VLF and hot rocks?

[TravyLeigh]

Hey all. So I'm a MXT and GMT user (wanting to learn my GMT better) and I was curious how you folks deal with and better recognize hot rocks?

An area I'm wanting to work is COVERED in hot rocks, and drives the detector and I both nuts, moreso with the sensitivity up. How can I pick out which is a hot rock signal compared to a good one?

Found a piece of bedrock that sounded off with low % of iron, strong signal, then it said high iron, then low again. Pyrite perhaps?

Any tips would be appreciated


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[Jim Hemmingway]

Hey all. So I'm a MXT and GMT user (wanting to learn my GMT better) and I was curious how you folks deal with and better recognize hot rocks?

An area I'm wanting to work is COVERED in hot rocks, and drives the detector and I both nuts, moreso with the sensitivity up. How can I pick out which is a hot rock signal compared to a good one?

Found a piece of bedrock that sounded off with low % of iron, strong signal, then it said high iron, then low again. Pyrite perhaps?

Any tips would be appreciated

Hi TravyLeigh… the guys have given you some good tips about how to use your ground balance to deal with hotrocks. I’ll add some comments and hope it doesn't muddy the water. Some of it may make more sense as you gain experience over different ground conditions.

Ensure the ground you select to ground balance is free from any targets. Initially select the presets for SAT and GAIN, and do your ground-balance routine. Initially stay away from using the GMT’s AUDIO BOOST feature because it’ll unnecessarily complicate ground-balancing due to the added sensitivity to ground minerals. The same applies to using excessively high gain on the MXT because if you've read the MXT Engineering Report by Dave Johnson, you’ll see that audio boost is programmed into the software for the upper + gain area.

On the other hand, once you’re comfortable with the various ground balance methods, use as much gain as you can without the detector being unstable and generating a lot of ground noise. How much gain that can be used will be dictated by the ground mineral magnetic susceptible strength, and will be markedly influenced by how much SAT is employed. Experiment and practice over clean ground and using your practice nugget as Hoser suggests. Take that test nugget with you into the field to double-check your settings, these few moments of testing will give you some added confidence that you’re doing things right.

Always pump that coil between the elevation above ground where you’ll be scanning it comfortably and about six inches above the ground surface, back and forth, to set your ground balance. Pump the coil regardless whether you’re initially setting your autotracking, manual ground balance, or pressing the GRAB button… until the ground signal is eliminated. And employ this same technique when ground-balancing over hotrocks. If you can’t get it to fully ground balance over clear ground, try reducing your gain a bit initially.

Then bury your test nugget at a reasonable depth and play with your SAT and GAIN adjustments to acquire the best possible signal while still able to sweep that coil over the terrain without undue ground noise. It sounds much more difficult to describe here than it actually is in practice. Best advice is to practice in the back yard until you’re confident about doing it. Bring back some of those hotrocks from you first trips, they may still act as hotrocks over your ground at home, and maybe some will not do so… but in either case you will gain a more practical understanding.

Below is some background information about hotrocks that you might like to have for reference, and I’ve included some suggestions about alternative techniques and under what conditions you might try them.

General Information about Hotrocks…

What follows initially refers to VLF responses to hotrocks using the motion all-metal (autotune) mode. Hotrocks are encountered as (a) essentially non-conductive iron mineralized rocks or (b) as electrically conductive hot rocks, usually in the form of sulfides, arsenides, and graphitic rocks in my area. Both hotrock types respond to VLF metal detectors. We’ll discuss non-conductive hotrocks first, followed by a brief discussion about conductive hotrocks.

Non-conductive iron mineralized hot rocks present themselves with one of two distinct responses to a metal detector. They will either respond with a positive target signal, or a negative “boing” signal due to autotune threshold “overshoot” reaction to a “negative” hot rock. A positive hot rock will have a ground balance compensation point below our operating GB setting, whereas the negative hot rock will have a ground balance compensation point above our operating GB setting.

The negative hotrock “boing” signal directly results from using a motion all-metal (autotune) mode. The autotune initially goes quiet over the rock but struggles to recapture its threshold level as the coil sweeps past the rock. In doing so, it “overshoots” the threshold level briefly and yields an audio response. Thus, as the coil is swept over a negative hotrock we get the familiar “boing” signal. By contrast, when using a true non-motion all-metal mode, negative hotrocks passed across the coil will simply cause the detector threshold response to go quiet. Magnetite within a rock structure is normally responsible for negative hot rock responses. These normally read very high into the non-conductive (sometimes referred to as ferrite range) ground balance range, are quite easy to recognize, and can be readily ignored by recognizing the “boing” signal produced.

Small positive hotrock signals can be frustrating to electronic prospectors because their signals sometimes mimic nugget signals. Stronger signals can sometimes target ID in the upper portion of the iron range, as do many small nuggets concealed in highly mineralized ground. Fortunately, hotrocks that respond to metal detectors tend to reside very close to or on the surface and lose their signal quickly as the coil is elevated. In fact this is a technique, in conjunction with reduced gain… to mitigate hotrock signals. Maghemite, a highly magnetic susceptible, red-brown iron oxide, is normally the culprit responsible for small but strong positive hot rock responses.

Techniques to Deal with Nuisance Hotrock Signals…

The foremost option is to learn to identify and consequently ignore most hotrocks in your area. As noted, most non-conductive iron-mineralized hotrocks can only exist at or near the surface… their signals tend to quickly subside with coil elevation compared to similar size metallic targets. They can usually be identified at a glance and kicked away. Unfortunately in my area non-conductive diabase hotrocks occur in very large sizes up to several hundred pounds and cannot be ignored or kicked away. They often dominate the substrate at some sites, and their magnetic strengths and ground phase are individually somewhat variable. Those that produce signals must be neutralized such that metallic signals can be more easily identified. Obviously in such areas, ground-balancing PI units are an excellent alternative to VLF units in tolerably low trash conditions.

In higher magnetic susceptible iron mineralizations, one must either contend with hotrocks individually as described later, or apply some amount of iron discrimination to be determined in the field for the hotrock types encountered. The more persistent positive signals may require discrimination settings slightly above the level required to eliminate small bits and pieces of metallic iron. Just how much discrimination is required really does depend on how the detector model and operating mode employed processes these signals. For example, the F75 will eliminate all negative hotrock signals, regardless of magnetic susceptible strength, at zero discrimination in all discrimination modes. Positive signals produced by those same high magnetite-bearing rocks require a discrimination setting of “1” in DE / PF modes and a maximum iron setting of “15” in JE mode. Positive signals are rarely encountered with these rock types unless searching in substrates dominated by them, for example... our massive diabase beds here in northeastern Ontario's silver country.

The level of discrimination required may also eliminate some small nuggets that fall into a similar conductive range, and possibly deeper larger nuggets on the threshold of detection depth in highly mineralized substrates. Check your discrimination adjustments… bury a suitable test nugget at a reasonable depth… and determine the effect of your discrimination settings on it.

In lower magnetic susceptible iron mineralizations, there is no reason why discrimination cannot be used as described above. But many operators prefer to ground balance to an area’s prevalent hot rock type and search at that GB setting if it is satisfactory. In lower mineral conditions this is acceptable practice because GB settings have wider windows of utility than exist with higher magnetic susceptible mineralizations where spurious ground noise will be generated as a result of even minimal offsetting of GB values. Of course using small coils can mitigate these conditions, many prefer DD coils because comparable sizes see less ground mineral. So while this technique in such conditions will leave you improperly ground balanced to some extent, either above or below that required for the surrounding terrain depending on the hotrock type… it is not nearly so critical as it would be if used over more highly magnetic susceptible ground minerals. Check your GB adjustments, bury a suitable test nugget at a reasonable depth… and determine the effect of using offset ground balance settings on it.

Autotracking plays a useful role over variable ground and areas littered with hotrock signals or noise. For example detecting streamrun cobbles, autotracking conveniently serves to reduce or eliminate such signals. But remember to disable the autotracking feature by pulling the trigger on either of your detectors when evaluating potentially genuine target signals to avoid possibly tracking-out such signals. In abundant hotrock areas where the target size is sufficiently large to respond and trash levels are tolerably low, a suitable ground-balancing PI unit is a more likely alternative.

Over any normally encountered ground mineralization conditions in prospecting country, particularly where hotrocks are not terribly plentiful, some prefer to use the “GROUNDGRAB” or sometimes called the “GRAB” or “FASTGRAB” feature available on many of the modern prospecting-capable detectors to assist with target signal evaluation. This is generally my preferred method particularly in my area where massive rocks are encountered and can’t be ignored. The FASTGRAB feature simplifies searching hotrock areas properly ground-balanced in the all-metal motion mode because it takes only a moment to check out a suspect hotrock signal and then quickly re-balance to the ground and continue searching. If an unidentified ‘fastgrabbed’ rock continues to produce a positive signal, it should be investigated further.

For example, our positive diabase hotrocks that produce good strong positive signals will read consistently in the upper iron range at target ID “14” on the F75. But when ground-balanced using the GROUNDGRAB technique, they lose their positive signal, their target ID disappears, and there is always a small reduction in the ground balance readout. Other positive non-conductive iron-mineralized hotrock types in this area react similarly, but experience more variable GB reductions according to the ground phase compensation point for any given rock.

Conductive hot rocks respond with positive target signals to a VLF metal detector. The most frequently encountered electrically conductive rocks in Ontario’s silver country result from sulfide or arsenide ore responses, for example pyrrhotite, cobaltite, and niccolite. Those located on the surface can usually be identified and ignored. Many other ‘collectible’ conductive minerals, such as galena or iron and copper sulfides, can give variable strength responses depending on specimen size and structure, amount of a conductive mineral in a rock, and the sensitivity of the VLF unit. Some conductive minerals such as larger pieces of pyrrhotite produce broad signals, more easily distinguished from typically narrower precious metal responses.

Pyrrhotite is the most widespread, frequently encountered conductive hotrock here in northeastern Ontario’s silver country. It occurs in variably shaped, frequently multi-pound sizes. Although field specimens are weathered to a rusty or deep brown appearance as can be seen below, a fresh surface is a pale brass / bronze color with a metallic luster. Pyrrhotite has a slight, variable magnetism, a feature that easily distinguishes it from commonly occurring tarnished iron pyrite and chalcopyrite. It yields a very distinct sulfurous odor upon impact with a rock hammer. Incidentally in response to your initial post, it is highly unlikely that anomalous signal would result from iron pyrite.

There is nothing that can be practically done to mitigate pyrrhotite’s wide, blaring signal other than to recognize and ignore it when found on the surface. Rocks containing sufficient amounts of pyrrhotite cannot be effectively ground-balanced on a VLF unit but these normally discriminate within the iron target ID range. Pyrrhotite is the bane of electronic prospectors here because its abundance can render entire sites unsuitable for detecting with either a VLF or PI unit.

Jim.

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[TravyLeigh]

Very interesting! I will need to try this out, thank you very much!


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[TheHunterGT]

Fantastic post Jim. Going to print this and laminate it for the field.

My Scorpion is no GMT...but valuable info for any VLF user wanting to prospect.

 

[Jim Hemmingway]

Thanks guys... I guess the risk is coming across sounding like a know-it-all but that isn't the case. I like to be thorough is all... I still remember when I had the same questions and had to figure out what seemed to work for me in this area. No doubt others have developed their own ideas about what works in their areas. Over time we become familiar with the various rocks that go beep and soon learn to recognize and pretty much ignore them. But for newcomers, I think it a good idea for them to have some background knowledge of the whys and wherefores. It can't possibly hurt in any event...

Jim.

 

[GoldpannerDave]

Wow, great stuff, Jim. Thanks for all the information.

 

[johnedoe]

Jim, You are a cornucopia of information. Thank you for such great and informative posts.

 

[TravyLeigh]

Ok so i tested some of this stuff today (GMT). There's bedrock at the beach down the street (was looking for lead to practice on) and to run it smoother over this stuff, I turned audio boost OFF, turned sensitivity to around 7, turned the sat speed to about 8, AUTO GB and I was finding .177 pellet size lead, maybe a LITTLE smaller, but not much. I even cut up a piece of .177 pellet into 3 pieces (decent picker size) and couldn't find them if they weren't directly under the detector :/ the ground is even worse up where I look for gold though, so I'm sure I'd have to turn it down real low, meaning the gold will need to be even bigger :/ I'd probably pass over rice sized gold up there. With my GMT on a hydro mining area, I can turn the gain to max, with audio boost on, and get TEEEENY birdshot. With my MXT I found a 1/3rd the size of a piece of rice, sized piece of gold. I honestly thought the GMT would be MUCH more sensitive than the MXT, but honestly I've only found gold with the MXT and not the GMT yet :0


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[Jim Hemmingway]

Your outing really does illustrate that VLF detectors are very much influenced by the ground mineralogy present at any given location. Higher operating frequencies are more vulnerable to higher mineralizations. A smaller / DD coil will help, and should allow you to increase your gain a bit from what you were using there. In lighter mineralizations the GMT will beep over tiny material that MXT will not see, but MXT will handle higher mineralizations better than a GMT. Finding tiny gold, specie gold, and wire gold in tough ground isn't easy, this is why the SDC 2300 PI unit has found a market niche despite the pricetag.

Jim.

 

[TravyLeigh]

Thanks Jim! You sure know your stuff, I'd love the SDC.

I use the smallest DD coils for both the GMT and the MXT.

Question: how much "chatter" do you deem as tolerable? Of course with higher gain/audio boost, there's much less chatter. But with the SAT way high, it does quiet it down a bit, but kinda makes it harder to hear those teeny pieces, I feel. A trained ear could probably pick them up though!


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[Hard Prospector]

Running in high SAT (some call "hyper-SAT) will tune the tiny gold right out. When trying to get the machine to quiet down in bad ground I would increase SAT and decrease gain (back and forth) til it became tolerable OR had come down to a gain of around 5 or 6 and a increase in SAT to no more than 6 or 7. Beyond that the GMT got swapped out for the TDI. I hate to say it but I believe the GMT's days are numbered, there are just too many newer machines now available that will find those tiny specimen pieces in nasty ground at better depth, run much quieter and don't have the learning curve the GMT always had. Don't get me wrong as I plan on using my GMT and GMZ until they wear out as they're worth about .50 cents on the dollar that I paid. Actually, I've found that older but still very effective gold VLF"s make great backpack-drop camp detectors. For example, my GMZ right now is packed in a sealed large plastic box with hand tools, pan and sluice buried near some good diggs 15 miles way back in the bush. When I backpack in for a week of digging it sure is nice to have all that stuff already there.

 

[TravyLeigh]

What size pieces have you found with the gain turned down like that and the SAT turned up? And that is a good idea, keepin everything out there already! Makes the hike much less sucky haha


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[Hard Prospector]

What size pieces have you found with the gain turned down like that and the SAT turned up? And that is a good idea, keepin everything out there already! Makes the hike much less sucky haha


Sent from my iPhone using Tapatalk[/QUOTE The GMT will find the crumbs even with the gain backed off BUT the gold needs to be near surface and ground conditions not too extreme.

 

[TravyLeigh]

Okay, maybe I'll try my spot again tomorrow with the detector, if I'm feelin up to hiking all this stuff in! hahaa


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[TheHunterGT]

For example, my GMZ right now is packed in a sealed large plastic box with hand tools, pan and sluice buried near some good diggs 15 miles way back in the bush. When I backpack in for a week of digging it sure is nice to have all that stuff already there.

Man HP...I would chew my fingernails to the quick if my stuff was buried out there. Just could not do it!

 

[Hard Prospector]

Man HP...I would chew my fingernails to the quick if my stuff was buried out there. Just could not do it!

Having gold fever with a 50 year old body has forced me to get creative.

 

[Jim Hemmingway]

Thanks Jim! You sure know your stuff, I'd love the SDC.

I use the smallest DD coils for both the GMT and the MXT.

Question: how much "chatter" do you deem as tolerable? Of course with higher gain/audio boost, there's much less chatter. But with the SAT way high, it does quiet it down a bit, but kinda makes it harder to hear those teeny pieces, I feel. A trained ear could probably pick them up though!

Hi Travyleigh… Rob (HP) has given you good feedback. I really do think that you just need to get out and practice… it’ll all come together and make perfect sense.

I've highlighted your comment above, because in actual fact higher gain / boost increases the chatter produced by ground minerals. As I mentioned earlier, turn-off the audio boost for now… it just complicates matters because it also increases sensitivity to ground minerals. Increasing SAT does mitigate the chatter but higher SAT levels also decrease depth and sensitivity. I prefer not to exceed the preset SAT level. It seems to me to be a good compromise for smoothing out ground noise without incurring unacceptable sens / depth loss.

As Hoser suggested above, make good use of one or two test nuggets of different sizes and see what settings produce the best signal at your site, while still enabling you to hunt without excessive ground noise. What he suggests is the best advice anyone can give a newcomer detecting over unfamiliar ground. It is important because you have to have confidence that you can find gold with your detector settings.

In regard to your specific question… if the chatter is excessive to the point of distraction, or you can’t reasonably distinguish small target signals… then the gain and SAT need to be adjusted accordingly. Go out into the backyard and play with these controls over a few different size (lead or pieces of a nickel) nuggets buried at reasonable depths. With a little practice you’ll soon get comfortable with how these SAT and GAIN adjustments interact.

Jim.

 

[TravyLeigh]

I'm sorry, I meant much more chatter. I was half way asleep when I posted that** haha


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