• Volume/Page
  • Keyword
  • DOI
  • Citation
  • Advanced
   
 
 
 

Journal of the Acoustical Society of America

SECTION LISTING

BROWSE VOLUMES

Year Range: 
Search Issue | RSS Feeds RSS
Previous Issue Next Issue

May 1989

Volume 85, Issue S1, pp. S1-S156

Return to All Sections
back to top
RSS Feeds
back to top Session JJ. Underwater Acoustics V: Acoustic Bottom/Interface Properties
Invited Paper
FREE

Geoacoustic properties of the seabed sediment critical to acoustic reverberation at 50 to 500 Hz: A preliminary data set (A)

Tokuo Yamamoto, Morris Schulkin, and Richard Bennett

J. Acoust. Soc. Am. Volume 85, Issue S1, pp. S86-S87 (1989); (2 pages)

Online Publication Date: 13 Aug 2005

Full Text: | Download PDF

Show Abstract
As much as 80% of acoustic reverberation is produced through bottom interactions. Deep‐sea sediment have high porosity (or low velocities), therefore, a waveguide is often formed from strong bottom interactions. In addition to the bottom roughness, the random variation in the geoacoustic properties (the compressional wave velocity in particular) of the sediment is responsible for generation of the incoherent propagation/acoustic reverberation. Our existing data [e.g., Bennett et al., Handbook of Geophysical Expro. (1983)] indicate that very strong spatial variations (vertical and horizontal) usually exist in the deep‐sea sediments. A preliminary analysis of these geological data by the Biot‐Yamamoto theory [Yamamoto and Turgut, J. Acoust. Soc. Am. 83, 1744–1751 (1988)] reveals that strong velocity variations, as much as 20% in velocity magnitude, usually exist in the deep‐sea muds at horizontal and vertical scales of 0.3 to 30 m, which is very critical to acoustic reverberation at 50 to 500 Hz. [Work supported by ONR Code 11250 A.]
Contributed Papers
FREE

High‐resolution investigations of seismoacoustic propagation in shallow‐water areas (A)

George H. Sutton, Noël Barstow, Jerry A. Carter, John I. Ewing, and David Harris

J. Acoust. Soc. Am. Volume 85, Issue S1, pp. S87-S87 (1989); (1 page)

Online Publication Date: 13 Aug 2005

Full Text: | Download PDF

Show Abstract
Data from a newly developed 30‐element accelerometer/hydrophone array are used to study seismoacoustic propagation in shallow‐water locations off New Jersey and Martha's Vineyard. The elements of the array have 1‐m spacing; each consisting of three orthogonal accelerometers (flat from 2 to 500 Hz), a hydrophone, and a vertical direction sensor. The 120 accelerometer/hydrophone signals and 30 vertical direction signals are digitized and transmitted via 1 km of fiber optic cable to a PC‐type recording system. The A/D converters digitize all hydrophones at 2048 samples/s and all accelerometers at 512 samples/s. Good coupling to the bottom is obtained since sensor symmetry has been maximized; coupling to the water has been minimized; and density is matched to the sediments. There are 10‐m extensions between the 30‐m sensor section and the A/D converter housing at either end of the array. Units containing port‐starboard paired, shotgun‐shell sources were mounted within the array in these extensions and fired through the array circuitry, providing precise time and distance control and reverse profile capability. Differenced shot pairs emphasize transverse horizontal shear (SH) and Love wave signals; summed pairs emphasize P, SV, and Rayleigh/Stoneley/Scholte waves. Shear boundary wave energy is observed with frequencies greater than 60 Hz and velocities and wavelengths as low as 20 m/s and 1 m, respectively. Considerable energy appears to result from lateral heterogeneity and/or anisotropy. [Research supported by ONR.]
FREE

Low‐frequency measurements of bottom reflectivity at a thin sediment site in the North Pacific (A)

Patricia L. Gruber and Ronald L. Dicus

J. Acoust. Soc. Am. Volume 85, Issue S1, pp. S87-S87 (1989); (1 page)

Online Publication Date: 13 Aug 2005

Full Text: | Download PDF

Show Abstract
Bottom reflectivity measurements were made in the North Pacific, during Pacific Echo I, using a low‐frequency, towed source. A MK VI source transmitted 50‐s FM sweep signals (5 to 15 Hz) to a vertical array of 16 hydrophones. The FM ramps were deconvolved and beamformed to separate direct and bottom arrivals in time and angle. In addition, ray theory predictions of the arrival structure, scaled by spreading loss, were beamformed. A comparison between the time integrated energy in the measured and theoretical bottom arrivals, led to a measure of the bottom reflectivity. FM ramps were processed along the source ship track to vary the bottom grazing angle from 10 to 75 deg. Reflectivity as a function of bottom grazing angle, smoothed over angle to reduce variance, will be presented. Theoretical computations of reflectivity from a simulated bottom that includes a sediment layer with sound‐speed gradient and an elastic basement, with shear and compressional speed gradients and attenuations, will be presented. Theory and measurements are compared to infer the sediment thickness and the wave speed profiles in the upper 200 m of the oceanic crust.
FREE

Bottom shear modulus profiler (BSMP) measurements of the shallow‐water seabed geoacoustics properties in Japan (A)

Mohsen Badiey, Tokuo Yamamoto, Hideo Suzuki, and Hiroshi Kanai

J. Acoust. Soc. Am. Volume 85, Issue S1, pp. S87-S87 (1989); (1 page)

Online Publication Date: 13 Aug 2005

Full Text: | Download PDF

Show Abstract
The method of bottom shear modulus profiler (BSMP) was used to measure the seabed properties of several locations in Japan. In this method, the seabed admittance, which is defined as the ratio of the wave‐induced particle motion to the wave pressure on the seafloor is measured [T. Yamamoto and T. Torii, Geophys. J. K. Astron. Soc. 85, 413–431 (1986)]. These data are then used as input to a linear inversion scheme for calculation of the shear modulus profile. The collected data are presented in a complete form of shear and compressional wave velocity profiles. An attempt has been made to improve the numerical analysis of the inverse method used in the BSMP software. By applying a tapering Hamming window in the truncation of the order employed in the singular value decomposition (SVD) technique, the truncated expansion equation was modified and the higher‐order eigenvalues were used stably. A numerical test as well as the analysis of the data using this method is presented.
FREE

Results from two shallow‐water experiments utilizing synthetic bandwidth (“stepped FM”) signals to estimate compressional wave speed in the seabottom (A)

Neil J. Williams and Harry A. DeFerrari

J. Acoust. Soc. Am. Volume 85, Issue S1, pp. S87-S87 (1989); (1 page)

Online Publication Date: 13 Aug 2005

Full Text: | Download PDF

Show Abstract
Two experiments were performed to investigate the utility of synthetic bandwidth or “stepped FM” signals [H. A. DeFerrari and H. B. Nguyen, J. Acoust. Soc. Am. Suppl. 1 82, S74 (1987)] for acoustic remote sensing of geoacoustic properties of the seabottom. In each experiment, pseudorandom codes with carrier frequencies ranging from 100 to 1600 Hz were transmitted consecutively at a fixed range to a moored vertical array. After every other complete sweep of the entire bandwidth, the range was increased and the transmission was repeated. The experimental sites were 42 nautical miles SE of Corpus Christi, TX and 75 nautical miles E of Lewes, DE. The water depth in each case was about 60 m and the range from source to receiver varied from 300 m to 2 km. The data have been reduced to pulse responses with a resolution of 6 ms. This, combined with the angular resolution of the array, aids in the identification of eigenrays and inversion of the data to yield estimates of compressional wave speed in the seabottom. [Work supported by ONR.]
FREE

Gaussian beam algorithm for bottom reflection and head waves due to extended aperture sources (A)

X. J. Gao and L. B. Felsen

J. Acoust. Soc. Am. Volume 85, Issue S1, pp. S87-S88 (1989); (2 pages)

Online Publication Date: 13 Aug 2005

Full Text: | Download PDF

Show Abstract
Gaussian beams furnish a useful algorithm for high‐frequency sound propagation in complex environments but in their paraxial implementation, they are flawed by spectral defects that obscure certain wave phenomena. Critical reflection and head wave generation due to a fast bottom belong in this category. In a separate study, methods for “fleshing out” the deficient spectrum for the prototype structure of a homogeneous ocean and a homogeneous fluid bottom, have been explored with excitation due to a line source [[X. J. Gan et al., 2nd IMACS Symposium, Princeton University (March 1989)]. In the presentation here, these methods of excitation from a continuously distributed extended aperture source and from an array of line sources have been applied. In the Gaussian beam modeling, not only are the ad hoc discretization that involves arbitrary parameters employed but the self‐consistent Gabor phase space stacking are also employed. It is found that distributed sources deemphasize the above‐noted spectral defects but do not remove them entirely. [Work supported by ONR.]
FREE

Propagation of Rayleigh and Scholte waves along edge of quarter‐space (A)

Jacques R. Chamuel and Gary H. Brooke

J. Acoust. Soc. Am. Volume 85, Issue S1, pp. S88-S88 (1989); (1 page)

Online Publication Date: 13 Aug 2005

Full Text: | Download PDF

Show Abstract
A surprisingly large leaky Rayleigh wave component has been observed propagating along the edge of a laboratory nonsaline‐ice quarter‐space in water with both source and receiver positioned along the edge of one of the two surfaces of the quarter‐space. Studies on different solids (aluminum, Plexiglas, limestone) indicate that all surface and interface edge waves travel slower than their corresponding waves on a solid half‐space. The measured ratio of the edge Rayleigh wave velocity to the half‐space Rayleigh wave velocity is 0.9617 for aluminum 6061, 0.9665 for Plexiglas, and 0.9954 for limestone. Ultrasonic experimental results are presented on edge Rayleigh waves, and on edge leaky Rayleigh waves and Scholte waves for the liquid/solid case. The edge wave has no geometrical spreading and dominates the received signal when the receiver is moved slightly away from the water loaded edge of the quarter space. The Rayleigh waves along the horizontal and vertical faces at the edge are out of phase influencing the radiation of the leaky Rayleigh wave for the water‐loaded case. Near the edge it is observed that a decrease in the Scholte wave signal is accompanied by an increase in the leaky Rayleigh wave signal. The leaky Rayleigh wave velocity for the water/ice is slightly smaller than the shear wave velocity at the onset of the signal. [Work supported by DREP and ONR.]
Return to All Sections
Close

Home Publications Meetings Contact ASA Site Map Back to Top

Copyright © Acoustical Society of America

close