INTRODUCTION The Story of Airborne Electromagnetics After the end of World War II, the reconstruction of war-ravaged economies fueled a great demand for natural resources. The emerging Cold War caused explorationists to seek secure supplies in countries geographically and politically close to the United States. With vast areas that were then little explored, Canada was one obvious choice. These circumstances provided a great incentive to develop geophysical methods whereby a sparsely populated country, where the climate is often harsh and frigid for part of the year, could be scanned quickly and effectively for deposits of strategic base metals, such as copper, lead, zinc, and nickel. Airborne magnetometer systems that were developed from early war-time prototypes used in submarine detection became widely used in mineral exploration in Canada. However, it soon became obvious that the magnetic information was of more value indirectly in aiding geologic reconnaissance than it was directly in ore exploration. The abundance of magnetic bodies in deformed metamorphic terrains with base metal potential made it difficult to select specific targets for more detailed exploration on the ground. An alternative or additional technique was, therefore, required to carry out prospecting from the air.

Abstract The aim of geophysical surveys is to obtain information on subsurface geology. While execution of surveys using specific techniques may differ in detail, it will almost invariably consist of three steps: surveying, data processing, and data interpretation. A successful survey will yield more information on the geological target—its existence, location, shape, size, etc. New information is obtained by interpreting geophysical data. The success of a survey depends to a large extent on decisions made before the survey initiation. An exploration geophysicist working for a mining company is often asked the following question: Can we use geophysics in prospecting for this particular commodity? If yes, what techniques should we use and how do we specify survey parameter. Decisions that are usually based on experience often cannot be justified scientifically. The proper approach would be to carry out test surveys to investigate the physical properties of the target and other bodies that might interfere with its response. In recent years, exploration geophysics has progressed beyond target finding to mapping subsurface geology. Analyzing the sequence of geophysical survey steps as shown in Figure 1, the main flow (surveying, processing, interpretation) and the associated areas of research can be identified. To make an intelligent decision on the use of a technique, the geophysicist should have at least a rudimentary knowledge of the physical properties of the target and the surrounding media the response of which might interfere with target identification. Most physical property studies have been done in the laboratory on samples collected in the field. While this approach may be satisfactory for some geophysical methods (gravity, magnetics), it is not for others. Electrical properties of earth materials vary substantially (by several orders of magnitude) depending on whether they are measured in situ or in a laboratory. It is virtually impossible to simulate real conditions in the laboratory. An attempt can be made to recompose the original water content, but microinhomogeneities typical of many geological environments (e.g., rock fractures and their frequency and variation with depth) cannot be duplicated.