To the Cruise Report for R/V Longhorn Cruise No. 736.
R/V Longhorn Cruise No. 741 took place from August 11 to 13, 1999 in the northwestern Gulf of Mexico. The cruise was the second phase of a two-cruise program with dual purposes: (1) to test a prototype BBOBS (broadband ocean-bottom seismograph) we are developing at UTIG and (2) to give students an opportunity to have shipboard experience in marine geophysical research.
During this second cruise (the recovery phase), we first repeated shooting of two short crossing seismic lines over the instrument with a small air gun to locate and orient the instrument on the sea floor, and then sent an acoustic signal to release the instrument from its anchor on the sea floor and to retrieve the instrument when it surfaced. We also conducted a 3.5kHz echo-sounder survey to investigate the shallow sedimentary structure during the transit to and from the BBOBS deployment site.
The cruise went relatively smoothly, although we did encounter some large swells, which forced us to change courses often to avoid excessive rolling during the last day of the cruise. As usual, the ship's crew was always eager to help, and with every member of the science party contributing positively, we completed a successful cruise.
The recovered instrument contained full seismic data since its deployment on July 14 during the first cruise. A quick look at the recorded data on board showed several clearly recorded teleseismic events. It was noted, however, that the general low-frequency background noise was significantly higher than at our Hockley seismic station. The recorded air-gun signals were not very large - about the same amplitude as the background noise. However, this should not be a problem because the former are of significantly higher frequency than the latter.
The ship track for the 3.5kHz survey had been selected to fill some gaps in previous surveys in the general area between Port Aransas and the BBOBS deployment site, as requested by Hilary Olson. The data quality may be somewhat lower than optimum. This may be due to the limitation of the equipment, due to adverse sea condition, or due to inexperience of the operators.
The acquired BBOBS data were down-loaded to a SUN workstation on board and displayed as simulated drum records. Post-cruise processing and analysis of the data are being performed with student participation. They include precise location and orientation of the instrument on the sea floor using the recorded air-gun signals, identification of recorded earthquake signals, reformatting of the recorded earthquakes into standard format for further processing, and assessment of the quality of the recorded signals in comparison with those recorded at our Hockley seismic station. The latter includes characterization of the ocean floor for teleseismic observations as well as performance of a PMD seismometer in ocean-bottom environment.
Participants
Scientific Party:
Yosio Nakamura - Chief
Scientist
Ben Yates - Engineer
Peter Abel - Student (12-4)*
Nathan Andrews - Student (12-4)
John Bacon - Student (4-8)
Carrie Beveridge - Student (8-12)
Deanna Combs - Student (12-4)
Melissa Gunter - Student (4-8)
Christy Kohl - Student (8-12)
Randy Schumitz - Student (4-8)
Jonathan Skaggs - Student (8-12)
*hours of watch assignment in parentheses
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.
Wednesday, August 11 - Left UTIG shortly after 6 a.m. and arrived at UTMSI in Port Aransas shortly after 10 a.m. Setup on board R/V Longhorn. Conducted a safety instruction and fire drill at 11:30 a.m. Sailed from MSI dock at 1:05 p.m. At 4:40 p.m., slowed down to about 5 knots and started 3.5kHz echo-sounder survey at 27°42'N, 96°29'W (W.P. #1, see Table 1/Figs. 1 & 2). Reached 27°30'N, 96°00'W (W.P. #2) and resumed cruising speed at 10:27 p.m.; 3.5 kHz left on.
Thursday, August 12 - At 6:15 a.m., arrived at 27°15'N, 94°45'W (W.P. #3), about 20 nautical miles WSW of the BBOBS deployment site, and slowed down to deploy air gun at 6:50 a.m. Pressurized the air gun, first to 1750 psi, and after a brief test increased the pressure to 2000 psi and started shooting at 9:28 a.m. at about 2 km SW of BBOBS (W.P. #4), going NE towards BBOBS at about 4 knots, shooting every 2 minutes, or about 250 m. See Fig. 3 for shot locations. Completed shooting at 11:10 a.m. at about 2 km SE of BBOBS (W.P. #9). Returned to BBOBS deployment site (W.P. #10) and sent an acoustic release signal at 12:47 p.m. BBOBS surfaced at 1:21 p.m. and was recovered on board at 1:27 p.m. Continued 3.5kHz survey while working on the recovered instrument and the acquired data. At about 17:40 p.m., reached 27°24'N, 94°45'W (W.P. #11), but decided not to make a course change because of unfavorable sea condition. At 19:43 p.m., reached 17°25'N, 94°56'W (ENE of planned W.P. #12) and resumed cruising speed.
Friday, August 13 - At 02:35 a.m., reached 27°24'N, 95°48'W (W.P. #13) and slowed down to about 5 knots for additional 3.5kHz survey, although large swells degraded data quality. At 6:50 a.m., reached 27°33'N, 96°12'W (W.P. #14) and changed to a further northward course. At 8:55 a.m., reached near 27°42'N, 96°18'W (W.P. #15), but because of unfavorable sea condition decided to take a more southerly course than originally planned. At 12:44 p.m., terminated 3.5kHz survey near 27°47'N, 96°34'W (ENE of planned W.P. #16) and resumed cruising speed to head back to Port Aransas. Port engine developed problem, which kept the top speed to about 6 knots with only the starboard engine. Returned to the MSI dock at 4:45 p.m. Unloaded equipment, cleaned up staterooms and labs, and left Port Aransas at around 5:30 p.m. Returned to Austin shortly after 9:30 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:
Channel 1
Channel 2
Channel 31, 2 and 3
vertical, 0.03-30 Hz
PMD horizontal 1, 0.03-30 Hz
horizontal 2, 0.03-30 HzNominal sensitivity of sensors (unit digitizing level):
2.6 nm/s
Anti-alias filters:
10 Hz Butterworth high-pass, 24 dB/oct roll off
Sampling interval:
25.000 ms
Recording mode:
continuous with a short (~22 s) inter-record gap every 36.6 minutes
Recording started:
July 15, 1999, 12:00:00.5 CDT
Recording ended:
August 12, 1999, 12:08:49.1 CDT
Data acquired:
549.5 MBytes
Key clock calibrations:
Time, ddd:hh:mm:ss | UTC Clock error, s |
196:12:19:00 | 0.3234917 |
196:12:52:00 | 0.3240081 (Pre-deployment) |
196:13:05:30 | (BBOBS deployed) |
224:18:34:00 | 0.9121016 (Post-recovery) |
BBOBS deployment and recovery coordinates and uncorrected water depths*:
Deployed: | 27°18.36'N, 94°23.49'E | 1478 m |
Recovered: | 27°18.31'N, 94°23.46'W | 1470 m |
*Sound speed of 1500 m/s is assumed. The mean sound speed to this depth is estimated to be 1497 m/s according to the Matthews' table.
Parameters related to the sound source and navigation:
Air-gun chamber volume: | 60 cu in. (1.0 ?) |
Air-gun firing pressure: | 2000 psi (13.8 MPa) [was 1500 psi for the first cruise] |
Air-gun depth: | 3 m, estimated |
Ship's draft: | 2 m |
Source setback from navigation reference: | 22 m [was 18 m for the first cruise] |
First shot*: Time: | 224:14:28:00 UTC |
Passed over BBOBS at: | 224:14:46 UTC |
Last shot: Time: | 224:16:10:00 UTC |
Shot interval: | 120 s |
Shot spacing: | ~250 m |
**Complete shot times and coordinates are available in files shottimes.b2 and ShotCoord.b2, respectively, in utig subdirectory /disk/obsdata6/pmd/gom/stcruise.
Explanation of Attached Table and Figures
Table 1/Fig. 1. (spreadsheet/chart) Planned way points.
Fig. 2. Ship's track of R/V Longhorn cruise No. 741. The black diamond indicates the location where the BBOBS was deployed. The small gray rectangle around the BBOBS deployment site shows the area covered by Figs. 3.
Fig. 3. Shot locations (small circles) and BBOBS deployment (black square) and recovery (gray square) locations overlaid on depth contours as obtained from the 3.5k survey during the first cruise.
Fig. 4. Sample seismogram recorded on BBOBS from a magnitude 5.8 earthquake near coast of Ecuador on August 3, 1999.
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