The research line entitled Non Destructive Testing (NDT) aims to inspect the interior of objects of interest without physically modifying them. This goal is similar to that of geophysics. The main difference, that prevents a direct transfer of methodologies from the field of geophysics, lies in the much smaller scale of the examined targets.
The size of sensors and active elements decreases and the boundary conditions plays a more relevant role, which is an important complication.
Our inspection methods dealing with wave propagation use both elastic (in the sonic range) and electromagnetic waves (GPR at frequencies higher than 1GHz). In the inspected object, energy is input –for instance through the use of an instrumented hammer or a GPR antenna-. Energy moves inside the object producing deformation that propagates depending of the elastic (electromagnetic) behavior of the material. These motions are measured and recorded at certain positions using adequate sensors (i.e. accelerometers / GPR antennae).
Both the times of arrival and the shape of the recorded signal carries information about the internal structure: possible defects, mechanical properties change, degradation, presence of water or other fluids, presence of conductor portions, etc.
In real applications it is too difficult to infer the internal properties of the object from the observation of the recorded temporal signal alone. It is usually too complex. To increase the possibilities of success, the signal is transformed following established procedures collectively known as signal processing methods.In processing the raw signal, the goal is to enhance and magnify certain interesting properties at the expense of damping or even deleting others not so relevant for the occasion.
The work with the processed signal allows detection but not quantitative estimation, with the exception of a few cases. To achieve quantitative estimates the mathematical modeling of the physics of wave propagation has to be coded in a computer. The model includes, it is assumed, the relevant geometrical and physical characteristics of the real process. Its output gives sensible images of the interior honoring the observations. Again, this is the inverse problem. This last step is less frequent in GPR.
For sonic methods, the energy sources are hammers (both instrumented and not instrumented) as well as different types of impactors. As sensors, accelerometers and microphones are used. The environmental noise can also be used as energy source.
Our projects deal with impact-echo, guided waves propagation and the interferometry of elastic waves to be able to use random and spatially dispersed energy sources.