The effect the metal casing of a vertical borehole may exert on transient electromagnetic (TEM) responses has been studied in a field experiment. Eddy currents in the casing affect transients only slightly at early times, but the casing effect predominates at late times. Therefore, early-time TEM response measured near a borehole can provide information on shallow subsurface. The late-time TEM signals induced by the eddy currents in the casing show exponential behavior b⋅ exp(–t/τ). The time constant τ refers to the rate of eddy current decay in the casing; the amplitude b is M12⋅ M23⋅ L–1⋅ τ–1, where L is the casing self-inductance, and M12 and M23 are the mutual inductances between the transmitter loop and the borehole and between the borehole and the receiver, respectively. Both M12 and M23 are controllable, while M23 is especially important for survey applications: by reducing it, one can reduce the casing effect on the TEM data.
© 2014, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved.