Refraction seismic velocity analyses from multichannel seismic data acquired during Expedition ARA04C on the IBRV Araon in the Beaufort Sea

Abstract

Expedition ARA04C (conducted from September 10 - September 26, 2013 in Canadian waters) on the Korean icebreaker IBRV Araon was laid out to investigate the Beaufort Sea shelf and slope region and collect geo-scientific data for various aspects relevant to the GSC's mandated regional geo-hazard assessment of the offshore Beaufort region. A critical element of the geohazards is the distribution of permafrost across the submerged shelf. To address this question and to verify data from earlier attempts made by the GSC in the mid 1980ies, a set of multichannel seismic (MCS) lines were collected across and along the shelf edge. Refracted arrivals were picked on all MCS lines acquired and velocities for the first and where occurring, second and even third refractions were determined. The depth to the first refractor was determined using simple ray-path geometry for a refracted, planar 2 layer case, with a fixed velocity of the upper layer (ocean water) of 1450 m/s determined from measurements of physical properties in the water column using a standard Conductivity-Temperature-Depth (CTD) tool. Using this velocity value creates a fully flat direct arrival when the shot-gather is reduced by this velocity value, confirming that this value is appropriate. Depths to 2nd or 3rd refractor where not determined in this study. Arrivals from deeper refractions are often masked by the occurrence of refractions generated by multiples and from strong linear noise occurring along the streamer. The results from the refraction velocity analysis allow verification of major boundaries of permafrost or icebearing sediment occurrences from previous work, but also define several critical corrections, especially near the shelf edge zone. Correlation of the MCS refraction data and the coincident 3.5 kHz sub-bottom profiler data reveal that a regional unconformity (~5 to ~15 meter below seafloor) often is the source of the first detectable refraction instead of the seafloor.