Experimental Study On Ultrasonic Velocity Detection In Attenuation Medium

Accurate detection of media's ultrasonic velocity is very important.The ultrasonic wave-field is a function of propagation distance.Therefore,we must consider the effect of sample size on velocity detection.We measured the velocity of six groups of lucite,epoxy and sandstone samples with different aspect ratio(the ratio of length to diameter)at three frequencies.Measured data show that as the aspect ratio increasing,velocity increases gradually from the "slab" velocity to "block" velocity first,and then reduces to "bar" velocity.If the sample is short,near-felid effect may cause the increase in velocity as distance decreases.When the length of sample>2.?,the effect of near-field is negligible.In addition to sample length,measurement error will directly affect the accuracy of velocity measurement.Therefore,we should average several measurements to reduce the measurement error.We discussed the relationship among group velocity,phase velocity and other four popular laboratory velocities.Experimental results indicate that one of the requirements for accurately calculating group velocity and phase velocity is to insure high spectral similarity between reference sample and test sample.Group velocity is more sensitive to signal to noise ratio than phase velocity and is more sensitive to frequency.For velocity detecting methods,the applicable conditions for picking up the propagation time at the starting point of the first-break wave are,attenuation is small,main frequency of the transducer is high,the length of two samples is close,and measured velocity is close to the maximum group velocity.The applicable condition for picking up the propagation time with the maximum amplitude of the initial wave are,low attenuation,low main frequency of transducer,small length difference between two samples,and the measured velocity is not the maximum group velocity.The applicable conditions for calculating the propagation time with the correlation method are,attenuation is not big,main frequency of transducer is low,the measured velocity is slightly less than the maximum group velocity.The physical significance of the velocity obtained by Hilbert transformation method is unknown so we do not recommend.