Image Processing Reference
In-Depth Information
SNR
2DWVT
(dB)
SNR
ini
(dB)
4 bands
experiment 7
Est. Meas. Est. Meas. Est. Meas.
0 0 4.93 0 8.64 0 12.88
1 6 8.90 9 12.81 12 18.08
2 12 11.91 18 19.01 24 25.31
3 18 16.76 27 28.02 36 38.92
4 24 21.63 36 35.70 48 50.45
5 30 27.65 45 45.32 60 64.33
6 36 34.63 54 56.13 72 80.90
7 42 41.53 63 63.17 84 94.67
8 48 48.91 72 78.46 96 111.31
9 54 56.88 81 90.69 108 127.91
10 60 64.24 90 101.73 120 142.04
Table 2. SNRs of the 2D representations obtained by means of the experiments 5 to 7.
2 bands
experiment 5
3 bands
experiment 6
The estimated and measured values of the
SNR
2Dtime
(Table 1, columns 2 and 3) and
SNR
2DTFlinear
ratios, obtained for 2 bands (Table 1, columns 4 and 5), 3 bands (Table 1,
columns 6 and 7) and 4 bands (Table 1, columns 8 and 9), present a very good agreement.
Finally, the
SNR
2DWVT
(Table 2) for different bands number show a high correlation between
estimated and measured values, but in some cases small differences appear. These are due
to the fact that the estimated expression for
SNR
2DWVT
was obtained by means of
approximations, but in any case, the global correspondence between estimated and
measured values is also reasonably good.
Apart from SNR improvements, the three techniques described in this chapter allow the
accurate detection of flaws inside pieces.
A second type-I experiment was realised to show this good accuracy in the defect detection
capability inside the pieces. A new set of ultrasonic traces was generated, simulating again a
hole in a rectangular piece as it is depicted in figure 4. In this case, the selected
SNR
ini
of the
initial A-scan was 3 dB.
The echo is the real 4 MHz trace sampled at 128 MHz, and the noise contained in the initial
eight traces was composed by white noise and coherent noise with amplitudes of 50% each
one. This set of simulated measures is displayed in figure 7, being the units shown in
horizontal axis micro-seconds. In these graphics, it can be appreciated that noise and echo
amplitudes are similar, thus it is very difficult to distinguish the reflector echo from the
noise. In fact, the echo only appears in graphics corresponding to transducers H3 and V2.
The real echo-pulse of H3 transducer is located in the middle of the noise beginning
approximately at 5.5 microseconds whereas the echo-pulse of V2 transducer begins around
10.75 microseconds.
Using the ultrasonic registers of figure 7, the three combinations of the traces by applying
the different techniques exposed in the chapter were performed. The first combination was
done using the time domain method and the resulting 2D representation is shown in figure
8.a., where the 24x24 mm inspected area is displayed (the axis units are in mm). The
searched hole location is around 8 mm in horizontal axis and 15 mm in vertical axis. It can
