Naturally occurring marine sediments can possess poorly sorted or multimodal particle size distributions. Furthermore, while well-sorted coarse grained sediments are typically characterized according to their grain size, for mixed or fine grained sediments the shape of the particles can have significant influence on the acoustical properties. In this work, particle size and morphology are investigated using scanning electron microscopy (SEM). Images of dry particles are analyzed to determine the sphericity and angularity of sand and silt grains and to estimate the shape (e.g. platelets or needles) and aspect ratio of clay particles. SEM images of wet samples are analyzed to understand the sediment microstructure, including the interaction between sand, silt, and clay particles. Additionally, the effect of the proportionally varying composition of sand, silt, and clay particles on the bulk properties of the sediment, including the porosity and compressional wave speed, will be examined using a multiphase effective medium model. These techniques are applied to sediments collected in the New England Mud Patch as part of the Seabed Characterization Experiment. [Work supported by ARL:UT IR&D and ONR.]