A three-component borehole seismometer for earthquake seismology

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Abstract
We have developed a three-component borehole seismometer, using 2-Hz electromagnetic moving-coil geophones as sensing elements. The main achievement of our new borehole package is a compact internal device for leveling horizontal geophones. Horizontal moving-coil geophones with a period longer than 0.5 sec need to be leveled to better than 1.5° to avoid nonlinear response and asymmetric clipping. In boreholes, where the axis of the package may well deviate from the vertical by more than 1.5°, a special device is required for leveling the horizontal geophones. A gimbal stage was adapted from the design of ocean-bottom seismometers for leveling the two orthogonally oriented horizontal geophones: the geophones can be leveled to better than 0.1° from an initial tilt of 10.5°. The external package is 14 cm in diameter and approximately 40 cm long. The gimbal stage uses free-flex flexural pivots and, during the leveling operation, suspends a brass pendulum that houses the horizontal sensors. In comparison with borehole seismometers that use electronic-feedback principles, the disadvantages of our system are a lack of response at long periods and, because of coil contacting stops, its inability to record strong motions. Nonetheless, it has an inherently low noise level, consumes no power, is simple to build, and is inexpensive; it is therefore suitable for rapid deployment for aftershock studies and for low-noise operation at remote sites. Such seismometers, using Mark Products L-22D 2-Hz geophones, have been installed at several sites in California: at the bottom of adjacent boreholes ∼ 150 and ∼ 300 m deep, in granite at station KNW near Keenwild and at Piñon Flat of the Anza digital seismic array, and at the bottom of an 88-m-deep borehole in hydrothermally altered serpentine in the Marina District of San Francisco. The effects of cable impedance on the geophone output have been analyzed and found to be negligible for a cable length of 335 m consisting of four pairs of shielded #20 gauge wires. Azimuthal orientation of horizontal geophones can be determined using first P-arrival particle motion generated by regional earthquakes. Earthquake data recorded by these borehole seismometers have been employed by several investigators for studies on the effects of near-surface weathered rocks on the corner frequencies of earthquake spectra near Anza, California, on the seismic-shear-wave-polarization characteristics of seismograms recorded in the southern California batholith at 300 m depth, and on site-resonance amplification caused by sedimentary deposits overlying bedrock in the Marina District of San Francisco.