Geophysical Imaging of Critical Zone of Trinity/Edwards Aquifer over Haby Fault of Balcones Fault Zones, Texas, USA

Abstract:

Geophysical surveys (resistivity and natural potential (NP)) were conducted across the Haby fault, which is located in the eastern Medina County of San Antonio, Texas. The fault is located within the Balcones Fault Zone, a recharge zone, which is a 25- to 30- km –wide en echelon system of mostly south-dipping normal faults that formed during the middle to late Tertiary. The Balcones Fault Zone includes the Edwards and Trinity Aquifers, which both are primary sources of water for south-central Texas communities, including the city of San Antonio. The Trinity Aquifer underlies the Edwards Aquifer through the Balcones Fault Zone. The Upper Glen Rose member is considered to be the Upper Trinity Aquifer, and also a confining zone underlying the Edwards Aquifer. However, recent geological studies have documented a hydraulic connection between the Edwards and Upper Trinity Aquifers: The upper-most portions of the Upper Trinity and the Edwards Aquifer, in some places, operate as a single aquifer system. The purpose of these geophysical study, thus, was to test this hypothesized connection, and to determine locations of karst anomalies (caves, subsidence, conduits, and faults/fractures) on either side (up and downthrown) of the Haby fault, and evaluate the fault in terms of its significance on the Glen Rose (Trinity) and Edwards Aquifer unit’s interconnectivity (T-E) (Critical Zone!).

Four resistivity surveys with different electrode spacing (20, and 33 feet) were conducted across the Haby fault, which juxtaposes the Lower Edwards Aquifer unit with the younger Upper Cretaceous rocks of the Eagle Ford Formation with a ~300 feet throw. The resistivity data with the 33 feet (10 m) electrode spacing displays the Glen Rose and the Edwards Aquifer boundary at about ~300 feet below the surface, and also maps the fault location precisely. To authors’ knowledge, this is the only resistivity work that has ever been applied to reveal the nature of the T-E critical zone across a major fault.