Seismic-Reflection Surveys of the Blake Ridge, 1992
and 1995
INTRODUCTION
As part of an ongoing study, seismic-reflection profiles were collected over the Blake Ridge in 1992 and 1995, in order to map the volume and distribution of methane hydrate. Faulting and seafloor instabilities appear to be related to methane hydrate processes at the Blake Ridge. Seismic profiles display a prominent collapse structure at the crest, which is inferred to have resulted from the mobilization of sediment that was associated with methane hydrate dissociation.
.
PROJECT DESCRIPTION AND PURPOSE
In 1992 and 1995, the U.S. Geological Survey (USGS), in
cooperation with the University of North Carolina at Chapel Hill, Woods Hole Oceanographic Institution and the U.S. Naval Research Laboratory, conducted three scientific cruises to the Blake Ridge, offshore South Carolina. Funding for the ship operations was provided by the U.S. Department of Energy, Office of Naval Research, and the National Science Foundation. These
expeditions were part of a project whose purpose was to decipher the volume and distribution of methane hydrate contained in the ocean sediments of the Blake Ridge. These expeditions had two primary objectives: (1) to
obtain seismic-reflection profiles of selected areas of the
ridge, for interpretation of the deep structure, and; (2) to obtain high frequency
(sidescan sonar) data to correlate the deeper structure with the bathymetric expression of the collapse feature.
The seismic-reflection profiles were collected to image the sedimentary section where gas hydrate is stable (the base of gas hydrate stability (BGHS) is approximately 500 meters below the seafloor (mbsf)). Of equal importance was the need to identify gas accumulations below the BGHS.
The faulted region of the seafloor collapse was discovered on seismic profiles collected on 1992. This led to the collection of deep-towed sidescan sonar imagery in 1995, to allow surficial mapping of the collapse feature.
All seismic, navigation, and related equipment were provided by the
USGS. All three expeditions were conducted aboard the R/V Cape Hatteras. The purposes of these CDROM's are to provide the data for use by other scientists, to describe the data collection and processing methods, and to
present examples of the data.
.
DATA COLLECTION
The 1992 data set consists of profiles oriented dominantly normal to the axis of the Blake Ridge. Work in 1995 consisted of collecting deep-towed sidescan imagery and seismic reflection profiles of the collapse structure identified in the 1992 data. A walk-away vertical seismic profile experiment was also performed in 1995 by placing hydrophones in drill holes created by the Ocean Drilling Program.
The unprocessed seismic profiles on this CDROM were collected as standard SEG-Y data (Barry et al., 1975) using moderately small, high-pressure, pneumatic seismic sources to penetrate below the BGHS, while maintaining the highest possible stratigraphic resolution. Profiles collected in 1992 used a 160-cubic-inch air gun, whereas profiles collected in 1995 (disk 2) employed a generator-injector (GI) gun (Seismic Systems Inc.) as the seismic source. In the latter, the generator chamber (primary signal) was 105 cubic inches and the injector chamber (controls bubble pulsing) was 105 cubic inches. Data were collected digitally with a 2.0-milliseconds (ms) and 0.5-ms sampling rate for the 1992 and 1995 datasets respectively.
Sidescan sonar were collected utilizing a Datasonics SIS-7000 30 kHz CHIRP (swept frequency) system. Port and starboard sonar transducers, which sweep from 26-33 kHz, were triggered at a 1 second rate (60 ms pulse width) which provided a swath width of 6000 meters for the entire survey.
.
DATA PROCESSING
Seismic data processing included band-pass filtering, predictive deconvolution, true-amplitude recovery and memory-stolt migration. Reverberations were prominant in the upper section of the profiles and had to be removed by applying deconvolution twice. The sonar mosaic stored on this CD-ROM was constructed from the raw tape files using the USGS Xsonar sonar processing package (Danforth, 1997), which is available via anonymous ftp at boomer.er.usgs.gov (look in pub/sonar for details). The sonar data were processed using standard sidescan sonar processing schemes (see Danforth, 1997, Fish and Carr, 1991, Paskevich, 1992a).
.
Acknowledgements
We thank those who have helped in the preparation of this CDROM. D. Blackwood and J. Zwinakis for preparation of the seismic profiles; S. Colman and D. Foster for helpful advice; W. Poag and J. Robb for thoughtful reviews.
.
References Cited
Barry, R.M., Cavers, D.A., and Kneale, C.W., 1975, Recommended standards for digital tape
formats, Geophysics, v. 40, p. 344-352
Danforth, W. W., 1997, Xsonar/ShowImage: A Complete System for Rapid Sidescan Sonar Processing and Display. U.S. Geological Survey Open-File Report 97-686, 77 p.
Fish, J.P. and H.A. Carr, 1991, Sound Underwater Images: A Guide to the Generation and Interpretation of Side Scan Sonar Data. Lower Cape Publishing, Orleans, MA, 189 p.
Paskevich, V., 1992a, Digital Processing of Sidescan Sonar data with the Woods Hole Image Processing System Software. Open-File Report 92-204, 9 p.
[an error occurred while processing this directive]
|
