8 meters deep chamber

[bushido]

hello, i'm looking for the best solution to locate an 8 meter deep ancient chamber in Rocky soil, dimensions 3*3. Gpr or anomaly detector ? or any suggestions. thanks

[Shojadel]

Use a Cesium vapor magnetometer or a dipole antenna gpr

[Comrad]

Hi! To solve your problem, the SIR-3000/4000 georadar and 200 and 270 megahertz antennas are suitable

[alanson]

Hi, I believe ny Dipole-Dipole EM would be sensitive enough to differentiate between the the chamber void and the surrounding rock or earth. It is very portable and very sensitive to variations in resistivity. I have it for sale on this web site. I would be pleased to further explain the workings of this in-phase out-of-phase instrument made by the makers of the Apex Parametrics Max-Min.
Robert Lee
alanson@hotmail.com

[EMC8388]

Alanson
Do not waste your time with GPR. The 8 meter depth you need is too deep for any system we have ever used. Our company uses GPR everyday and we know the capabilities. We mainly use GSSI systems with antennas ranging from 100 mhz to 1600 mhz. If you plan on using electromagnetics you would need to use either the Geonics EM31 or EM34. The quick moving EM31 is a great system but has a max depth of 8 m , which could be too shallow if your target starts at 8m. The EM34 is a slow moving unit but will penetrate much deeper. There are other geophysical applications that may work better for your project.Good luck. Joe

[radarsolutions]

Hello,
So, from my understanding, the chamber is approximately 3 METERS by 3 METERS and 8 meters deep? The GPR is easy enough, and the first choice to try. The success will depend upon 1) conductivity of the soil (the more conductivity, the less resolution and penetration), 2) the size of the cobbles, boulders at ground surface and within the soil, and amount of fracturing within the bedrock, which creates noise on the radargram (and "scatters" GPR signal, redirecting it away from the antenna), and 3) the amount of iron in the soil.

However, because the method is quick-- you can collect a lot of data in a short of amount of time, I would do GPR, regardless.

EM terrain conductivity using a Geonics EMI-34 or equivalent, would likely work-- and really well, if the surrounding material was more conductive, either from a high iron or residual clay content.

Electrical Resistivity using a DIPOLE-DIPOLE array, using a 10 to 20 meter electrode spacing, and collecting data so that you can perform a tomographic inversion (using Geotomo's INVRES2D program, which is free for academic use), you would get excellent resolution of the cavern.

Seismic Reflection Tomography (but you could also do Refraction tomography)- uses acoustic waves input from a source, with a system that uses 24 channels or greater. If you use a geophone spacing of 2 meters, then the spread length would be about 480 meters, or so, with the source changing positions, from off-set, to end, to shots every 20 meters or so along the spread. This would give enough redundancy for a tomographic inversion, which would produce a velocity tomogram that would show the location of the air-filled (or water-filled) cavern.

Last method, which is not so accessible to the general public, is gravity: Again, there is a huge density contrast between the surrounding soil (and bedrock) and the cavern. If you have gravity stations every 2 to 4 meters, along a single profile, you should be able to detect the 3 meter x 3 meter void space at 8 meters down in the raw gravity data. if there is topography, try and orient the lines parallel to the elevation contours. If flat, all you need to keep track of is the diurnal variations and occupy the same "base station" on an hourly basis. I would recommend a minimum of 3 lines that crosses where you think the feature is located.

My opinion... If you wish to discuss this further "off-line" then you're welcome to email me directly (doria@radar-solutions.com).
Good luck!

[RossGroom]

Standard cavity detection would be resistivity. Quickest is gradient resistivity. put the Tx just outside the area of interest and measure with 50m dipoles and 50m station spacings. Do line spacings at 100 to 50m depending on your need for resolution. Gravity is OK but wont give the same resolution.
TDEM fixed loop is obviously also good and quicker than resistivity if you have a system. All of these techniques will show the cavity just by mapping the data but you will need software to get the right depth. EMIGMA will work for any of these techniques.

[Marie143]

Another alternative is to use a combination of both methods, first use GPR to get a general idea of the location, shape and size of the chamber, and then use an anomaly detector to confirm the presence of metal objects.

Keep in mind that,photographers in Austin TX regardless of the method, the accuracy of the results will depend on the quality of the equipment, the experience of the operator, and the specific conditions of the site. It may be a good idea to consult with an experienced geophysicist or archaeologist to help design the survey and interpret the results.

[bushido]

hello, i would like to thank everybody for the suggestions.
the situation is very difficult, the surface to be surveyed is really narrow 6 meters*10 meters.it s located in the backyard of a house surrounded by brick walls.
Magnetometery is excluded.
ERT resistivity is excluded.
Gpr is not able to reach that depth.
What about VLF-EM instruments ? i found a german company called EMFAD selling vlf em devices that uses a 6 frequency transmitter and a receiver. the company claims that their device is able to reach 12 meters depth.
Is it possible ? how good it is ?

[geoschwarck]

hello, i would like to thank everybody for the suggestions.
the situation is very difficult, the surface to be surveyed is really narrow 6 meters*10 meters.it s located in the backyard of a house surrounded by brick walls.
Magnetometery is excluded.
ERT resistivity is excluded.
Gpr is not able to reach that depth.
What about VLF-EM instruments ? i found a german company called EMFAD selling vlf em devices that uses a 6 frequency transmitter and a receiver. the company claims that their device is able to reach 12 meters depth.
Is it possible ? how good it is ?

Hi Bushido,

The VLF-EM is widely used for conductive anomalies, buried chambers have opposite electrical behavior (they are very resistive with respect to their surroundings) so the VLF-EM method will not help you much. I would opt for the ERT method with a dipole-dipole arrangement, which can locate both conductive (metal pipes) and resistive (hard rocks, chambers) anomalies, their location and dimensions, And it's "cheaper" than other methods.

Bye!

[kaminae]

I agree geoschwarck regarding DC and EM method difference. Indeed EM is not so good in mapping resistive targets. But I affraid you can't reach depth 8 m on such limited area (6x10 meters). You need at least 20 meters in one dimension and pole-pole array to get realible results with ERT.