DETECTABILITY OF AN INTERMEDIATE LAYER BY PERPENDICULAR AND VERTICAL COPLANAR ELECTROMAGNETIC SOUNDING SYSTEMS EMPLOYING DIFFERENT PRIMARY EXCITATIONS*

Abstract

AbstractThe detectability of an intermediate layer in a three‐layer earth model in the time domain has been investigated. The calculations were made for the perpendicular loop (designated system II) and vertical‐coplanar (designated system III) electromagnetic (EM) sounding systems. The primary excitation employed is a train of half‐sinusoidal and square waveforms of alternating polarity. The time‐domain response has been determined by Fourier transformation of the matched complex mutual coupling ratios into the time domain and by linear digital filtering. Top and bottom layers have equal resistivity. EM responses have been computed for conductive and resistive intermediate layer with a wide range of thickness and for two values (500 m and 1000 m) of loop‐separation. For the detectability analyses, the root mean square (rms) difference between three‐layer and homogeneous‐earth responses is adapted. The threshold value for detectability is defined as an rms difference of 10% and the measurement error is arbitrarily assumed to be of the order of 3%. It is observed that the perpendicular‐loop system is better than the vertical‐coplanar system in detecting thin intermediate layers (either conductive or resistive). For a loop separation of 1000 m and half‐sinusoidal pulse excitation, the detectable thickness ratio (h2/h1) is 0.10 by system II for the conducting middle layers; for square pulse excitation the corresponding thickness ratios are 0.06 for system II and 0.12 for system III. For a loop separation of 1000 m and half‐sinusoidal pulse excitation in detecting the resistive intermediate layers, the corresponding thickness ratios are 0.9 for system II and 2.25 for system III; while for square pulse excitation the thickness ratios are 0.55 for system II and 1.55 for system III.Results in the frequency domain and time domain (for half‐sinusoidal and square pulsed field) have also been presented for systems II and III for detecting conducting layers by considering an earth model where p1≠ p3 and p3 > p1 (p is the resistivity). The loop separa‐ tion used is 1000 m. Direct comparisons between the frequency domain and time‐domain results clearly demonstrate the superiority of frequency‐domain systems for detecting con‐ ducting intermediate layers.