Geometry Determination of a Foundation Slab Using the Ultrasonic Echo Technique and Geophysical Migration Methods

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 family of techniques (SAFT) is used for imaging. These algorithms have difficulties in imaging steeply dipping interfaces and complicated structures such as steps and lower boundaries of voids. As an alternative two geophysical migration methods, pre-stack Kirchhoff depth migration and pre-stack Reverse-time migration (RTM) were tested in this paper at a reinforced concrete foundation slab. The slab consists of various reinforcement contents, different thicknesses and two pile heads. In a first step, both methods were evaluated with synthetic 2D data. In the second step, ultrasonic measurement data recorded with shear wave transducers on a line profile on the foundation slab were processed. The use of an automatic scanner simplified the measurements. A comparison of the geophysical migration results with those of SAFT shows, in particular for RTM, 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 were reproduced better. Limitations still exist in imaging the piles below the slab.