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Marine deep seismic sounding off the Coast of British Columbia

University of British Columbia

A marine seismic system for recording near-vertical incidence to wide-angle reflected waves and refracted waves with penetration to the bottom of the crust (deep seismic sounding or DSS) has been developed. In a two ship-operation, signals from six individual hydrophones are recorded in digital form on the receiving ship. To provide origin times and facilitate subsequent processing, signals from a single hydrophone are recorded in FM mode on the shooting ship. During 1973, DSS profiles about 20km in length were recorded in three regions off the west coast of Canada: the Hudson '70 survey area, west of the Queen Charlotte Islands near 51°N, 133°W; off Queen Charlotte Sound; and in Cascadia Basin west of central Vancouver Island. The recorded data were processed with various digital techniques such as autopower spectrum analysis, band-pass filtering, deconvolution, velocity spectrum analysis, and stacking. After compilation in record sections, the data were interpreted in terms of velocity-versus-depth models of the oceanic crust. Two kinds of models were derived for the refraction data, models are based on a traveltime and amplitude interpretation made by comparing the observed data with synthetic seismograms. For the reflection data, the models are based on a T²-X² analysis of seismic phases. The crustal models derived from the two approaches indicate the same basic crustal layers, but the reflection models show detailed velocity changes within these layers. The models show the complexity of the structure of the oceanic crust and relate to regional geology. The crustal model of the Hudson '70 area shows thin sediments over the basement which consists of either two layers or a layer with a velocity gradient. The model compares well with the results obtained in the same area by Keen and Barrett (1971). The model for the region off Queen Charlotte Sound indicates six sedimentary layers of different velocities, basement at a depth of 2.4 km sub-bottom, and the oceanic layer at a depth of 4.5 km sub-bottom. Velocity reversals within the sediments possibly show the influence of Pleistocene glaciation on the deposition of sediments underneath the continental slope. The model for northern Cascadia Basin shows four layers within the sediments of thickness 1.9 km, a velocity transition between the sediments and the basement, the basement at a depth of 2.7 km sub-bottom, and the oceanic layer at a depth of 4.2 km sub-bottom. Proposed interbedding of volcanic material with high velocity sediments at the top of the basement correlates with geological formative processes observed at the crest of the near-by Juan de Fuca Ridge. The results have shown that the marine DSS system is an efficient technigue for detailed investigation of the oceanic crust and is an inexpensive alternative to the multichannel common depth point techniques used in the oil industry.

Text

2010-02-11

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http://hdl.handle.net/2429/20120

Geophysics

University of British Columbia

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