Accuracy and reliability of tomographic two-dimensional seismic velocity models to map karst structures and infer hydrogeological processes in karst.

摘要

Identification of conduits that control groundwater flow is extremely important in understanding karstic systems. This study tests the hypothesis that seismic refraction tomography may be used to identify and map karst features that control groundwater flow, recharge and discharge. Tomography provides velocity images of the subsurface, from traveltimes of seismic waves generated at the surface. Rock properties, other geologic structures and water levels near recharge and discharge points were also investigated using refraction tomography. This study was conducted at two locations: Bear Creek Valley of the Oak Ridge Reservation, TN, and Perry Farm Park, Bourbonnais Township, Kankakee, IL.; P-wave velocity refraction-tomography models at Bear Creek Valley mapped lateral variations in the velocity of subsurface rock units and thickness variations of the overburden (0 to 30 ft). Inverted velocities are consistent with the two distinct bedrock groups at this field site---the Nolichucky Shale and Maynardville Limestone.; Distinct low-velocity zones have the appearance of "fracture-like" structures. They represent either sediments filling bedrock depressions, or thick zones of highly weathered bedrock along faults and joints. These zones may be water-saturated or partially water-saturated, and probably play an important role in the hydrogeology of the area. Some of these zones also lie directly beneath surface streams, suggesting fault control of these streams, including Bear Creek. In many such locations well logs report the presence of actual fractures, cavities, and water-level fluctuations that cannot be explained by seasonal variations alone.; Seismic refraction tomography over thinly mantled (7 to 30 ft) karst at Perry Farm Park may be used to delineate bedrock highs and lows, probably associated with different facies in the dolomite bedrock and/or fracture zones. Conventional refraction surveys, especially with S-waves, appear to delineate fractures. Results suggest that near-vertical fractures may also act as ground water recharge, discharge, and transmission features at the Perry Farm Park site. Borings and piezometers are needed to confirm this hypothesis, however.; This study demonstrates that tomographic models are able to estimate subsurface velocity changes associated with geologic structures with better spatial resolution than conventional seismic refraction methods. Tomographic models also provide standard errors and model covariance. The tomographic models, however, are strongly dependent on the choice of initial 1-D model and the nature of shot-point and geophone geometry. They also may include computational artifacts. Thus, independent borehole, geological, and geophysical data must be used to verify tomographic models.