Experience with SQUID Magnetometers in Airborne TEM Surveying

High temperature superconducting quantum interference device (SQUID) magnetometers have been developed in a collaborative project between BHP and CSIRO specifically for application in airborne time domain electromagnetic (TEM) surveying. The objective of this development was to improve the performance of the system in detection of conductors with longer decay time constants, particularly in the presence of a conductive overburden. The sensors were incorporated into a specially designed receiver system and successfully tested as receivers for the GEOTEM system with the assistance of Fugro Airborne Surveys. Their performance was shown to be comparable with, but not superior to current induction coil TEM systems. The development of the receiver system required solutions to a range of problems for the sensor devices and for the receiver system. The principal obstacle for the receiver was in overcoming the high dynamic environment of the towed receiver bird and the consequent high level of noise associated with motion of the sensor in the Earth’s magnetic field. The high dynamic range of the magnetometer response, which arises from this motion, was addressed by a combination of modification of the sensor fluxlocked loop and periodic resetting of the sensor offset. A digital stacking filter was used to eliminate low-frequency noise associated with motion and a specially designed suspension system was developed to isolate the sensor from higher-frequency motions of the towed bird.