Environmental Engineering Reference
In-Depth Information
Table 5.1 Summary of some geophysical techniques for determining soil-water content. Application
regime refers to location of measurements: land surface (S), subsurface (SS), or borehole logging (BH).
Quantitative indicates that quantitative estimates of water content can be obtained (if not checked,
technique provides only qualitative information). Measurement scales are from Ferré
et al
. (
2007
).
Method
Measured property
Spatial measure-
ment scale (m)
Application
regime
Quantitative
Time-domain reflectom-
etry (TDR)
Dielectric permittivity
0.1-1
SS
X
Ground-penetrating
radar (GPR)
Dielectric permittivity
1-10
S, SS, BH
Borehole only
Electromagnetic induc-
tion (EMI)
Electrical conductivity
1-10s
S, SS, BH
Temporal gravity
Total mass
10s-100s
S
X
Neutron moderation
Hydrogen density
0.1-1
SS, BH
X
The gravimetric approach consists of collect-
ing a known volume of soil sample at selected
depths, weighing the sample, and then drying
the sample in an oven at 105ºC for 24 to 48
hours and reweighing (Topp and Ferré,
2002
).
Accurate measurements can be obtained, but
the approach is time consuming and, because
the method is destructive, measurements can-
not be repeated at a specific location. Collection
of samples at depths greater than a couple of
meters can be expensive. Samples must be
weighed quickly after they are obtained; other-
wise, evaporation of soil water may affect the
accuracy of the measurement. Periodic gravi-
metric measurements of water content can be
used to develop calibration curves for electronic
water-content sensors.
Geophysical techniques can provide indir-
ect measurements of soil-water content. The
techniques actually measure a certain property
(
Table 5.1
), such as electrical conductivity of the
soil-water mixture that varies with water con-
tent. A number of techniques have been devel-
oped in recent years to relate soil-water content
to the dielectric permittivity of a soil/water mix-
ture as determined, for example, with trans-
mission line type electromagnetic (EM) sensors.
These sensors measure phenomena such as EM
wave travel time, impedance, capacitor charge
time, oscillation frequency, and frequency shift
(Blonquist, Jr.,
et al
.,
2005
). The techniques have
similarities in terms of probe size, installation
methods, and analysis of data, but there are
substantial differences in sensor accuracy and
cost (Blonquist, Jr.,
et al
.,
2005
; Robinson
et al
.,
2008b
).
Time-domain reflectometry (TDR), first pro-
posed by Topp
et al
. (
1980
), is perhaps the most
familiar of the EM methods. This paragraph
describes some of the features of TDR; many
of these features apply to other EM methods as
well. TDR probes typically consist of two or three
metal electrodes, 150 to 300 mm in length. The
probes are installed more or less permanently
at selected depths in the unsaturated zone;
installation can be through an open borehole,
but more commonly a trench is excavated, the
probes are installed horizontally into a trench
wall, and the trench is backfilled. Wires run
from the probes to an analyzer and data log-
ger on land surface for automatic sensing and
recording. TDR and other fixed-depth probes
offer a fairly high degree of accuracy and the
ability to automatically record at high frequen-
cies (in the order of minutes). Disadvantages of
the TDR method include the fact that measure-
ments are obtained over a small sample size,
a limited number of depths are sampled, and
installation techniques can alter natural water
movement patterns. Temperature fluctuations,
high clay content of soils, and high dissolved
solids concentrations in soil water can also
complicate determination of soil-water con-
tent, although TDR appears to be less affected
