Outline
• Website notice
• Where were we?
• Measuring soil wetness with TDR
• Water
Soil Physics 2010
Website notice
The class website is:
www.agron.iastate.edu/soilphysics/agron577.html
If you don’t include the “.html”, you won’t
get there.
Soil Physics 2010
Where were we?
Neutron Scattering
(thermalization, moderation)
Insert access tubes in soil
Lower neutron probe down
the tube
Record the count ratio
Convert count ratio to q
Soil Physics 2010
Probe emits fast neutrons
and counts slow neutrons.
Neutron Scattering
(thermalization, moderation)
Insert access tubes in soil
Advantages:
Measurements repeated at
exact same location
No temperature issues – even
Lower neutron probe down works in frozen soil!
the tube
Pretty reliable
Record the count ratio
Convert count ratio to q
Soil Physics 2010
Neutron Scattering
(thermalization, moderation)
Disadvantages:
Insert access tubes in soil
Radioactive material: need
special training & licensing
Lower neutron probe down the
tube
Indirect: need soil-specific
calibration
Record the count ratio
Slow & labor-intensive
Convert count ratio to q
Doesn’t work near surface
Issues with non-water H, O,
C, Al, Fe, etc.
Test volume varies with
wetness
Soil Physics 2010
Alternative Neutron Scattering
(cosmic ray version, Zreda et al.)
= Primary cosmic ray
Soil Physics 2010
Alternative Neutron Scattering
(cosmic ray version, Zreda et al.)
Soil Physics 2010
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Footprint 102 – 103 ha
Installs above ground
Requires calibration
Hourly reading
Depth varies with q
Methods overview
Thermogravimetric
Neutron thermalization
Electrical conductivity
Electromagnetic Induction (EMI)
Direct current (DC) resistivity
q confounded with sb
Dielectric properties
Time domain reflectometry (TDR)
Frequency domain reflectometry (FDR)
Ground penetrating radar (GPR)
Improving
Thermal properties
Photons
Microwave remote sensing
Infrared remote sensing
Acoustical methods
Soil Physics 2010
Emerging
Time Domain Reflectometry
(TDR)
Principle, part 1:
An electrical pulse propagating along a wire reflects
back from the end of the wire:
Knowing the speed of propagation (around c), we can
figure out the distance to the end – hence “Cable
Tester”
Soil Physics 2010
Animation courtesy of Dr. Dan Russell, Kettering University
Time Domain Reflectometry
Principle, part 2:
An EM field propagates through a non-conducting
medium with a velocity determined by the material’s
dielectric permittivity:
v
c
er
…where it can
be detected by
another wire
The dielectric permittivity er (sometimes called the dielectric
constant, which it isn’t!) is expressed relative to the
permittivy of a vacuum (1 by definition), so it is unitless.
Soil Physics 2010
Animation courtesy of Dr. Dan Russell, Kettering University
Dielectric permittivity?
Dielectric permittivity is a measure of how susceptible a
material is to being polarized in the presence of an
electrical field.
A material with a high dielectric permittivity is
generally (1) an insulator, and (2) polar.
Because the individual atoms do not polarize or align
instantly, there is a delay. Consequently, permittivity is
frequency-dependent.
Permittivity can also depend on temperature, humidity,
etc.
Soil Physics 2010
Permittivity values
Material
vacuum
air
hexane
charcoal
wood (dry)
cereal grain
sand
water
ice
Soil Physics 2010
Relative permittivity er
1.0
1.0006
1.9
1.5
2-6
3-8
3-5
80
3
Around 20 °C and 1 kHz
Permittivity is complex!
Soil Physics 2010
Robinson et al., VZJ 2008
TDR setup
Cable Tester
1) A pulse is sent
through the cable
to the probe
2) The material
between the
needles is subjected
to an EM gradient
5) The returned
pulse shows the
effect of this delay
+
-
4) The pulse also propagates
through the soil at a velocity
v
Soil Physics 2010
c
3) The pulse reflects off
the ends of the needles.
er
Animation courtesy of Dr. Dan Russell, Kettering University
TDR in practice
Montmorillonite
trace q
a 4
b 11
c 22
d 30
Soil Physics 2010
TDR in practice
time
d
c
v 
t
e
Soil Physics 2010
Montmorillonite
trace q
a 4
b 11
c 22
r
d 30
TDR in practice
Advantages
Easy to install
Easy to multiplex
Fairly strong signal
Repeated, nondestructive in-situ
measurements
Soil Physics 2010
TDR in practice
Disadvantages
Cable reader is expensive
Tricky waveform analysis
Fussy
Frozen water gives
different signal
Sensitive to temperature
Affected by clay
Affected by salinity
Soil Physics 2010
Best practice still debated
Water
Soil Physics 2010
Water
Effects of the hydrogen bonding
400
Boiling, K
350
Melting, K
300
Dielectric
250
200
150
100
50
0
CH4
Soil Physics 2010
NH3
H2O
HF
Ne
H2S
Ice (diamond lattice)
www.boston-audio.com
Soil Physics 2010
Why ice floats
Water and ice. www.nyu.edu
Soil Physics 2010
Back to the dielectric
+
-
r
The force F between two charged particles in a
fluid is
1 QQ
F
where
1
4e r r
2
2
Q is the charge,
r is the separation distance, and
er is the dielectric
Soil Physics 2010
Note the resemblance to Coulomb’s law,
Newton’s law of gravitation, etc.
Water’s dielectric in action
Soil Physics 2010
disordered. slac.stanford.edu
Effect of the dielectric
+
-
r
1 Q1Q2
F
2
4e r r
For a large dielectric (e.g., water),
the force is small.
When the force is small, particles of opposite
charge can be pulled apart more easily.
Large dielectric dissolves
ionic compounds well
Soil Physics 2010
Solutes lower the water’s energy
Fresh
water
Salt
water
Water moves from higher (pure) to lower (salty)
energy state
Soil Physics 2010
How do we know it’s energy?
Dh
Fresh
water
At equilibrium, the
higher pressure balances
the energy-lowering
effect of the salt.
Salt
water
This is the osmotic pressure, P
Soil Physics 2010
2500heat
Water and
Latent heat of fusion
latent heat of vaporization
2000
1500
1000
500
0
CH4
NH3
H2O
HF
Ne
H2S
Water is resistant to temperature
change, including phase change
Soil Physics 2010