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Cruise reports
 UTIG research
scientists are leading a scientific cruise along the Lesser Antilles Subduction Zone in
support of the project, "Structure, Tectonics, and
Sediment Flow into the Lesser Antilles Subduction Zone." The UTIG research team
consists of four research scientists (Nathan
Bangs, Gail Christeson, Yosio Nakamura, and
Tom Shipley), two systems analysts (Mark
Wiederspahn and Kevin Johnson), a research scientist associate (Ben Yates), and two
students (Alex Lopez and John Stachowiak) from The University of Texas at Austin Department of Geological Sciences. The team plans to
collect 1,600 km of multichannel seismic reflection data. These data will be acquired both
along strike and across strike of the subduction zone and are designed to image crust and
sediment structures from the seabottom. |
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| Here is a location map of the Lesser Antilles Island Arc. |
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| Here are the proposed lines along which the seismic data will be collected. We plan on collecting 4 dip lines and 8 strike lines. |
Monday, March 16.
The cruise is underway from Barbados. Stay tuned for reports...
Despite delays at the start, the data acquisition has been going smoothly so far and we are right on schedule. The last of the OBS shipment did not arrive at the ship until Friday afternoon, ~40 hours before leaving port. This did not allow Yosio and Ben enough time for all of the normal OBS preparation, testing, and sleep. Nearly continuous work in port by Ben and Yosio put us back on track. By the time we arrived at the first site for deployment, most of the OBS were ready to go and the deployment began on schedule. All 20 OBSs were deployed and we finished with a half hour to spare.
The streamer work required approximately 18 hours to get the whole system operational. The streamer required lots of oil and sections were moved or removed. In the first half of the experiment we shot four of the 8 strike lines and one of the four dip lines. The strike lines were shot in relatively calm seas, and the data look reasonably good. Winds were generally 15 to 20 kts. At the beginning of the dip line, the seas picked up and winds maintained 25-30 kts. The heavy seas and our course directly into the seas contributed to noise in the multichannel data. The first half of our acquisition finished about 6 hrs ahead of schedule, and we spent this time working on the streamer to better balanced it and make it more controllable. The first 12 OBSs released on time. They returned to the surface and have been recovered. So far, all appear to have good data except one that looks like it had problems with instrument noise. Currently 4 of them have been redeployed, and the other 8 are being refurbished. They will be redeployed tonight.
Tomorrow we will redeploy the streamer and shoot the remaining 7 lines. All OBSs will be retrieved by early morning April 1, and we will then head for Norfolk. Our current schedule will put us into Norfolk on April 7th.
Everything seems to be going ok so far. I'll send you more in a few days.
Things are going fine so far, and we are almost done. Here is another update:
The second half of the data acquisition has gone very smoothly, with no major difficulties with equipment or weather. Twelve OBSs and the streamer were redeployed for the last half of the data acquisition. The streamer work succeeded in making it better balanced and easier to control and except for a few noisy traces and some other noise problems, it has behaved well towing both into the sea and with it. The last four of the eight strike lines were acquired in calm seas, and from initial examination the data looks reasonably good. After the strike lines, our initial plans were to shoot the three remaining (out of four) dip lines. At the end of the shooting the strike lines, however, we were ahead of schedule and planned an extra dip line. The weather remained calm for the rest of the dip line shooting, and we were able to finish all of the planned dip lines as well as the extra one. The streamer and airguns have now been recovered, and we are steaming to pick up the first of the 20 OBSs.
During the second half of the cruise we have acquired 1150 km of multichannel data, making a total of 1850 km for the whole cruise. MCS system failures throughout the cruise were limited to an occasional missed shot, and one period of ~15 minutes when the recording system crashed. OBS recovery should be complete by the morning of April 1, and we will head for Norfolk.
We are now all done except for the boat ride home. We picked up the OBSs without any problems and are steaming for Norfolk. Below is a final cruise report that fills in some of the details. See you back at UTIG.
The final OBS pickup has gone very smoothly and the data acquisition is over. The 20 OBSs deployed for the second half of the experiment were all recovered successfully. The recovery went very smoothly with all OBSs popping up nearly on time except for one. Dead batteries on one OBS caused it to default to the backup timer and delayed resurfacing for 45 minutes. However, the batteries on this instrument lasted long enough to record for the duration of shooting. All of the OBSs recovered in the second half of the cruise appear to have recorded data successfully.
For the cruise as a whole we were very successful. We acquired 1850 km of seismic reflection data, more than we planned, and deployed 32 OBSs. Of the 32 OBSs, all were recovered sucessfully and only one appears to have had instrument problems that make it unusable. Currently we are headed for port on very calm seas and making 12.5 kts. We are scheduled to arrive in Norfolk, VA early morning April 7th. -- Nathan