Specific properties of the pole-pole array for depth investigations in geoelectrical mapping

Mapping a great number of resistivity measurements on a surface (mainly in archaeology and pedology) must be done with arrays which are easy to move in the fields. For a long time the most popular was the Wenner (normal or dipole-dipole) array. Then came the square array, the twin electrode and the pole-pole. The necessity of shortening the measuring time leads to use a frame which holds the electrodes together. The towed arrays with automatic recording were then introduced for dense spatial sampling. The square array was the first to be employed in this manner owing to its low apparent anisotropy (Hesse & al., 1986). The polepole can also be towed with the drawback of a long wire fastened to the system. The complete description of the subsoil over the first meters is increasingly needed by civil engineers, archeologists, pedologists ... The electrical resistivity methods can provide this information since the investigation depth is a function of the spacing (the spacing "d" is the shortest distance between a current electrode and a potential electrode). Depths investigations consequently require multi-mapping with different spacings. Then the problem is to find a multi-depths system in order to give the best description of the subsoil. Solutions have recently been given with the PACEP method (Serensen, 1996) or the MUCEP method (Panissod & al., 1997). This paper suggests an alternative choice using a multi-pole-pole array. Due to the specific properties ofthe pole-pole array, this kind of3D investigation ofthe subsoil can yield electrical apparent resistivity maps which clearly distinguish the anomalies from vertically superimposed structures. rs demonstrate this effect we compare the responses of the pole-pole array and of the square array through field tests and 3D direct modelling of horizontal resistivity maps.