T104 Combining Geology and Geophysics (Posters)

Monday, 10 October 2011
9:00 AM - 6:00 PM
Hall C

DELINEATION OF AQUIFER HETEROGENEITIES USING TRANSIENT ELECTROMAGNETIC SOUNDINGS IN THE GREAT SAND DUNES NATIONAL PARK

Paper number: T104-2
Program: Technical Sessions
Time: 9:00 AM – 6:00 PM

Authors

Abstract

During the Pleistocene epoch a laterally extensive aquitard composed of clay was deposited in the bed of Lake Alamosa. Despite the lateral continuity of the “blue clay,” as it is colloquially known, hydrological and geophysical evidence suggests that there are heterogeneities in the thickness of the clay within the San Luis Valley siliciclastic aquifer system. Possible causes for this heterogeneity may include faulting and/or variations in sedimentary thicknesses within the blue clay unit. These heterogeneities can cause variations in the confining pressure affecting groundwater flow and complicate the extraction of groundwater resources. Mapping the geometry and continuity of the blue clay in the subsurface will help to define the hydrogeologic framework in the San Luis Basin.

Transient electromagnetic (TEM) soundings were used to delineate the continuity of the blue clay near Antelope Springs in southern Saguache County, Colorado. TEM data were collected along profiles by inducing electrical currents into the ground. Resistivities of lithological features were modeled using mathematical inversion and were used to develop a cross section of the subsurface. The soundings were conducted where previous data had been collected, but were refined by decreasing sounding spacing to provide enhanced resolution. In the interpreted cross section, the top of the blue clay unit has a 50 m decrease in depth over a distance of approximately 1400 m along the profile and then terminates at the interpreted shoreline of the ancient Lake Alamosa. Preliminary analysis suggests that the topography of the blue clay unit could possibly be due to faulting. Further data acquisition and analysis are needed to further investigate whether the heterogeneity in the blue clay is related to a fault or to sedimentary processes.