Interpretation of SIROTEM data from Elura

Abstract

A SIROTEM survey of the Elura area was made by CSIRO in February 1978. This survey consisted of six traverses across and near the Elura orebody. Traverses across the ore-body show a clearly defined anomaly in the time interval of approximately 2.6 to 50 ms. The size of the orebody producing this anomaly is deduced from the value of 200 m for the half-width of the anomaly peak. Assuming the orebody to be a sphere buried at a depth (to the top) of 100 m, a value of 75 m for the radius is obtained. The time constant of the decay curve of the Elura orebody is 6.0 ms and a value in the range of 2.9 to 6.5 Sm?1 for the conductivity of the body is calculated from this time constant value. The data are presented in the form of voltage profiles and pseudo-sections of apparent resistivity values. A summary of all these data is given. When transient decay curves recorded at Elura are transformed to apparent resistivity values, these show a tendency to fall off with delay time. Resistivity values that increase with delay time may be obtained by displacing the receiver loop approximately 1.5 m relative to the transmitter loop. For example, the apparent resistivity value calculated at one station for a delay time of 22 ms changes from 5.5 to 52 Om. Apparent resistivity pseudo-sections are corrected by measuring the percent decrease in signal when the transmitter and receiver loops are displaced at one station, and applying the correction factors calculated at this station to the measurements with coincident loops at all the stations of the survey. Corrected apparent resistivity sections of the Elura environment are interpreted in terms of a two-layered medium. A fit to data obtained at off-orebody stations deduces the presence of a top layer 70 to 140 m thick, of resistivity 10 to 17 Om, over an infinitely thick layer of resistivity 340 to 1200 Om.