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Das Ultraschall-Echo-Verfahren ist eine klassische zerstörungsfreie Prüftechnik zur Geometriebestimmung sowie Schadensanalyse von Betonkonstruktionen. Um die Abbildung der Ultraschalldaten von komplexen Betonstrukturen zu verbessern, haben wir die Reverse-Time Migration (RTM) aus der Seismik auf die zerstörungsfreie Prüfung im Bauwesen übertragen. In einer Vorstudie haben wir einen 2D akustischen RTM Code verwendet und an realen Ultraschalldaten, die an einer Betonfundamentplatte aufgenommen wurden, getestet. Im Vergleich mit dem herkömmlichen Rekonstruktionsalgorithmus für Ultraschalldaten, der Synthetic Aperture Focusing Technique (SAFT), zeigten die akustischen RTM-Ergebnisse eine deutliche Verbesserung in der Abbildung der inneren Struktur der Betonfundamentplatte. Vertikale Reflektoren konnten rekonstruiert werden, was mit dem herkömmlichen Abbildungsverfahren nicht möglich war. RTM ist, im Gegensatz zu SAFT, eine wellengleichungsbasierte Migrationsmethode und beruht auf der numerischen Lösung der vollständigen Wellengleichung. Die Vorteile sind, dass der Algorithmus sämtliche Informationen des Wellenfeldes verwendet und daher verschiedene Welleneffekte wie z.B. Multipathing berücksichtigt werden können. Die RTM bietet damit die Möglichkeit auch stark geneigte Reflektoren sowie Streukörper mit komplexen Geometrien darzustellen. Ein Nachteil ist jedoch die lange Rechenzeit und der hohe Bedarf an Speicherkapazität. Ein RTM-Algorithmus, der die elastische Wellengleichung anstatt der akustischen verwendet (wie in unserer Vorstudie angewandt), hat das Potenzial, die Abbildungsergebnisse noch weiter zu optimieren. Das liegt daran, dass unsere Ultraschallmessdaten durch Anregung elastischer Wellen generiert werden. In einem ersten Schritt haben wir zwei elastische 2D-RTM-Algorithmen an synthetischen Ultraschalldaten getestet. Diese wurden mit einem Betonmodell, bestehend aus mehreren Stufen und kreisförmigen Lufteinschlüssen, erzeugt. In einem zweiten Schritt wurden reale Ultraschalldaten mit Scherwellenprüfköpfen an einer Betonstufenplatte mit integrierten Hüllrohren aufgenommen. Die Auswertung der realen Daten mit unserem elastischen RTM-Code war ebenfalls erfolgreich. Wir konnten die Abbildungsqualität der Stufen und Hüllrohre im Vergleich zur akustischen RTM und SAFT verbessern.
The ultrasonic echo technique is widely used in non-destructive testing for investigation and damage analysis of concrete constructions. Important applications include thickness measurements, geometry determination, the localization and characterization of built-in components as well as the detection of quality issues (cracks, honeycombing, low concrete strength).
To improve ultrasonic data imaging of complicated structures in concrete, we transferred a seismic migration technique, the Reverse Time Migration (RTM), to non-destructive testing in civil engineering. In a preliminary study, we tested a 2D acoustic RTM algorithm on measured ultrasonic echo data acquired at a concrete foundation slab. Compared to the conventional used synthetic aperture focusing technique algorithms (SAFT) for ultrasonic data reconstruction, our acoustic RTM results showed a significant improvement in imaging the interior structure of the concrete slab. Vertical reflectors were reconstructed which was not possible by traditional imaging.
In contrast to SAFT, RTM is a wavefield-continuation method in time and uses the full wave equation. RTM is, thus, able to include multiple reflections and to handle multi-pathing as well as many other complex situations. As a drawback RTM requires extensive computing power and memory capacity. Nevertheless, due to progresses in parallel processing and other computational technologies RTM has become appealing for the application in the field of non-destructive testing.
An RTM algorithm, which uses the full elastic wave equation instead of the full acoustic one (as applied in our preliminary work) has the potential to optimize the imaging results even further. This is due to the fact, that our ultrasonic measurement data are generated by exciting elastic waves. Thus, in a first step, two 2D elastic RTM algorithms were tested on synthetic ultrasonic echo data generated with a concrete model consisting of several steps and circular shaped air inclusions. In addition, two imaging conditions were evaluated to reduce migration artifacts. Our synthetic elastic RTM results showed an enhancement in imaging the features inside the test model compared to acoustic RTM and SAFT. In a second step, we acquired ultrasonic measurement data at a concrete test specimen consisting of three steps and four air filled tendon ducts. The evaluation of the real data with our elastic RTM code was also successful and the reconstruction of the geometries of the steps and tendon ducts could be improved. With our study we have shown that elastic RTM is a step forward for ultrasonic testing in civil engineering.
To improve ultrasonic imaging of concrete structures, we transferred a seismic migration technique, the Reverse Time Migration (RTM), to non-destructive testing. A 2D elastic RTM algorithm was tested on synthetic ultrasonic echo data. Compared to the typically used synthetic aperture focusing technique (SAFT) as well as our acoustic RTM algorithm, the presented elastic RTM results show an enhancement in imaging vertical reflectors and complex features inside the 2D numerical concrete model.
Das Ultraschall-Echo-Verfahren ist ein klassisches Verfahren in der zerstörungsfreien Prüfung zur Bestimmung der Geometrie von Bauteilen. Die Prüfaufgaben beinhalten unter anderem die korrekte Dickenbestimmung der Baukörper sowie die Lokalisierung von Einbauteilen und Fehlstellen. Stand der Technik bei den Abbildungsverfahren ist die SAFT–Rekonstruktion (Synthetic Aperture Focusing Technique). Diese Verfahrensfamilie hat Schwierigkeiten bei der Darstellung von steilen Grenzflächen und komplizierten Strukturen, wie beispielsweise Stufen oder Unterkanten von Hohlräumen und Hüllrohren. Als Alternative werden seit einiger Zeit geophysikalische Migrationsmethoden evaluiert. Am Beispiel eines Stahlbetonfundamentes mit verschiedenen Bewehrungsgehalten, unterschiedlichen Dicken sowie zwei Pfahlköpfen wurden die Kirchhoff-Migration und die Reverse-Time Migration (RTM) getestet. Die strahlenbasierte Kirchhoff-Migration arbeitet ähnlich wie die SAFT-Rekonstruktion. Die RTM basiert auf der vollständigen Wellengleichung.
In einem ersten Schritt wurden die Methoden an einem synthetischen, auf der akustischen Wellengleichung basierenden zweidimensionalem Modell getestet. Im zweiten Schritt wurden reale Messdaten, die mit Scherwellenprüfköpfen an der Fundamentplatte aufgenommen wurden, bearbeitet. Der Einsatz eines Scannersystems vereinfachte die Messungen. Die Experimente, die in diesem Manuskript vorgestellt werden, sind von bedeutendem Interesse für die Evaluierung geophysikalischer Migrationsmethoden an analogen Modellen.
Ein Vergleich der Migrationsergebnisse mit den bisherigen SAFT-Ergebnissen zeigt insbesondere für die RTM eine deutliche Verbesserung in der Abbildung der Bauteilgeometrie. Vertikale Kanten konnten dargestellt sowie die Lage und Struktur der Rückwände exakter reproduziert werden. Grenzen bestehen noch bei der Darstellung der Pfahlköpfe, da die von dem zylinderförmigen Pfahl kommenden Signale verrauscht sind. Ursachen dafür sind u.a. die Bewehrung, Randeffekte sowie Mehrfachreflexionen an dem Pfahlschaft.
For decades, the low-strain impact integrity testing using a hammer blow is well established as a method of quality assurance for various pile types. However, this method has its limitations. Our research and development focuses on improving the excitation signal using a shaker system in contrast to the standard hammer method. Another approach is to increase the amount of sensors used during testing. The purpose is to identify the direction of wave propagation which gives advantages under difficult conditions, such as piles below structures.
Pile integrity testing (PIT) using a shaker system was performed on two 11 m long piles of 90 cm in diameter. While one pile was intact, the other one showed a flaw at approx. 3.5 m below pile top, which was confirmed by standard PIT in 2012. A logarithmic sweep between 500 Hz and 1 KHz of 0.1 s was used as the input signal, being vertically injected into the pile. Prior to that, simulations on similar pile geometries showed that the depth of the pile toe as well as flaws within the pile can be extracted by applying regularized deconvolution. The result is the impulse response in the time domain.
The application of deconvolution on the measured signals shows that it is possible to identify the pile length but it is more difficult to clearly extract the flaw’s position in the pile. Additional digital signal processing techniques and the improvement of the regularized deconvolution method as well as the experimental setup need to be investigated.
Another way to improve the PIT method is to use a multichannel sensor arrangement. By arranging several accelerometers vertically along the accessible part of the pile shaft, it is possible to distinguish between downward and upward traveling waves. Furthermore, it is possible to estimate the unknown wave speed, which gives the possibility of more accurate pile length calculations. The method was evaluated successfully during a measurement campaign of a slab foundation with subjacent piles. In 20 of 28 cases the pile length could be detected accurately.
Ultrasonic echo testing is widely used in non-destructive testing in civil engineering to investigate concrete structures, to measure thickness, and to locate and characterise built-in components or inhomogeneities. Currently, synthetic aperture focusing techniques are mostly used for imaging. These algorithms are highly developed but have some limitations. For example, it is not possible to image the lower boundary of built-in components like tendon ducts or vertical reflectors. We adopted reverse time migration for non-destructive testing in civil engineering in order to improve the imaging of complicated structures in concrete. By using the entire wavefield, including waves reflected more than once, there are fewer limitations compared to synthetic aperture focusing technique algorithms. As a drawback, the required computation is significantly higher than that for the techniques currently used. Simulations for polyamide and concrete structures showed the potential for non-destructive testing. The simulations were followed by experiments at a polyamide specimen. Here, having acquired almost noise-free measurement data to test the algorithm, we were able to determine the shape and size of boreholes with sufficient accuracy. After these successful tests, we performed experiments at a reinforced concrete foundation slab. We obtained information from the data by reverse time migration, which was not accessible by traditional imaging. The imaging of the location and structure of the lower boundary of the concrete foundation slab was improved. Furthermore, vertical reflectors inside the slab were imaged clearly, and more flaws were found. It has been shown that reverse time migration is a step forward in ultrasonic testing in civil engineering.
Die beiden in dieser Arbeit dargestellten Beispiele zeigen, dass sich die Low-strain-Pfahlintegritätsprüfung noch über den bisherigen, schon sehr erfolgreichen Stand hinaus entwickeln kann. Der Einsatz von Vibratortechnik und passender mathematischer Methoden bietet das Potential, Prüfungen auch unter bisher nicht lösbaren Randbedingungen durchzuführen. Dazu gehören sehr schlanke Pfähle und Messungen bei hohem Störpegel. Der erhöhte Mess- und Auswerteaufwand ist jedoch im Einzelfall gegenüber dem erzielbaren Erfolg abzugleichen. Apparativ müssen noch Entwicklungsarbeiten geleistet werden, um einen einfachen und zuverlässigen Einsatz in der Praxis zu ermöglichen. Die Messung mit mehreren Sensoren entlang des Pfahls kann schon heute in der Praxis eingesetzt werden. In vielen Fällen lassen sich damit auch Messungen an Pfählen im Bestand durchführen, bei denen die konventionelle Pfahlprüfung aufgrund von überlagernden Signalen aus der aufgehenden Struktur versagt.
Advanced ultrasonic imaging for concrete: Alternative imaging conditions for reverse time migration
(2018)
Ultrasound echo is a widely used NDT technique for determining the internal geometry of structures. Reverse-time migration (RTM) has been recently introduced to NDT applications, as an imaging method for ultrasound data, to overcome some of the limitations (e.g. imaging steeply dipping reflector) experienced by the Synthetic Aperture Focusing Technique (SAFT), the most commonly used imaging algorithm for these measurements.
The standard implementation of RTM also experiences some drawbacks caused by its imaging condition, which is based on the zero-lag of the cross-correlation between source and receiver wavefields and generates high-amplitude low-frequency artifacts. Three alternative imaging conditions, developed for seismic data applications, were tested for their ability to provide better images than the standard cross-correlation: illumination compensation, deconvolution and wavefield decomposition. A polyamide specimen was chosen for the simulation of a synthetic experiment and for real data acquisition. The migrations of both synthetic and real data were performed with the software Madagascar. The illumination imaging condition was able to reduce the low-frequency noise and had a good performance in terms of computing time. The deconvolution improved the resolution in the synthetic tests, but did not showed such benefit for the real experiments. Finally, as for the wavefield decomposition, although it presented some advantages in terms of attenuating the low-frequency noise and some unwanted reflections, it was not able to image the internal structure of the polyamide as well as the cross-correlation did. Suggestions on how to improve the cost-effectiveness of the implementation of the deconvolution and wavefield decomposition were presented, as well as possible investigations that could be carried out in the future, in order to obtain better results with those two imaging conditions.
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 Reversetimemigration (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.