Archaeological ProspectionArchaeol. Prospect. 14, 142–145 (2007)Published online 23 February 2007 in Wiley InterScience

302

(www.interscience.wiley.com) DOI: 10.1002/arp.

* Correspondence to: J. LeObstgartenstrasse 7, CH –E-mail: Terra@terra.ch

Copyright # 2007 John

Short ReportPull-up/Pull-downCorrections forGround-penetrating Radar Data

J�RGLECKEBUSCH*

terra vermessungen ag, Obstgartenstrasse 7, CH ^ 8006 Zurich, Switzerland

ABSTRACT Ground-penetrating radar (GPR) is one of the best methods for detailed and high precision three-dimensional imaging of the subsoil. A range of specific processing steps are available to correct thegeometricaldistortionof thegeoradardata.Neverthelessit isnot yet possible to correct for pull-up/pull-downeffectsdue to chancesof velocity in target features.Unfortunately theseeffects caneasily changethe lower boundary of a structure bymore than 30% of the preserved height. Hence pull-up/pull-downisoneofthemost importantgeometricaldistortionsinarchaeologicalprospection.Afteradetailedthree-dimensionalinterpretationof the datacube, the calculationofanindividualcorrectionbecomespossibleand can be applied to each structure separately.Copyright# 2007 JohnWiley & Sons,Ltd.

Key words: pull-up; pull-down; geometry; corrections; interpretations; georadar

Introduction

Detailed and high-precision prospection methodsare becoming increasingly important for archae-ological services as information about the heightand width of the preserved structures is a criticalparameter. With the knowledge of these, the stateof conservation can be estimated as well as thevolumes of earth that have to be excavated.One of the best-suited methods to answer such

questions is ground-penetrating radar (GPR) orgeoradar. There are several well-known tech-niques that are available to correct for the specificimaging geometry of georadar. These includestatic corrections to account for the topography,migration and time to depth conversion withone-, two- or even three-dimensional velocityprofiles (Leckebusch, 2003) and also three-dimensional topographic projection to correct

ckebusch, terra vermessungen ag,8006 Zurich, Switzerland.

Wiley & Sons, Ltd.

for the attitude of the antenna (Leckebusch andRychener, 2004; Goodman et al., 2005; Lecke-busch and Rychener, 2005).

Even when these corrections are applied, thedata still show some discrepancies on the verticalaxis (Leckebusch and Rychener, 2004): wallsbelonging to a room seldom reach the depth ofthe corresponding floor (Figures 1 and 2). This isa well known effect of georadar and seismic datacalled pull-up (Martinez et al., 1998), which is notyet possible to correct. Pull-up or pull-downoccurs because the electromagnetic waves travelat different speed inside the archaeologicalstructures compared to the surroundingmaterial. The lower boundary of an anomaly istherefore shifted towards earlier or later times onthe traces.

Geometry of the structures

Archaeological structures embedded in the soiland detected with georadar often have walls

Received 13 June 2006Accepted 12 November 2006

Figure 1. Three-dimensional, conventional interpretation ofpart of a Roman town. The data is migrated and a static shiftfor the topography was applied. In many cases the walls of aroom do not reach the corresponding floor (see arrows): a neteffect of the pull-up because of strong vertical velocity vari-ations.

Pull-up/Pull-down Corrections 143

with a very strong and abrupt velocity variationalong the vertical interface between the struc-ture and the surrounding material. It is possiblethat this variation may not be constant, butvaries with depth (Figure 3). Proper dataprocessing with migration and time to depthconversion would require the knowledge ofthese searched structures in advance. Thisinformation may then be incorporated into avery detailed three-dimensional velocity pro-file. Unfortunately this geometry is what we arelooking for with the prospection method. Testshave shown that even multi-offset data are notable to cope with these high spatial variations.However, pull-up or pull-down effects signifi-cantly change the lower boundary of the

Figure 2. Migratedanddepth convertedprofile showing thepull-udepth of the floor from the left side, markedwith anarrow.

Copyright # 2007 John Wiley & Sons, Ltd.

structures. Let us assume that a wall has beenfound, and that it has been preserved from up to0.3m below the surface and preserved down toa depth of 1.0m. The velocity of this structuremay be 0.11mns�1 compared with the velocityof 0.08mns�1 of the surrounding material (seealso Figure 3). This would result in a reflectionof the lower boundary at 20.2 ns instead of25.0 ns. The pull-up in this case is 0.2m oralmost 30% of the preserved height. However,this is too much for a precise imaging of thearchaeological remains and factors other thanmigration normally change the geometry (Lecke-busch, 2000).The final processed data of any geophysical

prospection should be interpreted to help thenon-specialist understand the results. In the caseof georadar, a detailed three-dimensional vectorinterpretation can be extracted from the datacube. This means that the archaeological remainsin the subsoil are then known in great detail. Onemight now use this information as a refinementof the original velocity field (Alaei, 2006), but thevertical velocity variation would introduce a vastnumber of artefacts. This precise geometry nowallows us to correct for a pull-up or pull-downeffect of the original georadar data.

Correction strategy

To be able to correct for pull-up and/or pull-down, several steps have to be followed. First ofall, a detailed velocity profile (two- or three-dimensional) should be constructed to migratethe data and to properly convert the time scale to

peffect.Thewallbetweenmeter 3.5 and 4.0 doesnot reach the

Archaeol. Prospect. 14, 142–145 (2007)

DOI: 10.1002/arp

0.080

0.075

0.070

0.11

A B0.0

0.3

0.6

1.2

depth(m)

withcorrection

recorded

pull-up

velocities(m/ns)

anomaly depthon GPR trace

Figure 3. Schematic representation of a typical depth andvelocity profile (in italics) of a buriedwall (in grey).The velocityis often decreasing with depth and shows an abrupt changealong a vertical interface between the soil and the structure atpoints Aand B.

Figure 4. Effect of thepull-up corrections (addedin darkgrey),shownon the same dataset as Figure1.Thewalls and floors ofa roomnow formacompleteandcontinuousanomaly.The cor-rections differ from structure to structure because they aredirectly related to the preserved height.

144 J. Leckebusch

depth. The velocities can be determined by fittinghyperbolas to the pattern in the data or byconstand velocity migration tests (Leckebusch,2000). It is clear that with all the methodsavailable, normally the relative dielectric con-stant for the soil is estimated, but not for thestructure itself. This information is then used in asecond step for normal processing, includingfiltering, binning (or rubber banding) as well asmigration and time to depth conversion. Adetailed archaeological interpretation of the datacube in three dimensions will finally allow for apull-up correction. If the velocity of the structureis lower than the surrounding material, it will bea pull-down. This would be encountered on aditch for example. The specific pull-up correctionfor each anomaly can be calculated by integratingover the velocity variations with depth of thestructure

Zpull�up ¼Pn

i¼0

DZi � vcorr�vivi

with vcorr as the correct velocity and vi thevelocity used during data processing. Zi is thedepth extent of the structure in the depth range ofconstant interval velocity, as specified in a two-or three-dimensional velocity field. From directmeasurements on samples, we know that thevelocity of a wall, the most frequent structure ingeoradar, is between 0.1 and 0.12mns�1. Withthis assumption, the depth corrections for the

Copyright # 2007 John Wiley & Sons, Ltd.

lower boundaries of any of the interpretedstructures can be calculated. The effect on theinterpretation is significant and very important(Figure 4).

Conclusions

All of the known and very advanced processingsteps are not able to correct for the pull-up orpull-down effect in georadar data. If a detailedand three-dimensional interpretation of a data setis available, then a depth correction that adjuststhe lower boundary for the difference in thevelocities between the structure and the sur-rounding material becomes possible. Verificationusing real data proved that the proposed strategyis efficient and correct. In some cases though, thecorrections do not seem to be large enough,which shows that the amplitude reduction has asignificant effect (Leckebusch and Peikert, 2001)and therefore complicates the proper depthdetermination.

Acknowledgement

The author would like to thank Alex Furger fromthe archaeological service ‘Ausgrabungen Augstund Kaiseraugst’ for the permission to use thedata from his site for this publication.

Archaeol. Prospect. 14, 142–145 (2007)

DOI: 10.1002/arp

Pull-up/Pull-down Corrections 145

References

Alaei B. 2006. An integrated procedure formigration velocity analysis in complex structuresof thrust belts. Journal of Applied Geophysics 59:89–105.

Goodman D, Schneider K, Piro S, Hongo H, HigashiN. 2005. GPR and geophysical archaeometry.American Geophysical Union, Spring Meeting,New Orleans, Louisiana; NS31A-03.

Leckebusch J. 2000. Two- and three-dimensionalgeoradar surveys across a medieval choir: a casestudy in archaeology. Archaeological Prospection7(3): 189–200.

Leckebusch J. 2003. Ground-penetrating radar: amodern three-dimensional prospection method.Archaeological Prospection 10(4): 213–240.

Leckebusch J, Peikert R. 2001. Investigating the trueresolution and three-dimensional capabilities of

Copyright # 2007 John Wiley & Sons, Ltd.

ground-penetrating radar data in archaeologicalsurveys: measurements in a sand box. Archaeolo-gical Prospection 8(1): 29–40.

Leckebusch J, Rychener J. 2004. Kein glasernerBoden? Ein kritischer Vergleich zwischenBodenradardaten und Ausgrabungsresultatenin der romischen Stadt Augusta Raurica.Jahresberichte aus Augst und Kaiseraugst 25:197–214.

Leckebusch J, Rychener J. 2005. Three-dimensionalcomparison between GPR and excavation results.6th International Conference on Archaeological Pro-spection, Rome, Italy; 223–226.

Martinez A, Kruger JM, Franseen EK. 1998. Utilityof ground-penetrating radar in near-surface,high-resolution imaging of Lansing-Kansas City(Pennsylvanian) limestone reservoir analogs.Kansas Geological Survey, Current Research in EarthSciences 241(3): 43–59.

Archaeol. Prospect. 14, 142–145 (2007)

DOI: 10.1002/arp