Mark Vendl

Ground penetrating radar for the detection of liquid contaminants

Abstract The significance of ground penetrating radar (GPR) as a tool to detect near-surface contaminants is illustrated at a site in the Midwest representing petroleum product above the water table. Tests show that ground penetrating radar may provide a means of mapping hydrocarbons in the vadose zone. Results of controlled surveys in a sand test pit at The Ohio State University demonstrate conclusively that there is a clear GPR anomaly over containers of diesel fuel and containers containing the host sand material saturated with diesel fuel. Comparisons of GPR data measured at different times of the year (summer, fall, and winter) at a gasoline spill site in northern Indiana shows direct information on layers and lenses in the vadose zone that tend to accumulate water during times of high moisture and subsequently lose the moisture during dry periods. GPR data collected in the winter over partially frozen ground provided measurements that were more sensitive to the presence of the gasoline than measurements that were made during the summer and fall. The relative propagation transparency of the near-surface zone for GPR measurements from winter data over fozen ground allowed detection of the water table that could not be confidently identified from two of the other data sets. Four important points can be concluded from a comparison of the four data sets: (1) the quality and repeatability of GPR measurements over a clean sand depends on the amount of moisture located in the unsaturated zone above the water table; (2) reflections from sedimentary features can be distinguished from reflections from percolating groundwater; (3) GPR measurements made during the dry month of August are nearly devoid of reflections above the gasoline product, indicating that the water in the unsaturated region may have been displaced by liquid gasoline, or by gasoline vapors; and (4) most importantly, these tests illustrate the resolving power and sensitivity of ground penetrating radar.

Demonstration of using crossed dipole GPR antennae for site characterization

Crossed dipole (cross-pole) and parallel dipole (co-pole) GPR data were acquired at an industrial site that formerly operated as a creosote wood treating facility in order to locate buried pipes and tanks or other possible contaminant-filled subsurface structures. Cross-pole data are not typically considered during GPR field studies, but proved essential for accurate site characterization at this location, as images produced using co-pole data had a poor signal to noise ratio. Data interpretations were confirmed through exploratory trenching conducted subsequent to this study. The GPR data proved successful in locating back-filled trenches that contained creosote-filled drainage tile, as well as vaults and a pit filled with pure creosote product at the site.

Coincident Antenna Three‐Dimensional GPR

Coincident antenna three‐dimensional ground penetrating radar (GPR) consists of making measurements over an area on the surface using transmitting and receiving antennas that are located very close together. Measurements may be made using antennas that are towed behind a vehicle, or in a fixed‐station (fixed‐trace) mode of operation. In either case, the accurate location of each trace is critical to producing accurate 3D displays. Obtaining a good three‐dimensional display is a critical part of interpreting GPR data. Targets of interest are generally easier to identify and isolate on three‐dimensional data sets than on conventional two‐dimensional profile lines. Simplifying the image by eliminating the noise and clutter is the most important factor for optimizing the interpretation. Image simplification may be achieved by: 1) carefully assigning the amplitude‐color ranges, 2) displaying only one polarity of the GPR signal, 3) using a limited number of colors, 4) decreasing the size of the data set that is...

A Controlled Experiment to Determine the Water Table Response Using Ground Penetrating Radar

Ground penetrating radar (GPR) data were explored during a physical model experiment that utilized a polyethylene tank and an abiotic sand matrix to simulate water contamination in the vadose zone under fluctuating water table conditions. The main objective of this experiment was to determine the source of GPR reflections from a changing water table in a controlled experimental environment. During the experiment, the water table was varied by injecting water into the bottom of the tank and subsequently draining it. The experiment was conducted over a number of days, during which various conditions (unsaturated, saturated, residual saturation) were characterized using GPR measurements. As a result of this approach, a more comprehensive comparison was possible for each step and results were compared with mass balance information. Results of the study indicate that, under saturated conditions, the main reflector of GPR energy is indicative of the capillary fringe and not the actual water table. Well readings and estimates of the mass balance of water input into the system confirm this interpretation.

USE OF BOREHOLE-RADAR METHODS TO MONITOR THE MOVEMENT OF A SALINE TRACER IN CARBONATE ROCK AT BELVIDERE, ILLINOIS

Common-depth (CD) radar surveys and cross-hole radar tomography methods were used to monitor the movement of a saline tracer in a dual-porosity dolomite aquifer at Belvidere, Illinois. The tracer test was conducted using an array of six open-hole bedrock wells at the Parson’s Casket Hardware Superfund site. The injection and recovery boreholes were about 20 m (meters) apart, and the imaging boreholes were arranged to provide planar coverage across and along the anticipated tracer path. A hydraulically conductive zone identified during previous investigations was isolated using straddle packers and pumped to establish a hydraulic gradient between the injection and recovery wells. A sodium chloride (NaCl) solution was continuously injected into this zone to move the tracer across the tomographic image plane. CD cross-hole radar surveys and cross-hole tomography surveys were conducted before and periodically during the tracer injection. Background tomograms contain similar radar velocity and attenuation changes with depth, consistent with a layered dolomite that has variable porosity and electrical conductivity. Slow changes in attenuation associated with low tracer velocity permitted the acquisition of multiple CD surveys and two cross-hole tomography surveys during injection. CD surveys were used to rapidly identify the presence of tracer between wells. Attenuation-difference tomograms contain attenuation increases that delineate the spatial distribution with time of the saline tracer and show the progressive movement of the tracer within the tomographic image plane. Formation porosity and resistivities calculated from radar velocity and attenuation tomograms were used to estimate changes in fluid resistivity and tracer concentration in the tomographic image plane. INTRODUCTION Cross-hole radar tomography methods have been used in conjunction with saline tracers to interpret permeable zones and identify transport paths in fractured rock (Ramirez and Lytle, 1986; Niva and others, 1988; Olsson and others, 1992; Lane and others, 1996; Wright and others, 1996) and in unconsolidated sediments (Kong and others, 1994). Introducing a saline (electrically conductive) tracer .into an aquifer such that an electromagnetic pulse propagating between the radar transmitter and receiver intersects a saline region will increase the apparent attenuation of the wave with respect to a background attenuation. Attenuation-difference tomograms that contain regions of increased attenuation identify the interwell spatial distribution of the saline tracer. Attenuation-differencing methods assume steady-state conditions so that significant electrical conductivity changes do not occur during the radar data acquisition. Steady-state methods provide a means of identifying permeable zones, but these methods do not show changes in tracer concentration or distribution with time. The ability to monitor the movement of a saline tracer as it crosses the tomographic image plane would provide important insights and constraints on the nature of fluid flow and solute transport in an aquifer. In the absence of rapid (or multi-channel) radar systems that are able to record attenuation changes in real time, imaging tracer movement using cross-hole radar methods with currently available hardware requires a modification of the tracer injection and/or radar data acquisition procedures. Possible modifications to the experimental procedures include (1) reduction of the injection and pumping rates to

Electromagnetic Induction and GPR Measurements for Creosote Contaminant Investigation

Multifrequency EM induction and GPR parallel dipole (co-pole) and orthogonal dipole (cross-pole) surveys were conducted to assist in the characterization of a former industrial site prior to it being remediated by the Ohio EPA and the U.S. EPA. The site has been a major concern to both agencies for the past decade due to high concentrations of creosote present in clay-rich surficial soils, resulting from many years of wood treating at the site. Information provided on the approximate extent of contamination at the site and the locations of several contaminant-filled structures determined through the use of quadrature phase EM data and cross-pole GPR data served as the basis for an efficient, comprehensive and cost-effective site remediation plan. Geophysical data interpretations were confirmed through exploratory trenching and soil sampling subsequent to the completion of this study. This study demonstrates the potential for mapping the extent and variation with depth of resistive compounds under circumst...

COMBINING MULTIPLE GEOPHYSICAL DATA SETS INTO A SINGLE 3D IMAGE

Traditional geophysical interpretations of multiple data sets have been carried out by interpreting the data on an individual basis and painstaking comparison of the overlapping registration of anomalies on different types of data. This paper illustrates a visualization approach to combining multiple data sets. The individual data sets can consist of any quantified data on a two dimensional (2D) grid (e.g., EM, gravity, magnetics, digital coded geology, topographic maps, model responses, etc.), or three dimensional time-dependent data (e.g., seismic, or GPR). The approach is based on computer software that automatically registers the coordinates of the data sets to the same base map. The interpreter then assigns a vertical position in a 3D block and a color scale to each individual data set. The resulting 3D block is displayed on the screen for further manipulation of opacity and color scales to provide an optimum image for the interpretation of the fused data sets. The interactive interpretation phase is further enhanced with an ability to generate cross section slices and smaller 3D blocks that highlight individual anomalies. Multiple data sets that are handled in this manner provide the interpreter with the optimum environment for visual comparison and interpretation of diverse and complex data sets. One of the keys to the interpretation of multiple data sets is the ability to manipulate both the color assignments and the opacity so that individual features can easily be seen from one data set to the other. In addition, the digital display must be completely interactive to allow interactive control of the color, the opacity, and the view. A good interactive display also allows the interpreter to easily select and switch between block views or slices, with the capability to view a small portion of the block. In short, the interpreter needs to have full control of the 3D block of data to change any display parameter. The final display is the interpretation.

Seasonal variations and ground‐penetrating radar data repeatability

A ground-penetrating radar (GPR) study conducted over sandy soil during the fall of 1990 and winter of 1991 reveals important conclusions concerning the climatic effects on the repeatability and interpretability GPR data. Despite significant changes in the degree of ground surface saturation between the GPR data sets collected in October 1990, December, 1990 and late January, 1991, much of the data show a high degree of repeatability. GPR data collected during the coldest day over partially frozen ground yielded the highest resolution data. Direct comparison between seasonal data sets allowed the identification of a local low permeability zone likely associated with concentrated fine-grained sediment. The transparency of the near-surface low electrical permeability zone from the January data over frozen ground allowed detection of the water table which could not be identified in the other data sets. This study suggests that long-term GPR investigations over unconsolidated sediments that incorporate seasonal data sets can greatly enhance the interpretability of GPR data. In addition, the tests indicate the potential usefulness of the GPR data for long-term monitoring of hydrogeolgic conditions.