Setup of a bistatic ultrasound vibrometer system. The origin of the coordinate system is located at the mean surface, at the intersection of the source and receiver beam-pattern axes.
Magnitude of ultrasound beam pattern in decibels at intercepted by a surface for (a) , and (b) incidence of beam axis with surface. The transducer is a circular disk of diameter and is positioned from the surface. The equivalent beamwidth of the transducer is approximately .
Realizations of randomly rough (a) medium sand and (b) gravel surfaces. Spectrum of received ultrasound field, , reflected and scattered off harmonically oscillating (c) medium sand and (d) gravel surfaces for an ultrasound vibrometer continuously scanning the surface at a speed of . The corresponding results for the stationary vibrometer are plotted for comparison. (e) Medium sand and (f) gravel are the corresponding coherently processed spectrum of low-pass filtered mixed signal, . The spectra are plotted for an ultrasound source level of re at ( , , , , , ).
Spectrum of ultrasound field at reflected and scattered at normal incidence off a (a) smooth and nonoscillating surface for the stationary vibrometer (same result for scanning system), (b) smooth and harmonically oscillating surface for stationary vibrometer (same result for scanning system), (c) nonoscillating rough gravel surface with stationary vibrometer, (d) oscillating rough gravel surface with stationary vibrometer, (e) nonoscillating rough gravel surface with scanning vibrometer, and (f) harmonically oscillating rough gravel surface with scanning vibrometer. In (a), (b), (e), and (f) the scanning vibrometer is moving at . Unless otherwise specified, the sonar, measurement, and surface parameters are the same as in Fig. 3(d) .
Spectrum of received ultrasound field reflected and scattered off an oscillating (a) medium sand and (b) gravel surfaces. Results were for ten independent realizations of the sand and gravel surfaces, respectively. The average spectrum is obtained by incoherently averaging the intensity of the individual spectra. The ultrasound scan speed is . Unless otherwise specified, the sonar, measurement, and surface parameters are the same as in Figs. 3(c) and 3(d) , respectively.
Time domain wave form of ultrasound received field reflected and scattered off harmonically oscillating (a) sand and (b) gravel surface for the continuously scanning vibrometer. These were used to compute the spectra shown in Figs. 3(c) and 3(d) for the continuously scanning system.
Effect of adjusting the ultrasound source frequency on the spectrum of the received signal. The measurement setup is the same as in Fig. 3(d) . The axis should be scaled according to the ultrasound frequency for each of the cases.
Effect of adjusting the (a) measurement time , (b) vibrometer stand-off distance , and (c) angle of incidence , on the spectrum of the received signal. Unless otherwise specified, the measurement setup is the same as Fig. 3(d) .
Effect of adjusting the vibrometer scan velocity on the incoherently averaged spectrum of received signal from (a) sand and (b) gravel surfaces. All other parameters are the same as in Figs. 5(a) and 5(b) .
Averaged spectrum of scattered noise level vs scan speed in (a) sand and (b) gravel for sidebands corresponding to ground resonance frequencies of 140 and for a ultrasound vibrometer. The scattered noise level is converted to equivalent displacement amplitude using Eq. (27) .
(a) Anisotropic rough surface with five times longer correlation length in the direction than in the direction. (b) Similar to Fig. 3(d) but for the vibrometer continuously scanning in the direction or direction at .
Estimates of surface displacement amplitude from incoherent processing for a ultrasound vibrometer continuously scanning the surface at a speed of . The estimates are obtained by either using data from a single scan, averaging estimates over ten independent scans at a given location, or estimating from the average spectrum for ten independent realizations. The quantities in parentheses are the accuracies for the estimation.
Article metrics loading...
Full text loading...