Application of MT Forward Modeling Responses for Time-Lapse Monitoring of the Subsurface Electrical Resistivity Changes

Monitoring fracture developments in the rupture area of an earthquake or unconventional energy reservoirs (ex: enhanced geothermal systems (EGS), coal seam gas (CGS) or shale- gas reservoirs, where massive fluid injection enhances ground permeability) are crucial to determine the stress field direction and optimize well placement and energy production. In addition to microseismic tomography, magnetotelluric (MT) monitoring method provides an independent verification tool to determine more constraints on fluid distribution and migration in target lithologies. MT phase tensors (PT) and apparent resistivity tensors (RT) are calculated from impedance tensor. Assuming that geological and geo-engineering processes leave an electrically anisotropic volume in their corresponding damage zones, we investigate the time variation of RT and PT residuals for time-lapse MT monitoring purposes. First, we see how the PT and RT are influenced by layered models containing dipping and azimuthal anisotropy and then two synthetic models, representative of real earth situations including general 2D anisotropic features are studied. The results of our numerical experiments show that despite the phase tensor ellipses, the real part of apparent resistivity tensor could discriminate between isotropic, azimuthally and generally anisotropic half spaces. Furthermore, the PT and RT residuals provide complementary tools for MT monitoring of the variations in the subsurface electrical resistivity structure. Although PT residuals could confine the anomalous region more accurately, the RT residuals determine whether a conductive or resistive variation has been occurred in the anomalous region.