Geothermal Prospect Imaging Through Joint Airborne ZTEM and Ground Magnetotelluric Data Inversion Analysis

Abstract

Electromagnetic (EM) geophysical methods have been a priority technology in both enhanced geothermal systems (EGS) and hydrothermal exploration for imaging fluids, permeable pathways, and controlling structures. Natural EM field methods are the only practical option for probing depths of 2 km or more for geothermal resources. A DOE/GTO exploration R&D priority has been to develop an MT/EM/AFMAG airborne system capable of exploring to 2 km or more. Such a platform could cover large ground areas with high sampling density. The ZTEM (TM) airborne platform is simply MT tipper estimation with the vertical field sensor towed behind a helicopter and with the horizontal coils at a fixed ground location; it is otherwise processed and modeled similarly. Utilizing only the magnetic field, however, ZTEM requires an assumption for the host resistivity to correctly estimate body position and properties. Sparse MT sites in principle can help to resolve this, and the concept is largely validated in our experience. However, a consideration is the necessity to mobilize two survey types and to develop algorithms for the joint interpretation. This work was supported by U.S. Dept. of Energy/GTO funds to University of Utah/EGI under contract DE-EE0007697 and to Los Alamos National Laboratory under contract no. 89233218CNA000001.