Electrical anisotropy and seafloor EM exploration ? a forward modelling algorithm

Abstract

In regions where cross-beddings or faults in the earth are well developed, an isotropic earth might be an inadequate geophysical model. The parallel alignment of these structures results in the change of conductivity of the earth with the direction of the current flow (electrical anisotropy). Due to the huge memory and time requirements, general 3D isotropic modelling for these fine structures is practically impossible, so that some global parameters, e.g., the electrical anisotropy, can be very helpful in the interpretation of EM measurements over these structures. In this paper, the EM field is represented by two scalar potentials, describing the poloidal and toroidal part of the magnetic field, for which I obtain two coupled ordinary differential equations in the vertical coordinate. To stabilize the numerical calculation, the wave number domain is divided into two parts. For small wave numbers, the EM field is continued in the anisotropic earth from layer to layer using the continuity conditions. For large wave numbers, the EM field is calculated by a Green?s function. For seafloor EM exploration, where the transmitter and receiver (T-R) are usually positioned at the seafloor, the EM field is solved in the air half-space and in conductive seawater. At the bottom of the sea, they are connected to the field in the anisotropic earth. The apparent resistivities defined from the EM impedance are introduced to present the calculation results. Numerical experiments show that when the medium above the T-R is very conductive, as is the case of salt water, the earth anisotropy under the sea can only be explored at relatively high frequencies. At low frequency, the EM field concentrates in the conductive sea, so that the apparent resistivity reflects the true resistivity of the salt water. Polar plots of apparent resistivities at different frequencies can identify the anisotropic character of the earth, e.g., the principal anisotropic orientations.