7.5.1 Display Processing
Processing and display are an integral part of being able to effectively interpret GPR data. GPR data can rarely be interpreted without some type of processing to improve the resolution of coherent signals that represent the targets that are the objective of the survey, and a display that enables the interpreter to easily identify the anomalies that identify the time-space location of the targets. Processing and display can be conducted in the field, but in many cases, it is more convenient to process and display data at a later time in the office or laboratory.
Data display is a critical step toward providing an effective interpretation of GPR data. A poor display masks anomalies, and a good display enhances the target anomalies above the noise and coherent clutter. The spatial distribution of the field data determines the lateral resolution of the data. Three-dimensional displays can only be produced if field data are measured on a two-dimensional grid. The objective of GPR data presentation is to provide a display of the processed data which closely approximates an image of the subsurface, with the anomalies that are associated with the objects of interest located in their proper spatial positions. Data display is central to data interpretation, and an integral part of interpretation.
There are three types of displays of surface data, including a one-dimensional trace, a two-dimensional cross section, and a three-dimensional display. A one-dimensional trace is not of very much value until several traces are placed side by side to produce a two-dimensional cross section, or placed in a three-dimensional block view.
The wiggle trace (or scan) is the building block of all displays. A single trace can be used to detect objects (and determine their depth) below a spot on the surface. By towing the antenna over the surface and recording traces at a fixed spacing, a record section of traces is obtained. The horizontal axis of the record section is surface position, and the vertical axis is round-trip travel time of the electromagnetic wave. A GPR record section is similar to the display for an acoustic sonogram, or the display for a fish finder. Two types of wiggle-trace cross sections of GPR traces are shown in Figure 7.10. Wiggle-trace displays are a natural connection to other common displays used in engineering (e.g., oscilloscope display), but it is often impractical to display the numerous traces measured along a GPR transect in wiggle-trace form. Therefore, scan displays have become the normal mode of two-dimensional data presentation for GPR data. A scan display is obtained by simply assigning a color (or a variation of color intensity) to amplitude ranges on the trace, as shown in Figure 7.11. Scan displays are generally used for GPR data because of the high data volume (large number of traces/m).
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