Edge detection techniques for bedrock investigation in 3D electrical resistivity tomography data

Abstract

Edge detection techniques for depth to bedrock investigations have been applied in 3D electrical resistivity tomography data acquired from two study areas in Chiang Mai province. The first study area is located in Huai Hongkhrai Royal Development Study Center, Doi Saket district, and the second study area is in an area of Office of Regional Livestock 5, Mueang Chiang Mai district. In the first study area, full 3D electrical resistivity tomography data were collected using the 3D pole-pole array, 8x8 electrode grid system, with an electrode separation distance of 5 m covering a survey area of 35 X35 m . In the second study area, the 3D electrical resistivity tomography was obtained by a collection of 11 parallel 2D electrical resistivity tomography lines. All the 2D electrical resistivity tomography lines are 200 m in length with a line separation of 10 m and acquired using a gradient electro array. The parallel 2D electrical resistivity tomography profiles were combined to generate the 3D data covering an area of 200x100 m2. The 3D data from both study areas were inverted using the AGI EarthImager'M 3D software. To make more accurate depth estimation of the bedrock, the edge detection techniques were applied to the inverted 3D electrical resistivity tomography data using (1) the steepest gradient method by finding the steepest value of the first derivatives of an image and (2) the Laplacian method by searching for values of zero in the second derivatives of the image. The zero values of the Laplacian (Laplacian zero-line) correspond to the abrupt changes (i.e., the edge) in the inverted 3D electrical resistivity tomography data. The results from this study can indicate that the method for finding zero in the second derivative method is suitable for application in electrical resistivity tomography data in areas of less complex geological structures especially for bedrocks with smooth surface. In addition, the method for searching the steepest gradient could provide a better result than the second derivative method in areas with more complex geological structures for bedrocks with rough surface causing electrical resistivity tomography data variations in both vertical and horizontal directions.