Measuring Sub-Seabottom Seismic Velocities - Some Unusual Experiments

During the last 30 years, we have performed a number of unusual experiments to measure P- and Swave<br>velocity structure of unconsolidated sediments below the seafloor at sites to water depths of 700<br>m. These experiments were designed to aid in specific geotechnical problems related to soil stability<br>along pipeline routes or beneath bottom-founded structures as well as to provide regional information<br>for military purposes. Some of these experiments were done in open-water conditions, but many were<br>performed beneath ice-cover.<br>Early testing was directed towards mapping sub-seafloor ice-bonded permafrost in the Beaufort Sea<br>and elsewhere in the Arctic Archipelago, mainly using dynamite sources. Continuous ice-cover at high<br>latitudes presented specific challenges for refraction methods and various devices and hydrophone<br>arrays were designed for deployment beneath the sea-ice through open leads or drilled holes. Later<br>testing evolved from seabottom-laid hydrophones to towed, near-bottom arrays and non-dynamite<br>sources. As well, mapping of shear wave velocity structure was tested using seabottom coupled<br>interface waves.<br>All of these experiments (successful or otherwise) were learning experiences in one form or other,<br>and it is hoped that documentation of these experiments herein will be of some future research value.