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StrataView R96 Used in Marine Shear Wave Survey
There is new attention to the use of surface waves, on land and in the water. Shear waves can be used for measuring stiffness and ground strength, important measurements before construction of large structures like buildings. But most shear wave studies have been done on land, since water does not support shear vibrations. However by the clever use of Sholte waves, shear wave dispersion can be measured indirectly and shear velocities calculated.
Placing a platform for producing oil is a tricky and dangerous business. Many site studies are performed in advance to determine ground stability, whether shallow gas pockets exist that might prove dangerous and where to position the 'glory hole', the central point where several wells converge under the platform. The glory hole can be of enormous diameter, 10m or more, and locating soft sediment for drilling saves much time and money.
The Geological Survey of Canada, in conjunction with the Kansas Geological Survey, has started an experimental program to estimate ocean bottom softness by the indirect measurement of shear waves. A streamer is 'flown' behind a tow ship and shots fired with an air gun. Keeping the streamer within a critical distance of the bottom is essential, requiring around-the-clock observation by the scientific team.
Jim Hunter and Robbie Burns from the GSC, along with Jiang Xia, Rick Miller and Choon Park rounded out the scientific crew. They faced high seas and cold weather to collect the data and report some preliminary successes.
Dr. Jim Hunter files this report:
"Attached is a picture of Rob Burns operating two StrataView R-48's on board the CCGS Hudson, in the White-Rose/ Hebron/Hibernia block of the Jeanne D'arc basin, northern Grand Banks. The spare unit sits below the bench. We used as splitter from the output resistance box of our seabottom eel to go to each instrument. One is recording 100 ms at 0.0625 ms sample rate, and the other is recording 4 seconds at 1 ms sample rate. There is no cross-talk, as you predicted.The other equipment on the right is the navigation system which tracks the eel above the ocean floor.
We used a 300 cu inch airgun at 1800 PSI for most of the survey, offset 50 meters from our 24-channel 5 m group-spacing array. Firing rates were 1 minute for reconnaissance lines (end-to-end coverage) and 15 seconds for detailed site surveys (4 fold overlap). A total of 7248 records in all, with good refraction front ends and believable Sholte waves. We tried to keep the eel flying horizontally about 3 meters off bottom, but inexperienced novices from Kansas sometimes turned the system into a plow (quite desperate to leave their mark somewhere!). In the midst of a gale with 35 knot winds and 6 meter seas, Rick stood on the poop deck declaring that this was as comfortable as riding a 5-gaited saddle horse. Choon and Xia saw the world from a somewhat different angle."
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