Physical Pore Properties and Grain Interactions of SAX04 Sands

During the 2004 Sediment Acoustic eXperiment (SAX04), values of sediment pore properties in a littoral sand deposit were determined from diver-collected cores using traditional methods and image analysis on X-ray microfocus computed tomography (XMCT) images. Geoacoustically relevant pore-space properties of sediment porosity, permeability, and tortuosity were evaluated at scales ranging from the pore scale to the core scale from “mud-free” sediments collected within the 0.07-km² study area. Porosity was determined from water-weight-loss measurements to range from 0.367 to 0.369, from 2-D image analysis to range from 0.392 to 0.436 and from 3-D image analysis to range from 0.386 to 0.427. The range of permeability from all measurements was 2.8 × 10^-11 m² to 19.0 × 10^-11 m², however the range of permeability within each technique was much narrower. Permeability was determined using a constant head (CH) apparatus (k_range = 2.88 to 3.74 × 10^-11 m²), from a variant of the Kozeny-Carman (KC) equation (k_range = 12.4 to 19.0 × 10^-11 m²), from an effective medium theory technique (k_range = 5.60 to 13.3 × 10^-11 m²) and from a network model (k_range = 8.49 to 19.0 × 10^-11 m² ). Permeability was determined to be slightly higher in the horizontal than in the vertical direction from the network model. Tortuosity ranged from 1.33 to 1.34. Based upon the small coefficients of variation for the conventionally determined pore-space properties, the sand sediment within these core samples was deemed homogeneous at all of the SAX04 sites. Additionally, grain interactions, specifically grain coordination number and grain contact areas, were determined from XMCT images. Grain contacts ranged in size from small point- - contacts of 136 μm² to large-area contacts the size of grain faces ( >4500 × μm²). The mean coordination number was similar to that of a cubic packing (six), but sometimes exceeded 12, which is the coordination number for a hexagonal close packing of spheres.