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Interpretation of Out-of-loop Data in Large Fixed-loop TEM Surveys

Publisher –
SEG, San Antonio 2011 Annual Meeting

Author –
Les P. Beard*, Zonge International, Tucson, Arizona, USA

Case Study – [pdf] MIN_SEG_InterpLargeLoopTEM_LPB_2011

Summary
Interpretation of out-of-loop data from fixed-loop transient EM surveys can be enhanced by using plate models. Numerical modeling shows it is possible to distinguish flat-lying conductors from vertical conductive sheets by comparing the appearance of the vertical and along-line out-of-loop measurements. Where multiple, steeply-dipping conductive sheets exist, the sheet nearest the transmitter may diminish the responses of more distant sheets, but will usually not cause the more distant sheet to be undetectable. Screening increases with closeness of adjacent plates and with increasing conductance. If the screening effect is not taken into account, estimates of conduct-ance, and by inference, ore tonnage, may be underestimated.

Conclusions
Numerical modeling is useful in obtaining a better understanding of any set of geophysical data, but it is indispensible when the particular data type is

infrequently encountered. With out-of-loop TEM data, the extra information obtained by having both Z- and X-component data was well worth the marginal extra cost in obtaining two components rather than one. With the out-of-loop data encountered in this field study, modeling helped us determine whether the conductors were flat-lying or vertical, and helped in understanding the degree of screening that might be taking place, and how superposition can effect interpretation of Z-component data by obscuring the expected crossover. We also were able to rule out a third conductor sandwiched between the two conductive bands. The level of screening increases as plate conductance increases and as plates get closer to one another. Although screening may diminish an anomaly’s response only marginally, if not taken into account it can reduce the estimate of conductance and by inference, ore grade or tonnage.