The marine controlled-source electromagnetic (CSEM) method is most commonly used in targeted exploration or appraisal studies as a tool for detecting and delineating hydrocarbon reservoirs (see, for example, Constable and Srnka 2007). Whereas seismic surveys can detect the structures that may contain hydrocarbons with great accuracy, distinguishing hydrocarbon fluids from water within these structures is more problematic. Originally developed in the late 1970s (Young and Cox, 1981), the CSEM method uses a high-powered horizontal electric dipole (HED) to transmit a low-frequency electromagnetic signal through the sea floor to an array of multicomponent electromagnetic receivers. Bulk electrical resistivity of the sea floor can be determined at scales of a few tens of meters to depths of several kilometers. In many cases this resistivity is driven by the fluid content and saturation of subsurface formations.