An Experimental System for Audio - Magnetotelluric Measurements.

Abstract : An experimental, frequency domain system for audio-magnetotelluric (AMT) measurements is described along with appropriate electromagnetic field theory and data processing techniques. The system employs an induction coil magnetic field sensor, lead metal electrodes for electric field detection, and a pair of synchronized wave analyzers for frequency selection. Data is displayed on a storage oscilloscope and stored on Polaroid photographs. This instrumentation is emphasized in the report with detailed descriptions of sensors and electronic equipment. Sources of the earth's natural field are discussed with particular reference to possible seasonal variations in the strength of the audio-magnetotelluric signal. Also, source distance requirements are included, and the surface impedance of a stratified earth model is derived. Finally, results of AMT measurements in Central Texas are presented. Raw photographic data and computer-estimated apparent resistivity information is displayed, and recommendations for system improvements are made. (Author)