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ArchoSciences33 (suppl.) (2009)

Mmoire du sol, espace des hommes

................................................................................................................................................................................................................................................................................................

Meri A. Berge et Mahmut G. Drahor

Optimum Electrical ResistivityTomography (OERT) approach usingcombination of different arrays inarchaeological investigations

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Rfrence lectroniqueMeri A. Berge et Mahmut G. Drahor, Optimum Electrical Resistivity Tomography (OERT) approach usingcombination of different arrays in archaeological investigations , ArchoSciences[En ligne], 33 (suppl.) | 2009, misen ligne le 30 octobre 2011, consult le 03 janvier 2013. URL : http://archeosciences.revues.org/1667

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AS, revue darchomtrie,suppl. 33, 2009, p. 263-265

In this study, the optimum electrical resistivity tomogra-phy (OER) concept was analyzed by combining differentelectrode arrays in terms of sensitivity in archaeologicalcases. More than a hundred electrode arrays were classifiedby Szalai & Szarka (2008), investigating the superposi-tion, focusing and linearity problem. Also, Dahlin & Zhou(2004) compared images resulting from various configura-tions. However, the diffi culties posed by apparent resistivitydue to different characteristics of electrode arrays, resultingin different anomaly shapes and resistivity values, revealedthat some optimisation techniques in tomographic inversionare necessary to obtain more interpretable results.

Te electrical resistivity tomography method has beenwidely used recently together with magnetic and ground-penetrating radar for detailed investigation of archaeologi-cal sites. Faster data acquisition has been facilitated by thedevelopment of multi-electrode towed and wheeled systems.However, in 3-dimensional surveys, the inversion techniquedemands much computing time. Also, these new systemspermit the use of different electrode configurations for singleor multiple usage in data acquisition. Hence, data sets areimportantly increased, and the usage of optimum electricalresistivity tomography optimisations might give more usefulresults than the standard single array utilisations.

Commonly used arrays (Wenner, Wenner-Schlumberger,dipole-dipole, pole-pole) and non-conventional arrays (gra-dient, square, etc.) could also be inverted by commercialsoftware using joint or weighted inversion described bysome researchers (Athanasiouet al., 2007; de la Vega et al.,2003; Stummer et al., 2004). Sensitivity analyses led us toselect suitable electrode configurations for a field survey. Tesubsurface sensitivity between the electrodes inserted on thesurface gives the degree of resistivity change in the subterra-nean section of the investigation line. Sensitivity values arealso used to obtain the depth of investigation for differentelectrode arrays (Roy & Apparao, 1971; Edwards, 1977;Barker, 1991). Te superiority of the arrays (horizontal-ver-tical resolution, data coverage, depth of investigation) playsa critical and important role in choosing a suitable array. Inorder to image an architectural plan using ER studies, thisconcept should be had very important role because of thearchaeological context is very complex.

wo and three-dimensional ER studies have been car-ried out in Bayrakl Hyk (zmir/urkey) showing thecomplex structural settlement in three dimensions of thesubsurface. Sensitivity analyses on a real case were alsoperformed. It should be pointed out that the highest sen-sitivity values are observed as very near to the surface. For

Optimum Electrical Resistivity Tomography (OERT)

approach using combination of different arraysin archaeological investigations

Meri A. B *,** and Mahmut G. D *,**

Key words:Optimum Electrical Resistivity omography, Electrode array, Sensitivity.

* Center for Near Surface Geophysics and Archaeological Prospection, Dokuz Eyll University. ([email protected]) ([email protected])** Dokuz Eyll University, Engineering Faculty, Department of Geophysical Engineering.



264 Meri A. BERGE, Mahmut G. DRAHOR


different arrays, these values demonstrate critical changesin the sensitivity sections. Data acquisition was performedwith a 30-channel resistivity meter using electrode and lineintervals of 1 m and 0.5 m, respectively. Resistivity datawas processed using robust inversion algorithm minimi-

zing the absolute values of differences between observedand calculated data in an iterative manner. Tese proces-ses were achieved by Res2Dinv and Res3Dinv softwares(Geotomo, 2005). Inversion of the field data was carriedout up to 10 iterations and absolute error values (ABS) didnot exceed 6.5%.

In Figure 1, 2D ER and sensitivity results are given fora measuring line using Wenner and dipole-dipole electrodearrays. We used 96 apparent resistivity data for the Wennerarray, while the dipole-dipole included 105 datum points.Te results show that resistive structures (>75 ohm.m) lieapproximately between 0.25-2.25 m depth. Te conductive

layer following resistive strata starts at 2.25 m depth (Fig.1). Tere are slight differences between the sensitivity sec-tions of the Wenner and the dipole-dipole arrays in Figure1. Compared to the dipole-dipole array, the Wenner arraypresents low sensitive values in shallower layers (between 0

and 1 m) and higher sensitive values in medium depths (1and 2 m). Both arrays also have less sensitive values in thedeeper parts (> 2m) of the sections (Fig. 1).

A larger data set (519 datum points) was obtained aftercombining Wenner, Wenner-Schlumberger, dipole-dipole,

pole-pole and pole-dipole arrays. Te model resistivity sec-tion of combined data was changed importantly accordingto single array uses (Fig. 2a). Tis case might be related todiscrepancy of relative sensitivities according to used arraysin ER sections. In order to avoid this, we therefore calcula-ted the arithmetic means of whole model sections and theirsensitivities (Fig. 2b). Whole sections demonstrated thatthe resistive and conductive structures increase or decreaseaccording to the number and types of array used in combi-ned array modification. Tis situation might produce somepseudo-structures related to array type and data quality inthe model sections. Although, the relative sensitivities of

combined usage are slightly high from the arithmetic meansection, the arithmetic mean results are rather stable in termsof sensitivity. Terefore, we think that this stability mightproduce more reliable models.

Figure 1 (see color plate): Inversionand sensitivity model sections of a)

Wenner and b) dipole-dipole arrays.

Figure 2 (see color plate): Inversionand sensitivity model sections of a)combined form and b) arithmeticmean of arrays (Wenner, Wenner-Schlumberger, dipole-dipole, pole-poleand pole-dipole).



Optimum Electrical Resistivity Tomography (OERT) approach using combination 265


Tree-dimensional ER investigation data sets were col-lected from the parallel measuring lines for different arrays,and then processed by 3D robust algorithm up to six ite-

rations. o obtain more reliable models and sensitivity sec-tions, similar optimisations were also applied to the 3Dresults. Te model slices obtained from combined usage andarithmetic mean approach are presented for different depthslices in Fig. 3. As can be seen from this figure, the arithme-tic mean process gives more successful results than the com-bined usage of ER. Especially, the depth slices obtained bycombined process are still complex, and architectural plan

Figure 3 (see color plate): Inversion and sensitivity model depthslices of a) combined form and b) arithmetic mean of arrays(Wenner, Wenner-Schlumberger, dipole-dipole and pole-pole).

ABS error values are 3.5% and 3.75%, respectively.

could not be distinguished as well as the arithmetic meanapproach for first three slices.

Terefore, the optimum electrical resistivity tomography(OER) experiments revealed that the combined usage ofdifferent electrode arrays would not be useful in every situa-

tion. Tese experiments showed that an optimum approxi-mation such as arithmetic mean approach might producemore reliable results than single array and combined usagesin archaeological applications.

References

ATHANASIOU, E. N., TSOURLOS, P. I., PAPAZAC HOS and C. B.,

TSOKAS, G. N., 2007.Combined weighted inversion of elec-trical resistivity data arising from different array types.Journalof Applied Geophysics, 62: 124-140.

BARKER, R. D., 1991.Depth of investigation of collinear symme-

trical four-electrode arrays. Geophysics, 54: 1031-1037.DAHLIN, T. and ZHOU, B., 2004. A numerical comparison of

2D resistivity imaging with 10 electrode arrays. GeophysicalProspecting, 52: 379-398.

EDWARDS, L. S., 1977.A modified pseudosection for resistivityand IP, Geophysics, 42: 1020-1036.

DELAVEGA, M., OSELLA, A. and LASCANO, E., 2003.Joint inver-sion of Wenner and dipoledipole data to study a gasoline-contaminated soil.Journal of Applied Geophysics, 54: 97-109.

ROY, A. and APPARAO, A., 1971.Depth of investigation in directcurrent methods. Geophysics, 36: 943-959.

GEOTOMOSOFTWARE, 2005.Res2Dinv, Res3Dinv. From http://www.geoelectrical.com, 02/2009.

STUMMER, P., MAURER, H. and GREEN, A., 2004.Experimentaldesign. Electrical resistivity data sets that provide optimumsubsurface information. Geophysics, 69 (1): 120-139.

SZALAI, S. and SZARKA, L., 2008.On the classification of surfacegeoelectric arrays. Geophysical Prospecting, 56: 159-175.

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