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7.5.1.2 Three-Dimensional Displays

Three-dimensional displays are fundamentally block views of GPR traces recorded at different positions on the surface. Data are usually recorded along profile lines, in the case of a continuous recording system, or at discrete points on the surface in fixed-mode recording (Figure 7.12). In either case, the antennas must be oriented in the same polarization orientation direction for each recording position if linearly polarized dipole antennas are used.

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 carefully assigning the amplitude-color ranges, displaying only one polarity of the GPR signal, using a limited number of colors, decreasing the size of the data set displayed as the complexity of the target increases, displaying a limited time range (finite-thickness time slice), and carefully selecting the viewing angle. Further image simplification in cases of very complex (or multiple) targets may also be achieved by displaying only the peak values (maximum and minimum values) for each trace. Finite-thickness time slices and cross sections have many advantages over the infinitesimal thin slices routinely used for interpreting GPR data. These principles are illustrated in a stepwise fashion in Figure 7.13.

Trace spacing

Line spacing ; i

Trace spacing along the line

(a) Surface plan view of discrete (b) Surface plan view of continuous travel trace measurement locations GPR measurements along lines

FIGURE 7.12 Grids and lines for fixed mode and moving mode for three-dimensional data measurements. Note that the polarization orientation of the antennas is the same for each measurement point on the grid, or along the profile lines.

FIGURE 7.13 The process of constructing a three-dimensional display from a series of two-dimensional lines: (a) series of two-dimensional lines and (b) three-dimensional full-color (or gray scale) block view, (c) horizontal slices and vertical cross sections, and (d) adjustment of color scale (gray scale) and opacity to emphasize the important features in a three-dimensional block.

FIGURE 7.13 The process of constructing a three-dimensional display from a series of two-dimensional lines: (a) series of two-dimensional lines and (b) three-dimensional full-color (or gray scale) block view, (c) horizontal slices and vertical cross sections, and (d) adjustment of color scale (gray scale) and opacity to emphasize the important features in a three-dimensional block.

7.5.2 Interpretation

GPR can be used to locate any object that has an electrical properties contrast with the surrounding ground, is within the detection range of the radar waves, and is not masked by clutter or noise. Scattering reflections are caused by an abrupt change in the electrical properties (primarily electric permittivity) in the subsurface. Some common features that have a high contrast include empty cavities, voids, or tunnels; changes in rock porosity; the water table; metal objects (e.g., barrels, tanks, pipes, etc.); plastic containers; concrete foundations; oil, petroleum, dense nonaqueous phase liquid (DNAPL) spills; or changes in geology.

Interpretation is the intellectual (human or intelligent computer) process of identifying anomalies on the GPR data and determining the nature (size, shape, and physical properties) of the object in the subsurface that is causing each anomaly. A good interpretation is the result of the skill of the interpreter (or sophistication of the pattern recognition algorithms), the quality of the data recorded in the field, and the clarity of the processed display used for interpretation. The interpretation begins with a good display that makes it easy to identify anomalies, with interpretation and processing inevitably overlapping each other. Data processed to the point where ready for interpretation should contain a minimum amount of noise (either random noise or coherent noise). Coherent noise can consist of features that are a part of the system (e.g., antenna ringing) or objects that are not a target of the survey (e.g., geologic features, overhead cultural features, etc.). The objects that are not a target of the survey are often called clutter. GPR data interpretation should progress along the following stages, with some overlap and feedback between stages:

1. Optimize the two-dimensional display to isolate the distinctive anomalies in the data.

2. Identify and classify the anomalies on the two-dimensional displays. Isolate the anomalies of interest from the clutter in the data.

3. Formulate the three-dimensional display and optimize the display to isolate the trends in the data.

4. Plot out time slices and cross sections of the three-dimensional data display to provide a final interpretative view of the anomalies.

5. Classify and identify features on the two-dimensional and three-dimensional data displays.

6. Map the map location and depth of identified objects.

Reflection anomalies on GPR records caused by linear objects (e.g., fences, overhead powerlines, corners of buildings, etc.) located above the surface of the ground are easy to identify by calculating the velocity from the reflection hyperbolas. The velocity of a radar wave in air is approximately 1 ft/ns in English length units (0.305 m/ns in metric units), and the velocity of electromagnetic waves through all other materials is much slower than the velocity through air. Linear surfaces (e.g., parallel fences, walls of buildings, overhead pipes, etc.) are more difficult to identify directly from the data and may require the use of good field notes in order to identify them.

Noise from external radio wave sources can be identified by the fact that it is usually semicon-tinuous and tends to contaminate a series of traces. Radio wave, or microwave, frequency noise on GPR records can sometimes be minimized by digital signal processing, if the frequency of the noise is outside the operating frequency range of the antennas.

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