Appraisal of the Spatial Resolution of 2D Electrical Resistivity Tomography for Geotechnical Investigation

Abstract

In the past decade, the 2D electrical resistivity tomography (ERT) has been extensively used in the investigation and monitoring of geotechnical engineering and environment engineering, but there are many uncertainties hidden behind its vivid color earth-resistivity profiles. In order to use the 2D ERT in the scale of geotechnical engineering effectively, the accuracy and spatial resolution capability of measurements must be enhanced, or at least these uncertainties should be mastered to avoid overreading the measurement results. There were seven common geological models built in this study to discuss the variance in spatial analysis capability of 2D electrical resistivity profiles under different geologic conditions. The findings show that the resolution capability of 2D electrical resistivity profiles was influenced by depth, and in different strata, it may be influenced by the resistivity ratio, layer depth, covering depth, interlayer thickness, tilt angle, medium size, and noise intensity. Generally speaking, the relatively low resistance stratum had better resolution capability; if the relatively high resistance stratum was located under the relatively low resistance stratum, its resolution capability declined. In different strata, the resolution capability may be degraded under the effect of different factors. In addition, any noise in the course of measurement resulted in a random jump of the electrical resistivity profile, which worsened as the noise increased. These circumstances should be paid special attention to avoid misrecognition of electrical resistivity profile images.

Geophysical Research on Rock Mass Quality Evaluation for Infrastructure Design

“…In these surveys, electrical resistivity is acquired in Ωm via measurements on the ground surface. ERT is one of the leading geophysical methods (Hung et al., 2020). In ERT surveys, direct current is injected into the subsurface via a pair of current electrodes placed on the surface and the subsequent drop in potential is measured by another pair of potential electrodes.…”

“…Therefore, the seismic detection cannot evaluate the rock mass quality efficiently, and hence may cause geological ambiguities in the subsurface models. On the other hand, ERT can provide 2D and even 3D plane information of the geological model with wide range of resistivity values for the subsurface layers, gives the necessary investigation depth using the spread length, and provides high correlation for formation resistivity and subsurface structures (Hung et al., 2020). So, at present, ERT is being used widely in many geotechnical investigations.…”

Application of electrical resistivity tomography (ERT) for rock mass quality evaluation

“…In ERT surveys, electrical resistivity is measured (in Ωm), which is the inverse of electrical conductivity. The basic principle of ERT is based on the varying electrical conductivity of the subsurface materials, which depends on many factors, such as rock type, porosity, permeability, connectivity of pores, temperature, salinity, cation exchange capacity, clay content, nature of the fluid/water, weathering degree, fractures/faults, rock association, rock deformation, water–rock interaction and alteration, etc 3 , 9 , 42 . Generally, a wide range of electrical resistivity for most materials suggests the varying water content.…”

“…Hence, a contrast in resistivity values of the saturated weathered/fractured rock and fresh/unweathered rock is clearly observed 9 . 2D/3D ERT is a modern form of the conventional 1D VES (vertical electrical soundings) method, which provides high resolution imaging of the subsurface both laterally and vertically 42 . The modern electrical surveys are conducted using the most efficient and non destructive instruments, which can provide measurements of ERT, IP (induced polarization) and SP (self-potential).…”

“…Thus, such methods cannot evaluate the subsurface accurately and leave uncertainties in the interpretation of geological models. However, compared with other geophysical methods, electrical resistivity tomography (ERT) has wide range of resistivity values, shows high correlation between electrical resistivity and lithology of the subsurface layers, provide the required depth of investigation, and evaluates the subsurface via 2D and 3D imaging 42 . Therefore, at present, ERT is one of the leading geophysical methods in geotechnical research.…”