RESISTIVITY MAPPING WITH AN AIRBORNE MULTICOIL ELECTROMAGNETIC SYSTEM

Abstract

Dighem is a helicopter‐borne 900 Hz multicoil electromagnetic survey system. The EM device consists of a 30-ft towed bird containing a transmitter coil in the front and three mutually orthogonal receiver coils in the rear. Resistivity contour maps can be prepared from the EM data using any of several half‐space models. In this paper, two such models are selected and field examples of apparent resistivity derived from them are shown. The multicoil system has encountered areas of widespread conductivity while surveying for metallic minerals. In such areas, EM anomalies can be generated by changes of less than 10 m in survey altitude. EM anomalies of apparent significance, therefore, can reflect decreases in survey altitude as well as increases in conductivity of the earth. Under such conditions, apparent resistivity contour maps can aid the interpretation of the airborne data. The advantage of the contour maps is that anomalies caused by altitude changes are substantially reduced, and the contours reflect mainly the conductive anomalies. Resistivity contour maps improve the interpreter's ability to differentiate between conductive trends in the bedrock and those patterns typical of conductive overburden. Airborne resistivity mapping can be applied to a number of engineering problems. The multicoil system has been used for permafrost delineation and gravel detection. To be useful, the geologic units being mapped should have a resistivity less than 1000 Ω-m (for the 900 Hz frequency) and a surface extent of several acres.