Slingram-type horizontal-loop electromagnetic induction systems are popular for environmental and engineering investigations of relatively shallow subsurfaces. The Slingram method is effective for the detection of anomalous zones by mapping and profiling large areas. However, analogous to airborne electromagnetic and induction borehole logging methods, zero level adjustment is crucial for further inverse analysis to estimate subsurface resistivity structures. To address this problem, we propose a simple and practical procedure for on-site measurement and correction of bias noise. To attenuate the response generated by electromagnetic induction in the ground, we raise the Slingram-type sensor vertically off the ground and measure background noise as the bias noise. By subtracting the bias noise from the original raw data, bias correction is implemented. Two applications, a high-salinity groundwater investigation and a levee assessment survey, demonstrate that this procedure is effective for quadrature data, but is inadequate for in-phase data. For a more precise correction, a more sophisticated method should be developed.

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