An acoustic microscope for microflaw inspection and subsurface imaging

The representation of an ultrasonic signal coming from the
interior of a material using A mode, i.e., a single line of time behaviour, is
widely applied in ultrasonic flaw inspection. Much more information on the
material can be gained by creating an image in the B representation mode from a
large number of A-scan lines. The use of high-frequency ultrasound for creating
B-scan images permits the imaging and examination of material structures and
inhomogeneities which used to be invisible to date, down to the depth of several
millimeters under the sample surface. This paper describes a laboratory model of
an acoustic microscope designed for imaging material interior using the B-scan
representation mode. The microscope works in the frequency range of 20-70 MHz
where images are created on the computer monitor screen or printed out using a
video copy processor. On the one hand, the application of the microscope signal
processing methods permit the image quality and resolution to be improved, and,
on the other, they allow the acoustic properties of materials to be measured.
The signal is processed for the purpose of reducing grain noise and eliminating
the effects of image representation on a screen with low dynamics compared with
the dynamics of the signal. Attempts were also made to find the characteristic
properties of the medium under study based on a comparison of the frequency
compositions of ultrasound pulses penetrating samples with different grain sizes
and attenuation.