MAPPING LATERAL CHANGES IN CONDUCTANCE OF A THIN SHEET BY INVERTING TIME DOMAIN INDUCTIVE ELECTROMAGNETIC DATA
Michal Kolaj, Richard J. Smith · 2013
The laterally varying conductance of thin sheet models can be estimated by inverting time domain inductive electromagnetic data. The advantage is that it only requires off time data and the result is independent of the transmitter location, the waveform, and the delay time. The inversion requires solving a simple, linear regularized least-squares problem with input values of dHzs/dz, Hys, Hxs and dHz/dt. The measured vertical gradient has been used in our previous work, but we simplified the problem by assuming that the product of the horizontal fields with the corresponding horizontal derivatives of resistance were zero and hence that the sheet had a uniform conductance. Through forward modeling we show that removing these assumptions and using all the fields we get better results when the spatial gradient of the conductance is strong and the vertical magnetic field gradient and horizontal fields are comparable. A comparison of the simplified and full inversion in an in-loop survey collected overtop a dry tailings pond in Sudbury, Ontario, Canada revealed that there were small differences around large resistance contrasts. Overall, the full inversion is more reliable, but the simplified approach is recommended as it is simpler, and can be performed in the field if the survey is designed to minimize the horizontal magnetic fields and if caution is taken around large resistance contrasts.