3-D inversion of MT impedances and inter-site tensors, individually and jointly. New lessons learnt

Abstract

A conventional magnetotelluric (MT) survey layout implies measurements of horizontal electric and magnetic fields at every site with subsequent estimation and interpretation of impedance tensors Z or dependent responses, such as apparent resistivities and phases. In this work, we assess advantages and disadvantages of complementing or substituting conventional MT with inter-site transfer functions such as inter-site impedance tensor, Q, horizontal magnetic, M, and horizontal electric, T, tensors. Our analysis is based on a 3-D inversion of synthetic responses calculated for a 3-D model which consists of two buried adjacent (resistive and conductive) blocks and thin resistor above them.The (regularized) 3-D inversion is performed using scalable 3-D MT inverse solver with forward modelling engine based on a contracting integral equation approach. The inversion exploits gradient-type (quasi-Newton) optimization algorithm and invokes adjoint sources approach to compute misfits’ gradients. From our model study, we conclude that: (1) 3-D inversion of either Z or Q tensors recovers the “true” structures equally well. This, in particular, raises the question whether we need magnetic field measurements at every survey site in the course of 3-D MT studies; (2) recovery of true structures is slightly worse if T tensor is inverted, and significantly worse if M tensor is inverted; (3) simultaneous inversion of Z and M (or Z and T) does not improve the recovery of true structures compared to individual inversion of Z or Q; (4) location of reference site, which is required for calculating inter-site Q, T and M tensors, has also marginal effect on the inversion results. [Figure not available: see fulltext.].