Development of Wideband Downhole Triaxial Seismic Detector

点击翻译按钮获取中文摘要

We have developed a downhole triaxial sonde, which consist of cable head, an electronic circuit package, a transducer housing and two-arm mechanical fixcers. The fixers are set at the upper and lower parts of the sonde. Thesonde was designed for the maximum operating temperature of 150ﾟC and pressure of 20 MPa. A shear-mode piezoelectric triaxial accelerometer has been developed and calibrated. The cross sensitivity between three components os less than 2 %, while the absolute sensitivity is 2000 pC/g. An electronic compass, in which a hall device is rotated by a motor driven potentiometer, and preamplifiers have also been developed. Overall sensitivity and frequency range are 3.16 V/g and 7-1000 Hz, re- spectively.A new calibration method for the downhole triaxial detector has also been developed. Detectability of the three dimensional particle motion associated with the seismic wave is reasonably estimated in a frequency and time domain by using the spectral matrix analysis. The method has been applied to the in-situ calibration test of the sonde in Higashi Hachimantai field, where downhole air gun was employed as a wave source. The particle motion direction at the onset of the P-wave and the effect of resonance against that were quantitavely evaluated as a function of both time and frequency. The in-situ cross sensitivity of the sonde has also been estimated by the method. The feasibility of the new measurement system was confirmed in field AE measurements of geothermal reservoir.