SURFICIAL CONDUCTIVITY MAPPING WITH THE AIRBORNE INPUT SYSTEM.

The Geological Survey of Canada has undertaken to assess the airborne, time-domain EM system, INPUT (INduced PUlse Transient) in terms of its usefulness as a surficial conductivity mapping tool. In 1967, an 11-channel, minimum-coupled INPUT system was used to obtain airborne data from a small survey area near Hawkesbury, Ontario. As shown by ground DC resistivity data, the area is characterized by an electrical conductivity distribution dominated by the Champlain Sea clay. A method of quantitative interpretation has been developed using theoretical INPUT decay responses to various two-layer conductivity models. The average transient amplitude and the transient decay time are two quantities which are representative of the decay curve and are most diagnostic of the subsurface paramaters. These, therefore, form the basis of the interpretation chart which is used to interpret the field data. In the Hawkesbury survey, it is apparent that the INPUT response is sensitive to changes in thickness (which ranges from zero to very thick) and conductivity (about 0. 5 mho/m) of the highly conducting clay layer. Over a test profile the INPUT results show good agreement with the ground resistivity measurements and provide much more detailed and continuous coverage.