Groundwater search in North-Africa, which technique?

[Waterman]

Dear specialists,

I am interested in helping(non-profit) farmers in North-Africa (Morocco) to find drilling well spots more efficiently. Now they mostly use dowsing rods to find spots to drill. I have read about different techniques and devices to prospect/measure water aquifers.
The depth ranges from 50 to 200 meters, mostly they find water around 130/150 meters, sometimes salty. The soil is mostly dry sand/rock.
What is the best technique? Is resistivity the best option? It should be easy to operate and not expensive.
I have found some devices and would like to have your opinion as professionals.(I am just a medical doctor with some phisics knowledge)
- PQWT (for example the s500 model)
- GER River G
- Syscal system from IRIS

Please help me on my quest and many thanx for your reply!!

Greets
Noa

[geophix]

I would choose one with an active energy source instead of relying on passive sources. It feels more reliable. So I would not choose PQWT. I have used a Syscal system from IRIS so I have confidence in it for shallow investigation (around 50 feet to 100 feet), but I don't know whether it has corresponding cable lengths and power sources for deep penetration. I don't know how GER River G works, but it looks like it's designed for easy use for water exploration.

[Waterman]

Thank you for your reply geophix!

Is there someone else who has experience with any device on the depth of 50-200 meters?

[markg21ca]

Speaking to your comment about the possible salinity of your target, there could be a significant enough of a resistivity contrast between the aquifer and the surrounding material to consider adding EM soundings to your short list of methods. There are plenty of easy to use instruments out there that will do the job (i.e. Geonics) at the depth range and with the target geometry you mention.

[99thpercentile]

There are a number of legitimate geophysical methods to look for groundwater, and a number of fraudulent instruments sold as water detectors. There is no such instrument as a water detector, so if someone tries to sell you something called a water detector you should run away quickly.

I have years of experience doing this and there are a handful of methods that work, but there are several questions to ask first. Do you want to put a well at a specific location, if so you may either not be able to drill deep enough if there is water there or it may just be a bad location. The correct approach is to have a general area, say a 5 to 10 mile radius, then follow the standard workflow to pick the best location. You start with remote sensing methods such as Google Earth Imagery and geologic maps.

Then after deciding on a smaller area, you probably want a screening tool like mobile time domain electromagnetic induction (TDEM). The Loupe instrument can image to about 25 m, while the Aarhus Geoinstruments tTEM can image to 100 m. If you can refine your locations from the screening survey, you may need to follow up with electrical resistivity tomography (ERT).

It is easier in much of the USA as many states have a database of all water wells. This information can be used to estimate locations and depths for new wells.

[Waterman]

Thank you very much 99th!
I need to search specific plots of land (personal of the farmers) mostly ranging from 1 hectare to 10, average 2 hectares.
So you say a combination TDEM and ERT.
I see that the Loupe TDEM has a range around 20-30meters but the water level is mostly around 120-140 meters.

Would you please be so kind and helpful to give advice in brand and type of the devices. Price needs to be between $4000 and max €8000 for the project to be a succes.
Again Thanx for all your help guys!

[99thpercentile]

Based on your rather small budget, I would suggest looking for a used very low frequency (VLF) instrument such as a Geonics EM-16 or ABEM Wadi. VLF can be a powerful screening too looking for conductive targets at depth or a linear conductive anomaly. The VLF method can cover large areas quite quickly.

The next tool to get based on your limited budget is a used vertical electrical sounding (VES) instrument such as an ABEM SAS-300C resistivity instrument. Any other suggestion will be out of your budget very quickly. A VES survey can tell you if you have changes in electrical resistivity with depth that may indicate the presence of water.

[searcher]

Considering your soil types, Ryan may be exactly right about using EM as a screening technique. But, I know of many NGO's using basic resistivity measurements for successful water well locating in Africa and Central and South America. It can be labor-intensive at the basic level, but as a proven technique, it's hard to beat in a place where labor is relatively cheap. You can field a very portable, reliable single-channel instrument like the L&R Instruments Ultra Minires for less than US$7,000 plus the cost of wire. It's a great instrument for those environments because it has a really high front-end impedance and that allows it to make measurements in difficult soils.

It takes time and effort to collect a data set at a single measurement at a time but it's good value. Such an instrument is capable of making accurate measurements to depths of 100's of meters and the data can be pushed into fairly sophisticated 1D or even 2D inversion programs if you want to take the time to do it manually. It kind of comes down to how much value can you squeeze from the investment in capital equipment plus available on-site labor resources. I am not an agent for L&R, but I've owned 24 Minires units over the years and can attest to their accuracy and reliability.

[kaminae]

Africa is very different in grouding conditions. I would recommend not a method but combination of two methods.
First method should be fast, not sensitive to grounding conditions and cover depth range of perspetive horizons.
It will be used to map perspective areas. Let's me suggest TDEM (central/coincident loops) with loop side 50 meters.
When all area is mapped with TDEM and you can see perspective areas you can map details with help of another method. It is very
recommended(Vozoff) to involve VES in such investigations. VES is very flexible method and does not require in expensive
instruments. Combined interpretation of TDEM and VES data for zone of interest will give you realible and detailed vertical
profile of perspective area.

[99thpercentile]

I agree with the comment above about combinations of methods. That is always the best approach to take in geophysics. I also agree with the comment about basic vertical electrical soundings (VES) being useful. If you have little money but lots of time, VES is very cost effective. I also suggested VLF above as it can be a great screening tool to guide where to do your VES surveys.

You are correct that the depth of investigation of the Loupe instrument is only 25 m. The much more expensive TDEM option is the the TEMcompany (previously Aarhus GeoInstruments) tTEM towed sensor. It can image much deep, possiblely to 130-180 m depending on the configuration. The TEMcompany in cooperation with several non-government organizations (NGO) and Aarhus University have been performing lots of surveys in Africa for groundwater exploration.

https://www.temcompany.com/

They also have a new instrument called the sTEM, that is just a simple TEM sounding device that can do soundings in 10-15 minutes per location so it is faster than VES, but I'm not sure what the price is.

[kaminae]

Ryan,
1)Indeed VES can be time consuming but it may be used only for intresting zones mapped by TDEM. Moreover combination of DC and EM methods means equivalence suppression and more reliable models.
2) There are a lot TDEM instruments on the market, more or less expensive.