Patent ID: 7987074

Claim:
A computer-implemented method for efficiently simulating the electromagnetic field measurements in a controlled source (transmitter) electromagnetic survey obtained from a subsurface region, said simulation being for use in determining the resistivity structure of the subsurface region, said method comprising: (a) constructing an initial multi-dimensional (2-D or 3-D) resistivity model of said subsurface region on a global grid, said model including a water layer and optionally an air layer, using available information on resistivity, formation structure and bathymetry in said region, and specifying the configuration of sources and detectors used in the survey; (b) constructing a one-dimensional background resistivity model for the subsurface region, said model being a 1-D approximation of the multi-dimensional model; (c) separating Maxwell's electromagnetic field equations so that the solution generated is the sum of a background field solution and a scattered field solution, said background field solution to use the one-dimensional background resistivity model and said scattered field solution to use the multi-dimensional resistivity model; (d) invoking reciprocity for solving for both the scattered field solution and the background solution, wherein transmitter locations and receiver locations are interchanged, actual receiver locations thereby becoming computational transmitter locations; (e) selecting a frequency in the transmitter's frequency spectrum; (f) selecting a computational transmitter location; (g) constructing a non-uniform computational mesh (grid) within a domain surrounding said selected transmitter location, with a maximum mesh spacing near domain boundaries based on electromagnetic skin depth for the selected frequency and a minimum mesh spacing near the computational transmitter location where grid spacing is reduced to account for steeply varying electromagnetic field values, and said domain extent being based on skin depth considerations; (h) interpolating resistivity values from the global grid model to the non-uniform computational grid; (i) computing a background field solution in the frequency domain by numerical methods or analytically; (j) solving the electromagnetic field equations for the scattered field solution in the frequency domain on a computer by finite difference or finite element methods on the non-uniform computational grid for one or more electromagnetic field components, the solution being for at least one computational receiver location and taking as given information the interpolated resistivity values, transmitter signal generation parameters, and selected transmitter and receiver locations; and (k) combining the scattered field solution with the background field solution, and outputting the results or storing them in computer memory.