Time domain electromagnetic (TEM) surveys using inductive sources generally measure and interpret dB/dt data. However the electric fields (E-fields) also carry information about the conductivity and this has prompted renewed interest in collecting and interpreting them. Inductive source resistivity (ISR) surveys have recently been carried out at several exploration sites. Because much of the ISR signal arises from galvanic currents the ISR technique is applicable to geologic regimes exhibiting conductivity contrasts but not necessarily high conductivity. Thus even poorly conductive bodies at depth can sometimes be detectable. We investigate the ISR method through use of 3D forward modelling and inversion. This allows us, with synthetic models, to simulate the basic patterns of response, and evaluate the sensitivity of ISR data to different geologies and different transmitter and receiver configurations. We also invert ISR field data from Shea Creek, a uranium deposit in north Saskatchewan, Canada. Our study shows E-field TEM is a promising technique to map the 3D distribution of ground conductivity and can be an effective alternative to the dB/dt TEM survey in some resistive geologic settings.
Information about the electrical conductivity of the earth can be coded in the magnetic or electric fields arising from a geophysical survey. Typically only the magnetic field, or its time derivative, is interpreted when using inductive sources. However, recently there has been more interest in recovering the underground conductivity by collecting electrical field (E-field) data in time domain. Inductive source resistivity (ISR), first proposed in the early 1980s (MacNae, 1981) and now commercialized by Lamontagne Geophysics Ltd., employs large ground loops as transmitters, and electrical dipole receivers to measure the transient E-field. Because the data are sensitive to conductivity contrasts, ISR has been proven to work well in resistive regions to map deep alterations (MacNae 1988, 1989), and is invaluable as an alternative to magnetic field based TEM. Previously, ISR data were interpreted by analyzing data with 1D/1.5D conductivity depth imaging (CDI) algorithms. These methods are easy to implement but can produce artifacts due to 3D effects. Here we gain insight about the ISR technique by working in 3D. We use the forward modelling and inversion algorithm of Haber, Oldenburg and Shekhtman (2007). We begin by evaluating characteristics of ISR responses from some representative models, such as buried conductive and resistive prisms. This illuminates how locations of the anomalous body and survey design impact the ISR responses. Additional forward modelings can be used to identify the underground space illuminated by an ISR survey and the sensitivity of E-field measurements to a deep resistive target.
A typical survey geometry for ISR uses a square ground loop as a transmitter and a series of continuous grounded electric dipoles along a survey line as receivers (Figure 1). The ISR loop transmits a standard UTEM triangular waveform at a base frequency of 65 or 31 Hz. ISR receivers measure the inline component of transient E-field at 20 UTEM time channels with 25 or 50m dipoles at spacings of 25 or 50m.
