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High-resolution seismic-reflection surveys (Mosher and Simpkin, 1999) have been shown to be important tools in interpretation of active faults (for example, Barnes and Audru, 1999; Johnson, and others, 1999). Our investigation of active structures in the northern Puget Lowland and eastern Strait of Juan de Fuca relies heavily on a network of multichannel seismic-reflection data collected in 1995 by the U.S. Geological Survey (USGS) and in 1996 by the Geological Survey of Canada (G.S.C.). Collectively, we acquired several hundred kilometers of data in the eastern Strait of Juan de Fuca and Skagit Bay (figure 5, on pl. 1), crossing the Devils Mountain fault 15 times and the Strawberry Point and Utsalady Point faults 6 times.

For the 1995 USGS survey (figure 5A), the seismic source consisted of two 0.655-L (liter) (40 in.3) airguns fired at 12.5-m intervals. Data were digitally recorded for 2 s using a 24-channel (6.25-m group interval; 150-m active length) streamer. Resulting common mid-point (CMP) stacked data are six-fold and have a 3.125-m CMP spacing. Profiles were located using GPS satellite navigation with an accuracy of ± 100 m. Relative locations from fix to fix are accurate to within a few meters. These data were deconvolved and filtered before and after stack, then time-migrated using a smoothed velocity function. Deconvolution was only partly effective in suppressing water-bottom multiples.

For the 1996 G.S.C. survey (figure 5C), the seismic source consisted of a 0.655-L (40 in.3) sleeve airgun fired at 16-m intervals, and data were digitally recorded for 1 s using a 24-channel (8-m group interval, 192-m active length) streamer. GPS satellite navigation with an accuracy of ±20 m was used to locate profiles. CMP stacked data are four-fold and have a 4-m CMP spacing.

Both USGS and G.S.C. data are typically of high quality in the upper approximately 0.5 – 0.7 s and degrade significantly with greater depth. On both the USGS and G.S.C. cruises (figure 5A, C), higher frequency seismic-reflection data were collected concurrently with the airgun data. The U.S.G.S. used a geopulse boomer source and recorded data for 500 ms on a single-channel streamer. The G.S.C. used a Huntec hydrosonde deep tow boomer system and recorded data for 250 ms on a single "internal" hydrophone and a short external streamer trailing behind the towed source. The quality of these higher-frequency data is highly variable due to local nonreflective and (or) gassy sub-bottom sediments. Maximum penetration is about 150-200 ms (120-160 m).

The U.S. Geological Survey, the Geological Survey of Canada, and several academic institutions collaborated in 1998 on a deeper multichannel seismic-reflection survey, "Seismic Hazard Investigation of Puget Sound” (SHIPS; Fisher and others, 1999), that traversed the eastern Strait of Juan de Fuca (figure 5B). These data were collected using a 79.3-L (4,838 in.3) airgun array and recorded on a 2.4-km-long, 96-channel streamer. To enhance the shallow (< 10 km) geologic section for the purposes of this report, the near-offset 48 channels of these data were stacked (24-fold, 12-m CMP) and velocity migrated. We also examined several proprietary industry seismic-reflection data (3 to 5 s records) profiles from the eastern Strait of Juan de Fuca (figure 5B), most of which were collected in the late 1960’s or early 1970’s with airgun sources of various sizes, when the area was considered a frontier for petroleum exploration. Examples of these industry data included in this report have been stacked but not migrated.

Contact: Susan Rhea
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