Calculation of the engineering properties of marine sediments from acoustic reflection data using Biot theory

Abstract

Summary form only, as follows: This decade has seen unprecedented advances in seafloor mapping. Advancements in sonar processing and analysis allows for the more accurate and comprehensive inspection the seafloor through high accuracy swath bathymetry, on-the-fly side scan mosaics, acoustic seafloor classification systems, and improved systems for high resolution subbottom profiling. Traditionally, acoustic subbottom profile surveys have contributed to our understanding of the distribution of marine sediment environments by providing stratigraphic information obtained in a continuous manner. Sediment descriptions are then provided with strategically placed sediment cores. A method is presented that provides for the extraction of physical sediment characteristics from acoustic reflection data collected as subbottom profiles. The approach involves detailed acoustic surveys, conducted as quantitative tests rather than for simply creating qualitative images, and can provide descriptions of bulk sediment properties. The response characteristics of acoustic data are correlated to bulk sediment properties such as density, porosity, and mean grain size, and sediment thickness through assessment of acoustic impedance, velocity, and absorption using proven Biot-based analytical techniques. The Biot theory was developed to explain acoustical behavior of sedimentary material accumulating on the seafloor. Applicability of the Biot theory has been demonstrated for the entire suite of sedimentary materials, from surficial materials with porosities as high as ninety percent, to well-consolidated materials with porosities as low as one percent. The Biot model predicts that sound velocity and attenuation in sediments will depend on frequency, on the elastic properties of the sediment grains and pore fluid, and on porosity, mean grain size, permeability, and effective stress. This paper presents geoacoustic modelling and data inversion tools developed to more quantitatively assess in situ sediment characteristics by correlating measured acoustic responses with actual and predicted sediment properties. The technique has been used successfully on a wide variety of projects, including; beach nourishment studies, navigation dredging projects, assessments of navigable depth, lake sediment surveys, and general marine sediment investigations.