Imaging Shallow Structures Using ۳D Pole-Pole Array

Imaging Shallow Structures Using 3D Pole-Pole Array

(A Case Study in Shahriar, Iran)

مژگان استواری٬کارشناس ارشد ژئوفیزیک Email: mjostovari@yahoo.com

Abstract:

Geoelectric measurements are nowadays an indispensible tool for investigating the shallow subsurface because the deduced conductivity distribution often provides valuable information for environmental, hydrogeological and engineering applications. Although the geoelectric equipment has developed rapidly over the last few decades, most geoelectric surveys are still based on 2D surveying strategies involving conventional co-linear electrode configurations. Although this approach often provides useful subsurface information, it may lead to serious misinterpretations in the presence of pronounced 3D subsurface features. At sites with complex resistivity structure, the 2-D assumption may introduce significant error and the resulting 2-D images can contain significant distortions. Three-dimensional electrical resistivity can be a practical tool for resolution of complex subsurface heterogeneity. Perhaps the most important reason though, why application of 3D geoelectric surveys are not yet well established, is the lack of appropriate 3D experimental design strategies. 3D geoelectric surveying typically requires a large number of electrode positions to be occupied at the grid points of the rectangular area under investigation. This number is usually much larger than the number of electrodes available. Resistivity images are created by inverting hundreds to thousands of individual resistivity measurements in order to produce an approximate model of the subsurface resistivity. In this paper, the objective is the study over shallow structures using 3D pole-pole array. The site is situated in Shahriar County which is located 22 km to the southwest of Tehran in Iran. Due to technical limitations, the number of current and potential electrodes was limited in the field. Therefore, the choice of the appropriate array for the survey which depends upon optimizing resolution capabilities and the signal-to-noise ratio was crucial. The survey was conducted in a 3D pole-pole survey with an electrode spacing of 25 cm covering 1.5 m by 1.5 m area over a rectangular cubic box made of aluminum with dimensions of one and a half meters in length and twenty-five cm in width and height. After data acquisition, resistivity data was inverted with the RES3DINV program and the vertical and horizontal sections were provided. Then a 3D plot of subsurface layers was obtained with the Slicer/Dicer program. The results indicate high ability of this method in order to detect Shallow Structures.

Keywords: 3D Pole-Pole array, Shallow Structures, RES3DINV, Slicer/Dicer program