Effect of cable capacitance on in-situ borehole geophone calibration

Using 2-Hz electromagnetic moving‐coil geophones as sensing elements, we have constructed and deployed three‐component seismometers in boreholes at various sites for wave‐propagation studies associated with earthquake hazards (Liu et al., 1991). For example, one such seismometer has been deployed in a 88-m deep borehole reaching bedrock in the Marina District of San Francisco since 1990 (Liu et al., 1992) for the purpose of comparing ground motions in the bedrock and those at the surface. Periodic calibrations for such geophones are necessary to check if the geophone parameters have changed because of decreased magnetization of the geophone ferro‐magnet. For example, the coil transductance of the vertical‐component geophone of the borehole seismometer mentioned above was calibrated to be 121 V-s/m using phase‐ellipse test and step test before deployment. Sixty six months after the deployment, the coil transductance, when calibrated in situ and with a 100-m intervening cable between the geophone and the calibration instrument, was found to be 114 V-s/m.