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The approximately 100 km of seismic survey tracklines are shown in Figure 1. While laying out the survey tracklines, a minimum depth of 25 m was used to allow for R. V. Thomas G. Thompson’s draft (Figure 15). Trackline turn angles were limited to > 50° to avoid difficult ship maneuvers, minimize hydrophone streamer drift, and facilitate a smooth transition between adjoining tracklines. Throughout the 14 hour survey, one-minute NAVSTAR Global Positioning System (GPS) fixes (estimated positional accuracy of ± 50 m) and water depth (accuracy of about 1.0% the measured depth) were recorded. One-minute GMT time fixes were annotated on the airgun seismic and 3.5 kHz sub-bottom profile paper records. The ship’s speed was maintained at approximately 4.5 knots throughout the survey resulting in approximately eight time-positional fixes per kilometer.
Figure 15: The ship of opportunity; University of Washington’s R. V. Thomas G. Thompson. The airgun sound source was extended from the fantail of this 85-meter (274-ft) long vessel while the hydrophone array trailed aft off the port side. The sea-state throughout the survey was calm and similar to the condition shown (Online, Internet, Accessed June 1, 1998; http://www.ocean.washington.edu/ships/tgt.gif).
The seismic sound source was a single 655 cm3 (40 in3) Bolt PAR 600B airgun equipped with a wave-shape kit (or ‘bubble suppresser’). The airgun was fired at 2.2-second intervals using an inlet pressure of 124 bar (1800 psi) and towed 30 m directly astern of the ship at a depth of 6 meters. An AQ-1 titanate-zirconate, 200- element hydrophone streamer (15 cm-spacing) was suspended from a 4-m-long boom extending off the ship’s port quarter. The streamer was deployed approximately 70 m astern of the vessel and maintained at a tow depth of approximately 5 meters. The low and high band signals (30-90 and 40-120 Hz) from the streamer were routed through 30 dB linear amplifiers, and Krohn-Hite band-pass filters. An EPC 9600 digital thermal graphic recorder displayed the seismic data on paper at a vertical exaggeration of 2.6 for both 1-and 2-second sweeps. A TEAC digital audio tape (DAT) recorder was used to archive the raw acoustic, trigger signal, and amplifier data.
Post-processing of the seismic reflection dataset took place from November 6-10, 1997 at the University of Washington, Seattle with Dr. Thomas L. Pratt (USGS). Raw DAT tapes were played back with the TEAC DAT recorder, re-formatted from tape analog to a digital format, re-digitized at a rate of 2000 samples/sec into seg-y format using the USGS data acquisition software program ‘MUDSEIS’, and then displayed graphically as postscript files (Figure 16A). The seismic dataset was then bandpass filtered at 10, 20, 160, 320 Hz and deconvolved (Figure 16B), gained (amplitude-balanced), and shot-point migrated (Figure 16C). A UNIX-based workstation and a two-way traveltime (TWT) of 1500 m/s were used during this process.
Computer-drawn line interpretations were completed for all 37 seismic records and included the labeling of geologic structures, erosional seismic stratigraphic unit contacts, and faulting patterns (Figure 17 and Appendix). During the interpretation process all three profiles (raw, filtered and deconvolved, and migrated; Figure 16) were used to trace seismic-stratigraphic units and to verify record multiples when necessary. A scaled fence diagram was then constructed using transparency copies of the interpreted seismic records positioned along the ship’s trackline layout (Figure 18). The model’s design allowed seismic-stratigraphic unit contacts and lateral faulting patterns to be correlated and mapped in a pseudo-‘3-D’ context. In addition, a detailed bathymetry dataset (approximately 130 data points/km2; from 47° 28¢ to 47° 38¢ north latitude) was obtained from the NOAA facility at Sand Point, Washington and aided in seismic record interpretations.
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Figure 16: Dataset post-processing showing successive postscript printouts for trackline 18. ‘A’ ¾ raw acoustic data from the archived DAT tapes converted to seg-y format. The background noise from R. V. Thompson’s onboard electrical system is quite apparent. ‘B’ ¾ postscript file following bandpass filtering of 10, 20, 160, and 320 Hz and deconvolution; note the considerable reduction in background noise and clarification of seismic reflectors. ‘C’ ¾ final processing printout; shot-point migration ("slope correction") and another filtering pass yielded substantial record improvements. The horizontal axis is one-minute time-position fixes (about 8 fixes/km) in Greenwich Mean Time (GMT). The vertical scale is depth below the water surface in kilometers.
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Figure 17: Uninterpreted and interpreted printouts for trackline 13. Seismic stratigraphic units are designated ‘H’; Holocene, ‘Q’; Quaternary, and ‘Cr’; Crescent Formation. On the uninterpreted record (left), seismic reflectors between a-a¢ through e-e¢ are vertically offset, disrupted, or horizontally truncated. Solid lines are used where structural trends are traced with confidence while dashed lines indicate less apparent, discontinuous features, or reflectors possibly associated with water bottom multiples (labeled ‘M1’). The variable amplitude, discontinuous reflector labeled "Cr-Cr¢ " is inferred to be the upper Crescent Formation’s erosional surface. On seismic tracklines further north, the Cr-Cr¢ reflector is of higher amplitude and more easily distinguished.
Each firing interval of the airgun sound source produces a broadband bubble pulse. The shape and duration of this bubble is determined by the time interval between the initial bubble formation and final collapse (approximately 50 ms; Telford, 1990), engineered airgun design parameters, and the airgun supply air pressure. The airgun device used in this study was fitted with a bubble suppresser chamber insert. This modification delayed the violent collapse of the bubble thereby reducing the ‘ringing’ effect and reverberation observed on the seismic records between surfaces of different acoustic impedance (e.g. air-water, water-seafloor, sediment-bedrock). Although the chamber insert reduced the effective peak acoustic pressure by about 3 dB, the enhanced seismic record resolution that resulted was considered an acceptable trade-off (M. L. Holmes, 1998; personal communication).
The initially recorded return signal is commonly followed by groups of acoustic reflectors that bounce a second time between the air-water interface and the water bottom. These reflectors take twice the time (two-way traveltime; TWT) to reach their recorded position and appear on the seismic records as sub-bottom multiples. The amplitude of each multiple is proportional to changes in acoustic impedance defined as the product of density and velocity (units of kg/m2s). Strong multiples are the result of large contrasts in acoustic impedance (Telford et al., 1990). Geologic units in Hood Canal such as lower density unconsolidated/semi-consolidated glacio-lacustrine sediments (r 1.6-2.0 g/cm3) and higher density basalt and sedimentary interbeds of the Crescent Formation (r 2.7-3.3 g/cm3) present significant impedance contrasts. Although most of the record multiples are removed during post-processing, including shorter-path ‘peg-leg’ multiples, some can still be observed on the printouts. Throughout the seismic record interpretation process, multiples were identified, labeled, and due care was taken so that they were not misinterpreted as actual acoustic reflectors.
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Figure 18: Seismic profile fence diagram showing interpreted profiles copied onto transparencies and supported by plastic drinking straws in their relative trackline positions. Figure 18A shows the completed model mounted on a 3 m-long wood shelving board. Bathymetry contour maps were placed beneath the suspended lines and shoreline geologic features (e.g. river deltas) were labeled to facilitate seismic-stratigraphic interpretations.
Seismic velocities in the geologic section can vary considerably from one point to another. This is especially true in the vertical direction where increasing pressure is combined with rapid changes in lithology and corresponding in situ variations in density and elasticity. Due to the great expense associated with deep borehole drilling and the acquisition of detailed down-hole seismic velocity data, this study uses an equivalent average velocity between the water surface and the lowest discernable reflector horizon. Geophysicists and professionals commonly apply the equivalent average velocity method when processing relatively shallow seismic data. With deeper penetrating seismic data (> 1 sec TWT) a more complex and systematic ‘velocity function’ is typically used where velocity varies with depth along a series of defined horizontal reflection horizons (Telford et al., 1990).
Relatively shallow marine seismic surveys in the Puget Sound area typically use constant average P-wave velocities between 1500 and 1800 m/s. For this study, a P-wave velocity of 1500 m/s was used to convert two-way traveltime (TWT) into a seismic record vertical depth scale. This velocity resulted in the recognition of coherent acoustic reflectors down to depths of approximately 0.7 km and a vertical resolution of about 2.3 m/pixel (Figure 16). Both parameters are very close to or exceed the 150-400 m penetration depth and 1.0-1.5 m resolution criteria typically used to classify ‘high-resolution’ seismic datasets (McCalpin, 1996).
Time gaps observed on seismic records are periods in which the ship is underway but the DAT tape recorder has stopped; for example, during DAT tape change-outs. These time gaps represent short periods of data loss. Where editing of the affected post-processed records was required, the distance between time-positional fixes across these data gaps was averaged and spaced accordingly to facilitate seismic reflector stratigraphic interpretations could be made across them. Unfortunately, the DAT tape recorder was not started for DAT tape #4. This resulted in the loss of recorded seismic data for most of tracklines 21-24 and the inability to post-process that data. Although the raw paper EPC records for these tracklines were recorded they allow only limited interpretation to be made relative to the post-processed records.
The ship momentarily lost its NAVSTAR GPS signal twice during the approximately 14 hour of data acquisition. These periods were between tracklines 3 and 4 (21:49 to 21:59 GMT) and along trackline 41 (05:09 to 05:18 GMT). During data analysis, the ship’s position along each affected trackline was dead reckoned using the last known positions for geographic reference. The corresponding seismic record time-position ticks were then annotated and spaced over these dead reckoned sections at one-minute intervals assuming a constant ship’s speed.
Throughout the Puget Lowland, highly irregular sediment thickness patterns overlie an equally complex Crescent Formation bedrock surface (Gower et al., 1991). Hence, defining the bedrock erosional surface on seismic records without the use of in situ borehole data requires correlation with other sources of geologic data. Yount et al. (1985) interpreted high-resolution seismic reflection records from the Hood Canal-Dabob Bay area and characterized the Crescent Formation bedrock as displaying "…high-amplitude returns from an irregular surface with no apparent internal reflectors". Yount et al. (1985) used the Crescent’s apparent lack of stratification coupled with observed basalt outcrops and correlative borehole logs along Hood Canal’s western shoreline as sufficient evidence that this buried reflector was indeed bedrock.
Geologic support for the author’s seismic interpretation of Crescent Formation basalt is based on criteria similar to that used by Yount et al. (1985). It is tied primarily to projecting 20-30° east-dipping Crescent Formation basalt outcrops (Figure 14) into the waters of Hood Canal. This is coupled with the distinct and unique seismic character of the interpreted bedrock unit observed on seismic and 3.5 kHz sub-bottom records of this study. The most convincing seismic-stratigraphic unit relationship is seen along the beach of Seal Rock Park (Figure 1) where east-dipping Crescent basalt outcrops (Figure 14B) are observed < 1 km from the end of the northwest trending trackline 45 (Figure 1). Stratigraphic and spatial correlation can be made between the beach outcrop (Figure 14B), the 3.5 kHz record (Figure 19), and the inferred Crescent basalt seismic unit on the corresponding trackline 45 profile (see Results). Both the sub-bottom record and trackline 45 seismic profile show a thin sediment layer overlying an east-dipping, high amplitude reflector that projects beneath Quaternary sediments infilling Dabob Bay.
This east-dipping seismic unit, interpreted to be Crescent Formation basalt, exhibits high-amplitude returns with only minor internal reflectors and strong multiples; acoustic properties very similar to those observed by Yount et al. (1985). These strong multiples indicate large contrasts in acoustic impedance exist along this unit’s erosional surface. The unique acoustic character exhibited by this unit, although not entirely consistent throughout the dataset, is inferred to be that of Tertiary bedrock and is used to identify bedrock-sediment contacts on adjoining seismic profiles throughout Hood Canal.
Figure 19: 3.5 kHz sub-bottom record recorded simultaneously with the trackline 45 seismic record. It shows Dabob Bay bottom sediments on-lapping the seismic unit interpreted to be east-dipping upper Crescent Formation bedrock. Crescent Formation outcrops along Seal Rock beach (Figure 1) are located about 0.5 km to the W-NW of this figure. Depth is annotated on the record at 50 m intervals (horizontal dashed lines). Vertical exaggeration is 16 times.
Throughout the Seattle 1:100,000 quadrangle, which includes mid-to-northern Hood Canal and Dabob Bay, bedrock is presumed to be Tertiary volcanic rock, conglomerate, sandstone, or shale (Gower et al., 1985). This study does not attempt to distinguish between these lithologies on the seismic records because the record resolution of about 2.3 m/pixel makes distinguishing subtle differences in bedrock acoustic character difficult. It is assumed that sediments overlying the irregular bedrock surface are Quaternary in age; however no direct dating of this seismic-stratigraphic unit has been done to support this assumption.
The trackline 13 seismic profile is shown in Figure 17. This figure is intended to help clarify the general methodology used by the author during seismic stratigraphic unit interpretation of erosional contacts, inferred faults, and sediment shear zones. These features are line-drawn following identification of water bottom multiples (labeled ‘M1, M2’) and correlation of erosional contacts using both observed seismic character and the fence diagram (Figure 18). Delineation of unit contacts and deformation structures are not always straightforward, however truncated reflectors and obviously disrupted structural features are common. For example, the near-vertical fault line labeled "c-c¢ " on Figure 17 clearly disrupts bedding plane reflectors and vertical offset can be traced down to a depth of about 0.6 km below sea level. The depicted Cr-Cr¢ bedrock-sediment erosional contact (Figure 17) is made with less confidence than seismic profiles further north (e.g. lines 26-50) where stronger impedance contrasts are observed between Tertiary bedrock and overlying Quaternary glacio-marine sediments. However, the approximate depth of this contact corresponds well with trackline 12 to the south and trackline 14 and the north.
