Grounded-source TEM modelling of some deep-seated 3D resistivity structures |
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| Authors: | Poddar |
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| Institution: | National Geophysical Research Institute, Uppal Road, Hyderabad 500 007, A.P., India. |
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| 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. |
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