It is known that electromagnetic ("EM") data, properly acquired, can be useful in the exploration for hydrocarbons ("HC"), since low-frequency EM waves in rocks are controlled by the electrical resistivity of the rock, and since HC-saturated rocks possess anomalously high electrical resistivity. It is further known that seismic data-processing techniques may, be used to process EM data, properly acquired. This invention presents a new class of data-processing techniques, particularly suited to EM data (with high attenuation and dispersion) which reveal the subsurface resistivity directly. The output of the new transform amounts to a map of subsurface resisivity, interpretable as a map of subsurface HC saturation.