Geoscience Reference
In-Depth Information
The diminished values of apparent conductivity reflect the sensitivity of the arrays this thickness of
nonconductive air (inside the instrument) in the sampled depth.
Where the H/P-1 lies above 0.3 m layer of sand, the HCP apparent conductivity will decrease
to 43 mS/m; the greater sensitivity of the PRP array through this depth will decrease its apparent
conductivity to 28 mS/m, about midway between the actual conductivities of the sand and native
soil. Where the sand thickness increases to 0.5 m, the HCP apparent conductivity will decrease to
37 mS/m, and the PRP apparent conductivity will decrease to 21 mS/m.
The modeling suggests that sand-related changes in 1 m HCP apparent conductivity will be
significantly above the level of environmental noise, but undefined local changes in the conductivity
of the native soil will likely add significant uncertainty to the interpretation of actual HCP measure-
ments. Where the sand is 0.3 m thick, for example, modeling predicts that the HCP apparent con-
ductivity will fluctuate between 26 and 59 mS/m, where the conductivity of the native soil fluctuates
between 30 and 70 mS/m. The sand-related changes to 1 m PRP apparent conductivity will be about
twice those of the HCP array, and the PRP measurements will be less affected by sub-sand fluctua-
tions in native-soil conductivity. With greater sensitivity to depths typical of the sand beneath golf
greens, the PRP apparent conductivity should be effective at mapping the extent of the sand, and
perhaps fluctuations in the thickness and moisture of the sand.
21.5 SURvey exAMpleS
Surveys at two golf courses in Dublin, Ohio, tested the effectiveness of EMI for golf-course applica-
tions. At the Golf Course of Dublin (GCD), measurements were made over a practice green con-
structed using the California method. At the Muirfield Village Golf Course (MVGC), measurements
were made at the seventeenth hole, over both the USGA-method green and the professional tee.
At each site, the H/P-1 was pulled in a sled at walking speed along east-west traverses. Mea-
surements were at 1/8-second intervals, which yielded about eight measurements per meter along
the survey traverse. Closely spaced measurements are essential for mapping golf-course features
that might include an irregular boundary between sand layers and native soil. The survey traverses
were visually controlled according to markers at each end of the traverse, and the nominal spacing
between traverses was 1 m.
Figure 21.4 presents profiles of apparent conductivity from traverse 10 N, through the center
of the GCD California-method practice green. The apparent conductivities for both HCP and PRP
are around 55 mS/m at the ends of the traverse, indicating that native soil of about this conduc-
tivity extends to the surface. Over an apparent
transition zone of about 2 m (between eastings 1
to 3 and 20 to 18), the PRP apparent conductiv-
ity decreases from native-soil levels to about 30
mS/m. This decrease is consistent with a thick-
ening to about 0.3 m of a subturf layer of sand,
with conductivity of about 10 mS/m. A decrease
in PRP apparent conductivity of a few mS/m at
easting 11, and perhaps at easting 6, may indi-
cate thickening of the sand or other coarse mate-
rial coincident with drainage structures.
Figure 21.5 and Figure 21.6 are contour
maps of the H/P-1 apparent conductivities from
the traverses on the GCD green. The PRP con-
tours of Figure 21.5 reveal a rectangular area
of apparent conductivities below 34 mS/m,
enclosed by apparent conductivities in the 40s
and 50s of mS/m. In this and subsequent maps,
70
HCP
PRP
60
50
40
30
20
10
0
5
10
15
20
Easting (m)
fIGURe 21.4
Traverse 10 N at the Golf Course of
Dublin (GCD).
