Long question about FBS/coils/resonance/depth...

Hi all...

This is going to be long-winded, so bear with me please.

Background: I have relatively low-mineralization in my soil here in central OK -- but what mineralization there is, is in the form of iron (red clay predominates in this area). As a result, depth with ANY machine, and ANY coil, seems to suffer somewhat here -- more specifically, proper ID at depth. In all-metals mode, coin-sized targets can be seen plenty deep here with most machines, but ID tends toward iron with depth. The best ID machines in my soil (and I have tested roughly 8 different machines) are the FBS Minelabs (and, it would stand to reason, other multi-frequency units), and the Fisher Gold Bug Pro. Lately, I have been testing a SunRay X-5 5.5" coil on my Minelab Explorer, and have had some interesting observations, which I'll share, and then I want to speculate as to why this might be, and see if my thinking may be appropriate, or off base. Beforehand, though, I want to state some assumptions that I have, which I THINK are correct based on my understanding; please correct me if I am wrong. These assumptions are:

1. Low frequencies "resonate" best with high conductors, in general;
2. High frequencies "resonate" best with low conductors, in general;
3. Low frequencies can generally "penetrate" the ground better, giving generally better depth, as compared to higher frequencies, all other things being equal.

Also, I will, in this post, refer to what I call "lock-on" -- which is a "phenomenon" that I seem to notice with my Explorer. I am curious to know if others notice this, too. To describe what I mean...in using this machine over the past 9 months, I seem to have noticed a "characteristic" for the machine to, after a second or two, sort of, "figure out" what type of target it is seeing, and then "lock on" to that target from an "ID" perspective (both tonally, and graphically/numerically). To elaborate: as I am hunting along, and I get a decent-sounding chirp on an initial pass over a target, and then stop to "wiggle" and investigate the target further, the signal often times degrades -- even to the point of a "null" many times. It's almost like the machine said "I saw an unknown target initially, but now as you wiggle, I have been given time to see that this target is most resonant with an iron-favoring frequency, so I'll now stay in that frequency and spit out an "iron" ID, or a "null" for you." On the other hand, I have had exactly the opposite happen; I get the same decent-sounding chirp on that initial pass, and then when I stop to "wiggle," the signal starts to quickly improve, and sound like a good coin (i.e., the type of "lock on" I look for). It's almost like, in THESE cases, the "wiggle" allows the machine to say "I have now determined that THIS target is most resonant with low frequencies, so this is more likely a high-conductive, good target -- so I'm going to stay in this lower frequency, and give you a nice "coin"-type ID." Has anyone else noticed this tendency? I do NOT notice this with any other single-frequency detector I use, or have used. It seems specific to the way these Explorers work.

SO, based on what I observe, in my mind I try to kind of "reverse engineer" what might be going on, just based on how I observe that the machine seems to behave, so that I can hopefully understand the machine better and ultimately, improve my hunting. (Since this information is apparently proprietary, Minelab doesn't let us in on it; this makes it hard to utilize these machines to their highest potential, and forces a longer "learning curve" than is probably necessary...and likewise fuels a need to "speculate!" )

Anyway, from my observations, here's my guess at how these units may work:

When sweeping the machine in search of a target, the unit resides in some "neutral" or "middle-ground" range of frequencies, so as to get the best combination of resonance to both lower and higher conductors, and a good bit of depth (possibly biased toward a somewhat "lower than neutral" frequency range). Then, however, when you "hit" a target, the machine starts quickly "sampling" -- using the different frequencies, or harmonics, or whatever it does. It processes all of this, and then when it determines which frequency "hits hardest" on that target, it then "settles" on that frequency or range of frequencies, and gives you ID information using that "best" frequency range for the given target -- and thus the "lock-on" that seems to happen in terms of ID of the given target.

Now, I may be TOTALLY out to lunch here with all this speculation on what goes on inside these units, and if so, someone please correct me.

But, to continue with this...since I do not observe as good of depth ability in my soil with my Explorer as others claim to, in THEIR soil, I will guess that it has to do with my supposition that the "middle-ground" frequency range I speculated that the machine runs at as a default, until it "sees" a target, is not the DEEPEST frequency range available, for my soil. It would be fine in more NEUTRAL soil, but for me, I will still have some "loss" of depth.

NOW...to explain a bit more WHY I speculate that there must be some more "neutral" and "slightly more limited-depth" frequency range that is employed by default with the FBS machines prior to detection of a target, I will talk about my X-5 coil observations, and how this all relates.

I have hunted with the X-5 for roughly 8 hours; in this time, I have not hit a dime/penny-sized target deeper than about 5 or 5 1/2". Many folks who use this coil discuss hitting dimes down to the 7-8" range as a maximum; I'm a good inch or inch and a half less than that, in my limited experience with the coil thus far. So, to try and figure out what may be happening, I went to my test garden (though realizing that my observational time/sample size is very limited, and that it is QUITE possible I simply have not SWUNG OVER many 6"-plus coins so far, as we do have very DENSE clay here and coins often don't GET any deeper than 4-5" ).

To test, I entered the test garden with the X-5 as if in "normal hunting" mode -- i.e. normal sweep speed, acting as if I didn't know a coin was there. I headed for the shallower coins, to start (my 6" coins). Almost invariably, the machine would NOT report any of the coins on a "normal" sweep over the coin. It got a "chirp" or "blip" out of them MAYBE 10% of the time. Based on my experiences with other coils in the garden, a "chirp on only about 10% of the sweeps" means that you are near "fringe depth;" I also knew from prior experience that stopping and doing the "Minelab wiggle" over such a "fringe-depth" target for a given coil would get you a bit more consistent hits, but not really the good "lock-on" that would otherwise happen at shallower depth. SO, assuming that I had discovered that I was simply at "fringe depth" for this coil in my soil (unfortunately), I decided to "wiggle" some of the targets anyway. What I found out, though, surprised me. I quickly observed that EVEN THOUGH I would completely miss a 6" dime or 6" penny (or even the 6" quarter) on a "normal" sweep, within 1-2 seconds of "wiggling," I would get a GREAT lock-on. Now, a target that was COMPLETELY MISSED before, started giving outstanding tones/ID, repeatably, from all directions! Definitely, this seemed different than what I had experienced with other coils. So, I tried some deeper coins. I need to mention here that I have previously determined that some coins in my garden are harder to detect than others at the same depth (likely due to the fact that I didn't "sanitize!" ) For one, my 8" silver quarter has always been a "tough" target, and not surprisingly this coil struggled with that 8" silver quarter. But, it did better at "locking on" to the 8" silver dime; it was able to "lock on" pretty well to a 9" penny; it could even semi-lock on to both the 10" and even the 12" clad quarter! Bottom line is, I expected (hoped) that this coil would reliably hit a 6" dime/penny, and a 7" to 8" quarter for me -- given the testimony from other X-5 users of 7" to 8" dimes being found. What I discovered though, is that while it seems I would generally MISS that 6" dime or 8" quarter that I hoped to be able to hit, I DID find that I could EASILY "lock-on" to these coins with the "wiggle." Hmm...why? I would like an answer to this; the ability to "lock on" at deeper depths doesn't help much if I can't at least get a chirp on an initial sweep over the target; I tried all different sweep speeds and all the different machine settings I could think of, but no joy.

Here's my guess on what may be happening, building on all that speculation I did above -- and I'm interested to see if this makes sense to other, sharper electrical minds than mine...

This little coil, sending out that earlier-speculated-upon "neutral" or "mid-range" set of frequencies into my irony soil when in normal "search" mode, is -- proportionately-speaking -- even more limited in depth at that speculated-upon frequency range than a larger coil would be. That range of frequencies simply won't penetrate this soil enough, ESPECIALLY when transmitted by such a small coil, to get a "usable" target return from any deeper than about 5" or so. HOWEVER, when doing the "wiggle," the coil is apparently getting JUST ENOUGH signal back so that the machine's algorithms (as I have assumed them to be in this post) were able to start "doing their thing," and as they recognized my test garden coins as "high-conductive targets" -- then the assumed switch to that lower range of frequencies occurred. And then, WHEN THAT OCCURRED, since these presumed lower frequencies would be capable of better depth penetration, the resultant was not only a "lock-on" of very good, stable ID, but a VERY SOLID lock-on -- on a target that, just seconds before, was at the very fringe of detection for the "prior-to-lock-on" range of frequencies I presume were being employed by the unit.

I know this is long-winded, and probably convoluted, but if anyone has any thoughts on any of this, I'd love to hear them. All I know is, preliminary observations show this coil to have a much larger difference in pre-lock-on depth and ID capability vs. post-lock-on, as compared to other, larger coils I've used. Ideas?

Steve



Edited 6 time(s). Last edit at 01/12/2012 10:34AM by steveg.

I've noticed this "degrading of the signal" over a target with my E-Trac. I've also noticed the "Lock-on" of target as you work over the spot. I just assumed it was caused by the slow processor or slow processing of the target to get the depth. I have been using the X-8 coil lately with good results. Depths of 7-8" inches or deeper. The soil here is mostly mild hard clay but there is some loathy rich soil in some locations. You've brought up some good questions...

Obviously I'm not a Minelab man so my thoughts could be totally wrong but I think that with the smaller coil you are "seeing" less ground which in turn gives an initial target response that is going to be more accurate because there is less information being analyzed. Can that make a subconscience difference in what grabs your attention? Once the detector gets your attention and you begin to inspect the target much more intimately you are able to get closer to completely eliminating the air gap with the small coil than with a slighltly larger coil(does a millimeter of airgap matter under part of the coil even if most of the coil is completely flat on the ground?) so you get the better lockon.

I pass it off thinking that the coils' sweetspot is not over the target....that's using the concentric of the 3d. But you're saying that the more you pass the coil over the target, it seems to "light it up" so to speak. We'll have to wait for the gurus on this one.

I have found that part of the reason for your results is sweep speed.The fbs machines are a motion type machine and you will loose depth at slower sweep speeds.The wiggle is generaly much brisker than normal sweeps and gives more depth. Faster speeds hinders unmasking abilities which is the primairy reason for using the small coils.If it is trashy enough for me to feel the need for my x-5 I slow way down and do not worry much about depth because I am trying to seperate the good from the bad.

Steveg,
Have you tried adding manual switching of channels on your ET to see what different results you may or may not get? I've read somewhere that out of the marketed 28 frequencies used by the machine, it will only utilize 2 or so at the most and this might be by channel selection. I asked the following in a post several months ago over on findmall.

"So I'm still very currious about the 28 frequencies, what each one is, and how the ET lumps a select few of these frequencies into their channels. Is this "lumping/selection" a constant, or hard coded into each channel, or does the ET dynamically assign a few frequencies per channel based on some sort of intelligent algorithm? If hard coded per channel, and the math is simply selection of 2 frequencies per channel, does anyone know what frequencies are per channel?
I'm new to this hobby, and see the beauty of letting the detector choose what channel is best based on different ground makeups, but I wonder if the channels were hard coded to frequencies, and it was clear what frequencies were being used per channel, the user could override selection of frequencies by mannually selecting a specific channel, and thus hunt specifically for gold, silver etc."

**** What I discovered though, is that while it seems I would generally MISS that 6" dime or 8" quarter that I hoped to be able to hit, I DID find that I could EASILY "lock-on" to these coins with the "wiggle." Hmm...why? ****

The key appears to be in the 'wiggle' and not in a change/modification in the transmitted freqs and not due to the introduction of a unique or new processing routine response for hard to ID targets from the detector. The 'wiggle' technique is used to improve target response and therefore improve target ID. The idea is to find a target, probably in 'All Metal' mode since it has more depth, then switch to 'Disc' mode and wiggle over the center. If the tone goes away, decreases, or breaks up it is likely iron/junk if it is a solid tone or rises it is probably deep non-ferrous. This technique works with different makes of detectors and does not appear to be related to a processing scheme. The assumption why it works is that the wiggle puts the densest portion of the transmitted magnetic field directly over the target. In this there is less warping of the field away from the target by nearby iron (which acts like a 'cloaking' device) and also less confusion returning to the machine from illuminated iron (you can also help this by using a smaller coil). It also helps to 'wiggle' while moving perpendicular to the centered point so that the magnetic field's orientation (field lines) hit/intersect the target differently at different angles (the target itself may be at odd angles).

The goal is to intersect only the conductive target and do so perpendicularly (target surface perpendicular to the field lines) so as to induce the maximum eddy growth (depth) and gain the maximum returned signal response to work with. Broad coil sweeps won't normally do that - too much iron/junk illumination to process and separate, too little time-over-target, and too much field distortion in a high-junk ground matrix. Isolating the target by illuminating only the target (if possible) that is the operator's best recourse and the 'wiggle' helps. The rest is up to the detector algorithm to respond properly to the return.

Of course there are other factors at work. Some detectors respond better because they request more information from the suspected target (multi-frequency usage) which is more likely to yield an improved discernment of the target's makeup. Other factors are a better built coil, better programming design, tighter manufacturer tolerances or better components, all give certain detectors an edge and make a difference on fringe detection. The detector's transmit freq(s) may just happen to correspond better to a particular target's makeup and hit deeper because it drives more/deeper Foucault currents - thus using the right detector on the right target helps. So there are lot's of cause/effects going on under the ground unbeknownst to the detectorist. The 'wiggle' is one way/tool to give whichever detector you use a fighting chance to better discern those fringe targets.

interesting,thanks for shareing

Interesting observations but personally try to understand the hobby from a laymans standpoint and certainly adnit the Explorer series may have the best guts in the hobby but is not a fast unit which may answer some of your questions and observations.

Seems like some units just work better than others at certain locations and like to mentally insert in my neck of the woods while reading a post usually works for me to better understand as perhaps not applicable to my neck of the woods.

Nice informative forum from reading the posts and would advise all to experiment in the field as we get ideas from the post but its in the field where we learn and what works for one many not work for another.

Lets face it we are not all playing on a level field considering the U.S. and across the waters and sort of smirked last summer when an observer said what an easy hobby..hear a beep, watch a meter and dig which indeed is not true so keep the posts coming as we can never learn enough whether it be in the field or perhaps reading a post.

Johnnyanglo,
I've come to understand through reading Andy's book and general research that All Metal and use of Discrim patterns have nothing to do with depth increases or decreases on the E-Trac, but, I'm not a ML engineer, so I could have my interpretation wrong. Also, I've found by starting out with discrim pattern utilization and then double checking a target by moving to quick mask with no descrim helpful in ID.

Randy Kight

Very interesting, all. Thanks for the thoughts. Sounds like others have seen this "lock on," or else "degradation of the signal" when you stop and "wiggle" the target -- which helps to discern "good" targets vs. "bad" ones. It is very noticable on my Explorer -- almost like that "first chirp" is nothing more at times than a signal to "stop and investigate." It is, then, in that investigation, that the target ID begins to take shape. It sure does seem to me that something is going on during that time where the FBS machine's algorithms are "doing their thing;" I would THINK it would have to do with frequency switching by the unit for the best target ID, but maybe not.

jrk -- you asked about switching the "noise channels." I did try that, in my garden with this coil, but didn't expect much; I have seen it said enough times by enough people I trust to believe it, that those different frequencies are only TINY changes, and that it does NOT affect the performance of the units (i.e. this channel will give you much better depth, etc.) Incidentally, changing noise channels in my garden did not seem to me to have any effect at all on the response, while I was testing this coil. It is interesting, though, what you bring up, as I HAVE on occasion seen others SWEAR that manually picking the right channel can make significant changes to your machine's ability to see certain targets. I usually just "trust" the noise cancel...but I wonder if what you bring up, DOES matter at times (manually switching channels).

silverhound -- you bring up an interesting point also, about sweep speed. I did test a number of "sweep speeds" in the garden, which didn't seem to change my ability to lock on to fringe targets. However, the "Minelab wiggle" is certainly more brisk; further, I usually use VERY SLOW sweep speeds -- especially when I am hunting carefully looking for deeper targets. I DO know that with some of my Fisher machines, best depth comes with faster sweeps, but I hadn't gotten that impression with the FBS units. IF...you are right, then that's an issue -- because when I'm trying to hit deep targets, I EXPECT only a chirp at best on an initial sweep, so I usually swing REAL SLOW when deep coin hunting, trying to make sure I concentrate on all the "chirps" I hear, to figure out which ones to investigate further. If, by moving so slow, I'm actually COSTING myself some depth, that is to some degree counteracting my PURPOSE (finding deep coins). Hmm...almost sounds like you are advocating a slow sweep speed IN TRASH, for best unmasking (makes sense), but if less trashy, and you want deep coins, sweep a little faster? That's an interesting though, one that I hadn't considered, with this unit. I've been of the "slow no matter what" perspective with my Explorer...

Johnnyanglo -- great information, thanks for posting. Very, very informative. It makes good sense what you are saying. A question, though...while what you say about the "wiggle," and electromagnetically inducing the best response from the target makes sense, I'm not yet convinced that there isn't something going on there with the algorithm, also. While the "wiggle" may help you to get a more "CONSISTENT" response, on other platforms, a more REPEATABLE response, I've not seen the idea of "target ID improving" OR ELSE "target ID degrading," over the course of a couple of seconds of wiggling, with any other unit besides the FBS Minelabs. I may be wrong on this, but that's my experience. SO -- I DO think it has something to do with how these FBS machines are using the multi-frequencies and "sampling" the target -- I would expect that it MAY be algorithm related. While you seem to maybe disagree (and you may be correct), you DID say this:

Quote

Some detectors respond better because they request more information from the suspected target (multi-frequency usage) which is more likely to yield an improved discernment of the target's makeup

...which kind of supports my speculations. I would doubt that in the fraction of a second the coil spends over a target on a "normal" sweep, that a multi-frequency unit can utilize all of its ID algorithm capability, and that during the "wiggle," the algorithms would have more time to analyze more info from more of the frequencies (or "harmonics" or whatever), which could explain some of what happens in that "lock on" thing that seems to happen. Anyone else who can comment here? Bruce Candy?!?!

Dan-Pa. -- obviously you are correct that some units work better than others in a given location, and that things are NOT homogeneous across the U.S., and into Europe. In my area, the Minelab FBS units are, as NASA-Tom would say, the "trump card." I have done enough experimenting to feel confident with that statement. A CZ or F-75 could be, I think, if my soil didn't have the iron mineralization that it does (which hurts the performance of ALL the detectors I've used, INCLUDING Minelab FBS units -- they just seem to be affected "a bit less negatively.") Bottom line though, is, we all take a different approach to the hobby, based on our personalities and how our minds work. I am an analyzer, and not surprisingly, I am also a scientist by trade (but actually began college in Electrical Engineering before making the switch). So, the type of brain I have obviously "colors" or influences, the approach I take to detecting (and, everything else in life). Having said that, you call yourself a "layman," in terms of approach -- and yet your finds speak for themselves. Experience, over the course of years, beats plain old "knowledge," any day. Bottom line -- this is a great forum for all, "layman" or "tech-minded" folks; I am glad ALL the different types of people that are here, will chime in -- I can learn things from ANYONE.

Steve



Edited 1 time(s). Last edit at 01/13/2012 03:36PM by steveg.

SteveG - Go watch those videos I just put up comparing the E-Trac to the V3i on measured 7" - 9" targets in the field. Until I got the E-Trac getting past 7" on a coin was near impossible for years.
And Guess what my ground reads at with my V3i? Yes, 1%-3% iron mineralization at a very high level. So, even though my overall mineralization is low, that rusted iron from 100's of years of use, if not thousands, just kills performance. And it seems to affect my V3i as well and that has shocked me. Tom said the V3i keeps it's frequencies separate, then Carl at Whites said that is not true, that it combines them. But after one day of comparing signals it's clear, something is different. They are both hot machines but the E-Trac did noticeably better at around the 7" mark. 8" definitely.

Nice thread...
Albert

BBS also will do better on the wiggle thats why they call it the ML wiggle. It may be recovery speed... the more passes the more the target is magnetically influenced giving a better target response. Might be filters as well the faster the wiggle the less weak signals come thru. The coil size does limit the depth but can improve the separation. The MLs do not GB per sa they cancel out ground noises because they have the ability to differentiate between ground noises and metals.. There may well be a preset in ground with little mineralization.... the DFX will do that on the beach if it cant GB. Takes you right back to the post i did: [www.treasurelore.com] it has a lot of info.

Dew

I have to say, this post has my attention and interest.

Keep these posts coming Steve...sort of a meeting of minds...and certainly enjoyed you taking the time to comment on the posts and indeed we are all individuals....and come from all walks of life...

To add some levity my dear old Dad always said two heads are better than one even if they are cabbage heads...

PS: getting cabin fever already and only Jan.....come on Spring.....

Yes...... very good technical thread....... and very good observation Steve. The Minelab Sov will do nearly the same thing. When the unit is in 'auto' Sens upon initial turning on............. the unit is 'asleep'. I can pass the coil over a 11", 10" and 8" dime....... and nothing is detected/reported. If I pass over a 7" deep dime........... the unit reports it............ THEN will report the 8", 10" and 11" dimes. Steve..... I have no validated scientific proof of your phenomenon; yet, I have witnessed exactly this. If you are in manual Sens..... I do not believe it will happen to this extent.

Tom, once the 7" dime was detected, and the machine detected deeper dimes, did the unit stay capable of detecting the deepies. Or was it like a short window of time and then the unit had to be 'primed' again with a shallower dime to get the deepies. What a strange behavior for a detector.

*** If I pass over a 7" deep dime........... the unit reports it............ THEN will report the 8", 10" and 11" dimes. ***

I've noticed a similar phenomena with single freq detectors too. The detector is quiet (asleep) but sharpens-up after getting a non-ferrous 'hit' and after interrogating the target the sensitivity seems to have increased to other nearby targets. Now it can locate targets that it had just passed by with nary a whimper. It can't be a function of freq (when there is only one).

It might be the multiple averaging/differentiation of early and late responses that is used to compare/eliminate the ground signal. That is, the detector has a 'history' of comparing signals that are mundane (reactive wise) but when the coil is repeatedly interrogating a non-ferrous target the trend 'averages up' (the time-averaged amplitude of the signal rises).

Once the averaged voltages are raised by repeated sampling over the first target subsequent non-ferrous targets (especially low-response fringe targets) can get a response - perhaps one they wouldn't make if the averaged voltages were much lower.

This is just a theory. It could well be something entirely different.

TerraDigger...... you ask a question......... with a painful answer. The Sov would go back to 'sleep'..... after a certain period of time (unknown/unspecified time-frame). I have not noticed this phenomenon with Garrett, Fisher or White's units. So far...... only Minelab. I believe it is isolable to a specific design engineer's 'mindset' and conceptual architect.

I am with Dew on the Sov's as their wiggle seemed to suck the targets audio out of the ground. Although they were heavy and ill weighted and probably the most modded unit ever had the sweetest sound on silver and although not having the depth of the Explorer series many old Sov's are still producing. Thinking back they sure worked well on saltwater shores and land as well and got me some nice silver and gold along the way.