HR: 0830h
AB: We will present test results for a PMD model 2123 broadband seismometer installed in UTIG's OBS package, including side-by-side comparisons to a Streckeisen STS-1 and a Guralp CMG-3T as well as ocean bottom recordings from the Gulf of Mexico. PMD's innovative broadband three-component seismic sensor utilizes molecular electronic transducers instead of the traditional force-balance mechanism, which results in an inexpensive, small, lightweight, and rugged seismometer with low power requirements that is highly tolerant of tilt. Our strategy focuses on developing a small, inexpensive, and highly reliable broadband OBS that is easy to deploy and recover. These characteristics promote redundancy and dense deployments designed to target particular features and address specific scientific questions. While our OBS rests on top of the seafloor and is therefore subject to higher background noise levels than if the seismometer were buried in sediments, modern processing and imaging techniques, in conjunction with dense deployments, can more than compensate for the higher noise levels. Furthermore, in some areas of interest, such as midocean ridges, burial in sediments is often not an option. Materials to construct our broadband OBS cost less than \$16,000; with the new electronics package under development, we anticipate that batteries for a year's deployment would cost a few hundred dollars. All thirty-one earthquakes recorded by the GSN/USNSN station HKT during a four-week Gulf of Mexico test deployment were also recorded by our OBS, which was located 323 km SSE of HKT. HKT consists of a highly sensitive Streckeisen STS-1 seismometer installed at 475 m below ground in a working salt mine and, despite occasional blasts related to mining activities, has extremely quiet noise characteristics. As expected, the noise level on the seafloor was consistently higher than at HKT. Nevertheless, detection levels for earthquakes range from an mb=4.3 event at a distance of 22\deg \ to an mb=5.3 event at 96.6\deg \ to an mb=5.7 event at 145\deg, for which core phases appear clearly. Noise levels on the horizontals varied from 1.8 to 2.1 times the noise on the vertical but these levels do not appear to have compromised shear wave detection significantly, due to the larger amplitudes of shear phases. Rayleigh waves appear distinctly for the shallow larger events; Love waves are clearly recognizable on the horizontals for at least the ten largest events. One event, located on the California/Nevada border (mb=5.6, delta=19.6\deg), produced a large Lg phase at the continental HKT station that does not appear on the BBOBS, which was located 263 km offshore, at the edge of a presumed extended continental crust. Shear modes in the sediment layers were confined to frequencies above 1 Hz, whereas the lowest background noise coincided with the dominant energy from earthquakes in the band 0.03-0.1 Hz. We were able to exploit the shear modes to model the shallow sediment layers of our deployment site. Overall we found the data quality to be encouraging, given the low cost of the BBOBS and inexpensive means of deployment and recovery.
AN: S51B-06
TI: Development of a Small, Lightweight Broadband Ocean Bottom Seismograph Suitable for Dense Array Deployments
AU: * Pulliam, J
EM: jay@ig.utexas.edu
AF: Institute for Geophysics, Univ. of Texas at Austin, 4412 Spicewood Springs Road, Bldg. 600, Austin, TX 78759-8500 United States
AU: Nakamura, Y
EM: yosio@ig.utexas.edu
AF: Institute for Geophysics, Univ. of Texas at Austin, 4412 Spicewood Springs Road, Bldg. 600, Austin, TX 78759-8500 United States
AU: Huerta-Lopez, C
EM: huerta@ig.utexas.edu
AF: Institute for Geophysics, Univ. of Texas at Austin, 4412 Spicewood Springs Road, Bldg. 600, Austin, TX 78759-8500 United States
AU: Yates, B
EM: yates@ig.utexas.edu
AF: Institute for Geophysics, Univ. of Texas at Austin, 4412 Spicewood Springs Road, Bldg. 600, Austin, TX 78759-8500 United States
DE: 7294 Instruments and techniques
DE: 7299 General or miscellaneous
DE: 7200 SEISMOLOGY
SC: S
MN: Fall Meeting 2000