How To Conduct An Electrical Resistivity Survey In 9 Easy Steps
Electrical Resistivity : 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 supporting. 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 to space, you have to examine the mathematical relationship between the sensors, the depth, and the area of your site.
1. 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.
2. 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.
3. 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.
4. 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?