Coil selection 4 soil type

[Piero franchi]

Hi, if you was lucky enough to have all three coils for the x terra 70. 3kHz 7.5kHz and the 18.75kHz.

how can you tell what soil type (mineralisation) you have in front of you, so you can them make an informed decision as to which coil to use????

many thanks

 

[Digger]

If the three coils you mentioned are all the round, 9-inch concentric coils, it probably don't matter what your mineralization is. They should all perform about the same. Better, of course, in low mineralization due to the concentric design. You may find that the 18.75 is a bit more sensitive and you will have to lower the sensitivity on your X-70. But that is not due to mineralization. That is simply the characteristics of the higher frequency coil. For the most part, they will all do equally well.

All soil is mineralized. All natures elements impact that level of mineralization, with iron (FE) being the biggest factor affecting my detector in this part of the Country. To what degree that soil has become mineralized is the determining factor impacting the performance of a metal detector. With that said, if your soil is highly mineralized, you will likely have better results with a DD coil. Fortunately for you, Minelab has made them available for the X-Terra.

This ain't science, but I would suggest a simple process to see if your concentric's are doing the job. Take a large sized coin and see how far away from the detector coil you can sweep it before you lose the audio. (air test) Now, bury that same coin in an area where you are worried about the mineralization. If your soil is moderate to lightly mineralized, you should be able to bury that same coin (and detect it) to nearly the same depth as your air tests detected it. The more mineralization in your soil, the less deep you will be able to detect it with the concentric coils.

DD coils will perform better in highly mineralized soil than the concentric coils due to their design. And, if you try the same test (as above) with the DD coil in soil with higher levels of mineralization, your depth results should be better than with the concentrics. HH Randy

 

[Piero franchi]

Hi,
Plz don't take this to hart when I question your opinion, it just me trying to learn.

My understanding is a DD coil has a consistent footprint as the search pattern travels down from the coil, the concentric coils are just that, they cone down to a point.
so for a given kHz, a DD is searching a bigger area, I would conclude from that, that the DD would have a hardest time with mineralisation just because it has more area to deal with.

I thought the kHz plays the bigger part in the search coil working better on a given site

 

[Jackpine Savage]

My understanding is a DD coil has a consistent footprint as the search pattern travels down from the coil, the concentric coils are just that, they cone down to a point.
so for a given kHz, a DD is searching a bigger area, I would conclude from that, that the DD would have a hardest time with mineralisation just because it has more area to deal with.

The design of DD coils make them less susceptible to the ground. While they do have a wider search swath (or footprint) compared to a concentric, the overlapping double D pattern of the receive and trasmit coils sees less ground volume than a concentric coil. I have seen it quoted from 30% to 50% of the volume of a concentric.. coil size for coil size.

Tom

 

[Monte]

have a fair idea of the mineralization level of the ground you'll hunt, as well as the types of targets (conductivity) that you are after.

Rule #1: It is THEORY that suggests we use a Double-D (wide-scan) coil in highly mineralized ground.

Rule #2: It is THEORY that suggests a Double-D (wide-scan) coil will work a little better in higher trash levels.

Rule #3: It is THEORY that suggests a concentric coil will have more problems in "bad ground" than will a D-D coil.

Rule #4: It is THEORY that suggests a higher operating frequency will always be better on lower-conductive targets and not so good on high-conductors.

Rule #5: It is THEORY that suggests a lower operating frequency will handle high mineralization better and be more responsive on the higher-conductive targets.

Rule #6: It is FACT that most of these "theories" might be true from a book-learning sense, but not necessarily when they are all combined with other factors having to do with the actual size, shape and placement on the coils internal windings; the amount of sensitivity or detecting power; the type and efficiency of filtering used; the the operating mode selected; and other variables such as how well the operator adjusts the detector for the task at hand, and let's not forget the coil SIZE can make a lot of difference, too.

In short, there are some makes and models designed around a particular coil type and size, and with some accessory coils of different sizes and/or types they just don't work as well.

One way to help you decide is this:

If the three coils you mentioned are all the 9" concentric types, then THEORY would have you believe that the higher-frequency coil might work a little better in mild or mellow ground and that the low-frequency coil might punch through very mineralized soil much better. Also, that the higher frequency coil would be the one to pick if you were mostly after gold jewelry, but the low frequency coil might be hot hitting on silver and other high-conductive coins.

Well, I had all three and I was really hoping that would be true, but here is what I found after evaluating all the coils in the field and comparing them with the main competitive multi-purpose model from White's, the MXT.

 

[NH Bob]

No disrespect but didn't we both come to the same basic conclusion?
Honestly, do you write for a magazine or something?

 

[B.T]

I use the X Terra 70 over here in Oz for chasing gold and coins on the goldfields of the Victorian Golden Triangle. We have the worst ground conditions I am told in the world. Mostly I use the elliptical 18.75khz coil when hunting gold and find it works no probs anywhere.
I rarely have to turn it down, the sens, below 18. It will find gold down to a mere 1/40th of a gram, specks, in quiet ground when wound up to 30 on the sens scale. The cocentric coil which is the standard, 7.5khz, is great for the coins but struggles too much in hot ground and creates too many ground noises when used in Prospecting mode. It can handle hot ground in the coin/relic modes though but rarely does the sens get to be set higher than 18. The 7.5khz 10" DD is great for those dog areas where the ground is real mineralised and does not pick up as many hot rocks as the 18.75 does. Sensitivity differences are marked. The 18.75 will pick up gold in specimen form which the 7.5 will not even register on.
These can be bits containing a couple of grams in small to fine form gold. The concentric coils do have more sensitivity than the DDs and if your hunting in super quiet ground the 18.75 concentric should outperform the same khz in the DDs. But mineralisation is their cryptonite and thats where the DDs rule. The 7.5khz DD can seem, next to the 18.75, pretty insesnitive but begins to catch up when the gold reaches around the 7 gram or 1/4 oz range then begins to overtake it as the gold increases in size. It will outperform the 18.75 on gold in the 1 to 2 ounce range and upwards by roughly 2 to 3 inches.
I would expect simialr results with coins and relics anywhere.
Hope it helps.

 

[desertdon]

The overlap area in a DD coil acts as a neutrlizer to more mineralized soil better than a mono and also will give a lower signal response. Don

 

[Monte]

Well, we almost said the same thing.

You said: "DD coils usually handle mineralized soil better than concentric."... This is sort of true, but like I said, it is all theory and you have to learn each detector and coil type and how they can/can't work for you in the environment you're hunting in.

You used the word 'usually.' Other words you could have used are: 'might' or 'sometimes' or 'do not always' , and the readers interpretations could be different. Some of the other terms, perhaps, could leave them asking a questions as to when one might be better than another, while it is natural for many readers to sort of read past the word 'usually' and be left with a statement that is almost absolute.

"Beyond that its anybodies guess."... Until we confirm something to satisfy our own inquisitiveness, it is anybodies guess, and I tend to prefer not to have to 'guess' whenever possible.

"Maybe I can learn something from this post."... Since the first day I started replying to posts on various Internet forums, my goal has been to try and help others get as much enjoyment out of this hobby as I have. Knowledge is the key to success, because the better we understand our detectors and their strengths and limitations, the better we can do in the field.

ALL OF US can learn from these posts, just as I have. Read BT's response above and you'll see how it confirms much of what my initial post tried to convey.

Here you have a detectorist who is experienced with the X-Terra 70, as well as with an assortment of coils. He has used them and learned their strengths and weaknesses as they apply to him for his environments and his applications.

The high-frequency elliptical DD is hotter on the tiny gold nuggets, but the 7.5 kHz DD coil catches up and surpasses it when the nugget size (mass) gets larger and it will outperform it at that point.

The conductivity of the metal (gold) hasn't changed, but the mass that can disrupt the EMF makes the difference. Additionally, the mid-frequency might just handle the bad ground better, too, and that can help.

Besides, my post was somewhat in response to other posts that had been made, and in the end I addressed Piero franchi's question. That dealt with how can you tell how mineralized the ground is to determine which coil might be the better choice.

White's MXT provides a Ground Phase Readout, but on the X-Terra 70 you can compare mineralization from site-to-site based, somewhat, upon the GBRN (Ground Balance Reference Number) as I explained.

Let's all hope for spring to arrive soon, for weather an hunting conditions to improve, and for some success on our next forays afield!

Monte

 

[Neil]

I would also like to see a lighter 7.5khz coil that is DD. I used the 10" one, the one that looks like the explorer stock coil and it had great performance but sure made the lightweight Xterra unbalanced and nose heavy. In contrast the 18khz eliptical DD coil is a breeze to swing, really works well with the overall weight/balance of the 70 but does give up some depth to the larger explorer like coil.

I havent even tried my 3khz coil yet. The stock coil and the high freq DD eliptical are good ones for jewelry hunting.

HH
Neil

 

[Neil]

Tom what your saying about volume seems true for a stationary coil but once you start swinging the coil the concentric actually needs to be overlapped more closely as it is actually covering less ground compared to a DD of the same size.

I know when I tested a target buried 4" or so in the ground with both a CZ w/8" coil and a Sov w/8" coil, the Sov would hit the target almost the full length of the coil while the CZ would hit it a less than half the coils outside diameter. So for beach hunting which is my main type of hunting the DD coils are a much better choice, even though they dont get the volume they sure get better coverage.

Do you see any advantage to the volume of a concentric versus the coverage of a DD coil?

HH
Neil

 

[NH Bob]

I also am a hands on type of person. I read alot but find the best experiance is in the field. I have over 250 hours on the XT70 none of which has been on the use of the prospect mode. But I am working toward that for this coming summer. I don't have all the technical jargon that seems to flow from you but I do know what my machine can and can't do, real time in the field under most conditions.
Whether I use the words maybe, usually or in most cases. I think the average guy on this foum understands what I'm trying to say.
Again, with no disrespect intended and understanding that you are a learnered person. One could easily get lost in the way you presint your answers. And I am guilty of not enough infomation.
My posts are also intended to help where I can and just maybe I have helped someone too.

 

[Jackpine Savage]

Hi Neil,

I'm no coil expert just know what I see using them. The bowl shape field of the Concentrics do have some advantages. Response to on-edge coins comes to mind as one example. Coil size for coil size the concentrics, in my opinion, seem to light them up a little better at depth. Now when going after the deeper coins co-located with iron, with the right detector, generally I'll take the larger DD's over a big concentric.

Fishers 10.5" concentric with the elliptical receive coil is one of the best designed "big" concentrics available.

In a nutshell, both types will find targets the other won't and using different sizes will also add to your pouch.

Tom

 

[Piero franchi]

As to coil coverage, What I mant was, and I said, "consistent".
What I mean buy that, is its no good looking at a coil at, lets say the first inc of depth.
If you was to measure the area/footprint of a DD coil, for ever mm it traveled down into the earth, so lets say, 10 x 4, that bit is just a guess on a DD coil.
now that 10 x 4 for the 10 in depth would give a figer.

the concentric coils have a bigger footprint, BUT only some of the way down, then is drops of very rapidly.

with that in mind, do you see what I am saying (think it was way back in second reply)

 

[Jackpine Savage]

I see what you are saying. The problem is the misconception people have of a concentric coils field as being cone shaped when it would be more accurately described as bowl shaped. There is a big difference in volume when comparing a tapered cone to a rounded bottom bowl. Does that help?

Tom

 

[NH Bob]

n/t

 

[parrott]

B.T... Most impressive... I understand the Victorian Golden Triangle
separates the successful gold prospectors from those that can't.

Gosh one question: what headset are you using?

Again... Thanks for sharing your finds and to give encouragement. I hope you also find a "big one."

 

[Monte]

One reason why I will sometimes make lengthy posts is because I decide to express the same thought in more than one way so that it might be better understood.

There is no doubt that YOU or I would understand what either of us was saying if we used "usually", "maybe" or "some of the time", but I can assure you that many people do not really appreciate what the intention was of the statement.

Like you said, you're a hands-on type of person and some people can learn well by any sense. Still, some are great auditory learners, some are visual learners, and still others might require actual hands-on demonstration in order to understand a concept.

Yes, some might get lost in my replies, but I can assure you that others learn from them. Repetition can be a very good tool for teaching/instructing. I know a lot of the readers of this and other forums have a good understanding of detectors and terminology, but there are a lot who don't. That's why they seek out the forums in order to try and learn.

No, you're not guilty of not enough information. We are all individuals with different quirks. One of mine is a burning desire to help others learn and understand what I have over 42 years of detecting. Why? Because knowledge is a key and, like you, I would like to provide an answer that might be helpful.

As you stated, you have a good 250+ hours on the X-Terra 70 and it shows in your posts. Quite a few newcomers, and sadly there are many who've had an X-Terra for quite a while but haven't yet learned it. That, to me, is very saddening.

Let's hope for some good summer weather soon. All the best to you and I trust I didn't offend you with my lengthy reply.

Monte .... Way over here in NW Oregon.

 

[NH Bob]

n/t

 

[Neil]

Hi Tom,

I usually am digging with a scoop(beach) or a shovel(woods) so I rarely ever find coins in the position they are actually at when detected. Im glad you pointed that out about coins on edge, I would imagine that would be the same with a ring on edge.
Ill have to try both at the ocean one day and see if indeed the concentric might be a better choice. Ive never compared one to the other, just DDs to DDs which is mostly what I use(Sov/Explorer).

I notice Tesoro is coming out with the new small coil DD, same size as the one they have in concentric. It will be interesting to see how those two stack up against each other, same exact size and all.

Thank you and HH,
Neil