Ocean-bottom seismometers in Japan

Transcription

1 Stanford Exploration Project, Report 113, July 8, 2003, pages Ocean-bottom seismometers in Japan Ioan Vlad 1 ABSTRACT Ocean-bottom seismometers are well-tested, functional tools commonly used in crustal seismology. They can be deployed much deeper and are more robust than ocean-bottom cables, being the only type of instrument for 4-C surveys at depths greater than 1500m. I present the state-of-the-art of the Japanese OBS technology and the logistics associated with it to the seismic industry reader. INTRODUCTION Unlike Ocean-Bottom Cables (OBCs), in which receivers are strung on a cable, Ocean-Bottom Seismometers (OBSs) package the receivers as individual units. While OBCs do not go deeper than 1500m, OBSs are routinely operated at depths down to 6000m, which makes them the only currently available option for recording 4-C (three-component geophone plus hydrophone) data above deepwater reservoirs. They have been continuously developed and are currently employed by the crustal seismology research community, but only occasionally by the seismic industry. I became acquainted with the Japanese OBS technology during a crustal seismology cruise 2 and I present its state-of-the-art to the seismic industry readers. I describe the short-period OBS model developed at the University of Tokyo in the early 90 s, owned by Japan Marine Science & Technology Center (JAMSTEC) and currently operated and improved by Nippon Marine Enterprises Ltd. (NME). I refer to it as the Japanese OBS. I selected it among the 14 OBS models known to me because of several factors that make it interesting to a seismic industry audience: like an OBC, it has 4-C recording capability in the frequency band of active seismic surveys, but unlike an OBC, it can operate at depths down to several thousand meters. Additionally, I considered for my selection the criteria of logistical robustness 3 and of extremely high reliability 4 proven in hundreds of deployments of a uniform pool of instruments. 1 nick@sep.stanford.edu 2 On R/V Kaiyo, cruise KY-03-01, conducted as part of the U.S. - Japan Collaborative Research Project "Multi-Scale Seismic Imaging of the Mariana Subduction Factory". R/V denotes Research Vessel. 3 No moving parts to hamper recovery, small size of the instrument even with the sinker weights on, hydrodynamic shape to minimize: 1. drifting while sinking and 2. water current induced movement while recording. 4 Greater than 98% recovery rate from the ocean bottom and greater than 95% data recovery rate. 433

2 434 Vlad SEP 113 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. 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. nick1-sphere_closed [NR] The glass sphere (Fig. 1) 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. 2). Because the OBSs record the data locally, they use accurate clocks such as Seascan Precision Timebase, with drift rates of 1 : (< 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. 5 5 The price of a short-period OBS manufactured in Japan is of the order of magnitude of $40k. However, the sphere may be cheaper to manufacture if it were to withstand only to drillable depths rather than 6000 m. Manufacturing more instruments would also drive down costs. At present only slightly more than 100 Japanese short period instruments have been produced, and the number of OBSs of any kind whose existence has been made public does not surpass 500 for the entire world.

3 SEP 113 Ocean-bottom seismometers 435 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. nick1-sphere_open [NR] 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. 3). 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. nick1-view [NR] 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. 4). 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

4 436 Vlad SEP 113 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. Figure 4: Japanese OBSs being prepared for deployment, shown from the opposite side than in Fig. 3, to get a better view of the strobe. nick1-view_opp [NR] Table 1: Weights of Japanese OBS components, after Ito et al. (2002b) Item Weight in air (kg) Weight in water (kg) Sinker iron cylinders Base frame without sinker Sphere with sensors, recording unit and batteries Sonar transponder Sonar transducer Strobe Radio beacon Hydrophone Total at deployment Total at retrieval OBS LOGISTICS ABOARD R/V KAIYO OBS deployment is quite simple: the OBS clock drift is measured, the whole assembly goes through a last quality check, the retrieval system switches are set on automatic, and the OBS is lowered into the water (Fig. 5). The clock drift is measured along a period of several hours before the launch of the OBS, and just minutes before the launch the clock is re-synched with the GPS time signal. The instrument is lowered into the water with a small crane. This operation is not likely to be hampered by bad weather because of its simplicity. Sometimes sea currents may cause the OBS to land at a different position than planned. The average drift was

5 SEP 113 Ocean-bottom seismometers 437 Figure 5: The OBS is gently lowered into the water using a small crane. The items closest to the viewer are the sonar transceiver and transducer. The strobe is also visible as a horizontal appendage on the upper part of the OBS. Withought the sinker weights, the center of gravity of the OBS shifts so that the radio beacon and the strobe are standing upright. nick1-launch [NR] 6.5% of the water depth for the U.S. 2-C OBSs (Gunther et al., 2002). The position is found with a precision of 2% of sea depth by watching the OBS at descent and at retrieval with the sonar array of the ship, or with a precision of 5 to 25 m by triangulating water wave arrivals (Gunther et al., 2002). Currently there are no provisions for guiding the OBS during descent. Japanese OBS descent rates are between 79 and 85 m/min. These rates are not constant (Ito et al., 2002b), which may indicate turbulence caused by the lack of hydrodynamicity of the base frame, especially since the ascent rates, when the base frame is absent, are constant. Brainstorming for potential future measures to improve the accuracy of the drop reveals two main methods: guiding fins remotely controlled from the surface by acoustic modem and increasing the descent speed. Descent can be accelerated by increasing sinker weight and improving the hydrodynamic properties of the OBS ensemble, by including a simple compressed-air propulsion system or even by reducing the water resistance through super cavitation. Care should be exercised so as not to damage the instrument by a hard impact with the seafloor. After the OBSs are deployed, the survey is shot. The ship may tow streamers as well. For retrieval, the ship sends to each individual OBS a particular signal by sonar. The electrolytic corrosion process starts, lasting on average 13 minutes. The OBS then rotates 90 degrees to adjust to its new gravity center, which will bring the strobe and the radio beacon into an upright position. It then lifts towards the surface at a constant rate of 66.5 m/min. As at descent, the ship uses the sonar array to monitor the position of the OBS, so that it will be very close to the OBS when it surfaces. In most instances of OBS retrieval during the cruise, the

6 438 Vlad SEP 113 ship was able to maneuver so that the OBS would surface within 50 m in front of the ship. 6 The OBS is then simply fished out with a net (Fig. 6), and samples of sediment on it are collected if there is any such scientific interest. The OBS is then washed, the clock is checked Figure 6: The OBS is lifted aboard with a small net. nick1-recovery [NR] again with respect to the GPS time reference, then it is dismantled. The current data collection procedure involves taking apart the glass sphere, but that may be made unnecessary with future improvements. Even before improving the instruments, the recovery rate of Japanese OBSs was 97.2%, computed on 472 total individual OBS deployments. Instruments that did not surface were recovered by a Remotely Operated Vessel (ROV) in order to analyze the reasons of failure (Ito et al., 2002b). The three main causes of failure were: 1. Water leaks in the glass sphere due to improper sealing and/or sphere shaping defects. This was addressed by placing a pressure gauge inside the sphere to check the integrity of the seal before launch. 2. Leaking into the transponder pressure housing due to a structural defect of the O-ring seal at the pressure housing end cap. Mandatory checks of that component before launch were instituted. 3. Release devices failing to operate. Other improvements are currently under study (Ito et al., 2002a). They may include increasing transponder battery life above the current level of three months, raising transmission rate above the current 180 db, and retrieving data without opening the glass sphere. After improvements, the recovery rate is expected to be above 99%. The retrieval of about 100 OBSs (Fig. 7) placed along a 2-D line at a distance of 5 km and 10 km apart from each other took nine days, which is three times longer than their deployment and two times longer than shooting the 2-D line with the airguns. Much of the time was spent by the ship waiting for the electrolytic corrosion to finish and for the OBS to surface from 6 R/V Kaiyo is a very maneuvrable catamaran with thrusters at all four corners.

7 SEP 113 Ocean-bottom seismometers 439 depths sometimes greater than 4000 m (approx. 1 hour ascent time). The preparation time of an OBS for deployment was about 10 minutes, using a team of four technicians. Figure 7: Stacks of OBSs on the deck of the ship. After the base frame is left on the seafloor, they will take only 1/4 of the previous space. The ergonomic arrangements on the ship permit operations with hundreds of instruments. nick1-many_stacked [NR] OTHER OBS MODELS AND COMPARISON WITH OBC TECHNOLOGY The first OBS, built by Maurice Ewing in 1937, used rock salt for the release mechanism, containers with gasoline (incompressible and more lightweight than water!) for flotation and a pocket Hamilton watch for timing. It went as deep as 4500 m and is on display at the SEG Virtual Museum. 7 Today, NME operates a catamaran with a sonar array that can deploy 106 short-period OBSs for three months and retrieves them in 99% of cases. The technology has changed in the meanwhile, spinning off along the way broadband submersible instruments in the seventies and OBCs in the mid-nineties. Currently the U.S. National OBS Instrument Pool (OBSIP) administers 135 short-period, 2-C (hydrophone and vertical geophone) OBSs and C large broadband ones, designed for extended deployments longer than one year at a time. These instruments have been built and are maintained by Scripps Institute of Oceanography, Lamont-Doherty Earth Observatory and Woods Hole Oceanographic Institute, and are available for US academic and industrial use. 8 Broadband ( Hz- 50 Hz) OBSs are also produced by Guralp Systems. The U.S. instruments that the Japanese ones resemble most are the 19 OBSs maintained by the University of Texas Institute of Geophysics (UTIG), developed there in 1976 by scientists formerly involved in the lunar seismograph program. Another significant pool of instruments exists in Germany. It consists of approximately 40 long-period OBSs built by Geomar GmbH by adding an external moving arm that drops the 3-C geophone on the seafloor to their Ocean- Bottom Hydrophone (OBH). The OBH was originally designed as a ocean-bottom buoy, tall enough to be moved by water currents, which did not matter for the hydrophone, but does for the geophones. The University of Cambridge, UK, has developed a 4-C short-period, shortdeployment (20 days) OBS of which 25 or more have been built. It resembles the Geomar design taller than it is wide, and with a external arm that drops the geophones so that they are not shaken by water currents together with the rest of the tall instrument. Dalhousie University

8 440 Vlad SEP 113 from Canada has a small number of short-period 4-C OBSs developed in-house. University of Durham, UK, has six in-house built short-period OBSs and the Monterey Bay Aquarium Research Institute has a few ROV-operated OBSs developed in-house too. With the exception of the Japanese OBS, among the seismometers described above only the broadbands (Guralps and the long-deployment OBSIP ones) have both 4-C recording capabilities as well as reliability and recoverability demonstrated on a large pool of instruments. However, they are very expensive. The OBSIP short-period instruments are cheap (approx. $20k) and reliable (> 99%), but are only 2-C (vertical geophone and hydrophone). The UTIG ones seem less rugged and reliable than the Japanese OBSs, with instrument loss rates such as 3 instruments out of 33 (1997 Iberia experiment). The German short-period OBSs exhibit a lower recovery rate than the Japanese ones and, like the Cambridge ones, have external moving parts in order to correct for a design-induced problem. Other instruments (Dalhousie, Durham) look promising but they are larger and more complex than the Japanese OBSs, and very few of them have been built, so that extensive field testing (hundreds of individual deployments) has not occured. Should the seismic industry need an existing deep water 4-C receiver technology solution, the Japanese OBS seems to be the most appropriate among the instrument designs known to the author. A comparison between OBCs and Japanese OBSs yields interesting results. They share many advantages. Both can be used for data acquisition in areas with obstacles such as platforms, reefs, and transition zones that do not allow for streamer operation. Both record 4-C data, which can be used to image through gas clouds and in anisotropic media, to map fractures, and to predict overpressure. Neither is sensitive to noise: the seafloor is quiet and as a bonus the upward going signal and downward going noise (including the receiver ghost) can be separated by processing in the manner developed by Barr and Sanders (1989). It is in the disadvantages areas that the comparison shows dissimilarities. OBCs are currently limited to water depths of 500 m, with the most rugged going down to 1500 m. OBSs in contrast typically have a depth limitation of 6000 m. Whereas the cables sometimes have problems with withstanding deepwater pressures for extended periods of time, the OBSs have no problem. While OBCs suffer damage by strain from repeated retrievals due to repairs and moving surveys, the small and rigid OBSs are not harmed at retrieval. A localized event that destroys part of an OBC (submarine landslide, shark bite) can affect the entire cable, but OBSs are totally independent from each other. OBCs sometimes need burial by ROV for good coupling, which increases the costs and troubles at retrieval, while the weight of an OBS is sufficient to couple it well. But OBSs have disadvantages too, the first being a higher cost per deployed receiver. This may be alleviated by economy of scale if more units are produced, and by the longer life of a OBS as compared to a OBC. Another issue is that of drift during sinking, especially in deep water. The position can be determined satisfactorily with sonar arrays, but for industry purposes the deployment precision needs to be increased.

9 SEP 113 Ocean-bottom seismometers 441 CONCLUSIONS OBSs are well-tested, functional tools commonly used in crustal seismology. The cost per deployed receiver is higher than that of OBCs and there are still issues to be solved regarding the accuracy of landings on the sea bottom, but they can be deployed much deeper and are more robust than OBCs. They have potential for improvement, and can be the tool of choice for 4-C surveys at depths greater than 1500m. ACKNOWLEDGMENTS I would like to thank Bryan Kerr for useful information, for feedback, and for his comradeship during the cruise; to Simon Klemperer for logistical support and for reviewing this paper; to Shuichi Kodaira and Narumi Takahashi for arranging the practical details of our cruise, before and during it; to JAMSTEC s technician team, especially to Kazumi Baba (Fig 4), chief technician on R/V Kaiyo, for the great cooperation during our work, and for their patience and understanding of people of a completely different cultural background, such as Bryan and me; and to the National Science Foundation and the Stanford Graduate Fellowships program for funding. REFERENCES Barr, F. J., and Sanders, J. I., 1989, Attenuation of Water-Column Reverberations Using Pressure and Velocity Detectors in a Water-Bottom Cable: Expanded Abstracts, Gunther, R. H., Klemperer, S. L., Goodlife, A. M., and Kerr, B. C., 2002, Effect of Seafloor Topography on MCS Reflection Images and OBS Locations in the Mariana Subduction Factory Seismic Experiment: Eos Trans. AGU, 83(47), Fall Meet. Suppl., Abstract T72A Ito, M., Baba, K., Tanaka, H., No, T., Kashiwase, K., Kodaira, S., and Momma, H., 2002a, Developing New OBS: INMARTECH2002, 7, (2002). Ito, M., Baba, K., Tanaka, H., No, T., Kashiwase, K., Kodaira, S., and Momma, H., 2002b, Operation of Ocean-Bottom Seismometers for Multi-Channel Seismic Survey: The Twelfth International Offshore and Polar Engineering Conference, Kitakyushu, Japan, May 26-31, 2002.

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