XA (1991-1991): Piñon Flats Small Aperture Array, Broadband Triggered Array

• University of California, San Diego (UCSD), United States of America

During the winter and spring of 1991 the Kirghizia Seismic Array Committee (KSAC) of IRIS's Joint Seismic Program conducted a small aperture broadband array experiment in the vicinity of the Piñon Flat Observatory (PFO) in Southern California. This report is intended primarily to document the technical details of these data, their organization on data tapes that are the companion to this report, and any problems that are known to exist with these data. Piñon Flat was chosen as the site for this experiment because it is conveniently located near IGPP, UCSD, where key personnel in the experiment lived; because of the long history of this site as a geophysical observatory; and because existing seismic stations of the Anza and Southern California networks could provide related observational data. The PFO was also the site of the high frequency array experiment conducted by the same group during the Winter and Spring of 1990 (XA-90, see PASSCAL Data Report #91-002), which provided additional con­ straints on the site condition, expected source characteristics, and wavefield properties at higher frequencies and wavenumbers than were possible with this experiment.

The Piñon Flat broadband array experiment was deployed with 28 sensors arranged in 5 concentric rings. The array configuration is similar to that of the NORESS array, but has an additional three-sensor ring with a diameter of 6 km. Stations are spaced at equi-angular increments around each ring.

The sensors used for this experiment were Streckeisen STS-2's and Guralp CMG-3ESP's. These are both broadband, 3-component sensors. The Streckeisen STS-2 has a velocity response that is flat from 0.00833 Hz (120s period) to 50 Hz. From 1 to 6 Guralp CMG-3ESP's were used when there were insufficient STS-2's to occupy all 28 sites. Guralp CMG-3ESP's have a velocity response that is flat from 0.033 Hz (30s period) to 30 Hz.

Sensors were housed in mini-vaults designed and made at IGPP-SIO. These vaults consisted of a PVC tube with 0-ring sealed, aluminum tops and bottoms, and an aluminum leveling plate. The purpose of these min-vaults was to provide a second level of defense against water for the STS-2, which was originally designed for installation in a conventional vault.

The experiment used Refraction Technology [Reftek] digitizers. The array was designed to have the A, B and C rings (16 sites) linked to the central recording site via wire lines, and the D and E rings (12 sites) linked via radio telemetry. Delays in radio deliveries resulted in the outer rings being operated as stand-alone PASSCAL stations for several weeks. Because of this mix of recording/communication methods, three different Reftek Data Acquisition Systems [DAS] configurations were used during the experiment:

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