Airborne electromagnetic systems - 50 years of development

The initial successful test flights of the Stanmac-McPhar fixed-wing airborne EM (AEM) system in Canada during the summer of 1948 can nominally be called the birth of this branch of exploration geophysics. The discovery of the Heath Steele deposit in New Brunswick, Canada in 1954, as a result of an AEM survey, proved to be the catalyst for the development of additional AEM systems and the eventual application of AEM surveys worldwide.The challenge in developing AEM systems has been to balance the desired geophysical parameters with the realities of safe and effective aircraft or helicopter operation. The decade from 1950 to 1960 can be considered the period of survey platform and system geometry development. In 1955, the first towed, rigid-beam helicopter system was introduced with a 6 m (twenty foot) bird. By the end of that decade, most of the basic AEM system geometries in use today, both fixed wing and helicopter, had been developed, and at the end of the decade the first time-domain INPUT surveys were flown. AEM development had also taken the separate paths of "rigid transmitter-receiver" systems and "large separation, towed bird" systems, which still exists today. During the decade, the first "passive transmitter" system, AFM AG, was developed in Canada and semi-airborne, ground transmitter systems were used in the USSR.Developments during 1960–1970 reflect the level of mineral exploration activity: relatively quiet during the first half; with a number of new systems developed during the balance of the decade. The innovative and versatile F-400 system, with both fixed wing and helicopter installations, was introduced. INPUT was further refined with the introduction of the Mark V in 1965, and began to establish itself as the fixed-wing AEM system of choice. Several new helicopter, "towed-rigid-beam" systems were introduced and the trend to multi-orientation receiver coils was established with the introduction of the three-component-receiver DIGHEM system. A significant development during the latter half of the decade was the introduction of four airborne systems utilising the worldwide network of VLF stations as transmitters. Turair was also introduced as a semi-airborne variant of the Turam ground system.The decade 1970–1980 can be considered as a period of system refinement. The Mark VI INPUT was introduced not only as an improved system but also, significantly, using new survey platforms. Three-and five-frequency versions of the F-400 were introduced and successfully flew surveys in both fixed-wing and helicopter installations on five continents. Two new fixed-wing systems were developed, COTRAN and EM-30, neither of which became standard for survey application. TRIDEM, a three-frequency version of the successful Rio-Mullard system, was introduced. A fixed-wing, time-domain system with B-field output was developed and test flown by the NGIR in India. The IGGE of China developed the SFAEM system. Helicopter system development in Canada focused upon multi-frequency multi-orientation, matching coil pair, towed rigid-beam systems. More advanced VLF systems were also introduced.Buoyed by the successful application of AEM for uranium exploration and the presence of major oil companies in mineral exploration, the first half of the decade 1980 to 1990 saw significant activity, both improving existing systems and with new developments, primarily in the area of instrumentation. However at mid decade, with cutbacks in mineral exploration budgets and reduction in the number of surveys funded by international assistance agencies, an international conference in October 1985 questioned the future viability of the AEM industry. In the fixed-wing arena, digital technology time-domain systems were introduced during this decade. In China development of frequency-domain fixed-wing systems continued with the introduction of the digital acquisition DFAEM system. Towed rigid-beam helicopter system development continued on the basis of additional frequencies and coil pairs mounted within the tubular towed birds. A helicopter INPUT system was introduced as well as an experimental Unicoil cryogenic helicopter system. By the end of the decade system developments were separately focused on either multi-coil, multi-frequency, high-resolution helicopter systems or deep-penetration, time-domain, fixed-wing systems.The present decade has seen a resurgence in mineral exploration, and environmental applications of AEM with a concomitant increased activity in AEM development. Much of this has been in the area of digital systems and processing. In fixed-wing surveys it is now standard to measure three directional components of the response, and have a selection of pulse widths and pulse frequencies. The option exists in some systems to also measure the B field as well as dB/dt. Similarly, helicopter systems have developed with nominally five coil pairs and five frequencies. In both fixed-wing and helicopter, systems have been developed to address specific non-mineral exploration problems.For the balance of the decade to the year 2000, it is postulated that developments in fixed-wing AEM systems will look back and focus again on new survey platforms as well as the quest for greater effective depth of penetration in conductive terranes. In helicopter AEM, the focus will be on high resolution and well defined resistivity (conductivity) mapping systems employing multi coil pairs and a broad spectrum of frequencies.