Use of frequency-difference beamforming for scatterer localization

Abstract

Acoustic fields interacting with discontinuities or environmental variations lead to secondary scattered fields superimposed onto incident fields. As a result, the signal measured at a remote receiving array will be modified by the scatterer’s presence. Prior work has shown that the effects of strong random scattering are reduced using frequency-difference beamforming to downshift the analysis to a below-band frequency known to be less impacted by scattering. In some applications, information about a scatterer is desired, such as location, shape, or composition. However, a relatively weak scattered field from a single scatterer is difficult to detect when the incident field dominates the measurements, as both conventional and frequency-difference beamforming ambiguity surfaces primarily provide the incident field’s source information. Here, a subtraction-based algorithm is implemented with frequency-difference beamforming to locate the position of a single scatterer near a source. Simulations and water tank experiments with a 110 kHz center frequency signal are considered for incident and scattered fields with a 4 cm diameter spherical scatterer, where the scattered field’s received energy is roughly 2% that of the incident field. The ability to downshift the frequency demonstrates a more robust implementation of subtraction-based algorithms for locating weak scatterers without prior location knowledge. [Work supported by ONR.]