The overall task is to measure and model scattering by sediment volume inhomogeneities. In order to model the scattering there are three power spectra that will be measured in situ: (1) the sound speed spectrum will be measured by an acoustic tomography system, (2) the density spectrum will be measured by a sediment conductivity probe system, and (3) the cross spectrum between the sound speed and density will be inferred by co-locating the two systems and taking simultaneous measurements. Both systems are new and are currently being constructed, and both build on experience with previous versions [1].
ATOMS consists of a ring with an inner diameter of 25 cm on which 60 transmitters/receivers are mounted. The working frequency band is roughly 130–180 kHz. ATOMS will be mechanically driven into sediments to specifiable depths. At each depth, a two-dimensional tomographic inversion will be performed to obtain sound speed and attenuation coefficient variabilities with 1 cm by 1 cm resolution. A series of such 2-D inversions taken at successive depths with 1 cm spacing will provide us a three-dimensional view of the sediment. Currently, we are working on integrating the acoustic and electronics components into an underwater system and developing software for system control, data acquisition, and data analysis. FY00 will commence with the field experiment itself. Data will be acquired with the aid of divers, and the approach will be to take data at multiple stations in order to build up an ensemble for averaging the results. Following the experiment, the work will primarily focus on data analysis to provide required spectra for volume scattering models. In addition, tomographic data will be used to estimate the sediment attenuation coefficient as a function of frequency in the available frequency band. If a sufficient signal-to-noise level is achieved in the data, the same data sets will be used to study forward scattering issues, including multiple scattering. Since NRL will have sound speed and attenuation coefficient measurements both in situ and in the laboratory, collaboration is anticipated with NRL scientists to cross check and verify results.
This system has 16 independent electrodes arranged in a line with 1 cm spacing. By penetrating the sediment at successive depths, and repeating this process along a horizontal rail, a 3-D data set of sediment-sea water conductivity ratio with 1 cm voxel size will be obtained. The entire system is self-contained after divers deploy it and start the data acquisition process. For SAX99, we plan to take measurements on multiple slabs of sediment with dimensions of 15 cm (width), 11 cm (depth), and 50–100 cm (length). In order to convert the conductivity data to sediment density, we will work with several scientists in the DRI community. Pete Jumars and Jill Schmidt of UW Oceanography, using the weigh-dry-weigh method on cores, will provide a bulk density baseline. Tim Orsi of PSI will use CT scans on cores to obtain a higher resolution 3-D density grid. Rob Wheatcroft will take X-radiographs on cores from which sediment density will be inverted. Aided by these techniques on cores for calibration, the IMP will provide 3-D in-situ data of sediment density variability, from which the density spectrum will be calculated.
Since the cross spectrum between density and sound speed is required in volume scattering models, we will make an effort to measure both density and sound speed variability for the same volume of sediment. We are integrating the IMP with ATOMS in order to make co-located measurements. The approach to be taken in the field experiment is to insert the ATOMS and the IMP into the sediment together, with the former always about 10 cm below the latter as they move down together in small increments. Thus, in a given sediment volume, the sound speed will be measured first, followed very shortly by the conductivity measurement.
Subsequent to the experiment, work on data analysis will be directed first toward providing the density spectrum for use in acoustic modeling. After this, a significant effort will be needed to estimate the cross spectrum between the sound speed and density. These two quantities will be measured on different spatial grids, so that properly combining them is non-trivial.
When the IMP is used alone, the current estimate is that it will take roughly 8 hours to complete measurements on one slab of sediment. ATOMS can finish one set of measurements in about 4 hours when used alone. When the two systems are used together to measure cross spectra, the time needed is less certain, but is estimated to be roughly 5 hours for each station.
[1] D. Tang, "Small scale volumetric inhomogeneities of shallow water sediments: Measurements and discussion," in High Frequency Acoustics in Shallow Water, edited by N. G. Pace et al. (NATO SACLANT Undersea Research Centre, La Spezia, Italy, 1997), pp. 539–546.
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