Abstract

Electrical resistivity tomography (ERT) is a popular geophysical subsurfaceimaging technique and widely applied to mineral prospecting, hydrological exploration, environmental investigation and civil engineering, as well as archaeological mapping. This chapter offers an overall review of technical aspects of ERT, which includes the fundamental theory of direct-current (DC) resistivity exploration, electrode arrays for data acquisition, numerical modelling methods and tomographic inversion algorithms. The section of fundamental theory shows basic formulae and principle of DC resistivity exploration. The section of electrode arrays summarises the previous study on all traditional-electrode arrays and recommends 4 electrode arrays for data acquisition of surface ERT and 3 electrode arrays for cross-hole ERT. The section of numerical modelling demonstrates an advanced version of finite-element method, called Gaussian quadrature grid approach, which is advantageous to a numerical simulation of ERT for complex geological models. The section of tomographic inversion presents the generalised standard conjugate gradient algorithms for both the l1- and l2-normed inversions. After that, some synthetic and real imaging examples are given to show the near-surface imaging capabilities of ERT.

Keywords: resistivity, electrical current, geotomography, numerical modelling, subsurface imaging

## 1. Introduction

Direct-current (DC) resistivity exploration is a traditional geophysical method. It employs two electrodes to inject electric current into the ground and other two electrodes to measure the electric potential difference. The measurements are often carried out along a line or in an area on the earth surface, and then the observed potential differences are converted into sounding curves or pseudo-sections of apparent resistivities, which indicate the resistivity changes of subsurface rocks. Analyses of these data enable us to find the underground resistivity anomalies or outline the subsurface geological structure. With development of computer technology and numerical computational techniques, accurate numerical simulations of subsurface electrical field and acquiring a large amount of data in fields become possible [1–3], so that the traditional DC resistivity exploration was developed to a computerised geotomography technique, called electrical resistivity tomography (ERT), which employs a multielectrode equipment or system to automatically acquire a large number of data [4, 5] and applies a computer software to the

reconstruction of subsurface resistivity structure with the observed data [6–10]. Due to its conceptual simplicity, low equipment cost and ease of use, ERT is now widely applied in mineral exploration, civil engineering, hydrological prospecting and environmental investigations, as well as archaeological mapping [11]. This chapter provides an overall review of ERT techniques, which consists of four sections: (1) fundamental theory, (2) electrode arrays, (3) numerical modelling and (4) tomographic inversion. In each section, diagrams and formulations are used to illustrate basic concepts and principles of ERT techniques. Some synthetic experiments and practical imaging applications are also given to show the imaging capability of ERT.
