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Reverse-Time Migration of Ultrasonic-Echo Data on a Foundation Slab

The ultrasonic echo technique is a frequently used method in non destructive testing for geometry determination of concrete building elements. Important tasks are thickness measurements as well as the localization and characterization of built-in components and inhomogeneities. Currently mainly the Synthetic Aperture Focusing Technique (SAFT) is used for imaging. This algorithm is closely related to the Kirchhoff migration method and has difficulties in imaging steeply dipping interfaces and complicated structures such as steps and lower boundaries of voids. A reinforced concrete foundation slab with various reinforcement contents, different thicknesses and two pile heads was used for testing the prestack reverse-time migration (RTM) method. In a first step, the RTM was evaluated with synthetic 2D data. In the second step, ultrasonic measurement data recorded with shear wave transducers at two mutually perpendicular line profiles on the foundation slab were processed. The use of an automatic scanner simplified the measurements. Experiments such as the one detailed in this paper may be of interest to evaluate seismic migration methods on analogue models. A comparison of the RTM images with those of SAFT shows a significant improvement in the imaging of the geometry of the foundation slab. Vertical borders were reconstructed and the location and structure of the lower boundary of the foundation slab was reproduced better. Limitations still exist in imaging the pile geometry. The real data images show very noisy signals from the piles due to the reinforcement, edge effects and multiple reflections at the pile shaft.