Grounded‐source TEM modelling of some deep‐seated 3D resistivity structures

Abstract

Long‐offset transient electromagnetics (LOTEM) is now regarded as a suitable electrical method for deep exploration along with magnetotellurics (MT). In this method, the vertical magnetic‐field impulse response and, occasionally, the horizontal electric‐field step response of a grounded‐wire source on the surface of the earth are measured. Here, these two responses are computed for 3D models of three deep resistivity structures of interest in hydrocarbon exploration: (i) a faulted graben in a resistive basement rock at a depth of 4 km beneath a conductive overburden; (ii) a facies change in a resistive layer buried at a depth of 2 km in the conductive overburden above a resistive basement; and (iii) an anticlinal uplift of a resistive layer at a depth of 1 km in the conductive overburden above a resistive basement. The results show that the sensitivity of the electric‐field response to model perturbation is generally greater than that of the magnetic‐voltage response. Further, the electric‐field sensitivity is confined to early and intermediate times while that of the magnetic‐voltage response is confined to intermediate and late times. Hence it is recommended that both electric and magnetic recordings are made in a LOTEM survey so that the final results can be presented as apparent‐resistivity curves derived from the two responses jointly as well as separately.