Abstract: A geophysical survey system comprising: a first sensor system towed by an aircraft, having at least one airborne sensor for measuring electric components of a low frequency natural electromagnetic field in a survey area; a second sensor system for positioning at a fixed position on the ground during a survey, having at least two ground based induction coil sensors for measuring magnetic components of a low frequency natural electromagnetic field in or near the survey area, the ground based sensors each being oriented to sense the magnetic components in different directions; and a processing system for calculating a set of first vector values over time in dependence on the electric components measured through the first sensor system and calculating a set of second vector values over time in dependence on the magnetic components measured through the second sensor system and comparing one or more characteristics of the first vector values and the second vector values to identify geophysical information about the