Attenuation of Low-Frequency Sound Waves in Sediments

Abstract

Magnetic tape recordings of seismic profiler signals were made across an acoustically transparent wedge of sediments varying from 80 to 500 m in thickness on the flanks of Bermuda. The data were processed using 13-oct bandwidth filters. The average attenuation coefficient of the sediment, α, was determined from the equation d (y)/d (z) =α, where y is the ratio expressed in decibels of the peak pressure amplitude of the bottom to the peak pressure amplitude of the subbottom returns, and z is twice the sediment thickness. Possible errors resulting from spreading losses and changes in subbottom reflectivity due to increasing velocity and density with depth were considered and appropriate corrections made to the computed attenuation coefficients. For filters centered at 200, 250, and 315 Hz the average attenuation coefficients for the sediment were found to be 0.0031, 0.0069, and 0.0126 dB/m, respectively. For a filter centered at 400 Hz, the minimum value of the attenuation coefficient is 0.0110 dB/m. Outside this frequency range, the attenuation is too high or too low to be determined by this method. With attenuation expressed as being proportional to the frequency raised to the power n, the value of n is close to 3 for the frequency range 200–400 Hz. This high value of n is attributed to the effect of intermediate reflecting layers.