Experimental estimation of the longitudinal-wave backscatter coefficients for ultrasonic interrogation of weak scattering materials

The starting point for solving inverse scattering problems in acoustics is an estimate of the target’s scattering function. A typical procedure for estimating a scattering function is to apply an inverse filter to the Fourier transform of the backscattered signal from the target. The inverse filter is based on an estimate of the measurement system response function that includes electronic, transducer, and wave propagation related effects. In this paper, we address the problem of estimating the backscatter coefficient for scattering from a large number of randomly distributed scatterers. A method is presented for estimating the backscatter coefficient for weak scattering polycrystalline metals for the case in which the backscatter coefficient is independent of depth into the material. Measurement system response function estimates were based on volume integrations over velocity fields calculated using Gauss–Hermite polynomials. Ultrasonic measurements were made for direct backscatter in an immersion mode at normal incidence using both planar and focused probes operated broadband. Results are presented that show close agreement between the frequency dependence of the backscatter coefficient estimates and the frequency dependence of the classically estimated attenuation coefficient. The backscatter coefficient estimates are shown to be reasonably independent of the system configuration and depth into the material.