Selected applications of a biaxial tiltmeter in the ground motion environment

Reply to the comments by Wyatt and Berger on 'Use of a bubble tiltmeter as a horizontal seismometer' by Miller et al. (1 978)

“…In each of these examples where electronic means are used we find that the electronic noise exceeds that of the intrinsic noise of the inertial sensor to the extent that the intrinsic noise is nearly negligible in comparison. Note that this is true also of the tilt noise threshold for the bubble, given by Wyatt and Berger, quoting Cooper & Schmars (1973), of 4.6 x Rad*/Hz, for this is equivalent to a noise spectral density of 6.8 x 1 0-' Rad/Hz" for the amplifier and bubble combination where the amplifier's contribution to the total is 6 x lb-' Rad/Hz". The Benioff Seismometer is inherently one of the lowest noise seismometers currently in use because of its large mass and the high value of the generator constant of its variable reluctance transducer.…”

“…In Fig. 1, curve C is the noise of the bubble and its amplifier when the amplifier has a noise specification (ena = 250nV/Hz") of the value used by Cooper & Schmars (1973) in their study of the limits of sensitivity of the bubble. The noise of the resistive bridge consisting of the bubble electrolyte and external resistors (enb = 25 nV/Hzy2) is negligible in comparison to the noise of the amplifier.…”

Comments on 'Use of a bubble tiltmeter as a horizontal seismometer' by Miller et al. (1978)

“…1 shows the spectrum of the out- put of a bubble tiltmeter, corrected for system response, and spectra taken from Fix (1972) and Murphy & Savino (1975) which represent the horizontal seismic noise at quiet sites. Also presented in the figure is the theoretical noise level of the bubble transducer, 4.6 x Rad2/Hz, reported by Cooper & Schmass (1973). Clearly from this figure, the sensitivity of the instrument is severely limited by the fundamental Brownian motion and thermal noise of the bubble transducer, so that the instrument is incapable of detecting ground noise outside of the microseismic band.…”