3-D parallel inversion of multichannel transient electromagnetic data using a moving footprint

SUMMARY The multichannel transient electromagnetic method (MTEM), sensitive to both near-surface and deeply buried geoelectric structures, is a useful geophysical tool to map resistive targets. A typical MTEM survey consists of a large number of receiver and transmitter positions. The produced data volume is much larger than a traditional transient electromagnetic method survey, posing computational difficulties for its data inversion and interpretation. We proposed a 3-D parallel inversion algorithm for MTEM data based on nonlinear conjugate gradient method and the moving footprint technique. The electromagnetic response and Fréchet derivative are first calculated in the frequency domain with integral equation method and then cosine transformed into time domain. We not only fully parallelize the forward modelling and Fréchet derivative calculation but also use a moving footprint to speed up the inversion. The footprints for different offsets are carefully evaluated through synthetic models. The effectiveness of the developed algorithm has been demonstrated through synthetic and case studies.