The ratio of B-field and dB/dt time constants from time-domain electromagnetic data: a new tool for estimating size and conductivity of mineral deposits
Kun Guo 1 James E. Mungall 1 3 Richard S. Smith 21 University of Toronto, Department of Earth Sciences, 22 Russell St, Toronto, Ontario, Canada M5S 3B1.
2 Laurentian University, Department of Earth Sciences, 935 Ramsay Lake Rd, Sudbury, Ontario, Canada P3E 2C6.
3 Corresponding author. Email: mungall@es.utoronto.ca
Exploration Geophysics 44(4) 238-244 https://doi.org/10.1071/EG13042
Submitted: 1 May 2013 Accepted: 1 August 2013 Published: 12 September 2013
Abstract
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 rτ = τB/τdB/dt as a tool to estimate the size and conductivity of mineral deposits. Using the VTEM base frequency, the magnitude of rτ 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 rτ to increase to 2 or more.
Key words: B-field, dB/dt, decay constant, time constant ratio, time-domain EM, overburden.
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