AeroTEM characteristics and field results

Abstract

After half a century of rapid AEM (airborne EM) development and application, the 1980s were a “decade of uncertainty” (Fountain 1998) in which improvements focused mainly on increased bandwidth, multiple coil systems, and other advantages attendant on improved electronics and signal processing. An exception was the University of California Berkeley UNICOIL cryogenic helicopter system which adopted a single coil as both transmitter and receiver. Morrison et al. (1998) showed that this array maximized the ratio of target-to-host response in conductive environments. UNICOIL development was suspended in the early 1990s, but the same principle was used by AeroQuest in the AeroTEM transient (time domain) AEM system, which places the receiving coil centrally within the transmitting loop, thus achieving the same coupling with ground conductors simultaneously in both coils. This configuration has been shown (e.g. Buselli, 1977) to have significant advantages over the loop-loop method in ground TEM (transient EM) applications, particularly in conditions of high ground conductivity. Among the advantages are maximization of target-to-background response, simpler and sharper anomalies, enhancement of discrete conductors, and insensitivity to conductor strike direction. In addition, it has been shown by numerous authors (e.g., Smith and West, 1989) that the central loop configuration is optimally configured to excite a unique, negative response from bodies of modest polarizability. This feature is, in itself, a breakthrough of major importance in airborne prospecting because it is another step along the road to AIP (airborne IP). AeroQuest began development of AeroTEM in 1996. A key feature is the rigid mounting of the receiving coil, centrally within the transmitting loop. Other features include a triangular current pulse of 1150 μs, a base frequency of 150 Hz, a transmitting loop diameter of 5 m, and orthogonal receiving coils (Z and X). Three hundred records are recorded digitally per second. Each record holds …