How To Conduct An Electrical Resistivity Survey In 9 Easy Steps

1. Select your field site.

2. Decide on an interval for your electrodes.

3. Lay out the tape measure.

4. Hammer in the stakes.

5. Connect the stakes, electrode cable, switch box, and SuperSting.

6. Run a contact resistance test to check that all is connected right.

7. Begin the resistivity survey scan.

8. Visualize the data scan in real time.

9. Turn data into a model representing the subsurface.

1. Select your field site.

Your first step, of course, is to identify where the field study will take place. This may be a local site (i.e., looking for sinkholes at a building site) or a remote location (i.e., looking for the best place to drill a remote well for drinking water).

You may have only seen the survey site from photographs provided by your client or via satellite imagery, but with AGI’s SuperSting, you’ll still be able to deploy your survey quickly.

2. Decide on an interval for your electrodes.

Your electrodes are stainless steel stakes that can transmit currents and measure voltage. To properly conduct an electrical resistivity survey over the area you want to measure, the interval between each electrode is critical (as it is related to the maximum resolution).

So to choose spacing, you have to examine the mathematical relationship between the sensors, the depth, and the area of your site.

Understand that the depth you can see into the ground depends on the length of the electrode spread. For example, you can typically see down 20% of the electrode spread length when using the dipole-dipole electrode array—so if you spread your electrodes in a straight line over a 100 meter distance, you can expect to see 20 meter down.
You need to know whether there’s sufficient contrast in resistivity from the subsurface material to be surveyed. Different material like clay, sand, gravel, and bedrock all cover different resistivity ranges (and some are overlapping). For a successful result, the surveyed feature should have a contrasting electrical resistivity. For example, an air-filled void has extremely high resistivity (since air essentially does not conduct electrical current) in contrast to the typical host rock and therefore makes a good target for a survey.
How deep do you expect your target? Is it one meter below the surface or 100 meters? If something is very deep and very small, it may be difficult to see. A good rule of thumb is that the target can not be detected if its size is less than a quarter of the depth. For example, a target of one meter cannot be seen deeper than four meters depth.
What size of target can you detect? You cannot expect to detect a target smaller than half the electrode spacing. For example, if your electrode stakes are placed at five meter interval, the smallest object to be detected would be 2.5 meter near the surface.

For example, let’s say your target is 10 meters deep. To get the total electrode spread length you would divide the expected depth by 0.2 (or 20%), which is 50. That means you need to spread your electrodes over at least 50 meters to get to the depth you want to examine. An average-sized measurement system uses 56 electrodes , so you would then divide the 50 meters by 55 (which is the number of spaces between those electrodes). This leaves you with 0.9-meter spacing between each electrode, which you then round to the nearest meter to make it easier to deploy using a tape measure. So, for you to see a target at 10 meters depth, you’d need a spacing of at least one meter between each electrode when using a 56 electrode system.

As a final step, you’d need to double check that the target is detectable:

Is the expected size of the target greater than half the electrode spacing?
Is the expected depth to the target no more than 4x the size of the target?

3. Lay out the tape measure.

Now that you know what the spacing should be between each of the electrodes, it’s time to get them in the ground. You should use a very long tape measure—at least 100 meters—to mark off where each electrode stake should be hammered into the ground. (Be sure to use a non-metallic tape measure, as you do not want it to short circuit your electrodes in case it is forgotten on the ground.)

If needed, more precise electrode positions can be measured using a total field station or differential GPS while the equipment is in the process of scanning the subsurface. A scan can take only five minutes or much longer, depending how many electrode sensors are deployed.

4. Hammer in the stakes.

Use a small sledgehammer to pound the stakes securely into the ground.

5. Connect the stakes, electrode cable, switch box, and SuperSting.

Once your electrodes are ready to go, lay the multi-electrode cable along the line and connect each take-out to a stake. This allows the system to activate any of the stakes along the line. Once your cable is connected to each electrode, you’ll connect the cable to a switch box and the SuperSting, which controls the electrodes.

6. Run a contact resistance test to check that all is connected right.

Start the contact resistance test to make sure all electrodes are connected correctly. The instrument will issue warnings for electrodes which are not connected right or not planted firmly in the ground, so that they can be checked before the actual survey starts.

7. Begin the resistivity survey scan.

Once your system has been laid out, you’ll enter the survey filename, electrode spacing, and command file into the SuperSting and hit start for your survey to begin. This can take anywhere from a few minutes to a few hours depending on how many electrodes and what type of electrode array is used and.

For any given measurement during the survey, the system will transmit on only two electrodes at any given time and measure up to eight dipoles simultaneously. These measurements do not have to be adjacent and can be outside or inside the transmitter electrode pair. This is done hundreds to thousands of times, which helps define the electrical field for each transmission location.

8. Visualize the data scan in real time.

You can visualize the measured data on a graphical color plot, called a pseudo-section, in real-time using our app on any Android device. The pseudo-section is used for data quality control and assurance in the field. For a large survey, it’s advantageous to know if something you haven’t predicted has happened. That way, you can change any setting or repair any issues (i.e. connect a disconnected electrode or change instrument settings if noisy data is recorded).

9. Turn data into a model representing the subsurface.

Once the scan is complete, the data is immediately available on the SuperSting tablet. (It is also backed up into the SuperSting instrument, just in case.) You can then use a Bluetooth connection to transfer the data to your laptop while you’re still in the field and run AGI’s inversion software EaarthImager 2D or 3D to get your completed “X-ray” of the ground.

Do you need to conduct an electrical resistivity survey?

Advanced Geosciences, Inc. (AGI) is the leading developer and manufacturer of geophysical imaging systems. Since 1989, we’ve been producing state-of-the-art products for electrical resistivity and IP imaging. If you conduct your electrical resistivity survey using an AGI imaging system, our staff will help you start to finish with free technical support.