b'Ted Tynes best of Exploration geophysicsFeatureconference, Melbourne, 1989 (Asten and Denham, eds, 1989),who showed that vertical and horizontal gradients were related further developments in cross-hole seismics were introduced,through the Hilbert transform. Plotting vertical versus horizontal including the use of a swept-frequency source; also angradients for a contact (fault) anomaly leads to ellipses or circles application of the 30 seismic method, techniques using crookedfrom which the parameters of the contact could be derived.lines in mountainous areas, and trials of a high-frequencyA symposium on 20 gravity interpretation was held in Sydney vibrator, were described. The use of a downhole radar probein 1977 (Emerson and Falvey, eds, 1977). The usefulness of 20 was reported as useful in locating fractures in crystalline rockmodels in the real world, their applicability and limitations, and at ranges of 50 to 150 m, and more generally, radar was used atsome estimates of the errors incurred by departures from the the surface and in underground workings for horizon control,ideal conditions of the models, were discussed at length. Crustal locating old workings, and mapping geological structure in coalstructures, continental margins, and sedimentary basins were measures. among the problems considered.The use of radio waves at frequencies of 10 to 30 MHz for mapping orebodies from underground mine tunnels wasIn 1978, a symposium on magnetic interpretation was held, described by Nickel and Cerny (1989). In the example quoted,also in Sydney (Emerson, ed., 1979a). In some respects, this radio waves were transmitted from one tunnel to another atwas complementary to the gravity symposium. Emphasis was right angles to it; aerials were placed in boreholes from theplaced on the importance of realistic geological models rather tunnels, with at least 3 locations for both transmitter andthan computer-generated depths to and shapes of sources, receiver. A body with anomalous electrical properties appearsthough of course these are an essential step in the interpretation as a radio-wave shadow. Rock thicknesses of the order of 100 mprocess. In order to arrive at models helpful to the explorationist, were penetrated, and an anomalous zone was found, whichmuch more information was needed on the susceptibility and proved to be the extension of a vein. remanence of minerals and rocks, their relation to petrophysical and petrological properties, and the modes of occurrence of Turner (1989) showed how stacking procedures could be used tomagnetic minerals in ores and host rocks.improve the signal in ground-probing radar surveys. The standard procedure is to move transmitter and receiver along a profileHorizontal and vertical magnetic gradients of the total field at constant separation. A localized object gives a hyperbolicanomaly were also used as the basis for interpreting magnetic series of traces on the radargram. These can be concentratedanomalies by Atchuta Rao and Ram Babu (1980, 26; 1981, to a few traces nearly above the object by procedures similar to553). In the first paper, they combined the two gradients into seismic-reflection diffraction-stack migration. The signal can bea complex vector, and studied the derivation of the model further improved by using the coherence of signals around theparameters of a horizontal cylinder from the anomaly profile. In hyperbola, or by using tau-p migration. the second paper, two functions derived from these gradients are used to interpret the anomaly due to a buried sloping step.Potential Fields Mohan et al. (1980, 472) used Fourier transform methods in a study of the gravity effect of a 2D inverted triangular prism. Gravity and Magnetic Interpretation Atchuta Rao et al. (1982, 554) derived the Fourier transform of the magnetic anomaly of a 2D prismatic body. Properties of Papers on this topic include modelling the effects of geometricthe energy density and phase spectra were used to derive the shapes of anomalous bodies, signal-to-noise ratio improvement,parameters of the model body. Mohan et al. (1982, 471) used and analysis of field data covering large areas. the Fourier transform of the square of the magnetic anomaly of a buried sphere to interpret the parameters of the body.The gravity field of a circular disc with vertical axis is not expressible in terms of elementary functions; it involvesThe Olympic Dam copper-uranium deposit in the Stuart Shelf elliptical integrals. Lee (1971, 400) showed that the formulaof SA, one of the major discoveries in recent years, was found may be transformed into a simpler integral involving Besselby drilling of coincident gravity and magnetic anomalies. It is functions; by applying an inverse Hankel transform, ancovered by some 350 m of Adelaidean sediments. Anderson expression was found from which the parameters of the disc(1980, 11) discussed the basement features corresponding could be readily obtained. to these anomalies, and shows that there are many other Moore (1972, 474) gave a comprehensive review of thevariations in density and magnetic susceptibility giving rise to methods proposed by various authors for direct and indirectanomalies in the area, so that interpretation of similar anomalies interpretation of gravity and magnetic anomalies. Thiscannot be conclusive. Rutter and Esdale (1985, 575) also gave was followed shortly by Crain (1972, 122), in a paper whichan account of this discovery, including the geological thinking complemented rather than duplicated Moores discussion inwhich led to analysis of the existing geophysical data and to that it described computer methods of reduction, processing,further surveys.applying corrections, and display methods such as automaticA very comprehensive set of formulae for the magnetic effects contouring and stacked profiles. of bodies of various shapes and directions of the earths field Stanley (1977, 624) proposed a method of interpreting potentialand of magnetisation were presented in a special issue by field anomalies due to dykes or contacts. It was based on theEmerson et al. (1985, 207). Programs were given for evaluating similarity of the formulae for various derivatives of gravitythese formulae on a HP 41C hand-held computer.and magnetic effects of these bodies, and he claimed thatA method for extracting weak magnetic signals from a noisy only relatively short lengths of traverse need be measured.background was devised by Dass et al. (1986, 545). The filter Horizontal gravity gradients, based on the difference betweenused was derived from the horizontal gradient of the anomaly pairs of stations, were measured, rather than the total gravityfield and its Hilbert transform. This was successfully applied to a field. These ideas were developed further by Stanley (1978, 622),survey in the Cuddapah Basin in India.41 PREVIEW FEBRUARY 2020'