b'Ted Tynes best of Exploration geophysicsFeatureA preliminary version of a CSIRO-built transient EM systemPrevious calculations on TEM response over a layered earth (later to become SIROTEM) was described by Buselli (1974,assumed a step-function cut-off. Raiche (1984, 538) considered 77). Improvements over the Russian TEM equipment MPP01the effect of a ramp-function cut-off. It was shown that the included the use of multi-channel signal averaging andramp-derived voltages are higher, particularly at early decay recording signals to longer decay times, and use of a secondtimes.receiver loop to aid in rejection of artificial or natural noise.Extension of TEM surveys to depths of 12 km was tested in Comparison tests with MPP01 were made in conjunction withthe Sydney Basin (Strack, 1984, 629) using a newly developed BMR near Cloncurry. The signal-to-noise ratio was improved by asystem using a fixed grounded wire transmitter and a movable factor of 44. Further comparisons were conducted at Woodlawn,loop receiver. 300 transients were recorded at 8 stations in two NSW, in which noise sources and the effect of different loopdays, and the geoelectric models correlated well with basin sizes were investigated (Buselli, 1977, 80). topography.The first completed SIROTEM model was described by BuselliIrvine and Staltari (1984, 344) found a distinctive anomaly at and ONeill (1977, 78). The instrument was portable, an orderMt Windsor (NE Queensland), which appeared on the basis more accurate than other models, could record signals toof SIROTEM and Geonics EM 37 measurements to be due delays of 150 ms, could use a variety of loop geometries,to a steeply dipping bedrock conductor. However further and had a high tolerance to artificial noise. A built-inmeasurements with different loop arrangements showed that it microprocessor controlled the operations, and could convertwas caused by the edge of a conducting surficial layer.the measurements, e.g. to apparent resistivities. Examples of two field tests were presented. McCracken et al. (1984, 438) showed that noise recorded by The variation with height above ground of the response of aEM prospecting systems comprised natural geomagnetic and TEM loop system was investigated by Raiche (1978, 541). HeEM variations, and mechanical movement of the receptors. concluded that measurements from a helicopter should beSeasonal, diurnal, and regional variations could give an overall feasible. variation of about 104.Lewis and Lee (1978, 410) showed that the transient electricWhichello (1986, 700) developed an iterative computer program field excited by a TEM loop above a half space is concentratedfor inversion of SIROTEM data in a horizontally layered model, in a toroidal zone which moves slightly inwards at first, andand tested this on two coal deposits in South Australia. The then moves outwards along a 30 cone, eventually reachingresults agreed satisfactorily with the known geology.a maximum, at which time the rate of change of magneticA workshop on downhole EM (DHEM) methods was held in induction is zero. Melbourne in Dec. 1985 (Eadie and Staltari, eds, 1987). DHEM The Newmont TEM system was described by Dickson and Boydhad become widely used where surface methods were limited (1980, 158). The effects of a uniform half-space and of a buriedby large depths or conductive overburden. However, many tabular body were calculated and compared with field results. DHEM profiles were not being interpreted properly because of lack of knowledge and/or experience. The special issue based Srinivas et al. (1980, 615) used scale modelling to study the TEMon the workshop illustrates some successes and problems in effect of truncation and of non-perpendicular profiling on theapplication of DHEM, and its potential as an ore finder. Case anomaly curve due to a vertical sheet conductor. histories and theoretical papers were included, together with a comprehensive review by the editors.Several field applications of SIROTEM were reported by BuselliBuselli et al. (1987, 75) presented a method for calibrating a (1980, 74,) including profiles over the Elura deposit near Cobardownhole SIROTEM probe by placing it inside a solenoid in NSW, at Teutonic Bore in WA, at Mt Bulga, and a deposit in thewhich a decaying current is produced. The same apparatus can Willyama Complex. Interpretation was assisted by a modellingbe used for checking the polarity of connections to the probe.facility, which used SIROTEM equipment to record the response over a thin dipping layer in air or in a conducting medium. Lee and Thomas (1988) reviewed the basic phenomena related Lewis and Lee (1981, 409) showed that conducting materialto decay of transient EM signals and the methods used to around an orebody modified the transient EM field in two ways:analyse them. Decay curves were affected where permeability in the early stages of decay it strongly directed the primary field,of the rocks was frequently dependent, such as in areas where and later it swamped the target field. It was possible to designsuperparamagnetism existed, and negative effects could be field measurements so as to make use of these effects. explained. Anomaly separation was justifiable for a body more conductive than its host rock. Supplementary data might be Nagendra et al. (1981, 489) analysed the transient responsenecessary to define a model adequately. Smith and West (1988) of a horizontal conducting cylindrical shell, and prepared aexplained the negative response in terms of a larger initial nomogram for estimating the optimum measuring time for apolarisation followed by a rapid decay, causing a negative given model. vertical-field response if the receiving loop is coincident with or inside the transmitting loop. In a complementary paper, the Displacement currents are usually neglected in TEMTEM response of a thin conductive dipping sheet was tackled calculations. Lee (1981, 402) showed that the effect of theseby Wait (1989), who carried out a first-order analysis for a step-currents would be noticeable (but small) only at early decayfunction source. He also found that the IP slow tail could be of times for highly resistive rock. opposite polarity to the main EM response.Ramaprasada Rao and Kabra (1983) gave results of modelledAt the Perth 1987 conference (Middleton and Pridmore, eds, curves over sheet conductors with varying depths and1987), there was continued emphasis on down-hole EM inclinations for dipole-dipole TEM measurements. methods. Several mathematical models and techniques of FEBRUARY 2020 PREVIEW 44'