Micro-Scale Characterization of Marine Sediment Structures: The Potential and Challenges of X-Ray μCT Imaging

Natural marine sediments are heterogeneous with respect to sediment-physical properties, and have a wide range in composition and structures. For many years, sediment-physical characterization has relied primarily on laboratory experiments. However, the investigation of small-(grain-)scale sedimentary structures, which appear to control many sediment (re-)depositional and emplacement mechanisms, requires new analytical methods. Here, we test high-resolution X-ray synchrotron micro-tomography (μCT) to qualitatively and quantitatively investigate structural differences, in 3D, between two lithological end-member types of marine sediments: a coarse-grained, sandy sediment and a fine-grained, silty-clay sediment. Our results show clear compositional and structural differences between the two end-members, as well as between samples taken from the same lithological unit. These differences can be attributed partly to different sediment types, that is, coarse-versus fine-grained sediments, but also reveal a dependency on the sedimentation regime. We find that pore space distribution is highly spatially variable, even down to a sub-millimeter scale. Such high variability in porosity would be missed by standard geotechnical experiments, which only provide information averaged over far larger sediment samples. The identification of small-(grain-)scale changes in pore space, however, directly impacts sediment properties such as permeability, which in turn is crucial for the understanding of geological processes such as fluid flow and storage capacity of sediments and assessing hazards such as the preconditioning of submerged slopes to collapse. Our results therefore demonstrate the potential of μCT to investigate the internal structure of natural sediments, obtaining information that is not resolved or lost in data acquired through other analytical methods.