We have developed an instrument with the capability of measuring the in-situ compressive wave velocities (Vp) and density of sand in the upper part of the sediment column. This instrument will be deployed in conjunction with the ONR/DRI field program in July 1999. The purpose of these measurements is to rapidly carry out a large-scale site survey in order to locate and characterize a region of sediment with uniform acoustic impedance (Vp x density) where detailed acoustic experiments on a much smaller scale can be performed. The UW instrument is a large (3 x 3 meter base with a 3 meter height; weight approximately 1000 lbs) tripod that uses hydrojet technology to insert a sensor probe at precise increments to a 1 meter depth into sandy sediments. This probe is lowered over the side using (the ship's) steel support cable, and data and control commands are relayed (in real time) using a separate electrical umbilical cable. Real time color video is available, using a camera attached to the tripod foot. Control over insertion depth uses an optical encoder wheel (60 pulses per revolution, 5 threads/2 cm), which provides repeatable vertical positioning to better than 1 mm. The basic tripod has three probe options: (a) the (radioactive) density probe, (b) the permeability sensor, and (c) the acoustic (only) probe. Acoustic (Vp) measurements can be made using (a) and (c).
A wide-spectrum impulsive sound source and receiver (hydrophone) are used to measure compressive wave velocity and attenuation at frequencies of 3.5 kHz and 11 kHz. Sound paths are quasi-vertical through that portion of the sediment that is undisturbed by the hydrojet, and Vp data are acquired from the water/sediment interface to the full extension depth of 1 meter.
A Cesium-137 radioactive source and a pair of scintillation counters are used to measure gamma ray backscatter, for direct sediment density measurements. The gamma source intensity was chosen (for safety in handling) to be 10 mCurie, a factor of 15 less than that normally used for well logging determinations, but counting periods of several minutes/station will provide the same counting statistics. The density probe is being calibrated commercially, using a range of calibration materials that bracket the expected sediment densities.
In order to test one of the fundamental models of the DRI, direct measurements of permeability of the upper 1 meter of the sandy sediments are needed. To make these measurements, we are using a system that over-pressures the sediments and measures both the flow rate and the surrounding super-hydrostatic pressure near the instrument probe. These measurements are being made in conjunction with Dr. Richard Bennett (SeaProbe). Since both the flow rate at a given pressure and the external pressure field are strong functions of sediment permeability, this technique provides two independent measurements of this parameter. By conducting these experiments over a range of over-pressures and extrapolating to zero flow, we should be able to very accurately determine the permeability of the sediments. This technique also has the advantage of determining the critical flow rate (shear strength) where the sediment fluidizes.
We anticipate that each deployment station (vertical profiles through the top 1 meter of sediments) will take approximately one hour. In the present configuration, we plan to take Vp and density measurements simultaneously (on one lowering of the tripod), and then complete a second set of lowerings using the permeability probe. Assuming two parallel sets of site survey profiles that bracket the most probable high resolution area in orthogonal directions, we anticipate that the proposed 72 stations (100 meter spacing) would require six 12 hour days (a total of 75 hours, including set-up). If higher density spacing or shorter survey lines are required, this time requirement can be adjusted accordingly.
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