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Soil Physics 2010 Outline Website notice Where were we? Measuring soil wetness with TDR Water
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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.
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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) Where were we? Probe emits fast neutrons and counts slow neutrons.
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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) Measurements repeated at exact same location No temperature issues – even works in frozen soil! Pretty reliable Advantages:
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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
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Alternative Neutron Scattering (cosmic ray version, Zreda et al.) = Primary cosmic ray
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Soil Physics 2010 Alternative Neutron Scattering (cosmic ray version, Zreda et al.) Footprint 10 2 – 10 3 ha Installs above ground Requires calibration Hourly reading Depth varies with
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Soil Physics 2010 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 confounded with b Improving Emerging
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Soil 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
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Soil Physics 2010 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: 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. …where it can be detected by another wire Animation courtesy of Dr. Dan Russell, Kettering University
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Soil 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.
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Soil Physics 2010 Permittivity values Material Relative permittivity r vacuum1.0 air 1.0006 hexane1.9 charcoal1.5 wood (dry)2-6 cereal grain3-8 sand3-5 water80 ice3 Around 20 °C and 1 kHz
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Soil Physics 2010 Permittivity is complex! Soil Physics 2010 Robinson et al., VZJ 2008
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Soil 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 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 + -
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Montmorillonite trace a 4 b 11 c 22 d 30 Soil Physics 2010 TDR in practice
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Soil Physics 2010 TDR in practice Montmorillonite trace a 4 b 11 c 22 d 30 time
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Soil Physics 2010 TDR in practice Advantages Easy to install Easy to multiplex Fairly strong signal Repeated, non- destructive in-situ measurements
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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
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Soil Physics 2010 Water
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Soil Physics 2010 Water Effects of the hydrogen bonding
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Soil Physics 2010 Ice (diamond lattice) www.boston-audio.com
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Soil Physics 2010 Why ice floats Water and ice. www.nyu.edu
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Soil Physics 2010 Back to the dielectric The force F between two charged particles in a fluid is - + r where Q is the charge, r is the separation distance, and r is the dielectric Note the resemblance to Coulomb’s law, Newton’s law of gravitation, etc.
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disordered. slac.stanford.edu Soil Physics 2010 Water’s dielectric in action
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Soil Physics 2010 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
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Solutes lower the water’s energy Soil Physics 2010 Fresh water Salt water Water moves from higher (pure) to lower (salty) energy state
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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. hh This is the osmotic pressure,
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Soil Physics 2010 Water and heat Water is resistant to temperature change, including phase change
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