Typical setups are networked fields of multiple axis sub nanotesla magnetic sensors with processing to reduce noise and subtract interference in order to extract a differential change across time and space to convert to a deep earth image.
You know, the usual stuff.
The essential principal is a fixed sensor (network) that records the diurnal (daily) magnetic flux and specifically locals for local 'drag' caused by local features (deep metal deposits, more generally volumes with varying magnetic properties).
