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IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING, VOL. 42, NO. 8, AUGUST 2004

1683

A Phase Signature for Detecting Wet Subsurface

Structures Using Polarimetric L-Band SAR

Yannick Lasne, Philippe Paillou, Thomas August-Bernex, Gilles Ruffié, and Gilles Grandjean

Abstract--In this paper, we investigate the ability of L-band syn-

thetic aperture radar (SAR) systems to penetrate soils to retrieve

information about subsurface wet structures. Our experiment site,

the Pyla dune, is a bare sandy area allowing high radar penetration

and known to have large wet subsurface structures (paleosoils) at

varying depths. Buried paleosoils, which act as moisture tanks, are

detectable with radar, since they present a high permittivity due to

their water content. By analyzing airborne polarimetric SAR data,

we established that a phase signature is correlated to the buried wet

palesoils: a phase difference of 23 between the horizontal (HH)

and vertical (VV) channels was clearly observed. It allows detec-

tion of the paleosoil down to a larger depth (5.2 m) than when only

considering HH and HV amplitude signals (3.5 m). In order to con-

firm this result, field measurements were performed that led to the

same observed phase difference. We could fit our observations to

Fig. 1.

Aerial view of the Pyla sand dune.

the semiempirical model proposed by Oh and Sarabandi, and we

reproduced the observed phenomenon using a two-layer integral

equation method (IEM) model of the Pyla dune, which was com-

information about surface parameters such as topography, sur-

pleted by finite-difference time-domain (FDTD) numerical simu-

face roughness (vegetation cover, geology, waves), and the di-

lations. We show that the soil moisture significantly influences the

electric properties mainly related to soil moisture [1][3]. Over

radar response in terms of phase difference between the copolar-

ized modes. Our study also shows that the single-scattering IEM

arid areas, L-band SAR can explore the subsurface down to sev-

model reproduces the observed phase difference fairly well for a

eral meters when covered by dry material such as sand [4][8].

natural outdoor site when combined to FDTD simulation results.

Using SIR-A data, Schaber et al. [4] estimated the penetration

This phase signature could be used as a new tool to map subsurface

depth of L-band SAR to be 1.5 m through sandy sediments in

moisture in arid regions.

the southern desert of Egypt. Based on SIR-B data, other studies

Index Terms-- Finite-difference time-domain (FDTD), integral

discussed L-band radar penetration capabilities in Saudi Arabia

equation method (IEM), L-band, moisture, phase, polarimetry,

[9] and in the Nevada desert [10] to reveal buried scatterers.

subsurface, synthetic aperture radar (SAR).

More recently, other authors showed the ability of SIR-C multi-

frequency polarimetric SAR to map subsurface geology below

I. INTRODUCTION

sandy materials [11][13]. In this work, we considered an ap-

proach consisting of a combination of ground-penetrating radar

VER THE last two decades, remote sensing using polari-

(GPR) and SAR data. GPR measurements provide us with in-

metric synthetic aperture radar (SAR) has been widely

formation on the subsurface geometry and the dielectric values

used for study of the earth's surface. Spaceborne synthetic aper-

of the buried reflector, which is then used as input for SAR data

ture radar (SAR) (e.g., Spaceborne Imaging Radar-C/X-band

modeling and interpretation. Such a technique was successfully

Synthetic Aperture Radar (SIR-C/X-SAR), Japanese Earth Re-

demonstrated by Paillou et al. [5] over the Bir Safsaf region in

sources Satellite 1 (JERS-1), European Remote Sensing Satel-

southern Egypt and by Grandjean et al. [14] over the Pyla sand

lite 1 and 2 tandem (ERS-1/2), RADARSAT) allows retrieval of

dune in France.

All these experiments demonstrated that low-frequency radar

has penetration capabilities that can be used to map subsur-

Manuscript received September 23, 2003; revised April 6, 2004. This

face heterogeneities such as geological interfaces or wet layers.

work was supported by the French National Program for Remote Sensing

(INSU/PNTS).

As regards soil moisture, it is well known that the presence of

Y. Lasne and P. Paillou are with Observatoire Aquitain des Sciences de

water influences the radar response of a terrain. Experimental

l'Univers, L3AB-UMR 5804, 33270 Floirac, France (e-mail: lasne@obs.u-bor-

and theoretical studies, based on empirical or semiempirical

deaux1.fr; paillou@obs.u-bordeaux1.fr).

T. August-Bernex is with the European Organisation for the Exploitation of

models, have investigated this phenomenon [15][19]. Never-

Meteorological Satellites, 64295 Darmstadt, Germany.

theless, very few authors used the phase information from SAR

G. Ruffié is with the Laboratoire de Physique des Interactions Ondes-

data to detect moisture [20][22]. Moreover, even if some exper-

Matières, Ecole Nationale Supérieure de Chimie et de Physique de Bordeaux,

UMR 5501, 33405 Talence, France.

imental studies of the phase shift induced by moisture changes

G. Grandjean is with the Bureau de Recherches Géologiques et Minières,

were performed by means of indoor radar measurements, no de-

45060 Orléans, France.

finitive results were obtained for natural outdoor sites.

Digital Object Identifier 10.1109/TGRS.2004.830645