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 2010

Permittivity is complex!

Soil Physics 2010 Robinson et al., VZJ 2008

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

+

-

Montmorillonite

trace a 4b 11c 22d 30

Soil Physics 2010

TDR in practice

Soil Physics 2010

TDR in practice

Montmorillonite

trace a 4b 11c 22d 30r

c

t

dv

time

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

Soil Physics 2010

Water

0

50

100

150

200

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300

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400

CH4 NH3 H2O HF Ne H2S

Boiling, K

Melting, K

Dielectric

Effects of the hydrogen bonding

Soil Physics 2010

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

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.

disordered. slac.stanford.eduSoil Physics 2010

Water’s dielectric in action

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,

Soil Physics 2010

0

500

1000

1500

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2500

CH4 NH3 H2O HF Ne H2S

Latent heat of fusion

latent heat of vaporization

Water and heat

Water is resistant to temperature change, including phase change