Field Testing and Characterization of a Transmitter-receiver System for Surface to Borehole Electromagnetic Surveys

A prototype data acquisition system and surface high power source have been implemented to perform surface to borehole electromagnetic (EM) measurements. The system has been field tested and characterized evaluating background noise, source stability and downhole profiles of the signals excited using the surface source. Background measurements yield amplitudes ~10-6 V/m and ~4 10-4 nT for the electric and magnetic field recordings in the frequency range of 8-12 Hz, of interest to STB-EM. The stability of the source waveform was measured to better than 1% and better than 1 degree in amplitude and phase, within a time window (2 minutes) expected for STB-EM logging. Measured downhole vertical profiles repeat better than 2% and 2 degrees in amplitude and phase. The data yields the response of the formation as well as the distorting effect of the metallic casing and wireline above the receivers. The latter was further analyzed by comparing it to numerical simulations performed under representative conditions, including a resistive formation, a conductive borehole and the metallic infrastructure.