1. A method for determining time-dependent changes in the hydrocarbon content of a subsurface reservoir from controlled-source electromagnetic survey data obtained from the subsurface region containing the reservoir, said method comprising: (a) obtaining electromagnetic field data recorded at a plurality of online and offline, relative to a survey source line, receiver locations from an initial survey of the subsurface region, and corresponding survey results from at least one later survey of the same region conducted under substantially the same conditions, said data comprising a combination of field component and receiver location sensitive at least predominantly to vertical resistivity and a combination of field component and receiver location sensitive at least predominantly to horizontal resistivity, wherein each said combination depends on electromagnetic source design; (b) for each survey, using a transitory, non-human computing apparatus to solve Maxwell's electromagnetic field equations for horizontal resistivity and vertical resistivity at a plurality of (x,y,z) positions in the subsurface reservoir, using survey acquisition parameters and the measured electromagnetic field data; and (c) comparing calculated resistivity results between the surveys.