Using Multiple Stress Signals to Characterize Fractured-Rock Aquifers

Monitoring imposed stresses and the subsequent aquifer response to those stresses is a common practice for evaluating specific aquifer properties. At the Fractured Rock Research Site in Floyd County Virginia, we investigate the use of multiple monitored stresses to characterize a fault zone aquifer system contained within a granulite gneiss including atmospheric loading, Earth tides, and aquifer tests coupled with flowmeter, extensometer and tiltmeter logging. The measurement of both hydraulic (water levels) and hydromechanical (strain and deformation) data in response to imposed stresses can yield valuable information about the aquifer characteristics and allow for the estimation of such parameters as specific storage, Poisson’s ratio, bulk porosity, drained formation compressibility (Young’s modulus), fracture compliance, transmissivity and storage coefficient. The use of multiple stresses with the collection of hydraulic and hydromechanical data creates the opportunity for the implementation of a broad range of analyses and provides a mechanism for corroboration of parameter estimates from different modes of analysis.

Three separate four-hour pumping and recovery tests were performed at the study site in well EX-1 that is connected to well W-03 27.7 m away along the fault plane aquifer. During these three tests the extensometer, located in W-03 was anchored over 2.1 meter long sections of (1) a fracture in hydraulic communication with EX-1 and (2) a fracture not hydraulically connected with EX-1, but connected to other far-field wells at the site. Results from the pumping tests yielded a fracture compression expressed as compliance ranging from 0.1-.16 µm/m. When coupled with areal strain calculated from Earth tide analyses an average volumetric specific storage of about 4.3x10^-7 /m is calculated along with an average Poisson’s ratio of .20. Using this value with a calculated barometric efficiency of 0.45 calculated from multiple frequencies of atmospheric loading produces an overall porosity of 0.006 at well W-03.