A liquid-filled resilient cylinder is placed in a borehole and intimately coupled to the surrounding rock.When the strain in the rock changes, liquid is displaced, contracting or extending a thin-walled bellows, into a gas-filled volume, the pressure of which is substantially independent of the rock strain changes.

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An infrasonic generator utilizing a single bellows driven by a linear displacement transducer has been constructed. The generator has a dynamic range that extends from 0.5 dyn/cm9 to 600 dyn/cm2 and a frequency response of +0.5 dB, −0.6 dB from 0.001 Hz to 10 Hz. The infrasound generated by this device will be utilized for the study of the effects of infrasound on human beings. [This research is supported by National Science Foundation Grant 6K 1588].

Earthstrain is usually measured with extensometers installed in tunnels or mines. A new strainmeter for installation in a 6-in.-diameter borehole has been developed. It measures volume strain over the frequency range of 0–1.0 Hz. The strain amplitude range is from less than 10−11 to 10−5. The developmental units are operating in Washington, D. C., and there is a network of three volume strainmeters gathering near-earthquake data near Matsushiro, Central Hoshu, Japan. In a free field, a volume strainmeter would not respond to deformation caused by shear waves. However, at a boundary SV energy is partially reflected as P energy, and hence induces volume strain. Horizontally polarized (SH) motion such as Love waves are not recorded. Coupling analysis is outlined, and strainmeter response as a function of emergence angle is given. The response to surface waves is given, and a primary noise source, fluctuating atmospheric pressure, is treated.

An omnidirectional borehole volume strain-rate meter was concreted into a 412-in.-diam 150-ft-deep hole in competent rock. Noise (10−11 to 10−10 strain) related to atmospheric disturbances was observed in the period range 20 to 3000 sec. A microbarograph was buried nearby. The microbarograph and the strain rate meter both employ solion transducers. Visual observation of background on microbarograph and strain-rate meter records indicates high correlation between the two over a wide-period range. Coherence values computed indicate that S/N improvement up to 26 dB can be achieved by subtracting the microbarograph signal from the strain-rate-meter signal after suitable filtering. The latest experiments with vertical and areal strain-rate meters confirmed infrasound as the stimulus for the observed seismic noise. In addition, the areal data shows less short-period noise and thus provides better S/N for long-period surface waves. In order to achieve the best noise correction, a microbarograph must be used with spatial weighting proportional to the spatial sensitivity of the strain-rate meter to infrasound.