Siegfried Siegesmund - Monument Future

Figure 3: tomography of a water filled bucked with a POM cylinder.

Figure 4: tompgraphy of an artifically aged marble cylinder.

243The software of the system offers the possibility to calculate tomographies by algebraic reconstruction technique (ART), simultaneous iterative reconstruction technique (SIRT), Kaczmarz method, and inverse radon transformation of the velocities calculated by converter positions and times of flight.

The new ultrasound system was first tested in the laboratory on so-called ultrasound phantoms. The first phantom was a cylindrical bucket filled with water in which a rod (Ø = 90 mm) made of polyoxymethylene (POM) was placed. Tomography was performed with 24 ultrasound transducers.

Figure 3 shows the result of the evaluation. The rod made of POM is clearly visible as a yellow area. The sound velocity in water was calculated with approx. 1.500 m/s and the sound velocity in POM with approx. 1.900 m/s was calculated correctly. The second phantom was a cylinder made of Carrara marble which was artificially aged. The cylinder was then heated to 150 °C and thrown into cold water. Thus, the average speed of sound could be reduced to approx. 3.200 m/s. Figure 4 shows the results of the evaluation. These correspond well with the expected values.

After successful measurements in the laboratory, the system was tested on the columns (Ø = 550 mm) of the Marmorpalais in Potsdam (see Figure 2). These are heavily weathered and show cracks, some of which have been filled.

Figure 5 shows the found structure of the marble. Although signals could be received at this object, they had large interference components. An automatic detection of the signal beginnings and thus an evaluation of tomography was not possible. The performance limit of the system was reached because of the strong fissuring of the stone.

Figure 5: typical structure of the marble columns of the Marmorpalais in Potsdam.

The measurements on the ultrasound phantoms prove the principle functionality of the new ultrasound system in the creation of tomography on stone. The positioning of the ultrasonic transducers with the holding system takes approx. 30 min. and the recording of the positions approx. 10 min. The actual measurements and the calculation of the tomography take approx. 20 min. with max. 32 transducers. The determination of a tomography is thus easily possible within one hour. The transmission voltage of 1,000 Vpp at 9 mm aperture of the transducers is not sufficient to receive evaluable signals in case of strongly weathered and fissured marble. In addition in all cases, the high transmit voltage on the multiplexer generates a coupling to the receive channels. The resulting signals allows a reliable measurement only from 150 mm marble thickness.

The principle feasibility and the associated time saving could be demonstrated. However, a device that can be used practically requires the revision of the multiplexer and the electronics. As mentioned before, the high transmitt voltage generates a coupling to the receive channels and therefore the begin of the signal is difficult to define and structures thinner than 150 mm cannot be examined. This problem can be solved by a new layout which carefully separates the transmit and the receive parts of the electronic and their electrical grounding.

Furthermore, the holding system of the transducers 244should be optimized to allow a strong and secure mounting of the transducers on the marble surface. The existing system software has many useful features and is easyly to handle. Together with an optimized hardware, the system could accelerate the tomography of stone made cultural heritage.

The development was supplied by the Federal Ministry of Education and Research of Germany, VIP+00291.

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David Martín Freire-Lista, Luís Sousa

IN: SIEGESMUND, S. & MIDDENDORF, B. (EDS.): MONUMENT FUTURE: DECAY AND CONSERVATION OF STONE.

– PROCEEDINGS OF THE 14TH INTERNATIONAL CONGRESS ON THE DETERIORATION AND CONSERVATION OF STONE –

VOLUME I AND VOLUME II. MITTELDEUTSCHER VERLAG 2020.

UTAD – Universidade de Trás-os-Montes e Alto Douro, Quinta de Prados, 5001-801 Vila Real, Portugal

CGeo – Centro de Geociências da Universidade de Coimbra, Rua Silvino Lima, Universidade de Coimbra, Polo II, 3030-790 Coimbra, Portugal