The aim of microseismic monitoring is to evaluate the compactness level of the examined masonry, that is the possible presence of holes, the martor’s state and its constitutive elements.
The method’s principle is based on the waves propagation theory through a medium and particularly on the fact that their propagation speed is strictly related to the elastic features of the medium they have crossed.
The measurements are performed by applying a pressure impulse on a masonry point and observing the effects in the diammetrically opposite position.
In fact, once the distance of the wave’s itinerary is found and the related transit time is measured, it is possible to calculate the transaction’s speed of the wave in the analysed medium.
The acquisition chain employed for the sonic tests consists of:
- A metal stamping, employed to locate the exact poit to be energised, equipped with a geophone serving as time recorder at the wave’s starting.
- A hammer, serving as sonic impulse’s source
- A geophone, diametrically opposed to the metal stamping serving as waves receiver
- A laptop equipped with a software for acquiring, monitoring, storing and processing dynamic data
The hammer hits the instrumented stamp, that in turn will transmit the impulse to the rebar; in the meantime the impulse imput is perceived by the sensor in the stamp and recorded by the software. The wave crosses the masonry and reaches the second sensor, which perceives it and senti t to the software, where it is stored.
The result of this test consists in a reflexogram, where it is possible to read the transit time and, known the distance, to calculate the speed.
Through the realization of a rectangular grid of measurement points, the sonic investigations can provide a sonic tomogram, where, thanks to the different colour shades, it is possible to highlight the areas having different physical and mechanical features.
- Establishing the consistency state of the masonry
- Individuation of discontinuity, gaps and break-lines
- Check of the efficiency of consolidating interventions made through cement injections into a masonry, by comparing the conditions before and after.
- Non-invasive technique
- Establishing immediately the test values
- Check on masonry masses even of a considerable thickness