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FIGURE 28.4 Ground-penetrating radar (GPR) 9 to 15 ns (0.38 to 0.68 m depth interval) time-slice amplitude map comparison of different field survey setups: (a) interpreted schematic map showing golf course green areal extent (gray shaded area) and drain line locations (solid black lines for strong evidence and dashed black lines for weaker evidence); (b) bidirectional survey with a 1 m spacing distance between adjacent measurement lines; (c) bidirectional survey with a 2 m spacing distance between adjacent measurement lines; (d) bidirectional survey with a 3 m spacing distance between adjacent measurement lines; (e) unidirectional survey based on one set of parallel northwest-southeast transects with a 1 m spacing distance between adjacent measurement lines; and (f) second unidirectional survey based on the other set of parallel southwest-northeast transects with a 1 m spacing distance between adjacent measurement lines.

adjacent measurement lines. The main conveyance line and the lateral drain lines in the lower half of the Figure 28.4d time-slice amplitude map are still evident but not as distinct as in Figure 28.4c; however, the rest of the subsurface drainage system is not interpretable. It is obvious from the results presented in Figure 28.4b through Figure 28.4d that the spacing distance between adjacent measurement transects should be no greater than 1 m in order to effectively resolve a golf course green subsurface drainage system on a GPR time-slice amplitude map.

The Figure 28.4e time-slice amplitude map is based on a unidirectional survey (one set of parallel northwest-southeast transects) with a 1 m spacing distance between adjacent measurement lines. Because the orientation of the measurement transects for Figure 28.4e is essentially perpendicular to the orientation of the drainage pipe laterals shown in the bottom half of the map, these drainage pipe laterals show up extremely well. There are plenty of indications regarding the main conveyance line and the drainage pipe laterals in the upper half of the Figure 28.4e time-slice amplitude map, but they are not nearly as discernable as those in the lower half of the map. The Figure 28.4f time-slice amplitude map is based on a unidirectional survey (one set of parallel southwest-northeast transects) with a 1 m spacing distance between adjacent measurement lines. The drainage pipe laterals in the bottom of the Figure 28.4f time-slice amplitude map do not show up well at all, because the orientation of the measurement transects is essentially parallel to the orientation of the drainage pipe laterals in this part of the green. When they are oriented in the same direction, a GPR measurement line has to follow along directly over top of the drain line in order to detect it. Given circumstances in which they are parallel, if the GPR measurement line and drain line are offset from one another by a small amount, then the drain line will not be identified. One interesting aspect of the Figure 28.4f unidirectional survey GPR time-slice amplitude map is that the main conveyance line and the drainage pipe laterals in the upper half of the map show up remarkably clear, perhaps even better than in Figure 28.4b, which was based on a bidirectional survey. Consequently, a bidirectional GPR survey for a golf course green is required in order to have the best chance of detecting all the drainage pipes present. Although, plotting out the bidirectional and both unidirectional GPR time-slice amplitude maps and then comparing the three is probably worthwhile with respect to getting as good of an interpretation as possible for the overall golf course green subsurface drainage pipe network.

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