SkyTEM - a new high-resolution helicopter transient electromagnetic system

SkyTEM is a time-domain, helicopter electromagnetic system designed for hydrogeophysical and environmental investigation. Developed as a rapid alternative to ground-based, transient electromagnetic measurements, the resolution capabilities are comparable to that of a conventional 40 x 40 m(2) system. Independent of the helicopter, the entire system is carried as an external sling load. In the present system, the transmitter, mounted on a lightweight wooden lattice frame, is a four-turn 12.5 x 12.5 m(2) square loop, divided into segments for transmitting a low moment with one turn and a high moment with all four turns. The low moment uses about 30 A with a turn-off time of about 4 mu s; the high moment draws approximately 50 A, and has a turn-off time of about 80 mu s. The shielded, overdamped, multi-turn receiver loop is rigidly mounted on the side of the transmitter loop. This is essentially a central-loop configuration with a 1.5 m vertical offset. In vertical hover mode the SkyTEM responses were within 2% of those from a conventional ground-based system. Instrument bias level is not a concern as high-altitude tests showed that the background noise level is higher than the instrument bias level. By inverting a sounding from a test site to a standard model and then applying the SkyTEM system parameters to compute the forward response, conventional measurements were within 5% of SkyTEM responses for flight heights of 7.25, 10, and 20 m. Standard field operations include establishment of a repeat base station in the survey area where data are acquired approximately every 1.5 hours, when the helicopter is refuelled, to monitor system stability. Data acquired in a production survey were successful in detecting and delineating a buried-valley structure important in hydrogeophysical investigations.