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DeepTech

I hear alot of fluff about depth...

A

Anonymous

Guest
...from folks. A penny at fifteen inches??!! A model A Ford at 16 feet??!! As far as the electro physics of the thing is concerned, the field generated by a detector's coil CANNOT extend in any vertical direction much beyond the width of the coil itself. Even then, it's not cylindrical but rather cone like in shape, being governed by the parabolic nature of transmitted electromagnetic waves. Thus, your "detection cone" or "bubble(as some have referred to it)" will be narrowed to a pinpoint at it's extremity, no matter what you do. OK, so what can we DO?
We can overlap our sweeps to cover more area down at the limit of our "cone".
We can increase emitted power within the field to enhance target reaction TO the field.
We can extend the diameter of the coil to give us a bigger, thus longer(deeper) "cone".
We can increase gain of receive circuits so that fringe signals can be included in the phase response of the unit( that is really what a discriminator is: a phase-shift detector). However that is fraught with its own problems: we then have to filter for induced circuit noise and such things as EM interference. <STRONG>And</STRONG> then be forced to deal with soil mineralization and "hot spot's"( rust areas, foil bits, small junk items, etc.) within the matrix.
Think about it. If a coil is 8-9.5" in diameter and we have the best low noise amplification coupled with todays digital contollers and software to control the detectors functions in real time for us -well then, doesnt that sound like sufficient depth?
Face it, using a "standard detector" and without resorting to trickery of some kind, how many of you can honestly claim to detect anything of coin size at more than that 8-10" with any regularity? No matter whose detector you use. I'll take a Cortes with that kind of depth anyday!
 
The 15 inch penny remark was made by a rookie, my self and I was informed I missed the target and it fell. Thank you very much.
 
<body background="http://wiredcafe.net/jbms/tesorobg2.jpg"><center> <img src="/metal/html/biggrin.gif" border=0 width=15 height=15 alt=":D"></center>
 
...please accept my apologies if I offended you. It was not intended nor were my remarks directed at your post. I was commenting on this depth obsession we have, when if we carefully select or SITES <STRONG>or</STRONG> modify our expectations based on our sites, we will find we have most all the depth we need on coins.
Again sorry if I offended you
 
You said <span style="background-color:#ffff00;">".....the field generated by a detector's coil CANNOT extend in any vertical direction much beyond the width of the coil itself....."</span>
I guess that would depend on the definition of "much beyond". There are also other things to consider.
How does my Shadow X5 with the FIVE inch coil readily detect my wedding band at 12+ inches? Or my Compadre with a 4 inch coil conversion detect coin-sized objects at 7 to 8 inches? The electromagnetic field being induced by the coil obviously has to "get there" to cause the target response in the first place and to be able to detect the distortion in the field. So if the field of the coil is not reaching beyond 5 inches or 4 inches on these machines respectively, what IS happening? And why won't a 40 inch coil detect a coin-sized target at just a few inches, while a 4 inch coil can easily detect the same coin?
Granted, these are in optimum conditions (air-tests) with no ground matrix entering the equation, and things will be different "in the ground" depending on the level of ground mineralization, the positioning of the target, trash levels, etc. etc. But there is no "trickery" involved..... it's a matter of "<u>either it will or it won't</u>", with alot of outside influences and variables involved when it comes right down to performance in real-life situations. Too many think of performance in terms of what they have experienced rather than "optimized" levels of performance based on the potential of the machines design.
The term "trickery" is bantered around alot these days where metal detector performance is concerned. But stop and think about that for a minute. If one machine has the "option" or the "capability" of doing something that enhances the performance of that particular machine by use of a particular manual control, how is that "trickery"? If one machine has the "capability" while the next machine does not, are there "tricks" being used to get the closer-to-optimum performance out of the better performing machine? Hardly. A good analogy might be one from my days of racing motorcycles on the drag strips of Southern California. If bike A and bike B are identical, but one used a nitrous injection system while the other didn't, was "trickery" being used? Of course not. It was just a matter of using a specific tool to increase the performance of one bike over the other. One bike was "faster" as a result, just as one detector might be "deeper" as a result of the use of specific circuit designs or available tuning controls (tools) that increase it's performance. But like you say, there are alot of variables to consider as well, including external or internal noise, ground signal, and other "problems" that are compounded along with the use of some of those "tools" that increase the overall performance of the machine. I'm not saying that there are no "physical limits" of what the detector can do, but that it is sometimes dangerous to make "absolute" statements. <img src="/metal/html/wink.gif" border=0 width=15 height=15 alt=";)">
I think the more likely reason that most of us don't find more "deep" coin-sized targets than we do is that there are just not many at those depths to begin with, especially in areas (such as here in the states) where human occupation and activity is relatively new in comparison to Europe or Asia. Unless there has been natural erosion or man-made disturbances to the areas where coins are lost, there just aren't many beyond the 6-10 inch levels to begin with. One exception might be sandy beaches, but in a technical sense, wave action could be counted as "erosion" I suppose.
Personally, the reason that I find myself using the smaller coils more often than not, is that there are an exponentially greater number of good targets "hidden" from the detectors electromagnetic field due to masking effects than due to depth. No doubt there are times and situations where "depth" is the primary concern, and where the larger coils are the best tools for the job. But all too often, we are concerned with "depth" when "target separation" is really the most common problem for detectors and operators. That is one of the reasons I use and prefer Shadow and Tesoro machines above anything I've used in over 30 years of detecting. Larger coils have specific advantages on one hand, but another consideration on the other hand is the loss of sensitivity to smaller objects as coil size is increased and "field strength" is "diluted" due to the same signal being spread over a larger area and the lessening of target signal strength in the process. Field strength (concentration) differences between smaller and larger coils will normally create a situation where using a smaller coil will not reduce "depth" and signal strength to a proportional degree as increasing the size of the coil and the related drop off in small target sensitivity as a result. In other words, while getting range (or depth) equal to twice (or more) the diameter of a small coil is a real possibility on some machines, that capability with larger coils becomes less likely as the coil diameter is increased, all things (target size) being equal. So in a way, you are right in saying that there are actually very few "standard sized" coils in the 8 to 10 inch range that will detect coin-sized objects at much more than their relative diameters. It all boils down to a matter of physics, coil design, field transmission strength, receiver circuit capability, and all of the myriad external factors that come into play in the process.
Just a couple of thoughts......
 
Very good post Ralph. I am amazed at the small bits and pieces of trash that the old Tesoro will ring up not to mention the "good" targets. I suppose that the 40 inch coil is made for one of those Sovereigns as weight wouldn't matter. <img src="/metal/html/lol.gif" border=0 width=15 height=15 alt=":lol">
 
Those arent falling down deeper into the hole, they a lot of times are buried by people. They are often well beyond the width of a coil.
And would what your saying apply to pulse detectors also?
Im not trying to squabble with you, just my experience has been a little different as I can and do dig coins with an 8" coinsearch coil from 10" to 12" in the wet salt sand. 10" ones reguarly.
There must be more to it than what your saying otherwise this wouldnt be happening.
Ive also given up on signals deeper than my scoop which is 14" long, those I would guess are cans or something else rather large.
Neil
 
Hi Dave,
There are some primary differences between pulse and IB type machines as I'm sure you know. Pulse machines operate by sending momentary electromagnetic pulses into the ground, and then listening for decaying eddy currents that remain in the target. IB machines operate with a constant transmit and receive function that has no breaks like the pulse machine does, so are actually operating in a much more "noisy" atmosphere than a pulse machine, one of the big advantages to pulse technology.
On the other hand, pulse machines still have no dependable method of discrimination, at least not at the levels of induction balance machines. They can discriminate by determining differences in target decay rates, or in "reverse discriminating" iron from non-ferrous materials, but VLF/IB is still far superior in the area of discrimination.
As far as the physics of the coils in relation to size, pulse and VLF/IB are really no different.... both transmit or produce an electromagnetic field at the coil that is limited in range by the size of the coil. One difference however, is that pulse "mono coils" have a larger mean (average) diameter than do VLF/IB concentric coils, those most often supplied as "stock" on todays detectors. Since there is a combination of separate transmit and receive loop windings, the "effective diameter" of the concentric coil is smaller than a like-sized mono coil. A true mono coil of 12 inch diameter is producing the optimum field size from the coil based on the single 12 inch loop winding. On the other hand, a 12 inch VLF/IB concentric coil might have an outer transmit loop winding of 12 inches with an inner receiver winding of only 8 inches, making the "effective mean" (average) size of the coil only 10 inches in relation to the 12 inch mono coil. This is something alot of people fail to consider when comparing pulse and VLF/IB type machines, and the advantage, as you can see, is usually in favor of the pulse machine for the reasons shown, not because the technology is any more "superior" as far as the depth capabilities of the actual design. They are both putting out similar electromagnetic fields at the coil, but "receiving" and determining the presence of a target in different ways.
Wet salt sand is another advantage on the side of the pulse machine, since the eddy current of the wet salt decays much faster than most pulse machines are tuned to receive (via the gate pulse delay circuits - the time between switching off the transmitted signal and turning on the receiver circuit - measured in millionths of a second). By "ignoring" the wet salt sand, the target signals are more distinct and the pulse machine is capable of detecting deeper targets in those types of ground conditions than the VLF/IB. In some cases, the wet salt sand can actually act as a kind of "carrier" of the coils electromagnetic field, allowing the pulse machine to detect beyond what "physics" might otherwise dictate. Pulse is a "different animal" with alot of future potential for someone willing to put the detection circuits to microprocessor technology, garnering the maximum amount of data that is available in the target's signal. Considering pulse is true "broad band", operating across a very wide range of the frequency spectrum, the possibilities are really endless. That technology is just a matter of time in my opinion.
But in the end, there are some "limitations" to consider in relation to target detection range vs. coil size. But simply stating that the diameter of the coil is the maximum range doesn't work. There are optimum coil sizes to different sizes of targets as far as range or depth goes. But smaller coils will always give greater "sensitivity" to smaller targets than large coils, and visa versa, due to the more concentrated field produced by the smaller coils.
Lots of variables involved other than just coil size.
Just a few thoughts......
Ralph
 
The absolute deepest target I have ever dug was an empty pop can in moist (not saturated) sand in a dune near Lake Michigan. It was around 30" give or take a couple. It was located with a White's GM4B gold detector using the stock 10" elliptical concentric coil. That taught me what a deep big target sounds like. They have a very broad signal area probably due to the large surface area.
Tom
 
it follows that since the pop can gave a broad signal response, the field from the coil was actually extending out a bit further. In my case in that clean moist sand, perhaps roughly the size of a 40 gal drum?
I baffles me how White's make those Goldmasters as sensitive as they are without encountering a lot of the problems usually associated with high gain machines. Ralph if you read this, are they strictly analog designs? Thanks
Tom
 
Hi Tom,
Those earlier GoldMasters are still considered some of the best "workhorse" gold machines ever made. I still have a GM-3 that I use for most all of my gold hunting, that is when I can get out west which isn't too often these days. I used to live in Southern California, and was alot closer to the gold bearing areas back then.
The GoldMasters from the original Goldmaster-II to the v-Sat, GM-3, and the 4B were all basic analog machines, with each newer unit being an improvement on the original circuit. I used all of them at one time or another, but stuck with the GM-3 as it just "fit the bill", and I got along with that particular unit and preferred it over the others. To this day, I still prefer a good manual machine over the later auto-tracking or automatic ground balancing machines for gold hunting. The only real difference between the GM-3 and the 4B (other than the physical configuration) was the added (1000:l resolution) "ramping" ground balance of the 4B, which I always thought was a bit of "overkill" on White's part. 40:1 resolution was always plent (and then some) since that is the one control that you "work" the most when gold detecting anyway.
Here's a write-up I did on the Goldmaster a few years back that you might enjoy (link below) over on my NuggetShooter's Forum at Voy.com:
Ralph
 
Hi Ralph,
I remember reading that report of yours sometime back. I agree they were and still are a great machine.
Early on I came close to whacking that thing on a tree before I finally got a handle on it. You see it was my first detector and I had no idea what I was getting into with that machine. It might have been different if I would have had someone to teach me, but I was "on my own". I had to teach myself everything (GB'ing, difference in sounds of various hot and cold rocks, how to compensate for mineralization with the Gain and SAT..everything was new to me! The manual was some help, but being written by Jimmy S for experienced gold hunters, I had no idea what some of the terms he was using even meant!! I had to reread most sections multiple times. I would go over them after each outing and apply my experiences from that day against what in the manual, trying to make things click in place in my mind. Kind of like reverse engineering. <img src="/metal/html/lol.gif" border=0 width=15 height=15 alt=":lol">
It has had one benefit tho, after that ordeal, "normal" discriminating VLF's were a piece of cake! One thing for sure, I will never forget that machine!! <img src="/metal/html/wink.gif" border=0 width=15 height=15 alt=";)">
Tom
 
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