Please School Me on Magnetometers

[awemawson]

First a little back ground - I need to install land drains in a field that is cris-crossed by cast iron water pipes - some deep some not so deep. The water authorities (both Southern Water and South East Water) don't have accurate plans of where their pipes run, so I thought I'd dig out a little gizmo that I acquired many years ago to see if I could detect them - an ex-military Magnetometer.

I have no idea what application this magnetometer was intended for - but maybe E.O.D. implies Explosive Ordinance Detection 

[Joules]

Some local metal detecting nut to trace your pipes ?   as long as he doesn't dig them up and claim treasure trove.   



p.s
     how deep are the pipes ?

If you detect them with that, will they be classed as pipe bombs 

[awemawson]

Now the detector head is connected by a cable of about 25 meters, and has a mount for a tripod incorporating an angular adjustment roughly calibrated in degrees of magnetic dip, and even a marker to show the dip angle for the UK.

It is unbelievably sensitive - slightly moving a small screw driver 5 meters from it will trigger its alarm, and I cannot hand hold it still enough for it not to trigger (no I don't have the D T's !). It seems to be sensitive to changing magnetic fields rather than absolute ones, so with the screwdriver waving test, if the screwdriver is stationary the machine electronics returns the meter to zero and the alarm ceases.

I can remember when I got it (at least 15 years ago) mounting it on a tripod and being able to detect cars moving on a road about 1/2 mile away 

I don't think its suitable for my purpose as it's too sensitive and I cannot imagine how I could mount it to move across a field to scan for pipes without the very movement triggering it in the earths magnetic field.

I have always thought that it was intended for nautical use as the lid incorporates a compass bearing engraved lines marked up keel direction, however on reflection it is probably an off the shelf compass so the line are irrelevant.

The electronics looks to be very simple - a dual low power cmos op amp, four logic gates and a couple of transistors, so maybe I could reduce it's sensitiveness electronically, but before I muck it up has anyone come across anything like this before?  What the heck was it for?

[awemawson]

Joules,

Conventional metal detectors and CAT (cable avoidance tools) go hay wire as an 11 kV three phase line runs down the field 

I don't think this unit is going to help me, but really I'm posting to see if anyone knows what it's application originally was 

[John Rudd]

Andrew,
Go over to the EPE.com forum,
http://www.chatzones.co.uk/cgi-bin/discus/discus.cgi
there are some threads on there about magentometers....some of the guys there might be able to help....you might need to sign up for an account to post your question.....

[Joules]

Ground penetrating radar, maybe the local university or archaeology department would like to train some students in your field ?

Hmmmmm you never mentioned the    11kv

[modeng200023]

The simplest answer Andrew is a pair of dowsing rods 

[Will_D]

Whats in th ecast iron water pipes? Its not a daft question! are they empty of full of water?

Reason I ask is we had a survey done in the rugby club which found Electric/Water/gas pipes under abouy 1 meter of backfill.

If the pipes are empty and so don't respond to a water pipe check could you apply a vol;tage across them somehow and pick then up with a field-strength-meter.

BTW: last magnetometer I saw was mounted on a Nimrod sub hunter and called a M.A.D. (magnetic anomoly detector).

Know anyone with a working Nimrod?

[awemawson]

Will, if I knew that I wouldn't need to worry!

This farm at one period (early 1930's) was owned by Hastings Borough Council, who had water bore holes all over the place piped to a pumping station. In the late 1960's it was purchased by the then tenant farmer under the leasehold reform act. Since then the pumping station, which was excluded from the sale, has gone through several hands ending up now with South East Water. Although the deeds carry some wayleaves for pipes laid over the years the attached plans are not at all accurate, not are they complete.

Certainly some are, in their words, 'disused, abandoned' however equally certainly there is at least one huge pipe feeding three local villages, and I believe another one importing water from Southern Water to SE Water. In places pipes cross streams and are visible, but don't seem to go in straight line so are not following predictable route 

Theoretically ALL the pipes (by deed in the way leaves) are deeper than I need to go, however I KNOW that in places they are not 

In one case there is a 200 mm pipe in an early plastic so not detectable, however it is one that I know is redundant as it's been cut where it crosses a neighbours ditches.

All a bit of a mess really. At the far end of the field there is a linear undulation feature of raised hump running parallel to a depression - for all the world like a filled in pipe trench that has settled over the years, but local legend says it's the remnants of a horse drawn railway running from a powder mill in my neighbours field (mill stones still in situ) to 'Magazine Farm' about a mile away where the powder was stored after there had been a huge (well documented) explosion. It's a pain in the neck when mowing and I'm tempted to level it with the JCB, but perhaps I should persuade Time Team to look at it and do a geophysical survey of the whole field 

[vintageandclassicrepairs]

Hi Andrew,
A Cat and Genny should work ok on the pipes that have a part exposed where you can sit the genny on
or connect the signal cable to the pipe
if the pipes are all connected than one genny location should work
Theres about a 5M radius from where the genny is positioned where it wont work, due to the strenght of the signal
With the genny setting  the cat should not get interference from the power line
I used to work in a power station and did ground searches on hundreds of digs in the grounds around it
It took quite a lot of practice to differentiate between power cables and random bits of metal due to the effects from the overhead HV lines and the generators runnning
The trick was to set the cat as low as possible on a known power cable before scanning (and after to ensure it was still detecting)

Cheers
John

[Will_D]

Hi John,

What on earth are "a Cat and Genny"

BTW: The detector used in the club survey could detect water in plastic pipes!

[DMIOM]

Hi John,

What on earth are "a Cat and Genny"

BTW: The detector used in the club survey could detect water in plastic pipes!

Not so much as what on earth as what's in the earth!

CAT = cable avoidance tool - standard device to sweep a site before excavating to look for live power cables (detect 50/60Hz radiated signal). For pipe-hunting, if its metallic, you can couple a generator ("genny") with one wire to the pipe & one to an earth rod, this puts perhaps 33kHz signal on the pipe. Some gennys have an induction coil in the base of the housing and can induce a modest signal by just sitting on top of the pipe. If you're trying to differentiate between power cables, you can also get kits to live-couple to a 13A socket or similar to inject direct into a live cable.  For drains, there are sondes which are effectively a small self-contained transmitter/antenna which you can push up a drain using drain rods.

[wgw]

Years ago I had a similar problem with a water pipe running to an isolated garage and house, about a mile of pipe. After a few ideas did not work I managed to persuade a mate of mine that worked for the local council to help. He turned up with a radio-active tube on a string and a radiation detector, this worked well, not sure if allowed now. I've had good results with dowsing rods- couple bits of bent wire.

[modeng200023]

wgw, I have made the rods work as well.

[eskoilola]

Just a thought - although this thread seems to be quite old....

It should be quite easy to adjust the sensitivity just by adding iron around the sensor.