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Themes visible in the above map: USGS TRACKLINES 1995, LATITUDE/LONGITUDE, COASTLINE and LAND.

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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123°45'

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