Data Report for Cruise
G3-95-PS, June, 1995
Samuel Y. Johnson, Shawn V. Dadisman, Jonathan R. Childs,
and Susan B. Rhea
In June of 1995, the U.S. Geological
Survey collected over 700 km of high-resolution, multichannel
and geopulse seismic-reflection data across the submerged portions
of known and proposed upper crustal faults in the eastern Juan
de Fuca Strait, and adjacent inland waterways of northwestern
Washington. Cruise G3-95-PS was designed to image the upper 1
km of active or possibly active fault zones. A reconnaissance
scale survey, within U.S. waters, at roughly 5-km or greater spacing
was completed over the Devils Mountain fault, the southern Whidbey
Island fault and potential fault zones offshore of the northern
Olympic Peninsula.
Cruise G3-95-PS was conducted aboard
the R/V Robert Gray, a 120-ft research vessel, operating 24-hrs/day.
The multichannel data source consisted of two Bolt air guns with
0.655 liter (40 in3) chambers,
fired at a 12.5-meter interval. At speeds of 3.5 knots or less
the 12.5-meter fire interval could be maintained at full capacity
resulting in common-depth-point (CDP) data that were 6-fold, with
a 3.125-meter CDP interval. These data were received by a 24-channel
(6.25-meter group interval; 150-meter active length) solid-core
streamer manufactured by Innovative Transducers, Inc (ITI). The
seismic data were recorded with a Geometrics Strataview seismograph
recorder, which uses a PC-based, floating point computer system.
Field sampling rate was 1 ms, and field record length was 2048
ms.
Navigation fixes were based on a single
Global Positioning System (GPS) receiver. Because this GPS system
was not operational in differential mode, absolute position accuracy
was approximately + or - 100 m. However, relative positions from
fix to fix, are accurate to within a few meters. Shots were fired
at 12.5-meter intervals based on relative GPS fixes.
Data were originally recorded in
SEG-2 format but were converted to SEG-Y formatted files at sea.
These data were then processed using a Landmark Graphic Promax
processing system. Determination of accurate normal moveout (NMO)
velocity was difficult because very little NMO was observed due
to the limited offset and the low fold of the data. After extensive
velocity analysis, the decision to apply a standard velocity function
to all the data was made. The standard function was derived from
velocity functions applied to conventional industry seismic reflection
data collected in the region. The function was modified for differences
in the waterbottom travel time for each line. These data were
then deconvolved and filtered before and after stack, then time-migrated
using a smoothed velocity function. Data is typically of moderate
to high quality in the upper 1 second and degrades significantly
between 1 and 2 seconds.
Higher frequency geopulse data (1-2000
Hz) were collected concurrently with the multichannel data along
most of the survey lines. The geopulse system employs a "boomer"
transducer towed on a sled at the sea surface providing 105 joules
per shot. The reflected energy was received by two different streamers
and recorded on separate channels by an USGS-developed, PC-based,
two-channel, seismic recording system. The Benthos streamer contains
30 elements over 5 meters and the ITI streamer contains 12 elements
over 15 meters. Both streamers were positioned parallel to the
boomer sled. The sample interval of the data was 0.25 ms and the
total record length was 500 ms. The fire rate was every 0.5 s
which resulted in a shot spacing of about 1 m based on survey
speeds of 3.5-4 kts. The data quality is highly variable due to
local non-reflective and(or) gassy sub-bottom sediments. Maximum
penetration is about 150-200 ms.
Reference
citation
Johnson, S.Y., Dadisman, S.V., Childs, J.R., and Rhea, S.R.,
2000. Data
Report for Cruise G3-95-PS, June, 1995, 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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