Feasibility studies of TFMMIP and TFEM surveying with sub-audio magnetics

Abstract

The Sub-Audio Magnetics (SAM) technique measures parameters related to electrical and magnetic characteristics of the sub-surface. A galvanic andor electromagnetic source is used to excite current flow within the sub-surface. Measurements are taken with a rapid sampling, total field magnetometer while continuously traversing within the survey area. By selecting the transmitted waveform appropriately, magnetic signals related to separate physical phenomena become distinguishable and can be observed within the measured magnetic field variations. Signal processing techniques are then used to obtain the following measurements: Total Magnetic Intensity (TMI); Total Field Magnetometric Resistivity (TFMMR); Total Field Magnetometric Induced Polarisation (TFMMIP); Total Field Electromagnetics (TFEM). The measurements indicated are used to map variations in sub-surface resistivity, chargeability and magnetic properties. While software and hardware limitations have previously restricted measurements to the TMI and TFMMR parameters, ongoing development of acquisition and processing software now allows all four parameters to be measured, potentially from a single survey. Application of the SAM method to the Flying Doctor Lead/Zinc/Silver deposit showed that the mineralisation produced significant anomalous responses within the TFMMR, TFMMIP and TFEM parameters. Surface TFMMR and TFMMIP data are transformed to equivalent Magnetometric Resistivity (MMR) and Magnetometric Induced Polarisation (MIP) data, removing the data's dependence on the direction of the Earth's ambient magnetic field direction. A survey over an unexploded ordnance (UXO) test site showed that Total Magnetic Intensity (TMI) and transient electromagnetic (TFEM) data may be simultaneously acquired and used to locate these types of targets.