Help.. Magnetometer settings Forster

Has anyone tried a Speake FGM3 Gradiometer ?

I bought the electronics and am currently soldering away…

The various models of Foerster / Ferex are indeed useful tools – However I always bear in mind what they were designed to find! 😮 i.e. a single very solid lump of metal all on its own not a mass of smaller items shed on the way down to a (hopefully) larger lump – therefore I could never see how the various tables and calculations in the manuals could ever be accurately applied to a crash site.

I too have been misled by readings – fortunately not with any great financial loss. A solitary Spitfire ammeter magnet about 2’ down once looked very promising – fortunately a trail trench soon exposed it. On another occasion a collection of numerous mass balance weights and a solitary 20mm Hispano looked like a very promising and apparently intact site – Actually I was quite happy with the Hispano, but we could have extracted it with a lot less fuss had we been able to accurately predict what we were dealing with.

I have been asked many times to go over sites for other groups and have to say I am reluctant to do this – I do not proclaim myself to be any sort of expert and feel that other (often less experienced in this hobby) people’s expectations of what they are going to find can lead to misunderstandings.

For my own sites I tend to use a combination of deep-seeking detectors – a Whites TM808, A Fisher Gemini III and two different models of Foerster – all tend to have their own characteristics and despite not being an expert I feel through experience I can build up a fair idea of what I am dealing with – our last dig – Mustang KH838 it was very important to reassure the landowner exactly where the aircraft lay, at what depth and that what we were going to find would justify disturbing their front garden! It all worked out perfectly – phew!

Finally – a good example was a site where several groups believed one of the four engines still lay buried and certainly a quick pass with the Fisher did indeed indicate a “large object” was apparently present – however the Foerster though giving a good readings did not seem quite right, there appeared to be more than one target, but very close together and at a fairly shallow depth. Finally the Whites showed a much smaller target than we would expect from an engine – Permission for a limited trial trench was negotiated and showed that whilst the engine cowlings and sections of the engine bearers were present, the engine had clearly been recovered. Obviously the Fisher picked up the large surface area of the cowlings, the Foerster, the steel engine bearers, but the Whites seemed to be indicating the actual volume of metal? Crude and unscientific maybe? But I am simply going off observations made at the time.

I’ve also been given conflicting advise on settings & I agree with junkcollectors comments as to the usefulness of these machines when it comes to predicting what lies buried and at what depth, its complete nonsense.
I’ve attended countless digs where ‘expert users’ have
mis- interpreted the readings and made wild claims of say, five tons of wreckage buried at 20 feet & when we’ve excavated the site there’s not enough wreckage to fill a small shoe box! ……..
One of the biggest problems you will find is debris lying close to the surface, which can be mis-interpreted or can hide weaker readings from items buried at greater depths, so get your spade to work and double check before booking that digger.
Buried magnets are your very worst nightmare, aircraft crash sites and airfield dumps are full of them, even a small one just a couple of inches across will read like a merlin engine lying just below the plough line!
My best advise is don’t even try to interpret the readings from the magnetometer alone, use it as junkcollector suggests, purely as a tool to pinpoint and back it up with something like a fisher Gemini.
Don’t listen to the self proclaimed ‘experts’ instead use it with some common sense, don’t rely on it entirely or it will cost you a fortune.

yes Rocketeer,

I have to agree, it is a helpful thread, as i have no manual, so the words a tips from Ross, and junk collector were very helpful.

the forster i have is not digital, but the old EOD type, with needle/meter a sound
, I imagine too , it is of great help to others who dont get to use there mag’s very often.. but you know what they say… practice makes perfect 🙂 😀

Greetings Tony it is I.

I had a hip replacement op last year and the mag should be ok to use, only a small amount of ferrous in the socket joint. Will have to check out if the Whites will work and not be affected by the large chunk of metal in my upper thigh and pelvis. Someone else could use it when it is set up keeping away from me. Contact me via PM to arrange any dates you want to use the mag. I lost your phone details
Ian

I dont think this is a wasted thread at all…..I could do with this knowledge or better still, someone with this equipment to run over our two sites…..if that is the Ian I think you are (old fruit from M32 who had the swift b4 me)….hope you are well? Do you fancy coming and having a go over the sites….or Tim with his Whites?

Many Thanks for your help ,all who replied.

And I dont have a computer Ian, But I have use of my girlfriends laptop. and pretty much live nr Weston S Mare . lots to do , and have been busy so not much time on the laptop.. Su-7 is still in Storage.. and I hope to get on with her in the new year.. as you know ive had alot to contend with in my personal life.
but now things are looking up.
I hope your keeping well mate 🙂

Chris
Ask Danny or Gareth they are the experts on using the mag, I’m sure they both have the knowledge you are wanting. The manual is a good place to start. The Whites TM808 will do the job just as well as it will pick up more metal than the mag ensuring a better coverage, it might not go deeper than the mag but its a useful tool.
Seeing that you are back online now and have got a computer, whats happened about the su7.

Ian

Rarther than posting on here, why not just ask Danny or Gareth as you know they can help. Seems a waste of a post imho.

Depth with the Ferex 4 is relatively easy with about 10% error.

Use a close search on 0 to 100nT to identify peak signal value.

Lift sensor up about a foot and carry out a pass. If the signal peak value has decreased significantly then the anomaly is close to the surface.

If no significant decrease then the signal is deep and the following method will work.

Starting from peak signal with the sensor tip close to the ground slowly move directly out until the signal strength drops to 70% of peak. Mark with a non ferrous marker. Return to centre and move directly out 180 degrees to the first direction. Again mark where signal strength drops to 70% of peak.

The distance between the two non ferrous flags will be depth with about 10% error.

As Junk Collector says quantity is a completely different matter with no real correlation to signal with compass correction magnets and magnetised carbon rods giving strong returns way beyond their actual size.

I get better results in the 1 mtr to 5 mtr depth for ferrous with this device
http://www.schonstedt.com/index.cfm?page=GA-72Cd

Ferex I use for deeper.

Regards
Ross

Which model do you have ? Some people seem to have versions which supposedly predict the quantity of wreckage and the depth, personally i think they don’t really have a clue what they are talking about. I have heard many conflicting tales of what means what, i find them little more use than a pin pointer, and 1 part of many steps of a process, i have seen what happens when you stupidly rely on the reading alone and have a german magnet that cost a me hefty dig contribution to prove it.