Suggested methodology for direct and inverse characterization of marine sediments that contain mud

Abstract

The measurement and prediction of compressional wave speeds and attenuation in mud sediments are discussed. The principal defining properties are due to clay particles within the mud. Because such particles carry electrical charges, theories for sandy/silty sediments are not applicable.. The mixture theory of Mallock and Wood works moderately well and, because of the high porosity, predicts that the sound speed is less than that for sea water. Data implying substantially higher sound speeds should be carefully assessed. Theory also predicts monotonic increase of sound speed with frequency, so data contrary to this should also be assessed. A correction to the Mallock-Wood formulation that includes electrostatic pressure within the mud raises the sound speed by 1%, and this explains some data assembled by Jackson and Richardson and explains the shift in the minimum in the sound speed versus density curve discovered by Holland. Large discrepancies in the measured values of attenuation are reviewed, and it is suggested that the reasons for such be found. The rudimentary theory under development needs experimental guidance, but seems valid in its predictions that the attenuation must vary as the square of frequency at low frequencies and, in any event, increases monotonically with increasing frequency.