b'Ted Tynes best of Exploration geophysicsFeatureconductivity which lies along the eastern edge of the CobarMerrick (1974, 462) presented a pole-multidipole array, in Trough. which only a remote current electrode is moved, while several Lilley and Barton, eds (1986, 414) edited an account of apotential electrodes remained stationary with a range of Workshop on Geomagnetism held in Canberra in May 1985.separations. This technique was tested successfully in several This issue contains several papers on ionospheric magneticshallow groundwater investigations in NSW.fluctuations, which may seem remote from practical explorationTyne (1977, 662) developed a set of nomograms for calculating geophysics. However, these fluctuations on the one hand formthe geometric factor for resistivity logging of inclined drill holes.the source of induced currents in the Earth, which are used toA method for interpreting IP soundings over horizontally study subsurface variations in conductivity by magnetotelluriclayered media was presented by Dixon and Doherty (1977, 162). and related methods; and on the other hand, they are theThey showed that use of linear filters simplified the calculation cause of magnetic disturbances for which corrections must beof chargeability functions.made in magnetic surveying. Hence it is important that the nature of these phenomena should be understood for effectiveDavis et al. (1980, 147) derived a filter function for use in applications in both the above cases. Several examples ofinterpreting resistivity soundings over layered structures. It can induction studies of crustal conductivity were reported, as wellbe used in digital convolution interpretation with any of the as magnetic surveys at sea, in the air, and from satellites. Thestandard collinear arrays.latter led naturally into a discussion of global reference fields, and the origin of the Earths main field as a result of dynamoEdwards (1983) calculated modelled curves for magnetic IP and action in the Earths fluid outer core. magnetometric resistivity for a dipping dyke under a conductive Barton (1988) defined the international adopted referenceoverburden. He gave a FORTRAN program for calculation of field (IGRF) for magnetic charts and secular variation. He alsofurther curves.explained the need for a more detailed reference field for theUsing digital filter methods, Pal (1984, 509) derived formulae for Australian area (AGRF), and presented the charts for 1985.0the apparent resistivity measured by Wenner and Schlumberger for the total field and declination components and theirsurface arrays over an anisotropic, inhomogeneous earth. Banerjee secular variation. A workshop on magnetic field variationsand Pal (1986, 29) investigated the contribution to measurements and problems in their applicability to exploration data wasby various resistivity arrays made by thin vertical sheets extending held in Canberra in December 1987 (Barton et al., 1988), andto infinite depth. Sheets at some locations do not make any the proposed Australia-wide Array of Geomagnetic Stationscontribution, and even negative contributions were found.(AWAGS) was introduced. Improvements in methods ofAt the Adelaide Conference, 1988 (Middleton, ed., 1988), presenting the data were also discussed. measurements of the IP effects of several minerals were reported to aid in inversion of survey data, and the advantages Electrical and disadvantages of various electrode arrays were examined. At Melbourne, 1989 (Asten and Denham, eds, 1989), a technique Resistivity and Induced Polarisation for removal of an EM coupling effect from dipole-dipole IP data Direct current or very low frequency alternating current methodswas presented, and it was suggested that the effect could be of measuring resistivity are among the oldest methods ofused to gain more information about an anomalous body.mineral prospecting. The more recent method of includingAn alternative to standard methods of resistivity mapping measurements of time variations in response caused bywas presented by Macnae and Irvine (1988). Using UTEM electrochemical phenomena, known as induced polarisation (IP),instrumentation, an ungrounded inductive loop source was gives valuable additional information in appropriate situations. combined with a grounded electric field receiver. The advantage A very thorough discussion of the theories of the factors causingwas that the primary electric field was independent of the IP phenomena was given by Lynam (1972, 425). Treatment ofresistivity structure. In the one survey, electric and magnetic univalent interfacial electrochemical reactions for both metallicfield data could be used to explore for both resistive and and normal IP effects led to the presence of secondary currentconductive bodies. The method was tested at Mt. Aubrey, NSW, generators at the metal-electrolyte interface, which produce theand successfully mapped two quartz veins.IP voltage decay. Merrick (1989) developed a method for direct interpretation The various array configurations used in resistivity and IPof resistivity soundings of a horizontally layered earth. The prospecting were reviewed by Whiteley (1973, 709), whotop layer was resolved by two-layer curve matching and concluded that the response of different systems in complexthen stripped off. Successive two-layer curve matching was geological conditions was not well understood, and thatapplied to the whole section. Merrick did not claim better there was considerable scope for further research into theirinterpretations than other methods, but the process could be properties and possibly for design of improved systems. Thedone on a calculator rather than a computer.characteristics of many array configurations were calculated by Pratt and Whiteley (1974, 528) using surface-integrals as a basisElectromagneticfor computer modelling. The response in two distinct geological situations were calculated. Transient, Time DomainWhiteley (1974, 705) described an offshore resistivity methodOne of the major spheres of activity in mineral exploration has for continuous profiling in shallow water. In contrast to previousbeen the development and application of transient EM (TEM) methods, this development used electrodes towed near the seamethods, largely in an attempt to penetrate the ubiquitous surface instead of dragging a cable along the bottom. near-surface high-conductivity layer.43 PREVIEW FEBRUARY 2020'