An instrument for measuring complex magnetic susceptibility of soils

Abstract

To improve the success of electromagnetic induction (EMI) metal detectors in identifying anti-personnel land mines buried in slightly ferromagnetic natural soils, we need to know what range of soil physical properties must be dealt with. We have therefore built a laboratory instrument for measuring complex magnetic susceptibility in inch-sized samples over a frequency range from 100 Hz to ~ 70 kHz with errors of a few percent of the sample susceptibility in a sample of ~1 milli-SIU volume susceptibility, (i.e. ~30 micro-SIU). The instrument is a symmetrical, six coil, induction spectrometer. A pair of transmitter coils in Helmholtz configuration generates a uniform magnetic field over the sample region. The magnetic moment induced in the sample is detected (mainly) by a pair of receiver coils which are closer to the sample than the transmitter pair and also (nearly) in Helmholtz configuration, so as to provide uniform sensitivity over the whole sample region. The coupling of the main receiver pair to the transmitter pair is annulled with a second pair of coils (called the reference receiver pair) situated outside the transmitter pair. The transmitter coils are energized with a wideband current. Data acquisition is by a PC computer with a 192 kHz, 24 bit, 2 channel sound card using software in written in MatLab. Although our instrument is still a prototype and its design continues to evolve, we have measured susceptibility spectra of some samples from de-mining projects in areas where false alarms are a problem and have found dispersive susceptibilities.