Erich D. Guy

Pitfalls in GPR data interpretation: Differentiating stratigraphy and buried objects from periodic antenna and target effects

Periodic events in ground penetrating radar (GPR) data may result from antenna and target effects rather than reflections from geologic features. One of the most common pitfalls in GPR data interpretation is to identify each event on a radar cross-section as scattering from a discrete horizon, without considering other possible sources of these events. Soil electrical properties and surface roughness affect ground penetrating radar antenna radiation and waveform characteristics. An impedance mismatch occurs over soils with electrical properties different than those for which the antennas were designed to perform optimally over, and results in periodic ring-down, which can be misinterpreted as stratigraphy or multiple reflections. In addition, target resonance can introduce additional periodic features that can lead to misinterpretation in regards to the number of targets present. Co-pole and cross-pole antenna configurations can be combined with polarization dependent scattering characteristics of subsurface objects to recognize and reduce antenna ring-down for improved imaging and interpretation.

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.

SIGNIFICANCE OF CROSSED-DIPOLE ANTENNAS FOR HIGH NOISE ENVIRONMENTS

Crossed-dipole antennas can be used to reduce clutter and improve the signal-to-noise ratio of ground penetrating radar (GPR) surveys, depending upon field conditions and the targets of interest. The crossed-dipole antenna consists of transmit and receive antennas oriented orthogonal to each other, and is sensitive to field components oriented parallel to the long axis of the receive antenna. These cross-polarized components can be introduced by scattering from subsurface targets or may be composed of scattered cross-polarized components present in the field incident on the target. The physical shape and composition of targets will influence the polarization of the scattered field, and this enables cross-pole and co-pole antenna configurations to discriminate between different classes of targets for clutter removal. The crossed-dipole antenna configuration also improves isolation of the receive antenna from the direct arrival of the transmit antenna. The improved isolation and ability to discriminate between different targets can therefore result in an improved signal-to-noise ratio. INTRODUCTION Co-pole antennas are commonly used for environmental and engineering GPR investigations (Davis and Annan, 1989; and Daniels et al., 1997). Crossed-dipole antennas are not commonly used in GPR surveys because of their lack of sensitivity to stratigraphy. Crosseddipole antennas can be used to improve the signal-to-noise ratio in environments characterized by antenna ring-down and when the objects of interest include linear targets (pipes and rebar), or objects that produce a strong scattered cross-component (rough surfaces and small targets). Physical model examples are used to demonstrate which targets can be imaged with crosseddipole antennas. Field examples from a site characterized by antenna ring-down are also presented to illustrate the potential effectiveness of using crossed-dipole antennas. ANTENNAS AND ANTENNA RING-DOWN Dipoles and bow-tie antennas are widely used in impulse GPR, because they are relatively easy to design, are wide-band, are non-dispersive, and are linearly polarized (de Jongh et al., 1998). Most GPR antennas are located on or just above the ground to couple electromagnetic energy into the ground efficiently. The close proximity of the antenna to the ground causes the current distribution and antenna impedance to be strongly influenced by the ground. The antenna impedance changes with different soil types, moisture contents, and surface roughness. Most GPR antennas are designed for use over ground having a specific impedance. An impedance mismatch between the antenna and feed cable occurs when a GPR survey is conducted over ground having an impedance other than what the antenna was designed for. This causes the currents to bounce back and forth between the antenna feed and the ends of the

Recognition of Borehole Radar Cable-Related Effects Using Variable Offset Sounding

Conductive cables can influence borehole radar measurements and introduce artifacts into data and therefore must be considered during data analysis and interpretation. This study presents examples of some cable-related effects in data acquired with a radar system that relies on conductive cables for signal transmission. Data show that measurements can be affected when energy radiated from the transmitter antenna induces currents on conductive cables, which can function as an electromagnetic waveguide, allowing fields to propagate along cables and be detected by the receiver antenna. Additionally, periodic artifacts can result when currents traveling on cables reflect at system impedance mismatches.Variable offset soundings (VOS) are not typically conducted during borehole radar studies, but can be useful for recognizing cable-related effects on recorded data and studying propagation characteristics in a borehole. In addition to single-hole VOS measurements, VOS measurements made on the ground surface using E-Plane and H-Plane configurations are shown to have the potential for providing additional insight in regards to coupling mechanisms between borehole antennas and cables.

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.

Non-geologic events in single- and cross-hole radar data

Summary Accurate analysis of borehole radar data depends upon the proper identification of events and the precise measurement of amplitudes and travel times. This paper discusses and provides data examples of non-geologic events that can influence borehole radar measurements and complicate data interpretation. Data demonstrate that refracted air events can arrive prior to direct arrivals in cross-hole surveys, and that conductive cable-related effects can introduce artifacts and multiple events into records. Additionally, nontraditional variable offset soundings (VOS) are shown to be useful for studying propagation characteristics, recognizing possible cable-related effects, and providing insight in regards to coupling mechanisms between antennas and cables.