Detecting Caves and Voids

[gwzd]

You can measure by gravity. This demmand a sensor build by yourself. This is a sensor extremely sensitive to weight, exceptional, a small grain of metal deflects a microamp. strongly. This acts as an extremely sensitive variable resistor. This must not exposed to air, because the air produces strong variation. Also must be not exposed at vibrations. Now, you have my very useful copper plates-conductive rubber sensor.

I see only one little problem with this, how do you calibrate it? I find very difficult to find a reference for it. Of course, you could walk around with a very tall ladder, a well known weight and a chronometer, but that's just silly, isn't it? Another thing, how do you know your readings are not a result of vibrations?

If I'd would do something that tedious, I'd go for a resitivity meter which is much more easier to construct, test, calibrate and operate than something that complex as a gravimeter.

[Geo]

I see only one little problem with this, how do you calibrate it? I find very difficult to find a reference for it. Of course, you could walk around with a very tall ladder, a well known weight and a chronometer, but that's just silly, isn't it? Another thing, how do you know your readings are not a result of vibrations?

If I'd would do something that tedious, I'd go for a resitivity meter which is much more easier to construct, test, calibrate and operate than something that complex as a gravimeter.

Hi.
I believe that there is not need for accurate calibration. If you will pass up a known cave, you can calibrate it for audio output and nothing else....
For me the question if, if it is so sensitive to "see" the gravity difference of a small cave or a grave at 1... 1.5m depth.

Regards

[gwzd]

Hi.
I believe that there is not need for accurate calibration. If you will pass up a known cave, you can calibrate it for audio output and nothing else....
For me the question if, if it is so sensitive to "see" the gravity difference of a small cave or a grave at 1... 1.5m depth.

Regards

Geo,
That is exactly my point about calibration. For a gravimeter to detect such small voids at such shallow depths you'll need an instrument called "microgravimeter", the resolution being of the order of microgals. Relative measurements as you suggests, for units like those working very near the noise floor are unreliable, any noise can trigger a false alarm. The same concept used for other methods with much more stronger contrasts in the parameters they measure is not applicable to gravimeters. More on that, terrain topography and diurnal earth tides will complicate things further more. The point is that these equipment, even professional ones costing tens of thousands of dollars, is out of the reach of the average treasure hunter by the complexity and demand on training they present.
That's why I was amused at the simplicity of the schematics presented and was a little bit sorry for Esteban being frustrated about the thing not working.
Now, on the other hand, think about the resistivity even a small cave presents to an ER meter, nearly infinite! You won't miss that! Which is what I wanted to say, if you cannot use GPR for any given reason, then use ER methods. Nearly the same amount of work in the field as with gravimeters, but easier to put together, calibrate and test.
Regards,
RA

[Geo]

Geo,
That is exactly my point about calibration. For a gravimeter to detect such small voids at such shallow depths you'll need an instrument called "microgravimeter", the resolution being of the order of microgals. Relative measurements as you suggests, for units like those working very near the noise floor are unreliable, any noise can trigger a false alarm. The same concept used for other methods with much more stronger contrasts in the parameters they measure is not applicable to gravimeters. More on that, terrain topography and diurnal earth tides will complicate things further more. The point is that these equipment, even professional ones costing tens of thousands of dollars, is out of the reach of the average treasure hunter by the complexity and demand on training they present.
That's why I was amused at the simplicity of the schematics presented and was a little bit sorry for Esteban being frustrated about the thing not working.
Now, on the other hand, think about the resistivity even a small cave presents to an ER meter, nearly infinite! You won't miss that! Which is what I wanted to say, if you cannot use GPR for any given reason, then use ER methods. Nearly the same amount of work in the field as with gravimeters, but easier to put together, calibrate and test.
Regards,
RA

Hi qwzd.
I have not experience on instruments for cave detecting.
I want to find a simple detector for detect small caves or graves at small depth. GPR is not useful at my occasion because there are stones, small trees and also i want to take the results the same time (and always at night..... you understand ). From the other side the resitivity meters needs to put the metallic rods in the ground every 15...20 meters distance, a not practical work....
So i look for something else....

Regards

[gwzd]

Hi qwzd.
I have not experience on instruments for cave detecting.
I want to find a simple detector for detect small caves or graves at small depth. GPR is not useful at my occasion because there are stones, small trees and also i want to take the results the same time (and always at night..... you understand ). From the other side the resitivity meters needs to put the metallic rods in the ground every 15...20 meters distance, a not practical work....
So i look for something else....

Regards

For a cave at 1 to 1.5 meters depth you need only 3 to 4.5 meters separation between the electrodes, or 9 to 12.5 meters in a four electrodes configuration. It might be still impractical for you.
About the trees, well no geophysical instrument can go into those , to my knowledge anyway. The stones should not be a problem for GPR, there are "air launched" antennas specifically developed for such occasions, look here:
http://www.geoscanners.com/antennas.htm
Check the FLB-390 air launched antenna, specifically developed for that kind of surveys as cavities, voids, layers etc.
Although, if you are in US you won't be able to use it, not yet anyway, it has to be certified with the FCC and we are still working on that, second quarter of 2010 if we are lucky.
Regards,
RA

[gwzd]

Sorry Geo,
I wasn't paying attention, you are from Greece! No problems then, the FLB-390 is CE certified, compliant with the R&TTE directive 1999/5/EC and the EN 60950 CENELEC harmonized standard. So, no problems!
Regards,
RA

[Esteban]

Deepers MD is manufacturer of cave detector. The problem is this: you need to carry a bag with 6-7 kg of sand. You drop it and listen...

http://www.metaldetection.net/spanis...undSounder.htm

[Esteban]

Two box

For example, the 08-MI 2 box by Mineoro (the old "direct conversion") with control of audio in transmitter can detect caves and holes. You need to put a small audio and see the microamp. When cave is detected (the depth deppend of the size of the cave) the audio decreased or vanishes and microamp work in opposite.

[Geo]

The same is happening with Gemini III. You calibrate it so to hear a small sound and when you are up from a cave or grave, the audio decreases the volume. But if up the cave or grave there is a fat ceramic plate, then the Gemini locates the plate, the stones etc....

Regards

[Alexismex]

Hello Forum ,
To see very well the echo of the sound when you drop the weight i use
- a sensor which is a used GEOPHONE
http://psn.quake.net/geophone/index.html
-a preamp with o without low fcy filter (i make with a 3 band filter)
- a visual 100uamp meter or better for 25 Dlls !!!!this very good interface
http://www.dataq.com/products/startkit/di194rs.htm
with this you can see perfectly the echo produced in the display of your laptop......
I will post a photo of my equipment

THE SIMPLE FACT IT IS TO DROP A WEIGHT AND YOU SEE the echo IN THE MICRO OR DISPLA laptop LIKE IN MY EQUIPMENT YOU CAN HEAR THE SOUND also....
To drop the weight at the same height i make a bloc of steel around of an axe because it is important to drop always the same height i will post also photo of this tool
Geo it is not complicated to make i have tell the more important: geophone and logger...for the rest it is simple transistor fet amplifier and with a amp (741, tl071 etc...)
another thing
it is like a pulse detector
you make the pulse (weight choc)
1)and you measure the time after to have echo= this data is the DEPTH of the cave...(more time more depth)
2) the more strong or amplitud of the echo = it is the size aprox of the cave....a big cave give a big echo
it is "a priori " for cave to 5 meters ...to go more depht you must per-cut with more weight or with a hydraulic hammer like for archeologist surveys

[Alexismex]

AND if you want more sophisticated but eeasy you will see your cave if you make a grid and take data in every case after the choc weight with a fantastic data logger , thank you TIM :
http://lrlman.com/LRLMAN.htm
It is not complicated because you work in the sound range!!!!
Alexis.

[wam]

Surface Wave

It detects caves via microphones with buildin GPS.
http://www.micromentis.com/index1.html

Regards,
wam

[Grinok]

It detects caves via microphones with buildin GPS.
http://www.micromentis.com/index1.html

Regards,
wam

Thanks for this contribution!

[wam]

ultrasonic Fish finder

Hi, this weekend I have try ultrasonic Fish Finder. I placed it in the ground and above ground (I know that in this spot about 1 m underground is ceramic pipe) - … and I detected nothing.
Then my kitten started asking for food so I took his plate and came with this idea.
What about if I place ultrasonic head on top of upside down plate?
I have position this in the same location and bingo – on my Fish Finder appeared quit clear line indicating reflection about 3 foot under. Then I have moved all about meter away and Fish Finder made no detection. It worked every time.
Later I made experiments with different plates – some came with better some with worst results. My conclusion is that it works!!!! Is any one having similar observation or can try to duplicate my experiment?
Wam.

[Alexismex]

Hello Wam,
Very interesting, tell me the model of your fish finder and ultrasonic head frequency and i will make the experiment soon
Alexis