How do detectors work

What do we have - all different types of detectors and coils. You think you know how they work but then you get corrected. Thats fair enough but for us not so technically minded, we just switch in on, make sure the knobs are in the right place and go.

Then someone says,- you should be using a small coil, a large coil, a mono, or DD, elliptical vs round, Pulse induction vs VLF or is it VHS, VHF - Help!!!!!!!!!!!!!!!!!!!!!

Ok I have some idea, but we have experts here who can tell us in simple language the difference between the SD series and the GP series and how detectors have advanced.

OK Jef,

I think it's widely known that you're a detector operator yourself with much experience, so the question you're asking is rhetorical and meant to educate - and good for you.

Anyone who reads these posts will quickly come to the conclusion that there are many different answers to the questions of which coils, which batteries and a heap of other technical questions which need to be answered. Most of these questions ARE answered - according to the interpretation and experience of the operator and bloke who replied to the question. At this point it's up to the reader to sift through the information returned and work out what's best for his particular situation.

A recent poster to the forum remarked that he was a 'newbie' and had learned a lot about being in the business of prospecting by being a member of this forum. To me that's the finest accolade this website can receive.

Mike Wellington

Nice Question Jeff
And if it has been answered I for one have never seen it on this site.We have many techs on this site ,some even mod the sd and gp series.So this is a post that really needs a reply.What change was made to the Sd2000 to make it a Sd2100,then what change was made to a SD2100 to make it a SD2200 etc
Is a SD2200 the same as a SD2100 with the only change being auto ground balance?
Did they keep the same Frequencies on pulse rates and timings
Is a gp3000 the same as a Gpextreme but with higher frequencies and slightly different timings
Is the gp4500 effected by EMI due to using higher frequencies than the gp4000
Does using an array of higher frequencies make latter machines make the more deadly on small gold and less deadly on deep targets.Is the Gp3500 the same as a Gp3000 with the only difference being that you can manually dial the frequency range.If someone can explain the differences I and many others would be extremely
interested.
Cheers Dig

G'day,
The biggest difference between the SD series and GP series was DVT which as everyone knows stands for Dual Voltage Technology. Both detector series have MPS (Multi-Period Sensing) technology, effectively a pulse train of varying transmitter on/off transitions, (one long and several short pulses of electromagnetic energy) which was a novel idea of Bruce Candy’s to make difficult goldfield soil stand out against true metal targets so that the former could be subtracted, leaving just the metal target.

DVT was a means to overcome the weaknesses inherent in MPS which caused gold nuggets which had a decay signature similar to the ground to be hidden in the subtracted ground signal.

The return magnetic fields of target and ground during the series of short pulses was small in relation to the long pulses, so the idea was to increase the voltage applied during the short pulses to increase the quanta of received energy so that when a subtraction process was performed between the responses from the long and short pulses, the values would be more precisely matched, causing nuggets once hidden to the SDs to be revealed to the GPs.

That’s it in a nutshell Jeff from my perspective.

Cheers
Grey

G'day Dig,
Lots of questions I don't know the answers too. But roughly, and others may be able to confirm or deny them,
SD2000 -> SD2100 *additional receiver channels giving better target selectivity, and timing changes?
SD2100 -> SD2200 *auto ground tracking?
SD -> GP *DVT and selectable timings?
GP -> GPX *Analogue/digital hybrid giving operator control of formerly fixed parameters, extra timings and audio filters?

The GPX series seems to be more susceptible to EMI because of greater sensitivity due to uncompromised parameters such as gain and motion filters.

Model increments seem to be minor refinements, or extra features i.e
4000 -> 4500 *stabilizer adjustment, GB off, and more timings etc.

I’d be curious to hear from someone who knows definitively too!

Cheers
Grey.

Hi Grey
Looks as though the only ones that really know the answers are the engineers that design them,The silence is deafening on this post
Cheers Dig

Thanks Grey

Pulse trains - are they similar to other railway trains - grin. Now to see if I can understand what you said.

Pulses are sent out -(one long and several short pulses of electromagnetic energy) via the coil into the ground. The detector interprets the signal somehow when they bounce of a piece of metal or other object in the ground. It is the return of the magnetic waves that determine the target. Similar to radar but much more sophisticated

Jeff

You won't see any of the ML engineers here. A few were rumored to have turned up online some time ago but were treated very badly on another website (!) and have not been back since.

The Pulse Induction (PI) detectors function by emitting a very intense pulse of high frequency energy at approximately 5000 pulses per second. Gold and other metals are struck by the energy pulse (EMF) and briefly emit their own electromagnetic signal. The signal has a different rate of decay depending on the composition of the target... and the detector receives and then translates the speed of this decay to determine the nature of the object.

It's interesting to ponder that the ability of the detector to measure this rate of decay is important. One of the reasons why modded detectors have trouble performing is because they are often unable to measure the rate of decay (of the target's EMF) as accurately if they are putting out too strong an emission. And it's worse if the detector is overclocked because the delay of the decay is altered via higher pulse rates (assuming the pulse frequency is increased). This tends to result in a denser EMF near the coil but an inability for the machine to accurately gauge decay at depth. The more recent models are extremely sensitive to EMI spikes (eg: from weather or power lines) and this is why they are unsuitable for modding when compared to older, simpler detectors.

I thought the pulse energy is transmitted into the ground via coil and is absorbed by both ground and targets and then the detector switches from transmit to receive and the electrical or magnetic energy from the ground has already dissipated but the target still has measurable energy left and that is what the detector picks up before it fades away.
All this takes place in a fraction of a second and the different adjustable timings are what can be used to make different sized targets to be heard.
I’m not 100% sure but that’s how I have interpreted what is going on.
Mark

MS: That's probably about right. I was simplifying the process in my description above. I'm sure JP would have contributed by now if he wasn't out hunting.

Hi Nero
I didn't see your post it's wasn't there when I started to reply , you covered it in more detail and I only got that understanding when trying to get my head around how they work for myself.
Mark

Just to add on to this, The ground to target effect is also altered with different sized and makes of coils, I have spend a lot of time and money testing this out on my unit and have now come down to 3 coils which give the best results with my non standard detector and in my view hopefully pick up targets that others mostly miss. There is a lot to try and work out and at the end of the day I would hate to think how much I have spent on coils but I prefer to take things as far as I can and if I miss out it’s due to bad luck and that alone.
Another point I also would like to make is I thought all the aftermarket coils were a bit of a gimick like the thousands of fishing lures in a tackle shop but I now no longer have that view and have found measurable differances that are worthwhile in the field and can make the differance between hearing a target in the ground or not.
mark

Jefgold wrote:Thanks Grey
Pulse trains - are they similar to other railway trains - grin. Now to see if I can understand what you said.
Pulses are sent out -(one long and several short pulses of electromagnetic energy) via the coil into the ground. The detector interprets the signal somehow when they bounce of a piece of metal or other object in the ground. It is the return of the magnetic waves that determine the target. Similar to radar but much more sophisticated
Jeff

G’day Jeff,
Almost there. It’s not the detectors’ primary field that is bounced back from the target, such as in a radar system which utilises radio waves, Pulse induction utilises electro-magnetic fields, a much different kettle of fish, let me try to explain.

When we turn on the transmitter at the beginning of a pulse, the current begins to flow through the coil producing a magnetic field around it, similar to that of a magnet, the difference being our coil is more like an electro-magnet. Because of the large open coil, the field is not very dense, so you don’t feel an attraction to large iron objects. As the current builds in the coil, metallic compounds exposed to the field are excited so that the atoms in them begin to speed up their exchange of electrons, we then start to see small circulating currents generated in the soil minerals and any metal, topically known as “eddy currents”.

The current flowing through the coil grows exponentially until it begins to near the maximum value and the rate of change asymptotically begins to slow. Remember it is only during a change in the magnetic field that eddy currents are generated in the metallic objects. All this time the transmitter has been switched on, we now switch it off marking the end of the pulse. A very high voltage spike occurs in the coil as it tries desperately to maintain the current flow. We snub and dampen this spike and any ringing as quickly as possible and then switch on the receiver circuit.

The very same turmoil that occurred in our coil also happened in the target and the ground, such that the extremely rapid change in the magnetic field, caused by the termination of current flow in our coil at transmitter switch off, caused the target to respond to this change by a massive increase in electron flow such that the generated eddy currents produce a counter (opposing) magnetic field around the target.

The roles are now reversed, the target and ground is producing the magnetic field, and our coil is the target. As this field cuts our coil it causes a small current to flow in it which is interpreted by the receiver amplifiers as a voltage. The wonder in all this is how the receiver circuit has to amplify an absolutely minuscule response from some targets. I dug a flat 0.55 gram nugget at a measured 10 inches, no it didn’t drop down from the side of the hole, it was sitting in a crack in bedrock. Admittedly the ground was very benign, but this still speaks volumes about just how good our Minelab machines are at their job.

As for pulse trains, imagine the pulses, One long, three short, and a short time delay, now repeat that about 1,260 times a second, that’s the train, and the frequency is about 1.26 kiloHertz .

G’day Mark,
True, most of the ground signals have decayed before the receiver is switched on, but certain constituents have a reactive ferromagnetic effect (X) that can have very long decay times. Ever noticed how deeply some ferrous trash is detected, it’s due to this effect such that the results of successive interrogating fields sum, especially with higher frequencies, and ferrous minerals can have remanence and hysteresis too, all adding to the problems of detecting a small nugget encased in such stuff.

Cheers
Grey.

Good subject Jeff
The further this subject has gone the more you realise and appreciate the physics and electronic complexities that have had to be understood and overcome to give us what we use today.
It’s a bit like the more answers you get, the more questions arise , it would have been good to get some input from coil manufactures or people with an understanding on how they work beyond the standard transmit and receive.
My question would have been why are coils flat when you would think a parabolic shape would be better especially on the receive, and what about the wasted energy emitted from the top of the coil, wouldn’t it be of benefit to shield this or reflect this energy back down into the ground where it would be of more use, could this be done by a electronic or magnetic field to focus the energy to give deeper detection depth.
Mark

Hi Grey
Would a pulse train that had for example more low and medium frequencies obtain a different result than a pulse train with less low but more medium and high frequencies.
Cheers Dig

G'day Dig,
Sure, that's what you are doing when you flick from Extra to Normal, going from shorter pulses (higher frequency) to longer pulses (lower frequency). On the 4k5, Sharp has the longest pulses and uses up to 20% more power than Normal, and should come pretty close to the SD2000 for depth on large gold........that is if you can handle the chattery threshold.

With the longer higher current pulses, we have to wait a lot longer after turning off the transmitter before switching on the receiver and sampling the input waveform, thus the signal from smaller gold will have decayed away by this time. Alternatively a short pulse although good on small pieces, does not energise large bits enough to generate a sizable response, catch 22.

Hopefully the day is coming when a sweeping pulse train is employed and all sizes of gold will indicate their presence on a single pass. Additionally, with digital signal processing the possibility arises whereby broad responses from large deep gold will be processed by a time compression algorithm, and presented to the operator as a more normal peaky response that we experience from surface pieces. We have probably passed by these large bits because of not recognising the response as anything more than groaning ground noises.

Cheers
Grey.

That's a great reply, Grey.

Whilst the short and long pulses result in different targets responding, so do the combination of small and large coils - as does (to a lesser degree) the TYPE of coil (mono Vs DD).

Plenty of room for people to mess this up and come home with little to show for their time and effort.

EDIT: Grey, do you have any opinion or observation on Round Vs Elliptical coils?

alchemist
"and the frequency is about 1.26 kiloHertz ."
Having posted this info can you tell us the frequency range the 4500 has on its manual tune?
I know its not much but to know the frequency of use would help those of us set up the detector/coil/setting for the days operations given the amount of EMI present.
Also, does the frequency change much from one timing to the next?
While im at it, lol enhance has been optimized for small gold as we have been led to believe ( no argument here ) software does a lot of work whilst in enhance, but does the frequency change to the highest available or is this timing dependent? in other words, does the frequency for enhance select the highest available for this timing? example freq only .....2.4Khz?
The reason for the questions is whilst performing a manual tune close to another detector one can here the effects of your tuning upsetting the nearby operator, leaving one to think the tx & rx are both altered not just the rx, meaning that one can set the tx up to suit the area given the other variables present.

nero_design wrote:That's a great reply, Grey.
Whilst the short and long pulses result in different targets responding, so do the combination of small and large coils - as does (to a lesser degree) the TYPE of coil (mono Vs DD).
Plenty of room for people to mess this up and come home with little to show for their time and effort.

EDIT: Grey, do you have any opinion or observation on Round Vs Elliptical coils?

Spot on Marco….the variables are so varied and to add to the pie, two coils of the same make and model are not always equal, slight variations in manufacture can have marked performance differences when mated to one particular machine.

As for the Round Vs Elliptical debate, I posted quite a lot on a theory I had, but it wasn’t borne out empirically.
So my latest thoughts are, in a nutshell; 2 coils of equal area i.e. 14x9 inch elliptical mono, and an 11 inch round mono, given equal resistance, inductance and capacitance. I believe the elliptical will achieve slightly less depth, but will be slightly more sensitive to smaller pieces at depth, and will have a very slight blade pattern at depth giving a small amount of overlap. The reason being the two sides that are closer together concentrate the field which increases sensitivity, but it also has a marginal bucking effect which looses depth, a bit hard to explain how I visualise it. Of course its only my theory and could be yet again wrong.
Cheers
Grey

Gi’day Narrawa,

Narrawa wrote:
"and the frequency is about 1.26 kiloHertz ."
Having posted this info can you tell us the frequency range the 4500 has on its manual tune?

I’m afraid I’m not sure of the exact frequency, it hasn’t been a priority to know this so I’ve not looked into it. I suspect the tuning range will be very small probably around 50Hz either side of the fundamental so that a quiet spot can be found between mains harmonics.

Narrawa wrote:
Also, does the frequency change much from one timing to the next?

Yes it must do, but I’m unsure how the tuning is maintained across all timings?

While im at it, lol enhance has been optimized for small gold as we have been led to believe ( no argument here ) software does a lot of work whilst in enhance, but does the frequency change to the highest available or is this timing dependent? in other words, does the frequency for enhance select the highest available for this timing? example freq only .....2.4Khz?

Where are you Philip Beck? Care to help us out here?
I’m floundering in deep water here, all I know is Enhance is quite a low frequency timing, in that there is a lot of dead time between each set of pulses, I’m unsure why but I guess it is to sample for hot rocks and highly susceptible magnetic ground minerals, this could explain why the timing is weighted towards smaller gold even though it has relatively long pulses. The subtraction of late time ground signals also cancels out the responses from larger metal objects?

The reason for the questions is whilst performing a manual tune close to another detector one can here the effects of your tuning upsetting the nearby operator, leaving one to think the tx & rx are both altered not just the rx, meaning that one can set the tx up to suit the area given the other variables present.

I would expect that the tuning circuit would alter the master system clock governing the TX/RX block so yes you would be altering the pulse lengths while tuning the receiver for a quiet channel. I read that there is a sweet spot mid band, but it may have just been the size of gold the person was testing with happened to be better suited to the mid range.
The changes in frequency are only marginal, but if I’m after smaller gold I sometimes select a high number in the tuner, it certainly gives confidence and that’s worth a lot more than any settings.
Cheers
Grey

MS wrote:Good subject Jeff
My question would have been why are coils flat when you would think a parabolic shape would be better especially on the receive, and what about the wasted energy emitted from the top of the coil, wouldn’t it be of benefit to shield this or reflect this energy back down into the ground where it would be of more use, could this be done by a electronic or magnetic field to focus the energy to give deeper detection depth.
Mark

G'day Mark,
I don't know how you could wind a parabolic coil.
The wasted energy is just a fact of life I’m afraid, for the field to extend downwards to any depth it must likewise extend upwards.
If you’ve ever done the bar magnet under iron filings thing, then you will understand how the field passes from one end to the other along the outside of the magnet and then through the centre of the magnet, it’s similar to our coils, if we interfere with the field at one end it affects the other also.
Focusing the primary field with a secondary one is a good idea, although an increase in flux density will not necessarily solve any problems, it may just worsen them. Sure we would get a much stronger response from the target, but along with this is greater ground noise and delayed recovery…..it could be a case of more is less? Maybe this same issue would discount the parabolic TX coil as well.
Cheers
Grey

Wow this is great information guys and will take some time to digest. Grey you said G'day Dig,
Sure, that's what you are doing when you flick from Extra to Normal, going from shorter pulses (higher frequency) to longer pulses (lower frequency). On the 4k5, Sharp has the longest pulses and uses up to 20% more power than Normal, and should come pretty close to the SD2000 for depth on large gold........that is if you can handle the chattery threshold.
Grey would you say that in theory a sd2000 should go deeper on larger targets than those that came after ?

Grey, thx for you in depth reply.
"it certainly gives confidence and that’s worth a lot more than any settings."
This above statement is true to form whether or not the theory behind ones thinking is right or wrong, stay positive throughout the day, if you dont get any gold then you can swear all you like, the day is over.
When detecting for small gold using a smallish coil 11" mono, i use this theory, i tune for a lower set of numbers, as low as i can go within the limit of the EMI present.
The use of smaller coils exhibits by nature of design more sensitivity to smaller gold, so by using lower numbers in the tuning range my theory is that this will give that particular coil more ability to hear gold up to its range of depth, be it much more or not.
If im using the 16" mono given its design is for deeper gold I'll opt for a higher set of numbers in the tuning range giving the coil a better chance at smaller gold down to its range of depth, this works for me the majority of the time but there are some circumstances where this in not feasible due to EMI, ground, or other.
The theory works well on un dug targets but to test it on targets placed on the ground yields little in confidence to anyone trying it due to the amount of veritable's & the close proximity to the coils field.
The questions about tx frequency has plagued me for a long time, im sure it will be revealed sooner or latter, but your replies & posts are always a good read so keep up the good work.

Fisherman wrote:Grey would you say that in theory a sd2000 should go deeper on larger targets than those that came after ?

G'day Fisherman,
Tests have shown that it does, but the operator has to really know what he's doing.
Cheers
Grey

Hi Grey
Thanks for your valuable input on this post.My main concern with the latter minelab machines is that we have been left in the dark a bit.I have not been overly concerned on small coil shallow setups.Its the area of using large coils on deep ground that the operator could be defeating himself without an understanding of the right settings to use.
Cheers Dig

This thread has been a great revelation to a newbie. I have increased my understanding tenfold. Thanks to everyone who has contributed.

Dave

Hi All,
..my question is... have the frequencies changed from the early model GP series to now that make the GPX series more susceptable to shallow gold as opposed to the GP series? Given that most gold located is shallow and small. Or is it my imagination that makes the 2200 a better machine when looking for gold at depth.


Ray

I was of the impression that If I turned up the gain then I was turning up the sensitivity of the recieving part of the detector. Not actually turning up the output power. Is this correct?

Dicko..

.
LINK: GPX (with Mono) Vs SD & GP (with DDs)

Might be worth a look Ray. Not sure if anything was resolved but I think the theory is still sound.
I don't think it's your imagination. Seems that the preferred coil-choice for different machines may be the cause for different results in targets and their size/depths.

Cheers,

Marco

Hi Marco...

I really think what I wrote on my last thread is the case... also u posted information about the DD -v- Mono coils some time ago on this site and how under certain conditions mono coils COULD be deflected. Although your theory was shot down by some .. I really wonder if it has merit..

Cheers
Ray

PS Sorry the link u provided is the thread I was referring to...