High‐precision multicomponent borehole deformation monitoring

An instrument capable of deep borehole measurement of vector plane strain to 0.3 nstrain and tilt to 1.0 nrad has been developed for deployment in crustal deformation and earthquake prediction studies. The instrument has been deployed in California where shear strains dominate the deformation. The 125‐mm‐diam package is grouted in 175‐mm boreholes at depths of approximately 200 m. The wall thickness and the grout thickness are chosen to match instrument strength to expected rock parameters. The instrument is capable of flat response from dc to 10 Hz on any single channel. The electronics package is stable to three parts in 10⁸ over the temperature range 10 to 45° C. Reliable shear strain data is available immediately on installation when simple volume strain meters show only bond curing effects or thermal recovery signals.