Geoscience Reference
In-Depth Information
well above the position of the drainage pipes. The western-most of the four drainage pipe reflection
hyperbolas is extremely subtle in Figure 29.4a. The banded linear feature representative of a bur-
ied drainage pipe oriented directly along trend of the GPR data collection line is almost absent in
Figure 29.4b. Additionally, the subsurface drainage pipe system is difficult to discern on the western
end of the Figure 29.4c GPR amplitude map. With the water table above the drainage pipes, the
dielectric constant contrast between the saturated soil surrounding the pipe and the water inside the
pipe is such that the effective radar reflection coefficient for the pipe is fairly low, and there is less
reflected radar energy from the pipe that returns to the receiving antenna.
A better shallow hydrologic condition for GPR drainage pipe detection occurs with a very moist
soil profile surrounding air filled drainage pipe (Figure 29.4d through Figure 29.4f). All four of the
reflection hyperbolas are apparent in Figure 29.4d. The banded linear feature showing the complete
length of a drain line is very clear in Figure 29.4e. The complete subsurface drainage system is
well defined in Figure 29.4f. Consequently, the dielectric constant contrast between the wet soil
surrounding the pipe and the air inside the pipe is such that the effective radar reflection coeffi-
cient for the pipe is fairly high and there is more reflected radar energy from the pipe that returns
to the receiving antenna. Here it is worth noting that the strength of the 250 MHz antenna GPR
drainage pipe response for a dry to moderately dry soil profile and air-filled pipes, as exhibited in
Figure 29.3c and Figure 29.3d, falls in between the strengths of the GPR drainage pipe response due
to a wet soil profile and water-filled pipes versus a wet soil profile and air-filled pipes.
The results for this portion of the research project provide some important guidelines as to when
a GPR drainage pipe mapping survey should be conducted with regard to the shallow hydrologic
conditions present. Clearly, GPR surveys should be avoided when the water table is above the eleva-
tion of the drainage pipes. This shallow hydrologic condition takes place most often in the hours (or
day or two at most) directly following a substantial rainfall event and before much soil drainage has
occurred. Although less typical, the water table is also elevated above the drainage pipes, usually
for prolonged periods of time, at locations where controlled drainage and subirrigation methods
are in use. Moderately dry to dry soil profiles with the water table below the drain lines (pipes are
completely air filled) are an acceptable shallow hydrologic condition for the use of GPR to locate
drainage pipes. The moderately dry to dry soil profile, low water table condition is fairly common
during periods where there has been little rainfall, especially during the growing season when
evapotranspiration rates are high. A better shallow hydrologic condition for GPR drainage pipe
detection occurs with a very moist soil profile and a water table at or below the drain lines (pipes are
completely or at least largely air filled). The very moist soil, low water table conditions often occur
during wet periods, especially after a day or two following a significant rainfall event, during which
most of the excess soil water has had a chance to drain. It is important to point out that substantially
increased soil wetness and the corresponding increase in soil electrical conductivity, which reduces
radar signal penetration depth beneath the ground surface, does not in itself preclude using GPR to
find buried agricultural drainage pipes.
Although perhaps not strongly emphasized in the previous discussion, Figure 29.3 and Figure 29.4
provide important insight on the impacts of pipe construction material and drain line orientation
with respect to GPR drainage pipe detection. The Figure 29.3a, Figure 29.3c, Figure 29.3e, 29.4a,
and Figure 29.4d GPR profiles were generated from measurements obtained along a transect ori-
ented perpendicular to the four clay tile and CPT north-south trending drainage pipes at the ESL
test plot (see Figure 29.2). Importantly, none of these profiles when viewed separately depict any
noticeable difference in GPR response between adjacent clay tile and CPT pipes. Appearances of the
clay tile and CPT drain lines are likewise similar on the Figure 29.4c and Figure 29.4f GPR time-
slice amplitude maps of the ESL test plot. Therefore, the pipe construction material, clay tile or CPT,
seems to have very little influence on the GPR drainage pipe detection response.
Figure 29.3a, Figure 29.3c, Figure 29.4a and Figure 29.4d show that an upside-down U-shaped
feature called a “reflection hyperbola” is the typical drainage pipe response depicted on a GPR
