HR: 17:30h
AN: H14C-07    [Abstracts]
TI: Comparison of Three Geophysical Methods for Characterizing a Gravel Aquifer
AU: * Bentley, L R
EM: lbentley@ucalgary.ca
AF: Department of Geology and Geophysics, University of Calgary, Calgary, AB T2N 1N4 Canada
AU: Hirsch, M
EM: marcus.hirsch@student.uni-tuebingen.de
AF: Center of Applied Geoscience (ZAG), University of Tuebingen Sigwart Str. 10, Tuebingen, 72076 Germany
AB: Seismic refraction, ground-penetrating radar (GPR) and electrical resistivity tomography (ERT) were used to characterize a gravel aquifer. The aquifer is part of a terrace that is bounded on the North by the Bow River and on the south by the valley wall that consists of lakebed silts. The gravel is underlain by shaley sandstone of the Paskapoo Fm. and has a maxium saturated thickness of 1.5 m. Depth to bedrock is between 6 and 8 m. Borehole logs provided a method for validating the interpretations. Inversion of refraction data yielded a gravel velocity of approximately 800 m/s and the velocity of the underlying bedrock varied between 2400 and 4000 m/s. Refraction inversion provided accurate depth to the gravel-bedrock interface, but it is unclear how to interpret the valley margin. Paskapoo Fm. resistivity was in the range of 17 ohm-m whereas the gravel resistivity was greater than 2000 ohm-m. The gravel-bedrock interface could be interpreted from the GPR data as a strong reflection or as the beginning of the transparent portion of the record due to the strong signal attenuation in the relatively conductive bedrock. However, the interpretation of the gravel-bedrock interface was ambiguous. GPR also imaged structure within the gravel layer that was not visible with the other methods. The ERT results were extremely good due to the strong resistivity contrast between the gravel and the bedrock. In addition, variations from gravel to sandier or siltier regions were well imaged by the ERT. The ERT derived interpretation of the exact depth to bedrock was not as accurate as the interpretations of the GPR or refraction due to the lower resolution of the method. Using all three methods provided an interpretation with detail and reliability not possible using any single method.
DE: 1829 Groundwater hydrology
DE: 1894 Instruments and techniques
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting