Field Survey Setup FAQ
Which array should I use? (super secret hint: Always use the Dipole-Dipole AND the Strong Gradient Arrays ALWAYS ALWAYS ALWAYS!)
Doing surveys in the rain is both a safety hazard for persons operating the equipment and most often leads to noisy data. Here is why you don't want to perform surveys or play golf in the rain for that matter!
1) the risk of electrocution from batteries, generator (120/220V 50/60Hz 10A) or power cables, resistivity cables (400V/2A), the instrument or anything near these items which are likely wet and positioned on wet ground !!!
2) getting struck by lightning somewhere along the cable which attracts lighting and channels the strike back to the instrument which is often where operators will be standing!
3) damaging equipment and/or collecting noisy data. Rain percolating through the near surface may cause enough charge imbalance in the ground to create significant SP noise compared to non-rain days
Besides the obvious safety hazard we have to think about keeping the connectors clean from any moisture because they must handle both 400V and 2A transmitter signals on the same wires (passive cable design) and then handle mV level receiver signals on connector pins which are millimeters apart. Dirt and moisture create small pathways for electrical leakage which can contaminate surveys with noise. Contact AGI for instructions on cleaning (never use a liquid cleaner without special instructions), filtering noise, dealing with weather issues or any of the other items mentioned in the FAQ.
The depth of penetration is dependent on the distance we separate the transmitter and receiver electrode. Is there a limit to how deep I can see? Yes, as the transmitter and receiver dipoles are separated by an increasing distance the signal strength lessens and approaches the background noise. The depth of penetration has been emperically found to be around 18% that of the distance separating the two dipoles. Please see the very scientific paper "A Modified Pseudosection for Resistivity and IP" by Edwards (1977).
Wood fences will not introduce conductive pathways that could affect the survey. Be aware that many older wooden post fences may have metal barb wire buried along the ground. When the farmer/rancher fixes the fence, they will often leave the remaining wire on the ground and it may not be visible any more.
Conductive objects such as metal fences can affect surveys with an unaccounted pathway for the electricity to travel. Simply being conductive will not cause an issue. It is important to consider if the object is grounded well? As example, is the metallic fence post isolated from the ground by a thick concrete foundation? This can be tested by measuring the resistance from the metallic fence to the ground using your resistivity meter and an electrode stake. A simple 2 point test/contact resistance test mode can be used (V/I).
Perpendicular or Parallel: Other factors are how close are these objects to the survey area and are these linear objects (fences and pipes) perpendicular or parallel to the survey line? Measuring perpendicularly to a fence may only introduce a very shallow anomaly at the point of intersection.
Buried drill stem from old (an unknown) oil wells can cause problems if an electrode stake is pounded into the ground and touches or is very close (1 electrode spacing) to the drill stem. The drill stem will effectively become part of the electrode stake however the inversion software will not take in account the new longer electrode without a-prior model parameters.
It depends on what depth you need to image. A rule of thumb would be to divide the depth you need by 0.15 (note: 15% of spread length).
i.e If you needed to image 12 meter in depth we would need 80 meter in length to lay out your electrode cable.
Calculation: 80m = 12m/0.15
This 15% spread depth factor can be significantly expanded by utilizing remote/infinity electrodes
Surveys can be a few inches/centimeters to several miles/km long. Often lab studies are done on scale models such as water tanks with electrode spacings of a centimeter. Deep mining and Geothermal studies have electrode spacings of 100m or more.
You should always collect Dipole-dipole and the Strong Gradient Arrays to reduce filtering out important features in the ground. There is a very good reason for this. Electrical imaging surveys are made to explore the subsurface as completely as possible. This means that the equipment must measure both the potential field in between the two transmitting electrodes as well as the potential field surrounding the two transmitting electrodes. To simply measure only one data point for every transmission point in itself a filter to the data. We expand on these topics in the seminar and present the latest arrray optimization research. The Dipole-dipole array should be the primary array for any high resolution survey. Adding Strong Gradient (includes wenner and schlumberger and gradient in a faster multi-channel sequence) assures complete data coverage and higher signal levels which helps when some dipole-dipole may not be retained in the final model due to misfit filtering/noise. The Strong Gradient is a much faster array to collect (80% faster) than the dipole-dipole array and has higher signal levels, but lower resolution. In general, it is good insurance to collect the Strong Gradient in a merged or hybrid command file of dipole-dipole+strong gradient. Contact AGI for instructions on loading this array type.
Mathematically we cannot calculate the response of the ground at any better than one half of the electrode spacing. As example, if the electrode spacing for a particular measurement is 5 meters, the best possible (neglecting any noise) resolution is 2.5 meters. Does that mean that my resolution will be 2.5m at any depth? No, it means that we have to look at the "size" of the receiver dipole. To get better depth penetration, the transmitter and receiver dipoles are separated until the point where no signal can be recorded accurately (this is usually 8 dipole separations, in 2D, hence why we have an 8 channel SuperSting instrument). At this point we increase the receiver size to increase the signal and continue to separate the transmitter and receiver dipoles. However, increasing the receiver dipole size also decreases the resolution (remember that we can only resolve features in the ground at least half the size of the receiver dipole).
Why would I want to use Wenner? You never want to use Wenner (unless you are doing an ASTM G57 Soil Test) because your receiver dipole has to be 1/3 the size of the transmitter which means your resolution will be very poor. It is an often repeated myth propagated by other instrument manufacturers of lesser quality that make equipment that simply cannot measure the small signals generated by the Dipole-Dipole required to effectively image the subsurface. A good test when evaluating any electrical resistivity imaging instrumentation is if the featured case studies are using the Dipole-Dipole array, or better yet, the dipole-dipole+ Strong Gradient array or just the Wenner. Wenner only = low sensitivity receiver and a bad instrument. Wenner also tends to artificially flatten all responses which makes everything appear to have a layer cake like geology or flat horizontal bedding plane. As you can imagine, this is not going to be good for geotechincal surveys or any survey focused on finding complex features like fractures, voides or variations in geologic materials related to hazards, water or mineral content. Further, the wenner and also the schlumberger array will force an advanced and expensive multi-channel instrument into a 1 channel instrument which will take 8X longer on the SuperStingR8 for example. This is ~20min vs 2.6 hours on 1 line.
Yes, you must always account for topography (variations in elevation), but only if it is greater than half the electrode spacing. Well that doesn't make sense? It goes back to the fact that we must input the exact location of each electrode or the inversion calculations will introduce positional error in the final solution. What if I'm just doing a 2D line? Even then you will need to make the terrain correction. What if I have really uneven terrain close to the line in all directions? In that case you will need to invert the data with terrain using the EarthImager3DTM Inversion modeling software.