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The authors have developed a USCT system that can be used for testing wood and stone structures. It comprises a Proceq Pundit Lab+ non-metal ultrasonic device, an amplifier between the receiving probe and the detector, a sensor diameter convertor, a multi sensor fixator and the USCT analysis software. We have 20 Sonotec L40 54 kHz sensors of diameter 50 mm, and the convertors transmit the diameter to 10 mm when contacted the tested object. That makes the coordinates of each contacting points more precise. Non couplant is used for testing thus avoiding the penetration of couplants into the object through open cracks.

Laboratory tests were made with several wood and limestone samples, and the USCT images correspond very well with the visible appearance of the samples commendably. Fig. 7 shows four of them. All the equipment can be packed into one suitcase and easily transported for on-site testing.

For the on-site testing of the statue, a section of the cranial region of the statue was chosen as shown in Fig. 8, and sixteen probes were used. Fig. 9 shows the USCT detecting array and USCT image.

Figure 5: Probe for crack depth detection.

Figure 6: Cracks in the marble of the Soong Ching-ling statue.

The ultrasonic velocity in fresh marble similar to 177the material of the statue is about 4,500 m/s, and the measured velocities shown in the result image range between 1,000–4,500 m/s.

According to the USCT results no penetrative severe cracks were found. The depth of the superficial cracks on the top of the head was not more than 50 mm. However, a clearly deteriorated zone with a thickness from 10 to 50 mm, caused by the disintegration of the crystal fabric, was found around the head.