Ground tilt seismic spectrum measured with a new high sensitivity rotational accelerometer

Abstract

We describe a new rotational accelerometer exhibiting excellent rotational acceleration sensitivity of 2-3 X 10(-9) rad s(-2)/root Hz for frequencies below 2 Hz and increasing as the square of frequency for frequencies higher than 2 Hz. The sensitivity to horizontal linear accelerations and static tilts is 60 dB less than its rotational sensitivity. The accelerometer has been used to characterize the seismic ground tilt spectrum. Measurements of the performance of the device and of the seismic rotational spectrum are presented. (C) 1997 American Institute of Physics

Ring laser detection of rotations from teleseismic waves

“…We report here the successful extension of this to the detection of teleseismic waves (of period • 18-22 seconds) in two more advanced square HeNe ring lasers C-II and GO now at Cashmere. As we show, the noise floor is significantly below that for reported rotational accelerometers [Luiten et al, 1997]. The increase in sensitivity over our earlier work is associated with better engineering (for C-II) or with greater size (for GO); C-II has a monolithic construction in a solid piece of Zerodur (-0.65 tonne), a beam area of 1 m 2, and a quality factor 6 x 10 xa [Stedman, 1997] Other major improvements were important in securing the present results.…”

Anomalous low frequency dissipation processes in metal springs

“…3.10. Development of at least three generations of mechanical tiltmeters ended in disappointing performance [35,36], thousands of times the level that could be extrapolated from higher frequency behavior, growing at least with 1/f at the low frequency side. Additionally, it is well known that high precision seismometers need to settle for a "thermalization" time of many hours, which is much longer than the thermalization time constants that one could expect from the heath capacitance and conductivity of their structure.…”

Low-frequency seismic spectrum measured by a laser interferometer combined with a low-frequency folded pendulum

“…Araya et al [12] used DC-2 kHz sensitive seismometer with a sensitivity of 3 × 10 −11 m Hz −1/2 at 1 Hz using a laser interferometer to measure seismic noise acceleration. The VIGRO GW group [13] used a rotational accelerometer with a sensitivity of 2-3 × 10 −9 rad s −2 Hz −1/2 for frequencies below 2 Hz to measure the seismic ground tilt spectrum. The Australian GW group measured twodimensional seismic tilt spectra in their basement laboratory using a double-flexure two-axis tilt sensor with an optical walkoff sensor [14].…”