Problem, beach stones registering as non ferrous targets.

[Giz]

Hi all. No this is not April 1st
Decided today to face "the beast from the east" and with spring tides, conditions looked good on my local beach. No cuts, however some sand taken off.
Turned on my machine and away I swing low and slow no problems, picking up the low grunts of iron and finding modern coinage, then moved onto a patch of stones amongst the golden sand and here begins my problem, I start to get good non-ferrous signals that turn out to be "natural beach stones" of varying size, not all stones just random.
I used to own the Sovereign GT and had a similar problem with granite sea defences sort of falsing. Falsing today took place on dry and wet sand where there were stones. I don't want to discriminate the stones as I may loose good targets. Has anyone else experienced this? If so can anything be done?


This is the programme I used:
Discrimination 1, open screen
Discrimination 2, 5 bars blanked off
Sensitivity: manual 24
Response: Normal
Fast: Off
Deep: On
Saltwater: Off
Profile type: 2 tone Ferrous
Ferrous coin
Gain 24
Threshold level: 30
Volume limit:30
Threshold pitch: 15

[fred]

Hi Giz. I have only had the same problem with rocks imported from Norway for beach defences. I just do a manual ground balance over a hot rock and most of them do not register anymore. It doesn't affect the depth and rocks that do register should give such a strange signal that is so obvious that they can be ignored. I've never had a problem with natural stones so not sure if it will work on them but it is worth a try.

[Giz]

Hi Giz. I have only had the same problem with rocks imported from Norway for beach defences. I just do a manual ground balance over a hot rock and most of them do not register anymore. It doesn't affect the depth and rocks that do register should give such a strange signal that is so obvious that they can be ignored. I've never had a problem with natural stones so not sure if it will work on them but it is worth a try.

Thanks Fred, I brought a small piece of granite and 1 of the "natural Stones" home and will try your suggestion tomorrow as you say worth a try.

[fred]

Hi Giz. I have only had the same problem with rocks imported from Norway for beach defences. I just do a manual ground balance over a hot rock and most of them do not register anymore. It doesn't affect the depth and rocks that do register should give such a strange signal that is so obvious that they can be ignored. I've never had a problem with natural stones so not sure if it will work on them but it is worth a try.

Thanks Fred, I brought a small piece of granite and 1 of the "natural Stones" home and will try your suggestion tomorrow as you say worth a try.

I assume that the signal is proportional to the size of the hot rock so I try and ground balance over a fairly hefty bit. You should see if it works in principle though. Best of luck.

[Adam17]

I am stumped to read that the machine is picking up stones!!

[Machinist]

I use a pulse machine on the beach and get the hot rock problem a lot but usually can distinguish the sound and ignore them. There is one thing that does fool me sometimes and it is a red house brick that seems to be common on a lot of the beaches I search.

Lots of people think I am joking when I say my detector can find house bricks.

Ray

[Spanish digger]

Yes Ray I have same problem here in Spain,all old roof tiles,an bricks made with red clay,gives a digable signal,luckily there near the surface,mike

[Adam17]

Is it something in the clay/brick? I'd be tempted to bust em open and have a look..?

[sweepstick47]

This is a common phenomenon in metal detecting. If I can relate from memory, I'll attempt an answer. Please correct me if I've erred!

Note:- (with the exception of Pulse Induction units, most detector coils are in fact two coil windings in a single unit). A 'transmit' winding and a 'Receive' winding. These windings are 'balanced' when the detector is 'set-up for use.

The phenomenon is due to the fact that stones etc often possess 'capacitive' qualities (sometimes exacerbated by salt water). Unlike metal targets, these minerals (stones/pebbles etc) are unable to 'conduct' the tiny electric charge induced by the coils transmitted magnetic field, instead they temporarily 'store' an amount of that charge becoming in effect a 'capacitor'. This 'capacitance' is then recognised by the search coil's secondary 'receive windings' as it creates an imbalance between the two windings. so producing a 'positive' signal by the detector's circuitry. Regards ss47

[poly]

It should also be noted that some or many pebbles found on the beach ( location dependent of course) may be of igneous origin, eg. Granite, and may contain typically up to 3% of metal minerals such as iron, but also non ferrous metals such as copper, lead, tin, and aluminium. I wonder if this fact might also contribute to this issue.

Clays can also have a high ferrous and non ferrous content, which might also account for unusual signals from roof tiles, bricks etc..

[felixferal]

This is a common phenomenon in metal detecting. If I can relate from memory, I'll attempt an answer. Please correct me if I've erred!

Note:- (with the exception of Pulse Induction units, most detector coils are in fact two coil windings in a single unit). A 'transmit' winding and a 'Receive' winding. These windings are 'balanced' when the detector is 'set-up for use.

The phenomenon is due to the fact that stones etc often possess 'capacitive' qualities (sometimes exacerbated by salt water). Unlike metal targets, these minerals (stones/pebbles etc) are unable to 'conduct' the tiny electric charge induced by the coils transmitted magnetic field, instead they temporarily 'store' an amount of that charge becoming in effect a 'capacitor'. This 'capacitance' is then recognised by the search coil's secondary 'receive windings' as it creates an imbalance between the two windings. so producing a 'positive' signal by the detector's circuitry. Regards ss47

An excellent explanation, the only time I ground balanced was in a field I had done to death and I thought it might be worth an experiment. After ground balancing I had a load of excellent signals to dig, but each time turned out to be a stone. That baffled me for a while, but this would go a way to explain what was happening. I haven't touched ground balance since though!

[FUBAR]

Suggest you have a look at the Minelab Forum. You can get access to many well proven and tested programmes.

[Adam17]

Can I suggest using high trash target separation instead of the ferrous coin?

I have had several naff targets show as good'ns in ferrous coin....

Please let me know how you get on?!

All the best and good luck.