Читать книгу Monument Future - Siegfried Siegesmund - Страница 358

For about 50 years, various ultrasound-based methods have been developed for the examination of stone in the field of cultural heritage and building technology [1]. The review article by Chiesura et al. [2] and the book Architecture in Stone by S. Siegesmund and R. Snethlage [3] provide an informative overview. The speed of sound is the most widely used parameter in the investigation of marble, particularly the speed of sound of longitudinal waves used in 70 % of the articles sighted. The speed of ultrasound depends on the mineralogical, physical and mechanical properties [4], the degree of saturation of water [4, 5] and the degree of aging. The speed of sound in marble varies between 6000 m/s for freshly quarried material and 1500 m/s for heavily weathered material. Therefore, the determination of the speed of sound provides valuable information about the properties and condition of the marble. The most widespread method is the measurement of the sound velocity of longitudinal waves in transmission. Two ultrasonic transducers are mounted opposite each other on the stone to be examined and the sound velocity between them is determined. The transmission method can be modified in various ways by varying the arrangement of the transmitter and receiver in order to adapt them to different measurement tasks. There are, for example, the so-called radial transmission [6], the semi direct transmission [7] or refraction methods with surface waves [7]. A very meaningful technique is ultrasound tomography. It allows the representation of velocity distributions in cross-sections of marble objects and thus the assessment of the course of weathering [8] and the success of conservation measures [9]. Up to now, the measured values for the calculation of a tomography have been recorded manually with (digital) calipers and single-channel electronics with two ultrasonic transducers between 46 kHz and 250 kHz. First, the transmitter transducer is in any measuring position and the receiver transducer 240successively takes up the other measuring positions in the same plane. For each measuring position, the velocity of sound between the transmitter and receiver is determined. The amount of sound velocities that can be assigned to a transmitter position is called projection. After the data of the first projection has been acquired, the transmitter is moved to the next measuring position and the data set of the second projection is measured. A tomography of 32 measuring positions consists of 32 projections, i. e. 31 × 32 = 992 individual measurements. It is easy to understand that recording the data of an ultrasound tomography is very time-consuming and can take more than one working day. The ultrasound system presented here for the first time reduces the required time for a tomography with 32 measuring points to approximately 1 hour.