Absolute Calibration of EMI Measurements and Application to Soil Magnetic Susceptibility Inference
Beijia Zhang, Kevin O’Neill and Jin Au Kong
Journal name: Journal of Environmental and Engineering Geophysics
Issue: Vol 13, No 3, September 2008 pp. 223 - 235
Special topic: Unexploded Ordnance
Info: Article, PDF ( 1.92Mb )
This paper presents a method for determining a conversion factor for electromagnetic induction (EMI) survey measurements so that fields received in an instrument’s native units can be translated into standard ("absolute") EM units. The method is non-invasive, requiring only a rudimentary characterization of the instrument, such as loop geometry and the basic nature of the output such as the magnetic field, differencing of magnetic field, or time derivative of magnetic field. It does not require knowledge of the device’s internal electrical engineering features, such as amplification, filtering, or transfer functions between components. The technique proceeds by comparing data from controlled measurements to model results, and thus allows one to do similar comparisons henceforth. As an example application and reasonableness check, we use a particular frequency domain (FD) instrument to infer soil magnetic susceptibility value in situ directly from survey data. The same methodology is applied to a new time domain (TD) instrument. This allows validation of the basic scaling methodology via benchmark cases and also illustrates its transferability. Conversion of data from both instrument types into standard units, for measurements from the same soil, illuminates the contrasting nature of the soil responses to TD and FD sensors. It also points out the fundamentally different magnitudes of different components of soil magnetic susceptibility in the soil studied relating to instantaneous response and relaxation response.