Experimental investigation of sediment effect on acoustic wave propagation in the shallow ocean

Abstract

In shallow water the sediment layers have very strong effects on the propagation of acoustic waves. An effort to study the effects of the sediment has been made using 50- to 600-Hz continuous wave acoustic propagation data taken by Carey [‘‘Experimental verification and application of bottom shear modulus profile (BSMP) method,’’ Oceans ’91 Proceedings (1991)] at the Atlantic margin coring project (AMCOR) borehole 6010 off the coast of New Jersey combined with sediment properties measured at that site by Yamamoto et al. of the University of Miami Geoacoustic Laboratory using the bottom shear modulus profiler (BSMP) method. Excellent agreement was found between the model and data indicating the acceptability of BSMP sediment values as input for acoustic propagation studies. The introduction of shear or tangential stresses in the model was found to have no effect upon which modes propagated but only on their modal intensity. The higher the order of the mode the greater the penetration in the seafloor and the stronger the shear effect on the intensity. A sub-seafloor acoustic waveguide was investigated for the site and found to give possible explanation for enhanced intensity of modes that propagate strongly within that region. The intrinsic attenuation was determined using a method of matching modal intensity from model calculations with measured data. Biot theory was utilized and depth-dependent intrinsic sediment attenuation profiles were found for seven frequencies between 50 and 600 Hz. The depth-averaged attenuation for the first mode at each frequency and the first mode that penetrates to 100 m was found. The frequency dependence of the mode attenuation was determined. The attenuation values agreed well with previous experimental acoustic reflection data taken by Mitchell and Focke [J. Acoust. Soc. Am. 67, 1582–1589 (1980)] but were much lower than values for attenuation in this frequency range determined by Hamilton’s [Geophysics 36, 266–284 (1971)] extrapolation of high-frequency laboratory and field measurements to the ≤1-kHz range. The effect of interface and volume scattering on transmission loss was determined using the acoustic model and a thin layer representation of the scattering interface. The result showed scattering to significantly contribute to energy loss beyond a kilometer or so and to have increased significance as frequency is increased.