Data-based mode extraction with a partial water column spanning array

Abstract

In a shallow ocean waveguide the acoustic field can be characterized by depth-dependent modes propagating in range with an associated propagating wavenumber. Though recently developed methods for determining the modes from recorded acoustic data alone without ocean or bottom modeling have shown promise, they are only applicable when the acoustic field is sampled over the entire water column. This paper presents a method for determining the acoustic modes from measured data alone when the field is sampled over only a portion of the water column. The method requires broadband sources at many ranges, e.g., a moving source, in order to construct the frequency-wavenumber ͑f-k͒ structure of the waveguide. Because modal propagation is dispersive, the modes are characterized by a discrete set of wavenumbers that vary continuously with frequency. Due to the discreteness of the modal wavenumbers, it is possible to isolate the modes in the f-k domain and extract them individually with a singular value decomposition ͑SVD͒. Because the modes are extracted individually the full-spanning and degeneracy limitations of the SVD are removed. Theory, simulation, and laboratory data confirm the process.

Focal depth shifting of a time reversal mirror in a range-independent waveguide

“…In the second strategy, mode isolation is achieved by directly localizing the wavenumber characteristics of the modes. 7 By performing a wavenumber analysis, it is possible to isolate the modes according to their discrete propagating wave numbers. Though this requires the accumulation of pressure fields from many ranges, it has the advantages that it can be applied over a limited aperture TRM and that it results in depth shifting over many ranges ͑see Fig.…”

“…7. Experimental results are presented and used to illustrate the development of the wave-number isolation depth shifting technique.…”

“…As discussed in Ref. 7, the rotation transforms the location of the modal wave numbers in the f-k domain, k m ͑͒ → k 1 ͑͒ − k m ͑͒. In addition to exaggerating the intermodal intervals, this choice of rotation mitigates aliasing of the high order modes.…”

“…The method is an extension of the frequency-wave-number ͑f-k͒ mode extraction method introduced in Ref. 7. Although other methods for extracting the depth dependence of the propa-gating modes exist ͑such as the control feedback method introduced in Ref.…”

Dedispersion Transform Method for Extracting the Normal Modes of a Shallow Water Acoustic Signal in the Pekeris Waveguide

“…2b. The singular value decomposition (SVD) method (Neilsen et al, 1997;Neilsen, Westwood, 2002) and the f -k transform (Walker et al, 2005;Nicolas et al, 2006) have been presented to extract the mode depth function or to isolate the normal mode in the frequencywavenumber plane. The SVD can only extract the mode depth function.…”