Laboratory measurements of compressional and shear wave speed and attenuation in muddy sediments

Abstract

As demonstrated by William M. Carey's field measurements in Dodge Pond, muddy sediments are characterized by a slow compressional wave speed and a low compressional wave attenuation [Carey and Pierce, POMA (5), 7001, 2009]. To gain insight into the measured data, a theoretical treatment of muddy sediments, named the Card House theory [Pierce and Carey, POMA (5), 7002, 2009], was developed. According the theory, isomorphous substitution causes each mud platelet to carry a net negative charge, and the resulting electrical interaction between platelets is responsible for creating a card-house structure. In this work, we examine laboratory measurements of compressional and shear wave properties in mud. Compared to the Dodge Pond measurements, which were affected by the size and distribution of gas bubbles present in the mud, the volume of gas in the laboratory samples was reduced by applying a vacuum. The estimated compressional wave speed is consistent with predicted values for a relatively gas-free mud. The estimated shear wave speed compares favorably with predicted values from the Card House theory. The electrochemical basis of the Card House model and its acoustical implications are also investigated. [Work supported by ARL:UT IR&D.]