Challenges in the Separation and Analysis of Scattered Waves in Angle-beam Wavefield Data

The measurement of ultrasonic signals on a 2-D rectilinear grid resulting from a fixed source, referred to as wavefield imaging, is a powerful tool for visualizing wave propagation and scattering. Wavefield imaging provides a more complete picture of wave propagation than conventional single-point measurements, but creates more challenges for analysis. This work considers the development of wavefield-based methods for analyzing angle-beam wave propagation and scattering in plates. Methods of analysis focus on the separation of scattered waves from the total wavefield with the eventual goal of quantitative scatterer characterization in a laboratory environment. Two methods for wave separation are considered: frequency-wavenumber filtering and wavefield baseline subtraction. Frequency-wavenumber filtering is applied to wavefield data that are finely sampled in both space and time, whereas baseline subtraction is a technique that has typically been applied to individual signals recorded from fixed transducers rather than to full wavefield data. Baseline subtraction of wavefields, particularly for the frequency range considered here, is sensitive to both specimen alignment and temperature variations, whereas frequency-wavenumber methods are limited in their ability to separate waves traveling in the same direction. Results are shown for both methods with a focus on investigating and overcoming the challenges to full wavefield baseline subtraction.