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Themes visible in the above map: TRACKLINES PGC97007, CORE
SITES PGC97007, LATITUDE/ LONGITUDE and LAND. The SAMPLE field
for CORE SITES PGC97007 contains the core sample numbers. The
position of the each core can be found in the LATITUDE and LONGITUDE
field. For TRACKLINES PGC97007, the LINE field contains the trackline
numbers.
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Data Report for CCGS John
P. Tully PGC97007 - Leg I August 5-16, 1997
David C. Mosher
PGC97007 consisted of a grid of lines,
largely oriented northeast-southwest, of 850 line-km of single
channel seismic reflection data, and 26 piston and vibrocores.
It was planned to augment data collected during PGC96006. Single
channel seismic reflection and Huntec DTS high resolution subbottom
profiles were collected simultaneously over the eastern Juan de
Fuca Strait (Tables 1-4). The
seismic source consisted of two Bolt 10 cu.in.
airguns suspended in a frame, 0.5 m apart and towed at a depth
of 0.5 m depth and 15 m behind the stern of the ship. Airguns
were pressurized to 1850 psi and fired every 4 m, based on differential
GPS positioning. A single EDO model 141B hydrophone cartridge
was deep-towed below the airguns to monitor every shot point signature.
The relative timing of the firing of the two guns was controlled
by a 3-channel Bolt firing unit, permitting 0.1 ms accuracy in
firing delay. The shotpoint source signature was monitored on
a Zonic model 3525 spectral analyzer. The shot point hydrophone
and analyzer provided real-time display of the source signature
in time and frequency domains. The source signature was then tuned
to maximize the outgoing pulse for shape and frequency content
by adjusting the relative firing time of the two guns.
Two hydrophone streamers were used
to acquire the single channel seismic reflection data. 1) A Benthos
array, which is oil-filled and consists of a single group of 50
elements with 15 cm spacing. It was towed just below the surface,
30 m behind the ship; and 2) A Teledyne array, which has a 25
m active section with 50 hydrophone cartridges. It was towed at
3 m depth, 50 m behind the ship. Signal from these arrays and
the single cartridge shot point hydrophone were displayed as analogue
hardcopy on an EPC 9800 chart recorder, logged to a SONY
DAT recorder and digitized at 100 ms for a length of 1000 ms on
EXABYTE tape with a MUSE® digital sonar acquisition
system.
The Huntec Deep Tow System employs
a boomer source with an internal and external hydrophone receiving
array. The boomer source generates a broad band pulse with a centre
frequency of about 2.5 kHz, but spans 0.5 to 6 kHz relatively
cleanly. The system was run at 1.75 kJoules output. It is deep-towed
between 30 and 60 m below the surface in this survey area. Deep
towing the system puts the source and receiver closer to the seafloor
and away from surface heave. The system is depth and heave compensated.
The internal hydrophone is a single cartridge suspended from the
boomer plate and is best for high frequency acquisition. The external
array is a 4.6 metres long, 10-element oil filled streamer towed
from behind the fish. It acquires a broader frequency band than
the internal hydrophone, but is subject to heave and more external
noise. The internal and external signals were displayed in analogue
hardcopy on an EPC9800 chart recorder, recorded on a SONY
DAT 8-channel recorder and were digitized at 40 s for a record
length of 250 ms (deep-water delays were used to keep the record
length short) and written to EXABYTE tape with a MUSE®
digital sonar acquisition system. This sample interval yields
a Nyquist frequency of 12.5 kHz. The boomer source was fired at
every 1.0 m, but was interrupted for 1 second during firing and
acquisition of the seismic data.
Reference citation:
Mosher, D.C., 2000. Data Report for
CCGS John P. Tully PGC97007-
Leg I August 5-16, 1997, in: Mosher, D.C. and Johnson, S.Y. (Eds.),
Rathwell, G.J., Kung, R.B., and Rhea, S.B. (Compilers), Neotectonics
of the eastern Juan de Fuca Strait; a digital geological and geophysical
atlas. Geological Survey of Canada Open File Report 3931
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