time domain reflectometry soil moisture measurement for ... · – calcium carbide gas pressure...
TRANSCRIPT
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Time Domain Reflectometry Soil Moisture Measurement for
Hydrological Modelling Sustainable Urban Water Workshop
June 4, 2015
Xinbao Yu, Ph.D., P.E., Assistant Professor University of Texas at Arlington Arlington, TX, [email protected]
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TDR History
• Electrical Engineering (cable tester) Detect cable breaks • Agriculture and Hydrology Volumetric moisture content Topp- father of TDR • Geotechnical Engineering
Purdue University Research Group Water content and density, ASTM standard
• Various Area of Civil Engineering
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Current TDR Research at UTA
Thermo-TDR • Soil moisture, dry density,
thermal properties
Suction-TDR • Soil moisture and suction
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TDR Basics
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A TDR System
RG58
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TDR Instruments
• Different TDR devices
(a) 1502C (Tektronix Inc., Beaverton, OR); (b) TRIME-FM (IMKO, Ettlingen, Germany); (c) TRASE System I (Soil Moisture Equipment Corp., Goleta, CA); (d) TDR100 (Campbell Scientific Inc., Logan, UT)
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TDR Instruments (cont.)
• TDR probes
Different type of TDR probes Design of your own probes for your research needs
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A Field Monitoring System
• Data flow
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What TDR measures?
-1.25
-0.75
-0.25
0.25
0.75
1.25
0 1 2 3 4 5 6 7 8
Scaled Distance (m)
Rel
ativ
e V
olta
ge (V
)
−= 11
f
sb V
VC
EC2
=
p
aa L
LK
Apparent Length, La
-1.25
-0.75
-0.25
0.25
0.75
1.25
0 1 2 3 4 5 6 7 8
Scaled Distance (m)
Rel
ativ
e V
olta
ge (V
)
−= 11
f
sb V
VC
EC2
=
p
aa L
LK
-1.25
-0.75
-0.25
0.25
0.75
1.25
0 1 2 3 4 5 6 7 8
Scaled Distance (m)
Rel
ativ
e V
olta
ge (V
)
-1.25
-0.75
-0.25
0.25
0.75
1.25
0 1 2 3 4 5 6 7 8
Scaled Distance (m)
Rel
ativ
e V
olta
ge (V
)
−= 11
f
sb V
VC
EC2
=
p
aa L
LK
Apparent Length, La
Vf
Vs/2
o
Kεε
=Note: dielectric constant
•(Apparent) Dielectric constant: Ka •Electrical conductivity: ECb
permittivity
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Data Acquisition
•PMTDR
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Signal Analyses
•Detection of reflections
Topp et al. (1982)
Baker and Allmaras (1990)
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APPLICATION EXAMPLES
•Measurement of Moisture Content and Density •Infiltration Monitoring
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Moisture Content and Density
• Earthwork control • Pavement
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Current Methods
• Water Content – Oven Dry (ASTM D 2216) – Microwave (ASTM D 4643) – Nuclear Method (D 3017) – Calcium Carbide Gas Pressure (ASTM D 4944)
• Density
– Nuclear Method (ASTM D 2922) – Sand Cone (ASTM D 1556) – Rubber Balloon (ASTM D 2167) – Drive Cylinder ( ASTM D2937)
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Nuclear Method – the State of Art
Different Modes of Nuclear Gauge (from Hausmaan, 1990)
(a) Backscatter (b) Direct Transmission
Advantages: Easy to use and accurate for most situations Disadvantages: Regulation, nuclear radiation; biased by hydrogen and certain types of ions; different sampling zone
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-1.25
-0.75
-0.25
0.25
0.75
1.25
0 1 2 3 4 5 6 7 8Scaled Distance (m)
w=2.1% r d=1648 kg/m3
w=5.2% r d=1655 kg/m3
w=9.3% r d=1658 kg/m3
w=14.0% r d=1653 kg/m3
(a)
Rel
ativ
e V
olta
ge (V
)
Incr
easi
ng E
Cb Increasing Ka
Information from TDR Signal
Constant dry density; changing water content
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-1.25
-0.75
-0.25
0.25
0.75
1.25
0 1 2 3 4 5 6 7 8
Scaled Distance (m)
w=10.7% r d=1256 kg/m3
w=10.7% r d=1490 kg/m3
w=10.6% r d=1583 kg/m3
w=10.4% r d=1871 kg/m3
(b)
Rel
ativ
e V
olta
ge (V
)Information in TDR Signal
Incr
easi
ng E
Cb Increasing Ka
Constant water content ; changing dry density
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Soil Dielectric Constant, Ka
• Soil dielectric constant, Ka • Predominantly decided by water content • Topp’s equation relates Ka to volumetric water content (widely
applied in the current practice)
Soil Solids
Air
Water
222436 103.51092.2105.5103.4 −−−− ×−×+×−×= aaa KKKθ
( )ββ
βββ
θairasa
airasaa
KKnKKnK
,,
,,1−
−−−=
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Utilizing Electrical Conductivity
Analogy Ka=81
Ka=1
Ka=3-5 ECw
ECa=0
ECs
bwaKd
wa +=ρρ
Dielectric Constant
dwcECd
wb +=ρρ
Electrical Conductivity
Soil Solids
Air Water Soil
Phases
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0
0.05
0.1
0.15
0.2
0.25
0.3
0 2 4 6 8 10 12 14 16
Location No.
Wat
er C
onte
nt
TDROven DryNuclear
Glacial Till, INDOT Division of Research Testing Pavement
One-Step TDR Method versus Nuclear Method
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Water Content Comparisons
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35
Oven Dry Gravimetric Water Content
1:1 Line1:1 Line +1%1:1 Line -1%One-Step Method
Gra
vim
etric
Wat
er C
onte
nt b
y TD
R
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Density Comparisons
800
1000
1200
1400
1600
1800
2000
2200
800 1000 1200 1400 1600 1800 2000 2200
Dry Density by Total Density and Oven Dry Water Content (kg/m3)
One-Step Method1:1 Line1:1 +3% Line1:1 -3% Line
Dry
Den
sity
by
One
-Ste
p M
etho
d (k
g/m
3 )
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Rainfall Infiltration Monitoring
Flexible Flat Strip TDR Sensor
23
Coiled TDR strip Sensor Installation TDR strip Sensor in the Field
Cross Section View Sampling Area
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Sensor Calibration
Photo of calibration experiment for the TDR strip sensor
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Seepage Monitoring
Photo of the sand tank seepage monitoring Measured TDR signals during the seepage experiment
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Infiltration Monitoring
Test set up for monitoring infiltration process
Measured TDR signals during the precipitation test
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Conclusions
• TDR can accurately measure soil moisture content and dry density.
• TDR can monitor rainfall infiltration. • TDR probe sensor and flat strip sensor
will be used for soil moisture and infiltration monitoring.
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Thank you for listening!