oa Surface-Borehole Electromagnetic Method - A Review on the Technology Development and Potential for Geothermal Applications

Electrical resistivity of subsurface formations has proven to be the most useful geophysical property in the search and characterization of geothermal resources. Surface based Electromagnetic (EM), primarily Magnetotelluric, is the preferred method to map the resistivity structure and anomalies associated with a geothermal reservoir. Resistivity models are so obtained via regularized inversion of surface-based data, resulting in low-resolution images of the subsurface structure, as expected, owe it to diffusion of the fields traversing the medium. Recent developments of simultaneous joint inversion methods of multiphysics data has shown to significantly reduce the uncertainty in defining exploration and monitoring targets. Yet, sensitivity of the measured response is rather related to frequency of excitation and proximity to the target, which are to be addressed in data acquisition. DeepLook EM technologies, including single borehole high frequency, mid TX-RX separation and low frequency surface to borehole measurements, have proven very efficient in providing mid to high resolution images, in reservoir characterization, monitoring and well placement application in the O&G industry. This paper presents a brief overview of these technologies, further focusing on the capabilities and limitations of the recently deployed surface to borehole system for oil-water contact mapping and its potential applications in Geothermal problems.