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.