Recognition of electromagnetic overburden anomalies with horizontal loop electromagnetic survey data

An analog modeling program was conducted to investigate the electromagnetic (EM) response of nonuniform conductive overburden layers with the horizontal loop technique of geophysical prospecting. Several characteristics of the response that will help to differentiate between the EM response of nonuniform overburden layers and the response of bedrock mineralized conductors are reported. To recognize these characteristics, survey data at several frequencies and coil separations are required. It was observed that the response of a ramp‐like, a ridge‐like, and a valley‐like inhomogeneity in the overburden can be mistaken as caused by bedrock conductors. In the case of the ramp and ridge inhomogeneities, the migration of the negative peak anomaly observed in profiles surveyed at different coil separations was one of its more diagnostic characteristics. Prominent positive shoulders as well as an abnormal distance between crossovers of a given detected field anomaly might be the first indication of the presence of a valley‐like inhomogeneity undernearth the overburden. It was generally found that phasor plots of the studied overburden anomalies reveal clearly their basic characteristics.