My overall goal is to provide DRI modelers with an acoustically relevant description of seafloor sediments at the Panama City study site. Complete with continuum and discrete scatterer characteristics, this fine-scale description will eliminate ambiguities associated with uncertain sediment structure and permit calculation of statistical descriptors, such as correlation length, property variances, subsurface interface roughness spectrum, and property anisotropy (if present) on any plane.
X-ray computed tomography (X-ray CT) will be used for core characterization. Developed over 30 years ago by the medical industry to generate cross-sectional X-ray images of the brain, X-ray CT is a powerful analytical technique, well-suited for high-resolution geoacoustic characterization of marine sediment cores [1–3]. It is non-destructive and quantitative, has sub-millimeter resolution, and permits two- and three-dimensional visualization of sediment structure. Volume characteristics that can be quantified via X-ray CT analysis include:
•Size and location of discrete scatterers
- Shells and shell fragments
- Biological features (tubes, burrows, etc.)
- Gas bubbles
•Discrete scatterer geometry/morphology/geotechnical characteristics
- Surface area and volume
- Orientation
- Physical properties of scatterer and matrix
•Shape of scattering interfaces and volumes
- Subsurface interface roughness
- Inclusions
Development of an approach to quantify these parameters will enable examinations of the nature and environmental causes of fine-scale variability in marine sand properties and their effect on high-frequency sound interactions with the seafloor. Careful coordination with DRI modelers is critical so that the CT approach can provide all (or as many as possible) of the relevant parameters used in high-frequency modeling.
(1) Develop data processing techniques to derive relevant geoacoustic parameters.
Applicable to the DRI, CT data from the CBBL Panama City site (FWG’s acoustic backscatter area) are initially being used to develop processing techniques. These cores, collected by Briggs and Richardson (NRL-SSC) in 1993, have already been scanned and processed, resulting in a considerable amount of readily available information. I am presently working closely with DJ Tang (APL-UW) so that relevant acoustic parameters are derived. This effort is in direct support of the upcoming DRI October 99 field experiment, where representative sampling sites will be selected for CT analysis. Subsequent CT results will then be distributed to all interested DRI modelers.
(2) Calibration of the APL conductivity probes.
As requested by APL-UW, I am also assisting DJ Tang in calibrating his IMP (In Situ Measurement of Porosity) by providing CT-derived density spectra. Saturated sand samples will be prepared in small tanks (1 x 0.5 x 0.5 ft), and sample conductivity (porosity) will be measured using the APL-UW device before and after CT scanning. In addition to calibrating the IMP, we will examine the disturbance the probes cause in the sediment sample.
[1] Orsi, T.H. and A.L. Anderson, "Bulk Density Calibration for X-Ray Tomographic Analyses of Marine Sediments," Geo-Marine Letters, In press: 01-13-99.
[2] Lyons, A.P. and T.H. Orsi, "The Effect of a Layer of Varying Density on High-Frequency Reflection, Forward Loss and Backscatter," IEEE Journal of Ocean Engineering, vol. 23, pp. 411–422 (1998).
[3] Orsi, T.H., C.M. Edwards, and A.L. Anderson, "X-Ray Computed Tomography: A Non-Destructive Method for Quantitative Analysis of Sediment Cores," Journal of Sedimentary Research, vol. A64, no. 3, pp. 690–693 (1994).
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