2012 IRIS Workshop

A Site Comparison Between Shallow Vault-Deployed and Direct Burial Broadband Seismometers

Katherine E. Anderson: New Mexico Tech, Jacob F. Anderson: New Mexico Tech, Robert E. Anthony: New Mexico Tech, Julien Chaput: New Mexico Tech, Nicole D. McMahon: New Mexico Tech, Emily A. Morton: New

PDF median coherence for vault-vault and direct burial-direct burial (DB) sites from Dec 1-31, 2011 on BHZ components. Log10 frequency is on the x-axis and magnitude squared coherence is on the y-axis. Scale is in log units. Perfect coherence is one while perfect incoherence is zero. Vault and DB sites are coherent from 0.1-5 Hz. Decoherence occurs with both DB and vault sites near 1 Hz, with larger decoherence exhibited on DB sites, suggesting DB sites are more affected by strong winds.

Full-resolution graphics file in original format: 0070.png

Studies of Earth structure and seismic sources are widely facilitated by temporary deployments of broadband seismographs in compact vaults. Such vaults are designed to minimize temperature changes and surface noise, provide optimal coupling, and mitigate destabilizing environmental effects. Vault installations ma be impractical or suboptimal for some situations because the associated materials are bulky, the site preparation may be difficult, and the aggregated cost of material may be considerable. To increase the practical number of deployable sensors for broadband seismic imaging, a more rapid and cost effective methodology is desirable. For these reasons the research community has developed a renewed interest in direct burial broadband installation. The purpose of this study was to quantitatively compare the thermal and mechanical stability of direct burial stations and traditional vaults in a controlled experiment. The IRIS PASSCAL Instrument Center installed a test site in August 2011 near Socorro, NM (33.96°N, 106.85°W). Data were collected from two identical direct burial stations each with 1 Hz and 40 Hz sampling 3-component data streams. Four Guralp CMG-3T seismometers were deployed in a square configuration with center-to-center distances of approximately 1 m. Data were collected using two 6-channel Quanterra Q330 digitizers and telemetered to the PASSCAL Instrument Center. Data were analyzed for absolute and differential noise levels and characteristics, inter-sensor coherency, and signal-to-noise levels for December 1-31, 2011. During this period the site experienced several winter storms, strong winds, and numerous strong freeze-thaw cycles. Temperature, precipitation, and wind statistics were obtained from a nearby weather station to examine environmental correlations. We further examined event-based transient signals such as the November 6, 2011 M5.6 Oklahoma earthquake.

Acknoweldgements: IRIS 2012 Workshop student scholarship for funding to the meeting, IRIS-PASSCAL Instrument Center for site setup and data archiving, Noel Barstow of Passcal for helping with site setup and instrumentation monitoring.

Keywords: instrumentation, coherency, signal_to_noise, broadband_seismometers, direct_burial_installation, absolute_and_differential_noise_levels, site_comparison, vault_installation, pdf, psd

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