David A. Emilia, James W. Allen, Ronald B. Chessmore
Bendix Field Engineering Corporation
Advanced Technology Division
Grand Junction, Colorado
Robert B. Wilson
Simplec Manufacturing Co., Inc.
Dallas, Texas
ABSTRACT
The Department of Energy (DOE) and Bendix Field Engineering Corporation in conjunction with Simplec Manufacturing Company have developed a magnetic susceptibility (MS) borehole logging system. The probe employs a ferrite-core sensing coil in a basic Maxwell bridge circuit driven by a stable oscillator. Phase detectors in the surface unit independently measure the resistive (conductivity) and the reactive (susceptibility) components of the bridge output signal. System circuitry provides for minimization of temperature variations within the probe and compensation for changes in coil characteristics due to temperature variations; this is an attempt to overcome temperature drift common in previous MS systems. During controlled testing and under the extreme condition of step–function changes in temperature, the system showed abrupt MS excursions and some dependence of MS output on coil temperature and of coil temperature on borehole temperature. These observations do indicate further need for improved temperature stabilization and bridge compensation. However, under actual borehole conditions the system is more stable. Iong term stability tests reveal a low-amplitude system drift that is as yet unexplained in terms of borehole or coil temperature variations.
Calibration has been achieved by using a cubic (5x5x5 ft) granite block with an assigned magnetic susceptibility value of 385 + 30 mCGS. This block and other “models” designed to characterize the probe are available for public use at the DOE facility in Grand Junction, Colorado. The characterization models include those designed to provide borehole size correction factors, thin layer interpretation parameters, reproducibility limits, longitudinal resolution, and radius of investigation.