soil physics 2010 outline website notice where were we? measuring soil wetness with tdr water
TRANSCRIPT
Soil Physics 2010
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 2010Soil Physics 2010
Insert access tubes in soil
Lower neutron probe down the tube
Record the count ratio
Convert count ratio to
Neutron Scattering(thermalization, moderation)
Where were we?
Probe emits fast neutrons and counts slow neutrons.
Soil Physics 2010Soil Physics 2010
Insert access tubes in soil
Lower neutron probe down the tube
Record the count ratio
Convert count ratio to
Neutron Scattering(thermalization, moderation)
Measurements repeated at exact same location
No temperature issues – even works in frozen soil!
Pretty reliable
Advantages:
Soil Physics 2010
Insert access tubes in soil
Lower neutron probe down the tube
Record the count ratio
Convert count ratio to
Neutron Scattering(thermalization, moderation)
Radioactive material: need special training & licensing
Indirect: need soil-specific calibration
Slow & labor-intensive
Doesn’t work near surface
Issues with non-water H, O, C, Al, Fe, etc.
Test volume varies with wetness
Disadvantages:
Soil Physics 2010
Soil Physics 2010Soil Physics 2010
Alternative Neutron Scattering(cosmic ray version, Zreda et al.)
= Primary cosmic ray
Soil Physics 2010Soil Physics 2010
Alternative Neutron Scattering(cosmic ray version, Zreda et al.)
• Footprint 102 – 103 ha• Installs above ground• Requires calibration• Hourly reading• Depth varies with
Soil Physics 2010
Methods overview
ThermogravimetricNeutron thermalizationElectrical conductivity
Electromagnetic Induction (EMI)Direct current (DC) resistivity
Dielectric propertiesTime domain reflectometry (TDR)Frequency domain reflectometry (FDR)Ground penetrating radar (GPR)
Thermal propertiesPhotons
Microwave remote sensingInfrared remote sensing
Acoustical methods
confounded with b
Improving
Emerging
Soil Physics 2010Soil Physics 2010
Time Domain Reflectometry(TDR)
Knowing the speed of propagation (around c), we can figure out the distance to the end – hence “Cable Tester”
Principle, part 1:
An electrical pulse propagating along a wire reflects back from the end of the wire:
Animation courtesy of Dr. Dan Russell, Kettering University
Soil Physics 2010Soil Physics 2010
Time Domain ReflectometryPrinciple, part 2:
An EM field propagates through a non-conducting medium with a velocity determined by the material’s dielectric permittivity:
The dielectric permittivity r (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.
r
cv
…where it can be detected by another wire
Animation courtesy of Dr. Dan Russell, Kettering University
Soil Physics 2010Soil Physics 2010
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 2010Soil Physics 2010
Permittivity values
Material Relative permittivity r
vacuum 1.0
air 1.0006
hexane 1.9
charcoal 1.5
wood (dry) 2-6
cereal grain 3-8
sand 3-5
water 80
ice 3Around 20 °C and 1 kHz
Soil Physics 2010Soil Physics 2010
TDR setup
Cable Tester
4) The pulse also propagates through the soil at a velocity
2) The material between the needles is subjected to an EM gradient
1) A pulse is sent through the cable to the probe
r
cv
5) The returned pulse shows the effect of this delay
3) The pulse reflects off the ends of the needles.
Animation courtesy of Dr. Dan Russell, Kettering University
+
-
Soil Physics 2010
TDR in practice
Advantages
Easy to install
Easy to multiplex
Fairly strong signal
Repeated, non-destructive 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
Best practice still debated
Soil Physics 2010
Water
0
50
100
150
200
250
300
350
400
CH4 NH3 H2O HF Ne H2S
Boiling, K
Melting, K
Dielectric
Effects of the hydrogen bonding
Soil Physics 2010
Back to the dielectric
The force F between two charged particles in a fluid is
- +
r
whereQ is the charge,r is the separation distance, andr is the dielectric
221
4
1
r
QQF
r
Note the resemblance to Coulomb’s law, Newton’s law of gravitation, etc.
Soil Physics 2010
Effect of the dielectric
- +
r
221
4
1
r
QQF
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
Solutes lower the water’s energy
Soil Physics 2010
Fresh water
Salt water
Water moves from higher (pure) to lower (salty) energy state
How do we know it’s energy?
Soil Physics 2010
Fresh water
Salt water
At equilibrium, the higher pressure balances the energy-lowering effect of the salt.h
This is the osmotic pressure,