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In this study, a quick method using a digital elevation model (DEM) to obtain real terrain points for generating tetrahedral mesh has been developed, based on TetGen. Then threedimensional (3-D) forward modeling and inversion, based on the patching method of electrical resistivity tomography (ERT) and which have been compared favorably with those obtained using other software, were used to study topography effect. Asystematic research of apparent resistivity features of different topographies with pole-pole array and Wenner array has been conducted in this study. Based on that, the solutions for removing topography effect are given to the two-dimensional (2-D) survey and the 3-D survey, respectively. Comparing to the inversion result, the solution for the 2-D survey can effectively remove topography influence. A 3-D inversion algorithm incorporating topography is proposed at the same time. Two synthetic models incorporating real topography with fault and ellipse anomalies were created to test the 3-D inversion algorithm, and the results show that the relative image error is less than 30 % and the correlation coefficient is more than 90 %.

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This paper presents the imaging of contaminant plumes in Qinhuai River and its bed for the remediation strategies of the river. Qinhuai meets with Yangtze River at Nanjing area via inner and outer channels, coursing through urban districts of the city. The discharge of urban effluents has badly affected the water quality of the river. Generally, contamination assessment is carried out through traditional point-based sampling methods. However, these methods are often inadequate to estimate accurately the spatial distribution of contaminants and their extension. Therefore, electrical resistivity tomography (ERT) is applied because of its high quality imaging to investigate contamination and its extension in Qinhuai River and its bed. Eight sites were selected at outer and inner channels of the river. A 2-D inversion of the perceived data was carried out and the obtained resistivity values were converted into salinity after temperature correction. The acquired images of resistivity and salinity show that in low streaming conditions the river has large zones of low resistivity in the water body, which are extending toward underlying bed corresponding to high contamination. Conversely, in high streaming conditions, the low resistivity zone shrinks significantly at almost all the studied sites except Xishan Bridge (site E), which still showed high plume of contamination. This phenomenon reveals that in low streaming conditions the contamination level is high while in high streaming conditions salts are flushed out from the river. This study further demonstrates that ERT method can be a useful technique for the environmental studies of rivers, canals and surface water environments.

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