Geology topics

L.D. Slater

Publications and source records attributed to L.D. Slater.

3 recordsLinked to original sources

USGS2016

A fractured rock geophysical toolbox method selection tool

Geophysical technologies have the potential to improve site characterization and monitoring in fractured rock, but the appropriate and effective application of geophysics at a particular site strongly depends on project goals (e.g., identifying discrete fractures) and site characteristics (e.g., lithology). No method works at every site or for every goal. New approaches are needed to identify a set of geophysical methods appropriate to specific project goals and site conditions while considering budget constraints. To this end, we present the Excel-based Fractured-Rock Geophysical Toolbox Method Selection Tool (FRGT-MST). We envision the FRGT-MST (1) equipping remediation professionals with a tool to understand what is likely to be realistic and cost-effective when contracting geophysical services, and (2) reducing applications of geophysics with unrealistic objectives or where methods are likely to fail.

Groundwater↗

USGS2009

Geoelectrical measurement and modeling of biogeochemical breakthrough behavior during microbial activity

We recorded bulk electrical conductivity ( σ b ) along a soil column during microbially-mediated selenite oxyanion reduction. Effluent fluid electrical conductivity and early time σ b were modeled according to classic advective-dispersive transport of the nutrient medium. However, σ b along the column exhibited strongly bimodal breakthrough which cannot be explained by changes in the electrical conductivity of the pore fluid. We model the anomalous breakthrough by adding a conduction path in parallel with the fluid phase, with a time dependence described by a microbial population-dynamics model. We incorporate a delay time to show that breakthrough curves along the column satisfy the same growth model parameters and offer a possible explanation based on biomass-limited growth that is delayed with distance from influent of the nutrient medium. Although the mechanism causing conductivity enhancement in the presence of biomass is uncertain, our results strongly suggest that biogeochemical breakthrough curves have been captured in geoelectrical datasets.

Geophysical Research Letters↗

USGS2009

Hydraulic anisotropy characterization of pneumatic-fractured sediments using azimuthal self potential gradient

The pneumatic fracturing technique is used to enhance the permeability and porosity of tight unconsolidated soils (e.g. clays), thereby improving the effectiveness of remediation treatments. Azimuthal self potential gradient (ASPG) surveys were performed on a compacted, unconsolidated clay block in order to evaluate their potential to delineate contaminant migration pathways in a mechanically-induced fracture network. Azimuthal resistivity (ARS) measurements were also made for comparative purposes. Following similar procedures to those used in the field, compressed kaolinite sediments were pneumatically fractured and the resulting fracture geometry characterized from strike analysis of visible fractures combined with strike data from optical borehole televiewer (BHTV) imaging. We subsequently injected a simulated treatment (electrolyte/dye) into the fractures. Both ASPG and ARS data exhibit anisotropic geoelectric signatures resulting from the fracturing. Self potentials observed during injection of electrolyte are consistent with electrokinetic theory and previous laboratory results on a fracture block model. Visual (polar plot) analysis and linear regression of cross plots show ASPG lobes are correlated with azimuths of high fracture strike density, evidence that the ASPG anisotropy is a proxy measure of hydraulic anisotropy created by the pneumatic fracturing. However, ARS data are uncorrelated with fracture strike maxima and resistivity anisotropy is probably dominated by enhanced surface conduction along azimuths of weak 'starter paths' formed from pulverization of the clay and increases in interfacial surface area. We find the magnitude of electrokinetic SP scales with the applied N2 gas pressure gradient (??PN2) for any particular hydraulically-active fracture set and that the positive lobe of the ASPG anomaly indicates the flow direction within the fracture network. These findings demonstrate the use of ASPG in characterizing the effectiveness of (1) pneumatic fracturing and (2) defining likely flow directions of remedial treatments in unconsolidated sediments and rock. ?? 2008 Elsevier B.V. All rights reserved.

Journal of Contaminant Hydrology↗