Project goals are the quantification of selected sediment properties crucial to the modeling of high frequency sound interaction in ocean sediments. The research effort is two-fold. Part I is the in-situ three-dimensional measurement of sediment permeability. Part II is a sediment microfabric study of the pore fluid pathways, porometry, and bio-organic components. The ultimate goal of the microfabric investigations is the development of microfabric models that describe important sediment properties such as fluid flow characteristics, isotropy and anisotropy, stress-strain behavior, and pore space occupied by water, gas, and biogenic materials (Bennett et al. 1989, 1996; Bennett, Ranson, et al., in press).
Objectives are to (1) assist in the design and testing of an in-situ permeameter to be used in the full scale SAX99 field experiment and (2) quantitatively analyze (2-D and 3-D) sediment samples for microfabric modeling studies. One objective is to collect permeability data in a three dimensional configuration with a sand probe in cooperation with Dr. Paul Johnson (University of Washington). The University of Washington will collect in-situ wet bulk density/porosity data with the UW sand probe. These measurements will provide a statistical database of in-situ sediment properties for high frequency modeling.
An additional thrust is to develop techniques for high quality sediment sampling of sandy deposits and the laboratory processing of these samples for microfabric study using electron and optical microscopy techniques. The objective is to reconstruct and quantify the two- and three-dimensional microfabric and porometry including the bio-organic components. Interstitial organic material will be preserved during sampling for the study of the interrelationships between the solids (mineral grains) and the indigenous microbiota and organic debris occluding the pores (Baerwald et al. 1991; Bennett, Olsen, et al., in press).
Part I. In- Situ Permeameter. Research and modeling were completed on the technical design of the permeameter. A report was completed and delivered to UW and ONR detailing design requirements including probe size, materials, required flow rates and pressures, effective stress considerations, and estimates of the force required for probe insertion in sandy sediments (Bennett and Hulbert, 1998). In addition, limited sediment analyses were completed on samples provided by Dr. Michael Richardson, Naval Research Laboratory (Bennett, 1999). Special tests were made to determine a range in porosity for very low effective stress conditions for the Panama City coastal samples.
Part II. Microfabric and Porometry. Reconstruction of the microfabric, porometry, and microbiota for 2-D and 3-D analyses of sandy sediment requires techniques suitable for scales ranging from submicron to millimeter, a range of three orders of magnitude. Sands range in size from 0.0625 mm to 2.0 mm, and indigenous sedimentary bacteria are in the size range of ~0.001 mm (clay size). Thus the techniques found suitable for the reconstruction of the microfabric and pores will require optical microscopy whereas the reconstruction of the microbiota in relationship to the solid grains will require electron microscopy (SEM). Several phases are involved in the analyses: (1) Field Sampling and Agar Embedding, (2) Fixing (staining as required), (3) Epoxy Embedding, (4) Laboratory Analyses/Sample Prep. for EM and Optical Microscopy, (5) Imaging, (6) 2-D and 3-D Reconstruction of Microfabric and Microbiota, and (7) Quantitative and Qualitative Analyses of Microfabric and Microbiota. A summary of the Progress in these seven phases is summarized as follows:
(1) Field Sampling and Agar Embedding. Subsampling of high quality sediment cores in cooperation with UW (Dr. Jumars et al.) and OSU (Dr. Wheatcroft) researchers will be required. The first problem faced working with sediment for subsequent three-dimensional microfabric reconstruction was holding the sand grains together in their native configuration. Embedding the sediment in agar prior to embedding in epoxy appears to be promising. The agar percolates through moist sand relatively quickly and sets. Getting agar to percolate through saturated sand is more difficult. We had preliminary success by heating the saturated sand to about 60°C before adding agar and allowing the agar to percolate for at least 30 minutes. This allows us to impregnate with agar a core 2 cm in diameter and about 8 cm long. This is a larger sample than we anticipate using in the final analysis. Additional testing is presently ongoing.
(2) Fixing. Once the sand is held together in agar it can be processed as a standard biological sample for electron microscopy. The sample with its organic components is fixed in 2% glutaraldehyde for 2 hours (0.1M cacodylate buffer, pH 7.0), buffer rinsed, postfixed in 1% osmium tetroxide for 45 minutes, and water rinsed.
(3) Epoxy Embedding. The sample is then alcohol dehydrated, propylene oxide (PO) dehydrated, and gradually transferred into ERL 4206 (Spurr's) epoxy (50:50% PO: epoxy, 4 hours; 25:75% PO: epoxy, overnight; 100% epoxy three changes in 8 hours followed by 24–36 hour curing at 70°C).
(4) Laboratory Analyses/Sample Prep. for EM and Optical Microscopy. Samples have been cut with a diamond saw using standard geologic thin sectioning techniques. Both thick (ca. 100 mm) and thin (ca. 30 mm) sections have been created and examined with light microscopy. Polished surfaces of epoxy embedded sand have been examined with scanning electron microscopy. We found that dissolving the epoxy with a sodium hydroxide saturated solution of absolute ethanol allowed us to see some interesting surface relief. This will be useful for seeing detail of the interfaces at sand grain contacts and may be useful in observing bacteria and other biogenic material provided that preferential removal of the epoxy can be achieved.
(5) Imaging. The three-dimensional reconstruction of sand is basically a light microscope requirement. Epoxy embedded sand will be cut or, more likely, polished, removing known amounts of material (probably 30–50 mm). The image of the polished surface will be digitally imported into a computer for three-dimensional reconstruction and for some routine stereology. SEM will be used for the reconstruction and imaging of the organic material and microbiota.
(6) Two- and Three-Dimensional Reconstruction of Microfabric and Microbiota. We made diligent searches for software capable of three-dimensional reconstruction that will work on an existing Windows platform. R & M Biometrics produces a package called Bioquant that appears to do everything we anticipate needing. The package includes several modules for the 2-D and 3-D aspects of the study as well as a dedicated frame grabber board. Our requirements were discussed with Bioquant representatives and technical support people to ensure that the package will perform the analyses needed.
(7) Quantitative and Qualitative Analyses of Microfabric and Microbiota. Quantitative and qualitative data obtained from 5 and 6 above will be integrated with other sediment data to develop organic-inorganic microfabric models leading to the understanding of fundamental sediment properties such as porosity-permeability, stress-strain behavior, and other physical properties (isotropy-anisotropy), mechanical properties, and biogeochemical processes and transport pathways.
Baerwald, R.J., Bennett, R.H. and Burkett, P.J., 1991. Techniques for the Preparation of Submarine Sediments for Electron Microscopy. pp. 309–320, in: Microstructure of Fine-Grained Sediments: From Mud to Shale, Springer-Verlag, 582 pp.
Bennett, R.H., Fischer, K.M., Lavoie, D.L., Bryant, W.R. and Rezak, R., 1989. Porometry and Fabric of Marine Clay and Carbonate Sediments: Determinants of Permeability. Journ. Marine Geology, Vol. 89, p. 127–152.
Bennett, R.H., Hulbert, M.H., Meyer, M.M., Lavoie, D.M., Briggs, K.B., Lavoie, D.L., Baerwald, R.J. and Chiou, W.A., 1996. Fundamental response of pore-water pressure to microfabric and permeability characteristics: Eckernförde Bay. Geo-Marine Letters, Vol. 16, p. 182–188.
Bennett, R.H., 1999. Panama City Coastal Sands, DRI Sites 10 and GS-172, Sediment Analyses, Informal Report, SEAPROBE Technical Report No. SI-0099-03.
Bennett, R.H. and M.H. Hulbert, 1998, In Situ Mini-Permeameter Probe Design Considerations, SEAPROBE, Technical Report No. SI-0098-04.
Bennett, R.H., B. Ranson, M. Kastner, R. Baerwald, W.B. Sawyer, H. Olsen, and M.W. Lambert, (in press), Early Diagenesis: Impact of Organic Matter on Mass Physical Properties and Processes, California Continental Margin, Marine Geology.
Bennett, R.H., H.W. Olsen, M.H. Hulbert, R.J. Baerwald, W.B. Sawyer, and B. Ransom, (in press), Organic Matter and Geotechnical Properties Interrelationships: Marine Sediments, 13th ASCE Engineering Mechanics Division Conference, Johns Hopkins University, June, 1999. Proceedings.
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