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geowizard
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Electrical Coring ( 16:35:37 WedDec 30 2015 )

Question;

Has anyone ever heard of "Electrical Coring"? :confused:

- Geowizard

  
Fleng
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Re: Electrical Coring ( 18:08:16 WedDec 30 2015 )

It looks like a way to save money drilling. Electrical measurements including resistivity, temperature, and magnetic orientation at the bottom of the drill hole provides information about mineral formations. A few books are available but this paper was a quick read:
electrical coring

Seems to be more effective in the search for oil, gas and coal but it might add value for gold. Since the three most important items are Structure, Structure, and Structure, using electrical measurements at the bottom of the core hole one could reduce the risk of mistaking a transition layer.

This is a good example of revisiting an older technology (1930) with some precision electronics for adding value to your core sampling. Low leakage amplifiers are thousands of times more accurate and far cheaper than earlier.

  
Jim_Alaska
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Re: Electrical Coring ( 20:54:35 WedDec 30 2015 )

I am not sure I understand the benefit. If you have to core drill to get the electrical core reading, why not just examine the core?



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Jim_Alaska
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geowizard
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Re: Electrical Coring ( 04:38:42 ThuDec 31 2015 )

Jim,

On many occasions. I find core holes with no core data.

Drill holes can be logged with a very simple tool. I made a tool using 3/4" diameter pvc with 2" long brass pipe fittings spaced 16" apart for electrodes.

You can measure resistivity of the rock formations. With an added black box - you can measure IP. IP is important because it is a measure of disseminated metal in rock formations.

Fleng hit the nail on the head - as the Ore Deposits 101 series points out - it's all about three things; structure, structure and structure. Gold prospectors can benefit from understanding a few basic concepts that help them identify where mineralization occurs and then correlate mineralization with structure.

Electrical coring concepts are used on the surface too. No drill holes are needed to measure electrical properties of the earth and then place a few drill holes in high grade mineralization.

- Geowizard

  
geowizard
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Re: Electrical Coring ( 14:24:43 ThuDec 31 2015 )

Start with a battery;

Take a 12 volt battery and connect a light bulb.

What happens?

Take the same battery and connect it to a couple of metal stakes driven into the earth... :confused:

Electric current flows in the earth!

Are we having fun yet? :smile:

Now grab your handy-dandy volt meter and...

Drive two more stakes in the ground between the first two...

Clip the voltmeter onto the two stakes and...

Guess what?

There's a voltage produced because current is flowing in the earth!

The current and the voltage can be used to find GOLD.

Move the two current stakes farther apart and the current goes deeper into the earth.

Now, the voltage is related to deeper mineralization!

Move the current stakes farther apart and... you guessed it, the voltage reflects the mineralization deeper into the earth.

This is called Vertical Electrical Sounding (VES).

You can use this method to measure the depth to bedrock and also measure the depth to sulfide mineralization.

It's cheap, easy to do and saves drilling a hole!

- Geowizard

  
Jim_Alaska
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Re: Electrical Coring ( 14:43:24 ThuDec 31 2015 )

That explanation makes sense to my shallow mind Wiz.. Even just being able to determine depth to bedrock would be a big plus.

So, if I understand correctly resistivity is a function of less mineralization, is that correct?



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geowizard
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Re: Electrical Coring ( 16:21:42 ThuDec 31 2015 )

Jim,

That's right. :smile:

At Ophir, the remains of the community are located near the Innoko River. The surrounding area was never mined because - it was after all a community.

The depth to bedrock varies from zero 3 miles up the creek to about 40 feet at the Innoko River.

But, who knows for sure?

We know that "bedrock" is usually the final resting place for placer gold. It makes sense to "want to know" how deep bedrock is!

Get a drill?

or... Do some electrical drilling!

Bedrock has different resistivity than mud!

Water saturated gravel has different resistivity than mud or bedrock.

Frozen ground has high resistivity - current flows in water because of "ions. I learned that in 6th grade when the teacher put two lead electrodes in a tub of salt water and lit up a light bulb when the circuit was plugged into the wall.

But... Frozen water i.e. permafrost does NOT conduct electric current and has a high resistivity. This is a way to find out how deep permafrost is.

Most of the bedrock around Ophir is Graywacky. That is a greenish-gray sandstone-siltstone rock. It has a resistivity of about 250 ohm-meters.

Resistance is measured in "ohms" and Resistivity is measured in "ohm-meters". It's usually abbreviated "ohms".

So, the answer is a resounding "YES". Resistivity is related to mineralization in the earth. As mineralization increases, the earth is able to conduct better- it represents lower resistance. In barren sandstone or limestone, there isn't much conduction or conductivity, so the resistance (resistivity is high.

We are looking for conductive, mineralized ground. Conductive ground has low - very low resistance.

As we separate the battery probes farther, it is possible to measure a contrast between bedrock and overburden.

It's simple geometry to estimate the depth to bedrock!

- Geowizard

  
geowizard
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Re: Electrical Coring ( 16:28:15 ThuDec 31 2015 )

False bedrock?

Clay is a "conductive" mineral.

Conductive means "LOW" resistance or "LOW" resistivity.

Clay forms "false bedrock". GOLD often rests on false bedrock.

Putting all of this together means... we can measure the depth to a clay layer by using batteries, a set of four metal stakes, a volt meter, wire and a little imagination! :smile:

- Geowizard

  
geowizard
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Re: Electrical Coring ( 16:45:51 ThuDec 31 2015 )

Simple Geometry? :confused:

So, run back to the shed and grab your trusty measuring tape...

Measure the distance between your two metal stakes connected to the battery.

Distance between stakes = Depth of measurement (approximately)

I told you this was simple! :smile:

- Geowizard
[1 edits; Last edit by geowizard at 16:46:55 Thu Dec 31 2015]

  
geowizard
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Re: Electrical Coring ( 16:56:09 ThuDec 31 2015 )

Converting "volts" to "resistance";

We measured volts with a voltmeter. (remember?)

We need to measure "current" flowing in the battery circuit. If you have not measured current before, here's how...

Switch your voltmeter to "amps". Remove one of the wire connections at the battery or at one of the stakes and connect the current meter in series. That means the current meter is connected to one wire and a battery terminal or to one wire and a stake so the electric current flows through the current meter.

Run back to the shed and grab your trusty calculator. :smile:

It's a division problem...

Resistance = volts divided by amps.

So type in the volts and divide by the amps and the result is resistance!

There's a little more - but we're getting close.

- Geowizard

  
geowizard
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Re: Electrical Coring ( 17:29:08 ThuDec 31 2015 )

Converting Resistance (ohms) to Resistivity (ohm-meters);

We have four metal stakes.

Two stakes connect to the battery. They are called current stakes. They are labeled C1 and C2. They are also referred to as A and B.

Still with me? :confused:

Two other metal stakes were used to measure volts. They are labeled P1 and P2 for "potential". They are also referred to as M and N.

Above, we came to the calculation of resistance by dividing volts by amps to get resistance.

Resistivity (ohm-meters) uses a "fudge factor" called K.

So... Resistance is multiplied by the fudge factor "K" to obtain Resistivity.

Here's how we obtain the fudge factor, "K";

Measure the distance from stake A to Stake N. That is called distance AN.

Measure the distance between stake A and stake M. That is called distance AM.

Finally, measure the distance between stake M and stake N, That is called distance MN.

Holding your calculator with your left hand and typing with your right hand;

Multiply distance AM times distance AN and divide by distance MN.

Multiply the answer times 2. Then multiply that answer times "pi". Going back to Geometry 101... Pi is equal to 3.14 and change.

You are NOW looking at the required fudge factor. Multiply the fudge factor time the resistance and you have Resistivity!

That didn't hurt too much, did it? :smile:

- Geowizard
[4 edits; Last edit by geowizard at 22:12:23 Thu Dec 31 2015]

  
Jim_Alaska
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Re: Electrical Coring ( 00:40:27 FriJan 1 2016 )

Excellent explanation Wiz. I copied it into a file since this old mind will never remember all the steps. :confused:



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geowizard
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Re: Electrical Coring ( 02:14:31 FriJan 1 2016 )

Jim,

I uploaded a MS Excel worksheet at:

http://www.alaska-gold.com/ves_calc.xlsx

This will run on a laptop in the field.

-Geowizard
[1 edits; Last edit by geowizard at 02:16:08 Fri Jan 1 2016]

  
geowizard
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Re: Electrical Coring ( 03:20:10 FriJan 1 2016 )

For the curious DIY types:

A system schematic:

http://www.alaska-gold.com/ves_system.png

- Geowizard

  
geowizard
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Re: Electrical Coring ( 03:24:27 FriJan 1 2016 )

Still skeptable?

You can see application of a similar system at:

http://www.arctic-geophysics.com

- Geowizard
[1 edits; Last edit by geowizard at 03:25:50 Fri Jan 1 2016]

  
geowizard
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Re: Electrical Coring ( 18:13:34 WedJan 6 2016 )

Discussion;

Why have a "reversing" switch?

The current applied to the ground "charges" up the ground because minerals act like capacitors and the ground retains a certain amount of charge. The charge will dissipate to zero over a short period of time.

Reversing the current has the effect of balancing the "net charge" in the ground.

Is there a danger with high voltage?

Yes. As with many things in mining, high voltage should be recognized when it is used. Safety precautions are necessary when voltages greater than about 40 volts are used. Never touch electrodes or circuit components when the power switch is "on".

Is there a depth limit?

Depth is directly related to the "spread". The spread is defined as the distance between the A and B current electrodes. Increasing the spread, increases the depth of measurement. Note also, the increasing spread will require more current to be applied to the A and B electrodes. The applied current is directly related to the voltage needed to produce the current.

- Geowizard

  
kcm
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Re: Electrical Coring ( 02:22:04 MonJan 18 2016 )

Hey Geo. Just to be clear here, you're using this process in "placer", correct? While there is water that flows through rock formations making it conductive, I can't quite grasp that this might work in lieu of drilling in hard rock exploration - other than possibly in very limited circumstances/information.

kcm

  
geowizard
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Re: Electrical Coring ( 14:54:55 MonJan 18 2016 )

kcm,

Yes, water is conductive. Metals are MORE conductive. So, there is a measureable contrast. This method is used to find water in arid areas. There is a contrast in resistivity between dry sediments and wet sediments.

Minerals and metallic ore bodies have contrasting resistivity when compared to rocks that contain silica. Silica is an insulator. Insulators have extremely high resistivity. Sulfide ore bodies have very low resistivity. The contrast is measurable and can be used to identify rock types and degree of mineralization - in BOTH hard rock and Placer deposits. :smile:

- Geowizard

  
kcm
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Re: Electrical Coring ( 18:51:29 MonJan 18 2016 )

Thanks Geo. Still trying to wrap my head around the process. There's a device I saw advertised a few months ago that seemed similar. But apparently, there are quite a few folks who think it is just a lot of bunk. ...Can't remember the name of it. Some guy out in California is selling the device. Anyway, the concept is still very weak in this pea-brain of mine. :confused:

kcm

  
geowizard
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Re: Electrical Coring ( 20:40:53 MonJan 18 2016 )

kcm,

It isn't my idea. The concept of resistivity has been around for almost 100 years. I worked on these systems originally in 1975 in Kenai. The systems were used to measure resistivity in oil producing rock formations under the Cook Inlet. Schlumberger developed the method of measuring Resistivity in France. The Schlumberger Brothers, Conrad and Marcel toured across the oil producing regions of the US showing oil exploration companies how it could be used to find oil - mostly in oil wells.

In a well, the electrodes are mounted on a fiberglass (insulated) rod that is connected to a wireline cable. The wireline cable conducts voltage like a telephone cable. The resistivity systems manufactured in the past 70 years have electronics that does the measuring and in the case of oil wells, the measurements are transmitted over the wireline to a recording device in a wireline truck on the surface. The wireline truck has a winch and measuring wheel that measures the depth of the resistivity tool in the well. A recording - called a well log is made of the resistivity over a portion of the well that is believed to contain oil. The well log is printed out and interpreted to determine where the most oil is located. Then when the oil well is cemented and cased, the well can be perforated and oil comes out of the perforations and is pumped to the surface.

That's how oil companies know where to perforate oil wells! :smile:

- Geowizard
[1 edits; Last edit by geowizard at 20:42:02 Mon Jan 18 2016]

  
kcm
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Re: Electrical Coring ( 20:56:09 MonJan 18 2016 )

<Still trying to wrap my head around the process.>

Translation: I need to study on it more. Sorry if I wasn't very clear there.

kcm

  
geowizard
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Re: Electrical Coring ( 21:21:13 MonJan 18 2016 )

kcm,

I understand.

Think of a light switch. Have you ever wondered what happens when you get up in the morning and reach over to the light switch and move it, twist it, flip it, and...

The light comes on? :confused:

To many people, it just happens! I flip the switch - the light turns on and I flip it again, the light goes off.

There's a circuit like the one I gave the link to above. The circuit is an electric path that has wires (conductors) made of copper, a metal and those wires are connected to a source of electric power - usually a plug-in.

The plug-in has two pins. Why?

Well, here goes...

The electric power plug has alternating current that flows through the power cord and to the switch. In the "on" position, electric current can flow through the metal switch contacts, to the lamp and back through the power cord to the other pin on the power plug.

When the switch is in the "off" position electric current cannot flow across the open switch contacts because the resistance of "air" is too high for the current to flow.

Metal conducts electricity. Air and Glass including things like silica in rocks do not conduct electric current. Metals and minerals in rocks conduct very well and have very low resistance. The amount of minerals and metals in the rocks can be measured indirectly by measuring the current that can flow through the rocks.

Fun stuff, Huh? :smile:

- Geowizard

  
kcm
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Re: Electrical Coring ( 22:29:32 MonJan 18 2016 )

Ok, let me be even MORE specific - I am trying to relate all of this in my head WITHOUT a background in geology. Electricity I understand. The concept I understand. electricity moving through the Earth I'm not so familiar with. Above, yes. Below, not so much.

kcm

  
geowizard
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Re: Electrical Coring ( 01:31:00 TueJan 19 2016 )

kcm,

OK, so it works the same. The concept of ohms-law works above the ground and now we go into the earth and we are working with a "volume" of rock. The volume of rock has a length and a cross-sectional area. Like a box.

So, normally ohms law is written...

Resistance (R) = Voltage (E) / Current (I).

The units of measurement are "Ohms".

When we speak of "Resistivity", understand that we are talking about a "volume" of something like dirt, sand, rock, water, a wooden post, a block of concrete, etc.

So, we use a "fudge factor". The fudge factor is used to add "volume" into consideration.

Volume of a box is calculated by measuring the length and width and height. Width and height are also called cross-sectional area in square feet or square meters.

So, Resistivity is calculated with Length (L) and Area (A) added to the equation.

Resistivity (p) = L / A X E / I or... L X E / A X I

It's a multiplication AND division problem. :smile:

Length in meters and Area in square meters. So Resistivity is in units of ohm-meters.

The actual volume of earth involved is considered to look like a cylinder - actually a half cylinder. The factor has a semi-circular component in the case above = 2 X pi.

The "geometry" of the electrodes used in measuring resistivity of the earth can be different from one survey to another depending on the method the surveyor wants to use.

The geometry requires a different fudge factor called "K" for different layouts of the stakes. If you only use one layout - you can use one value for K based on that layout. I explained the calculation above for a Schlumberger layout.

- Geowizard

  
kcm
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Re: Electrical Coring ( 02:14:31 FriJan 22 2016 )

...is it anything like Induced Polarization?

kcm

  
geowizard
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Re: Electrical Coring ( 03:07:24 FriJan 22 2016 )

kcm,

I'm glad you asked! :smile:

I was getting ready to open some discussion on IP.

It is different. It is in some ways better. IP measures disseminated metal particles in rock and sediments. The system charges the ground with a voltage and then turns off the voltage. The ground stores voltage briefly because the metal particles are charged up like a capacitor. It is the "chargeability" that has meaning to prospectors. The more disseminated metal particles, the greater the chargeability. The reverse is also true. If there is little or no chargeability, then there is very little or no disseminated metal in the ground.

- Geowizard

  
kcm
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Re: Electrical Coring ( 07:20:14 FriJan 22 2016 )

How would either of these compare with, say, a magnetometer? I mean, the original posting was about Electrical Coring - finding out what you got in the ground with electric current.

...Geo, maybe you need to start a new thread about ore exploration techniques and devices, along with comparisons. :coffee:

kcm

  
geowizard
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Re: Electrical Coring ( 00:35:15 SatJan 23 2016 )

kcm,

You can find lots of past discussion on magnetometers here on the Alaska Gold Forum.

Do a search of the prospecting forum.

A magnetometer is not sensitive to disseminated mineralization. A magnetometer will show "structure" and massive magnetite ore bodies. I have elaborated on the use of magnetometers and included examples of surveys in recent past threads.

Magnetometers cost thousands of dollars. You need TWO in order to do a magnetometer survey.

The electrical survey systems can be fabricated for a few hundred dollars. :smile:

- Geowizard
[1 edits; Last edit by geowizard at 01:46:47 Sat Jan 23 2016]

  
kcm
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Re: Electrical Coring ( 03:06:28 SatJan 23 2016 )

As I said, my device appears to be something similar to a magnetometer, but it clearly isn't. I hope to go out this summer and test the thing in both South Dakota and in Montana.

kcm

  
geowizard
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Re: Electrical Coring ( 12:24:16 SatJan 23 2016 )

kcm,

Is it a Long Range (LR) detector?

- Geowizard

  

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