Detecting small‐scale targets by the 2D inversion of two‐sided three‐electrode data: application to an archaeological survey

ABSTRACTThe detecting capabilities of some electrical arrays for the estimation of position, sizeand depth of small-scale targets were examined in view of the results obtained from2D inversions of apparent-resistivity data. The two-sided three-electrode apparent-resistivity data are obtained by the application of left- and right-hand pole–dipolearrays that also permit the computation of four-electrode and dipole–dipoleapparent-resistivity values without actually measuring them. Synthetic apparent-resistivity data sets of the dipole–dipole, four-electrode and two-sided three-electrodearrays are calculated for models that simulate buried tombs. The results of two-dimensional inversions are compared with regard to the resolution in detecting theexact location, size and depth of the target, showing some advantage for the two-sided three-electrode array. A field application was carried out in the archaeologicalsite known as Alaca Hoyuk, a religious temple area of the Hittite period. The two-dimensional inversion of the two-sided three-electrode apparent-resistivity data hasled to locating a part of the city wall and a buried small room. The validity of theinterpretation has been checked against the results of subsequent archaeologicalexcavations.INTRODUCTIONThe two-dimensional (2D) inversion of direct current (DC)sounding data has become a standard tool for the investiga-tion of targets buried at shallow and intermediate depths. Theapplications include archaeology, environmental and hydro-geological works, mining exploration and engineeringstudies. When using the traditional electrode arrays thedata acquisition is usually carried out by expanding electro-des along a measurement profile. The apparent-resistivitydata obtained from all soundings are then used as the input ofa 2D inversion algorithm. Pelton, Rijo and Swift (1978) andSasaki (1981) first introduced inversion algorithms applied tothe dipole–dipole and Schlumberger arrays, respectively.Later, Loke and Barker (1996) introduced a fast 2D inversionmethod for the Wenner and dipole–dipole electrode arrays.This algorithm is based on a quasi-Newton method whichcalculates an approximate value of partial derivatives of theapparent-resistivity data with respect to model parameters.The finite-difference technique is used for the computation ofthe model response.One of the remarkable developments in DC methods is theuse of modern measuring systems with multicore cableswhich allows automatic switching between electrodes. Forpractical field operations, the electrodes are located at equallyspaced measurement stations. For example, Morris, Ronningand Lile (1997) described a data acquisition system thatsimultaneously collects Schlumberger and two dipole–dipoledata. However, they did not indicate a quantitative inter-pretation tool. Acworth and Griffiths (1985) and Griffithsand Turnbull (1985) described Wenner tripotential apparent-resistivity measurements to reduce the number of ambiguitiesarising in the qualitative interpretation of apparent-resistivitypseudosections. The tripotential method produces threeapparent-resistivity measurements for each position of acollinear equispaced quadripole of electrodes. They used the