Low induction number, ground conductivity meters: A correction procedure in the absence of magnetic effects

Abstract

Ground conductivity meters, comprising a variety of coil-coil configurations, are intended to operate within the limits provided by a low induction number (LIN), electromagnetic condition. They are now routinely used across a wide range of application areas and the measured apparent conductivity data may be spatially assembled and examined/correlated alongside information obtained from many other earth science, environmental, soil and land use assessments. The theoretical behaviour of the common systems is examined in relation to both the prevailing level of subsurface conductivity and the instrument elevation. It is demonstrated that, given the inherent high level of accuracy of modern instruments, the prevailing LIN condition may require operation in environments restricted to very low (<12 mS/m) conductivities. Beyond this limit, non-linear departures from the apparent conductivity that would be associated with a LIN condition occur and are a function of the coil configuration, the instrument height and the prevailing conductivity. Using both theory and experimental data, it is demonstrated that this has the potential to provide biased and spatially distorted measurements. A simple correction procedure that can be applied to the measured data obtained from any of the LIN instruments is developed. The correction procedure would, in the limit of a uniform subsurface, return the same (correct) conductivity, irrespective of the ground conductivity meter used, the prevailing conductivity or the measurement height.

Error Analysis in Measured Conductivity under Low Induction Number Approximation for Electromagnetic Methods

“…Above this value there are strong deviations from linearity indicating that we are close to singularities of the complex function (not only the quadrature term). Despite a correction procedure suggested by Beamish [4], we believe that under statistical fluctuations of the measured signal the response could depart from the expected model leading to very high errors in the estimated conductivity due to the proximity of the function singularity.…”

“…The recent literature contains some debate on which values of (and consequently ) are valid for low induction number approximations [4,5]. It also contains suggested corrections for nonzero height dependence and nonlinear departure for higher values of conductivity even above the low induction number regime [4].…”

“…It also contains suggested corrections for nonzero height dependence and nonlinear departure for higher values of conductivity even above the low induction number regime [4].…”

“…We also consider zero elevation of the coils and, as in Beamish [4], we discard medium magnetic and dielectric components as we are working in frequencies lower than 15 kHz. Those equations represent the ratio between primary and secondary fields as the response of a homogeneous earth.…”

Electromagnetic induction antenna modelling using a linear system of complex antenna transfer functions

“…Although the modelling approach is theoretically exact, it has not yet been applied to a real field scenario due to the limited dynamic range of the instrument. Recently, Beamish () proposed a simple correction routine for measured EMI data by examining the theoretical behaviour of the commercial EMI systems in relation to the dominating level of subsurface conductivity, coil configurations and the instrument elevation. The correction procedure provides consistent results subject to the LIN assumption.…”