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USGS Open-File Report 03-474, G1-03-GM, Gas Hydrates Cruise Report, R/V Gyre, 1-14 May 2003, Northern Gulf of Mexico


1. Navigation

Primary navigation for G1-03-GM was by Differential Global Positioning System (DGPS), from a Communications System, Inc. (CSI) DGPS Max receiver that utilized wide area augmentation system (WAAS) corrections. YoNav software (developed by the USGS, version 3.14) logged the DGPS positions together with the gyro-compass heading and water depth, provided a map display of position, distributed the navigation to other acquisition and display systems, and output a shot trigger for the seismic source. A separate computer off the YoNav server provided a graphical monitor to assist bridge steering along tracklines. Features included in YoNav are cross-track distance off line, distance to go, distance along line, speed, and heading. The DGPS antenna was installed in an open area on the bridge deck and was measured to be a horizontal distance of 26 m from the stern of the vessel. Mike Boyle and Larry Kooker were primary YoNav and DGPS technicians. Photographs of the navigation system are shown in Appendix 6 (Photos 4-6).

2. Multichannel Seismics

The components for the multichannel seismic system consisted of the source, the receiving array, and the digitizing and recording PCs.

Two sources were used: a Seismic Systems, Inc. 15 in3 water gun (operated at 2000 psi pressure), and a Seismic Systems, Inc. Generator-Injector (GI) gun (operated at 3000 psi pressure). A Bauer, 4-cylinder, 50-SFPM (standard cubic feet per minute) diesel compressor provided the high-pressure air for the guns. The GI gun is a dual-chamber air gun designed to minimize the bubble pulse. The "injector" chamber of the GI gun is timed to discharge a short time (typically 20-30 msec) after the "generator" chamber so as to suppress the bubble pulse and create an optimal signal. It was used with chamber inserts for a 13/13-in3 configuration (i.e., 13 in3 generator chamber, 13 in3 injector chamber) for most of the cruise and was fired at 20-m intervals. The water gun was used for a test line at the beginning of the cruise and could be fired at 10-m shot intervals because it has only a single chamber. The GI gun with a 24/24 in3 chamber was used for 4 lines in the middle of the cruise, but the larger source size could only be fired at longer space intervals (30 m). The larger source size was judged not to compensate for the lower fold stacking in the processing, and therefore the smaller chamber was reinstalled.

The GI gun was towed off the starboard stern 24 m aft of the stern and 50 m aft of the GPS antenna. It was suspended 1 m beneath the surface by a towing harness attached to a large inflatable buoy. Firing was by distance (10-m shots for the water gun, 20-m shots for the 13/13 GI gun; 30-m shots for the 24/24 GI gun). The firing pulse generated by YoNav went to a SEAMAP Seislink seismic interface box, then into a Sureshot computer system (version 3.06) which enabled optimizing firing between the generator and injector chambers. Hal Williams and Walt Olson were responsible for operation of the guns and compressor.

The receiving array consisted of an Innovative Transducers, Inc. solid-core 24-channel, 240-m array. Each channel had 3 "thin-film" cylindrical hydrophones of polyvinylidene fluoride (PVDF) plastic. Channel spacing was 10-m. A lead in section 54-m long was used. This yielded an offset of 30 m between the GI gun and the nearest channel. A polyform float at the end of polypropylene line formed the tail buoy. The streamer was weighted to tow about 1-m beneath the surface. At the beginning of the cruise, channels 16 and 18 were known to be dead. During testing, channel 24, which was at first intermittent, then also ceased working. In bad weather conditions, channel 23 was often too noisy to be used. The analog signals sensed by the streamer were brought into the StrataView acquisition system in the lab via a deck leader.

A Geometrics StrataView unit served as the multichannel data acquisition system. Location information from YoNav together with the 24-channel data were recorded in SEG-D format on 4 gigabyte Sony DDS 4-mm tape cartridges. Data were digitized and recorded at a sample interval of 0.5 ms. Record lengths were 4 s with a 1 s deep-water delay (i.e., 1-5 s record window) except for lines where the sea floor was shallower than 1 s. The StrataView consisted of two computers, one for data digitizing, the other with a graphical user interface for quality control and recording parameter selection. The at-sea display to monitor data quality consisted of multiple windows showing in real-time the near trace (generally the second nearest channel), the time between triggers (about 10 s for the small GI-gun configuration), a shot gather enlarged to show the water bottom return, a display of noise on all 24 traces, and various header information. The near trace monitor was printed at the end of every line showing ffid (field file identification) number and gain settings. These near trace plots provided an initial glimpse at the seismic stratigraphy along each line. The ffid number was generally kept sequential on all lines on one tape, and reset to 1 at the beginning of a new line on a new tape. Larry Kooker and Mike Boyle had primary responsibility for the multichannel acquisition system. Photographs of the multichannel seismic instrumentation are shown in Appendix 6 (Photos 7-15).

3. Bathymetry

The hull mounted 3.5 kHz transducer mounted beneath the water line on the bow of the Gyre provided the signal for the bathymetric record. This was triggered by a Knudsen 320B/R fathometer system with an external display showing gated windows (generally in 200-m increments). Recording was done directly to shared disk using the naming convention of "Line Number"_LF_000.sgy, where "Line Number" was manually entered at the start of every line. The "000" designator augmented when multiple files were written for each line. A new file automatically started each time a setting on the Knudsen was changed (e.g., to change the depth display range). Hence lines in changing water depth often had many files. The firing interval was every 2 s (approximately 4 m assuming 4 kt vessel speed). The sampling interval was 40 microseconds (25 kHz), and only the gated window in the monitor display was recorded.

Along lines when sea conditions were calm and the water-bottom return showed sufficient signal-to-noise ratio to enable the Knudsen's automatic water bottom picking algorithm to work, a digital depth reading was sent to the YoNav navigation recording computer to be logged with position in the navigation files. During these periods of relatively calm seas, the 3.5 kHz chirp record showed up to 80 m of sub-bottom imaging. Tom O'Brien, Mike Boyle, and Larry Kooker set up and tuned the Knudsen bathymetry system. Photographs of the Knudsen bathymetric system are shown in Appendix 6 (Photo 16).

4. Single Channel Seismics

Two single channel seismic systems were brought on the cruise, with the intent of using the one that provided the highest quality data. These were the EdgeTech Full Spectrum Sub-bottom (Chirp) Profiler and the Huntec Deep Tow System (DTS). Neither of these single channel systems were used during the cruise, after testing the first day in calm seas showed that neither system was achieving significant subbottom penetration in the 1300-m water depths of the test line. If these systems had been used, separate digital acquisition would have occurred on a Delph Seismic acquisition system. Graham Standen served as the primary technician for the Huntec DTS and Tom O'brien was the primary technician for the Edgetech profiler. Photographs of the Edgetech and Huntec systems are shown in Appendix 6 (Photos 19-25).

5. Local Area Network (LAN)

In order to facilitate sharing of data between computers and disks, a local area network was set up on the Gyre. This consisted of a 24-port Netgear network switch in the main lab, into which the various main-lab acquisition systems and computers connected. This switch also provided two gigabit Ethernet interfaces to additional network switches in the seismic processing lab and the GIS lab, both one deck above the main lab. The various computers in those labs connected into the network via those switches. Connectivity was via 10/100 megabit interfaces.

The backbone of data storage was supplied on two Snap Servers (each providing 320 gigabytes of disk space). One server was dedicated storage for the multichannel seismic data. The other served for the storage of the Knudsen data and other cruise needs (e.g., navigation files, cruise maps, and other digital files). A photograph of the LAN system is shown in Appendix 6 (Photo 17).

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Title Page / Abstract / Introduction / Acknowledgments / Geologic Settings / Cruise Objectives / Cruise Strategy / Instrumentation / Data Processing / Data Handling / Marine Mammal Mitigation / Operational Summary / Meeting Objectives Success / Summary / References / Appendices
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