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The Japanese OBS

Japanese short-period OBSs consist of a glass sphere inside a plastic shell, on which a base frame and a retrieval system are fixed.

 
sphere_closed
Figure 1
The glass sphere alone, unopened. Only the metal bands have been removed, their outline highlighted with a marker prior to removal. The manometer dial is very visible. The square recording unit sitting on top of the round battery pack is distinguishable through the glass. The sphere is seated in a special stand with a rubber collar on bearings and with two openings for the two halves of the sphere to be dismantled.
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The glass sphere (Fig. [*]) is manufactured by Benthos Technologies, Inc., USA, has a diameter of 17 inches and houses: 1. a self-gimbaling, three-component, 4.5 Hz Mark Products L-28 geophone, 2. a recording unit with a capacity of 2Gb on DAT or 8 Gb on hard disk and with circuitry for dynamic lossless compression, and 3. sealed batteries that do not generate hydrogen (Fig. [*]). Because the OBSs record the data locally, they use accurate clocks such as Seascan Precision Timebase, with drift rates of $1:3-5 \times 10^{-8}$ (< 0.5 ms/day before correction and < 10 ms/yr after correction). The sphere is actually an ensemble of two glass hemispheres, held together by a low-degree vacuum. The vacuum approximately corresponds to the atmospheric pressure at the altitude of 3700 m and allows the ensemble to be checked for leaks before deployment by a manometer readable from outside the glass sphere. The hemispheres are sealed at the joint with latex and electrical tape and engirdled by two metal bands placed along great circles on the sphere. The metal bands are rigorously checked to be orthogonal to one another so that they do not slip off the circle or exert a shearing force that would separate the hemispheres enough to damage the latex. The sphere is traversed by an electrical contact point for the hydrophone cable, one for reading and synching the clock, and by a valve for modifying the pressure inside. The sphere is designed to withstand depths to 6000 m and is responsible both for the buoyancy of the entire OBS ensemble (see Table 1 for exact figures) and for much of the extra cost over the price of a land three-component seismometer.[*]

 
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Figure 2
The 3-C geophone is visible inside the hemisphere on the left. The toroidal battery pack and the small square electronics box are visible on the floor, in the upper right corner of the picture.
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The glass sphere is placed inside a plastic shell, on which the base frame and the retrieval system are attached. The base frame is metallic; two heavy iron cylinders are welded to it. It acts as a sinker and it also ensures seafloor mechanical coupling. The shell is attached to the frame in only two points, which will be electrolytically corroded by the retrieval system, leaving the frame assembly on the ocean bottom and the OBS floating towards the surface. Leaving the frame on the seafloor has led to environmental complaints about rusting iron as a pollutant and to fishing industry complaints about nets being torn by frames, so in the future they may be replaced by concrete plates. The current horizontal size of the base frame is 1.2m by 1.0m, and the height of the shell-frame ensemble is 0.6m (Fig. [*]).

 
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Figure 3
Japanese OBS being prepared for deployment. The radio beacon and the hydrophone (horizontal cylinder) are visible in the foreground. The item on top of the OBS is a writing pad with the deployment checklist.
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The retrieval system consists of several components. The first is a sonar transceiver which listens for a certain signal from the retrieving vessel and can transmit back information. The sonar batteries are housed in a steel cylinder that protects the transponder as well. The transducer is fixed separately on the plastic shell of the OBS. The second retrieval element is the electrolytic corrosion system, which is also fed from the sonar batteries. The system is designed in such a way that should a short-circuit occur, the corrosion would begin, sending the OBS to the surface rather than leaving it stranded on the seafloor. The third element of the retrieval system is a strobe with its own C-cell batteries (Fig. [*]). The fourth element is a radio transmitter with its own C-cell batteries. Both the strobe and the radio transmitter have an automatic, pressure-triggered switch and an overriding manual one. Just before launching the OBS into the water, the switch is set on automatic. The strobe will flash and the antenna will send signals at atmospheric pressure, but will not function when the OBS is at depth. Also, the strobe has a light sensor so that it will only flash in low-light conditions.

 
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Figure 4
Japanese OBSs being prepared for deployment, shown from the opposite side than in Fig. [*], to get a better view of the strobe.
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Table 1: Weights of Japanese OBS components, after Ito et al. (2002b)
Item Weight in air (kg) Weight in water (kg)
Sinker iron cylinders 52.28 45.62
Base frame without sinker 3.22 2.58
Sphere with sensors, recording unit and batteries 31.00 -13.90
Sonar transponder 4.25 2.30
Sonar transducer 2.60 1.20
Strobe 1.80 1.00
Radio beacon 1.90 1.05
Hydrophone 0.40 0.20
Total at deployment 97.45 40.05
Total at retrieval 41.95 -8.15


next up previous print clean
Next: OBS logistics aboard R/V Up: Vlad: Ocean-bottom seismometers Previous: Introduction
Stanford Exploration Project
7/8/2003