Table of Contents Title 19. Public Safety Agency 30. Department of State Police Chapter 70. Motor Vehicle Safety Inspection Regulations Part III. Inspection Requirements for Passenger Vehicles and Vehicles Up to 10,000 Pounds (GVWR) 19VAC30-70-180. Clearance lamps, side marker lamps, and reflectors.
REFLECTOR 3 CRACK
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In the framework of non-destructive-testing advanced seismic imaging techniques have been applied to ultrasonic echo data in order to examine the integrity of an engineered test-barrier designed to be used for sealing an underground nuclear waste disposal site. Synthetic data as well as real multi-receiver ultrasonic data acquired at the test site were processed and imaged using Kirchhoff prestack depth migration reverse time migration (RTM). In general, both methods provide a good image quality as demonstrated by various case studies, however deeper parts within the test barrier containing inclined reflectors were reconstructed more accurately by RTM. In particular, the image quality of a specific target reflector at a depth of 8 m in the test-barrier has been significantly improved compared to previous investigations using synthetic aperture focusing technique, which justifies the considerable computing time of this method.
In order to better assess the possibility of imaging the cracks within the barrier for the specific acquisition geometry, a synthetic model representing the barrier has been generated in a first step. The corresponding forward-modelled ultrasonic data have been processed and the imaging results have been analyzed and compared for both applied imaging techniques, KPSDM and RTM, respectively.
The obtained imaging results for the synthetic and the real data are evaluated in terms of the capability of both methods to image the cracks within the barrier and conclusions are drawn from the results with respect to the overall frame of the study in the last section.
As a final step, all migrated shot gathers are summed up and yield the final migrated image. In that approach, the migration process is performed in the depth domain according to Kirchhoff theory before the summation (stacking) of various offsets (source-receiver distances) and is therefore referred to as prestack Kirchhoff Depth Migration. In an NDT context, a different terminology is commonly used, e.g. a similar version of pre-stack summation is known as total focusing method (TFM). In the TFM approach an unweighted summation along the diffraction hyperbola is performed [10]. The weighting function (Eq. (2)) in KPSDM originates from Kirchhoff theory [9]. Using a weighting functions in Kirchhoff migration schemes produces sharper reflectors by reducing their lateral extent and the formation of migration smiles. Migration smiles appear as remnants of the wavefield which is smeared along isochrones throughout the subsurface and appear at the boundaries of the illuminated part of the reflector due to insufficient constructive interference of these wavefield isochrones.
As mentioned above, the velocity model shown in Fig. 3 was used to compute the synthetic data set by FD modeling. For the FD wave field simulations involved in RTM, the monitoring device, the crack and the separating plate were removed from the velocity model since the aim of the migration procedures is to find these features. The FD simulations were therefore conducted in a velocity model containing the salt concrete block surrounded by halite as only a priori information which is the initial situation known before the reconstruction. It can be seen as an advantage of RTM that other a priori information can be easily added to the migration scheme by altering the velocity model accordingly.
In addition, at least some parts of the steeply dipping transition between salt concrete and surrounding halite are visible in the RTM image (feature 4), although this boundary has already been a part of the velocity model used for the FD simulation within the RTM scheme. Steeply dipping reflectors are elements having a large angle to the source-receiver line. In this case, the salt-concrete halite transition is positioned at an angle of approx. 80 to the front face. These reflectors are challenging for imaging algorithms since reflected wave energy is mostly directed away from the receivers and therefore barely recorded.
In the same way as for the synthetic data set, data processing preceding the migration was reduced to a minimum. In addition to a top mute of the direct wave, a frequency band pass filter between 5 and 80 kHz was applied. For the RTM algorithm, the velocity model shown in Fig. 9 was used as input for the FD simulations. The approximate position of the boundary between the salt concrete block and the surrounding halite was taken from a priori knowledge about the engineered test-barrier. Information about existing cracks or built-in monitoring devices inside the salt concrete were assumed to be unknown and therefore are not part of the model. Furthermore, the separating plate is only shown for orientation but was not used in the velocity model, since the aim was to reconstruct this element and its position is not known accurately.
For the synthetic as well as the real data sets investigated in this study, both imaging algorithms (KPSDM and RTM) yield comparable and plausible results as shown by the synthetic experiments. Most reflectors are imaged at the correct position up to a depth of 8 m. Since the cracks are air-filled and the separating plate is a metal item, the corresponding reflectors should have opposite polarity. This would ideally result in an inverted color sequence at the corresponding reflectors in the migration images, i.e. red-blue-red vs. blue-red-blue, respectively. Unfortunately, this is not evident in either migration images which is possibly due to delamination and/or perforation at the separating plate as well as the influence of the stacking process. Since the reflectors are located relatively deep in the material, very small inaccuracies of their location within the migrated single shot gathers lead to an imperfect constructive interference during the stacking process.
Cracks were identified mostly parallel to the front face. This supports the finding that the engineered barrier has been watertight for the last years, as mentioned in the introductory part. However, longitudinal cracks (with an angle of approx. 90 to the front face) can only be resolved close to the front face due to the acquisition geometry and the ratio between source receiver offset and imaging depth. The detection of deeper longitudinal cracks is the subject of subsequent investigations using a borehole probe.
Since there is a vanishing shear modulus in fluids such as water and air, shear waves are not transmitted, and shadow zones form behind the cracks. Small reflectors or parts of larger reflectors within those shadow zones can be hardly imaged by both approaches, as shown with the separating plate in Fig. 5. However, the real data set was acquired at a 3D structure. If a crack is considered as a one-dimensional structure or at least not extending over the complete cross section of the barrier, waves can also pass to deeper regions. This can explain the sharp reflection of the separating plate (feature 5) in Fig. 10 below the prominent reflectors 1 and 6.
KPSDM using a constant velocity seems to misalign the reflectors dipping towards the lateral boundaries of the model. This behavior was observed for the crack in the synthetic data set and has been likewise observed for the separating plate in the real data set. For certain details, e.g. the deepest crack, the RTM image shows significant advantages compared to the KPSDM image.
Further improvement of the migration images may be obtained by more advanced data processing, e.g. deconvolution for removing the influence of the source wavelet or additional methods of noise attenuation for further improvement of the signal-to-noise-ratio. Advanced Kirchhoff-based focusing prestack depth migration techniques (e.g. Fresnel-volume-migration [23]) can also be taken into consideration. Focusing type PSDM use information about the direction of recorded wavefronts to reduce the lateral extent of reconstructed reflectors by limiting the smearing along the isochrones resulting in a sharper reconstruction image.
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