Popular Post GB_Amateur Posted August 17, 2019 Popular Post Share Posted August 17, 2019 I started this project 2 or 3 years ago (so long I can't remember). Spurred on by recent field experiences and also a recent thread on Equinox settings I've finally finished it. I don't know if it's a completely new idea. I call it a 'test-stand' as opposed to 'test garden' just to distinguish it from the standard test gardens many of you either already have or at least are familiar with. There are other similar variable depth test gardens out there (seen on YouTube). This one has the advantage of continuous depth capability. It also allows 3-d target orientation angle (similar to pitch, roll, yaw of airplane). It's based upon the 30-60-90 triangle (remember that from geometry/trigonometry class in high school?): Here is a sketch which shows how to implement this concept: Shown in the sketch, buried at an angle, is a PVC pipe. A test target can be slid into the pipe a distance 2*d which will result in it being located at depth d. I used two sections of pipe (ID = 1.57 in., OD = 1.90 in.), side-by-side to allow me to put neighboring targets in the ground with some option of how close the two targets are separated. Think of this as burying a double barrel (side-by-side) shotgun with the stock end deep in the ground. All you see are the ends of the two barrels. The concrete (bag of Kwicrete) locks the pipes in place. Here's a closeup of those extruding barrels: Besides the tape measure (units of inches) you also see a hand-graded scale at left which I'll explain shortly. Here is an overall view: The two PVC caps, attached together, are for keeping water, dirt, and varmints out of the pipes when not in use. You'll notice a 1.5 in. diameter wooden dowel rod inserted into one of the pipes. More detail on that shortly, but the target is inserted into the dowel near its end and then the dowel is slid into the pipe. Holes for locating pins (you can see one of those -- gray plastic -- inserted to register the intended depth) are 1 inch apart leading to a depth resolution of 1/2 inch. (Again, refer to the 30-60-90 diagram to understand the relationship between insertion length and actual depth into the ground -- a 2::1 ratio.) Next I show the business end of the dowel rod: The black foam fills the chamber and holds the target (in this case a silver dime) in place. The hole in the dowel is actually lined with a plastic film canister (remember those from 35 mm film days?) which has been modified to conform to the circular cross-section of the dowel and thus be able to fit into the pipe. The second slightly smaller) large hole was put in there originally for a second target but so far I haven't used it -- likely of limited value. You can see the registration holes. The first one has a red '2' (difficult to see) just above it; the next (representing 2.5" depth) isn't labeled; the third one has a '3'; etc. These represent the resultant depth of the target when a registration pin is put in that hole and then the rod slid into the pipe until the registration pin keeps the dowel from going deeper. Although the chamber packing material can be made up of many materials, I chose ethafoam (polyethylene foam) high quality packing material. You typically find this in higher end electronics packaging such as with desktop computers. More commonly it is white but in this case I used black. I initially cut plugs with hole saw (see next photo) and then trim with a pocket knife as needed to fit the pipe: Ok, so now you're still wondering what that specially graded (homemade 'yardstick') is for. Again, referring back to the 30-60-90 triangle drawing, the 3rd side of the triangle is also related to the depth. It is squareroot of 3 times the depth. (Squareroot of 3 = 1.73.) That yardstick will indicate how far downrange (along the ground surface) that the target is located. This helps when you get an iffy response on your detector and want to confirm or deny that the surface location of the target is consistent with its depth. The units written on the scale are associated with the depth of the target. You can see from the sketch above that the max depth is 15". The largest common US coin that will fit the chamber (with some force...) is a half dollar. I didn't keep track of the cost but it's probably $30 or so, mostly for the PVC pipe and caps and the dowel rods plus a bag of Sacrete. (I'm counting labor as free. ?) OK, now that I (finally!) finished this test-stand it's time to get busy making measurements. I'll be posting those here on the forum as they become available. 13 2 Link to comment Share on other sites More sharing options...
Andyy Posted August 17, 2019 Share Posted August 17, 2019 Cool. I like seeing how others do it. I usually make small vials with the lid drilled out to accept a knotted string. If not going deep, then I can pull it up to different string lengths. But this gets really hard after 6 inches. That's why I have gone in your direction, at times, by using a vertical pvc pipe that I can raise the gold vial out on a string. But yes, a 30/60/90 or a 3/4/5 triangle would let you make a simple chart for this and you would have more ground above the gold (to improve accuracy). Andyy Link to comment Share on other sites More sharing options...
PimentoUK Posted August 17, 2019 Share Posted August 17, 2019 There is a German ( Austrian? ) depth test out there on the web, that does a pretty similar thing. I can't recall if they 'drilled' the hole for the pipe to go in, so as not to disturb the earth. I'll see if I can find it, there may be some tips to pick up from it. Link to comment Share on other sites More sharing options...
RickUK Posted August 18, 2019 Share Posted August 18, 2019 A similar setup was/has been used by Georgi who owns Nexus metal detectors,he used a small arbour/drill slightly larger than the coin buried,must admit he has not done it in the last few YouTube videos though.I guess with some of the crazy large GPX coils that he has been testing the holes at angles would be very deep. Link to comment Share on other sites More sharing options...
mn90403 Posted August 18, 2019 Share Posted August 18, 2019 This is a very worthy project. I'll spend time looking at some of those YouTubes. I think this type of 'test stand' would allow for someone to learn how to hear fringe targets. Each detector/coil combo will have an ultimate limit of depth on a target. What does it sound like just above that? (If you are directly over it.) What does it sound like when you are 'close' to it from the side? (Not directly over the target but kind of a side scan/off coil detection.) The second measurement could determine how you set your grid pattern if you know a layer the gold or relics are in then you could 'open up' the pattern to find more in the same amount of detection time. I think a third benefit of this type of 'test stand' would be to test the coil speed. (How fast/slow can you swing and still hear the target to maximize time and efficiency.) I'm like Simon. I need to do it so that I know what the coil sees and what I can hear. It has been a few years since I've done it. How much ground can I cover in an hour and be confident I didn't miss anything? A question I would have is one about possible lack of halo. You can't make a 'test stand' that is as natural as a long buried target or an unearthed target on virgin ground. If the target is dry and encased in the pvc tube and perhaps other wrapper ... how close to natural is this? Mitchel 2 Link to comment Share on other sites More sharing options...
GB_Amateur Posted August 18, 2019 Author Share Posted August 18, 2019 7 hours ago, mn90403 said: A question I would have is one about possible lack of halo. You can't make a 'test stand' that is as natural as a long buried target or an unearthed target on virgin ground. If the target is dry and encased in the pvc tube and perhaps other wrapper ... how close to natural is this? Good point, and it shows one of the weaknesses of this setup. I did bury two coins (US 5 cent 'nickel' at 6" depth, US 1 cent bronze at 5" depth) for calibration/sanity-check purposes. But those can have their problems, too, such as the depth and orientation changing over time. I will point out that three engineers most of us have heard of (Dave Johnson, Carl Moreland, and George Overton) are skeptical that the halo effect even exists except for possibly iron. But that's a whole 'nother discussion topic and doesn't need to be elaborated here. In the end I think any controlled setup has its limits compared to real in-ground targets. But I do think (and I see you and many others agree) that it's still informative and worth the effort. 1 Link to comment Share on other sites More sharing options...
Chase Goldman Posted August 18, 2019 Share Posted August 18, 2019 14 hours ago, mn90403 said: A question I would have is one about possible lack of halo. 6 hours ago, GB_Amateur said: will point out that three engineers most of us have heard of (Dave Johnson, Carl Moreland, and George Overton) are skeptical that the halo effect even exists except for possibly iron. This^. I was going to respond similarly, but you beat me to it. The target orientation variabity and lack of direct contact with the soil are problematic, but still better than an air test IMO. Can't wait to see your results. 1 Link to comment Share on other sites More sharing options...
mn90403 Posted August 19, 2019 Share Posted August 19, 2019 I just read a thread about mono vs DD coils that 'expanded' the usefulness of your 'test stand' features for users. The first 2/3s of it required slow, deliberate reading (just like overlapping a coil) in order to get to the truth of a swing and what I should be hearing. (The last 1/3 is more he said/she said which is a distraction.) The thread talks about the coil field (I liken it to a nerf ball) but when swinging any coil over your 'test stand' the weak fringe targets should lead to the centered targets which means 'dig me' vs ground noise! 2 Link to comment Share on other sites More sharing options...
GB_Amateur Posted August 19, 2019 Author Share Posted August 19, 2019 3 hours ago, mn90403 said: The thread talks about the coil field.... Thanks for the link, Mitchel. I went back and reread it (probably need to do that again) and, yes, once the new (May 2019) replies start talking about the X-coils the informative discussion gets derailed. Being the contrarian that I am, though, I'm reminded of Dave Johnson's article: http://www.fisherlab.com/hobby/davejohnson/SearchcoilfieldshapeApril2012.pdf where he warns: So to say what happens when you swing a target past the searchcoil of a simple motion discriminator requires computation of at least 26 variables. I doubt anyone has ever attempted to compute such a thing, although it’s theoretically possible. In any case, nobody will ever draw a picture of it. Kinda like quantum physics. You can get a pretty good idea of what's going on with a good verbal description, but to really understand it you need to get into the weeds (higher mathematics). Just like detecting. It's hard to swing a detector in the weeds, but sometimes that's where the really good targets are hidden. ? 1 Link to comment Share on other sites More sharing options...
mn90403 Posted August 19, 2019 Share Posted August 19, 2019 Wow! Not so simple by anyone's descriptions or calculations. I think your paper is an attempt by Dave Johnson to take the complicated science and reduce it to a few words as he says. Maybe this is really the 'artful' part of what we do. Most participation events or creations we do as humans requires both art and science. I'm hoping some of these details will help to recalibrate my personal algorithms. Now, if I could just hear it! haha I'm finding my hearing requires me to equalize the pressure on each side of my eardrums much too often even without elevation changes. Translation: I'm getting old. Thanks for the mind tease. I hope someone like Andy or Jason sees it. 1 Link to comment Share on other sites More sharing options...
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