Deep test results with the PPM-MarkIII

[Willy Bayot]

Hi all,

This is the nice results of a series of benchmark tests with the PPM-Markiii made by one of our good clients.
A tunnel has be dug at a depth of 2.7m under a road and a bunch of empty ammunition boxes have been put in the tunnel. Survey grids were then made over this deep target and the results plotted in 2D and 3D.
The rightmost plot is with 4 boxes, next is 7 boxes, next is 11 boxes and the last, leftmost plot has been made with 11 boxes elevated in the tunnel to reduce the depth to around 2m.

Note that an (expensive) EMI sensor device (those so-called 'deep target', two-boxes types of detector) has been used for making surveys in the same conditions and this did not even detect the presence of the last 11 boxes.

Willy

[WM6]

Nice test Willy. Well done.

But is hard to explain difference in graph presentation between elevated and un-elevated boxes (I expect more warm/white colours in graph on the left).

[Willy Bayot]

Hi,

Actually, shallower targets give higher, narrower peaks than deeper targets of the same weight. Deeper and larger targets give lower and wider peaks.
The 2D surface of the white color area is smaller but you can see that its 3D surface clearly shows closer contour lines.

Willy

[okantex]

,Hi Willy,

good pics , what about empty tunnel .do you have pics for same tunel without any metal
and any nonferrous metal tests?

kind regards

[Willy Bayot]

,Hi Willy,

good pics , what about empty tunnel .do you have pics for same tunel without any metal
and any nonferrous metal tests?

kind regards

Except if the ground is rather magnetic (containing a sizable proportion of magnetite or any other such material which is not the case here), a magnetometer will not detect a void (empty tunnel) nor would it detect any non-ferrous material.
Thus, it is useless to make any such test.
Sorry to disappoint you

Willy

[Boattow]

Now I'm confused

Willy,
You stated in your very early tests that it took you a while to get a proton ring out of your setup. One of the things you had to do was get the sensor off the ground. So this would imply that something in the soil was effecting the protons. If a void, especially as large as the one in the photo, removed a significant amount of soil, wouldn't it effect a reading? I've never tested it, but I've always heard a void could be detected by a magnetometer. This is the first time I've heard that it cannot detect a void.

[Willy Bayot]

Willy,
You stated in your very early tests that it took you a while to get a proton ring out of your setup. One of the things you had to do was get the sensor off the ground. So this would imply that something in the soil was effecting the protons. If a void, especially as large as the one in the photo, removed a significant amount of soil, wouldn't it effect a reading? I've never tested it, but I've always heard a void could be detected by a magnetometer. This is the first time I've heard that it cannot detect a void.

A bit of clarification now!!
It is true that it took me quite a while to get a first correct reading from my early mag prototypes and I had no clue at that time why I failed to get a signal. Now, I know better and a sensor on the ground is not really a problem except if, by bad luck, there would be a piece of magnetic material buried right under the sensor at shallow depth.
Such a void could probably be indeed detected with a sensitive mag like ours but it would require a gradiometer or a differential configuration to really see it by cancelling the earth field diurnal variations which would cover the weak field gradients generated by the void.
Again, I repeat that a soil with a very low magnetic susceptibility (like normal sand or limestone) is very much looking like air (void) to the eyes of a mag.


Now, for the sake of curiosity, I could still possibly ask my local friends to make an extra survey using a differential configuration over this tunnel without any known target planted in it. I know they own several PPM-MarkIII systems, thus, they are able to make this test.

Willy

[okantex]

Hi willy ,

so is the problem same for nonferrous metals like copper ,silver ,gold,bronze?
cause as I know they have almost similar reflection of earth filed as void does, am I right?

regards
okantex

[Willy Bayot]

Yes, those have no action on the earth magnetic field at all.

[okantex]

Willy ,
you are wrong , non ferrous and voids have opposite effecets on earth field movements
while ferrous materials attracts them and make fields come closer to metal , nonferrous and void does opposite, because their magnetic conductivity is poor than earth(soil)
regards

[Boattow]

I've not made any tests but all my reading has been that there is a certain amount of mineralization in the soil to conduct the earth's magnetic field. If there is a void, then the magnetic field reading would actually decrease. So with one sensor , the absolute field reading would decrease from the ambient. If a gradiometer is used, a ferrous object would cause a positive reading. If a void was detected, a gradiometer would give a negative reading. This is the first I've heard this is not so. Again, I've not come across a void in "normal soil" to test this though with my mag. Should your group decide to test this, it would be best to conduct a survey some fair amount larger than the void so as not to skew the mapping software.

I did map an area with a 1.8meter deep hole once. When I mapped the survey it showed an anomaly of -5nT. This area had a high clay content so I figured that it had a high iron content which probably is not a good test compared to an area composed of good soil. Not sure.

[Willy Bayot]

I've not made any tests but all my reading has been that there is a certain amount of mineralization in the soil to conduct the earth's magnetic field.
You are right but the difference between this ground and void can be very thin if this ground is weakly magnetic (sand or limestone) while the difference could be greater with clay-based or volcanous rock-based ground. In the first case, the differences could be so low as to be confused with the diurnal field variations.
If there is a void, then the magnetic field reading would actually decrease.
RIGHT
So with one sensor , the absolute field reading would decrease from the ambient. If a gradiometer is used, a ferrous object would cause a positive reading. If a void was detected, a gradiometer would give a negative reading. This is the first I've heard this is not so. Again, I've not come across a void in "normal soil" to test this though with my mag. Should your group decide to test this, it would be best to conduct a survey some fair amount larger than the void so as not to skew the mapping software.
I'll ask my friends to do a differential survey when their time and weather will permit. Then, we'll see if the field gradients are high enough in their ground to detect a void with or without the help of the differential configuration. Their ground is mainly made of a mix of rocks and soil, it is the perfect ground to grow olive trees and vineyards.
I did map an area with a 1.8meter deep hole once. When I mapped the survey it showed an anomaly of -5nT. This area had a high clay content so I figured that it had a high iron content which probably is not a good test compared to an area composed of good soil. Not sure.

Willy

[Willy Bayot]

Hi Boattow,

What type of mag do you use?

Willy

[Boattow]

I've got a Geometrics G-856.
Thanks,
Boattow