Geoacoustic modeling for acoustic propagation in mud ocean-bottom sediments

Abstract

This paper reviews research since 2007 developed at BU and RPI, with leadership by the late W.M. Carey, on acoustic properties of mud. Marine mud consists primarily of clay mineral platelets, comprised of crystalline layers and usually carrying charge because of isomorphous substitution. Because of resulting electrical forces, the physical nature of mud is considerably different from sandy sediments. In particular as platelets settle under gravity, electrical forces repel face-to-face contact, while strong van der Waals forces permit edge-to-face attachment. This platelet aggregation results in card-house structures, for which a tentative quantitative model has been analyzed (J. O. Fayton, RPI Ph.D. thesis, 2013). The model preserves basic physics of platelet interactions and leads to low shear speed predictions that are consistent with observations. Reasonably accurate compressional sound speed estimates follow from the Mallock-Wood formula; improved estimates result from including an electrostatic correction (2pAO5, this session). The basic physical concepts and semi-empirical formulas determined for compressional and shear attenuations and their frequency dependencies in sandy sediments do not apply to mud, nor do assumptions underlying the often cited Biot theory for poro-elastic media. A summary is provided of physical and geoacoustic mud properties for which measurements are needed.