On: “Comparison of apparent resistivity functions for transient electromagnetic methods” by A. P. Raiche (GEOPHYSICS, v. 48, p. 787–789).

Abstract

We have read with great interest this short note essentially showing some advantages of measuring the step response rather than the impulse response for depth sounding purposes. We have reached very similar conclusions in the interpretation of UTEM® transient data in depth sounding applications compared to pulse EM data: generally the step response itself or derived apparent resistivities show the effect of either deep or very conductive layers at earlier sampling times than does the pulse response, and the apparent resistivities are most often simpler in shape and single valued. For measurements outside the transmitter loop, both step and pulse responses are more complex, but in general the step response is relatively simpler and has its characteristic points at earlier times. To give two quantitative examples: for a vertical dipole source, the zero crossing for a thin horizontal conductive layer response occurs √3 times earlier for the step response; for a half‐space response, the ratio is quite close to 3 to 1. The asymptotic behavior used to define apparent resistivities by Raiche is therefore reached at earlier sampling times for the step response in spite of the generally smaller amplitude of the pulse response at late times. We are thus in complete agreement with the main thrust of the short note.