3D Phase tensor inversion and Joint inversion using impedance and phase tensor components aiming better subsurface mapping and interpretation in Dhanjori basin, Singhbhum craton, eastern India

Dhanjori formation, a significant member of Singhbhum Craton, Eastern India, is well-known for its complex mineralogical character and huge content of Au-mineralization. This 2.1 Ga old Paleoproterozoic formation is composed of siliciclastic metasedimentary rocks interlayered with mafic/ultramafic volcanic rocks. Traces of gold has been spotted by many researchers in the sulfide-hosted mafic-ultramafic volcanic rocks and quartz pebble conglomerate units by different studies. Regional Audio Magnetotelluric (AMT) survey was conducted in 10 kHz–10 Hz frequency range over various lithological units of Dhanjori and adjacent Iron Ore Group. The key challenge faced during the analysis of magnetotelluric (MT) data is due to the presence of near-surface heterogeneities. The amplitude of the MT response gets distorted by galvanic effects which further create difficulties in modelling and inversion. If the effect is not removed in advance, it might lead to erroneous subsurface modelling and interpretations. On the contrary, the phase tensor data does not show any change in values even in the presence of galvanic distortion. Though phase tensors are the computed values from impedance tensor, still the response is free from any shallow level ambiguities unlike impedance data. We have performed 3-D inversion of phase tensor data to generate a conductivity model of Dhanjori basin. The objective was to verify whether the inclusion of phase tensor components in the inversion routine was able to provide some extra information that might add value to the analysis. Model from phase tensor inversion indicates only relative variation in the subsurface conductivity structure due to lack of amplitude information in the data. Hence, we also performed 3D joint inversion including both impedance and phase tensor components for obtaining the final conductivity model of Dhanjori to recover the absolute subsurface conductivity structure unaffected by galvanic distortion. Correlation of the conductivity model with available geoscientific information in Dhanjori and adjacent geological units helped us to relate the conductors to the sulfide mineralization prospect.