tectonic history for the early Palaeozoic rocks of the region derived from integrating the aeromagnetic, regional mapping and other datasets. It provides a new geological model for exploration beneath the thin Murray Basin cover.

Two distinct packages of rocks are present in the western Victorian basement; the eastern Stawell Zone and the western Glenelg Zone. The Stawell Zone contains turbidites and oceanic basalts typical of much of the Victorian Lachlan Fold Belt and which host major gold deposits. They are weakly magnetic with the basalts and pyrrhotitic slates providing most magnetic expression. Recent Ar-Ar dating gives a deformation age of about 435 Ma. Their western boundary is the northern extension of the Moyston Fault.

A new tectonic framework is proposed for the Glenelg Zone. Island arc volcanic rocks and metasediments of the Zone have been variably metamorphosed and simply deformed in the 500 Ma Delamerian Orogeny. The rocks lie in a similar structural and tectonic position to those of the Mount Read Volcanics and have potential for VHMS deposits. Correlations can also be made with other Cambrian VHMS deposits and prospective regions in eastern Australia.

Along the South Australian border part of the Glenelg Zone, the Ozenkadnook Subzone, contains amphibolite grade rocks that have been complexly folded in a deformation that seems to be earlier than the Delamerian. These rocks are of uncertain affinity, but may be a northern correlative of the upper Proterozoic rocks seen in northwestern Tasmania.

Keywords:

Investigation of magnetic properties and magnetic modelling of the Porgera Intrusive Complex, Papua New Guinea, has established representative magnetic properties and the palaeomagnetic signature of the major rock types of the Complex.

Relatively unaltered intrusive rocks of the Porgera Intrusive Complex are moderately to strongly magnetic, whereas strongly altered intrusions, mineralised zones and country rocks are very weakly magnetic. For most intrusions, Koenigsberger ratios are substantially less than unity, indicating that induced magnetisation is predominantly responsible for anomalies associated with most of the Complex. Hornblende diorite and hornblende diorite porphyry tend to be the most magnetic rock types in the Complex. However, all fresh intrusive rocks sampled, from very mafic to intermediate compositions, have rather high magnetic susceptibilities.

Primary thermoremanent magnetisations are retained by most of the intrusive rocks, unless they are highly altered. Both normal and reverse polarity remanences are preserved in the Complex. Primary magnetisation is carried by multidomain (titano)magnetite. In addition, the remanence carried by the intrusions is weakly to heavily overprinted. Primary remanence directions have been rotated and are steeper than the Miocene reference field directions of corresponding polarity, thus demonstrating tilting of intrusions in the Complex since emplacement. The sense of tilting is reasonably consistent, although the amount varies up to 50° or 60°. Prior to tilting, there appears to have been variable rotations about the vertical, mainly in a clockwise sense.

Magnetic modelling incorporating measured magnetic properties and known geometry of the shallow intrusions has led to interpretation of a deeper mass of the Complex. Modelled tilts of the shallow bodies agree with the sense of tilting from palaeomagnetic data, but are less extreme. This suggests that rotations at two different scales are being detected.

The tectonic rotations are evidently in response to thin-skinned tectonic processes which have accompanied the rapid uplift of the Complex. Palaeomagnetism is a powerful tool for detecting such rotations and could be particularly useful for elucidating the structural history of young deposits in tectonically active areas. The present study has detected hitherto unsuspected structural complications, which may have implications for locating further mineralised zones.

High-quality airborne magnetic and radiometric data provide the cornerstone for the geological understanding of the relatively unexplored eastern highlands of Victoria. The Geological Survey of Victoria has obtained over 180 000 line kilometres of new airborne data in eastern Victoria. This data collection is a major component of the Victorian Initiative for Minerals and Petroleum (VIMP).

Eastern Victoria has an excellent geological section across the eastern Lachlan Fold Belt. The Early Silurian Benambran event deformed Ordovician turbidites. Two cycles of rifting during the Silurian and Early Devonian resulted in transtensional rift-like grabens. Silicic volcanic units and marine sediments filled these rifts. The Late Silurian Bindian deformation resulted in major northwest and northeast fault trends which are apparent in the geophysical data. Granites with varied magnetic and radiometric responses have intruded the area, with several reaching the surface to produce calderas.

The area is rich in potential mineral targets. These include porphyry style copper-gold mineralisation, dyke-associated tin-gold mineralisation, volcanic-associated massive sulphide mineralisation, epithermal gold mineralisation, and carbonate-hosted Irish style lead-zinc mineralisation.

The new geophysical data allow improved mapping of the geological units of eastern Victoria, interpretation of the regional structural features, and identification of exploration targets. The regional geophysical datasets provide an excellent basis for mineral exploration in the under-explored eastern Victoria.

Recent work has shown that in some cases hydrocarbon reservoirs are overlain by an associated alteration plume. If it were possible to detect these plumes with an airborne geophysical technique, then the technique could be used on reconnaissance surveys to identify prospective areas. More detailed geophysical (eg, seismic) or geochemical surveys would be required to further develop these prospects.

Hydrocarbon alteration plumes have been detected using the following geophysical techniques: magnetic; electrical; induced polarisation; and radiometric methods. Of these methods only magnetic and radiometric methods can be acquired from an airborne platform. However, the electrical method responds to resistivity or conductivity and this quantity can also be mapped with an electromagnetic method which can be mounted on an aircraft. The ALTREX method therefore consists of an electromagnetic system, a magnetic system and, optionally, a radiometric system installed on an aircraft. The method has been tested (with electromagnetic and magnetic installations only) in three onshore basins in the southwestern United States of America. Only one of the seven fields shows a near-surface magnetic anomaly interpreted to be associated with the alteration zone. The other six fields, however, do have magnetic responses which can be associated with deeper structural features, which may control the hydrocarbon traps. The electromagnetic data, which respond to the conductivity in the top 300 m of the subsurface, show an anomalous response in most cases. This anomalous response generally correlates with the surface projection of the producing field (which are generally at 1 km to 2.5 km depth). Results suggest that a combination of airborne magnetic and electromagnetic data will provide a useful reconnaissance tool for regional scale exploration for alteration zones. Whether these zones are indicative of producing fields or not would require further work.

Keywords:

Tin mineralisation occurs at Renison Bell, near Zeehan in Tasmania, as a group of cassiterite-bearing massive pyrrhotite bodies in Cambrian sedimentary rocks. Mineralising fluids from the underlying Pine Hill Granite have replaced carbonate rocks with massive sulphides and cassiterite. The discrete blocks of massive pyrrhotite have high density and high magnetic susceptibility contrasts with the comparatively barren country rock. They are strongly remanently magnetised and are excellent geophysical targets.

A detailed geophysical model of the mine was created from a database comprising 1:2500 scale geological mapping and geological logs of approximately 4000 drill-cores. The model comprises 93 parallel cross-sections, each 1.5 km long, spaced 20 m apart. Each cross-section is comprised of up to 150 polygons which represent all known individual rock types and ore bodies. The magnetic response of the model was computed and the residual between the model and the observed response, measured by a low-level helicopter magnetic survey, revealed several anomalies unaccounted for by the model. Drill testing of these anomalies led to the discovery of several hitherto unknown massive sulphide occurrences.

Surface gravity data of the mine and surrounding district were enhanced by applying digital terrain corrections. The corrected Bouguer anomaly revealed a hitherto unknown positive anomaly associated with the ore zones of the Renison mine. The gravity anomaly of the model was also computed. Unfortunately, the limited resolution of the observed data prevented the delineation of ore targets.

The terrain-corrected Bouguer gravity data were also used to model the shape of the underlying non-magnetic Pine Hill Granite. A computer inversion method was implemented to build a 3-D block model of the granite. Results agreed with previous conventional 3-D modelling techniques but yielded improved resolution of the granite form.

The project involved development of new computer software to build and edit the large and complex 3-D geophysical model of the mine, and to compute the model’s magnetic and gravity responses. Software was also developed to build and compute the gravity response of a 3D block model of the underlying granite.

During the first two years of the New South Wales Government’s six-year ‘Discovery 2000’ Exploration Initiative budget, approximately $9 million was spent on the acquisition and processing of over 850 000 line kilometres of airborne magnetic and radiometric data, and 14 000 gravity stations. Both mineral- and petroleum-prospective areas were targeted, covering an area of over 200 000 square kilometres — equivalent to 26% of the State.

The minerals industry has been provided with new high resolution geophysical datasets which constitute a world-class product. Airborne surveys have been read at low-altitudes at close interline spacings. The airborne data have been released in combination with a greatly improved density of gravity stations over all areas and limited seismic reflection data over sedimentary basins. The resultant high resolution data have assisted, and will continue to assist, selection of new exploration project areas. Already there is a significant increase in the level of new exploration.

The Wirrda Well Prospect is located approximately 20 km south-southeast of Roxby Downs and 500 km north-northwest of Adelaide, South Australia. The Wirrda Well deposit consists of a massive granite breccia. Within the breccia, hematite and magnetite are pervasive, and copper, uranium and gold are associated. Evidence suggests this distinctive deposit may be an analogue of the Olympic Dam orebody.

A variety of petrophysical measurements have been made on drill core samples from the Wirrda Well deposit, including density, gamma activity, compressional velocity and thermal and electrical conductivity. Detailed magnetic studies were also undertaken, comprising magnetic susceptibility, natural remanent magnetisation, partial demagnetisation, anisotropy and saturation magnetisation measurements.

Results of these studies enabled comparisons to be made between the physical properties of the host rocks, the cover sequences and the surrounding country rocks. Generally, the host rocks are of greater density, have a higher magnetic susceptibility and gross gamma ray activity, and are better conductors of thermal and electrical energy than nearby unmineralised rocks. The main carrier of magnetism was found to be multi-domain, coarse-grained magnetite and remanence was a significant component of the total magnetisation.

The Wirrda Well potential field anomaly consists of near-coincident magnetic and gravity highs. Modelling of the ground magnetic data, constrained by geological logs and petrophysical measurements, could not account for the observed anomaly, suggesting the main causative body of this anomaly has not yet been intersected by drilling.

From modelling of the gravity anomaly, it was demonstrated that only half of the magnitude of the anomaly can be resolved, utilising bodies constrained by drillhole data. It has been concluded that the remainder of the anomaly may be explained by the presence of a central vertical pipe with three apophyses radiating to the northwest, southeast and southwest, that are largely untested by drilling.