Ground Balanced On The GPZ

[russ]

 

I thought hawkeye might like to see the GPZ on tree roots.

[Norvic]

From videos, and own use, forget up and down with the GPZ coil, always side to side as per searching over a target. Constant height over ground, wider holes, no real difference from all other motion detectors I`ve used. Perhaps even consider it a habit to avoid especially with the GPZ. This is just my observation, will try up & down myself next positive signal out of curiosity.

[fredmason]

The manual specifically says not to poke the coil into the dig area...and to slope the side of the dig....a false signal on all the detectors I have used can be generated by poking at the signal area...

 

I of course have no actual experience yet....but, sometimes it really is best to follow the manufacturers directions......... and maybe even read those directions...

 

fred

[wirelessguy]

 

Gold Hound

Posted Yesterday, 12:03 AM

Reno Chris, on 20 Mar 2015 - 11:02 PM, said:

Radiant energy disperses into all three dimensions and so decreases in the cube of the distance. So it's the inverse distance cubed.

since the.energy goes out both from the coil and then from the target back to the receive coil, the total energy loss in both directions is the distance times 10 to the 6th power, or a million times.

Look up 'inverse square law' on Wikipedia and you will se what I am talking about.

You're right about energy traveling out from the source in 3 dimensions, it travels out in an ever expanding sphere.

My explanation and mathematics are correct the inverse square law takes into consideration all 3 dimensions.

 

 

 

Gold Hound,

 

The total energy across a unit area of an electromagentic field in the far field decreases proportional to 1/r^2 with r being the distance from the radiator.   Far field by all definitions and approximations = more than a wavelength.  If we approximate the frequency involved with metal detection as 20 kHz we get 15,000 meters wavelength using c=(freq)(wavelegth)  with c= speed of light = 3*10^8 m/s.   So that is a range left for charlatans and heretics with their divining rods and long range locators.   So 1/r^2 is valid for distances far far greater than we are talking about finding gold nuggets.

 

And this paragraph above has nothing to do with metal detection using induction balance, pulse induction or zero voltage transmission.   That is because all metal detectors are based on induction principles - magnetic field only.  NOT electromagnetic plane waves (radio waves) propagation.  The metal detector coil is not an "antenna".

 

In my 30+ yrs of radio frequency engineering practice I find the concept of near field and far field as a function of wavelength used widely in electromagnetic plane wave (radio wave) practice and are not so much used in inductive (magnetic) field practice such as in the design and winding of inductors, transformers.  Specific geometries come far more into play governing the magnetic field strength at points in the area of interest.   

 

I blame all of the detector manufacturers for spreading over simplifed information on this subject when it is not that complcated.

 

I am still a relative newbie to metal detecting however have devoured literature (including tons of lore on blogs) on the details of the transmitted magnetic field from an MD coil and the return eddy current from the target to the receive coil.   I guess I differ with your statement above.  I find it extremely complicated.  I find the best simplification that exists on field strength that has merit in the physics of magnetic fields is the 1/r^3 approximation Reno Chris and jasong note.   ( I don't find Chris' reasoning above for the 1/r^3 approximation accurate).   I have read a few different credible reasonings for the field strength approximation of 1/r^3 under many applicable scenarios of dipole approximation of a current loop and size of area this applies to (size of target) relative to the loop.  1/r^3 approximation is widely quoted by strong technical people in the metal detector field.

 

As Chris notes, returned field strength loss vs distance becomes 1/r^6 if you consider transmit magnetic field to energize the target and the the return path of eddy current from target to the receive coil.   

 

HOWEVER, 1/r^3 is an approximation with multiple ifs and thens and the actual math is complicated particularly close to the coil.   I think the metal detector manufacturers must simplify all this physics with best visual models of cones and blades when it comes to using the metal detector.  Humans don't generally visualize in terms of graphs and equations.   So, the way that I, an inexperienced user, experienced RF engineer, reconcile statements that Steve H and others make about those cone and blade approximations is that these experienced users are integrating a lot of experience on different size targets at different distances, with different coils, in different conditions and using a lot attention to detail in their practice and have observed the physics exceptions to the visual approximation.  Good info, more nuanced than a brand_new_first_times_in_the_field metal detector user needs.

 

http://www.geotech1.com is a great site for the engineering and physics side of metal detecting if you have not already found it.    

 

[Reno Chris]

Quote

HOWEVER, 1/r^3 is an approximation with multiple ifs and thens and the actual math is complicated particularly close to the coil.

 It is a rough approximation sort of thing, but the reality of the situation is actually much worse as we do not detect targets which are floating weightlessly in a vacuum. The minerals of the soil do react also with the magnetic field of the transmit coil absorbing some of the field energy going to the target, and then also absorbing energy as it goes from the target back to the receive coil. Unfortunately, gold is often found in mineralized areas. As a result,

"signals in the receive coil which can be millions of times weaker than the signal in the transmit coil"

Bruce Candy, Metal Detector Basics and Theory

[Marty]

 

 

If your in General/Difficult and want to switch over to say General/Normal , you have to do the BG for the next mode right ? Or are you able to keep swinging away once you have made the mode change, if a re BG has to be made every time you change modes would this not take a good length of time for the ground data to be back up to speed ?

 

Marty

[Steve Herschbach]

Wow, quite the rabbit hole going here! My flight was just delayed, flew all night, missed connection, new flight, been delayed, two gate changes so far and still no plane. Anybody need a chill pill, I have a bunch!

I am all about practical reality myself. When I say straight edged blades shooting straight down are a myth it comes from simple observation of reality. Guys, turn the coil over, and observe a small, medium, and large target under the coil. Forget all the theory. Just look and see. Let me know when you see a shooting blade pattern. Or a pointy cone. Reality is bowls and elongated bowls that break up into winding related hot spots with small targets.

I think it is important to know why things work the way they do until worrying about it gets in the way of finding gold.

[Steve Herschbach]

Marty, I personally ground balance again after changing modes. I also believe the 20 seconds it takes to do so is more than sufficient to ground balance the machine properly enough to operate well. I really think people have latched onto this improved automatic ground balance method as being magic. I have no issue personally with just putting the GPZ in fixed ground balance mode and using ut without the tracking. That used to be detecting 101. Now all the sudden tracking is all the rage and not only will the GPZ not work without it but only if given 20 minutes to chew on data points?

I do it think so.

I have found this machine to be very easy to operate and it seems to me as if everyone wants it to be much more complicated and difficult than it is.

Just my opinion and a general observation not aimed at anyone in particular!!

[G.B.]

Went back with GH re checked and re dug again sounded good detected this time without dipping into the hole result ground noise which didn't balance out this time until the hot pocket was dug out. 

 

l can honestly say the 4500 Enhance handled hot pockets and hot rocks better than the GPZ using audio smoothing off, it also loves charcoal and moisture rich tree roots.  

 

We have re-worked several old patches for minimal results confirming if you worked a shallow patch properly first time round with a 4500 there will be stuff all left for the 7000 and in some cases none left.

 

It took me at least 2 years to become proficient with the 4500 and that was with the help of great mentors and l have no doubt it wil take considerable time to become proficient with the 7000 to the point of hearing and understanding what it is telling me.

 

lt will not open up the Golden triangle again like the SD Series did because most of that easy gold is gone, will it do similar to what the GPX achieved by finding previously undetectable gold only time will tell.

 

If you think that this detector is going to be a majic wand and make gold instantly jump out of the ground without hard work, research and a lot of luck then you are living in fantasy land.

[Ashley100456]

Thanks GB & GH for the follow up, Was eagerly awaiting the outcome.

You are right this new Detector is going to take some listening and learning to get the best out of it.

I wonder if down the track some of these little glitches can be done away with by an updated from Minelab through a computer download ?

Anyway Thanks again for the follow up, Heading of to W.A. to hopefully get amongst it with a Zed shortly ( along with my trusty 5000 )

 

Cheers

Ashley