It has long been known that induced polarization (IP) transient voltages decay with time at a rate that is dependent strongly on the mean grain size of the metallic conducting particles in the IP source. The Cole‐Cole model provides a three‐parameter representation (M, τ and C) for IP responses. The time‐constant (τ), in particular, has been found to be very useful in resolving IP sources with differing mean particle size. The characterization of IP responses in terms of these parameters has been termed “spectral IP.” These parameters may be determined either through the analysis of the response of the earth to sequential transmission of ac currents of different frequencies (i.e., frequency‐domain IP), or through the analysis of the transient decays resulting from the transmission of interrupted square‐wave currents (i.e., time‐domain IP). The latter approach offers the major convenience of being applicable to data obtained in the course of routine production surveys, with no increase in survey time. In practice, spectral IP parameters are determined most readily from time‐domain transients through the computer matching of the observed data to the best fit in a family of precalculated Cole‐Cole curves. This may be done, off‐line, using a PC, or, in a recent receiver, essentially on‐line, using software imbedded in the receiver. Field case histories from Canada, Finland, and Australia are given to illustrate the application of spectral IP to the resolution of IP sources in the time domain. It is recommended that this approach to the processing and presentation of time‐domain IP data should be applied routinely, as a very cost‐effective enhancement to the exploration value of such data. An additional benefit from this presentation is that it will facilitate a sharing of experimental results with workers in the frequency domain who may also use the spectral IP approach.

Electromagnetic (EM) techniques are extremely important as a direct detection geophysical tool utilized in the base metal industry. They were developed in countries such as Canada, whose thin conductive weathering overburden did not hamper the penetration of EM signals and enabled exploration to depths on the order of 300 m. As a result, EM techniques were used widely in North America and Scandinavia for many years before they became common in countries with a thick conductive overburden, such as Australia. The 1980s and 1990s have seen the use of EM methods move from anomaly finding to mapping, as well as the development of better, faster and more accurate computer modelling algorithms. A review of EM papers, for the years 1998 to 2002, showed that most dealt with EM techniques as mapping tools. Airborne, ground and marine EM techniques are still being developed, as are data processing and interpretation software. The advent of robust 2-D and 3-D computer modelling and inversion algorithms has led to the acceptance of EM methods as a mapping tool for many environmental and petroleum industry applications, a trend which is expected to increase.

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An airborne survey was undertaken on the Mount Isa inlier in 1990–1992. During this survey, both airborne magnetic and gamma‐ray spectrometric data were recorded over 639 170 line-km. Because of perceived value of the radiometric data, stringent calibration procedures, including the creation of a test range, were adopted. In addition to the data from the newly‐flown areas, 76 760 line‐km of existing data were acquired from other companies, and were reprocessed and merged with the Mount Isa survey. The total area covered by the Mount Isa airborne survey was 151 300 km2. Over the last five years, several studies have been undertaken that seek to exploit the Mount Isa region gamma‐ray database and maximise the use of radiometrics for mineral exploration. This paper highlights the results of these studies by focussing on radiometric signatures of major mines in the Mount Isa Inlier, radioelement contour maps, geomagnetic/radiometric interpretation maps, lithological mapping, regolith mapping, geochemical sampling, and spatial modeling using geographical information systems (GIS). Due to the recent introduction of GIS technology and better techniques for handling high quality digital data, there has been a revived interest in making more use of image data sets. The integration of raster and vector data sets for both spectral and spatial modeling has maximized the potential of this approach.