WHOI Broadband OBS (BBOBS)

These instruments are equipped with a Guralp CMG-3T broadband seismometer and used for year-long monitoring of teleseismic and regional earthquakes. WHOI has 30 BBOBS available for use.

Design and Specifications

Picture B (labels)

WHOI-designed leveling system for the Guralp CMG-3T seismometer.

WHOI technicians Dara Tebo and Tim Kane attach a heavy tripod to a seismometer pressure housing prior to BBOBS deployment. The tripod helps couple the seismometer to ground motion.

Item	Specifications
Physical Attributes	Three 17" 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. One 17" Benthos ball for floatation. A 14" 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 WHOI-designed 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 intiating a leveling procedure if necessary. Air-weight (less 175 lb. anchor but with one-year battery pack) is 530 lbs. Water-weight on bottom (less sensor) is 65 lb. Buoyancy (less anchor) is 66 lbs. Descent rate: ~43 m/min. Ascent rate: ~ 43 m/min.
Data Logger	Quanterra 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.
Clock	Seascan low-power, digitally-temperature-compensated (DTCXO), clock with drift rate, before end-point correction, of <5 ms/day.
Sensors	Guralp 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 seisometer package is mechanically decoupled from motion of the Differential Pressure Gauge (DPG) long-period pressure sensor.
Recording Duration	Lithium battery pack for ~15 months recording at 40 Hz. Keep-alive battery for clock.
Operation	Operated 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.