Image Processing Reference
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be deduced that by using this time domain technique, the flaw is not very well marked and
a lot of noise appear, but it is must taken into account that, in the initial traces shown in
figure 7, the echo level was under noise level, in some cases.
The linear time-frequency transform used for second combination in this comparative
analysis was the undecimated wavelet packet transform with Daubechies 4 as mother
wavelet, as in the previous set of experiments. Figures 8.b, 8.d and 8.e show the 2D
representations obtained using wavelets with 2, 3 and 4 bands. In these graphics, which
amplitudes are in linear scale, it can be clearly distinguished the mark corresponding to the
hole. Figure 8.f represents the same result than 8.e, but with the gray scale of amplitudes
measured in dB, in order to appreciate with more detail the low levels of noise.
Finally figures 8.c, 8.g and 8.h show the 2D representations obtained using WVT with 2, 3
and 4 bands and using a linear scale for amplitudes. Figure 8.h and 8.i correspond to the
same results, but figure 8.i is displayed with its amplitude scale expressed in dB. Thus, in
figure 8.h, the noise has disappeared but in figure 8.i the low level noise can still be
observed. It must be noted that, for all the cases, the 2D representations of figure 8 mark the
flaw that we are looking for, although in the initial traces, shown in figure 7, the echoes
coming from the flaw were very difficult to see.
Additionally, in the first strip of the figure 8, the 2D graphic resulting when time domain
method is used, is shown. It can be seen its performance in contrast with the wavelet
method with minimum quality ( L =2) and WVT option with minimum quality ( L =2), in such
a way that a quick comparison can be made among improvements applying the different
methods.
In that concerning to results of type-II experiments, displays of 2D representations, obtained
by combination of experimental traces acquired from the ultrasonic prototype described in
section 4 are presented in figure 9. Two scales have been used for each 2D result: linear and
logarithmic scales. With the logarithmic scale, the small flaw distortions and secondary
detection indications, produced by each combination method, can be more easily observed
and quantified. It must be noted that the logarithmic scales have an ample resolution of 60
dB, giving a better indication of techniques performance.
In all these cases, the initial traces had a low level of grain noise because these echo-signals
correspond to reflections from the small cylindrical hole drilled in a plastic piece made of a
rather homogeneous material without internal grains. The patterns of figure 9 were obtained
using similar processing parameters than those used with the simulated traces in the type-I
experiments, and only two bands were considered for frequency decomposition. The results
of the figure 9, using the time-combination method, present clear flaw distortions (more
clearly visible in 9.b) with shadow zones in form of a cross, but even in this unfavourable
case, a good spatial flaw location is achieved.
The mentioned crossing distortions appear already very attenuated in the results shown in
figures 9.c and 9.d, corresponding to the linear time-frequency combination technique
(wavelet using 2 bands), and practically disappear in the results of figures 9.e and 9.f
obtained by using to the WVT combination technique.
Similar good results could be also achieved in many practical NDE cases with isolated-flaws
patterns, but this performance could be not extended to other more complicated testing
situations whit flaws very close among them, i.e. with two or more flaws located into a same
elemental cell and thus being insonifyed by the same two perpendicular beams. Under these
more severe conditions, some ambiguity situations, with apparition of “phantom” flaws,
could be produced [Rodríguez et al 2005]. We are working order to propose the extension of
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