Abstract

Depositional muddy sediments are slow bottoms and pose a problem for the sonic detection of buried ordnance. This paper addresses the question: can the frequency dependent dispersion be predicted and verified by measurements in areas where buried object detection is required? Wood and Weston (Acustica, V14, 1964) have indicated that muddy sediments in the kHz range have a compressional speed 3% less than water with a frequency dependent attenuation (less than that of sand). A theoretical treatment of "muddy sediments", the Card House Theory (Pierce and Carey, POMA (5), 7001, 2009), estimates the slow sound speed and frequency dispersion proportional to mud porosity. Preliminary Dodge Pond results obtained with a buried array (1 to 10 kHz) are presented and illustrate the importance of micro-bubbles on the dispersion characteristic. The initial measurements on the disturbed sediment were found strongly influenced by scattering from larger bubbles whereas the results after a period of 10 months showed the effect of a smaller size distribution of bubbles. Estimates of the dispersion characteristic of mud and the effect of micro-bubbles are discussed. Finally the application of an impedance tube to the characterization of mud is discussed.

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