Nanometrics Trillium Compact, User Manual

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The calibration sensitivity is not trimmed with the same accuracy as the ground motion
sensitivity. The calibration input can be used to verify the frequency response but not the
absolute sensitivity. The calibration input sensitivity, combined calibration sensitivity, and
input resistance of the available models of the seismometer module are listed in Table 10-2 .
Model Calibration input
sensitivity at 1 Hz
Combined calibration
sensitivity at 1 Hz
Calibration input
impedance
120 s -0.011 m/(s²•V) ±7% -8.3 rad/s ±7% 120 kΩ
20 s -0.010 m/(s²•V) ±7% -7.5 rad/s ±7% 133 kΩ
Table 10-2 - Calibration circuit parameters for 120 s and 20 s models
The units of the combined calibration response are rad/s because the calibration input
produces an equivalent acceleration, while the sensor passband is flat to velocity. Therefore
for a sinusoidal calibration you must divide the sensitivity listed by 2πf , where f is the
frequency of the sinusoid, to determine the expected gain.
10.2 Self-Noise
Figure 10-2 and Figure 10-3 plot typical self-noise for Trillium Compact seismometers. Three
curves are included for reference: Peterson’s new low-noise model (NLNM) and new high-noise
model (NHNM), and McNamara and Buland’s probability density function (PDF) mode low
noise model (MLNM).
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The noise floor shown is the typical level of instrument self-noise in an ideal installation. To
achieve best performance for any seismometer, careful attention to detail must be paid to
choice of site; vault design , if applicable; and seismometer installation. See "Installing a
Trillium Compact Posthole Seismometer" on page 18 or "Installing a Trillium Compact Vault
Seismometer " on page 30 .
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See also:
Jon Peterson, Observations and Modeling of Seismic Background Noise, Open-File Report 93-922
(Albuquerque, New Mexico: U.S. Department of Interior Geological Survey, 1993).
Daniel E. McNamara and Raymond P. Buland, “Ambient Noise Levels in the Continental United States,”
Bulletin of the Seismological Society of America 94, 4 (August 2004): 1517–1527.
John F. Clinton and Thomas H. Heaton, “Potential Advantages of a Strong-motion Velocity Meter over a
Strong-motion Accelerometer,” Seismological Research Letters 73, 3 (May/June 2002): 332–342.
Chapter 10 - Performance
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