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Journal of Cultural Heritage 10S (2009) e83–e92

Original article

Novel tomographic based approach and processing strategies for GPRmeasurements using multifrequency antennas

Francesco Soldovieri a,∗, Luciana Orlando b

a Istituto per il Rilevamento Elettromagnetico dell’Ambiente, Consiglio Nazionale delle Ricerche, Via Diocleziano 328, 80124 Napoli, Italyb Dipartimento di Idraulica Trasporti e Strade, Sapienza University, Via Eudossiana 18, Roma, Italy

Received 30 May 2009; accepted 18 September 2009Available online 13 November 2009

bstract

This work is focused on an advanced processing strategy based on the microwave tomography and novel approaches able to exploit the data-iversity ensured by the exploitation of antennas at different frequencies for Ground Penetrating Radar (GPR) prospecting. The aim is to improvehe interpretability of the reconstructed images compared to the standard data processing and the interpretation of vertical profiles and time-slices.

he effectiveness of the proposed strategies will be shown for a synthetic example and a realistic GPR survey performed with 200 and 600 MHzntennas for archaeological prospecting in Palatino site (Rome) having the aim of detecting buried objects within the first two meters of theubsurface.

2009 Elsevier Masson SAS. All rights reserved.

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eywords: Ground Penetrating Radar (GPR); Microwave tomography; Archae

. Research aims

We present two advancements in the field of data processingiming at improving the interpretability of the results of theround Penetrating Radar (GPR) diagnostic procedure.In particular, we show the effectiveness of the proposed

dvanced data acquisition and processing strategies for therchaeological survey performed at Palatino site in Rome. Theim of the survey was to detect and localize buried Roman struc-ures along the Via Nova located in front of Domus Tiberianahat is affected by structural instability.

For the survey, we adopted an advanced measurement confi-uration using a two-channel unit able to acquire simultaneouslywo parallel profiles at two different peak frequencies (200 MHznd 600 MHz). Therefore, the first aim was to develop suit-ble processing strategies able to exploit the frequency diversity

rovided by the two peak frequency antennas.

The second topic was concerned with the exploitation ofn advanced processing technique, based on a microwave

∗ Corresponding author.E-mail addresses: soldovieri.f@irea.cnr.it (F. Soldovieri),

uciana.orlando@uniroma1.it (L. Orlando).

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296-2074/$ – see front matter © 2009 Elsevier Masson SAS. All rights reserved.oi:10.1016/j.culher.2009.09.001

al prospecting; Cultural heritage monitoring

omography approach, able to give high-resolution and detailedmages of the investigated scenario.

. Introduction

The usefulness of GPR in archaeology and cultural heritagesiagnostics is now well-assessed as pointed out by the largeumber of field studies [1–6].

On the other hand, the main requirement from archaeolog-cal community is concerned with the “visualization” of theata, i.e., the possibility to achieve images of the undergroundhat are reliable, easily interpretable, with high-resolution andtable with respect to the noise on data and uncertainties onoth measurement parameters and features of the investigatedcene.

In this work, we propose two advancements with the aim ofmproving the results of the overall GPR diagnostic procedure,hus mitigating the necessity of the “interpretation” of the recon-truction outcome, which is one of the crucial limitations in thexploitation of GPR as a diagnostic tool [7].

In particular, the feasibility of the presented advanced datacquisition and processing strategies is shown by considering theealistic situation of the archaeological survey at roman remainsf Palatino site in Rome; the survey had the aim of detecting and

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eqspembedded electric dipoles oriented perpendicularly to the pro-file direction; the distance between the centres of the antennaswas equal to 0.42 m in the cross-line direction and 0.085 m inthe in-line direction (Fig. 4).

84 F. Soldovieri, L. Orlando / Journal o

ocalizing buried structures along the Via Nova located in frontf Domus Tiberiana affected by structural instability [8].

For the survey, we adopted an advanced measurement confi-uration based on a two-channel electronic unit able to acquireimultaneously two parallel profiles at two different peak fre-uencies (200 MHz and 600 MHz). Therefore, the first aim waso develop suitable processing strategies able to exploit the fre-uency diversity provided by the two peak frequency antennas.

The second topic is concerned with the exploitation of andvanced processing technique, based on a microwave tomog-aphy approach, able to give high-resolution images of thenvestigated scenario when the aim is the detection and the shapeeconstruction of localized objects.

The microwave tomography based approach faces an inversecattering problem where a contrast function, accounting for thedifference” between the electromagnetic properties of the tar-ets and of the host medium, is determined starting from thecattered field measurements. In particular, simplified modelsf the electromagnetic scattering are adopted with the aim ofpproaching the realistic scenarios where the investigation ofarge domains should be addressed in a reasonable computa-ional time and in a reliable way. Besides the exploitation ofhe tomographic approach to provide the reconstructed imagesf the vertical profiles, here we present two simple strategiesble to combine the tomographic reconstruction results fromhe datasets collected by two different peak frequency antennas.he effectiveness of the microwave tomography based strategy

s tested against synthetic data and against the measurementsollected during an archaeological survey at a Palatino site inome.

Therefore, the paper is organized as follows. Section 3escribes the archaeological survey at a Palatino site in Romend the measurement instrumentation. Section 4 presents theata analysis and interpretation based on the classical migrationechnique and proposes a strategy to combine the data acquiredy two different antennas. Section 5 deals with a brief descriptionf the microwave tomographic approach and presents the strate-ies to combine the tomographic images. Section 6 presentshe results of the microwave tomographic approach applied tohe measurements at Palatino site and finally conclusions fol-ow.

. The archaeological survey: description of the site andeasurements

A GPR survey was performed at a Palatino site in Rome alonghe ancient road named Via Nova to gain information about therchaeological setting in the near surface layer. The road runslong the NE most recent part of Domus Tiberiana buildingFig. 1). At the time of its construction, the Via Nova surroundedhe northern slope of the Palatino hill. Today, the first part of theoad, paved with basalt stones is still visible. Probably, afterhe fire occurred in 64 B.C., the second part of the road was

odified so to become straighter than before. Across the road,here are massive arcs connecting the Domus Vestali with theomus Tiberiana that were probably built in order to ensure the

einforcement of the structures (Fig. 2).Fp

Fig. 1. The red arrow indicates Via Nova site.

The GPR survey was performed in an area located in frontf Domus Tiberiana near the arcs connecting it with the Domusestali (see Fig. 2) and close to a site where an archaeologicalxcavation revealed an ancient water drain (Fig. 3).

GPR survey was carried out with IDS instrumentationquipped with a double antenna system with nominal peak fre-uencies of 200 MHz and 600 MHz, respectively. The data wereimultaneously acquired using a two-channel unit along parallelrofiles. The 200 and 600 MHz antenna were moved with the

ig. 2. The black squared area was surveyed with x- and y-direction GPRrofiles. The location of the water sewer is reported as a red line.

F. Soldovieri, L. Orlando / Journal of Cultural Heritage 10S (2009) e83–e92 e85

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Fig. 3. Water sewer detected by excavation test.

The data were acquired with a time sampling of 0.195 ns and12 samples per trace, thus leading to an overall time-window of00 ns; a nominal spatial step of 0.024 m and 12 trace stackingere adopted.The 15 m by 4 m survey area was investigated along a grid

f orthogonal 0.5 m spaced profiles for a total of eight pro-les parallel to the road direction and 31 normal to the roadirection.

The data were preprocessed by means of a conventional signalrocessing procedure consisting of the following steps: settinghe zero time; background removal; time and spatial filtering;ain adjustment and migration. The time filters were chosen as0–500 MHz and 100–900 MHz band-pass filtering for 200 and00 MHz antennas, respectively. The gain was a linear func-ion with the time and constant for all traces; the migration waserformed with the Stolt algorithm [9].

The electromagnetic (e.m). velocity used for the depthonversion and migration was 9 cm/ns obtained from the usualnalysis based on the diffraction hyperbolas present in the ver-ical profiles.

Finally, the survey results were visualized in terms of a vol-me (3D cube) and as depth slices made up by the compositionf the individual vertical profiles.

ig. 4. Scheme of 200 and 600 MHz antenna relative positions during the acqui-ition.

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ig. 5. Modulus of the Fourier transform of the time domain traces for the sameertical profile. Blue line: 200 MHz antenna; red dashed line: 600 MHz antenna.

. Data analysis and interpretation

A spectral analysis of the field data collected by the twontenna systems was performed.

Such analysis provides for the 200 MHz antenna a peakrequency of about 150 MHz and a frequency band from 50o 450 MHz. The same analysis performed for the 600 MHzntenna, provided a peak requency of about 300 MHz and arequency band from 50 to 700 MHz (Fig. 5). The frequencyands were estimated by assuming a reference level equal tobout 1/10 of the peak amplitude.

For the case at hand, the 200 and 600 MHz data, having partialverlapping of spectra, allow us an increase in the exploitablepectral band width. The enlargement of the allowable spectralidth can be exploited by different strategies such as a correla-

ion of data in time domain or summing the spectra in frequencyomain. This procedure is not a simple task as demonstratedn [10,11], especially in the case that not good overlap of thepectra occurs.

Usually in archaeology a good interpretation is obtained bynalysing the energy attribute as time-slices and isosurface. Theertical and lateral continuity of anomalies are improved withhe energy stacking on time-windows chosen as function of thehickness of the structures. The continuity is also improved byeducing the dynamic of the colour scale used in the visualiza-ion.

In this paper we adopt a strategy to improve the continuitynd energy of the anomalies by using the two data set acquired,uring the survey presented in the previous section, with 200nd 600 MHz antennas in x- and y-directions.

A fundamental requisite for multiple-frequency composit-ng is the spatial coincidence of the traces acquired with the

ifferent peak frequency antennas. For the case at hand, dueo the spatial dislocation (42 cm) of the different peak fre-uency profiles acquired simultaneously by the system and tohe distance between two lines acquired with the same frequency

e86 F. Soldovieri, L. Orlando / Journal of Cultural Heritage 10S (2009) e83–e92

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w600 MHz antenna. Also in this case, the slices are obtained bythe energy sum of the two data sets processed separately intoa window of 0.5 m. In this way, the continuity of anomaliesis improved due to the similar measurement sampling in both

Fig. 6. Vertical profiles acquired by 200 (upper panel) and

eak-antenna (0.5 m), we have that the profile acquired with onentenna is actually closer (8 cm) to the profile acquired with thether peak frequency antenna in the previous or next scan. There-ore, in our analysis we consider spatially coincident (althoughcm distance is between them) the previous said lines for the

requency compositing.The interpretation of data was performed on the vertical pro-

les and on the time-slices achieved from the stacked energy of.5 m thick layer at depth step of 0.25 m.

From the collected profiles, for the 200 and 600 MHz a depthf penetration of about 2–2.5 m and 1.5 m can be inferred,espectively (Fig. 6). The archaeological excavation and trial-oreholes have pointed out that the upper layers detected byPR are more or less the anthropic layers and that further GPRenetration is inhibited by underlying clay layers.

Fig. 6 depicts two of the 200 and 600 MHz vertical profilescquired simultaneously in the x-direction of Via Nova 1 m aparto the right side of polygon on Fig. 2 and near the archaeologicalxcavation. They clearly show an anomaly zone which is theesults of reflecting surfaces and, scattering objects. This zone isocated at depth 1–2 m and, from the archaeological excavation,t is interpreted as an ancient sewer.

Fig. 7 depicts two depth-slices at 1.25 (upper panel) and 1.5 mlower panel) of only the data acquired in transversal directionoth with 200 MHz and 600 MHz antenna. The slices are the

um of energy into a window of 0.5 m of the two data sets pro-essed separately. The images suffer from under-sampling in theirection of the larger side of area under investigation due to theot small profiles offset.

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Hz (lower panel) antenna along x-direction on Via Nova.

Fig. 8 depicts the time-slices at 1.25 m and 1.5 m obtainedith data acquired in x- and y-directions with the 200 and

ig. 7. Tomolayer obtained using 31 transversal profiles acquired with 200 and00 MHz antenna at the depth of 1.25 m (upper panel) and 1.5 m (lower panel).he location of the sewer is reported as a black line.

F. Soldovieri, L. Orlando / Journal of Cultural Heritage 10S (2009) e83–e92 e87

Fig. 8. Tomolayer obtained using eight longitudinal and 31 transversal datarwl

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eferred at a depth of 1.25 m (upper panel) and 1.5 m (lower panel) acquiredith 200 and 600 MHz antenna. The location of the sewer is reported as a black

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irections, as shown in [12], even if this procedure has decreasedhe vertical and lateral resolution.

The time-slices show several anomalies although onlynomaly indicated by the black line is known from the archae-logical excavation to be the sewer of Domus Tiberiana shownn Fig. 3.

. Microwave tomographic approach and strategies toombine different datasets

.1. The microwave tomographic approach

Besides the conventional approach to GPR data pro-essing described in the previous paragraph, an advancedicrowave tomographic technique was applied with the objec-

ive to improve the image quality. The novelty concernedith this paper is the adoption of strategies based on theicrowave tomographic approach able to exploit simultane-

usly the data acquired by the 200 and 600 MHz antenna;hese strategies will be described in the following subsec-ion.

The microwave tomographic approach has been largelyescribed in many papers and here we report only the keylements necessary for the full understanding of the results;he reader interested can find more detailed descriptions in13,14].

In order to set up a tomographic inversion procedure, the firsttep is the definition of a background scenario with respect tohe targets are considered as anomalies.

In the case at hand, since we are interested in objects buriedn the subsurface, the background scenario can be conveniently

odelled as made up of two homogeneous half-spaces (air andoil) with a planar interface at z = 0 (Fig. 9). The upper half-spaceepresents the air, while the lower one schematizes the soil that

n terms of electromagnetic properties has a relative dielectricermittivity εb and conductivity σb.

Assuming for the sake of simplicity a 2D geometry, the cross-ections of the embedded objects are assumed to be invariant

bua

Fig. 9. 2D geometry of the inverse scattering problem.

long the y-axis and supposed to be enclosed in a rectangularnvestigation domain D = [−a,a] × [zmin,zmin + 2b].

The unknowns of the problem are the relative dielectric per-ittivity εD(x, z) and the conductivity σD(x, z) functions inside

he investigation domain D.The source of the incident field Einc is supposed to be a

ime-harmonic (time dependence exp (jωt), where ω = 2πf ishe angular frequency and f is the frequency) filamentary y-irected electric current (TM-polarization), of infinite extentnd invariant along the y-axis. A multimonostatic measurementonfiguration is considered where the transmitting and receiv-ng antennas are located at the same point and are moved with

uniform spatial step at various positions xs within the inter-al � = [−xM,xM] located along the air/soil interface (at z = 0).he measurements are collected within the frequency band1 = [fmin, fmax].Under the above assumptions and within the Born Approx-

mation, the unknown-data relationship is provided by theollowing equation in the frequency domain [13,14]:

Es(xs, f )=k2s

∫Ge(xs, f, x

′, z′)Einc(xs, f, x′, z′)χ(x′, z′)dx′dz′

f ∈ Ω1 (1)

here Es(xs, f) denotes the scattered electric field probed at xs

nd collected at the frequency f; ks indicates the wave-numbern the lower half-space; Ge is the (known) Green’s function andhe “contrast” function χ is defined as:

(x′, z′) = εeq(x′, z′) − εeqb

εeqb

(2)

here εeq(x′, z′) = ε0εD(x′, z′) – j(σD(x′, z′)/2πfε0) andeqb = ε0εb – j(σb/2πfε0).

Therefore, the contrast function accounts for the relative dif-erence between the equivalent permittivity of the objects andf the host medium. The key ingredients of the linear integralquation in (1) are the incident field and the Green’s functionhat, once the background is prefixed, can be evaluated under

spectral form; their meaning and expression are detailed in13–15].

The inversion of the linear integral equation is performedy the Truncated Singular Value Decomposition tool [16] that isseful for mitigating the effect of the noise on data and achievingstable solution.

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88 F. Soldovieri, L. Orlando / Journal o

Finally, the results of the inversion are 2D tomographicmages depicting the normalized modulus of the contrastunction; the regions where the modulus of the contrast functions significantly different from zero account for the location andeometry of the buried objects.

The measurements have to be preprocessed before applyinghe inversion algorithm. In fact, as said, the datum of the inver-ion algorithm is the field scattered by the buried object in therequency domain whereas the GPR measurements account forhe total field (given as the contribution of the direct wave, theeld reflected by the interface and the field backscattered by thebjects) in time domain.

Since the microwave processing deals with the raw datahere no gain is applied, in the reconstructed images the

etrieved amplitude of the contrast function for the shallowerargets can mask deeper targets. This affects the detectabilitynd interpretability of the deeper objects. In order to avoid thisffect, the first step is to “gate” the first part of all the timeomain traces, which corresponds to erase the direct, surfacend first layers wave contributions; this step roughly providesn estimation of the desired scattered field in time domain.he gating consists in constraining equal to zero the initialart of all the traces of the radargram. Finally, after appro-riate choice of the time-zero, the application of the Fourierransform yields to the gated data scattered field in frequencyomain.

.2. A novel strategy to exploit the measurements by twoifferent peak frequency antennas

Since the archaeological GPR survey was performed throughhe simultaneous adoption of two 200 and 600 MHz antennas, weere concerned with the development of processing strategiesased on the microwave tomographic approach that are able toxploit this antenna-frequency diversity.

In particular, here we present some possible strategies aimingt exploiting the measurements from two antennas with differentork frequency band �1 and �2.The first strategy is based on the simultaneous inversion of

he data corresponding to the two work frequency bands �1nd �2. The processing strategy can be described as follow:or each spatial measurement (trace), we collect the scatteredeld Es1(xs, f) for f∈�1 and Es2(xs, f) for f∈�2; then the “addi-

ion” of the scattered field is achieved as Es(xs, f ) = 0.5 ∗Es1(xs, f ) + Es2(xs, f )) for f ∈ Ω1 ∪ Ω2 (i.e., more pre-isely their mathematical mean); finally, the inversion accordingo the eq. (1) is performed starting from the Es(xs, f ) data.

The above strategy is exemplified by the following numer-cal result where the tomographic approach is tested againstynthetic data. The synthetic data have been generated in timeomain via GPRMAX that is a finite difference time domainased code [17]. Subsequently, the time domain data have beenreprocessed according to the procedure described above 4 in

rder to obtain the scattered field data in frequency domain isbtained.

A soil with relative dielectric permittivity equal to 11 and con-uctivity equal to 0.01 S/m is considered. The buried object has

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cattered field for the 200 MHz antenna. Middle panel: modulus of the Fourierransform of the time domain scattered field for the 600 MHz antenna. Loweranel: arithmetic mean of the spectra at 200 MHz and 600 MHz.

elative dielectric permittivity equal to 4, circular cross-sectionith radius 0.1 m and depth of centre at 0.6 m.A measurement line of 1.6 m in 41 points with a spatial step

f 0.04 m is considered. The simulated filamentary antennas areed with Ricker signals having centre frequency of 200 MHz and00 MHz, respectively.

In the inversion we adopt a work frequency band [50,860]Hz with a frequency step of 30 MHz. An investigation

omain D = [−.8,.8] × [0.01,0.6]m2 is considered within whichhe objects are assumed to reside. A solution is achieved bydopting in the TSVD inversion scheme a threshold at −35 dBo cut off the lower singular values.

Upper and middle panel of Fig. 10 depict the modulus of theourier transform of the 41 traces for the two synthetic data setsorresponding to the 200 and 600 MHz antenna, respectively.e can observe that the two spectral contents have almost no

verlap; the arithmetic mean of the two datasets is shown in theower panel of Fig. 10. Therefore, due to the “independence” ofhe two datasets, the tomographic image reconstructed by thenion of the two data sets can be expected to be the “addition”f the images reconstructed under the single frequency bandsi, i = 1,2, as shown on Fig. 11.The upper left panel (panel A) of Fig. 11 depicts the modu-

us of the reconstructed contrast function |χ200(x′, z′)| startingrom the simulated radargram for the 200 MHz antenna (fre-uency data of Fig. 10). The upper right panel (panel B) depictshe corresponding reconstructed contrast function |χ600(x′, z′)|tarting from the data simulated for the 600 MHz antenna (fre-uency data of Fig. 10). While the maximum modulus of theontrast function coincides with the upper side of the object foroth images, we can observe a higher resolution in the image

btained from the 600 MHz data; for such a reconstruction, apot arises at a larger depth, which accounts for the lower sidef the object and is delocalized due to the fact that the Bornodel assumes an e.m velocity (equal to the background veloc-

F. Soldovieri, L. Orlando / Journal of Cultural Heritage 10S (2009) e83–e92 e89

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rthe reconstruction achieved by the multiplication procedure and,as expected, a more focused image is obtained.

On the other hand, in the case of objects characterized bya different level in the reconstructed contrast function (due to

ig. 11. Tomographic images of the single object. Upper left panel: reconstructiower left panel: reconstruction with the addition of data. Lower right panel: re

ty) that is different from the true one when the electromagneticave propagates inside the object.Lower left panel of Fig. 11 (panel C) depicts the reconstruc-

ion achieved by starting from the “added” scattered field data˜

s(xs, f ); in this case, the reconstruction can be also interpreteds the “arithmetic mean” of the modulus of the contrast func-ion reconstructed from the data collected separately by the twontennas.

In the lower right panel (panel D) of Fig. 11, theeconstruction is achieved as a spatial function 0.5 ∗|χ200 (x′, z′) | + ∣∣χ600 (x′, z′)

∣∣ (addition of the images); as cane seen, the reconstructions of the lower panels (addition of datand addition of images) are almost identical.

Since we have stated the “equivalence” of the two strategiesased on the “addition of the data” and on “the addition of themages”, when considering the experimental data, we will adoptnly the strategy based on the “addition of the images”.

For the other strategy to combine the images, i.e., “the mul-iplication”, the composite image is achieved by performinghe multiplication “pixel by pixel” of the modulus of the con-rast functions retrieved under the single frequency bands, by

eading to the spatial function |χ200(x′, z′)|*|χ600(x′, z′)|. Thistrategy inherently enhances the common parts of the objecteconstructed under the single frequency bands. Therefore, it isseful to achieve more focalized images of the object and to

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th 200 MHz antenna. Upper right panel: reconstruction with 600 MHz antenna.ruction with addition of the images.

emove artifacts present in the single inversions. Fig. 12 depicts

ig. 12. Tomographic image of the single object reconstructed as the multipli-ation of the images at 200 and 600 MHz.

e90 F. Soldovieri, L. Orlando / Journal of Cultural Heritage 10S (2009) e83–e92

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ig. 13. Tomographic images of the two objects. Upper left panel: reconstructioower left panel: reconstruction with the addition of images. Lower right panel

heir different depth and/or the relative positions with respect tohe antennas), the reconstruction through the multiplication hashe drawback that the object with the smaller “intensity” couldisappear in the image. This can be observed in the reconstruc-ion of synthetic data from two objects having relative dielectricermittivity equal to four and circular cross-section with radius.05 and 0.1 m with the centers at different depth (the soil hashe same properties as in the previous case). Fig. 13 depicts theeconstruction obtained with the single peak frequency anten-as and by means of the two strategies based on the additionnd the multiplication of the images. It can be noted that theeconstruction by means of the “multiplication of the images” isess affected by artifacts but the deeper object is reconstructed atlower level compared to the reconstructions of the other threeanels.

. Experimental validation of the tomographic

pproach

This section is devoted to presenting the reconstructionesults obtained through the application of the two solution

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h 200 MHz antenna. Upper right panel: reconstruction with 600 MHz antenna.nstruction with the multiplication of the images.

trategies based on the summation and the multiplication of theomographic images to the experimental data collected duringhe survey presented in Section 3.

In particular, we are concerned with the processing of the1 y-direction profiles, normal to the road direction, collectedy the 200 MHz and 600 MHz antennas. As said previously, theistance between the profiles is assumed to be equal to 0.5 m,hile the length of profiles is about 4 m. Thus, a 15 m by 4 m

egion is surveyed.The parameters adopted in the inversion of each vertical pro-

le are summarized in Table 1.Fig. 14 depicts the depth slices achieved through the

ummation of the reconstructed tomographic images at theepth of 1.25 m and 1.5 m. The tomographic reconstruction haso be compared with Fig. 7; as can be seen, the reconstructionn Fig. 14 is more richer of details than the slice on Fig. 7 andermits a higher continuity of the retrieved structures especially

or the sewer.

In addition, the quality of the results on Fig. 14 is compa-able with Fig. 8 where both the longitudinal and transversaladargrams were exploited.

F. Soldovieri, L. Orlando / Journal of Cul

Table 1Parameters of the tomographic inversion.

Parameter Value

Model relative dielectricpermittivity of the soil

11

Model conductivity of the soil 0.005 S/mSpatial step of the data 0.048 mObservation domain 4.8 m (101 points spaced

by 0.048 m)Frequency band 50–650 MHzFrequency step 25 MHz (25 frequencies

exploited in the inversion)Investigation domain 4.8 m

(horizontal) × (0.5–2.0 m)depth

Fig. 14. Tomolayer obtained by using tomographic approach and the additionof the images for the 31 transversal profiles acquired with 200 and 600 MHzantenna. The depth slices are at 1.25 m (upper panel) and 1.5 m (lower panel).The location of the sewer is reported as a grey line.

Fig. 15. Tomolayer obtained by using tomographic approach and the multiplica-tion of the images for the 31 transversal profiles acquired with 200 and 600 MHzantenna. The depth slices are at 1.25 m (upper panel) and 1.5 m (lower panel).The location of the sewer is reported as a grey line.

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tural Heritage 10S (2009) e83–e92 e91

Fig. 15 depicts the depth slices achieved through the multi-lication of the reconstructed tomographic images at the depthf 1.25 m and 1.5 m.

Comparing the results obtained from the two differentpproaches devoted to combine the data of 200 and 600 MHzntenna, we observe that the multiplication strategy (Fig. 15)s more suitable for enhancing higher energetic anomalies andherefore, could be an useful tool to clean the image of smallcattering objects.

. Conclusions

In this paper, we have presented some strategies to combinehe data acquired by two different antennas with partially over-apping in frequency bands, with the aim of improving the imageuality of depth-slices.

In particular, we have shown that the time-slice visualizationan be improved by a two-step procedure where the data of twontennas are focused with the microwave technique and then asummation” or “multiplication” of the tomographic images iserformed. The performances of the two strategies have beennalysed with both synthetic and experimental data.

The actual data were acquired at a Palatino archaeologicalite in Rome, where a 2–2.5 m thick anthropic layer was present.he tomographic results obtained from the data acquired with

wo peak frequency antenna and combining strategies have beenompared with depth-slices obtained from conventional process-ng.

In particular, we have observed that the quality of the tomo-raphic images achieved by the summation of the images isomparable with the image quality of the conventional approachhat however exploits both the longitudinal and transversal radar-rams.

In addition, as other main outcome of the paper, the multipli-ation procedure allows us to enhance the visualization of thenomalies with higher energy compared to the weaker ones and,t the same time, to reduce the artefacts.

As future development of the activity, we aim at addressingore in depth the implications of the proposed strategies for

requency compositing by investigating, from a more rigorousoint of view the advantages and drawbacks of the strategieslso in relation with the joint use of the microwave tomographicpproach.

eferences

[1] L. Orlando, Georadar and magnetic data for the planning of an archaeolog-ical excavation (case study in central Italy), Archaeological Prospection 14(2007) 1–13.

[2] L.B. Conyers, D. Goodman, Ground Penetrating Radar: An Introductionfor Archaeologists, AltaMira Press, Walnut Creek, London and New Delhi,1997.

[3] L. Orlando, F. Soldovieri, Two different approaches for georadar data pro-cessing: a case study in archaeological prospecting, J. Appl. Geophysics

64 (2008) 1–13.

[4] S. Piro, D. Goodman, Y. Nishimura, The study and characterizationof Emperor Traiano’s villa (Altopiani di Arcinazzo, Roma) using high-resolution integrated geophysical surveys, Archaeological Prospection 10(2003) 1–25.

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