Robert F. Gragg Daniel Wurmser
Naval Res. Lab., Code 7144, Washington, DC 20375-5350
This work advances the capability for modeling the scattering from near-surface bubble plumes beyond the static approximation. Plumes were modeled as evolving structures having finite lifetimes (~10 s) with distinct growth and decay stages. The scattering response of such a plume was formulated in terms of the theory of linear time-varying (LTV) filters. Complex scattering amplitudes within the frequency band of interest (50--500 Hz) were computed using either partial wave analysis [for (hemi-) spherical shapes] or boundary integral equation (BIE) methods (for more general shapes). For cw inputs, groups of pseudo-Doppler side peaks were observed in the scattered spectra as a result of the time evolution of the target's internal resonances. Several broadband source waveforms were considered to assess their effectiveness as experimental probes for observing transient near-surface plumes in the field. [Work supported by ONR, Code 4530.]