ASEG Extended Abstracts - ASEG2001 - 15th Geophysical Conference, 2001

The Cadia Ridgeway Au-Cu deposit is a deep (> 500 m) gold-rich porphyry copper system located in central NSW. A chargeability anomaly, recognised within a prospective NW-SE mineralised corridor motivated the drilling program which led to the discovery of the deposit.

In-situ and laboratory electrical tests were conducted to characterise the petrophysical properties of the deposit and host sequences. The ore is both chargeable and relatively conductive, although not sufficiently conductive for EM methods.

Significant scale variation of apparent resistivity was observed with in-situ measurements using electrode spacing of one metre or more up to two orders of magnitude lower than values determined from laboratory measurements.

Forward modelling of measured induced polarisation and apparent resistivity data was conducted using petrophysical properties determined primarily from in-situ testing. The chargeability anomaly identified over the Ridgeway ore body is due largely to disontinuous halos of pyrite alteration above the economic mineralisation, but a highly chargeable source component associated with the economic mineralisation at Ridgeway is also inferred.

The contrast in conductivity between ore and host rocks suggests that Ridgeway-style mineralisation may represent a target for magnetometric resistivity (MMR) surveying. Three-dimensional numerical modelling of the expected MMR response indicates that the Ridgeway deposit would be detectable by both surface and down hole MMR surveys.

The Tritton massive sulphide copper deposit was discovered using systematic moving loop surface EM surveys. The use of this technique was based on previous experience around the Girilambone copper mine. The deposit consists of an upper and lower lens, both of which are highly conducting. The lower lens was discovered through the use of downhole EM techniques.

The upper lens lies at a depth in excess of 160 metres below the surface and the lower lens is at a depth of greater than 400 metres. Both these lenses have EM time constants of about 10 milliseconds.

Apart from having a minor associated magnetic anomaly the Tritton deposit has not been seen from the surface by any other geological, geochemical or geophysical technique.

Approximately one kilometre north of Tritton is the uneconomic sulphide deposit, Budgerygar. This has similar geometry to the Tritton deposit but lies at the shallower depth of 60 metres. The Budgerygar deposit is readily detectable by a variety of geochemical and geophysical techniques. The Tritton and Budgerygar deposits are an interesting test site for development of new exploration technology, due to the similarity in their form but difference in depth and hence detectability.

We present a new method for calibrating a classification of a 3D seismic volume.

The classification process employs Kohonen self-organizing maps; the subsequent calibration is performed using one of more sets of borehole logs.

The method is described in some detail, and a case-study is presented using data from the North Sea.

The method shows promise as an alternative to current techniques for integrating seismic and log data, for seismic reservoir characterization

Analysis of susceptibility and density data from a borehole in the Bushveld igneous complex in South Africa was performed using statistical tools such as power spectra, wavelet analysis, and windowed histogram plots. This analysis has revealed significant patterns and cycles that correlate both with rock type and subtle layering within and between units. These results can now be used to test models of layering processes.

Image processing of high-resolution aeromagnetic data is an essential tool in mineral exploration and is finding increasing application in petroleum exploration. However interpolation of the flight line data onto a regular grid can lead to loss of high frequency information and introduction of gridding artifacts which can obscure subtle anomalies of interest. There is a high degree of anisotropy in sampling with closely spaced measurements along flight lines and much wider spacing between flight lines. For surveys conducted over shallow magnetic basement, there is significant high frequency information in the located total magnetic field data profiles which is lost or distorted in gridding. Gridding of line data is always a compromise between honouring closely spaced data along line and producing smooth and continuous interline interpolation between widely spaced flight lines. Cross-line aliasing is a minimum when flight lines are perpendicular to the geological strike but where multiple trends are present or where we have local trends, aliasing is directional and cross profile aliasing may be severe. A problem, common to all gridding methods, is that linear trends at an acute angle to lines tend to produce ‘bull's-eyes’ at line intersections. Linear and curvilinear anomalies appear as stepladder or string of beads effects. This paper examines methods of improving line to line correlation and concludes that gradient enhanced gridding using measured transverse gradient provides the best solution to interpolating between flight lines as this provides direct information on magnetic anomaly variation between flight lines. However, when measured gradients are not available it is still possible to use calculated gradients or other search techniques to improve line to line correlation and reduce stepladder and string of beads effects to an acceptable level.

Three-component downhole TEM probes can provide a unique service for mineral exploration surveys. Apart from assisting with target recognition the individual vectors can be used to resolve rotational ambiguities associated with filament inversion routines. However noise levels in the crosshole components are often considered to be extreme compared to data obtained with the more common axial probes.

Substantial improvements in data quality are unlikely in view of the physical limitations relating to probe construction. In particular core materials are relatively ineffective in the short crosshole sections and consequently there is no substantial gain in effective physical area. In addition background noise levels in the horizontal EM field may be 5 times larger than for the vertical field. Consequently digital filters and advanced numerical techniques are required to improve data quality in post survey processing.

Deconvolution involving exponential basis functions can provide substantial improvements in crosshole data assuming consistent decay curves scaled for variations in coupling. Apart from reductions in noise the verification of a uniform decay can eliminate the possibility of spurious axial anomalies associated with an extreme core response. More complex scaling factors are required for complex conductors with several iterations to establish the extent of coupling for each source.

The Copper Hill prospect is a well known porphyry copper/gold system. Investigation of geochemical assay data and gradient array IP data suggest that grid northwest and grid northeast structural directions have considerable control over mineralisation. Previous drilling has been oblique to these directions.

The sulphide rich mineralisation zones have moderate to strong IP responses. The intention of Golden Cross Resources Ltd, as tenement holders, was to verify the main trends and locations of possible extensions to mineralisation. A 3D inversion of IP data is a suitable approach to the problem of mapping the sulphide horizons in detail.

The primary aim of the survey was to gather sufficient IP/resistivity data over an area 1.2km by 1km to generate a reasonably detailed 3D-inversion model for Copper Hill.

Based on the knowledge that the data was to be processed in a 3D-inversion program, a modification of the pole-dipole IP survey geometry was used. The new array design resulted in fast collection of a large quantity of data. The high data redundancy inherent in the design allowed for editing prior to inversion.

3D inversion results highlighted the three dimensionality of sulphides zones at Copper Hill. Zones of high IP response are concentrated along northwest and north trending structures forming an annular zone terminating at depth.

A cost effective broad scale and detailed IP survey was successfully accomplished.

A review of the parameters used in processing aerial gamma-ray survey data from a number of recent surveys showed that many surveys were processed with incorrect values. Many of the problems arise from unrecognized variations in concentrations of airborne radon during the collection of the calibration data. These problems may be overcome by:

1. Testing values against expected values to flag potential problems,
2. Deriving the height attenuation coefficient for U from the average values for K and Th if the value does not fall between those for K and Th, and,
3. Calculating the sensitivity coefficients at various heights and plotting the K/U and K/Th ratios to reveal any radon effects.

If necessary, a U sensitivity value can be obtained by appropriately scaling the Th value.

Forward modelling of potential field data, combined with new geological mapping and deep seismic reflection transects acquired by the Australian Geodynamics Cooperative Research Centre (AGCRC), has led to testing of robust crustal architectural models for the eastern Lachlan Orogen of New South Wales. This integrated analysis has led to new subsurface inferences unlikely to be deduced solely from any of the individual datasets used.

The overall Palaeozoic history of the eastern Lachlan Orogen implied by the integrated analysis of geophysical and geological datasets, involves:

• formation of an Ordovician oceanic island arc ;
• early Silurian thin-skinned east-directed overthrusting of a continental backarc basin over the arc;
• generation of Silurian granites due to crustal thickening;
• early Silurian to early Devonian rifting, resulting in the dismemberment of the deformed arc-basin complex, and formation of a series of arc fragments with intervening thin basins;
• tight, asymmetric folding of these basins during the Early to Mid Devonian, with some granite magmatism;
• extensive Carboniferous thin-skinned thrust stacking involving both east-, and west-directed transport, and slicing of the upper structural level of earlier antiformal stacks, together with granite roof zones; and
• final granitic arc magmatism during amalgamation with the New England Orogen.

In December 1999 the TL/1 JV partners acquired a 2D walkaway VSP survey in the Monty-2 well in the Barrow Sub-basin, Western Australia. The survey was designed to analyse AVO effects and possible later calibration of surface seismic data. The survey was recorded using CGG's 12 level array tool, the SST-500, and comprised a total of 4 walkaway lines, with 2 azimuths at each of two array positions. Each of the 4 walkaway lines had a length of 6000 meters. Walkaway VSP data was acquired by positioning the array tool just above the two target reflectors, the North Rankin and the Mungaroo "B" sandstone formations.

The VSP data was processed using an ‘isotropic’ sequence, which allowed for a perfect preservation of the amplitude and phase relationships between the three receiver components. AVO estimates were computed from the reflected P-wavefield after shaping deconvolution, derived from the down going P-wavefield. P-wave polarisation information was used to compute reflection angles at the target horizon and the local reflector dip. Finally, synthetic P-wave reflectivity curves were computed for the North Rankin and the Mungaroo "B" sandstone formation and compared with the borehole seismic measurements.

The AVO/AVA response at the top North Rankin reflection was found to be strongly affected by a thin overlaying carbonate layer. The best match between measured AVO curves and isotropic modeling therefore was found for a shale/carbonate model rather than a carbonate/sandstone model. In contrast, the AVO response of the Mungaroo "B" reflection was found to be in very good agreement with isotropic modeling results.

BHP commenced exploration surveying with the world's first fully operational, airborne gravity gradiometer in October 1999. This gradiometer (called Einstein), together with a later one called Newton, was developed in conjunction with Lockheed Martin by BHP's FALCON project. Falcon data are acquired by Sander Geophysics Ltd., flying a Cessna Grand Caravan to survey specifications typical of aeromagnetic surveys.

The first FALCON survey was flown over a portion of the Bathurst mining camp in New Brunswick, Canada in order to compare system performance with existing extensive and detailed ground-gravity data.

The ground-gravity data, supplied courtesy of Noranda Minerals Exploration Ltd., were upward continued to the flying height and vertically differentiated to provide vertical gravity gradient data suitable for comparison with the airborne data.

The two data sets compare very well and the results demonstrate that FALCON airborne gravity gradiometer is capable of detecting sources with a vertical gravity gradient signal of greater than 10 Eo and a full-width at half-maximum of 500 m.

Environmental and cultural restrictions are increasingly impacting exploration efforts for mineral and energy resources. It is almost impossible to gather seismic data without being constrained by these factors. 3D land seismic data acquisition requires intense source and receiver sampling particularly for high resolution work where shot and receiver line intervals can be as little as 45 metres. Consequently, the acquisition method chosen is extremely important, as it will govern the degree of environmental impact imposed by the survey.

In the case of the Onley 3D survey, traditional sources such as dynamite and vibrator were deemed to be unacceptable, on environmental grounds. Mini-SOSIE, which utilises a light, portable surface compactor (Rammer) as a source was an acceptable alternative. After comparing the results from the ‘Rammer’ source with dynamite and vibrator sources, the Mini-SOSIE system proved to have the penetration and resolution needed to acquire useful 3D data. However, as could be expected, the signal bandwidth of the Mini-SOSIE (and vibrator) data was not as high as that of the dynamite data.

After extensive software and hardware development aimed primarily at increasing recording channels and receiver line numbers, a pilot 3D seismic survey was recorded. The aim of this survey was to delineate small faults and other geological features, to assist in mine planning. The Mini-SOSIE 3D data were of high quality. From the interpreted data volume it was possible to resolve a seam split as well as a likely small fault -critical information for mine planning.

The environmental impact imposed by the survey was minimal, with no negative feedback from relevant landholders.

Using synthetic seismograms, we study the influence of the widely observed strong scattered noises in complex near-surface areas. Our studies show that an intense scattering occurs in near surface regions, especially in the near-source and near-receiver regions. The scattering is dominated by rugged topography. These strong near-surface scatterings can seriously decrease the energy of valid signals and significantly increase noise levels that mask the interesting reflections. The study of simulating wave scattering by a low-velocity topographic structure in the western areas of China shows that the topographic effect is the most important factor to degrade the quality of seismic data in these areas.

The paper presents a f-k filtering method to reduce coherent seismic noise. The method uses an adaptive strategy that the f-k filtering automatically adapts to lateral variations in apparent dip and amplitude of coherent noise events. We first use orthogonal polynomials to fit apparent dip and amplitude of coherent noise events. We then incorporate the instantaneous variations in apparent dip and amplitude into narrow reject-band (NRB) f-k filters which are applied in a spatially and temporally varying way across seismic traces. The method is only applied to the area where coherent noises are located and will not affect the signal that is located outside the area in the same seismic record but has the same regularity as the noise. That is, in a given window, the method removes coherent noises using the differences between noise and signal in three aspects: apparent velocity, frequency, and amplitude. The data outside the window will keep intact. The method can remove any coherent noise in prestack and poststack seismic data, which will favor following seismic processing and modification of seismic sections. The novel method has been applied to field seismic data and shows good attenuation characteristics with minimal distortion of signal.

We have developed a simple approach to determine the depth and radius of subsurface cavities from the microgravity data. Horizontal location of cavity center is picked up by an algorithm as the projection of the minimum of gravity anomaly. Depth to the cavity center is determined using back propagation neural network. The cavity radius can be then calculated using the determined parameters if the density contrast between the host rock and the cavity filling materials is known or assumed according to the geological background. The present method is tested by several synthetic data sets and showed high ability to determine the cavity parameters in presence of natural noise. Applying the method to field data from Medford cavity site, Florida, the estimated cavity parameters are coincident the boring results. The method is proved to be fast and robust and can be used in field situation.

Geophysical methods have played an important role in the discovery and definition of porphyry copper deposits in the Indonesian Archipelago. Geophysical methods of particular value are aeromagnetics, induced polarization, and to a lesser degree airborne radiometrics. Some deposits that have recently benefited from the application of geophysical methods, within the Indonesian Archipelago, include Bukit Hijau, Batu Hijau, and Beutong.

One of the most important issues in searching for metallic ores is to discriminate between different types and textures of mineralisations. One of the methods capable of deriving such information is the spectral induced polarisation (SIP). With advances in the time domain IP receivers it is possible to get SIP data with no more additional costs. Therefore using Cole-Cole impedance model for the subsurface it is possible to calculate the apparent Cole-Cole parameters by a curve matching technique in a least squares sense. In the current study, SIP measurements have been obtained using six chargeability slices of the transient decay curve after a 2 seconds square wave current on time at the Irankuh lead and zinc ore deposit complex. It is shown that the SIP measurements will give additional valuable textural and structural information which can be of great help in determining and delineating ore blocks for a systematic exploration program. It was found that the increase in time constant is associated with the ore zones with continuous and greater grain size of IP sources. The average frequency exponent for the vein type ores was found to be about three times larger than that of the disseminated ore zones. Also from the true chrageabilities obtained from SIP analysis, the location of the vein type ores was delineated with a higher resolution.

Finally due to its cost efficiency and faster field operation it is suggested to collect SIP data for exploration targets where the textural information can play a very important role in dividing ore to the blocks with similar characteristics.

A large number of geophysical inversion methods have been published, but there are no published efficient technique applied to either of MMR or TFMMR data. One of the important aspect of inverting TFMMR data is its dependence on all three components of the anomalous magnetic field. In the case of inverting TFMMR data using gradient methods, we propose a preprocessing reduction of data to the pole to overcome the problems of the effect of the geomagnetic field direction and dependency of the TFMMR data on all three components. By doing this, we invert the data only using the sensitivity of the vertical anomalous magnetic field. After formulating the expressions for computing the Frechet derivatives for the anomalous vertical component on a homogeneous half space model, the Marquart-Levenberg method (damped least squares method) incorporating the quasi-Newton approximation for updating the Jacobian matrix in successive iterations is employed. As the most time consuming task in inversion process is to calculate the Frechet derivatives, therefore employing such scheme will considerably reduce the computing time and improve the stability of the inversion. An analytical solution for calculating the Frechet derivatives in wavenumber domain for all mesh nodes component for a homogeneous half space model is developed first. Next for recovering 2-D models, every 4 by 3 element in the x and z directions respectively constitute a resistivity block. This block parametrisation allows for recovering the sharp resistivity contrasts which occur in mineral exploration prospecting or shallow surface experiments. Assuming a linear variation of transformed electric potential and its gradient over the boundary of the elements, we may derive the discretised form of derivative equations. The efficiency and accuracy of the proposed scheme is demonstrated using a set of synthetic data from a double vertical dike model.

In this study we investigate variations of P-wave reflection coefficients with offset (AVO) and with azimuth (AVA) for a transversely isotropic medium with a horizontal axis of symmetry (HTI) in contact with water, and also with another HTI medium. Aligned, equidistant, vertical fractures, inbedded in an isotropic background, were used to represent a HTI medium. To grasp the effect of fractures their contribution was computed separately and then added to the isotropic background. The total response of a medium is then obtained by adding the two.

An approximate equation is used to compute partitioning of seismic energy at layer interfaces. The solution compares well to the results obtained in physical modelling experiments. To analyse AVO and AVA effects we used 3-D multi-azimuth transmission and reflection surveys. The results show that AVA effects are significant even at moderate angles whilst they are dominant for large incident angles. Routine 2-D AVO analysis, as commonly performed, is clearly unsuitable for HTI media.