archaeological prospection of wall remains using geoelectrical methods and gpr

Archaeological Prospection of WallRemains using Geoelectricalmethods and GPR

ERWIN APPEL1*, JORG WILHELM 1 AND MARTIN WALDHO R2

1Institut fur Geologie und Palaontologie, Universitat Tubingen, Sigwartstrasse 10,72076 Tubingen, Germany2Terrana Geophysik, Zeppelinstrasse 15, 172116 Mossingen, Germany

ABSTRACT Geoelectrical twinpole mapping clearly reveals shallow wall remains of a Roman villa complexbuilt from limestone blocks. A tripole array is introduced that may allow the estimation of thestrike direction of linear structures by electrical profiling on a single line. Further geoelectricalinvestigations, i.e. Wenner profiling and pseudosection, as well as geoelectrical forward model-ling have been carried out on a standard profile across prominent anomalies recognized bytwinpole mapping. Despite unfavourably low background resistivity (10–20 Ohm �m), groundpenetrating radar (GPR) surveys provide useful information. It is demonstrated that additionalqualitative information on the nature of archaeological structures (possible identification of acollapsed roof, and of a cellar with collapsed ceiling) and quantitative depth estimates (0.4 m foran isolated wall; minimum of about 0.7–0.8 m for another wall) can be made by integrating theresults obtained by all the methods mentioned. *c 1997 John Wiley & Sons, Ltd.

Archaeol. Prospect. 4: 219–229, 1997.

Key words: archaeological prospection; geoelectrical mapping; two-dimensional geoelectricalprofiling; geoelectrical arrays; ground penetrating radar profiling.

Introduction

Prospection of prehistorical and historicalremains with non-destructive geophysicalinvestigations has gained increasing interest inthe archaeological community (Wynn, 1986;Boucher, 1996). Magnetic surveys are used inmost cases (e.g. Scollar et al, 1986), but othermethods are also increasingly entering the field.Geolectrical measurements using conventionalfour-electrode arrays have also become a stand-ard method, even more frequently used in Britain(Hesse and Spahos, 1980; Clark, 1994). They areeither used in combination with magnetics or as

the only methods in sites where resistivitiessufficiently differ but the magnetic permeabilitycontrasts are low (e.g. Tsokas et al, 1994). This istrue in the case of wall remains, built from highresistivity limestones and embedded in a highlyconductive environment such as soil or clayeysediments.

The twinpole array is the most popularconfiguration for geoelectrical mapping ofarchaeological sites because only two electrodeshave to be moved, and because of the benefits ofhigh lateral resolution (e.g. Apparao et al, 1969;Apparao and Roy, 1971) and high depth ofinvestigation compared with the total spacing ofthe mobile electrodes (Roy and Apparao, 1971). Itintegrates over a broad depth range and thus isparticularly suitable for the detection of verticalstructures. On the other hand, its potential for

*Correspondence to: Professor E. Appel, Institut fuÈ r Geologieund PalaÈontologie, Sigwartstrasse 10, 72076 TuÈ bingen,Germany. Email: [email protected]

CCC 1075±2196/97/040219±11$17.50 Received 24 September 1997# 1997 John Wiley & Sons, Ltd. Accepted 18 December 1997

Archaeological Prospection, Vol. 4, 219±229 (1997)

discriminating objects and boundaries at differ-ent depths is comparatively weak. Consideringthe growing demand for the third dimension,configurations with a higher vertical resolution,such as the Wenner array (Roy and Apparao,1971), are more favourable for two-dimensionalprofiles or areal three-dimensional surveys.Such measurements may include qualitativeinterpretations of pseudosections (e.g. Slateret al, 1996) and quantitative forward-modelling(e.g. Aspinall and Crummett, 1997) or tomo-graphic inversion (e.g. Noel and Xu, 1991).

More recently, ground-penetrating radar (GPR)has entered the field of archaeology (e.g. Vaughn,1986). Ground-penetrating radar is recognized asa valuable tool because of its suitability for rapidmapping and depth determination using radarsections and time-slices (e.g. Malagodi et al,1996). However, successful GPR studies inpractice are quite limited due to high absorptionof electromagnetic energy in well conductingmaterial like clay minerals and mineralizedgroundwater (Davis and Annan, 1989).

This paper shows the results of twinpolemapping of an ancient Roman villa complexcompletely buried in soil. Further resistivitymeasurements and GPR profiling wereperformed on a single profile over prominentfeatures recognized from mapping. Geoelectricalpseudosection, two-dimensional geoelectricalmodelling, and radar reflections are combined inorder to obtain additional information on thedepth of wall remains and details of the archaeo-logical structures.

Investigated site and data acquisition

The site investigated is known as the ancientRoman villa complex `Burg', located about 0.5 kmwest of Reutlingen-Altenburg (4883204100N,00980902900E) at the so-called `RoÈmerschanze',approximately 30 km south of Stuttgart (south-ern Germany). Some excavations were made atthe end of the last century and more recently in1988 (Klein, 1989). Earlier magnetic measure-ments (Faûbinder and Klein (1991) identifiedsome principal structures. However, the perm-eability contrast between the soil environmentand the walls built from limestone was notsufficient to provide satisfactory information.

Recently,resistivity mapping was carried outby a twinpole array using a Geoscan RM15instrument. Measurements were performed witha spacing (AM) of 0.5 m on a regular grid of 0.5 mcovering an area of 100� 100 m2, with about36,000 data points in total. Figure 1 shows a partof the resulting map. Dark anomalies representhigher apparent resistivity values, resultingmainly from remains of limestone walls. Thestructures were interpreted by Wilhelm et al(1995) as the main villa (1), the bath (2), and awaste water channel (3). The dark spot at theupper left corner represents a tree, and theparallel, approximately north±south striking,light stripes can be related to a modern drainagesystem. The background resistivity is not homo-geneous due to the varying moisture content inthe soil during the measurements conducted ondifferent days.

Selection of a standard profile

The villa 1 shown in Figure 1, is the most strikingfeature and contains a lot of details. The groundplan is impressively distinct. Division intodifferent rooms and an internal passage can beidentified clearly. Even small wall buttresses atthe southeastern part appear. Two extendedanomalies of higher resistivity, labelled `r' and`c' in Figure 1, are of interest, but cannot beexplained by the twinpole data mapping alone.Furthermore, isolated points of increased resis-tivity values (dark spots) frequently occur withinthe left (southwest) side of the villa. We did notremove such outliers because they were causedby coupling problems and may contain valuableinformation.

For our further studies on a standard profilewe selected a line between the grid points A(x� 10 m, y� 25 m) and B (x� 45 m, y� 25 m),which intersects three major walls and also theunexplained anomalies `r' and `c'.

Lateral resolution of differentelectrical arrays

Figure 2 compares the results from Wenner(a� 0.5 m), twinpole (AM� 0.5 m), and tripole

220 E. Appel, J. Wilhelm and M. WaldhoÈr

# 1997 John Wiley & Sons, Ltd. Archaeological Prospection, Vol. 4, 219±229 (1997)

(A1A2�A1A3�A2A3� 0.6 m, MpNp�MtNt� 0.1 m) arrangements.

The tripole array (Figure 3a) is our owndevelopment based on the unipole concept (e.g.Brizzolari and Bernabini, 1979). Three currentelectrodes (A1, A2, A3) are arranged in an equal-sided triangle and all of them are supplied withthe same constant current (equipotential sources).The opposite polarity current electrode is placedat `infinity'. Potential electrodes are centred inthe triangle, and the voltage is measured in-line(parallel to the profile direction) between Mp andNp (parallel tripole: PaT), and perpendicular to itbetween Mt and Nt (perpendicular tripole: PeT).Measurements were performed with a Syscal R1instrument (BRGM), and an attached self-madeelectronic device was used for constant currentcontrol. All seven electrodes were mounted on awooden frame to enable rapid data acquisition. InFigures 2 and 4 we show the voltage plot(as usually done for symmetrical arrays) for the

PaT and PeT. The centre of isolated bodies canbe located by a zero intersection between acharacteristic maximum and minimum. This isdemonstrated by the anomaly of wall 1, but alsowall 2 is clearly identified. Figure 4 shows themajor advantage of the tripole array. For a profiledirection perpendicular to the wall (Figure 3b),the PaT gives a maximum response, whereas thePeT curve is insignificant. When the profileorientation is changed by 308 (Figure 3c), theamplitude of the PaT is reduced and a PeTanomaly appears, but is still smaller than thePaT. From these preliminary tests, we mayconclude that the tripole array can be used forestimating the strike direction of linear structuresby measuring the ratio of PaT/PeT anomalies on asingle profile. For further improvement, wesuggest the use of three pairs of MN electrodeswith axes that form angles of 608 (Figure 3d). Suchan arrangement would be really symmetrical,which is not the case in the present geometry.

Figure 1. Geoelectrical mapping of an ancient roman villa complex by a twinpole array with 0.5 m spacing (only part ofthe total area is shown here). Dynamic range of the grey-scale: 8--25 Ohm�m (black denote values 425 Ohm�m). Theanomalies represent the main villa (1), the bath (2) and a waste water drainage (3) (Wilhelm et al, 1995). Three majorwalls, aligned approximately in the y direction, are marked by w1--3. Some unexplained features are indicated by encircledareas ‘r’ and ‘c’. A standard profile line is shown between A (x� 10 m, y� 25 m) and B (x� 45 m, y� 25 m).

Archaeological Prospection of Wall Remains 221


For the electrode spacings shown in Figure 2,the depths of investigation are similar for Wenner(0.17 m) and twinpole (0.18 m) if estimated afterRoy and Apparao (1971) but they differ by afactor of about 1.7 (0.26 m and 0.43 m) followingEdwards (1977). There is no major difference inthe lateral resolution between the Wenner andtwinpole curves, and we can pass over to Wennermeasurements for our further studies to benefitfrom the increased vertical resolution.

The third dimension

Two-dimensional Wenner measurements

All Wenner measurements were carried outwith the Syscal R1 device using a multipolecable. A slight improvement is observed whenthe distance between data points is reduced to0.25 m. The data were sampled along the

standard profile with a point distance of 0.25 mand eight different spacings of a between 0.25 mto 2.0 m. For selected spacings of a, i.e. 0.5 m,1.0 m, 1.5 m and 2.0 m, the curves are shown inFigure 5. Corresponding depths of investigationare about 0.2 m, 0.35 m, 0.50 m and 0.7 m afterRoy and Apparao (1971), and about 0.25 m,

Figure 2. Geoelectrical profiling with different arrays (Wennera� 0.5 m, twinpole AM� 0.5 m, tripole AiAj� 0.6 m/MN� 0.1 m) along the standard profile AB shown inFigure 1. The point distance is 0.25 m for Wenner andtripole, and 0.5 m for twinpole. The positions of walls 1--3are indicated.

Figure 3. The twinpole array. A1, A2, A3 are constantcurrent electrodes. (a) In the version used, the voltage ismeasured between Mp/Np (parallel tripole, parallel to theprofile line) and Mt/Nt (perpendicular tripole). (b) Profileperpendicular to the wall. (c) Profile 308 inclined. (d) Twoalternative versions of a symmetrical tripole array.



0.55 m, 0.8 m and 1.0 m after Edwards (1977).Furthermore, the data have been processed into apseudo-section (Figure 9) with a commercialgridding software.

Ground-penetrating radar profiling

Ground-penetrating radar measurements werecarried out on the standard profile using aSIR10 system (GSSI) with a 500 MHz bistaticantenna. The data were recorded with time-dependant gain and a 100±600 Mhz electronicbandpass filter. Very low ground resistivities ofabout 10±20 Ohm�m can be deduced frombackground values shown in Figures 1 and 5.For such a well conductive environment, thepenetration depth of electromagnetic wavesshould not exceed a maximum of a few tensof centimetres for the frequency of 500 MHz(e.g. Daniels et al, 1988; Davis and Annan 1989)

and a successful radar survey seemed to beunlikely in this case. Using a lower frequencycould not be considered appropriate because ofthe very shallow targets at our site (51 m).Despite the unfavourable conditions, the resultsfrom GPR profiling pick up the structuresalready identified by the electrical investi-gations. This underlines that, although theoreti-cal performance predictions (e.g. Annan andChua, 1988) are useful, a `trial and error'strategy is advisable for GPR.

The radar-section in Figure 6 has been pro-cessed by a horizontal filter for removing thedirect wave and coherent electronic noise. Thefirst 3±4 ns are masked by the direct wave andcannot be interpreted. At places where betterpreserved wall remains exist, the resistivity is inthe order of about 50±100 Ohm�m (see Figure 7)and the attenuation of electromagnetic waveenergy is lower. Signals up to about 15±20 nstwo-way travel time (TWT) can be recognizedhere. A TWT of 10 ns corresponds to a depth of0.5 m, assuming a velocity of 0.1 m/ns (as shownin Figures 6 and 8). Velocity values of 0.1 m/nsare commonly estimated for soils and sediments(permittivity er� 9) and are confirmed in ourradar-section by analysis of hyperbolic features.

Figure 4. Tripole measurements across wall 1 (see Figure 1)on profiles perpendicular to wall 1 and on a 308 line (profilewith a 608 angle to the strike direction of the wall)respectively. The thick line represents the perpendiculartripole. The position of wall 1 is indicated.

Figure 5. Geoelectrical Wenner sections with differentspacings along the standard profile AB shown in Figure 1.For better distinction, the a� 2.0 m curve is drawn as athick line. The positions of walls 1--3 are indicated.



Figure 6. Ground penetrating radar section along the standard profile AB shown in Figure 1. The depth conversion is basedon a velocity of 0.1 m/ns. The positions of the walls 1--3 are indicated.

Figure 7. Results of geoelectrical two-dimensional forward modelling of the structure of wall 1. Three different models arecompared with the measured Wenner curve (of the standard profile AB shown in Figure 1) for a spacing a� 2.0 m. Thesmall figure shows the measured curve (solid line) and the modelled curve of model 2 multiplied by a factor of 1.12.Resistivities in the models are given in Ohm�m.



Figure 8. Expanded GPR sections of Figure 6 from 10 to 20 m (top) and 30--45 m (bottom) with wiggle traces.



Qualitative interpretation

Anomalies in the `a� 0.5 m' Wenner curve aregenerally the highest of all spacings (Figure 5).This indicates a very shallow upper boundary ofall observed archaeological features.

Anomaly `r' completely fades out for themaximum spacing of a� 2.0 m (Figure 5).Obviously, it originates from sources situatedvery close to the surface. No major structures areidentified in the GPR section in this range, butnumerous hyperbolic signals appear between 20and 30 m (Figure 6), pointing to an accumulationof small isolated objects. A collapsed roof of thebuilding is likely to be responsible for anomaly`r'. Fragments of bricks have been found here(Klein, 1989) probably causing the anomaloushigh resistivities in the twinpole mapping (blackdots in Figure 1). Anomaly `r' also correlates witha positive magnetic anomaly (Faûbinder andKlein, 1991).

In curves with a spacing of a� 2.0 m, wall 2and wall 3 are well separated, whereas fora� 0.5 m and a� 1.0 m, a central anomalyappears in between (designated as `c' in Figure 1).Anomaly `c' seems to arise from shallower objectsthan the neighbouring walls, and wall 3 isdiscriminated less well from anomaly `c' thanwall 2 for spacings of a� 0.5 m to a� 2.0 m(Figure 5). The radar section shows reflectedsignals up to about 15±20 ns TWT betweenwalls 2 and 3 but only to about 10 ns TWT forwall 1 (Figures 6 and 8), suggesting a deeperfoundation for the first part. This is alsosupported from the geoelectrical pseudosection(Figure 9). An important observation from GPR isthat a `depression' of reflected signals is identifiedbetween 34 and 38 m (Figures 6 and 8). Ground-penetrating radar signals from the `depression'are not as deep as from walls 2 and 3. Summar-izing our qualitative observations, the structure `c'(Figure 1) is likely to represent a collapsed cellar.

Figure 9. Geoelectrical pseudosection along the standard profile AB shown in Figure 1. The upper figure shows the samesection (with the same scale) with a different dynamic range for a better resolution of the parts having resistivities450 Ohm�m. The positions of walls 1--3 are indicated.



The walls of the cellar seem to have deeperfoundations than the cellar floor.

The depth of archaeological structures

Wall 1 is isolated and the structure is certainlymuch simpler than that between walls 2 and 3.Therefore it was selected for quantitativeinterpretation. Forward modelling was per-formed for two-dimensional bodies using aprogram that applies the finite-differencesmethod (Niederleithinger, 1990). From excava-tions at another part of the farm-house complex,the thickness of the wall could be estimated toabout 1 m. It is likely that the wall is markedlybroken and weathered and that the wall materialis mixed with soil. This is considered in themodels (Figure 7) by a higher resistivity of thecore (110 Ohm�m) compared to the outer parts(80±60 Ohm�m). A value of 30 Ohm�m probablyrepresents soil with mixed fragments from thewall. Values of 20 and 10 Ohm�m are assigned tothe top-soil layer (0±0.4 m) and the underlyingsection respectively. Figure 7 demonstrates thatmodels 1 (about 0.6 m depth) and 3 (about 0.2 m

depth) yield curves that are far from matchingthe measured curve for a� 2.0 m. Model 2, witha foundation depth of about 0.4 m, yields the bestapproach. It is worthwhile noting that thesystematic offset of modelled and measuredvalues is a matter of model resistivities (theresult of a simple correction by multiplying themodelled curve with a constant factor of 1.12 isshown in the small plot of Figure 7). Figure 10demonstrates that model 2 also matches theanomalies measured, with smaller spacings.

In our GPR section, the onset of reflectedsignals at lower TWT cannot be identifiedunambiguously because of the possible super-position with the direct wave (which is removedby the coherence filter). For wall 1, a secondreflection occurs at about 10 ns (Figure 6) whichcorresponds to a depth of 0.5 m. This estimationbasically coincides with the result of geoelectricalforward modelling. Of course, we cannot beabsolutely sure that the 10 ns signal is anindependent reflection and not part of a con-voluted wavelet starting at about 7 ns. For wall 2,a deeper reflection at a TWT of about 15 ns(corresponding to a depth of 0.7±0.8 m) is more

Figure 10. Results of geoelectrical two-dimensional forward modelling of the structure of wall 1. Curves for model 2 arecompared with the measured Wenner curve (of the standard profile AB shown in Figure 1) for different spacings. Theposition of wall 1 is indicated (for the exact model see Figure 7).



significant because the wavelet clearly differsfrom the earlier one at about 7 ns. However, it isnot confirmed that we see the base of thearchaeological structure here because the signalpossibly fades out for higher TWT.

Conclusions

(i) Geoelectrical twinpole mapping clearly re-vealed an ancient Roman villa complex byoutlining wall remains built from limestoneembedded in conductive soil.

(ii) A novel geoelectrical tripole array (paralleland perpendicular) is introduced, whichmay allow an estimation of the strike direct-ion of linear structures from a single profile.

(iii) Despite the unfavourable conditions of ahighly conductive background, GPRmeasurements with a centre frequency of500 Mhz were performed successfully. Thisunderlines that a `trial and error' strategy isadvisable, although theoretical performancepredictions also may be useful.

(iv) In addition to twinpole mapping, integratedinterpretation of Wenner sections, pseudo-section, forward-modelling and GPR pro-filing provided useful information on thedepth of wall remains and on the details ofstructures that could not be explained bytwinpole mapping.

Acknowledgements

The authors wish to thank F. Klein, J. Faûbinderand H. van der Osten for their introduction to thearchaeological site and many discussions on thegeophysical results, U. Asprion for his help inradar data aquisition, and B. Greiner for his helpin processing the twinpole data.

The electronic device for constant currentcontrol was constructed by E. Lippmann (Kor-nacker 4, 94571 Schaufling, Germany).

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archaeological prospection of wall remains using geoelectrical methods and gpr

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