The Components Of An ERI System & 6 Unique Applications

A basic electrical resistivity imaging (ERI) or electrical resistivity tomography (ERT) system is comprised of the following components:

 

  • One transmitter that creates an electric field using direct current (DC).

  • One or more receivers.

  • A switching mechanism located at either an electrode takeout or a centrally-controlled passive electrode cable system.

  • Electrodes and electrode cables.

 

ERI and ERT are more or less the same thing, but ERI is used more commonly in land surface surveys and ERT is used more in borehole surveys. ERI systems can be modified and applied in a number of different ways. Below, we’ve detailed six of them.

1. Borehole-To-Borehole Systems

Boreholes are dynamic environments that require rugged, specialized equipment that can handle high pressure, varying temperatures, water, and more.

 

There are two types of boreholes: open (rock-edged) and cased (lined with material).

 

  • You can conduct an ERT survey in either borehole, except in those encased in metal—as that would short out the electric measurement.

  • If the borehole is encased in PVC, it must be perforated or slotted along the whole length of the lining to allow for the electrical current to exit the borehole and go out into the rock formation.

  • The electrodes in the borehole must make connection to the formation. This may happen if the borehole is filled with water, which establishes electrical contact.

  • If the borehole is dry, connection to the formation can be established by filling the borehole with mud. The mud should have a resistivity that mimics the formation as closely as possible, and also helps prevent collapse of the hole.

  • To prevent corrosion of the electrodes through electrolysis, non-corrosive graphite electrodes should be used.

2. Manual Measurement Systems

 

In a manual measurement system, you have four cable reels with single conductor wires: one for each of the four electrodes needed for each measurement. For a manual survey, you typically place each of the four electrodes and take a single measurement, and then move them and take another. This process repeats until you have all the data you need.Of course, this is a very tedious, time-consuming process.

 

One of the applications that uses a manual system is Vertical Electrical Sounding (VES):

Vertical Electrical Sounding

 

The purpose of VES could be to locate water in the subsurface. It’s an inexpensive but laborious process:

 

  1. First, you pick a center point and expand your four electrodes around it. The electrode sequence should be A, M, N, and B. A and B are the current electrodes, M and N are the potential electrodes. The center point should be located midway between M and N. (You can use the Wenner or Schlumberger electrode array for VES. Schlumberger is recommended.)

  2. Using the Schlumberger array, move A and B while keeping M and N stationary. Start with small spacing—maybe a meter from the center point—and then move A and B out symmetrically.

  3. Each time you stop, you take a measurement and graph it on a logarithmic X-Y plot  (where the X axis is A/B distance divided by two and the Y axis is apparent resistivity).

  4. As A and B expand, you’ll be exploring deeper into the formation. At some point, the size of the A/B dipole in relation to the M/N dipole becomes too large, so the data becomes noisy. At this point, you expand M/N out while keeping A/B in the same location.

  5. You then continue to expand A and B until you lose signal again. Proceed to perform steps 4 and 5 repeatedly until desired depth has been achieved. The larger the A/B dipole becomes, the deeper in the ground your measurements will be.

3. High-Power Mineral Exploration System

 

The components of a high-powered mineral exploration system would consist of the following:

 

  • An electrical resistivity instrument with a built-in receiver and transmitter. This would likely be the SuperSting instrument interfaced with the PowerSting. This is offered in three models—5 kW, 10 kW, or 15 kW.

  • A large A/C generator. The 10- and 15-kilowatt PowerStings require custom-built three-phase SGS generators, and a 5-kilowatt PowerSting can use an off-the-shelf 220-volt generator.

 

While mineral resistivity and  induced polarization (IP) data can be collected manually, it’s less time-consuming to do an automatic ERI survey with combined resistivity and IP. To perform this imaging survey, you’d do the following:

 

  • Set out your electrode stakes in a line and connect to an active or passive cable.

  • Lay out a number of power-node-transmitting electrodes next to the active or passive cable, which are designed to handle the PowerSting.

  • Perform the imaging survey to get a scan of the ground.

  • Invert the data using the EarthImager 2D software to get a resistivity and/or induced polarization model of the ground.

 

When all of this is complete, you’ll have the true resistivity and IP values of the ground, along with the dimension of the mineral deposits.

4. CRP (Continuous Resistivity Profiling) Marine

 

To perform continuous resistivity profiling in a marine setting, you need the SuperSting R8, an electrode cable (called an electrode streamer), and the Marine Module with a Wi-Fi adaptor box.

 

CRP is performed using the following steps:

 

  • An electrode streamer with graphite, noncorrosive electrodes is towed in straight lines behind a boat in bodies of freshwater or saltwater. The streamer can either be towed right above the sea bottom or at the surface, depending on the depth you need to image. If images must be taken of the very deep seafloor, the ROV and the SuperSting OBEi1 are both required. The ROV also needs its own 24-volt power source.

  • Every 2.8 seconds, eight measurements are taken—so the instrument takes measurements continuously as the electrode streamer is towed forward.

  • The data gathered from these measurements is inverted using the continuous resistivity profiling module.

  • The Wi-Fi adaptor box sends the GPS NMEA stream to the SuperSting for positioning data, depth reading, and temperature reading.

  • The Android application displays the data in real time as it’s being collected—and if the GPS on the depth-finder experiences signal loss at any point, the Android device takes over until it’s recovered.

5. CRP (Continuous Resistivity Profiling) Land

 

You may need to perform continuous resistivity profiling in a land setting if your end goal is to:

 

  • Perform a soil salinity study.

  • Examine how much agricultural land has clay, silt, or sandy soil.

  • Perform an archeological electrical resistivity survey.

  • Lay an oil or gas pipeline.

 

Electrodes are towed behind an all-terrain vehicle (ATV) in a start-stop fashion that allows for many measurements to be taken. It allows you to build a very continuous resistivity profile but is very shallow—and only allows you to see a few meters below the earth’s crust.

6. Remote Monitoring

 

To monitor an area remotely to examine resistivity over time, you need the following components:

 

  • A SuperSting.

  • The remote module.

  • An active or passive electrode system.

  • A computer that acts as a server at the location which is connected to a persistent internet connection (like an LTE modem).

  • A/C power.

 

The remote monitoring system utilizes a controller that cycles two sets of deep-cycle batteries at the location. The battery that is used for measurement is totally isolated from the charger and A/C system during measurement in order to achieve low noise interference. It can monitor resistivity changes and induced polarization (IP) changes.