Fig: ERT image section showing subsurface rock types, its location and depth.

2D and 3D Electrical resistivity tomography (ERT) also known as electrical resistivity imaging (ERI) is a geophysical technique for imaging sub-surface structures from electrical measurements made at the surface. It is closely related to the medical imaging technique and mathematically is the same inverse problem. Due to many advantages as given below, ERT is being used widely worldwide for solving various problems.

ERT is being used in Nepal for:

  • Natural resources (mineral) prospecting
  • Hydro Electric projects
  • Geothermal prospecting
  • Ground water resource potential assessment
  • Geological mapping
  • Landslide Study
  • Bedrock quality
  • Geological structures
  • Archaeology
  • Mapping & monitoring of ground water flow and leakage at hazardous waste sites
  • Mapping of frozen ground/permafrost
  • Geotechnical investigation

Electrical resistivity depends upon several parameters like water content, temperature, ionic content, particle size, resistivity of solid phase, permeability, porosity, tortuosity, pressure, clay content, etc. (Gueguen & Palciauskas 1992).

Electrical Resistivity Tomography data acquisition system acquires a series of voltage and current measurements from surface (and under water) electrode arrays. The extensive data resulting from such measurements are processed to produce electrical resistivity

tomographs using state-of-the-art inversion algorithms. These tomographs show spatial variations in electrical resistivity. The tomographs show the location and shape of electrical resistivity zones, which can be used as a guide for focusing more detailed characterization and monitoring evaluations.

The modern equipments are capable of taking measurements both of Electrical Resistivity and Induced Polarization. These can complete 1D, 2D and 3D data acquisition more accurately, faster, and stores data in its built in memory. Data acquisition using various electrode configurations is possible in short time. The advent of efficient softwares can handle the large amount of data processing and analysis in short time. The development in equipment and data analysis software made ERT a very popular, important tool. It is non-invasive geophysical techniques, less costly, fast and more environmentally friendly etc.

Electrical Resistivity Tomography (ERT) has become an important engineering and environmental site investigation tool. Resistivity images are created by inverting hundreds to thousands of individual resistivity measurements (e.g., Loke and Barker, 1996a, b) to produce an approximate model of the subsurface resistivity.

Contaminant plumes usually have a sufficiently high contrast in physical properties against the host media due to an increase in dissolved salts in the groundwater and a resulting decrease in pore water resistivity; therefore, they may be detected by geo-electrical techniques.

In hydroelectric projects, it is important to know the thickness and constituents of overburden, bedrock depth and its quality etc. These can be mapped efficiently by ERT. Geological features like voids, weak zones, buried channels etc. usually have resistivity contrast with the surroundings and can be detected successfully.

Use of ERT has helped in understanding the subsurface geology, which is time and cost efficient. It provides important input to many civil design works. In many cases it has helped in effective management of further construction works in the project.