WSSS Annual Meeting, Park City, UT June 19-21, 20061

Development of Rapid TDR Measurement Capability for Mobile Water Content Mapping

Scott Jones, Utah State UniversityDavid Robinson, Stanford University

Shmulik P. Friedman, Volcani Institute, ARO, Israel

WSSS Annual Meeting, Park City, UT June 19-21, 20062

On-The-Go Mapping

• Precision Agriculture – Variable Rate Technology– It is especially important to develop and implement

sensor technology for those parameters, such as soil nitrate and soil moisture, which can change rapidly (both spatially and temporally) and must be measured in real-time or near real-time to be useful for input control.

• K.A. Sudduth, http://mpac.missouri.edu/pubs/eng_tech.pdf

s

nv

wb ECECEC +⎟⎟⎠

⎞⎜⎜⎝

⎛=

φθ

Mapped

Unknown Texture

WSSS Annual Meeting, Park City, UT June 19-21, 20063

Project Rationale• Soil water content measurements are needed for on-the-go

agricultural, engineering and environmental activities.– Plowing, planting, reclamation, management, irrigation– Road base materials, dam materials– Hydrological studies, ecological work

• We propose an invasive TDR-based water content probe– provide robust direct permittivity measurements– Capacitance probes previously suggested– Short TDR probes have been proposed– An open-ended probe has advantages

Blade w/ open-ended TDR probes/ and temperature sensors

Depth Sensor

Tractor/Implement

Hand-heldPC

EMI

GPS

Datalogger

TDR

Blade w/ open-ended TDR probes/ and temperature sensors

Depth Sensor

Tractor/Implement

Hand-heldPC

EMI

GPS

Datalogger

TDR

WSSS Annual Meeting, Park City, UT June 19-21, 20064

TDR Travel-Time Analysis

Travel time [ns]0 10 20 30 40 160 170

Ref

lect

ion

Coe

ffici

ent

-1.0

-0.5

0.0

0.5

1.0

ECw = 0 S m-1

0.1 S m-1

0.6 S m-1

t1 t2

V0 Vf

212

2)(⎟⎠⎞

⎜⎝⎛ −

=L

ttcbε

⎟⎟⎠

⎞⎜⎜⎝

⎛−= 12 0

fc VV

ZKEC

PermittivityPermittivity

WSSS Annual Meeting, Park City, UT June 19-21, 20065

Open-Ended Dielectric Probe

• HP – Network Analyzer– Dielectric Probe Kit

• Frequency-dependent permittivity– 300 kHz – 6 GHz

• Frequency domain advantages– Relaxation phenomena– Fixed frequency

Blade (-) (+) (-)

Soil

Blade (-) (+) (-)

Soil

WSSS Annual Meeting, Park City, UT June 19-21, 20066

Open-ended Probe Waveform Analysis

• Bilinear function (Berberian, 1993; Cole et al, 1989)– Laplace transform– Compute admittance values

• Sample, cell, coaxial line, etc.

– Reflection coefficient related to• Complex permittivity• Bilinear coefficients

– f (reference dielectric, TL admittance, connectors)

– Bilinear coefficients are determined from:• Short, open and 50 ohm termination • Or three known references (water, alcohol, air)

Castiglione and WraithMontana State Univ.

WSSS Annual Meeting, Park City, UT June 19-21, 20067

Probe 1- Near Surface Probe

• UHF to BNC adapter• UHF end machined flush• One-sided measurements

WSSS Annual Meeting, Park City, UT June 19-21, 20068

Testing Effects of Probe Travel Velocity

• Porous media– Dry, moist, saturated sand

• Travel Speeds – 0 to 5 mph

WSSS Annual Meeting, Park City, UT June 19-21, 20069

Open-Ended Probe Fringing Field

Blade (-) (+) (-)

Soil

Soil

Blade (-) (+) (-)

Soil

Soil

• Modeled using– ATLC (Kirkby, 1996)– Plots energy density– Darker regions

• Higher field density• Greater influence on permittivity

WSSS Annual Meeting, Park City, UT June 19-21, 200610

Prototype Probe 2 – Single-depth Probe

• Custom-made– Dual sided

• RG-178 cable• 0.66 cm dia. Rod

– Inner conductor

• 1 cm wide blade– Shield

1.55 cm

0.66 cm

1.55 cm

0.66 cm

WSSS Annual Meeting, Park City, UT June 19-21, 200611

Field Testing of Probe Travel Velocity

Open-endedprobe in blade

a) Open-endedprobe in plate

Open-endedprobe in blade

a) Open-endedprobe in plate

-0.10

0.10.20.30.40.50.60.70.80.9

1 51 101 151 201Waveform points

Ref

lect

ion

coef

ficie

nt

0m12 m24 m36 m48 m

Open-ended probe12 m spacing5 km/h

Wettedregion

0.15

0.2

0.25

0.3

0.35

0.4

0 20 40 60 80 100

Transect distance (m)

Wat

er c

onte

nt e

stim

ate

b)

c)

4 km/h

-0.10

0.10.20.30.40.50.60.70.80.9

1 51 101 151 201Waveform points

Ref

lect

ion

coef

ficie

nt

0m12 m24 m36 m48 m

Open-ended probe12 m spacing5 km/h

Wettedregion

0.15

0.2

0.25

0.3

0.35

0.4

0 20 40 60 80 100

Transect distance (m)

Wat

er c

onte

nt e

stim

ate

b)

c)

4 km/h

• Waveform consistency• Wet and dry

– Travel-time analysis– Relative water content

WSSS Annual Meeting, Park City, UT June 19-21, 200612

On-The-Go Probe in Action

• 12 kph• Single blade• Probe at 15 cm

WSSS Annual Meeting, Park City, UT June 19-21, 200613

Prototype Probe 3 – Multi-depth Probe

• 3 – BNC Tee’s– Dual sided

• RG-58 cable• Epoxied in place then machined flush• 1 cm wide steel blade

Probe 3

10 cm

0.78 cm

0.23 cm

Probe 3

10 cm

0.78 cm

0.23 cm

WSSS Annual Meeting, Park City, UT June 19-21, 200614

Multi-depth Probe Results

• 9 reps

• Water– static

• Soil– θv ~ F.C.– 3.2 km/h

Water Soil Blade

Waveform pixels

Ref

lect

ion

coef

ficie

nt5 cm

15

25

RMSE = 0.012

0.010

0.019

0.035

0.019

0.045

WSSS Annual Meeting, Park City, UT June 19-21, 200615

Probe/Cable Attenuation

• 1 UHF• 2 Custom• 3 BNC T• 4 UHF T

Probe 2Probe 1

Probe 3b

1 MHz 10 100 1000 1 MHz 10 100 1000

1 MHz 10 100 1000 1 MHz 10 100 1000

UHF Tee

High attenuation

Low attenuation

NA Probe - Reference

WSSS Annual Meeting, Park City, UT June 19-21, 200616

Summary

• On-the-go water content measurements would add valuable GIS information to many mobile activities.

• An open-ended design is the most feasible probe configuration but requires an advanced analysis approach that is being developed for TDR.

• Hardware testing indicate high quality cable and probe can reduce losses to levels observed in a network analyzer.

• Ongoing work is looking to provide real-time water content determination within the next few years.