Maximising geological information recovery from different magnetic instruments through the application of joint inversion

Abstract

Magnetic instruments such as cross-wing gradiometers, vertical gradiometers and full tensor SQUID magnetometers presented challenges for geological interpretation and geophysical inversion. In particular, the full tensor magnetometer presents many new challenges for an interpreter where only the vertical derivative of the vertical magnetic component presents a useful geological analogue for visual interpretation. With six channels of information how do we make practical use of the other five channels which implicitly contain useful information about the 3D distribution of magnetic properties? Joint inversion of all six channels is the logical solution whereby the data is inverted directly to a 3D magnetic susceptibility model. When compared with the scalar amplitude of the total magnetic intensity measurement, the magnetic tensor has valuable 3D information. For example just a few samples can provide sufficient information to immediately determine if an igneous pipe is on the left or right side of the flight line. A few more samples can locate the position and depth of a pipe that is off to the side of a flight line. Joint inversion can be used with various combinations of sensors and derived parameters. For example a cross-wing total magnetic field gradiometer can be used with the centre point total field value to derive important off-line geological information. The first vertical derivative derived from gridded data can be combined with total magnetic intensity measurements for two channel joint inversion to optimise the quality of depth, width, dip and depth extent inversions. Examples are provided to illustrate the improvement in geological information extraction when compared with single channel inversion of total magnetic intensity data. The methods provide new opportunities to look at the latest generation of instruments and new ways to look at old surveys.