Seismometer Acquisition and Alignment

After the data logger was calibrated, it was necessary to establish a stable seismometer subsystem test bed. Nine different STS1 instruments (six horizontal, and three vertical instruments) from a variety of sources were used. These sensors came from among the available BDSN sensors, from surplus sensors on loan from the Gräfenberg Array and from IGPP, UC San Diego. The instruments were set up and leveled, and the outputs to the Q4120 data logger were compared quantitatively. The sensors were systematically inspected and checked for nominal operation, and the ones that performed the best were selected for the testing procedures and installed in the test bed. The horizontal instruments were aligned along a single axis, toward the East, enabling coherency comparisons and evaluations. Misalignment of less than one degree across the six horizontal instruments caused unacceptable variances in signal coherency. It took a week to resolve these differences by rotating the individual instruments. Alignment of the vertical instruments was much easier. Only after all alignment incoherences were resolved were the new Metrozet electronics evaluated against the original Streckeisen factory electronics. The broadband STS-1 sensors listed in Table 3.16 were utilized during the testing procedures.

Table 3.16: STS-1 sensors. Notes: 1 - Gräfenberg (GRF) sent five seismometers total; three vertical, and two horizontal sensors. These BB instruments were some of the earliest produced by Streckeisen. While the sensors themselves are nearly identical to the later VBB instrument Streckeisen produced, the connector and electronics are sufficiently different that these were not used as part of the response test bed. These instruments were extremely useful in evaluation of the hinges, manufacture and assembly of these instruments. Gräfenberg also provided shielding, vacuum enclosures, and warpless base plates that were used. 2 - IGPP/UCSD sent two horizontal seismometers. 3 - These sensors were shipped from Streckeisen as 20 sec BB seismometers and later upgraded to 360 sec VBB seismometers. 4 - BSL provided STS-1 seismometers. 5 - These co-sited BKS STS-1's in the Byerly Seismic Vault are used as reference. 6 - Sensor was shipped with a mismatched Streckeisen electronics box (s/n 6824). 7 - These STS-1's were ultimately selected and used in the test bed.
Serial Number Component Source Note
9 V GRF 1
13 V GRF 1
14 H GRF 1
18 H GRF 1
unknown V GRF 1
18718 H IGPP/UCSD 2, 3
29201 H BSL 4, 7
29212 H BSL 4, 7
38502 V BSL 3, 4, 7
38505 V BKS Reference 3, 5
39010 H BSL 4
39039 H IGPP/UCSD 2, 3, 6, 7
48530 H BSL 3, 4
48531 H BKS Reference 3, 5
109112 V BSL 4, 7
109124 V BSL 4, 7

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