A helicopter-towed multicoil electromagnetic system was developed using a 30-ft bird and a transmitted frequency of 918 hz. Its primary features of interest are (1) low aerodynamic and geological noise, (2) three-dimensional vector recording of anomalies, and (3) quantity of geological information which is extracted by a computer-oriented interpretive procedure. The multicoil system was developed to obtain more diagnostic geologic information about a conductor than was possible with other aerial electromagnetic equipment. This led to two approaches: (1) three-dimensional recording of anomalies was developed to provide information on the electrical current distribution within the conductor and, hence, on the geometry of the conductor and (2) high-resolution, low-frequency design was chosen to bias the response of the system toward high-conductivity targets to minimize interference from the usual geologic noise sources, thereby enhancing the response of discrete massive sulfide conductors. A consequence of these design criteria is a system yielding an advance in the state-of-the-art of aerial electromagnetic exploration.
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Douglas C. Fraser (1972) studied this question.