b'Ted Tynes best of Exploration geophysicsFeatureAina (1986, 4) proposed the use of reduction to equator andResults of some older surveys suggested that there was scope orthogonal reduction of magnetic anomaly fields in additionfor increased use of the method, and that many of the newer to the usual reduction to the pole. Reductions to pole andhigh-resolution techniques should be introduced.equator give rise to symmetrical anomalies over a symmetricalThe concept of airborne magnetic susceptibility mapping was body, whereas orthogonal reduction (i.e. reduction to anpropounded by Silva and Hohmann (1984, 601). The total field inclination of 45) leads to an antisymmetrical profile with zeroanomalies were reduced to pole, and an equivalent layer of crossing over the centre of a symmetrical body. poles is computed. This is converted to a susceptibility map The formulae for the gravity and magnetic fields of uniformassuming that there is no remanent magnetisation present. The triaxial ellipsoids of arbitrary orientation were given by Clarktechnique works best for magnetic bodies with vertical sides etal. (1986, 100). These models are useful for general modellingextending to infinite depth and flat tops, but fair results could of compact bodies because of the flexibility of shape, andbe obtained with departures from these conditions.also because self demagnetisation can be treated analytically.Arafin (1984, 20) used a relatively cheap Develco fluxgate Programs for a HP41CX calculator were presented. magnetometer comprising 3 orthogonal sensors which could Topics on potential fields discussed at the ASEG Conference,record continuously when used in a moving vehicle. Accurate Perth, 1987 (Middleton and Pridmore, eds, 1987), includedangles between the sensor axes were determined by rotating presentation and analysis of magnetic and gravity anomaliesthe magnetometer about three axes. The total field and three over the WA goldfields, magnetic image processing bycomponents could then be calculated.computers, the use of an automatic gain control (AGC)Hoschke (1985, 329) developed a downhole vector fluxgate technique to deal with the large variation in amplitudesmagnetometer, which was used in the Tennant Creek area. It prevalent in magnetic gradient displays, and application of theconsists of 3 fixed orthogonal sensors, and 3 inclinometers used Australian Geomagnetic Reference Field in the reduction ofto correct for probe rotation and drillhole inclination changes.magnetic field data.At the Adelaide Conference in 1988 (Middleton, ed., 1988),Rock Magnetisationthe effect of magnetic fluctuations along the southeastStudies of the magnetic properties of rocks and minerals are Australian coast on magnetic surveying was shown to beessential for reliable interpretation of field data into geological significant. Papers were presented on the benefits of studyingfeatures; these are represented in the following papers.magnetic rock properties to improve geological interpretation, improvements in image processing, advantages andRidley and Brown (1980, 561) described a transformer bridge limitations of one-dimensional upward continuation applieddeveloped in CSIRO for measurement of low-field susceptibility to aeromagnetic data, and the use of terrace functions inof rock specimens. A low operating frequency reduces errors inversion of potential-field data to convert smoothly varyingdue to specimen conductivity; however conductivity can be data to step functions. inferred from in-phase and quadrature components of the Mudge (1988) developed a method of analysing magneticsignal.data measured on a sloping surface, as in steep topographyClark (1983, 99) summarised knowledge of the magnetic or an inclined borehole. Apparent dip and declination wereproperties of rocks such as susceptibility, remanent calculated from the induced and remanent magnetisation andmagnetisation, and their relation to the domain state of the the orientation of the survey plane. magnetic grains, e.g. superparamagnetic, single domain, or At Melbourne, 1989 (Asten and Denham, eds, 1989), magneticmulti-domain. Components carried by different mineral grains and gravity terrain modelling as used in interpretation werecan often be separated by palaeomagnetic cleaning techniques. compared. Magnetic petrophysics can assist in interpretation of anomalies, dating of mineralisation, fabric studies, stratigraphy, recognition Magnetic Surveys of chemical processes, and quantitative mineral analysis.A vehicle-borne alkali-vapour magnetometer, capable of highGeomagnetismresolution, rapid traversing and digital recording, was describedIf the present rate of decrease of the Earths magnetic dipole by Stanley (1975, 623). A Hilbert transform was automaticallymoment continues, it would become zero about 3900 A.D. Facer applied to the recorded data to facilitate interpretation. (1971) discussed the nature of the geophysical world when By plotting the trends of magnetic anomalies from three largequadrupole and octupole moments will predominate.areas (two in the WA shield and one in eastern NSW) on rosetteCorrections to regional magnetic observations for diurnal diagrams, Emerson (1976, 206) showed that trend analysis couldvariations have always been hampered by the sparsity of be useful in studying tectonic provinces and their boundaries,magnetic observatories in Australia, and a lack of knowledge and evidence for past relative movements. of the area of applicability of recordings at a fixed site. Lilley Aeromagnetic trends were displayed as simplified second- (1982, 413) analysed the recordings made as part of projects derivative maps in the Broken Hill area (Stewart and Boyd,to examine natural EM induction in the Earth. The effect of 1983, 628). These maps facilitated resolution of closely spacedthe conducting sea water was present at all coastal sites, but anomalies and recognition of trends. Long lineations in NE anddisappeared about 100 km inland. Smooth patterns persisted NW directions could be used to subdivide the basement of theover substantial inland areas, but two zones were identified Willyama Complex into a mosaic of blocks. where the pattern was not uniform. A study of the Cobar area (Lilley, 1984, 415) showed changes of up to 25% in the Retard and Butt (1983, 556) claimed that use of aeromagneticsamplitude of the fluctuations over distances of about 30 km. in oil exploration had dropped considerably since the 1960s.The changes were attributed to a slate formation of high FEBRUARY 2020 PREVIEW 42'