Exploration Geophysics - Volume 32, Issue 2, 2001

Interbed multiples are frequently a problem at target levels in seismic data. These multiples are often generated in high velocity layers, and consequently have little differential moveout compared to the primaries with which they interfere. In addition, statistical methods such as predictive deconvolution frequently fail to remove these multiples (for example as a result of geological complexity) or else they attenuate primary energy beyond a level that is considered acceptable. In such cases, the only remaining approaches to attenuating the multiples are based on modeling and subtraction.

This paper describes a modelling and subtraction based method that uses primary reflections in the data themselves to predict interbed multiples via a technique analogous to that used to predict surface multiples. The paper concentrates on the practical aspects of the method and includes synthetic and field examples. It is shown that reasonable predictions of interbed multiples can be obtained provided great care is taken at every stage of the data preparation. It is envisaged that, although presently in their infancy and computationally very expensive, methods similar to this will find their place in future processing flows when alternative algorithms are not successful.

Gravity data obtained at Hiles Lagoon, near Terowie, indicates the presence of a subtle gravity low. The 5 μm s^-2gravity low over the lagoon possesses roughly circular dimensions with a diameter of approximately 200 m and is the likely response of a concealed kimberlite pipe. These results are of particular interest since previous aeromagnetic and aeroradiometric geophysical surveys have had little success in detecting a kimberlite pipe at Hiles Lagoon. Density determinations typical of weathered Australian kimberlites and shales were employed to create a structural model of a kimberlite pipe and the surrounding country rock. Resulting gravity raster images from the kimberlitic model allow us to confidently interpret the field response as a kimberlite pipe and justify the application of the gravimetric technique as a tool for detailed exploration.

In drape-flown aeromagnetic surveys, operational factors such as weather and rugged ground topography may result in certain lines flown at average heights significantly different from the nominal terrain clearance. Such changes in the distance from sensor to magnetic source lead to variable attenuation of the magnetic field and result in spurious anomalies oriented along flight lines. A recent survey flown in Namibia was subjected to high wind conditions that led to some anomalously high flight lines and severe corrugation effects in the gridded data parallel to the line direction. Computational draping of the measured field to correct for the line-height variation was carried out on both gridded and line data using a Taylor series approach. The linebased approach produced the best results. Gradients calculated from the gridded data (perpendicular to the traverse lines) were overwhelmed by errors caused by the large line-to-line height variation.

The Operational Airborne Research Spectrometer (OARS) is a novel hyperspectral profiling reflectance spectrometer developed by the Commonwealth Scientific and Industrial Research Organisation (CSIRO) and Fugro Airborne Surveys (FAS) to examine alternative means to collect and deliver exploration data, based on the principles of reflectance spectroscopy. OARS is a component of CSIRO's "Airborne Mineralogy" concept.

OARS can be integrated with other airborne geophysical instruments that principally collect magnetic, gamma-ray spectrometry and digital elevation data, and can be flown simultaneously on exactly the same flight lines. OARS will measure ground reflectance spectra from contiguous 8-m pixels from an altitude of 80 m.

After data processing, interpolated images of relative mineral abundance can be reconstructed using differential GPS (DGPS) positioning data to produce a variety of mineralogical maps.

The OARS system consists of a downward and an upwardlooking spectrometer. Each spectrometer covers 190 spectral channels in the visible to short-wave infrared range, with an average spectral resolution of 12 nm. This wavelength range allows OARS to map most species of phyllosilicates, clays, sulphates, carbonates and iron oxides, that occur in alteration zones, regolith and host rocks. Green and dry vegetation can also be identified. A CCD camera (Charge Coupled Device – Silicon Array) allows monitoring of the flight path while flight attitude sensors and DGPS are used for geo-location.

The instrument was first tested in late 1998. Subsequent test surveys in early 1999, over a more challenging geological area in NW Australia, verified its ability to deliver new mineralogical data and its capability as part of an integrated system incorporating magnetic and gamma-ray spectrometry surveying. The resulting case study shows that this line profiling spectrometer can deliver useful interpolated mineral maps with potential synergistic benefits from co-registered magnetic and gamma ray images.

Field trials of a Soviet inspired seismoelectric method at an underground zinc mine in British Columbia, Canada, have demonstrated its effectiveness and confirmed some of the claims by the inventors of the method. The deposit investigated is a massive sulphide orebody principally made up of sphalerite, grading about 15% Zn in the area investigated. This deposit is a difficult target for standard geophysical techniques as it is virtually indistinguishable from the andesite/rhyolite host rock. However, we were able to demonstrate that the massive sulphide ore produces high frequency electromagnetic emissions when a strong seismic wave passes through the orebody.

An explosive seismic source (less than 1 kg) and wide bandwidth, 1-5000 kHz, EM sensors connected to a computer data acquisition system were used to produce and collect the seismoelectric signals. Our experiments show that there was a substantial increase in high frequency EM activity when seismic waves passed through the orebody. Thus, EM signal arrival time and shot point location could be used to locate the source of the EM signals. To demonstrate this capability shotpoints were placed around a known portion of the orebody in order to image the extent of the ore zones. A two dimensional image of the zones creating the EM signals was constructed from data gathered at 14 shot positions. The location and extent of a large anomaly in the image corresponds well with the drill-inferred position of the orebody. Another smaller anomaly appears to be coincident with the known location of a pillar of ore left behind by previous mining.

The impulse response moments of a conductive sphere in free space excited by a uniform magnetic field can be used to approximate the moments of a sphere in a dipolar field. The numerical computations are straightforward and the approximation is especially good for higher-order moments. The greatest discrepancy is seen on the zeroth-order moment when the radius of the sphere is large. It is possible to improve the accuracy for the zeroth-order moment by modelling the large-radius sphere (in a dipole field) as the combined response of multiple small-radius spheres (each in a locally uniform field). The small spheres are closely packed inside the larger sphere. The discrepancy can be reduced to less than 15 % in this manner.

The sphere in a uniform field can also be used to approximate the response of a body that has its currents constrained to flow in a plane with a specific orientation. This means that plate-like bodies or anisotropic spheres can also be modelled.

The third-order moment has been calculated from data acquired during a MEGATEM airborne electromagnetic survey of the Reid-Mahaffy test site. There is an anomalous response in the third-order moment that can be modelled by a sphere at 170 m depth with a conductivity of 15 S/m and a radius of 40 m. The currents flowing in the sphere are constrained to flow in a vertical plane. This model is consistent with the geology of the area and a hole drilled to test the anomalous zone.

Small but significant supplies of groundwater are found in weathered crystalline rocks. In areas where weathering is actively occurring as a result of tectonic movement, the weathering zones show major lateral variation. Electrical techniques can be used to detect zones of deep weathering because of the significant resistivity contrast between the different weathering zones. Bulk resistivity decreases as weathering intensity increases. Groundwater reserves are located in the intermediate weathering grades between fresh rock (high resistivity) and clay dominated (low resistivity) weathered material. The determination of accurate thicknesses and resistivities for intermediate zones is complicated by the problems of equivalence and suppression.

Small but significant supplies of groundwater are found in weathered crystalline rocks. In areas where weathering is actively occurring as a result of tectonic movement, the weathering zones show major lateral variation. Electrical techniques can be used to detect zones of deep weathering because of the significant resistivity contrast between the different weathering zones. Bulk resistivity decreases as weathering intensity increases. Groundwater reserves are located in the intermediate weathering grades between fresh rock (high resistivity) and clay dominated (low resistivity) weathered material. The determination of accurate thicknesses and resistivities for intermediate zones is complicated by the problems of equivalence and suppression.

The use of VES at anomalies located by CST is shown to be unsatisfactory. The most appropriate combination of geophysical techniques is electromagnetic profiling to produce a 2-dimensional map of apparent resistivity and electrical imaging to determine the depth of weathering at the regions of low apparent resistivity detected by the EM measurements. Recent advances in electrical image equipment and interpretation methods make the acquisition and interpretation of these data as rapid as that for VES for an equivalent depth of investigation.