Home – Home – Jornals – Russian Geology and Geophysics 2014 number 11

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.