Lessons Learned from Hydraulic and Pneumatic Tomography in Fractured Rocks

Difficulty in characterizing the fracture-rock matrix system, its hydraulic properties and connectivity at resolutions have led to the development of different conceptual models of fractured rocks. Over the last several decades, considerable effort has gone into creating maps of subsurface heterogeneity in hydraulic conductivity (K) and specific storage (Ss) of fractured rocks. In the unsaturated zone, maps of permeability (k) and porosity (ϕ) may also be obtained with pneumatic tests. Traditionally, numerous single-hole tests are conducted at successive intervals along boreholes to obtain information on such variability. If data from multiple test intervals in several boreholes are available, then they are amenable to geostatistical analysis. However, at some field sites, boreholes are sufficiently far apart that could lead to difficulties in conducting traditional geostatistical analysis as single-hole tests do not directly provide information on connectivity between boreholes. Recent research in the characterization of both unconsolidated and fractured rocks through synthetic, laboratory and field studies has shown that hydraulic tomography (HT) and pneumatic tomography (PT) are very promising approaches in characterizing subsurface heterogeneity. This is due to the fact that both HT and PT rely on multiple pumping or injection tests to generate signals that are detected in neighbouring monitoring intervals. In this paper, I discuss lessons learned from the various studies published in the literature on HT and PT in fractured rocks. In particular, some lessons learned include that: 1) both techniques allow for the mapping of heterogeneity between boreholes; 2) they are applicable at large scales as long as reliable drawdown signals can be detected; 3) PT has shown that improved mapping of heterogeneity leads to the suppression of scale effect in flow properties; 4) both techniques can map the connectivity of flow properties; and 5) improved mapping of heterogeneity and connectivity may lead to improved transport predictions. Overall, HT and PT both appear to provide the most reliable maps of subsurface heterogeneity in fractured rocks, but improvements can be made. Future research needs resulting from these lessons are also briefly discussed.