Relationships between sediment organic content and viscous grain shearing parameters in Mobile Bay, Alabama

Abstract

Whether deposited from the water column or generated by benthic infauna, interstitial organic matter is a ubiquitous constituent of marine sediment and is particularly prevalent in sediments with significant silt/ clay fractions. Interstitial organic matter suspends silt and clay particles in the sediment matrix, adsorbs onto mineral surfaces, and resides between mineral contacts, all of which are hypothesized to alter geoacoustic properties. However, the extent to which interstitial organics alter geoacoustic properties is understudied and warrants further investigation. To address this, diver cores were collected from a wide variety of sediments located in Mobile Bay, Alabama. The sediment was subsequently processed in the laboratory, where compressional wave speed (10 kHz to 1 MHz), compressional wave attenuation (100 kHz to 1 MHz), porosity, and organic content were measured. Mineral grain-size analysis and elemental analysis of the organics are ongoing. Next, viscous grain shearing (VGS) theory was fit to the data. Measured geoacoustic properties and VGS parameters were subsequently compared with sediment organic content. The porosity, acoustic impedance, grain shearing coefficient, and strain-hardening index correlated well with organic content, consistent with our hypothesis. The viscoelastic time constant, however, was lower than expected and constant with respect to organic content. [Work sponsored by ONR.]