Tomographic Imaging Using Multi-Channel Low-Frequency Ultrasonic Measurement Systems

Multi-element ultrasonic transducer arrays open the possibility for rapid and flexible tomographic imaging systems. By electronic switching of the individual array elements, different schemes of reflection tomography and transmission tomography can be realized. This contribution firstly introduces general tomographic measurement principles to some detail. It then describes two multi-channel low-frequency ultrasonic measurement systems and their application to tomographic imaging. Both systems comprise transducer arrays in which each transducer group can be electronically switched between transmission, reception, and deactivation. Tomographic reconstruction algorithms provide three-dimensional volume imaging. The first imaging system, the reflection-tomographic FLEXUS system for concrete evaluation, incorporates a 48-element/16-channel transducer array. The array is moved by a three-axes scanner in a range of 1.0 m x 0.8 m. The collected data is online processed by a SAFT (synthetic aperture focusing technique) algorithm for reflection-tomographic volume imaging. Different SAFT variants are implemented that are optimised towards image quality or measurement speed. The second imaging system, the transmissiontomographic FLEXUS 120 system for refractory testing, includes a 60-element transmitting array and a 60element receiving array. The two arrays are applied to opposite faces of the tested object for transmission measurements. By varying the excitation sequence and the reconstruction algorithm, different transmissiontomographic schemes in direct transmission or with angular diversity can be realized. For both imaging systems, measurement results are presented and discussed.