Analysis of GPR Data for Time and Frequency-Dependent Effects of Changing Water Table Levels
Frequency-dependent ground-penetrating radar (GPR) data are acquired every 2-3 years from 1998 to 2017 at a single location on an exposure of the Lewisville Member of the Woodbine Formation along the shore of Grapevine Lake in North Texas. The depth to the groundwater in the outcrop varies in position with the water level in the lake, and the short-term rainfall history. Thus, the distribution of fresh and ground water and their different dielectric permittivities and electrical conductivities produce visible time and frequency-dependent GPR velocities and attenuations. The lithologic interbedding of sands and thin clays, the presence of fractures, and the distribution of hematite in the uppermost layer, and a shallow unconformity, all contribute to variations in GPR amplitude and attenuation. The difference between the known minimum and maximum water table levels of ~3.38 m is consistent with observed changes in GPR velocity from ~0.063 m/ns to ~0.52 m/ns, and with the observed difference of 31.9 cm of rainfall and an average porosity of 20%. The presence of hematite in the near-surface part of the section attenuates the GPR signals and produces an amplitude variability of a factor of ~3.