| Summary: | Pore geometry and topology are important determinants of sediment physical properties, such as porosity and permeability. They also influence processes that occur in the sediment, such as acoustic propagation, attenuation, and dispersion, single- and multi-phase fluid flow, and hydrodynamic dispersion. This study uses images to evaluate pore geometry and topology of ooid (subspherical particles) and siliclastic (angular quartz) sand that was collected from the marine environment south of Bimni Bahamas and Ft. Walton Beach, FL, respectively. Image analysis techniques and predictive tools enable insight into the relationships among sediment pore geometry, topology, and physical properties for these differently shaped sands. High frequency acoustics utilize short wavelength signals to evaluate sediments. Correspondingly short length scales are then needed for sedimentary property predictions, which is possible with planar and volumetric image analysis of sand. This data was compared to data obtained by direct large scale measurements (e.g., water weight loss, constant head permeability) were made. Mean porosity differed by as much as 0.04 and mean permeability showed good agreement and differed by a factor of 2. Given that the image analysis predictions were made from much smaller samples (~equivalent to the length scale of the high acoustic frequencies used) than the bulk samples, a sediment characterization at acoustically relevant length scales is possible. It was also demonstrated that for these homogeneous sands (i.e., ooids and quartz) two-dimensional pore geometry and topology are quite similar to three-dimensional pore geometry and topology (i.e., pore connectivity). Additionally it was determined that pore network models typically overestimate the topology and therefore, in order to match image and bulk predictions of sediment properties, these models must underestimate the conductance of individual pore throats (i.e., conductive element in sand). Typically pore throats are depicted as straight cylinders. Image data suggests that pore throats are better represented by biconical shapes where conductance is as much as 3 times higher than conductance within the straight cylinders. These findings indicate that increased realism in pore throat shape (higher conductivity) and in topology (fewer pore throats) may significantly influence network model evaluations of fluid flow or acoustic propagation in marine sand.
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