Ultradense Topographic Correction by 3D-Laser Scanning in Pseudo-3D Ground-penetrating Radar Data:...

* Correspondence to: T. TeAndaluz de Geofísica, Gra

Copyright © 2013 John

Archaeological ProspectionArchaeol. Prospect. (2013)Published online in Wiley Online Library(wileyonlinelibrary.com) DOI: 10.1002/arp.1464

Ultradense Topographic Correction by3D-Laser Scanning in Pseudo-3DGround-penetrating Radar Data: Applicationto the Constructive Pattern of the MonumentalPlatform at the Segeda I Site (Spain)

TERESA TEIXIDÓ1*, JOSÉ ANTONIO PEÑA1, GLORIA FERNÁNDEZ2,FRANCISCO BURILLO2, TERESA MOSTAZA3 AND JULIO ZANCAJO3

1 Universidad de Granada, Instituto Andaluz de Geofísica, Granada, Spain2 Universidad de Zaragoza, Ciencias de la Antigüedad, Teruel, Spain3 Universidad de Salamanca, Escuela Politécnica Superior, Ávila, Spain

ABSTRACT Three-dimensional ground-penetrating ra
dar (3D-GPR) is one of the highest resolution geophysical methods forexploring the shallow subsurface and it is widely utilized in the diverse fields requiring this kind of information:for example archaeology, civil engineering and environmental studies. Currently, there are several ways to present3D-GPR results: 2D vertical pictures (radargrams), time- and depth-slices, a mixture of radargram-slice images,GPR reflectivity maps and GPR isosurface images. All of these techniques, however, require the maximum numberof details possible. When the recognition surface is not horizontal, the GPR image is distorted due the topographicirregularities. To eliminate these distortions, a classic topographic correction is applied to the GPR data set,particularly in 2D GPR profiles. Generally, this topographic information is obtained by laser levelling, total station,differential Global Positioning System (GPS) or similar equipment. This study uses a new method of topographiccorrection based on three-dimensional laser scanner (3D-laser scanner) technology that provides ultradensecoordinates of the terrain. A strategy for applying this topographic correction to 3D-GPR vertical traces isdiscussed and evaluated by comparing corrected images with other uncorrected images obtained using the samestandard processing flow. The GPR dataset used to test this method is from a monumental structure located inthe Celtiberian site of Segeda I (Mara, Spain). The data were acquired using a 400MHz antenna on 0.25m spacedprofiles. Although the relief of this structure is not overly complex, we demonstrate how the results obtained byapplying this topographic correction technique allow a better archaeological interpretation of the internal architecture.The technique is therefore presented as a new archaeological tool to obtain clearer images of buried structuresand/or their internal elements. Copyright © 2013 John Wiley & Sons, Ltd.
Key words: 3D-GPR method; 3D-laser scanner; ultradense GPR topographic correction; Celtiberian site of Segeda I

Introduction

Topographic data are an important aspect of ground-penetrating radar (GPR) surveying and, in order to beuseful, measured coordinates must be matched withGPRdata. This topographic information can be obtained

ixido, Universidad de Granada, Institutonada, Spain. E-mail: [email protected]

Wiley & Sons, Ltd.

using traditional surveying tools: laser levellers (Jol et al.,2004), total station (Heincke et al., 2002), or real-timekinematic Global Positioning System (GPS) (Urbaniand Baskaradas, 2010; Böniger and Tronicke, 2010).Usually these topographic corrections are applied to2D-GPR data when the relief of the profiles varies morethan 10% (Lehmann and Green, 2000), in which case thetraces are moved according to the measured elevations.These corrections, however, are rarer in pseudo-3DGPR or 3D-GPR sets because a dense topographic grid

Received 8 May 2013Accepted 11 September 2013

T. Teixidó et al.

is required in order to correct a large number of traces;however, the effort proves worthwhile as a significantimprovement can be seen in the results. (A pseudo-3DGPR set consists of data obtained by parallel profilesspaced 5–10 times further apart than the spacing inthe profile direction, and a 3D-GPR set refers to datacollected with full GPR resolution (Novo, 2009)).Figure 1 illustrates, in a conceptual way, the effect ofapplying a three-dimensional topographic correctionwhen the relief differences are significant, so that, whenthe correction is applied, the slice succession reveals aclear image of the buried body.When the underground structures are coarse and

fine details are not required, three-dimensional topo-graphic correction can be carried out utilizing morewidely spaced coordinates and using traditional tools(Peña et al., 2009). However, in those special caseswhere greater definition of the structures is required,an ultradense topographic correction performed witha 3D-laser scanning technique can be applied. Thefollowing sections explain how this correction isapplied and how the GPR images of a detailedstructure are enhanced.In general terms, the topographic correction process

consists of time-shifting GPR data to compensate forthe surface elevation effect. This means shifting eachtrace by the differential time (Δt) representing thepropagation time for excess, or defect, according to theelevation difference of the location from which eachtrace is acquired with respect to a horizontal referenceplane, called the reference datum (Yilmaz, 2001). Themost crucial aspect of this process is to correctlycalculate the time-shift Δt because time zero for eachtrace is repositioned on the vertical axis and thereflections adjusted accordingly.

Figure 1. Conceptual 3D-GPR topographic correction. The diagrams showtopographic correction has been applied, so the successive horizontal slicpartially reveals the shape of the body, and its interpretation is difficult. Onso the slope of slices has been modified according to the slope of the terr2009.) This figure is available in colour online at wileyonlinelibrary.com/jou

Copyright © 2013 John Wiley & Sons, Ltd.

Rugged topography usually means complex near-surface irregularities that can mask subtle importantchanges in GPR images, such as the relief and theshallow velocity model, which must be identified inorder to evaluate the rapid vertical and lateral velocitygradients. The surface topography and velocity modelare intrinsically linked when topographical correc-tions are applied, meaning that time-shift Δt is a resultof both. In this paper we present a procedure forobtaining these time-shifts (Δt) when an ultradensetopographic survey is available, ensuring precisecorrespondence between each three-dimensionalGPR trace and its elevation with reference to thedatum. We also assess whether the near-surfacevelocity model can be replaced by a constant velocitymodel due to the thinness of the surface layer andthe nature of the archaeological structure.Migration and antenna tilt corrections are two

processes that are also relevant when improvinginternal images of subsurface structures, because theyare algorithms that correct morphology of reflectors.Migration (Fisher et al., 1992) collapses the diffractionscaused by sudden changes in the geometry of thereflectors and also corrects the hyperbolae producedwhen the GPR antenna crosses a buried cylindricalobject (e.g. a cavity or pipe line). Meanwhile, theantenna tilt correction (Goodman et al., 2006) restoresthe ray projections of radar waves transmitted fromthe GPR antenna at sites with more varied topogra-phy, because crossovers can occur when topographicchanges are abrupt and/or the surface velocities arefast enough to cause ray intersections at depth. Bothprocesses are usually applied in the final stage ofprocessing and have also been taken into account inthis study.

a surface section hosting a three-dimensional body. On the left, noes are parallel to the ground surface. The succession of these slicesthe right, a topographic correction has been applied to the data set,ain revealing a clearer image of the body. (Modified from Peña et al.,rnal/arp

Archaeol. Prospect. (2013)DOI: 10.1002/arp

Topographical Correction using 3D-Laser Scanning

Case study: the monumental platform at theSegeda I site

To evaluate the effectiveness of this ultradense 3D-GPRtopographic correction method, it was tested on anarchaeological structure from the Segeda I site (Mara,Spain). This site has been identified as the ancientCeltiberian city of Segeda mentioned in Roman sources.In particular, historical documents from the warsbetween the Celtiberian tribes and Romans troops from153 to 133 BC make the identification of this oppidumpossible (Burillo, 2006).Archaeological research undertaken in this area

between 1986 and the present day has facilitatedan understanding of the different parts of the cityand enabled a reconstruction to be made of part ofthe urban model of Segeda I. In 2004, archaeologicalexcavation revealed a strange structure related moreto defensive or ritual functions than to productionor domestic use (Burillo et al., 2009). This 600m2

horizontal construction with a 120º corner at thesoutheast intersection is located outside the enclosuredelimited by the walls (Figure 2).The objective of the archaeological works in 2010

and 2011 was to define the constructive parts of thismonumental platform. The structure consists of atleast four constructive elements: three walls posi-tioned in a U-shape and a platform (PL) built to theeast that is limited by these walls, as seen in Figure 2.Wall M501 is the shortest and closes the structure tothe north. It is 10.52m long but its width has not beendetermined because it joins onto the platform. To thesouth, this wall connects to wall M502 forming a 120º

Figure 2. The monumental structure is composed of differentconstructive elements: the central platform (PL) and three wallsthat limit it (M501, M502 and M503). This study is restricted tothe platform zone. This figure is available in colour online atwileyonlinelibrary.com/journal/arp


angle stone, and northwards it connects to M503. WallM503 is 28.50m long and 3.60m wide and M502 is23.45m long and 3.50m wide; both measured at theend of the structure where there is no platform andthey run parallel, 4.45m apart. Through excavationsaround this structure it was estimated that the founda-tions could be between 1.75 and 2m maximumdepth, in particular, on the side of the M503 wall(Burillo et al., 2009).It should be noted that the monument is positioned

on a deposit of Quaternary clays and silt representingPleistocene fluvial terraces. The construction materialis predominantly Miocene gypsum arranged in blocksand slabs and evened out by gravels and boulders.As previously mentioned, excavations at this

structure have provided a great deal of informationon its exterior and surroundings. However, in orderto understand the internal structure, more appropriatenon-destructive methods such as geophysicalprospection were planned and GPR exploration wasfocused on the platform, which is the most logical zonein which to study the internal architecture.

Field data acquisition

Three-dimensional laser scanner

The archaeological interventions on the SegedaPlatform were carried out with a topographic gridand using a total station tool (3D Trimble®-GX, Inc.)that provides the reference base for the performance ofan ultradense topographic survey with a three-dimensional laser scanner device. To sweep the wholemonument, five spheres were placed at differentheights, four on the exterior of study zone and oneinside it. The visibility of at least four spheres wasensured at all stations. Table 1 details the mostcharacteristic parameters for this topographic dataacquisition and Figure 3 shows the results.

Three-dimensional GPR survey

Field work was carried out in September 2011. A400MHz (GSSI, Inc.) antenna was used due to the

Table 1. Main acquisition parameters for 3D-laser scanner set.

Range 350mScanning speed 3000 points per secondData density 10800 points / m2

Scan resolution 3mmPoints with Cartesian coordinates (xi, yj, zk) UTM


Figure 3. On the left are the 3D-laser scanner data field sets. The colour of each point cloud represents the base station used for each sectorsweep. On the right is the 3D topographic surface after data processing; the black line marks the limits of the studied platform. The errorunification of the clouds is 0.001m and the density of the processed grid is 1 point / cm2. This figure is available in colour online atwileyonlinelibrary.com/journal/arp

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optimumtrade-off between its estimated penetration(up to 1.75–2m depth) and the resolution required tobest define the archaeological features of thisplatform. The 162m2 (18m × 9m) study area wasexplored by parallel profiles spaced 0.25m in the sameorientation as the M501 and M502 walls (Figure 4a).The following acquisition parameters were selected:512 samples/trace, 4 stacks/trace, 60ns of timewindowand a trace trigger interval of 2 cm. The initial and finalcoordinates for each GPR profile were measured withthe total station and the profile distance was controlledby the odometer integrated into the GPR antenna.Before the data acquisition began, the odometer wascalibrated in order to adapt it to the platform relief.

Figure 4. (a) 3D-laser scanning topographic surface portion of the platforminitial and final points for each GPR profile have been marked on this surfacstation with ±5mm precision. (b) The same surface portion of the platformreduced to 1 point / dm2. This figure is available in colour online at wileyon


GPR data processing

This study focuses on two issues: first, how topo-graphic GPR data can be corrected from an ultadenseCartesian grid obtained with a 3D-laser scanner and,second, how this correction improves the quality ofresults. For the first question, three aspects should beconsidered: (i) the correct assignment of each trace toits corresponding elevation (Δzmn), where m and nrepresent trace and profile respectively; (ii) the time-shift calculation (Δtmn) for each trace through itselevation and near-surface velocity value; and (iii)the best processing stage at which to apply thisstatic correction.

with a cover colour texture according to the elevation levels (Z). Thee. The coordinates of theses GPR points were measured using a totalshowing the data grid; to improve the view the grid density has beenlinelibrary.com/journal/arp


Figure 5. The maximum elevation difference of the platform is around0.4m (Δzmax); this level defines the datum for topographic correc-tions. This figure is available in colour online at wileyonlinelibrary.com/journal/arp


Depth-shift calculation

The calculation begins by placing the initial (xni, yni, zni)and final (xnf, ynf, znf) coordinates for all GPR profiles(n) over the three-dimensional ultradense topographicgrid (1 point / cm2). This can be done because the initialand final coordinates were measured using a totalstation tool with ±5mm accuracy. As the GPR profilesare straight, the intersection points between theultradense topographic grid and these straight GPRlines can be calculated considering a step of 2 cm in theGPR profile direction (Figure 4b). This value is the same

Figure 6. (a) View of the west wall (M503) and the nearest radargram (b). Theof the constructive material to be calculated, particularly at the measuring pwileyonlinelibrary.com/journal/arp


as the trace trigger interval. Then a set of zmn elevationsis extracted for each trace (m) on each GPR profile (n)so that, when a datum reference level is selected for alltraces, the elevation-shift values Δzmn can be obtained.In order to make a realistic calculation, the dimen-

sions between the emissor and receiver parts of theGPR antenna used in this study must be considered,because the rigid displacement of the antenna smoothesthe real relief. Therefore, the ultradense topographic grid(1 point / cm2) can be simplified to 1 point /dm2. Thisprocessing step is not necessary if the GPR code usedin the data processing has a direct trace-to-trace topo-graphic assignation, as occurs in seismic codes. In thiscase the zmn coordinate is assigned by the intersectionof each trace (xmn, ymn) to the nearest point of theultradense topographic grid. However, there are severalGPR codes (e.g. Radan Version 6.5; http://www.geo-physical.com/software.htm) where the topographicassignation is performed through an auxiliary file thatcontains only the topography of significant traces andfor the intermediate traces the elevations are interpo-lated. Therefore, this simplification (1 point / dm2)becomes useful. In this context, it has been verifiedthat the simplification does not significantly affectthe results of the topographic corrections, taking intoaccount the fact that the data were acquired with a400MHz antenna. Obviously, the optimal density ofthe topographic grid will always depend on theantenna dimensions because they restrict the horizon-tal resolution, which suggests that the grid sizedepends on the central frequency antennae, but thisis a pending study that is beyond the present work.

proximity of this GPR profile with the wall allowed the average velocityoint shown in both images. This figure is available in colour online at


Table 2. Standard processing flow applied to raw three-dimensional data set.

3D geometry definition (xmn, ymn, zmn) for all traces of all profilesZero-time correction 0.16m T-R separation (400MHz)Background removal Average of all tracesPredictive deconvolution 21 ns time window, lag 7 ns, prewitening 10%Band-pass filtering 200–700MHzKirchhoff migration 0.1mns�1; hyperbola width of 31 tracesGain correction The same for all data setsHilbert Transform For 3D images generation (magnitude mode)Time to depth conversion With a dielectric constant of 7.17

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Time-shift calculation

Usually, in standard GPR data flow processing, thetopographic correction is applied in the final steps,once the signal enhancement algorithms have beenapplied: deconvolution, filtering, migration, etc.(Daniels, 2004). If it is assumed that the near-surfacevelocity is constant, then the ray-path between thereflectors and the subsurface is perpendicular, but thisis only true when the reflectors are parallel to thesurface. However, reflectors are usually inclined withrespect to the surface and the ray-path returnsperpendicular to the reflector, not the surface. As a con-sequence, the reflector will be offset both vertically andlaterally. The standard procedure for restoring this effectis the application of static corrections and a subsequentmigration of the data so that the dipping reflections arerestored to their correct positions. This is almost thesame processing procedure as used in post-stack seismic

Figure 7. (a) Raw radargram. (b) The same radargram after applying the proccorrection before the migration step. (d) The elevation and antenna tilt correcthe radargram in (d) shows better reflection coherence.


reflection (Yilmaz, 2001). These corrections operate intimemode and represent a trace time-shiftΔtmn that con-tains the equivalence of each elevation Δzmn correctedfor the local near-surface velocity value.Common mid-point hyperbolae (reflected-wave

method) or those given for an object at a known depth(direct wave method) are two tests that provide mea-surements of surface velocities and can ensure accurateconversions of travel time to depth over a study area,thus properly determining the depth of buriedstructures (Conyers and Lucius, 1996). In both casesthis is an expensive task because the measurementsmust be repeated at several points until the entiresurface velocity map has been determined. The effortmakes sense in studies where the surface layer is thickand heterogeneous, but in our case it is unnecessarybecause our near-velocity target is too shallow, themaximum is only 0.4m deep (Figure 5).

essing flow with no topographic correction. (c) The result of time-statictions have been applied after the time-to-depth conversion. Note that



Consider a layer of 0.4m thickness (Δzmax) and twomaterials with very extreme velocities: for example,air with an approximate velocity of 30 cmns�1 andwet clay with approximately 11.3 cmns�1. The timereflection difference due to the ray-path through thislayer is Δt= 2z(1/v2 � 1/v1) ≈ 4.4 ns; resulting in7.3% of maximum time-shift trace between theseextreme cases. In our study area, the near-surfacerelief is formed by very dry bricks of mud and gyp-sum slabs with estimated velocities of 8.6 cmns�1

and 10 cmns�1, respectively, giving ≈ 1.3 ns (2%) ofmaximum time-shift trace. Hence a constant near-

Figure 8. The GPR depth (or time) slices are horizontal maps on which the rewith a selected thickness interval (or time), and show the evolution of the refltopographic correction. The equivalent GPR depth-slices in (b) have been coreflector distribution (dashed lines) can be observed, and obviously these difin depth between the homologous slices is due to topographic correction. Thi


surface velocity model is calculated to find theequivalence between each elevation Δzmn and eachrespective time-shift Δtmn.

The following strategy was used: first the maximumelevation difference between the GPR profiles wasfound in order to establish the reference datum (Δzmax,

in Figure 5), which in this case is 0.4m. The last GPRprofile was then selected, which coincides with themost westerly part of the wall M503 (Figure 6), andthe average velocity was calculated. This averagevelocity (Vav) can be obtained using the relationship:Vav = z/(2TWT); where TWT is the two-way travel

flection amplitudes have been projected at a specified depth (or time)ectors at depth. The depth-slices in (a) have been constructed withoutnstructed applying topographic correction. A marked difference in theferences lead to different interpretations. Note that the slight differences figure is available in colour online at wileyonlinelibrary.com/journal/arp


T. Teixidó et al.

time of the reflection wave and z is the measureddepth. Here Vav = 11.2 cmns�1 which is equivalent toa dielectric constant of 7.17.

Selected processing flow

Both Δzmn and Δtmn suppose the same correction becausethey are calculated using a near-surface constant velocitymodel based on a vertical ray-path assumption. How-ever, the results may vary significantly according towhich stage of data processing this correction is applied.Because the raw radargrams show good signal

penetration and numerous reflectors, standard dataprocessing was applied. The main steps are shown inTable 2 and their effects on the raw data are displayedin Figure 7a and b.Given that the objective of this study is the optimal

application of topographic correction, both the time-shift correction (Δtmn) and elevation-shift correction(Δzmn) cases were tested. For the first case, Lehmannand Green (2000) propose an algorithm that allowsGPR data to be migrated directly from either slightlyor highly irregular acquisition surfaces. In accordancewith the findings of these authors, we propose anapplication of the topographic correction in time mode(Δtmn) before applying the standard Kirchhoff migra-tion. This time-static correction places the reflectors atthe corresponding real zero-times. Subsequently, the mi-gration algorithm restores the dipping reflectors and

Figure 9. Interpretation of the Monumental Platform from the individual rawileyonlinelibrary.com/journal/arp


provides greater amplitude coherence between theneighbouring traces. For the second case, the standardtopographic (Δzmn) and antenna tilt corrections areapplied at the end of processing, after converting theprocessed three-dimensional GPR data from time todepth. Figure 7c and d shows the two results: a detailedinspection has enabled us to confirm that the reflectorsin the radargram of Figure 7d have better continuity,enabling better understanding of the internal structure.The effect of this correction also can be seen if the

GPR depth-slice images without topographic correc-tion (Figure 8a) are compared with the equivalentimages with topographic correction (Figure 7b). Thedashed lines plotted on Figure 8b show the differentspatial distribution of the reflectors; obviously thesevariations lead to different interpretations of theinternal structure of the monument.

Results and interpretation

The interpretation of the GPR models was supportedby the archaeological record extracted from the exca-vations carried out around the Monumental Platform(Figures 2 and 5b).First, the radargrams obtained were analysed in

order to understand the internal elements of thestructure (Figure 9). In the radargrams two differentparts were distinguished, being from the lowest

dargrams (GPR B-scans). This figure is available in colour online at



upwards: a geological stratum comprised of silts andclays with low reflections, overlain by the culturalsection with reflections associated with constructivematerials (levels A, B and C in Figure 9). For theselevels the morphology and arrangement of reflectorshas been associated with the different kinds of con-struction used in the Monumental Platform. As wesee in Figure 2, the mud bricks form a superficial layer(A), which previously covered the entire structure butis now preserved only in certain sectors due to the ef-fect of the agricultural work that the site has suffered.Under this is found the gypsum slab layer (B) withvarying thicknesses; in the central part of the platformthe gypsum slabs are arranged in rows (generally two

Figure 10. Final GPR images of the Monumental Platform. (Left) Selected d(Right) Cover surfaces at different depth intervals. The cover surfaces reprrepresentation for the first cover surface (near-surface level) has been constrto display the effect of the ultradense 3D-laser topographic correction apavailable in colour online at wileyonlinelibrary.com/journal/arp


levels) and are 50 cm thick on average, but toward thesides the thickness of the slabs increases up to 80 cm,defining the perimeter walls. It can be seen that thetransition between the central reflectors and the wallreflectors is continuous because there are individualslabs that form part of both sections, providing goodcohesion between the two structural elements. Theradargrams also show the base of the gypsum in thewalls, reaching depths of around 1.6–1.9m, character-ized by smaller slabs that may be interleaved limestoneboulders. Therefore, the walls progress downwardsfrom carved, regularly arranged slabs to a coarser basewhere waste material is used. This implies that the con-tact between the bottom of gypsumwith the top of level

epth-slices showing the evolution of the reflectors at different depths.esent the cumulative reflectors between defined stretches. The scaleucted with the same colours as in the topography of Figure 4a, in orderplied to the GPR data. For more information see text. This figure is


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C is not clearly defined under the walls; the limit is setconsidering the fact that the shrinking size of the reflec-tors must match the size of the reflectors located underthe slab rows in the central part. Layer C, filled withgypsum, limestone boulders and gravel, is a foundationlayer and acts as levelling of the palaeoground surface.Last, a thin layer (level D in Figure 9) is detected, whichis characterized by short reflections corresponding tolevelled gravels arranged as a drainage bed placed ontop of impermeable clays and silts.Second, 2D and 3D-GPR images have been analysed

to help understand the spatial distribution of themonument. Figure 10, left-hand images, shows theselected depth-slices, and the reflections describe howthe materials are arranged in this construction. Forexample, it can be observed that the foundation ofthe side walls is irregular: while the east wall (M502)is 0.8m high, the north and west walls (M501 andM503) are up to 1.9m high. This height differenceleads to the assertion that the construction of thestructure was adapted to the palaeorelief and not viceversa, in the same way as other buildings excavatedat the site. Furthermore, two small transverse walls inthe central platform were detected between 0.8 and1m depth (see 80 cm slice in Figure 10). These wallsare below the central gypsum slabs and seem to becross braces, one strengthening the eastern wall at thesouth end of the platform, and the other at its northend, both running in the direction of maximum slopeof the land. The 3D images on the right of Figure 10are the covering surfaces (Peña and Teixidó, 2012) thatrepresent the cumulative reflectors for a given depthhorizon, so the uppermost cover surface (between0 and 0.4m depth) is the datum level containing theremains of mud bricks and some gypsum slabs. Inorder to show the effect of ultradense 3D-lasertopographic correction, the same colour scale as thatin Figure 4a has been used. These radar images clearlyshow that the north and east walls are the deepest andmore solid than the rest of the structure. These wallsbecome deeper towards the western boundary of thestructure, confirming that the original relief slopeddownwards in this direction, where this monumentwas built and its foundations dug. Another result thatcan be extracted from the images is the fact that thewalls around the platform are about 4.6m wide.

Conclusions and discussions

The implementation of classic topographic correctionsin 3D-GPR data sets is not normally applied during


the processing stage because it represents additionalwork, both in field, in order to obtain the topographicdata, and a computational effort to introduce the eleva-tion coordinates. In this context, this study evaluates theapplication of topographic corrections and concludesthat the effort is advisable as it can improve the results.However, we go further because we also present a newvision of topographic correction: applying ultradensetopographic information to 3D-GPR data.This idea stems from the archaeological work at the

Monumental Platform of Celtiberian site of Segeda I(Spain), where interest in this particular structure ledto the application of 3D-laser scanning. This technol-ogy provides an ultradense set of coordinates for thesurface of the monument (1 point / cm2), which wasused in our special 3D-GPR topographic correction.As a first step, we suggest prior simplification of the

topographic density grid in order to reduce computa-tional effort, where the density of the node mesh isrelated to the size of the antenna and geometry scanacquisition; in this case 1 point / dm2 is a useful den-sity. As a second step, we present the basic methodfor applying ultradense topographic correction anddemonstrate that the best results are obtained whenthis is applied in depth mode, after the time-to-depthconversion and prior to antenna tilt correction. Inaddition, we have assessed the radar image enhance-ment compared with the same, uncorrected data set.It is necessary to establish the application limits of

this type of correction. The literature suggests thatthe topographic correction in 2D GPR profiles beginsto be useful when the terrain dip, or relief difference,is more than 10%, but for 3D-GPR case there is littledocumentation. In this study the maximum elevationdifference of the explored platform is 40 cm and the to-tal target depth is around 2m, meaning 20% dip. Asimilar rate can be derived in timemode: with a dielec-tric constant of 7.17, using 60 ns as the time window,40 cm is equivalent to 6.98 ns, resulting in 17.4%. Inreality these values are lower, because the maximumdifference between adjacent traces is only 25 cm, andthe overall elevation is gradual (Figure 5): we estimatethat≈ 12–10% may be more representative. Therefore,if a significant change is seen in this structure, theprevious rate could provide a good reference value.In summary, 3D-GPR data corrected with 3D-laser

scanning information improves the resulting imagesand enables the establishment of better interpretativemodels. The new technique presented in this studycontributes to improving the knowledge of theinternal architecture of special structures through theapplication of a non-destructive GPR prospectionmethod.



Acknowledgements

This project has been developed as part of the researchand development project I +D+ i: HAR2010-21976,financed by Ministry of Education Culture and Sport-Govermment of Spain (MECD) and the equipmentused was purchased through the European RegionalDevelopment Fund (ERDF-2004).

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http://hdl.handle.net/10481/22949

http://hdl.handle.net/10481/22949

Ultradense Topographic Correction by 3D-Laser Scanning in Pseudo-3D Ground-penetrating Radar Data:...

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