Recent advances in hydrogeologic characterization of the shallow subsurface using direct-push technology

Abstract:

Risk assessment and site remediation efforts continue to be greatly hampered by our inability to characterize conditions in the subsurface at the resolution and efficiency required for practical investigations. The current generation of field methods has proven to be of limited effectiveness for obtaining information at the spatial resolution needed to quantify groundwater movement in heterogeneous media. Over the last decade, we have developed new direct-push (DP)-based, high-resolution field methods that allow characterization of the shallow subsurface at a speed and resolution that has not previously been possible. Our emphasis has been on the development of methods for characterizing hydraulic conductivity (K) and porosity (n) in shallow (< 30 m depth) unconsolidated settings. The high-resolution hydraulic conductivity (HRK) tool combines continuous DP injection logging (DPIL), a high resolution (0.015 m) but qualitative approach, with the DP permeameter, which provides reliable K estimates but at a relatively coarse resolution (0.4 m). We have demonstrated that DP profiling at the MADE site, using both the HRK probe and a DPIL-only tool, provides a depiction of the K field that is quite similar in structure to that provided by the much more costly and time-consuming flowmeter profiling done earlier at the site. Nuclear magnetic resonance (NMR) profiling, a widely used borehole technique in the petroleum industry, was recently adapted to DP equipment for characterizing K and n in unconsolidated deposits. The current NMR tool provides K and n estimates that are averaged over a 0.5-m vertical interval, although a higher resolution can be achieved by advancing the tool at a fine interval and analyzing the entire profile data with global optimization techniques. Ongoing efforts are directed at integrating these methods at collocated profiling locations to provide an unprecedented assessment of subsurface conditions in highly heterogeneous settings.