Imaging a shallow aquitard with seismic reflection data in Memphis, Tennessee, USA. Part II: Data analysis, interpretation and traveltime tomography

摘要

We collected P-wave seismic reflection data to image suspected breaches in the confining unit (aquitard) above the Memphis aquifer in Memphis, Tennessee, USA, where previous studies of water quality have suggested potential contamination. A 1-km-long reflection line was acquired across depressions of the water table that have been interpreted to reflect a breach in the aquitard. Although raw data are dominated by surface waves, after frequency filtering, shot static correction and f-k filtering, consistent reflections can be observed. Integration of constant velocity analysis (CVS) and super gather semblance analysis was used to determine the RMS velocity field. The stack section shows a set of shallow reflectors interpreted to correspond to the top and bottom of the confining unit. The aquitard thins by about half from the north end to the south end along the reflection line. The continuity of the top and bottom of the aquitard was disrupted by a few faults interpreted along the line. None of these faults juxtapose the surficial aquifer against the Memphis aquifer but it is possible that the faults themselves may act as a hydraulic connection between the surficial aquifer and the Memphis aquifer and thus serve as pathways for a potential leakage. The refraction tomography technique was applied to the first arrival data and it revealed three depressions that are interpreted as paleochannels on the upper part of the confining unit, which is consistent with the background geology of the area. These inferred paleochannels may suggest that ancient river channel erosion may contribute to the process responsible for the formation of aquitard breaches in this area.