1. A method of processing geophysical data including at least measured potential field data from a potential field survey of a surveyed region of the earth to provide a three-dimensional representation of the underlying geology of said surveyed region, the method comprising: inputting terrain-corrected potential field data for said surveyed region, said potential field data comprising data for a range of spatial wavelengths, geological features at different depths in said surveyed region being associated with different wavelengths in said range of wavelengths; filtering said potential field data by spatial wavelength to generate a first plurality of filtered sets of potential field data, each relating to a respective wavelength or range of wavelengths, each targeting geological features at a different respective said depth; processing each said filtered set of potential field data, to identify a set of spatial features comprising one or both of line spatial features and point spatial features in each said filtered set of potential field data, and to generate a set of plot data for each said filtered set of potential field data, a said set of plot data representing said identified set of spatial features for a said depth targeted by said filtering; and combining said sets of plot data to generate three-dimensional map data providing a three-dimensional representation of said underlying geology of said surveyed region.