Exploration Geophysics - Volume 25, Issue 4, 1994

The pseudo-lithology mapping technique developed by Dransfield et al. (1994) for use with coincident gravity gradient and magnetic measurements and demonstrated with synthetic data is applied here to measured field data. Application of the pseudo-lithology method utilised total magnetic intensity data (TMI) and gravimeter measurements, whereas Dransfield et al. (1994) required gravity gradient information which is not yet practically available. The ‘pseudo-magnetic projection’ of the gravity gradient is developed from Poisson’s relation and then correlated with the magnetic data, resulting in a ‘pseudo-lithology’ map of the ratio of apparent susceptibility to density contrast. One of the main advantages in using this correlation is that the information contained is independent of source geometry and depth.

The technique was applied to data from the Corsair locality in Western Australia (Price, 1993), proving very useful in the determination of the subsurface geology, and extending the interpretation of previous workers (Whitaker et al., 1987). We also provide an error estimate of the derived horizontal and vertical gravity gradients, exploiting the zero trace of the gradient tensor. This furnishes us with a noise estimate of the gravity data, and also an edge detector for continuous rock units. This edge detecting ability is also apparent in the pseudo-lithology maps, aiding the interpretation.

Modern civilizations are at risk from natural geophysical events and from industries whose accumulated wastes release toxic substances into the environment. These environmental hazards are increasingly man-made, increasingly voluntary and diffuse in their impact. Environmental geophysics addresses the quantification and monitoring of subsurface hazards, irrespective of their origin.

There are two major challenges for environmental geophysicists, firstly, to understand socio-economic and environmental factors which govern the demand for their services and, secondly, to use improved methods and develop practices which more effectively address environmental problems. Field studies on major environmental problems provide some perspective on these challenges.

Salinisation of agricultural lands is arguably Australia’s major environmental problem. A dryland salinity study in Victoria demonstrates the process by which electromagnetic responses of soils can be expressed in terms of crop productivities for appropriate plant species. This information is of direct use to farmers, land managers and agronomists.

Conventional geophysical methods have had limited success in delineating hydrocarbon LNAPL and DNAPL contamination, both major pollutants. In South Australia, the extent of a plume produced by a large diesel oil (LNAPL) spill was mapped with a new method of Radiowave electromagnetic profiling method (R-EM) which overcomes many of the limitations of conventional electromagnetic induction and ground probing radar methods.

It is concluded that environmental geophysics has a bright future provided geophysicists are willing meet these challenges, if not, other non-specialists will increasingly enter the field as has been the case in North America.

The aim of a cross-gallery seismic experiment in the Hillgrove Antimony-Gold Mine (New South Wales) was to find out whether variations in seismic velocities could be related to a known shear zone containing mineralised gold. A further objective was to test simple means of S-wave generation and recording. Using the impact of a 4.5 kg sledge hammer as seismic source, seismic signals containing frequencies above 500 Hz could be produced. P-waves were best recorded on horizontal-component geophones for horizontal impact. Most P- wave onset could be picked with a precision of 0.1 milliseconds. S-waves showed best up on vertical-component recordings of vertical impact shots. However, their onset cannot be easily picked with the same precision as those of P- waves. The resulting P-wave tomogram indicates significant velocity variations. That a straight correlation with the known geology is not possible indicates that the zone of mineralisation is too narrow to be resolved by our experiment.

Combined and interactive inversion of D.C. resistivity and time domain induced polarisation sounding data (i) appears to be an effective approach for quantitative interpretation of induced polarisation sounding data, (ii) shows better resolution of the subsurface for a certain class of earth models, (iii) provides more information about the subsurface, (iv) is one of the approaches for studying saline water and fresh water problems. Field observations, taken in a sedimentary terrain near Federicton Junction, New Brunswick, Canada, were analysed. Weighted and non weighted ridge regression estiamtors were used to solve the inverse problems. Two tier inversion algorithm is chosen where resistivities and thicknesses are modified in the first stage. Apparent chargeability sounding curves with negative values for certain elecrode spreads, in between, revealed the presence of negligibly polarisable layers. These zones, which also show low resistivities, are demarcated as saline water pockets.

Induced polarisation forward model curves are resistivity model dependent. Vertical resolution in an IP sounding curve is the best in Q type and worst in A type resistivity models for K type (m1 2> ma) layer chargeabilities.t