Contact resistance Problem in Resistivity survey explained.

It is obvious sometimes when we carry out electrical resistivity surveys in the field especially in complex and dry geological terrain to be faced with this problem. You can detect this issue when resistance reading in your measuring device becomes very high. Unfortunately if the issue is too serious sometimes it may cause the survey to stop for a while. It is the electrode contact resistance problem that I want you to know about. I want to share my experience with you through this post regarding the basic causes, how it affects electrical resistivity results and how we can overcome this problem in the field.

Without wasting time Let's move straight to the main point.

What is Contact resistance?

Electrode Contact resistance can be defined as the situation at which there is significant increase in measured electrical resistance at electrodes due to presence of poor conductive materials in the subsurface during electrical resistivity surveys.

As explained above, electrical surveys require direct plugging (injecting) of the metal electrodes into the ground. So the materials which are directly in contact with metal electrodes in one way affect the overall passage of electric current into the subsurface.

What are the causes of contact resistance Problem?

Although there are variety factors that may contribute the rise of this problem, but the following is mostly assigned in various electrical surveys,
Poor conductive geological materials such as dry sediments like sand, boulders or even icy materials. These materials will reduce conduction of applied electric current into the ground, as resistance varies inversely proportional to the current, this may cause significant rising of resistance at measured points.

How do you overcome contact resistance problem in the Field?

Inorder to Overcome this Problem, you can simply applying one or both of these method,

Wetting of electrodes: Here we have to ensure the electric current is smoothly passing into the ground, so applying a solution of water mixed with salts or bentonite at current electrode points. Water mixed with salts is used because salts ions trigger the increase of electrical conductivity. Also since bentonite is the type of clay, as mostly clay contains much water so has high electrical conductivity, then resistivity is reduced.
Parallel connection mode: Since the rule of thumb in parallel connection mode of resistance is that the value of effective resistance is lower than values of each individual resistance in that connection. In order to reduce an overall resistance, we have to connect another electrode parallel to each current electrode.

Let Rx and Ry are the two resistance associated with new connected current electrodes.

R1 and R2 are the resistances associated with original current electrodes.

The connection of new electrodes must be at 90° to the original electrode array. You may look the figure 1 below to get basic insight,

Then, in parallel connection mode,

1/Re = 1/Rx+1/R1+1/R2+1/Ry

Where, Re - effective resistance.

Figure 1: Simple Parallel Mode of current electrodes

All in all, solving contact resistance problems in the field is very important especially when conducting electrical resistivity surveys that are objected to explore sensitive layers that are related with significant resistance variation. So you have to stay arleted in your survey.

Thank you for your time!

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You can check here below the simple lists of equipment and tools that can suit your Project!

BASIC RESISTIVITY EQUIPMENTS and TOOLS

- Electrical resistivity meter

- Handheld GPS receiver

- glass-Fiber tape

- Geological Hammer (The friend of Geologists)

NEED METAL DETECTORS

- Lightweight, High accuracy Metal Detector.