Interpreting my Mag survey

[Unregistered]

Hi all,
I have a proton mag and did my first survey this past weekend. I have generated a contour map which shows a hi (red) area and to the northwest is a low (blue) area. I surveyed this 20ftX20ft area because I am hoping there is a box buried there. I am having some confusion as to where the object would be located. Is it in the center or transition between the 2 areas? or more toward the red, hi area? Also not quite sure the low area is to the NW rather than to the north of the high.

I'm hoping someone here has some experience interpreting these maps and can shed some light on where the iron object causing the magnetic dipole to be.
Thanks a lot

[magndig]

Mag results

Unregistered.
There are several things you can look at. Just like in the manual of any mag, you will notice the high and low gammas registered on the object you are looking at. What I have found is that using a contour map + a shaded vector map helps more definitely point out the shapes.
I use Surfer 7. I don't know what you use, however.
Good luck!

[Unregistered]

Magpick

Hi,
I have a Geometrics G-856 mag and used their Magpick to create the contour map. What I'd like to know is about how deep is the object is. Does Surfer estimate the depth? I can't figure out if Magpick can do it...I don't think so.
Thanks

[magndig]

That is tougher

The G-856 can detect items up to 20' deep in the roughest conditions, which makes it a bit tough to gauge depth. The rule of thumb is that the sharper the Gammas "bend" around the object usually the shallower it is. Go to Geometrics.com and see if you can download surfer7 from their site. That tool will help you out a bit more.
To answer your question, MagPick does not show depth. That is something I will speak to Gemetrics about. I know their software developer, she can probably figure something out.
I'll keep you posted.

[Unregistered]

Surfer 7

Hi again,
Did you download surfer 7? I thought that Surfer was a commercial package that you had to purchase. I have not seen it on Geometrics site but I'll take another look. Between magmap and magpick, I get what I would think is a pretty good contour map. I expect to find a box 6 feet down. I did a survey with my mag (first time to do that) because the spot is covered with some sort of clay that sets off the metal detector so I hoped the mag would "see through it". I don't believe that the clay is affecting the mag results. I got a strange looking anaomaly. When I went to the site this past weekend, I couldn't find anything when I sank some probes. I pulled out the metal detector to try detecting again. When I did, I found a piece of junk iron on the surface just under the leaves. I removed it and did the survey again. This time I got more round results with the transition between the red and blue area shifted a couple feet so I'll go back and try sinking a probe again in a week or 2. As you stated, I think the edges or slope is very sharp indicating a shallow target. The indicators I have lead me to believe it is suppose to be 6 feet deep. If the mag survey could tell me it could be around that depth, it would be encouraging.
Thanks alot for the input,
FJ

[ronjoly]

profile the data ..the highest mag spike in your profile will be close to your treasure

[Unregistered #2]

Question for Ronjoly

Ronjoly, Why do you suggest that the item is near the highest indication? I'm new to this but my understanding of the operation of a mag tells me that the item causing the profile will be located at the transition between the high and low readings. As I said, I'm brand new to mags and am trying to learn.
Thanks.

[Unregistered]

I ask the same question

I have read and even the geometrics software indicates it is in the center between the hi and low. With that said, you are the second person that has told me to check the high for the target and the manual for the mag at the www.quantrosensing.com site says to investigate the hi and the lows found. Seems to be a lot of conflicting advise.
Thanks,
FJ

[ronjoly]

you are seeking i assume something highly magnetic therefore the highest
value will be your high but in very high mag areas you will get what's called a
dipole which is a sharp drop in the magnetism beside a high mag value.

in exploration this is normal .. the low could also be a change in the properties of the mineral where you'll have a magnetic mineral like magnetite then adjacent to the magnetite you may have pyrite .. the mag high is the magnetite the mag low is the pyrite .. both worthy of checking out for gold etc..

for treasure i would say you are seeking ANY anomolie .. so any sharp change in the magnetism is a viable target .

make sure you mag well beyond your target so you get a good feel of the background.

[Jeff S]

Some more ideas...

So, there are a few things you could probably try.

First, take another look at your survey geometry. Set up one set of dense survey lines, parallel, along one azimuth (let's say due north). Record data only while you are on the lines (ie, don't record during turns). Perform a second set of lines at a 90 degree angle to your first set (ie, due east). Record that data separately. Even though you are only interested in like a 20 foot area, you might want to try expanding your search area to a 100 foot square area and making your survey lines about 5 - 10 feet apart if your GPS can give you positioning that is accurate enough.

Why do we do these lines like this? Because the proton magnetometers are sometimes affected by the directionality of the earth's magnetic field and it can screw with your results a bit.

Expanding the survey area helps you establish better boundary conditions in your area of interest. Without these boundary conditions, the surface generation programs may create artificial contours because there is no data out beyond the last survey line. By expanding your investigation you have a better chance of reducing these edge effects within the actual area of interest. Adding the more dense survey line spacing gives you a better chance at higher resolution surface generation and better target location.

When it comes to processing, the MagPick software can probably do it for you. Though Surfer gives you more capability when it comes to how you create your surface interpolation, MagPick should be adequate (plus, you've already paid for it). Just make sure you don't mix and match the survey data (ie, keep all of your "north" azimuth data separate from your east azimuth data). Extreme highs and deep lows are likely to be the sources of magnetic anomalies whether they are causing local increases or decreases in the magnetic field. Alot of it depends on what the background natural magnetism is and what the object of interest is doing to affect that field. Whatever it is, I would think that your target should create a peak or a pit, but not necessarily a linear feature.

Give that a shot, and let us know if you have any success.

[ronjoly]

post the data here and i'll work my magic and post JPEG'S of the results.. i use Oasis Montaj and have a ton of tools to play with to make 3d models of the mag,profile,contour and grid ,filter,trend ...you name it.

btw i am posting here after processing my days mag ,em data

in diamond,gold and base metal exploration we use other tools to find the depth of a mag target using
EM surveys like max-min ,utem,protem & vlf .. i'm not totally convinced of figuring out depth of a mag target using algorithms like a euler survey.

to be honest i've only used the very expensive $10000 mags so i really don't know the capabilities of cheaper mags.


read up on Gradiometer mag surveys ..it's a mag survey with different mag pole heights .. survey it with the sensor at one height then resurvey it at another height "2 feet higher" .

i don't know the theory but i've had to do Grad a million times for gold and uranium ..our units have 2 sensors on 1 pole 2 feet apart.

[Jeff S]

Oh, that was one thing I meant to mention, too. A magnetometer, by itself, can't really provide you with the depth of a target. You have the xy-position of the magnetometer and thus you can determine the xy-position or your target. However, the strength of the anomaly you record is a function of the material (or the magnetism) of the target and the distance of the target from the sensor. In your z-axis, these are two unknowns for your one known measurement, that being the magnetic field measurement at your magnetometer.

As ronjoly has suggested, a magnetic gradiometer can provide you with a solution. By recording the differential measurement between the magnetometers, at different heights, it is possible using the right software model to extract a depth. Of course, a gradiometer requires two magnetometers being run in the same xy-position in differing heights. You could also try, as ronjoly suggested, running the same survey twice while fixing the sensor at different heights, but that may only be an estimate as you may not exactly duplicate the xy-position of the sensor during the second survey.

[Unregistered]

Hi,
I have a Geometrics G-856 mag and used their Magpick to create the contour map. What I'd like to know is about how deep is the object is. Does Surfer estimate the depth? I can't figure out if Magpick can do it...I don't think so.
Thanks

As a rule of thumb...The width of the response at half the maximum signal is equal to the width of the buried object, or its depth if that is greater. Known as the Full Width at Half Maximum (FWHM). This is for total field measurments...not for gradiometer. So yes...you can estimate the depth of a magnetic anomaly. Some say this is quite accurate. GPR is the best method IMO though. But if you only have mag data...this method should work just fine.

Surfer is just for display of maps...not specifically designed for mag data,

[Geophyz]

As a rule of thumb...The width of the response at half the maximum signal is equal to the width of the buried object, or its depth if that is greater. Known as the Full Width at Half Maximum (FWHM). This is for total field measurments...not for gradiometer. So yes...you can estimate the depth of a magnetic anomaly. Some say this is quite accurate. GPR is the best method IMO though. But if you only have mag data...this method should work just fine.

Surfer is just for display of maps...not specifically designed for mag data,

That was my post...I don't know why it logged me as unregistered.

[dcyoung9]

Think about what you are actually detecting....

NOT the object. You are detecting the results of what it does to the earths magnetic field.

Picture the flux lines drawn for a bar magnet.

The earths magnetic field flux lines are not much different.

The FERROUS object causing the change, burried in the ground at some depth, is drawing these flux line to itself causing a concentration of the magnetic field.

In the northern hemisphere the magnetic flux lines come out of the earth at an angle from north to south. For a single sensor mag if you are detecting something and are standing at the point where you get the best, highest, signal where would it be. Probably to the NORTH. How far? That depends on the depth and the magnetic field inclination. There are maps on the net showing inclination. http://rst.gsfc.nasa.gov/Intro/magnetic_field_b.jpg

Now you need the depth. Back off directly south and start over. Walk south to north toward the object. At some point you will get a null (almost no signal) MARK THAT SPOT. Continue on north until you get that highest reading. MARK THAT SPOT. Continue on north a short distance and you should get another null. MARK THAT SPOT. You should now have three marks. South null, Center hight, North null. Half the distance between the nulls is the approximate depth.

From Scintrex here is a table of the offset by depth and inclination:
Est depth----Inclination Angle in Degrees
----------<40-----50-----60-----70-----80>
2 --------None----------------------------
4---------1.5---- 1.5-----1.0-------NONE
6-------- 2.5-----2.0-----1.5--- 1.0--- NONE
8 --------3.0-----3.0-----2.0--- 1.5 -----"
10------- 3.5---- 4.0-----3.0--- 2.0 -----"
12 -------4.5 ----4.5 ----3.5----2.5---- 1.0
14------- 5.5---- 5.0---- 4.0--- 2.5-----1.0
16------- 6.0-----6.0---- 4.5--- 3.0-----1.5
18------- 7.0-----7.0---- 5.0--- 3.0-----1.5
20------- 7.5-----7.5---- 5.5--- 3.5-----2.0

This is the table used for UXO/Mine detecting by our military so I would guess
it better be right!!! Works for me.

You can see that if you don't know where to dig you could dig all day and miss the target by just a couple of feet.

Good luck, Let us know what you find,
Don