Deep-towed Chirp Profiles of the Blake Ridge Collapse Structure Collected on Aboard the R/V Cape Hatteras in 1992 and 1995, USGS OFR 01-123

This study was completed as part of an ongoing project in the field of
natural gas hydrate research. Natural gas hydrates are an ice-like
crystalline combination of water and gas, most commonly methane.
The data included in this report were collected in an effort to understand
a site where we believe large quantities of methane, approximately 4% of
the present atmospheric total, was released from seafloor sediments. This
site is known as the Blake Ridge collapse structure, located 300 km off
the South Carolina coast at approximately 2600 m of water depth.

This CD-ROM contains copies of the navigation and deep-towed chirp
subbottom data collected aboard the R/V Cape Hatteras on cruises 92023 and
95023 in 1992 and 1995 respectively. This CD-ROM is (Compact Disc-Read
Only Memory UDF (Universal Disc Format) CD-ROM Standard (ISO 9660
equivalent). The HTML documentation is written utilizing some HTML 4.0
enhancements. The disk should be viewable by all WWW browsers but may not
properly format on some older WWW browsers. Also, some links to USGS
collaborators and other agencies are available on this CD-ROM. These links
are only accessible if access to the Internet is available during browsing
of the CD-ROM.

On cruise 92023, 58 km of deep-towed chirp data were recorded on 4 lines
and broken into a total of 8 files. 78 square kilometers of sidescan
mosaic and approximately 1000 km of air gun single channel seismic
reflection data were recorded as well but are not achived on this report.
On cruise 95023, 100km of deep- towed chirp data were recorded on 5 lines
and broken into 18 files. 152 square kilometers of sidescan mosaic and
244.3 km of GI gun single channel seismic reflection were also recorded
but are not archived on this report.

The archived Chirp subbottom data are in standard Society of Exploration
Geologists (SEG) SEG-Y format (Barry and others, 1975) and may be
downloaded for processing with software such as Seismic Unix or SIOSEIS.
The subbottom data were recorded on the ISIS data acquisition system in
QMIPS format. Chirp subbottom channel extracted from raw QMIPS format
sonar files and converted to 16-bit Int. SEG-Y format using the program
QMIPSTOSEGY. Even though the data are in SEG-Y format, it is not the
conventional time series data (e.g. voltages or pressures), but rather
instantaneous amplitude or envelope detected and therefore all of the
amplitudes are positive (though not simply rectified).

Seismic reflection profiles are acquired by means of an acoustic source
(usually generated electromagnetically or with compressed air), and a
hydrophone or hydrophone array. Both elements are typically towed in the
water behind a survey vessel, or some cases, mounted on side of the hull.
The sound source emits a short acoustic pulse, which propagates through
the water and sediment columns. The acoustic energy is reflected at
density boundaries (such as the seafloor or sediment layers beneath the
seafloor), and detected at the hydrophone. As the vessel moves, this
process is repeated at intervals ranging between 0.5 and 20 meters
depending on the source type. In this way, a two-dimensional image of the
geologic structure beneath the ship track is constructed. For more
information concerning seismic reflection profiling at the USGS Woods Hole
"http://woodshole.er.usgs.gov/operations/sfmapping/"