The importance of fracture geometry and matrix data on transient hydraulic tomography in fractured rocks: Analyses of synthetic and laboratory rock block experiments

Abstract

The accurate characterization of hydraulic properties such as hydraulic conductivity (K) and specific storage (Ss) for fractured geologic media as well as imaging of fracture patterns and their connectivity are of paramount importance to robust groundwater flow and transport predictions. Recently, transient hydraulic tomography (THT) has been suggested as a promising approach for imaging the K and Ss distributions of porous and fractured geologic media. This study investigates the importance of geological information on geostatistics-based THT analysis through synthetic and laboratory rock block experiments. Specifically, fracture geometry, connectivity and matrix hydraulic parameters of varying accuracy are utilized as priori information in THT analysis. Comparison of results from model calibration and validation indicates that: 1) THT analysis can capture the overall fracture pattern and their hydraulic properties (K and Ss), but the estimated values are higher within the matrix, where observations are limited when the inversion begins with effective K and Ss; 2) using correct fracture pattern, connectivity and matrix data as prior information can better preserve fracture and matrix hydraulic parameters and their patterns, especially where drawdown data are hard to obtain; and 3) THT analysis starting with homogeneous hydraulic parameter estimates without geologic information is more reliable than the one based on the wrong or incomplete description of geologic features. Overall, results from this study indicate the importance of incorporating accurate geological data in THT analyses when drawdown data are sparse and not available within the matrix. However, existing technology does not allow for the accurate mapping of fracture patterns and their connectivity between boreholes, hence further research is necessary in data fusion of other types of information to improve THT results.