Results And Discussion

Constructed soil layers and drainage pipes within a golf course green will reflect radar energy, and with proper computer processing, have a response that can be clearly exhibited on GPR profiles. Figure 28.2 provides some typical examples for the computer processing of golf course green GPR profiles generated with EKKO View Deluxe. The Figure 28.2 profiles represent the application of different computer-processing procedures to the same set of raw 1000 MHz GPR data collected along a line oriented southwest to northeast on the Muirfield Village Golf Club seventeenth hole USGA Method green. For each Figure 28.2 GPR profile, the left vertical axis represents the two-way radar signal travel time, the right vertical axis represents depth in meters, and the bottom axis is distance in meters along the line of measurement. The raw data are presented in Figure 28.2a. The Figure 28.2a GPR responses to the bottoms of the constructed sand and gravel layers are barely visible, and reflection hyperbolas representing buried drainage pipes cannot be detected. The Figure 28.2b GPR profile processing sequence began with a signal saturation correction filter (SSCF) followed by a constant gain function (CGF) with a factor of 25. The bottom of the constructed sand layer (indicated by the downward pointing arrows), the bottom of the constructed gravel layer (indicated by the upward pointing arrows), and three upside-down U-shaped drainage pipe reflection hyperbolas (one is highlighted by an oval-shaped gray line) are all clearly visible. The actual position for the top of a drainage pipe coincides with the apex position of its reflection

FIGURE 28.2 (see facing page) Ground-penetrating radar (GPR) profile computer-processing results, (a) raw data, (b) signal saturation correction filter (SSCF) and constant gain function (CGF), (c) SSCF and automatic gain control function (AGCF), (d) SSCF, CGF, signal trace enveloping (STE), and 2-D migration (2DM), and (e) SSCF, CGF, and spatial background subtraction filter (SBSF).

hyperbola response. Interpretation of the Figure 28.2b profile was aided substantially by modeling synthetic GPR profiles with GprMax2D (Giannopoulos, 2003).

The Figure 28.2c GPR profile processing sequence began with the SSCF followed by an automatic gain control function (AGCF). The AGCF, by equalizing amplitudes along the signal trace, oftentimes enhances unwanted signal or "noise." For golf course greens, much of this noise is due to "multiples." Multiples occur when the radar wave travel path from the transmitting antenna, downward into the subsurface, and back up to the receiver antenna involves reflections off of more than one dielectric constant discontinuity interface. The direct, single reflection radar wave travel path to and from a particular subsurface interface will always be shorter than any travel path for a multiple involving this same subsurface interface. Multiples produced by reflections off of two or more shallower interfaces can become a problem then they interfere with the single reflection primary response of a deeper feature. Because only a portion of the incoming radar wave is reflected at an interface, multiples are oftentimes, but not always, somewhat weaker compared to the primary GPR responses produced by direct, single reflection radar wave travel paths. By using an AGCF, multiples that appear in some manner to involve the constructed soil layers are amplified, thereby making overall interpretation of the Figure 28.2c GPR profile more difficult with regard to constructed soil layer thicknesses; however, the three drainage pipe reflection hyperbolas do show up quite well.

The Figure 28.2d GPR profile processing sequence began with the SSCF, then a CGF (factor = 25), followed by signal trace enveloping (STE), and finally, 2-D migration (2DM). The Figure 28.2d computer-processing sequence does not produce a GPR profile that is as easy to interpret as the one for Figure 28.2b. The Figure 28.2e GPR profile processing sequence began with the SSCF, then a CGF (factor = 25), followed by a spatial background subtraction filter (SBSF). The SBSF used for the Figure 28.2e GPR profile employed a seven signal trace moving average sequence. The SBSF removed most of the responses to the constructed soil layers, thereby more completely isolating the drainage pipe reflection hyperbolas. All in all, the results presented in Figure 28.2 indicate that for the purpose of determining constructed soil layer thickness and depth and drainage pipe positions, a computer-processing sequence of a SSCF and a CGF is the best alternative.

Again, drainage pipes tend to reflect radar energy, and with proper computer processing are displayed as lighter shaded linear features on a GPR time-slice amplitude map. Figure 28.3 provides some typical examples for computer processing of golf course green time-slice amplitude maps. These time-slice amplitude maps represent a two-way travel time interval of 9 to 15 ns or a depth interval of 0.38 to 0.68 m. It should be noted that the EKKO Mapper software used to produce the GPR time-slice amplitude maps did not allow input of signal gain functions, because in doing so, the results tend to become distorted. The Figure 28.3 time-slice amplitude maps represent the application of different computer-processing procedures to the same set of raw 1000 MHz antenna center frequency GPR data (bidirectional and 1 m spacing between lines of measurement) collected on the Muirfield Village Golf Club seventeenth hole USGA Method green. For each Figure 28.3 GPR time-slice amplitude map, both the left axis and the bottom axis provide a distance scale in meters. The raw data are presented in Figure 28.3a. Although the areal extent of the golf course green shows up well, there are almost no indications in Figure 28.3a of the subsurface drainage pipe network that is present.

The Figure 28.3b GPR time-slice amplitude map processing sequence began with the signal saturation correction filter (SSCF) followed by signal trace enveloping (STE). Besides clearly depicting the areal extent of the golf course green, the Figure 28.3b time-slice amplitude map begins to provide some very subtle evidence of the drainage pipe system present. The Figure 28.3c GPR time-slice amplitude map processing sequence began with the SSCF, then 2-D migration (2DM), followed by STE. The Figure 28.3c time-slice amplitude map exhibits some definite, although still subtle, indications of the drainage pipe network that is present. Again, in Figure 28.3c, the areal extent of the golf course green is quite apparent.

FIGURE 28.3 Ground-penetrating radar (GPR) 9 to 15 ns (0.38 to 0.68 m depth interval) time-slice amplitude map computer-processing results: (a) raw data; (b) signal saturation correction filter (SSCF) and signal trace enveloping (STE); (c) SSCF, 2-D migration (2DM), and STE; and (d) SSCF, spatial background subtraction filter (SBSF), 2DM, and STE.

FIGURE 28.3 Ground-penetrating radar (GPR) 9 to 15 ns (0.38 to 0.68 m depth interval) time-slice amplitude map computer-processing results: (a) raw data; (b) signal saturation correction filter (SSCF) and signal trace enveloping (STE); (c) SSCF, 2-D migration (2DM), and STE; and (d) SSCF, spatial background subtraction filter (SBSF), 2DM, and STE.

The Figure 28.3d GPR time-slice amplitude map processing sequence began with the SSCF, then a spatial background subtraction filter (SBSF), followed by 2DM, and finally, STE. Not only is the areal extent of the golf course green obvious, the herringbone subsurface drainage pipe pattern is easily distinguishable. The SBSF used for the Figure 28.3d GPR time-slice amplitude map employed a seven signal trace moving average sequence. The SBSF helped substantially by removing most of the constructed soil layer response, thereby enhancing drainage pipe signals. Consequently, for mapping golf course green subsurface drainage pipe systems, a computer-processing sequence of a SSCF, a SBSF, 2DM, and STE is the best choice.

Figure 28.4 depicts the effects of different golf course green GPR field survey setups in relation to a time-slice amplitude map (1000 MHz center antenna frequency data) depiction of the subsurface drainage system present. For reference, Figure 28.4a is an interpreted schematic map showing the Muirfield Village Golf Club seventeenth hole USGA Method green areal extent (gray shaded area) and drain line locations (solid black lines for strong evidence and dashed black lines for weaker evidence). This schematic map is based on 250, 500, and 1000 MHz center antenna frequency GPR data collected in July 2004.

All Figure 28.4 time-slice amplitude maps were produced using a computer-processing sequence of a SSCF, a SBSF, 2DM, and STE. The Figure 28.4b time-slice amplitude map is based on a bidirectional survey (two sets of parallel measurement transects oriented perpendicular to one another) with a 1 m spacing distance between adjacent measurement lines. The complete subsurface drainage pipe network shows up very clearly in Figure 28.4b, given the possible exception of the far left portion of the map. The Figure 28.4c time-slice amplitude map is based on a bidirectional survey with a 2 m spacing distance between adjacent measurement lines. The main conveyance line and the lateral drain lines in the lower half of the Figure 28.4c time-slice amplitude map still show up well, but the rest of the subsurface drainage system is not nearly as visible. The Figure 28.4d time-slice amplitude map is based on a bidirectional survey with a 3 m spacing distance between

0 0

Post a comment