Experimental Separation and Identification of Acoustic Normal Modes in Shallow Water.
Abstract
Normal-mode models of ducted propagation require knowledge of the attenuation coefficients for each mode in order to predict signal intensities. An experiment was conducted in the Gulf of Mexico in July 1969 to evaluate a technique for isolating, identifying, and measuring the intensity of signals propagated in individual normal modes. The measurements were made under downward refracting conditions in 30 meters of water with a level sand bottom. By projecting short (3-cycle) pulses and listening at a sufficiently long range, the differing group velocities of the modes permitted resolution of some individual mode arrivals. The resolved modes were identified by comparing the measured vertical intensity distributions and relative group velocities with those predicted by a normal-mode model. The model used assumes a semi-infinite, lossless, fluid bottom of constant velocity overlain with a constant-depth water layer having an arbitrary velocity profile. At 400 Hertz the observed intensity distributions and relative group velocities for the first two modes were in excellent agreement with predicted values. Results at higher frequencies were inconclusive, since the prevailing sound-speed profiles did not permit resolution of any individual modes. Results of this experiment show that the short-pulse technique is a practical means of making direct measurements of modal attenuation. (Author)
Document Details
- Document Type
- Technical Report
- Publication Date
- Oct 30, 1970
- Accession Number
- AD0715340
Entities
People
- Andrew L. Faber
- Frank Ingneito
- Raymond H. Ferris
Organizations
- United States Naval Research Laboratory