Electrical Resistivity Tomography

Electrical resistivity tomography (ERT) is a well-known subsurface-imaging geophysical technique and is widely applied to mineral prospecting, hydrological exploration, environmental investigation, and civil engineering, as well as archaeological mapping.

With the advancement in ERT from 1D to 2D and nowadays 3D, ERT has explored many areas. The applications of ERT include fault investigation, groundwater table investigation, soil moisture content determination, and many others. Let us jump into the blog to get some more knowledge on ERT and its applications.

Hers is images of  Electrical Resistivity Tomography

               WERT-60/120/4B 2D Resistivity Imaging System                                                           WGMD-9 2D/3D Resistivity/IP Imaging System

 

Where is ERT used?

      It is an electrical testing method where current is induced in the ground using two current electrodes. Here are few frequent areas where ERT is used,

• ERT tomography can be used to map geologic variations, including soil lithology (e.g., clay versus gravel), presence of groundwater, fracture zones, variations in soil saturation, areas of increased salinity, or, in some cases, groundwater contamination. 

• ERT tomography can be used to map bedrock depths and geometry, though MASW or SRT are better suited for mapping top-of-bedrock in most geologic settings. 

• ERT is often the best suited for mapping cavities such as caves and evaporation of dissolution sinkholes. Like seismic, the electrical method has the capacity to get 1 D (vertical electrical sounding), 2D (profile), or 3D (volume) imaging.

What is the basic principle of ERT?

   ERT is a geophysical technique in which DC electrical current is injected into the ground between one pair of electrodes and the measured voltage. A line (array) of electrodes is used, and an instrument called a terrameter acts as a switch box and a measuring device, sending energy to different sets of electrodes through a set sequence.

How to take an ERT survey?

   AS you are aware of the principle of ERT above, consider you are going to take an ERT survey. Here are the steps you should proceed with,

• Choose the spot on your site where you want the deepest measurement – that’s the mid‐point.

• Move the terrameter and its car battery source to the mid‐point and layout the cables away on both sides ( To know what a Terrameter, explained below).

• Push an electrode into the ground at each contact on the cable and join the cable connections to the electrodes using jumper cables.

• Select the set‐up on the tetrameter and start the survey,

• The depth of penetration of the ERT depends on the type and length of the electrode array.

• The greater the depth, the less the horizontal and vertical resolution. 

• Need space for the survey. 400 m is long, and it’s not always possible to lay it out at a given site.

What are the significant advantages of ERT?

• Adds a second dimension to frozen ground conditions to a single borehole measurement.

• It’s possible to create a 3D image using parallel survey lines.

• Non‐invasive technique: no disturbance and all equipment removed.

• A single person can set up and undertake a typical survey in half a day, but several surveys per day can be conducted with more workforce. Equipment can be carried in roughly four person‐loads.

• Inexpensive to undertake an ERT survey once equipment purchased

• ERT surveying is easier to learn and more accessible to interpret than GPR.

Is the ERT technique an easy one to undergo?

  It is not debated between easy or heavy, as every imaging technique is quite hard to get results. However, ERT is not new, and also the instrumentation and software needed have become much more user-friendly. Added to the advantage, it is also cost-efficient.

What is Terrameter and its uses?

      The instrument used in geophysical exploration is to measure electrical resistivity primarily in groundwater. Terrameter has been a popular device for electrical resistivity measurements in groundwater & some other geochemical investigations, especially in ERT resistivity surveys.

  Though ERT is a cost-effective way of surveying surfaces, it has certain limitations where it can provide you only the resistivity you have to make all the interpretations.