Investigations of tilt measurements using shallow borehole tiltmeters

An array of shallow borehole tiltmeters has been operated at Pinon Flat Observatory since early in 1977. The data from this array are examined for coherence between the individual instruments and compared with the corresponding data from three 732-m laser strainmeters. In general, there is no significant coherence between the signals from the tiltmeters outside the microseismic and tidal bands even though they are spaced as closely as 10 m apart. Comparisons with the strain records show that the observed tilt noise power exceeds the strain noise power by 25-40 dB over the band from 0.000001 to 1 Hz. Analysis of the coherence estimates establishes the necessity for an unacceptably large (approximately 1000) array of instruments in order to determine the common tectonic signals. The observed secular tilt rates are from 40 to 450 times the observed secular strain rates for the same period. The theoretical noise power limit of the tilt transducer renders the instrument incapable of recording background noise at a quiet site over the band from 0.0001 to 0.1 Hz.