Many steeply dipping massive sulfide ore bodies have a dike-like shape, and this has led to wide acceptance of the vertical half-plane model in the interpretation of electromagnetic data. This model assumes that the conductor is thin, but the restriction has not been considered critical and in practice has frequently been disregarded. Conductance and conductor depth estimates based on the results of towed-bird AEM surveys have been observed to be lower and less accurate than those obtained from helicopter EM and ground EM measurements. In order to explain the low reliability of the towed-bird estimates, AEM responses over 17 Canadian ore bodies were analyzed. In the study, field results obtained by the time-domain Input system and two dual-frequency quadrature systems were interpreted.Error in conductance and depth estimates results from the frequency-dependent, diffusive behavior of thick geologic conductors. This dependence makes invalid the basic assumption made in the interpretation of dual-frequency quadrature EM data, that of the equivalence of response parameters at two frequencies. The estimated conductance and depth are too small when applying the current interpretation procedure based on amplitude ratios at two high and widely separated frequencies. The error is smaller in the case of Input time-domain measurements, because the delay times are relatively long and the channels narrowly spaced. The vertical half-plane model has been found to hold for ore bodies less than 10 m wide. In the case of wide mineralized zones, which are more important economically, the vertical half-plane model could be successfully applied only at long delay times. Applying the vertical half-plane nomogram at short delay times, the conductance and depth were underestimated, and better values could only be achieved by fitting the field data to a horizontal ribbon model. The consistently low conductance values interpreted from towed-bird measurements for wide conductive zones have probably resulted in not selecting many potential massive sulfide targets for ground followup.

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