WHOI Keck OBS

Like the Broadband OBS, the Keck OBS is used for teleseismic and regional earthquake monitoring. It is equipped with a Guralp CMG-3T seismometer and a Kinematics Episensor strong-motion accelerometer for a combined dynamic range of ~220 dB. WHOI currently has 10 Keck OBSs in the laboratory.

Design and Specifications

Item

Specifications

Physical AttributesThree 17โ€ Nautilus glass ball pressure housings holding: (i) data-logger, storage device, controllers; (ii) acoustic-release board, WHOI recovery board, and lithium batteries; (iii) lithium batteries. Two 17โ€ Nautilus/Benthos balls for floatation.
A 14โ€ I.D. Al sphere holds seismometer and leveling system.
Leveling is via a gravity-driven, 2-axis gimbal system that can be locked in place by motor-driven disk brakes mounted on each gimbal axis in order to record ground motion with high fidelity. A controller is capable of locking/unlocking the seismometer masses, centering the masses, and activating/de-activating the disk brakes. The controller also carries a compass, temperature and tilt sensor. The controller is capable of automatically monitoring seismometer tilt and initiating a leveling procedure if necessary.
Air-weight (less 175 lb anchor but with one-year battery pack) is 745 lb. Water weight on bottom (less sensor) is 75 lb. Buoyancy (less anchor) is 42 lb.
Descent rate: ~40 m/min. Ascent rate: ~32 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 storage device with 20 GByte rotary disk-drive.
ClockSeascan low-power, digitally-temperature-compensated (DTCXO), clock with drift rate, before end-point correction, of < 5 ms/day.
SensorsGuralp CMG-3T seismometer with a velocity response that is flat from 120 s to 50 Hz. The CMG-3T model used draws less current than a conventional unit, at the cost of reduced dynamic range. When deployed, the seismometer package is mechanically decoupled from motion of the OBS frame. Seismometer package has a water weight of 21 lbs.
Kinemetrics Episensor strong-motion 3-axis accelerometer mounted on a gravity-driven, 2-axis gimbal system.
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-ball mode i.e. programming of acquisition parameters, offloading of data, clock setting, etc, all done through penetrators. Glass spheres carry an electronic vacuum gauge to ensure spheres are well sealed prior to deployment.