See below for some common questions to ask/actions to take before heading out into the field.
Know your field site:
Decide what you need to see in the subsurface and identify the site. Before you start your survey, consider these questions:
- Is the site open?
- If not, that's a problem. You'll need to find a way to access the site or reconsider the feasibility of your survey.
- Are there Obstructions?
- Trees, buildings, water, land formations, metal fences, pipelines, rocks, topography, etc., may hinder you from correctly setting up and completing a geophysical survey. Take note of the obstructions in the area and decide how to work around them.
- Is there pavement that you will need to drill through?
- If you're attempting to survey an area where the surface is covered by pavement, you'll need to think through whether or not it's possible (and permissible) to drill through the pavement. If drilling isn't an option, you may still be able to survey on hard surfaces.
- Do you have enough site access to lay out a sufficient length of cable?
- For some sites, you may not have enough room to pull out the many meters of cable you need in order to survey the area. If you don't have access—or if it's too difficult to get the cable line you need—you may have to reconsider the logistics of your survey.
- Is there surface water that might interfere?
- If you're surveying an area that has surface water, you will need to consider using marine cables instead of (or in addition to) land cables.
- Is there open ground nearby where you can deploy?
- The depth penetration is about 20% of the electrode layout. If the site is 30 by 30 meters, and you need to see down 12 meters, you need to spread out over 60 meters. Therefore, the site is too small to see down to 12 meters. A surrounding area must be used if accessible.
Determine how deep the target is:
It's important to know the depth of your target so you can plan how long your cable needs to be. For an electrical resistivity survey, 20% of the length of a cable is the maximum exploration depth. So, for example, a 100ft cable allows a maximum exploration depth of 20ft.
You'll have to have site access for your cables, so be sure to confirm you have the site access you need to bring in the length of cable required for your exploration.
Decide on an interval for your electrodes:
Electrodes are stainless steel stakes that 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).
To choose spacing, you must examine the mathematical relationship between the sensors, the depth of exploration, and the area of your site.
- Understand the depth you can see into the ground
- This depends on the length of the electrode spread.
- Know whether there's sufficient contrast in resistivity from the subsurface material to be surveyed
- Different materials 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 electricity) in contrast to the typical host rock; it makes a good target for a survey.
- Determine what size target you can 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 intervals, the smallest object you could detect would be 2.5 meters 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 (20%), which is 50. That means you need to spread your electrodes over at least 50 meters to reach the desired depth. 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 meters 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 10 meters deep, you would need a spacing of at least one meter between each electrode when using a 56-electrode system.
Double-check that the target is detectable:
Some final questions to ask are:
- Is the expected size of the target greater than half the electrode spacing?
- Is the expected depth to the target no more than four times the size of the target?