To the Cruise Report for R/V Longhorn Cruise No. 741.

Cruise Report, R/V Longhorn Cruise No. 736

We conducted a BBOBS (broadband ocean-bottom seismograph) test/student cruise on R/V Longhorn, Cruise No. 736, from July 14 to 16, 1999 in the northwestern Gulf of Mexico. The cruise was the first phase of a two-cruise program with dual purposes: (1) to test a prototype BBOBS we are developing at UTIG and (2) to give students an opportunity to have shipboard experience on marine geophysical research.

During this first cruise (the deployment phase), we first surveyed a preselected area on the mid-slope off the Texas coast, approximately 150 nautical miles ESE of Port Aransas, using a 3.5kHz echo sounder to find a location sufficiently flat to accommodate the instrument [unlike our regular OBSs, the prototype BBOBS is not mounted on gimbals, and thus cannot tolerate a slope greater than about 5 degrees], then deployed the BBOBS, and after the instrument had settled on the sea floor, shot two short crossing seismic lines over the instrument using a small air gun for the purpose of locating and orienting the instrument on the sea floor. The instrument was left on the sea floor, to be recovered during the second cruise (the recovery phase), which is scheduled for August 11-13, 1999.

This test site was selected based on the following considerations: (1) The area has sedimentary evidence of recent structural activity, likely to have been caused by tectonically active salt domes in the area [personal communication, Bill Behrens; also, Satterfield and Behrens, 1990]. (2) It is near one of the teleseismically determined earthquake epicenters in the central and northern Gulf of Mexico [Frohlich, 1982] - a shallow event of magnitude 3.2 on October 4, 1967 located at 27°N, 94°W. (3) It is on the mid continental slope, deep enough not to worry about the possibility of the instrument being caught in a fishing net. (4) It is within a day's transit from Port Aransas.

The cruise started with a transit over a relatively rough sea, which caused many of the participants to feel sick. However, by the time we reached the test site, the sea condition improved, and the rest of the cruise went smoothly. Every one of the scientific party contributed positively, and with the competent help from the crew of the Longhorn, we accomplished all tasks we planned without delay.

One problem we did not anticipate was that the ship's navigation data were not recorded digitally. As a result, we had to enlist students' help to manually write down every key location (e.g., shot coordinates) on paper. We hope that this situation will be corrected in the near future.

The instrument left on the sea floor at the test site continues to record seismic data until 12:00 CDT (17:00 UTC) of August 12, 1999, and then will wait for an acoustic release signal. If no recall signal is received by 00:00 CDT of August 25 (midnight of August 24), it will then release itself from the sea floor. Our plan for the second cruise, therefore, is to go back to the site and fire another series of air-gun shots over the instrument shortly before 12:00 CDT of August 12, and send out release signals to call it back to the surface for recovery. Since the weather condition in the Gulf is unpredictable at this time, we have the following contingency plans: a) If the weather condition in the Gulf prevents us form going back to the site as scheduled, we will have the following 12.5 day window to try an acoustic release. b) If we are unable to recall the instrument during this time period, we will go back to the site shortly after midnight of August 24 to retrieve the instrument. c) If the weather condition prevents us from following plan b, we will go back to the area as soon as the weather clears to search for the instrument.

References
Frohlich, Cliff, Seismicity of the central Gulf of Mexico, Geology, 10, 103-106, 1982.
Satterfield, Will M. and E. W. Behrens, A late Quaternary canyon/channel system, northwest Gulf of Mexico continental slope, Mar. Geol., 92, 51-67, 1990.

Participants
Scientific Party:

     Yosio Nakamura - Chief Scientist
     Jay Pulliam - Co-Chief Scientist (8-12)*
     Ben Yates - Engineer
     Judy Sansom - Accountant (4-8)
     Tom Hughes - Science Teacher (12-4)
     Nathan Andrews - Student (12-4)
     Mike Bradley - Student^G (4-8)
     Mike Edmonds - Student (4-8)
     Tom Fenstemaker - Student^G (12-4)
     Ben Phillips - Student (8-12)
     Emily Wharton - Student (8-12)

*hours of watch assignment in parentheses
^Ggraduate student

Ship's Crew
     Noe Cantu - Captain
     Haydon Abel - Mate
     Randy Pritchard - Mate
     Stan Dignum - Mate
     Chuck Rowe - Marine Technician

Chronology
In the following narrative, all times are in CDT, which is 5 hours behind UTC.

Tuesday, July 13 - First group of seven left UTIG shortly after 6 a.m. in UTIG pickup truck, loaded with equipment (B.Y. and J.S.), and in a DGS van (Y.N., M.B., M.E., T.F and E.W), and arrived at UTMSI in Port Aransas shortly before 11 a.m. Unloaded equipment from the truck and setup on board R/V Longhorn. With a help of Archie Roberts, tested air gun in water. Archie Roberts left Port Aransas around 3 p.m. to return home. Started BBOBS main clock at 6:04:20 p.m. Put OBS chassis in a sphere and sealed the sphere at about 10 p.m. Second group of four (J.P., T.H., N.A. and B.P.) arrived at UTMSI shortly after 10:30 p.m. All stayed on board for the night. No running water on board.

Wednesday, July 14 - Conducted a safety instruction and fire drill at 6:15 a.m. Sailed from the MSI dock at 07:48 a.m. At 09:57, arrived at oil rig F1 MU 754-A at 27°44.93'N, 96°46.10'W and released two loggerhead turtles, as requested by Tony Amos of MSI. Encountered a rough sea; and as a result, postponed planned lectures on earthquake and marine seismology till later.

Thursday, July 15 - Arrived at the test area shortly after midnight. Conducted 3.5k bathymetric survey from 00:55 to 04:20. Selected a BBOBS deployment site at 27°18.5'N, 94°23.5'W. Reprogrammed BBOBS with the final recording/release schedule. Deployed BBOBS at 08:05:30. Deployed air gun, brought pressure to 1500 psi, and test fired, completing the test shortly after 11 a.m. From 12:20:00 to 14:36:30, shot two cross lines over the instrument, manually at 90 second intervals and average speed of 3.7 knots. Retrieved air gun and departed the test site shortly before 3 p.m. to return to Port Aransas. After the dinner, Jay gave a 15-minute lecture on use of BBOBS to explore the earth's deep interior.

Friday, July 16 - Returned to the MSI dock at 7:18 a.m. Unloaded equipment, cleaned up staterooms and labs, and left Port Aransas around 8:30 a.m. Returned to Austin shortly before 1 p.m.

Technical Details
Major equipment used:
     OBS chassis: UTIG S/N 93-4
     Sphere: Benthos S/N 62109 with PMD Model 2123 broadband seismometer
     Submersible transmitter: Novatech Model RF-700A-1, S/N E08075
     Hydrophone: OAS Model E-2DP, S/N 17
     GPS receiver: TrueTime Model GPS-TMD, S/N 97232400
     Acoustic deck unit: Benthos Model DS-7000, S/N 137
     Compressor: MAKO Engineering Model K-51, S/N 5/615/4, 5.1 CFM (borrowed from MBI/UTMB)
     Air gun: Bolt Model 1900C, S/N 976, 60 in3 chamber volume
     Echo sounder: Knutsen Model 320M, 3.5k (on-board equipment)

Recording parameters:
     Active sensor channels:1, 2 and 3

0.03-30 Hz Channel 2 PMD horizontal 1, 0.03-30 Hz Channel 3 PMD horizontal 2, 0.03-30 Hz

     Nominal sensitivity of sensors (unit digitizing level):2.6 nm/s

Key clock calibrations:

BBOBS deployment coordinates:
     27°18.36'N, 94°23.49'E
     Water depth: 1478 m (uncorrected; 1500 m/s sound speed assumed)

Parameters related to the sound source and navigation:

*Complete shot times and coordinates are available in files shottimes.b1 and ShotCoord.b1, respectively, in utig subdirectory /disk/obsdata1/bbobs.

Fig. 1. Ship's track of R/V Longhorn cruise No. 736. The orange diamond indicates the location where the two loggerhead turtles were released, and the blue diamond indicates the location where the BBOBS was deployed. The small orange rectangle around the BBOBS deployment site shows the area covered by Figs. 2 and 3.

Fig. 2. Raw data plot of 3.5k bathymetry survey. The depths are given in meters assuming water-wave speed of 1500 m/s.

Fig. 3. Shot locations (small orange dots) and BBOBS deployment site (black dot), overlaid on depth contours as obtained from the 3.5k survey (Fig. 2).