Methods of geophysical prospecting are described herein. In the described methods, a data set of electromagnetic readings from a marine geophysical survey is obtained and loaded into a computer processing system. Using the computer processing system, a high-frequency subset of the data is selected based on a frequency threshold, and a seawater resistivity map is resolved from the subset for a seawater portion of the survey by inversion. The seawater resistivity map can subsequently be used to resolve a resistivity map of the seabed from the data set by inversion.