Deep Sea Fishing For BBOBS

Barbara Romanowicz, University of California, Berkeley, Adam Dziewonski, Harvard University

The June 1998 OSN1 recovery cruise was proceeding smoothly under Chief Scientist Ralph Stephen's watchful eye. Three broadband packages had been built as a joint project of Scripps and WHOI and successfully deployed in February 1998: one downhole broadband package and two Broadband Ocean Bottom Seismometer (BBOBS), one of which was destined to be completely buried in the sediments. The downhole package and the buried BBOBS had been recovered on previous days and secured aboard the oceanographic research vessel "Melville". Now it was the second BBOBS's turn to be hoisted from the seafloor, 4400 m below the ship, attached by a cord to its recording package. The recording frame was dangling on a grapple hanging 30m below the control vehicle (CV), a crucial element in the system, equipped with lights, cameras and thrusters, and lowered to the seafloor on a sturdy cable.

Lowering down the control vehicle for BBOBS OSN1 experiment.

At 30 meters/minute, it would take close to 3 hours to bring the last packages back to the surface, so most of us dispersed to resume other activities. We had plenty of time left before going back to watch the delicate final stage during which the heavy packages are brought onto the ship. The last stage involves a complex system of pulleys and ropes operated with skill by a joint Scripps/WHOI crew led by Gary Austin and Matt Gould, to break down the weights and prevent them from swinging wildly over the shark infested water.

We were sitting in the computer room, when suddenly Frank Vernon burst in and shouted: "we lost it". At 1000m above the seafloor, jerked by a sudden surge, the frame slipped out of the grapple's grasp and went tumbling back down. Then followed several hours of tense search for the lost packages as Gary maneuvered the CV's thrusters to let the camera explore their possible location and bring back images on monitors aboard the Melville. For a long time, there was hopelessly nothing in sight. A radius of 50 meters around the presumed location had been scanned, straight down from where the package should have dropped. We were settling for a lengthy wake...

Ocean floor recording package being recovered.

There must be a better way to do this, thought Cris Hollinshead, who had been in charge of recalling the automatic-release Scripps standard OBS's, using acoustic signals. How about trying to take advantage of the acoustic transponder on the BBOBS recording package to locate it with respect to the CV, and guide the positioning of the ship? An acoustic signal was then promptly sent several times down the cable to the CV, from there to the BBOBS recording package and back to the ship. The decreasing times it took for the pulse to come back gave an indication of the direction in which the ship should move. The package responded, and soon it was clear that it had fallen well outside of the circle of search. Owing to this ingenious triangulation method, guided carefully by John Hildebrand, the package was soon found and this time brought safely back up. It was already past midnight, when the last piece of equipment, the BBOBS's seismometer, came out of the water.

The OSN1 pilot experiment dates back over 10 years. Following the COSOD II conference in Strasbourg in 1987, a workshop was held at Woods Hole in the spring of 1988. During this workshop and in the following report, the scientific needs to deploy long term seismic observatories on the sea floor were spelled out, and related technical issues were reviewed. The principal motivation for sea floor broadband observatories is to complement the land-based networks and provide better global coverage for studies of the deep Earth's structure and tectonics. The Woods Hole workshop led to the formation of an Ocean Seismic Network (OSN) steering committee, chaired first by Mike Purdy and Adam Dziewonski, and later by John Orcutt, to prepare general plans for the deployment of a 25 station sea floor observatory network. The Ocean Drilling Program (ODP) responded positively by agreeing to drill a borehole specifically for the seismological experiment. The chosen site was conveniently located 250km south-east of Oahu (Hawaii), in deep water, not too far from land, offering the possibility of signal comparison with the Kipapa island site (KIP: a joint Geoscope/IRIS broadband station) (Figure 1) . The OSN-1 hole was drilled in March 1991 and the pilot experiment was designed by an ad hoc group under the chairmanship of Don Forsyth, in the summer of 1991.

Figure 1. Site of the OSN1 experiment, 250 km east of Hawaii. The ocean floor instrumentation included a borehole broadband package and its recording system, the BIP (PI's John Orcutt, Frank Vernon, Ralph Stephen and Ken Peal), two BBOBS (PI's John Collins, John Orcutt and Frank Vernon), one buried in the sediment and the other not. Each of the BBOBS had its own recording system (DARS). The control vehicle (CV) used to deploy and retrieve the broadband packages was built at Scripps by Fred Spiess, John Hildebrand and his group. Three standard short-period OBS's (1 WHOI, 2 Scripps) were also deployed, and later retrieved using an acoustic release system which makes them "pop-up" to the ocean surface, a technique widely used in ocean seismology campaigns.

Many technical challenges needed to be overcome. A crucial question, partly addressed by an earlier, unfortunately too short, French experiment in the mid-Atlantic (OFM), was to determine the optimal mode of installation of broadband seismic systems on the ocean floor: would the background noise level be lower deep down boreholes versus on the sea-floor, and, for the latter, whether significant improvements could be achieved by burying the sea-floor instruments in the sediments. The development of the instrumentation as well as the deployment and recovery cruises were funded by the National Science Foundation.

The deployment cruise took place in February 1998, nearly 7 years after the OSN1 hole was drilled. Three separate broadband packages were then deployed successfully, two on the seafloor (one buried, the other one not) and one 300m down the OSN1 hole. Four months of continuous data were acquired and retrieved, providing invaluable information on background noise. About 60 teleseisms, ranging in moment magnitude from 5.5 to 8.1 were also recorded with good signal to noise ratio. Preliminary comparisons indicate that the downhole recordings are less noisy in a period band from 15-100 sec than at KIP. In the same band, the buried BBOBS's teleseismic records track the downhole recordings wiggle by wiggle (Figure 2). At the lowest frequencies, the buried BBOBS perform much better than the borehole system, whereas it is the opposite at high frequencies. As expected, background noise is generally much higher on the surface BBOBS. Near the microseismic peak, the difference between borehole and seafloor or shallow buried sensors is dominated by shear wave multiples in the sediments. These cause the microseismic peak on the seafloor sensors to shift to 0.4 Hz, and they add a resonance to impulsive body wave arrivals. These shear modes are greatly attenuated for a borehole sensor, emplaced even just a few meters into the basement. On the other hand, understanding the source of the very low frequency noise in the borehole and high frequency noise on the buried BBOBS, is one of the issues that need to be addressed in the future. The technical and scientific results of the OSN1 project will be described in forthcoming publications. The data will soon be available to the community from the IRIS-Data Management Center.

Figure 2. Example of data recorded by the borehole instrument (OSN!: BHZ: vertical component; BH1, BH2: horizontal components), buried BBOBS (OSN1B) and seafloor BBOBS (OSN1S), for the magnitude 7.1 (Mw) deep (554 km) Fiji earthquake of March 29, 1988. The horizontal components in each system are orthogonal to each other, but have no preferred orientation with respect to North. Note that the short period background noise is lower on the borehole data than on the seafloor data which are affected by sediment resonances.

Even before all the data have been processed, it is clear that the successful OSN-1 experiment has confirmed that high quality seismic broadband data can be acquired on the seafloor over extended periods of time. These results justify proceeding with developing rapidly national and international plans for a global network of long-term sea-floor observatories. Many technological lessons will have been learned towards optimal deployment procedures, which can be applied to such projects in the future. Before deploying such systems permanently or semi-permanently (for durations of 1 to 2 years), future efforts will need to address the issue of power supply and data retrieval in places where ocean bottom cables are not available.

Principal Investigators on the OSN1 experiement

R.A. Stephen	Dept. of Geology and Geophysics	Woods Hole Oceanographic Institution (WHOI)
J.A. Collins	Dept. of Geology and Geophysics	Woods Hole Oceanographic Institution
J.A. Hildebrand	Marine Physical Laboratory (MPL)	Scripps Institution of Oceanography (SIO)
J.A. Orcutt	Institute of Geophysics and Planetary Physics (IGPP)	Scripps Institution of Oceanography
K.R. Peal	Dept. of Applied Ocean Physics and Engineering	Woods Hole Oceanographic Institution
F.N. Spiess	Marine Physical Laboratory	Scripps Institution of Oceanography
F.L. Vernon	Institute of Geophysics and Planetary Physics	Scripps Institution of Oceanography

Scientific team on the OSN1 recovery cruise

Other participants:
Adam Dziewonski, Harvard University and IRIS;
Joris Gieskes, SIO/GRD;
Masanori Kyo, JAMSTEC;
Barbara Romanowicz, University of California, Berkeley

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