WHOI Short-Period OBS (D2)

This compact, lightweight instrument has a fast ascent rate and is used for active-source experiments and micro-earthquake monitoring. WHOI operates 30 Short-Period OBSs.

Design and Specifications

d2snip

WHOI short-period OBS. The upper portion of the yellow hard-hat holds a 17” Nautilus glass-ball pressure housing that holds a Quanterra Q330 datalogger, Quanterra PB1-4 Packet Baler, WHOI-designed controller board, Seascan clock, acoustic release, VHF radio and flashers. A 12” McLane glass-ball in the lower part of the hard-hat holds a lithium or alkaline battery pack. The geophone housing drops to the seafloor when a corrodible link dissolves after a few hours in salt water. The geophone is a three-component GS-11D with a natural frequency of 4.5 Hz mounted on a gimballed leveling system with a 360° operating range. A High- Tech HTI-90U hydrophone completes the sensor suite.

D2 extended pack

Item	Specifications
Physical Attributes	17” Nautilus® glass-ball pressure housing holding data-logger, storage device, acoustic-release board, WHOI recovery board, and WHOI auxiliary controller. 12” McLean® glass ball pressure housing for battery storage. Custom-designed hard-hat. Air-weight (with battery pack but not 50 lb anchor) is 140 lb. Water weight on bottom (less sensor) is 26.5 lb. Buoyancy is 20 lb. Optional syntactic collar for additional buoyancy of ~8 lb. Descent rate: ~60 m/min. Ascent rate: ~70 m/min.
Data Logger	Quanterra Q330 6-channel data engine with 24-bit A/D. 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	High Tech® HTI-90-U hydrophone with built-in preamplifier, and a low-frequency corner at 5 Hz. Geospace® GS-11D, 3-component, 4.5 Hz geophone housed in its own titanium pressure vessel, and mounted on a two-axis gimbal system that allows the seismometer to level itself regardless of the tilt of the pressure housing. When deployed, geophone is mechanically decoupled from motion of the OBS frame with a water weight of 5.6 lb, not including the weighted cable.
Recording Duration	Alkaline battery pack for ~1 month active-source experiment. Separate keep-alive battery for clock. Standard lithium battery pack in 12” battery ball allows ~8 months recording at 100 Hz. Battery back-pack (16” ball) allows 12 months recording at 200 Hz.
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. NOTE: Weights/decent & rise rates will vary depending on choice of battery pack.