ARRA OBS

The ARRA OBS carries a Nanometrics Trillium Compact seismometer and a chip-scale atomic clock (CSAC), and is used for year-long monitoring of teleseismic and regional earthquakes. WHOI currently has 20 ARRA OBSs available in its instrument pool.

Design and Specifications

Item

Specifications

Physical AttributesTwo 7075-T6 aluminum pressure housings holding: (i) 8.25” ID x 23.25” internal - data-logger, storage device, WHOI-designed controllers; (ii) 7” ID x 38.6” internal - acoustic-release board, and lithium batteries.
Syntactic foam floatation: 52”x32”x15” / Finished Mass: 557 lbs nominal / Buoyancy 300 ± 27 lbs in seawater.
A 257 mm long, 158 mm diameter titanium tube holds seismometer and leveling system. Leveling is via motorized gimbals.
Air-weight (less 220 lb. anchor but with one-year battery pack) is 1,000 lbs. Water-weight on bottom (less sensor) is 93 lb. Buoyancy (less anchor) is 75 lb.
Expected descent rate of ~55 m/min. Expected ascent rate of ~48 m/min.
Data LoggerQuanterra Q330 6-channel data engine with 24-bit A/Ds.
Simultaneous sample rates of 200, 100, 50, 40, 20, 10, and 1 Hz.
Quanterra Packet Baler 44 storage device with dual 16 GB flash disks (optional 64 GB capacity).
ClockSymmetricom Chip-Scale Atomic Clock (CSAC). Drift : <~20 ms after 1 year.
SensorsNanometrics Trillium Compact OBS 3-axis seismometer with a velocity response that is flat from 120 s to 50 Hz. When deployed, the seismometer package is mechanically decoupled from motion of the OBS frame. Seismometer package has a water weight of 24.5 lb.
Differential Pressure Gauge (DPG) long-period pressure sensor.
Recording DurationLithium battery pack for ~12 months recording at 40 Hz.
Separate keep-alive battery for clock.
OperationOperated in closed-housing mode i.e. programming of acquisition parameters, offloading of data, clock setting, etc, all done through penetrators. Pressure tubes carry an electronic vacuum gauge to ensure tubes are well sealed prior to deployment.