Geophysical prospecting using rotationally invariant parameters of natural electromagnetic fields

A geophysical survey system comprising:

• a first sensor system including three magnetic sensors for measuring low magnetic field data in a survey area, the sensors each having a different relative orientation and measuring the magnetic field data in a different relative direction;
• a second sensor system spaced apart from the first sensor system and including two or three magnetic sensors for measuring low frequency magnetic field data in the survey area, the sensors of the second sensor system each having a different relative orientation and measuring the electromagnetic field data in a different relative direction;
• and
• a processing system for calculating a set of first vector values over time in dependence on the magnetic field data measured through the first sensor system and calculating a set of second vector values over time in dependence on the magnetic field data 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 survey area.

A method of geophysical surveying comprising:

• measuring along multiple axes at multiple locations within a survey area magnetic field components of a low frequency magnetic field resulting from naturally occurring electromagnetic sources using a first sensor system;
• measuring along multiple axes magnetic field components of a low frequency magnetic field resulting from naturally occurring electromagnetic sources using a second sensor system;
• and
• receiving information about the magnetic field components measured by the first sensor system and the second sensor system and in dependence thereon computing parameters from the received information that are independent of rotation of the first sensor system or the second sensor system about any axis thereof.