Geo-Marine Letters(1996) 16:219 225 © Springer-Verlag 1996 Peter D. Jackson • Kevin B. Briggs • Robert C. Flint Evaluation of sediment heterogeneity using microresistivity imaging and X-radiography Received: 17 December 1995 Abstraet Microresistivity imaging and X-radiography of 3-cm-thick sediment core slabs collected by divers from Eckernförde Bay, Germany, indicate subtle delineations and features created by hydrodynamic and biological pro- cesses within the top 20 cm of the sediment. Variations in the images of both techniques are controlled by the physi- cal properties of the sediment. The X-radiographs record attenuation by solids throughout the 3-cm thickness of the sediment slab, while microresistivity images depend on pore water in terms of amount, salinity, and distribution. The microresistivity method is shown to detect layered structures and the presence of shells within the high- porosity sediment. Introduction Measurement of sediment inhomogeneities and subse- quent statistical characterization of the sediment hetero- geneity is essential to successful acoustic modeling. Typi- cally, acoustic models have been developed to describe bottom scattering in terms of two components: interface scattering from bottom roughness and volume scattering from sediment inhomogeneities. Although the measure- ment of interface roughness has been accomplished re- cently by established methods of stereo photogrammetry and close-range high-frequency sonars (Briggs 1989), characterization of inhomogeneities buried within the sediments has remained an elusive goal. Characterization of subsurface inhomogeneities is espe- cially important in fine-grained sediments where the P. D. Jackson . R. C. Flint British GeologicalSurvey,EngineeringGeologyand Geophysics Group, Keyworth, NottinghamNG12 5GG, UK K. B. Briggs([~:~) Naval ResearchLaboratory,SeafloorSciencesBranch, Stennis Space Center, Mississippi 39529-5004, USA Approved for public release;distributionunlimited impedance difference between the overlying water and the high-porosity sediment is not appreciable. It is in these fine-grained sediments that acoustic energy is scarcely in- fluenced by interface roughness but is likely to penetrate and be influenced by subsurface structures (Jackson and Briggs 1992). Subsurface structures capable of scattering sound are typically shell lag layers or burrows but can be any discontinuity in the sediment fabric. In addition, gas bubbles may be present in a variety of coastal sediments (Schubel and Schiemer 1973) and may be a source of sedi- ment volume scattering. All these features affect porosity, and as described by Archie's Law, the electrical resistivity of the sediment. Furthermore, sediment porosity is theo- retically and empirically related to sediment density, which is one factor (sediment sound velocity is the other) in determining sediment acoustic impedance (Hamilton 1980). Differences in acoustic impedance within the sedi- ment are responsible for scattering of acoustic energy by the sediment volume. Thus, from this premise we proposed to use measurements of microresistivity to document inhomogeneities theoretically capable of creat- ing sediment volume scattering. We then confirmed the presence of electrical detection of these inhomogeneities with X-radiography. The development of microresistivity imaging as a tool for quantifying two-dimensional sedi- ment heterogeneity would be superior to existing methods of determining porosity fluctuations on cores, which pro- vide data solely in the vertical dimension (Briggs 1994). Eckernförde Bay muds High-porosity muds are found in the deeper parts of Eckernförde Bay in the Baltic Sea and are subject to bioturbation and to deposition of thin, coarser-grained, storm laminae (Milkert et al. 1995). The presence of gas bubbles or shells within these high-porosity muds repre- sents barriers to the flow of electric currents and should therefore result in anomalously high values of measured resistivity. These sediments have very high water contents