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A discrete conductive sphere model in which current paths are constrained to a single planar orientation (the 'dipping sphere') is used to calculate the secondary response from Geotech Ltd's VTEM airborne time domain electromagnetic (EM) system. In addition to calculating the time constants of the B-field and dB/dt responses, we focus on the time-constant ratio at a late time interval and compare numerical results with several field examples. For very strong conductors with conductivity above a critical value, both the B-field and dB/dt responses show decreasing values as the conductivity increases. Therefore response does not uniquely define conductivity. However, calculation of time constants for the decay removes the ambiguity and allows discrimination of high and low conductivity targets. A further benefit is gained by comparing the time constants of the B-field and dB/dt decays, which co-vary systematically over a wide range of target conductance. An advantage of calculating time constant ratios is that the ratios are insensitive to the dip and the depth of the targets and are stable across the conductor. Therefore we propose to use their ratio r(tau)=tau(B)/tau(dB/dt) as a tool to estimate the size and conductivity of mineral deposits. Using the VTEM base frequency, the magnitude of r reaches a limiting value of 1.32 for the most highly conductive targets. Interpretations become more complicated in the presence of conductive overburden, which appears to cause the limiting value of r(tau) to increase to 2 or more.

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258rz<br/>Times Cited:1<br/>Cited References Count:19