11. The method of claim 1 , wherein the electromagnetic field equations are solved for electromagnetic field component values using an assumed resistivity model of the subsurface region and space above it, comparing the calculated values of field components to measured values, adjusting the model's resistivity values to compensate for any differences, and repeating the process until agreement is obtained within a predetermined criterion.
11. The method of claim 1 , wherein the electromagnetic field equations are solved for electromagnetic field component values using an assumed resistivity model of the subsurface region and space above it, comparing the calculated values of field components to measured values, adjusting the model's resistivity values to compensate for any differences, and repeating the process until agreement is obtained within a predetermined criterion. 15. The method of claim 11 , wherein the model is anisotropic, i.e., resistivity depends on current flow direction, and the model is run once each iteration.