A comparative study of the imaging capability of quasi-null and dipole-dipole electrode configurations over an elongated, dipping, semi-infinite conducting body

Abstract

The effect of the dipping and the finiteness of a 2D prismatic body were investigated in Electrical Resistivity Tomography (ERT) measurements. For this purpose a free-air analogue modeling laboratory was designed. This enabled to control the parameters of the body and carry out measurements in field conditions as this is the most reliable method to verify the field applicability of an array. The imaging capacity of the best four-electrode traditional array, the Dipole-dipole (Dp) was compared to that of special configurations, three members of the series of the γ11n quasi-null arrays. This is the first attempt to study in field circumstances the imaging capacity of quasi-null arrays whose theoretical background is very different from that of the traditional arrays. In the lack of available processing codes that can invert the field data of these arrays a new routine, the Res2D-Hu code was used. Dp array proved to be robust, insensitive to both dipping and finiteness of the body, and very good in imaging shallow objects. The results of the γ11n arrays are less certain, but they may detect deeper bodies and resolve the model also vertically. Due to the complementary nature of the Dp and γ11n arrays, the joint interpretation of their images is recommended. In the present study - with a minimally increased time investment in relation to the individual Dp measurement - the joint interpretation enabled the following aspects: (1) separate clearly the real anomaly from possibly artificial ones; (2) make clear that the investigated model is not exactly 2D, it ends close to the first profile; (3) delineate the model better both horizontally and vertically and (4) detect the body from a larger depth than it would have been possible by the application of only the traditional array. In order to make the above points explicit, investigations had to be done besides the traditional Dp array by the γ116.