U.S. Geological Survey
Open-File Report 02-368
Version 1.0

High-Resolution Multichannel Seismic-Reflection Data Acquired in the Northern Gulf of Mexico, 1998-99

By Patrick E. Hart, Alan K. Cooper, David Twichell, Myung Lee, and Warren Agena

2002

This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards or with the North American Stratigraphic Code. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government

Contents:
Introduction
Cruise Summaries
1998
1999
Data Processing
SEG-Y and Image Data
Positional Data
Data Display
Contacts
FGDC-compliant metadata
References

Introduction

During June 1998 and April 1999, the U.S. Geological Survey (USGS) conducted two research cruises in the northern Gulf of Mexico to acquire high-resolution seismic reflection data across the upper and middle continental slope as part of an investigation of the seismic character, distribution, and potential effects of naturally-occurring marine gas hydrates and related free gas within the gas hydrate stability zone. Over 1600 km of two-dimensional multichannel seismic reflection profiles were acquired during these two cruises. The specific objectives of this investigation are (a) to produce high-resolution images of the gas hydrate stability zone; (b) to study the distribution and character of potential seafloor failures and their relationship to known and inferred gas hydrate deposits; (c) to look at systematic variations in subsurface structure in gas hydrate and non-hydrate areas; and (d) to estimate, if possible, the amounts of hydrates present within the gas hydrate stability zone. The multichannel profiles provide high-quality images with approximately 5 meters of vertical resolution and up to 2 km of penetration. This report gives an overview of the acquisition and data processing of the multichannel seismic reflection profiles and provides references and links to reports with more detailed information. Geologic interpretations of these seismic profiles regarding gas hydrate occurrence and distribution within the study areas of this investigation are given in Cooper and Hart (2002).

Figure 1. Map showing location of the 1998 and 1999 cruises in northern Gulf of Mexico

Cruise Summaries

1998 (USGS Cruise ID: M1-98-GM)

The USGS and the University of Mississippi Marine Minerals Technology Center (MMTC) collaborated on a 12-day cruise in June 1998 in the Mississippi Canyon Region of the Gulf of Mexico, collecting data in two areas on opposite flanks of the Mississippi Canyon. The R/V Tommy Munro, owned by the Marine Research Institute of the University of Southern Mississippi, was chartered for this cruise. A detailed summary of the field operations and equipment is given in Cooper et al. (1998). Several types of seismic data were acquired during this cruise, including surface-towed high-resolution multichannel profiles (the subject of this report), deep-towed boomer single-channel profiles, and ocean-bottom seismometer data. A trackline map of the multichannel profiles acquired during the M1-98-GM cruise is shown in figure 2.

The multichannel data acquisition system used for the 1998 cruise consisted of a 24-channel hydrophone streamer, a small air-gun or water-gun source, and a PC-based digital recording system. The streamer was 240 meters in length with a 10-meter group interval (three hydrophones per group), and was manufactured by Innovative Transducers, Inc. (ITI) with solid-core construction and "thin-film" cylindrical hydrophones of polyvinylidene fluoride (PVDF) plastic. There was a 30 meter source-to-first-channel gap and the streamer was intended to be towed at a 1 to 3 meter depth although there were no depth sensors or depth control system on the streamer. A problem with a streamer connection caused the far 7 channels to be very noisy, and these channels were eliminated during data processing. The source used was either a 35/35 cubic inch dual-chamber GI air gun (lines 1-15) or a 15 cubic inch water gun (lines 17-19 and 50-75). Line 16 was acquired using a larger 105/105 cubic inch GI gun. The "injector" chamber of the GI gun is timed to discharge a short time (typically 20-30 milliseconds) after the "generator" chamber so as to suppress the bubble pulse and create an optimal signal. The small GI gun was fired at a 14-or 15-second interval (approximately 30 meters at our survey speed of 3.5 to 4 knots), the large GI gun at a 42 second interval (approximately 90 meters), and the water gun fire interval was 5 seconds (approximately 10 meters). The data were digitized and recorded in SEG-D format with a Geometrics Strataview data recorder; a compact, PC-based instrument with approximately 24-bit (120 dB measured) dynamic range. Field sampling interval was 0.25 millisecond and field record length was 2 seconds plus up to 1.5 seconds of deep-water recording delay. A differential Global Positioning System (GPS) system provided navigational accuracy of 5 to 10 meters.

1999 (USGS Cruise ID: G1-99-GM)

The USGS conducted another northern Gulf of Mexico seismic data acquisition cruise in April 1999 using the Texas A&M University Research Vessel Gyre. This 13-day cruise was a continuation of the USGS northern Gulf of Mexico gas hydrate study and was run west of the 1998 cruise area in the Garden Banks and Green Canyon regions. (Cooper et al., 1999) A trackline map of the multichannel profiles acquired during the G1-99-GM cruise is shown in figure 3.

The multichannel data acquisition equipment and parameters during G1-99-GM were the same as M1-98-GM except the 15 cubic inch water gun source was used on all lines. In addition, the recording problem with the far 7 streamer channels was corrected.

Data Processing

The SEG-Y and gif image files of the multichannel seismic data profiles (for 1998 and 1999 cruises) were created using the following seismic data processing sequence. Note that the SEG-Y files have no post-stack processing applied, but the gif image files have a post-stack filter and balance. A noise-spike editing step was applied to the 1999 data to minimize high-frequency spikes created by the concurrent acquisition of deep-tow boomer data (Huntec). The water gun data had specialized deconvolution applied, as described in detail in Lee et al., 2000.

SEG-Y and Image Data

The seismic reflection profiles and associated data are provided with this report. The profile data are written as files in SEG-Y exchange format (Barry and others, 1975).

Stacked multichannel air gun profiles: the stacked SEG-Y data are written in 32-bit IBM floating point format at a 0.5 millisecond sample interval; record length is 2.0 to 3.5 seconds including up to 1.5 seconds of deep water recording delay. The EBCDIC header for each file contains additional descriptive information as shown in tables 1 and 2.

Table 1: 1998 EBCDIC Header

SEG-Y Data and gif images for Seismic-Reflection Profiles from M1-98-GM Cruise

Table 2: 1999 EBCDIC Header

SEG-Y Data and gif images for Seismic-Refection Profiles from G1-99-GM Cruise

Each profile is available as a digital data file (SEG-Y format), and a low-resolution image (gif).

CAUTION: SEG-Y files may be very large, and therefore long to download.

Positional Data

These ASCII files contain columns listing line number, CDP number, latitude and longitude in decimal degrees and UTM coordinates CDP stands for Common Depth Point and each seismic trace along a profile has a unique CDP number and are located at 5 meter intervals. Locations are listed for every 20th to 40th CDP along each profile. Interpolation of locations between listed CDPs would be necessary to obtain coordinates for every CDP. CDP locations are not listed in the SEG-Y seismic trace headers.

Data Display

No software is provided for preview or display of the SEG-Y, image or navigational data. SEG-Y data can be displayed, printed or further processed using a variety of seismic processing programs that run on UNIX, Windows and Macintosh systems. A public-domain seismic processing package for Unix systems, SU, is available via the Internet at: http://www.cwp.mines.edu/cwpcodes/.
Gif images of the seismic lines can be opened in any of numerous graphics packages.
Navigation files can be plotted using commercial GIS software such as ARC/INFO. The Generic Mapping Tools (GMT), a public-domain mapping package for visualizing geographic vector data on Unix systems, was used to create the trackline figures, and is available via the Internet at: http://imina.soest.hawaii.edu/gmt/

Contacts

For further information regarding these datasets contact:

Patrick Hart (650) 329-5157 or Alan Cooper (650) 329-5160
U.S. Geological Survey
M/S 999 M/S 999
345 Middlefield Rd.
Menlo Park CA 94025
hart@usgs.gov or acooper@usgs.gov

References

Barry, K.M., Cavers, D.A., and Kneale, C.W., 1975, Recommended standards for digital tape formats: Geophysics, v. 40, p. 344-352

Cooper, A. K., Hart, P.E., Pecher, I., 1998, Cruise report for a seismic investigation of gas hydrates in the Mississippi Canyon region, northern Gulf of Mexico – Cruise M1-98-GM, U.S. Geological Survey Open-File Report 98-506, 25 p.

Cooper, A. K., Twichell, D., and Hart, P.E., 1999, A seismic-reflection investigation of gas-hydrates and sea-floor features of the upper continental slope of the Garden Banks and Green Canyon regions, northern Gulf of Mexico: Report for cruise G1-99-GM (99002), U.S. Geological Survey Open-File Report 99-570, 19 p.

Cooper, A. K., and Hart, P. E., 2002, High-resolution seismic-reflection investigation of the northern Gulf of Mexico gas-hydrate-stability zone, Marine and Petroleum Geology, in press.

Lee., M. W., Hart, P. E., and Agena W. F., 2000, Processing Strategy for Water-Gun Seismic Data from the Gulf of Mexico, U.S. Geological Survey Bulletin 2181

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Created September 12, 2002 (cad)
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