LASNE et al.: PHASE SIGNATURE FOR DETECTING WET SUBSURFACE STRUCTURES

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Fig. 5. Two horn antennas used during field experiment.

is about 23 and corresponds to a buried

maximum

wet layer located 2.9 m under the dune surface. The correlation

Fig. 4.

profile of the southern part of the Pyla dune when crossing

between HH and VV signals remains high, which means that the

the buried paleosoil. The curves shown on the figure are obtained from averaging

observed phase difference is significant. It should be noticed that

several profiles along the paleosoil.

traces the buried paleosoil far after the HH and HV

amplitude signals have disappeared, allowing to detect it down

Besides classical polarimetric analysis of amplitude data that

to 5.2 m deep, which is almost 2 m deeper than with HH or HV

allowed detection of the wet subsurface layers down to 3.5 m

amplitude signals.

deep [14], we established that a specific phase signal was cor-

related with the buried paleosoils: a phase difference between

B. Field Measurements

HH and VV channels of 23 was clearly observed on RAMSES

In order to confirm the phase difference observed in SAR im-

L-band images as shown in Fig. 4.

ages, a field experiment was performed during June 2002 using

Physical optics tells us that an incident wave propagating

two L-band horn antennas in bistatic mode (cf. Fig. 5), oper-

through some homogeneous sand layers accumulates succes-

ating in the 1.2­1.4-GHz frequency range and connected to a

sive phase differences before reaching the reflecting paleosoil.

time domain network analyzer [24].

After it has been backscattered on the sand­paleosoil interface,

Several measurements have been performed along a profile

the wave goes back through the overlying sand layers along the

crossing the paleosoil shown in Fig. 3(c). Results are presented

same way and then loses the accumulated phase differences.

in Fig. 6, where two main phase behaviors could be observed:

It means that the overlying sand stratification, especially the

is close to 22 for points 6­8 for which the paleosoil

smooth air­dune interface, cannot be responsible for the ob-

is close enough to the surface to contribute to the backscattered

served phase difference. Moreover, the observed phase differ-

is close

signal (less than 2 m deep). On the contrary,

ence could not be explained by anisotropy in the material fa-

to 0 for points 4, 5, and 9 for which the paleosoil is to deep

voring a polarization rather than the other as the anisotropy re-

and where only the dry surface response can be measured. It

quired is too high for natural material.

should be noticed that the (HH-VV) correlation remains higher

We observed that the paleosoil's moisture content increases

than 0.9 in all cases, confirming that phase difference measured

as it sinks into the dune because the sand layer covering it pre-

in the field is significant [24].

vents water evaporation. The phase difference observed along

a profile crossing the paleosoil in Fig. 4 also increases with re-

IV. SEMIEMPIRICAL MODEL

spect to the thickness of overlying sand as the paleosoil moisture

In order to model our observations, we considered the

increases. We also see in Fig. 4 that the copolarized phase dif-

semiempirical model proposed by Oh and Sarabandi [16]­[18],

decreases as the attenuation of the subsurface

ference

which has been developed from experimental data acquired by

backscattered signal increases with sand thickness.

polarimetric radar under a variety of roughness and moisture

It is the first time such a phase signature, related to soil mois-

conditions for L-, C-, and X-bands (i.e., 1.5, 4.75, and 9.5

ture, was observed for a natural outdoor site. Fig. 4 shows that

computed from the average of

GHz, respectively) at varying incidence angles (20 to 70 ).

the phase difference

several profiles along the paleosoil [arrows in Fig. 3(b) and (c)]

This model has been proposed to empirically establish rela-

is close to zero for the ocean and presents a strong increase for

tionships between copolarized phase parameters on one hand

the first 10 m after the point where the paleosoil outcrops. The

and roughness and dielectric constant of surfaces on the other