Frequency Effects in Hand-held Electromagnetic Short Coil Spacing Data

Hand-held small coil spacing EMI systems are used for near-surface resistivity characterisation. In this study the LIN range validity and related approximate layer modelling scheme is studied with layer modelling examples. 2-layer model results were calculated with precise numeric solutions for 14 different cases and in frequency range 0.5 - 32 kHz. Depth penetration was analysed by comparing calculated apparent resistivity curves to layer models. Calculations were made both for HCP and VCP configurations and at coil heights 0.1 and 1.0 m.

Results indicated that approximate layer modelling fails frequently when more conductive ground situates below resistive surface layer. Differences where highest at the high frequency end used. Differences increased with coil heights and were larger for VCP configuration. Square root of the skin-depth yields good approximate value for maximum depth penetration for the HCP geometry. With multi-frequency hand-held systems depth penetration is typically much larger than used coil spacing. Use of exact numeric modelling is preferable because validity of approximate solution may vary considerably within area surveyed in the field.