Digital data for depth to basement in the deep-sea basins of the Pacific continental margin (cowbsm) based on data collected in 1984.

Additional information about the field activities fromwhich this data set was derived are available online at
https://cmgds.marine.usgs.gov/fan_info.php?fan=f184sc https://cmgds.marine.usgs.gov/fan_info.php?fan=f284nc https://cmgds.marine.usgs.gov/fan_info.php?fan=f384nc https://cmgds.marine.usgs.gov/fan_info.php?fan=f484wo
Although this Federal Geographic Data Committee-compliant metadata file is intended to document the data set in nonproprietary form, as well as in ArcGIS format, this metadata file may include some ArcGIS-specific terminology.

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(modified from Gardner and others, 1992)

The U.S. Geological Survey conducted a series of cruises in 1984, EEZSCAN 84 (EEZ-SCAN 84 Scientific Staff, 1986; U.S. Geological Survey activities F-1-84-SC, F-2-84-NC, F-3-84-NC, F-4-84-WO), to collect reconnaissance data on the newly proclaimed Exclusive Economic Zone (EEZ), the area out to 200 nautical miles from the coastline of the United States. The cruises systematically surveyed the entire conterminous United States west coast EEZ using the Geological Long-Range Inclined Asdic (GLORIA) side-scan sonar, a 160-cubic-inch airgun seismic-reflection profiler, a 3.5-kHz high-resolution seismic-reflection profiler, a 10-kHz echo sounder, and a proton-precession magnetometer. The nominal trackline spacing throughout the survey was 30 km.

Derivative maps of sediment thickness (Gardner and others, 1992a, 1993a, 1993b) and depth to basement (Gardner and others, 1992b, 1993c, 1993d)) in the basins of the west coast EEZ were compiled from both the sonar-image data and the deep-penetration seismic data obtained on these cruises. Only EEZ-SCAN 84 data were used for the map compilations because available data from other cruises in this region are sparsely located, have poor navigational control, or were obtained with seismic systems that were not powerful enough to resolve oceanic basement. Sediment thickness and depth to basement were determined only in the deep-ocean basin regions because the seismic system used on the EEZ-SCAN 84 cruises could not resolve oceanic basement beneath the thick sediments of the continental slope. All the data used to compile the maps are presented in the "Atlas of the Exclusive Economic Zone, Western Conterminous United States" (EEZ-SCAN 84 Scientific Staff, 1986) and are now available online (Paskevich and others, 2011).

Basement Outcrops

GLORIA imagery was used to locate areas of basement outcrop throughout the region. Where possible, sediment thickness on and immediately adjacent to basement outcrops was measured from seismic data. Where there was no bathymetric or seismic control, a seamount peak was assumed to have no sediment cover.

Data Reduction

Acoustic basement in the basins, invariably oceanic Layer 2, was observed on all of the seismic records. One-way traveltime was measured from the seafloor to acoustic basement. Because the trackline spacing of about 30 km is relatively large compared to the data density along track, the sediment thickness was measured every 0.5 hour or at an interval of approximately 7.5 km. Water depth was measured with a 10-kHz profiler. Depth to basement was calculated using the sea surface as the zero datum and adding the corrected water depth (Carter, 1980) to the sediment thickness. Acoustic travel times were converted to depths by first calculating a regression equation from the interval velocity versus depth data of Connard and others (1984). Their data base comprises a compilation of all available Deep Sea Drilling Project data plus wide-angle refraction data, which were collected in Cascadia Basin west of Oregon and represents the best data set available for the United States west coast EEZ. The regression equation was integrated to determine sediment thickness as a function of one-way traveltime. The resulting equation is

z = 1400t + 0 .5t ^ 2

where z is sediment thickness in meters, and t is one-way traveltime in seconds. Sediment thicknesses calculated using this equation were compared to values calculated from the general equation of Carlson and others (1986). Values for sediment thickness calculated by the two equations differed by no greater than 10 percent throughout the range of travel times.

References Cited

Carlson, R.L., Gangi, A.F., and Snow, K.R., 1986, Empirical reflection-traveltime/ depth and velocity/depth functions for the deep-sea sediment column: Journal of Geophysical Research, v. 91, no. B8, p. 8249-8266.

Carter, D.J.T., 1980, Echo-sounding correction tables: Taunton, United Kingdom, Hydrographic Department, Ministry of Defence, 150 p.

Chase, T.E., Wilde, Pat, Normark, W.R, Miller, C.P., Seekins, B.A., and Young, J.D., 1981, Offshore topography of the Western United States between 32 and 49 North latitudes: U.S. Geological Survey Open-File Report 81-443, scale 1:864,518 at 38 latitude, 2 sheets, http://pubs.er.usgs.gov/publication/ofr81443

Connard, G., Couch, R., Keeling , K., Roy, J., and Troseth, S., 1984, Abyssal plain and continental net-objective sedimentary thicknesses, in Kulm, L.D., and others, eds., Western North America continental margin and adjacent ocean floor off Oregon and Washington, Atlas 1 of Regional Atlas Series, Ocean Margin Drilling Program: Woods Hole, Mass., Marine Science International, sheet 7.

EEZ-SCAN 84 Scientific Staff, 1986, Atlas of the Exclusive Economic Zone, Western Conterminous United States: U.S. Geological Survey Miscellaneous Investigations Series I-1792, scale 1:500,000, 152 p., http://pubs.er.usgs.gov/publication/i1792