Structure-coupled joint inversion for magnetotelluric, seismic refraction and reflection traveltime data

Abstract

SUMMARY A method of joint inversion of Magnetotelluric, seismic refraction and seismic reflection (JIMRR) is developed especially for typical hydrocarbon or hard-rock mineral exploration. JIMRR includes two parts: jointed seismic refraction and seismic reflection; and its combination with Magnetotelluric (MT) method. The objective of the research is to enhance spatial resolution of the three model parameters: electrical resistivity, seismic velocity and reflector depth. Since horizontal coordinates of reflector are not treated as model parameters in existing travel time inversion algorithm, seismic forward modelling may loss the true reflection point locations at the side edges of reflector with limited extension. We developed the technology of extensible reflector to overcome this problem. JIMRR is completed by employing the cross-gradient function as constraints which enforces the structural similarity between the resistivity and the seismic velocities, so as to reduce velocity-depth ambiguity. The cross-gradient constraints are incorporated into the solution through least squares and Lagrange multiplier method. This method results in integrated symmetric square linear matrix that is solved by bi-conjugate gradient method (BiCG). Two example synthetic models show that our joint inversion can significantly enhance the spatial resolution of inversion; and also the velocity-depth ambiguity caused by reflection travel time inversion can be notably reduced by constraints from shallow lithologies.