Earth Resistivity

[Jim in Idaho]

I thought some of you guys (or gals) might find this interesting. To go back about 4 years...I started looking into resistivity because of the 2.2lb hunk of mearly pure copper I found in northeast Nevada. I'm still convinced there's a large lode there somewhere. So jump to now, and I had mentioned to an online friend last fall that I had used Resistivity down there, and had some, but limited experience with, but I'd built a power supply, and a system to work with a free 2d software package. He asked me to come up to the Helena area where he has several claims, and teach him how to use the system. So, Friday Chris and I loaded the Casita andm headed north. Saturday we went to one of his claims in the Marysville area. The metals, mostly copper and gold, in this area come either from quartz veins or are ancient pockets of sedimentary deposits. We wanted to determine if the oxydized copper we were finding on the surface came from a quartz vein, or were pockets of debris from farther up the hill. The image attached is what the software kicked out after we took 48 readings. Those 48 readings took the 4 of us about 6 hours to do, and then I spent another couple of hours doing the math so I could input the data in a form the software would accept. We only went to a depth of about 3 meters, as were trying to determine if there were rising quartz veins below the oxydized copper they'd been finding. In the image, the light blue is the low-resisitvity rock, and is what we want to see. The green/yellow is broken rock and overburden of various densities, and the orange/brown and deep blue/purpler is the undisturbed bedrock. You can see the pockets containing the oxydized copper they'd found, and you can also see they don't extend below about 5' of depth (1.5 meters). We covered about a 50 meter distance along an access road cut into the hillside.

 

[GB_Amateur]

Your plot appears to show us the depth range of 1.17 m to 2.3 m.  Does your detection device show anything from surface down to 1.17 m?  Also, does this measurement now answer your question as to how the copper got there?

[GotAU?]

This is an interesting paper on how it was used for a gold mining operation- they used it to find additional potential lode deposits near an inactive mine by taking measurements along three parallel lines, and were using quite an expensive professional grade unit with multiple sensors arranged in an array- much like the geophones used for seismic studies.  
https://www.scielo.br/j/remi/a/NP37ZfKM4yFkFrLYtCXSq8c/?lang=en#

I guess in your case Jim, we’re you running a line by hand with just 2 probes?   Could you share a bit more about your setup?

[Kostas 13]

I bought a professional resistivity meter to locate caves, while the principle of operation is relatively simple, the interpretation of the readings requires maps of the geological area and knowledge of the resistivity of the materials found underground. my opinion is that in order to have a good interpretation of the data there should be a professional geophysicist or we should have intensively dealt with this method at a theoretical and practical level otherwise we risk reaching the wrong conclusions.

[Jim in Idaho]

57 minutes ago, Kostas 13 said:

I bought a professional resistivity meter to locate caves, while the principle of operation is relatively simple, the interpretation of the readings requires maps of the geological area and knowledge of the resistivity of the materials found underground. my opinion is that in order to have a good interpretation of the data there should be a professional geophysicist or we should have intensively dealt with this method at a theoretical and practical level otherwise we risk reaching the wrong conclusions.

I've been using this for several years now, and find it a useful tool in my prospecting. Is it perfect?...No. But, I see no need to have a trained geologist on hand, though in this particular case, I was training a mining engineer.

Jim

 

[Jim in Idaho]

7 hours ago, GB_Amateur said:

Your plot appears to show us the depth range of 1.17 m to 2.3 m.  Does your detection device show anything from surface down to 1.17 m?  Also, does this measurement now answer your question as to how the copper got there?

There are issues with depths of less than a couple of meters. Mainly caused by surface distortion of the electrical field, and surface moisture. It did answer our question as far as we now know there are no conductive intrusions within the search zone. So, either the metal indicators were deposited in those place where found, or they may have eroded from an uphilll source, and moved down the hill. There are several prospect pits uphill of where we did the test. That particular spot was never commercially mined...just prospected. But it does have indications of copper.

Jim

[Jim in Idaho]

One thing I forgot to mention, GB...you can go as deep as you want, providing you have enough voltage to drive enough current through the current probes so you can see a voltage at the voltage probes. I usually use what's called a Dipole-Dipole array. in that array you have two sets of probes. The distance between the probes in each set have to  be the same. One set you apply power to. The other set you read the voltage across. The distance between sets determines the depth you are reading. With this type of array, the depth is equal to half the distance of the center-to-center distance between probe pairs. We used 2 meters and 4 meters, and 1 meter between probes in each set. Add 1 for the 1/2 meter to the center, and we had 3 meters and 5 meters. so depth was 1.5 and 2.5. The software massages it a little so we ended up with an indicated 2.36 max. We could have done another layer at 6 meters, but were worn out, and thought we had enough info to determine what we wanted. Doing this is boring as hell, and a lot of work, if you don't have a fancy commercial unit. And, as you know, I like to build my own stuff.😁

Jim

[Jim in Idaho]

9 hours ago, GotAU? said:

This is an interesting paper on how it was used for a gold mining operation- they used it to find additional potential lode deposits near an inactive mine by taking measurements along three parallel lines, and were using quite an expensive professional grade unit with multiple sensors arranged in an array- much like the geophones used for seismic studies.  
https://www.scielo.br/j/remi/a/NP37ZfKM4yFkFrLYtCXSq8c/?lang=en#

I guess in your case Jim, we’re you running a line by hand with just 2 probes?   Could you share a bit more about your setup?

LOL...I'd love to have one of those 3d units. I could buy a 3d version of the 2d software I'm using, but too spendy for this poor Idaho guy...Ha! The engineer I was training can do 3d CAD, so by running lines at 90 degrees can input the GPS results and build a 3d image. I was using 4 probes. Two pairs of two. I chose a spacing on the probes in each pair as 1 meter. The distance between the inner probe of each pair should be an integer value of the probe spacing. I started at 2 meters, and did a layout for 4 meters and 6 meters, but we never did the 6 meter in the field. I only had the 4 probes, so we had to keep moving the probes as we went along the 50 meter line. very slow, boring work. It would be better to have 55 or so probes and install them along the line and then just go from one to another, according to the desired layout. That would be faster, but it's still a slow process. When I was doing it in Nevada I was down there for 8 days, and working 8 hours/day, and then inputting data in the evening. The more modern (but expensive) units use a separate cable to each probe, and then you push "start", And it automatically, and electrically goes from probe to probe collecting the data....much nicer....LOL

Jim

[Jim in Idaho]

For those of you interested, here's an image of what the dipole-dipole array looks like. We were doing our readings at 2 meter intervals, so 25 readings at each depth along the 50 meter line. The formula for "apparent" resisitvity is as follows:

Pa= V/I*Pi*n*a*(na+1)*na+2)

Jim

[jasong]

Interesting stuff. What software are you using? Are you basically doing everything by hand by taking manual readings one at a time and then moving probes, repeating, and then inputting all the readings into the software at the end of the survey?

I've watched the commercial resistivity surveys from a distance on a project I optioned to an exploration company, but never really understood everything going on. One explorer lit a mountain on fire in AZ when something shorted out with a probe or something, which is probably why I was only let to watch from afar...😅