b'Ted Tynes best of Exploration geophysicsFeatureinterpretation were introduced. Papers presented for surfacecosine transform. Numerical solutions were slow to converge surveys included the use of surface-integral methods inbecause of the oscillatory nature of the cosine. Lee (1982, 404) EM modelling, a survey of step and impulse systems, and apresented closed expressions for the Greens function which description of a laboratory modelling facility. avoided this problem and enabled calculations on a mini-At Adelaide in 1988 (Middleton, ed., 1988), topics discussedcomputer.included filtering of SIROTEM data, and an attempt to explain theThiel (1984, 644) investigated the effect of anisotropy on the negative TEM anomalies often found with coincident-loop surveys.surface impedance measured using VLF radio waves. A simple Further responses of models of anomalous bodies were presented,technique was proposed to determine the degree and direction and a three-component down-hole TEM system was described. of the anisotropy.At Melbourne in 1989 (Asten and Denham, eds, 1989), a methodAt the Adelaide Conference, 1988 (Middleton, ed., 1988), the was outlined for preparing conductivity-depth sections formeasurement of EM impedances to high frequencies (100 kHz) TEM, and use of the finite element method for modelling thewas related to delineation of inhomogeneities in the ground, response of complex structures was described. The use of firstsuch as variations of permeability.and second spatial derivatives together with decay times for interpretation of TEM data was presented, and the problems ofGeneralinversion of layered models near a contact were analysed. A joint Australian/US workshop, held at Macquarie University Cull (1989) described some tests of a SIROTEM system mountedin Dec. 1977, was summarised by Braham et al. (1978, 63). in an airship. A transmitter loop 25 m x 12 m was mountedTopics included problems associated with deep weathering, on the airship, and a receiving coil, wound on a plywoodnoise reduction, modelling, and description and comparison of former, could be lowered by a winch to offsets of 20 or 40 m.various EM and MT equipment.Measurements were taken at a height of 200 m. Good signalsGunn and Brook (1978, 276) compared the results of surveys were obtained over a conducting body near Dookie, Vic., whichby several methods over a sulphide ore body in WA. The had been mapped previously by ground SIROTEM. mineralisation was detected by MPP01, Crone, SP, MIP, and Frequency Domain Turam methods, but the location and definition of the body varied considerably between the methods. Gunn (1980, 273) A full description of the TURAIR EM prospecting methoddescribed further test comparisons, this time between electrical was given by Seigel Associates (1971, 588). A fixed groundIP and Pulse EM (PEM). Murali et al. (1980, 479) compared telluric energising source was used in conjunction with an airbornefield anomalies with time-domain IP effects over a mineralised receiver. This led to lower efficiency and limited mobilityzone in India.compared with other airborne EM methods where onlyThe problem of finding ore bodies under the conducting moderate depths were involved. However greater depthweathered layer, prevalent in much of Australia, was penetration could be obtained with TURAIR, and the system wasreviewed by Smith and Pridmore (1989). The overburden less affected by near-surface conductivity, and could be usedwas highly conducting, and formed a barrier for electrical with a helicopter in mountainous terrain. and EM methods and led to spurious IP effects. Maghemite Raiche (1973, 542) developed a general method for calculatingwas formed, which caused erratic magnetic anomalies, and the EM fields due to either electric or magnetic dipole sourcessuperparamagnetic phenomena in TEM surveys. Radioactive embedded in a horizontally layered earth. Both horizontal andelements might also be present. High-resolution low-level vertical dipoles were treated. airborne magnetic surveys could smooth out the near-surface magnetic effects, and allow geological mapping and in some Hall and Davis (1974, 290) described briefly the principle of acases direct detection of ore bodies. Airborne EM had not rotating EM dipole source, which radiates in all directions, thusbeen widely used in Australia, though the INPUT system enabling both radial and azimuthal profiles to be surveyed, andhad been modified by increasing the transmitter power facilitating determination of dip of a conductor. An instrumentand number of channels recorded, digital recording, and for model experiments was described by Hall et al. (1978, 289).shorter integration time. INPUT had been used for massive Results of tests at various azimuths for the effect of a graphiticsulphides, geological mapping, and kimberlite exploration. model in an electrolytic tank were presented. Several ground EM systems had been used, with various loop A large scale-modelling facility for electromagnetic experimentsconfigurations. Superparamagnetic effects simulated those of was constructed in CSIRO Division of Mineral Physics. Duffina deep conductor, i.e. higher response and slower decay rate and Drinkrow (1976, 179) described the problems in devisingat later times, but offsetting transmitter and receiver loops models with appropriate electrical properties, and Drinkrowby 13 m, or using an in-loop configuration, was commonly (1976, 176) described the design and operation of the system. enough to eliminate these effects. Negative TEM response could be modelled by a weakly polarisable body with suitable Vozoff and Jupp (1977, 673) developed a theory of error boundsparameters. IP had not played a major role in Australia since for model parameters derived from geophysical measurements.the introduction of SIROTEM; the method was hampered They applied this to the resolution obtainable from DC and MTby inductive coupling in many weathered areas, but a few measurements in three cases: a buried permafrost layer, an EMsuccesses had been achieved with careful field procedures. waveguide, and a sub-Moho layer. Both DC and MT data wereDownhole measurements commonly included magnetic found essential for the waveguide case. DC data may be helpful insusceptibility, conductivity, and IP; these gave information the permafrost case, but add nothing to MT for the sub-Moho layer. which assisted interpretation of surface-mapped anomalies. EM methods used a large transmitting loop near the surface EM scattering was generally modelled by an integral equationand a downhole receiver; this combination might detect of which the Greens function needed for a solution is aconducting bodies not intersected by the hole.45 PREVIEW FEBRUARY 2020'