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Outline Website notice Where were we? Measuring soil wetness with TDR
Water Soil Physics 2010
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The class website is: Website notice
If you don’t include the “.html”, you won’t get there. Soil Physics 2010
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Where were we? Neutron Scattering Insert access tubes in soil
(thermalization, moderation) Probe emits fast neutrons and counts slow neutrons. Insert access tubes in soil Lower neutron probe down the tube Record the count ratio Convert count ratio to q Soil Physics 2010 Soil Physics 2010
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Neutron Scattering Advantages:
(thermalization, moderation) Advantages: Measurements repeated at exact same location No temperature issues – even works in frozen soil! Pretty reliable Insert access tubes in soil Lower neutron probe down the tube Record the count ratio Convert count ratio to q Soil Physics 2010 Soil Physics 2010
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Neutron Scattering Disadvantages:
(thermalization, moderation) Disadvantages: 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 Insert access tubes in soil Lower neutron probe down the tube Record the count ratio Convert count ratio to q Soil Physics 2010 Soil Physics 2010
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Alternative Neutron Scattering
(cosmic ray version, Zreda et al.) = Primary cosmic ray Soil Physics 2010 Soil Physics 2010
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Alternative Neutron Scattering
(cosmic ray version, Zreda et al.) Footprint 102 – 103 ha Installs above ground Requires calibration Hourly reading Depth varies with q Soil Physics 2010 Soil Physics 2010
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Methods overview Thermogravimetric Neutron thermalization
Electrical conductivity Electromagnetic Induction (EMI) Direct current (DC) resistivity Dielectric properties Time domain reflectometry (TDR) Frequency domain reflectometry (FDR) Ground penetrating radar (GPR) Thermal properties Photons Microwave remote sensing Infrared remote sensing Acoustical methods q confounded with sb Improving Emerging Soil Physics 2010
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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 Soil Physics 2010 Animation courtesy of Dr. Dan Russell, Kettering University
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…where it can be detected by another wire
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: …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 Soil Physics 2010 Animation courtesy of Dr. Dan Russell, Kettering University
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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 Soil Physics 2010
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Relative permittivity er
Permittivity values Material Relative permittivity er 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 3 Soil Physics 2010 Soil Physics 2010 Around 20 °C and 1 kHz
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Permittivity is complex!
Robinson et al., VZJ 2008 Soil Physics 2010 Soil Physics 2010
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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 3) The pulse reflects off the ends of the needles. Soil Physics 2010 Soil Physics 2010 Animation courtesy of Dr. Dan Russell, Kettering University
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TDR in practice Montmorillonite trace q a 4 b 11 c 22 d 30
Soil Physics 2010
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time TDR in practice Montmorillonite trace q a 4 b 11 c 22 d 30
Soil Physics 2010
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Repeated, non-destructive in-situ measurements
TDR in practice Advantages Easy to install Easy to multiplex Fairly strong signal Repeated, non-destructive in-situ measurements Soil Physics 2010
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Cable reader is expensive Tricky waveform analysis Fussy
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
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Water Soil Physics 2010
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Effects of the hydrogen bonding
Water Effects of the hydrogen bonding Soil Physics 2010
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Ice (diamond lattice) Soil Physics 2010
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Why ice floats Water and ice. Soil Physics 2010
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r Back to the dielectric
- + r The force F between two charged particles in a fluid is Note the resemblance to Coulomb’s law, Newton’s law of gravitation, etc. where Q is the charge, r is the separation distance, and er is the dielectric Soil Physics 2010
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Water’s dielectric in action
disordered. slac.stanford.edu Soil Physics 2010
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Large dielectric dissolves ionic compounds well
Effect of the dielectric - + 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
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Solutes lower the water’s energy
Fresh water Salt water Water moves from higher (pure) to lower (salty) energy state Soil Physics 2010
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This is the osmotic pressure, P
How do we know it’s energy? At equilibrium, the higher pressure balances the energy-lowering effect of the salt. Dh Fresh water Salt water This is the osmotic pressure, P Soil Physics 2010
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Water is resistant to temperature change, including phase change
Water and heat Water is resistant to temperature change, including phase change Soil Physics 2010
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