High-Frequency Sound Interaction in Ocean Sediments
2 July 1999

Sediment Characterization for Gas Content and Low-Frequency Geoacoustic Properties

L. Dale Bibee
Naval Research Laboratory, Code 7432
Stennis Space Center MS 39529-5004
(228) 688-5459, -5752 (fax)

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The goals of this effort are two-fold. (1) The presence of even a small amount of free gas in the surficial sediments can generate acoustic effects that will dominate the high-frequency volume scattering and propagation problems. While we do not expect free gas to exist at the Panama City site, a system using diver-sealed cores and co-located geoacoustic probes will be used to verify this expectation. (2) While the focus of this experiment is on high-frequency acoustic propagation, an examination of the physical processes involved in propagation in this porous medium requires constraints on the low frequency (< 1 kHz) geoacoustic properties as well in order to assess frequency dispersive processes. A linear array of buried three-axis particle motion sensors will be deployed to record low frequency Scholte, shear, and compressional wave data from seafloor airgun and hammer sources

Gas Content Monitoring

The system for measuring gas content in seafloor sediments consists of a special core tube which is inserted into the seafloor by divers, sealed in place, and transferred into a pressure vessel for return to the surface. The pressure vessel contains plumbing penetrations that allow the core to be pressurized above ambient seafloor pressure. The amount of fluid flowing into the core is then monitored to determine the compressibility of the material in the core tube. If free gas is present, the compressibility of the material is significantly increased.

During the survey conducted in August 1998, two cores were taken to assess the feasibility of this technique. In one case, a leak occurred in the core seal that reduced the accuracy of the measurement, and we were only able to place an upper bound on the amount of gas at < 0.033 ml (0.008% fractional volume). In the other case, 0.05 ml of gas was detected in the sample (0.01% fractional volume). Without additional measurements, we are reluctant to conclude that this value is typical, or even that this small amount was not in some way introduced during the sampling process. We propose to take a number of samples during the October experiment to confirm our expectation that the sands are fully saturated.

This system was developed for measurements in the surf zone sands. In that setting, we typically take a geoacoustic measurement co-located with the core using a pair of multi-node geoacoustic probes that measure sound speed and attenuation in vertical slices of sediments to a depth of approximately 45 cm. These probes operate at frequencies between 75 kHz and 300 kHz, and the data are recorded by a portable hand-carried acquisition system. For the October experiment, the data recorder will be made diver-portable to allow measurements coincident with the sealed cores and provide data for comparison with the NRL ISSAMS and the APL-UW tomographic system.

Low-Frequency Geoacoustic Sediment Properties

During August of 1998, a linear array of seismic three-component sensors was deployed. Four nodes at 4 meter spacing were deployed at a distance of 50 meters from the moored ship. Each node consisted of a pressure case containing three 4.5 Hz seismometers buried just below the seafloor and cabled to the ship. Signals from a small airgun source mounted to a plate and lowered to the seafloor were recorded as the ship was winched along its anchor lines to a range of 100 meters. Signals from a source at closer range were planned using a diver operated hammer source; however, difficulties in maintaining ship position with the anchor lines precluded these measurements. Data were recorded up to a frequency of about 200 Hz. As expected this source-receiver configuration generated strong Scholte wave signals from which shear wave speed can be extracted. Analysis is not complete at this time, but it is clear from the data that shear speeds are higher than those expected for an unconsolidated sand undergoing compression from effective stress. It is also clear that the data are sensitive to sediments at greater depth than those sampled by ISSAMS. It is important to replicate this experiment with the shorter range sources to adequately sample the shallower sediments.

These data also provide some constraints on the low frequency compressional wave speed of the sediments. This is important in determining the frequency dispersion of compressional waves as predicted by the Biot theory. The particle motion of the signals indicates that the first energy is arriving from below and traveling through the sediments. The average speed of the signal between source and receiver is near 1700 m/s, but the phase velocity across the nodes is higher (near 2000 m/s, indicating refraction of the signal through a shallow (less than 9 meters), higher velocity layer. Despite this relatively deep penetration of the compressional wave, we may be able to provide constraints on the compressional wave speed at shallow depths using the particle motions to define the angle of emergence of the wave. Difficulties in working with these data for the compressional wave problem include limited bandwidth, uncertainty in the source-receiver ranges, and the lack of the short-range source data.

In October we plan to replicate this experiment using an array with more nodes, an increased recording bandwidth, hydrophones on at least some nodes for accurate ranges, and shorter range sources.

Measurement Time Considerations

The experience of August 1998 indicates that the gas content cores each take approximately 15 minutes of diver time to take. The addition of acoustic probe measurements would probably add an additional 15 minutes for each sample. For the array measurements, one dive of approximately 30 minutes is required to emplace the array. The airgun source work is operated from the ship and requires approximately 2 hours to complete a profile. The diver operated hammer source would require one dive of approximately 30 minutes. A final dive is required to recover the array.

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