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TIME‐DOMAIN ELECTROMAGNETIC SOUNDING—COMPUTATION OF MULTI‐LAYER RESPONSE AND THE PROBLEM OF EQUIVALENCE IN INTERPRETATION *
Author(s) -
MALLICK K.,
VERMA R. K.
Publication year - 1979
Publication title -
geophysical prospecting
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.735
H-Index - 79
eISSN - 1365-2478
pISSN - 0016-8025
DOI - 10.1111/j.1365-2478.1979.tb00962.x
Subject(s) - equivalence point , time domain , computation , resistive touchscreen , mathematical analysis , frequency domain , computational physics , mathematics , geometry , optics , physics , geophysics , materials science , algorithm , computer science , electrode , quantum mechanics , computer vision , potentiometric titration
A bstract Computations of the time‐domain electromagnetic response of a multi‐layered earth have been carried out for different source‐receiver coil systems. The primary excitation is a train of half‐sinusoidal waveforms of alternating polarity. The conversion into the time‐domain involves Fourier series summation of the matched complex mutual coupling ratios of the layered earth models computed by a digital linear filter method. As an example, the response of a perpendicular coil system on the ground surface for two source‐receiver separations has been presented for a five‐layer earth model. This has been compared with the responses of homogeneous, two‐layer, three‐layer, and four‐layer models. Next, the investigations have been extended to study the problems of equivalence of three‐layer models, the intermediate layer of which is either conductive or resistive. For an intermediate conductive layer (H‐type), the studies show that in the early portion of the signal the interpretation of a true three‐layer earth is possible to some extent, whereas the ambiguity due to equivalence persists in the late samples. On the other hand, for an intermediate resistive layer (K‐type), the three‐layer earth and its equivalent model cannot be distinguished from each other over the entire sampling period. On the basis of a computational approach, equivalence has been empirically established as √ h /ρ= constant for H‐type earth‐sections, and as h 2 ρ= constant for K‐type earth sections, where h and ρ are respectively the thickness and resistivity of the intermediate layer.