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period for pieces that do not have clear chronological markers. In addition, the
semi-supervised training mode could be used to model the uncertainty that expert
archaeologists may have about the chronological period to which the pieces
belong.
6.2 Consolidation Diagnosis and Layer Determination
in Heritage Building Restoration
6.2.1 Introduction
NDT by ultrasounds is a very useful technique applied in fields such as con-
struction, food, and biomedicine. The technique has basically two operation
modes: pulse-echo (one sensor as emitter and receiver) and transmission (one
emitter and one receiver). An ultrasound pulse is injected in the inspected material
and a response of the material microstructure is received [
22
,
23
]. The measured
signal can contain echoes produced from discontinuities, inhomogeneities, borders
of the material, plus material grain noise (superimposition of many small echoes
due to the material microstructure). All of this information can be used for quality
control and characterization of materials [
25
,
26
]. The present study used the
pulse-echo technique, given that the inspected material consisted of a wall with no
possible access from opposite sides. This wall was a zone at the dome of the
Basilica de la Virgen de los Desamparados in Valencia, Spain.
This section includes two novel applications of ICA [
27
,
28
] as pre-processor in
ultrasound NDT applied to historical building restoration. The first application
consisted in using the mixture matrix to distinguish information about the con-
solidation process of the material. This was done by injecting a product to fill
cracks in the wall. The second application consisted in detecting interferences in
the recorded signals to cancel them thereby improving the quality of the signals.
This procedure was applied to recorded signals for estimating the thickness layer
profile of the wall.
Interferences can be due to the internal clocks of the measurement equipment,
interferences with other equipment, and so on. In many applications, the recording
of high quality raw data is a difficult task, especially in situations where the
conditions cannot be controlled during the experiment. One difficulty in obtaining
the measurements at the dome was the use of a plastic for covering the transducer
in order to avoid direct contact of the ultrasonic coupling gel with the artistic
paintings on the walls. This kind of measurement produced attenuated signals.
B-Scans diagrams were used to visualize consolidated and non-consolidated
material zones to check the quality of restoration and to detect interfaces between
the different materials in the wall. B-Scan is a 2D representation of a signal set.
The evolution in time windows of a parameter such as power or spectrum was
calculated for each one of the signals. Then, all of the calculated information was
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