A multifunction sonde for horizontal borehole logging

Abstract

A multifunction sonde designed to detect anomalous structure in horizontal gas drainage boreholes in coal has been developed. The sonde is manoeuvered in the borehole using the drill string and rig, and is remote-controlled to provide a semi-continuous suite of geophysical logs including seismic, caliper, natural gamma, spontaneous potential, resistivity, and survey. Logs are available in real time via a standard CCITT modem protocol. The seismic log is obtained from a piezoelectric source which can be pulsed at any rate from 1 to 30 Hz, detected by an array of six hydrophones with remotely switched gain in the range 24 to 90 dB, and bandwidth 300 Hz to 100 kHz. Selected sections of detected waveforms are sampled to 10 bit accuracy and averaged over up to 16,000 firings of the source providing good random noise rejection. Preliminary field testing of the sonde has shown that the combination of caliper, resistivity and gamma logs provide good correlations on basic structure. Performance of the seismic log was limited in gas-filled holes but where liquid-filled conditions were available both refracted path (normal velocity logging) and reflected path (sonic ranging) logs were obtained. Sonic ranging logs, allowing location of floor or roof of the coal seam at distances of up to 4 m from the borehole, have been demonstrated for crosshole logs and should also be possible from a single borehole.