relative range of detectors ?

[John Smith]

Hi,


Can anyone tell me the comparative detection ranges ( when trying to detect metal wreckage on or under the sand of a beach/ seabed ) of both flux gate and proton precession gradiometers , and a pulse type metal detector ?


John Smith

[Carl]

Re: relative range of detectors ?

>Can anyone tell me the comparative detection ranges ( when trying to detect metal wreckage on or under the sand of a beach/ seabed ) of both flux gate and proton precession gradiometers , and a pulse type metal detector ?


Don't know if there is even an answer to this. It probably depends very heavily on the quality of the sensor. For example, some PPMs use water cylinders and others use toroids which should be better. Also, PI range depends on the search coil. I would like to compare my fluxgate mag to a PPM but I don't know how to do it fairly. For now I suppose the only thing to do is an air test on a standard target.


- Carl

[John Smith]

Re: relative range of detectors ?

>>Can anyone tell me the comparative detection ranges ( when trying to detect metal wreckage on or under the sand of a beach/ seabed ) of both flux gate and proton precession gradiometers , and a pulse type metal detector ?


>Don't know if there is even an answer to this. It probably depends very heavily on the quality of the sensor. For example, some PPMs use water cylinders and others use toroids which should be better. Also, PI range depends on the search coil. I would like to compare my fluxgate mag to a PPM but I don't know how to do it fairly. For now I suppose the only thing to do is an air test on a standard target.


>- Carl


Thanks Carl,
what I really want to do is ( when I get the time ) build the best type of wreck detector I can. ( I havnt got the time to try them all )


The problem is that I have no experience ( we have just moved to a town that has 360+ recorded wrecks, of which only 60 odd have been located, so I am told )


The electronic side is what interests me, but I don't want to fly off at a tangent, and build a fluxgate when I should be building an overhauser gradiometer.


Incidentally, has anyone found out the best fluid mix ( and its exitation RF frequency ) for an overhauser mag ?


Only yesterday we spoke to a local lady who had spoken to another local lady ( 2nd hand information ) who has admitted that she supplied food to the U-boats that were patrolling around the Southern tip of Africa ( where we now live ) South Africa was part of the United Kingdom then, but many Afrikaaners were sympathetic to the Germans.
She reckons there are 3 U-boat wrecks very close to our lighthouse here. She is in her 80s but evidently still very bright, we hope to chat to her this week.


I digress --


So what is the best device to find a U-boat ( say ) under 5 meters of sand ( thats what I hear is to be expected )


John

[Carl]

Re: relative range of detectors ?

>what I really want to do is ( when I get the time ) build the best type of wreck detector I can. ( I havnt got the time to try them all )


I can't help you here - I have no experience with towables. You might try Tony Wells at SearchMasters.


Also, if you're interested in a towable PI unit Eric Foster has the Towtec board available pretty cheap. Search back through his forum for info.


- Carl

[Eric Foster]

Re: relative range of detectors ?

Hi John,


A magnetometer is by far the best instrument for detecting large objects at depth, such as your U boats. Even a powerful PI would not come close for the following reason:- A PI has to transmit a signal from the coil to the object and then the induced object signal has to make a return journey. This results in a inverse 6th power law which causes the signal to fall off very rapidly with distance. With a magnetometer, it is only a one way signal from the object to the sensor, so the signal fall off with distance is much less. This means that a magnetometer sensor can be towed some distance off the bottom, which avoids problems with snagging on obstructions. A proton sensor is also not orientation sensitive like a PI search coil would be.


Eric.

[John Smith]

Re: relative range of detectors ?

>Hi John,


[i]>A magnetometer is by far the best instrument for detecting large objects at depth, such as your U boats. Even a powerful PI would not come close for the following reason:-


>Eric.


thanks Eric,


I will concentrate on the mag to start with, although it is very easy to get distracted from the treasure ships and u-boats, towards a metal detector when somebody mentioned that we have 50,000 holidaymakers hitting our beach for 7 weeks in December ( Southern hemisphere here, summers just starting )


So much to do, so little time :-)


John

[Charles]

Re: relative range of detectors ?

I agree that a Proton mag would be a good tool for what you are trying to do. Using a Dalton Proton mag (probably one of the most cumbersome) was used to find a German U boat off the coast of Texas in 17feet of sand. Texas isn't big on letting you salvage ship wrecks and they certainly wouldn't give permission to salvage this wreck. As you probably know be careful because some U boats used Mercury for ballast. The State of Texas wouldn't allow salvage for this reason. To me if its done right they should have allowed it. That ship is going to rust through someday and its all coming out.


Also, some people believe that fluxgates aren't as sensitive as proton mags. That isn't true these days. Bob Marx happens to recommend a fluxgate in his book "Sunken Treasure-How to Find It". Of course, what does he know


Good Luck,
Charles


>>Hi John,


>>A magnetometer is by far the best instrument for detecting large objects at depth, such as your U boats. Even a powerful PI would not come close for the following reason:-
>>Eric.


>thanks Eric,


>I will concentrate on the mag to start with, although it is very easy to get distracted from the treasure ships and u-boats, towards a metal detector when somebody mentioned that we have 50,000 holidaymakers hitting our beach for 7 weeks in December ( Southern hemisphere here, summers just starting )
>So much to do, so little time :-)


>John

[John Smith]

Re: relative range of detectors ?

>I agree that a Proton mag would be a good tool for what you are trying to do. Using a Dalton Proton mag (probably one of the most cumbersome) was used to find a German U boat off the coast of Texas in 17feet of sand. Texas isn't big on letting you salvage ship wrecks and they certainly wouldn't give permission to salvage this wreck. As you probably know be careful because some U boats used Mercury for ballast. The State of Texas wouldn't allow salvage for this reason. To me if its done right they should have allowed it. That ship is going to rust through someday and its all coming out.


Thanks Charles, this is all wonderful stuff for a newbie, you will all have to excuse my running around different subjects like a chicken with its head cut off, but after years of mundane electronic design, I have finally found a challenge that has got some real end product. ( Including some real "pieces of eight" promise - OK I play the lottery too :-)


The U-boats are more of a practice run, what I fancy are the 300+ shipwrecks around this Southernmost point of Africa.


see http://www.shipwreckexplorer.com/breakingnews.htm


A mag helped find the Brederode presumably due to the cannon and anchors.


>Also, some people believe that fluxgates aren't as sensitive as proton mags. That isn't true these days. Bob Marx happens to recommend a fluxgate in his book "Sunken Treasure-How to Find It". Of course, what does he know


I will try and find this book, what is the grin? is he a participant in this forum?


When it comes to sensitivity, of a mag for instance, I am getting a bit confused.


If the mag is measuring the earths magnetic field, and I build a pair of them, positioned several meters apart,( gradiometer) then I will be hoping to see the "distortion" or changes in the earths magnetic field caused by some magnetic object ( hopefully a montain of cannons and anchors ) that is nearer to one mag than the other.


At the point ( or pair of points) where the measurements are taken, the influence of the target on the earths magnetic field is completely independant of how sensative my mag(s) is/are, or even if I am present at all !


In other words, if the magnetic field at that point causes protons to precess at (say) 2300 Hz, then as long as my instrument is sensitive enough to measure that fields precession frequency ( OK the signal to noise ratio will affect the absolute reading, ) increasing the signal sensitivity seems a bit futile - or am I missing the point ?


By the way, I am at the point of winding the Fischer Price Proton Precession Magnetometer ( well actually a far east copy of the Fisher Price plastic rings ) and am contemplating sticking a couple of tubes in the plastic toroid so that I can compare ( say ) kerosene and distilled water as the medium, without having to rewind the whole thing.


Cheers


John