Geoscience Reference
In-Depth Information
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 auto-
matic 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, down-
ward 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 mul-
tiple 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, multi-
ples 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 sat-
uration correction filter (SSCF) followed by signal trace enveloping (STE). Besides clearly depict-
ing 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.
