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Development of a very broad band (VBB) seismometer


This collaborative project between the Berkeley Seismological Laboratory (BSL) and Metrozet, Inc was aimed at developing a state-of-the-art mechanical sensor for the detection and registration of seismic vibrations, in order to replace the famous VBB STS-1 sensor. This remarkable sensor has been operating since the early 1980's and is now installed at more than 400 seismic observatories distributed all over the globe, to capture faint signals from distant earthquakes. The collected data are essential for the imaging of the earth's deep structure, illuminated by seismic waves, as well as for improving our understanding of earthquake physics. The STS-1s have proven invaluable in the last decade when a series of magnitude 9 earthquakes occurred around the world, generating unprecedented quality of recordings of these seismic events at low frequencies.

The sensor comes in 3 separate components, two that sense horizontal motion and one, vertical motion. This way the displacement due to earthquakes can be reconstructed in three dimensional space. The aging STS-1's were unique in that they had very low instrumental noise in the frequency band of the earth's free oscillations (several hundred seconds to one hour). However, they were built with 1970's technology, and their production has been halted due in particular to dwindling supplies of the alloy used to build their characteristic "leaf spring," which had optimal elastic as well as thermal expansion properties. Rather than inventing a completely new design, the team decided to build a sensor that would closely follow the design of the original STS-1, but using modern components, as well as machine tooling capabilities that would make it easier to produce in quantities of hundreds at an equal level of precision. Several other improvements in the design were implemented, in particular, to better shield the sensor from humidity.

Photo of seismometers and a feedback electronics module. Figure 1. Left: A set of 3-component Metrozet M2166 VBB seismometers on common warpless baseplate. The vertical component seismometer module is under metal cover (this module is further isolated with a mu-metal shield). Both horizontal seismometers will also be covered with metal vacuum covers and then all three will be evacuated by use of a single port. Right: M2166 feedback electronics module (similar to the Metrozet E-300 STS-1 replacement feedback electronics developed in a collaboration between Metrozet and BSL). This allows remote digital control of mass centering, calibration, and other functions (from ASL report on the testing of M2166).

The new instrument was built in successive iterations. The BSL provided space in its Byerly Vault (BKS) on the Berkeley campus, where the performance of the successsive versions of the new instrument built at Metrozet were tested against several original well calibrated STS-1's.

These instruments are typically installed in seismic vaults, buried deeply in the ground or the side of a mountain, in order to minimize disturbing influences of temperature and pressure fluctuations. Further shielding of the sensors from environmental disturbances involves careful installation on a rigid baseplate and covering them with heat insulating shields. In this context, a new design for a granite baseplace was developed and tested. In addition to providing improved coupling to the ground, pre-engineered grooves in the new baseplate allow the orientation of the two horizontal components so that they are precisely orthogonal to each other.

Unfortunately the BKS vault is not as quiet as one would want it, in particular because it is located relatively close to the California coast, so that background noise generated in the ocean is larger than what can be resolved by these seismometers. Once performance at Berkeley was satisfactory, an additional series of tests was performed at the Albuquerque Seismological Laboratory (ASL) of the USGS, where the vault is remote from any cultural noise and ocean disturbances, providing for a significantly quieter environment.

The final tests have been concluded satisfactorily. The new VBB instrument meets the noise specifications for the vertical component, that is, the instrument noise is lower than the "new low noise model" (NLNM), which is the lowest background noise observed anywhere in the world, in the frequency band 0.001 Hz to 5 Hz. For the horizontal components, the noise is slightly higher than the NLNM at frquencies below 0.01Hz. However, the earth's horizontal background noise on the horizontal components is significantly larger.

Figure 1 shows a picture of the 3 component sensors installed on the new common baseplate.

The new Very Broad Band sensor comes with an electronics package, which provides signal conditioning and digitization, the design of which builds upon that of the E300 replacement electronics for the STS-1 built earlier by the same team, also with support from NSF. The new VBB sensor is now available commerically; more details can be found at: http://www.metrozetvbb.com/

This project was partially supported by the NSF EAR Instruments and Facilities Program under grants EAR 0744021 and EAR 0744045.