Measurement of soil surface dielectric permittivity and correlated water content using
off-ground GPR
Sébastien Lambot(1,3), Evert Slob(2)
Diana Chavarro (4), Maciek Lubczynski(4)
(1)Forschungszentrum Jülich (Germany)(2)Delft University of Technology (The Netherlands)
(3)Université catholique de Louvain (Belgium)(4)ITC (The Netherlands)
Current GPR techniques for soil characterization
Subsurface
Surface
Propagation time to a known interface
Reflecting hyperbola of an inhomogeneity
Common midpoint (CMP) or WARR
Propagation time between boreholes (tomography)
Ground-wave propagation time between antennas
1D surface reflection coefficient
Full-wave inverse modeling
Modeled signalSoil EM properties
Optimization algorithm
Measured signal
Errorfunction
EM forward model
Maximize information retrieval from the available data
?
Ground penetrating radar system
• SFCW radar with a VNA → UWB, forward modeling
• Horn antenna (highly directive) → to be used off the ground
• Monostatic mode → accurate and efficient forward modeling
Low-cost and handheld radar
GPR forward modeling
Antenna equation in the frequency domain
phase centerJx Ex↑
a
b
HiHf
H
E↑(Distributed)
Distribution assumed to be independent of the target
(Lambot et al., IEEE TGRS, 2004)
Air-subsurface full-wave Green’s function
• 3D N-layered medium
• point source/receiver (dipole approximation)
Jx Ex↑
3D Maxwell’s equations solution
Integrand
Sommerfeld integral
Model validation
Frequency domain Time domain
Configuration ≠ θ
Surface dielectric permittivity
Objective function
tmin
h
εσ ≅ 0
tmax
zmax
with
Surface roughness: hmax<λ/8
(Lambot et al., WRR, 2006b)
(Lambot et al., WRR, 2006a)
Effect of electric conductivity
1-2 GHz
0.1-0.9 GHz
Effect of shallow layering
Aquiferex: Ben Gardane
• Gravimetric
• Theta-probe
• Off-ground GPR
Transect
Olive trees
Measurements and characterization scales
• Gravimetric
- 100 cm³ soil samples
- 0-5 cm depth, 20 cm²
volumetric water content
• Theta-probe
- 0-10 cm depth, 20 cm²
dielectric permittivity and electric conductivity
• Off-ground GPR
- 800-2600 MHz (frequency step 6 MHz)
- 0-5 cm depth (depending on soil properties), 1 m²
- Rayleigh criterion should be respected (hmax<λ/8 → hmax<2.5 cm)
dielectric permittivity
GPR data in the time domain for the transect (65 measurements)
No strong subsurface reflectors were observed• Length: 2.2 km
• 65 GPR measurements
• Average space step: ~30 m
Dielectric permittivity and water content over the transect
0
0,01
0,02
0,03
0,04
0,05
0,06
0,07
0,08
0 10 20 30 40 50 60 70
Measurement
Wat
er c
onte
nt
0,00
1,00
2,00
3,00
4,00
5,00
6,00
0 10 20 30 40 50 60 70
Measurement
Die
lect
ric c
onst
ant 3672243mN687119mE
3670591mN688533mE
• Dry
• Small variation range
• Some spatial correlation
Observations
Other measurements
• Road (end of transect): average εr=7.04
• Concrete platform: average εr=4.33
• Saline area
εr=4.01, σ=0.098 S/m
εr=?, σ=0.176 S/m
Electric conductivity contributes significantly to the surface reflection coefficient
Aquiferex: GabesMeasurements in different irrigated plots: 36 measurements over 17 plots
Water content
dry
Saturated
GPR TDR
Topp’s model
Topp’s model
Results
GPRSoil specific model
TDR
Results taking into account electric conductivity during inversion of GPR data
r=0.94Soil specific modelr=0.93
→ Microvariability→ Measurement scale→ Surface roughness
(gravimetric) (gravimetric)
Conclusions and perspectives
• An adequate full-wave radar model has been developed for air-launched GPR
• In the field, surface-focused inversion permits to measure surface dielectric permittivity and correlated water content
• Good correlations were observed between gravimetric, TDR, and GPR
• Off-ground GPR is appropriate to bridge the scale gap between ground-truth measurements and airborne and spaceborne radar remote sensing
GPR measurements should be compared to airborne and spaceborne data
References
- Lambot, S., E.C. Slob, I. van den Bosch, B. Stockbroeckx, and M. Vanclooster, Modeling of ground-penetrating radar for accurate characterization of subsurface electric properties, IEEE Transactions on Geoscience and Remote Sensing, 42, 2555-2568, 2004.
- Lambot, S., M. Antoine, M. Vanclooster, and E.C. Slob, Effect of soil roughness on the inversion of off-ground monostatic GPR signal for noninvasive quantification of soilproperties, Water Resources Research, 42, W03403, doi10.1029/2005WR004416, 2006a.
- Lambot, S., L. Weihermüller, J.A. Huisman, H. Vereecken, M. Vanclooster, and E.C. Slob, Analysis of air-launched ground-penetrating radar techniques to measure the soil surface water content, Water Resources Research, 42, W11403, doi10.1029/2006WR005097, 2006b.
