MODELING AQUIFER FRESHWATER/SALTWATER INTERFACE COMPLEXITIES VIA ELECTRICAL RESISTIVITY METHODS AND WATER CHEMISTRY ANALYSIS IN THE BARNETT SHALE PLAY (SOUTHEAST JACK COUNTY, TX)

The primary research goal is the development of theories to explain the complex interactions between freshwater and saltwater components of an aquifer system from which private landowners and petroleum companies vie for limited freshwater supply. The study site is a presumed recharge area adjacent to a creek located within the Barnett Shale play in southeast Jack County. Electrical Resistivity Tomography (ERT) is being used to map spatial and temporal variability in the shallow subsurface and Vertical Electrical Sounding (VES) employed to create a salinity depth profile for the local aquifer system. A water chemistry analysis from an abandoned shallow test well near the creek will be used to characterize temporal ion changes that may correlate with seasonal variations in resistivity due to rainwater infiltration. The research results could be used by petroleum and hydrology companies when exploring aquifer systems for optimum drill sites for oil and gas wells (identifying saline zones to utilize in compliance with new drilling regulations) and water wells (freshwater zones).

ERT data obtained Feb. – May 2014 before the rainy season (Jun. – Aug. 2014) suggest the possible presence of several shallow, isolated zones of very low resistivity initially interpreted to represent saline water lenses in close proximity to the creek. The VES data shows relatively low resistivity (15 µÙm) as shallow as 20 m, which provides supporting evidence for potential saline lenses in the shallow subsurface.

Reimaging of all previously recorded ERT lines and a second VES measurement will be completed by Dec. 2014 and analyzed for temporal variations to constrain the nature of the low-resistivity zones observed in the original measurements. Modeling analysis of temporal changes for each multiple-imaged ERT line will be performed in Google Earth, as well as time lapse inversion analysis of each line using AGI EarthImager 2D (ERT modeling software).