Pelase check this instrument https://www.campbellsci.com/tdr100 and this article: Article Seasonal variations measured by TDR and GPR on an anthropoge...

I think the 4point light 10W from Lippmann company is one of the best systems in the market for autonomous monitoring. The system is very compact and robust, easy to program, and very high resolution at a reasonable price. On top of that, the system has a very low power consumption which makes the system ideal for monitoring, especially for your case given a large number of electrodes that you are thinking to use. I am using it over a very resistive train in Antarctica and the precision and repeatability are quite reliable. Lippmann has recently built very small and compact switch boxes for the system which makes the monitoring setup easier.

for our case in Antarctica, he integrated a timer to the system which turns on the system during each survey to even further optimize the energy consumption. you could find examples of our compact system in this link

https://antermon.weebly.com/

and here is a link to the 4point light 10W

https://www.l-gm.de/en/en_resistivity.html

Dear Adrien

My group and I asked the same your question when, in the 2004, we started planning the ERT time lapse experiment for monitoring saline intrusion in the Venice lagoon area (Chioggia experiment)

In the linked article you find our instrumental solution and the results

Article Monitoring the saltwater intrusion by time lapse electrical ...

Despite the technological advancement in the instrumentation produced by the market in these last two decades, I think more than choosing an instrument, it is necessary to think about the acquisition system as a whole including also the logistics. However, a great work of customization (dedicated development) of the acquisition system will be necessary. We chose to develop a dedicated prototype system. In fact, the requests we made to the manufacturing companies or the solutions they proposed were either not feasible or too expensive as they required development by the manufacturing companies.

To plan a time lapse ERT experiment aimed at hydrogeophysical monitoring, in my opinion, it is necessary to identify

1) The minimum spatial resolution needed and if you want to have a spatial monitoring with different resolution scales. This constrains the number of electrodes and therefore the choice of their management. The spatial sampling also constrains the maximum investigation depth and therefore the current energization (the maximum amount of current injected into the ground). In our system we reached maximum 1.5 A.

2) The typical period of the hydrogeophysical process to be followed will instead constitute a constraint on the acquisition time of the system for performing a tomographic survey. It is clear that seasonal variations will not be a problem but the monitoring of transient (extreme events) or stationary short-period processes become a constraint on the execution time of the measures, which must be reliable, for the whole array.

3) The necessary requests for remote access to the system for: restoration of the system itself, operate the calibration, setting acquisition parameters, data download, pre-processing, visualization. This places constraints on the speed of the system. From my experience it is always better, at least that not strictly necessary, to eliminate for example any preprocessing and visualization unless you have fast embedded systems. Unless we have available and dedicated calculation power, it is advisable to dedicate the machine's calculation power only to the electrode configuration addressing, measurement (also redundant) and storage of the acquired data. For example, the download operations must be performed with a lower priority than the measurement.

4) Energy back-up characteristics which must guarantee the recharge of the energy necessary to carry out the energization and above all to protect the data acquired in the event of system failures. You have to be very conservative and redundant (mirroring systems) in configuring these aspects.

As in 2004, today I don't think that much has changed except for the part of the calculation and transmission speed which is already a good thing. The central problem concerns the choice of the type of system whether with a) diffuse electronics along the cable or b) centralized electronics.

a) Normally an addressable electrode system such as the one proposed before by Mohammad (Lippmann instrument), like that we built for our experiment, has the advantage of the versatility and lightness of the cable. In practice, the electronics are distributed along the cable and are not centralized. This solution allows derivations with light cables (in our case they were only eight poles), which can make possible configurations with variable interdistances and derivations with not too much work. They have the weak point that the electronics that manage the stake can have failures. Therefore attention must be paid to how they are made and their protection from the weather agents.

b) In this case the electronics reside entirely in the central unit, therefore if robust it is less exposed to the effect of external agents. On the other hand, the layout with derivations must be wired and this requires a great deal of work on the construction of a cable dedicated to the experiment. Obviously if the layout is with regular spacing this is not a problem.

Lastly, the problem of stakes should not be underestimated when monitoring for a long time. It is necessary to choose the material of the stakes that tend not to oxidize. Non polarizable electrodes are to expensive. Our experience with stainless stakes, even if positive, required their cleaning and backfilling on a quarterly basis. Cleaning the vegetation, in the stake areas can also be a problem when monitoring lasts multiple seasons. We have experienced that there is an influence in the soil-electrode coupling in case of thick vegetation. Also the protection from animal bites to be considered (a pipeline contining the cables, or resistent cables).

I hope this long answer is useful for you.

Dear Mohammad,

Thank you so much for your reply, I've visited your website presenting the fieldwork you and your team did in Antarctica! This is amazing work, congratulations!

The 4 points light 10W system seems really nice for long term electrical monitoring: we will definitely consider it as an option for our site.

Thank you again

Adrien

Dear Roberto,

Thank you so much for your complete answer, I find it really clear and helpful.

We will consider these aspects following your advice:

1) Spatial resolution (1m and 2m) and number of electrodes (roughly 800),

2) Time resolution (some areas daily and some areas weekly),

3) Remote managing: we will put the priority to measurements as you suggested,

4) Energy back-up: the system will be powered by an electric line

(but we will try to have a back-up energy source as solar and batteries),

5) Type of system: we planned a centralized system but if I understand correctly having derivation boxes could limit the number of cables between the system and the electrodes, (i.e. the price) which could be better for us,

6) Design of stakes: we chose rectangular plates (6cm x 2cm) made of stainless steel, they were buried and protected by PVC pipes (animals and vegetation) as you suggested.

Again, thank you so much for your help,

I should contact you directly for further advice when we will have a better idea of the system.

Adrien

I wish to introduce you and others to the IGIS resistivity meter series from India. Since when I lay my hands on this equipment, I have had reasons to abandon other other hitherto highly rated ones in our lab. Though it is comparably inexpensive, it is really very sensitive and you will love it.

I am referring to a resistivity meter that works in environments like clayey, shalely, and sandstone, where the high profile others failed.

In addition to professor Anthony E. Akpan's response, the referred meter is rugged and can display current along with earth resistance.