Douglas LaBrecque

Hydrogeophysical Case Studies in the Vadose Zone

The focus of this chapter is the characterization of the vadose zone, or the unsaturated section of the subsurface, using hydrogeophysical techniques. The regions of water saturation as they relate to the physical properties are shown for reference in Figure 14.1. Characterization below the water table (in the saturated section) is described in Chapter 13 of this volume and will not be discussed in detail here. From a physical properties perspective, the zones of variable saturation above the water table are transitional, and depend upon the soil or rock type and the lateral heterogeneity of the materials. In the vertical direction, the boundaries between all of these zones are dependent upon the types of soil, regolith, or rock that are present; the current and historical climatic conditions; and the regional and local geomorphology of the site. These same factors affect the heterogeneity of the vadose zone in the horizontal (lateral) direction and generally compound the problems of defining the different regions of moisture in the vadose zone.

Chapter 15 Three-dimensional monitoring of vadose zone infiltration using electrical resistivity tomography and cross-borehole ground-penetrating radar

Abstract We discuss results from field experiments conducted at the Sandia-Tech Vadose Zone experimental facility on the New Mexico Tech campus at Socorro, New Mexico, as part of a project to develop a joint hydrological-geophysical method to characterize fluid flow properties of the vadose zone. The site contains dense arrays of tensiometers, access tubes for neutron moisture and ground-penetrating radar probes, and arrays of surface and subsurface electrical resistivity tomography electrodes installed in shallow clays, sands and gravels. We collected electrical resistivity tomography (ERT), cross-borehole ground-penetrating radar (XBGPR) and neutron data before and during a controlled infiltration of water at the site. Using local, empirical relations, we estimated subsurface moisture contents from images generated with the XBGPR and ERT data. The XBGPR images provided an excellent comparison to the neutron-derived moisture contents along a plane through the center of the experimental site. The ERT results were limited in terms of resolution by the coarse electrode spacing and inversion mesh used at the site, but provided a full three-dimensional picture of the wetting front as it progressed.

A Hydrologic-geophysical Method for Characterizing Flow and Transport Processes Within The Vadose Zone

The primary purpose of this project was to employ two geophysical imaging techniques, electrical resistivity tomography and cross-borehole ground penetrating radar, to image a controlled infiltration of a saline tracer under unsaturated flow conditions. The geophysical techniques have been correlated to other more traditional hydrologic measurements including neutron moisture measurements and induction conductivity logs. Images that resulted during two successive infiltrations indicate the development of what appear to be preferential pathways through the finer grained materials, although the results could also be produced by cationic capture of free ions in clays. In addition the site as well as the developing solute plume exhibits electrical anisotropy which is likely related to flow properties. However the geologic significance of this phenomenon is still under investigation.The primary purpose of this project was to employ two geophysical imaging techniques, electrical resistivity tomography and cross-borehole ground penetrating radar, to image a controlled infiltration of a saline tracer under unsaturated flow conditions. The geophysical techniques have been correlated to other more traditional hydrologic measurements including neutron moisture measurements and induction conductivity logs. Images that resulted during two successive infiltrations indicate the development of what appear to be preferential pathways through the finer grained materials, although the results could also be produced by cationic capture of free ions in clays. In addition the site as well as the developing solute plume exhibits electrical anisotropy which is likely related to flow properties. However the geologic significance of this phenomenon is still under investigation.

Autonomous Monitoring of Fluid Movement Using Electrical Resistivity Tomography

The electrical resistivity tomography method (ERT) is seeing increasing use in long-term monitoring. Applications might include monitoring of advanced remediation methods, vadose zone fluid-flow monitoring, and monitoring below tanks at the Hanford reservation. For this method to be cost effective, future systems will need to be highly automated. This paper compares different strategies for collecting three-dimensional (3-D) data sets. We discuss the critical design aspects of the system and the importance of using integrated hardware for data collection, and software for data interpretation. An autonomous acquisition system was used to monitor a field experiment at the Idaho National Engineering and Environmental Laboratory. The system was successful at collecting data that were used to monitor infiltration of water into interbedded sediment and basalt layers. The results showed the advantages of autonomous systems for collecting data, and the need for robust operating systems designed specifically for autonomous operation.

Measuring Subsurface Conductivity Using Integrated ERT and MMR Measurements: Experimental Results

INEEL scientists developed MMR instrumentation to spatially resolve the surface magnetic field associated with an induced subsurface alternating current low between borehole ERT electrode pairs. The instrumentation consists of a variable frequency alternating current source and a synchronously detected, spatially resolved vector B-field measurement system. In the past year numerical codes were developed for the simulation of MMR data. Comparison of the results of numerical codes against a comprehensive set of field measurements at the Mud Lake, Idaho sediment beds allow for instrumentation and code enhancement as well as initial interpretations of MMR results in terms of subsurface structures.