Target In A Hole Not Detected

[Cody]

I have heard and read a lot of explanations for why a target is sometimes not detected in a hole. All of us have detected a target then dug a hole and the target seems to have gone away. We get to digging around in the hole and the target then is detected. I don't think there is a single answer but a general answer for this interesting problem.

The transmitter is dumb and happy generating electromagnetic fields. It could care less what happens to them as long as it gets to do its thing. Any conductive target in range of detection is detected by the receiver. The microprocessor is the brains of the receiver and decide what to do about signals in the receiver coil that is fed to the preamplifier and then the demodulators. I have read of signals bouncing around in the hole that in some way are to blame for the coin in the hold problem. (A TR detector at one time could not be used in a cave which was a major PR for the BFO to be used in caves as it would be detuned.)

I think we can take a basic approach to these kinds of problems that are general since there is not enough information to say why that specific coin was not detected in that specific hole with the setting of the detector. That basic approach is there are two reason why the microprocessor does not report a target as detected.

One is the targets looks like soil minerals and the other is the target matches a user input for a target to be rejected.

I have seen coins in a hole that could be detected if flat in the hole but could not be detected on edge. I have seen coins right on top of the pile of dirt from the hole that were not detected until a probe is used to push around in the dirt. The answer is the same in that the target looks like mineral or it is set to be rejected. A great test to me is to see if it is detected in pinpoint.

One is tempted to reply that the detector is not designed to detect coins in free air or in a hole but that really does not add anything to our understanding. I have seen all kinds of tricks, coins with a pulltab folded around them, 10 nickels out of 20 that are detected and the other 10 rejected with the exact same settings. I saw a kid do a card trick during the Tet Offensive in Saigon that I am still trying to figure out. Best one I have seen to date that I could never figure out how it was done.

I am very curious as to what the Capt thinks on this one and the rest of you. We use to have a guy posting, Ralph, that was the Chief of Police, in a town not far from me that came over and purchased a DFX from me. He had no formal training in electronics but I I recall he was friends with Erick Foster and had really great answers to these types of questions. I have not seen his post for a long time and seem to recall he is building a house. I bet Jeff Foster on the DFX forum has a good answer and it would be the coin is rejected as ground minerals.

This is ramblings at its peak,

 

[Captain Kirk]

Cody,

I think that most of us have buried a coin and observed that It can not be detected as well as a coin that has been buried for a significant time. I do not have an explanation for that for which I have much confidence.

I have also experienced that a coin placed in an open hole does not detect as well as a coin in air. I do not have a good explanation for that, but here is a thought. We do know that the ground matrix has an effect on the shape of the magnetic field. When the coil is swept over an empty open hole, then the detector sees a change in the environment. This change could be confusing the detector electronics. This is just a "WAG".

In the final analysis I do not worry too much about not being able to detect a coin in an open hole because I do not think that I will ever run across that situation in the places where I hunt (That's a joke).

HH,
Glenn

 

[Cody]

I agree completely in that we don't look for coins in the air or in a hole. I have no idea what happens to the detector when it sees soil then all of a sudden a hole. This is like like tying to prove the halo one way or the other. There does not seem to be a firm answer but it can be confusing to a new user that may think their detector is not working properly.

 

[cspene]

the confidence thing. When you have a detector(s) that you have put a lot of trust in, then just for S & G's drop a dime in a hole 5" deeps and not a peep...that kind of blows your confidence in the machine. I know Minelabs go deep and I've been told that due to the design of the detector (Exp.ll) the minerals are not a problem, and no discrimination, it makes you kind of wonder "why?".

Cody, I understand for the most part what you said about coil size, but am I right to assume a small coil may see something a big coil won't even at IM 16 because of the clutter in the ground and all of the trash objects it is trying to separate?

Thanks for all of your responses.

 

[Cody]

Yes, your are correct. IM-16 is all metal detecting and in opinion the best test of a detector other than how well it discriminates. We get all that discrimination out of the way and see what happens if we are reporting all targets. The smaller the target volume, soil minerals and targets is considered to be the target volume, the easier it is to separate metal targets. I don't consider a co-located target to be one that are contact but co-located in a way that they affect how the detector sees the targets.

If we measure the electromagnetic footprint of the stock it is about the size of a 5 gallon trash can lid. We can detect metal to a good two or three feet depending on the size of the metal. The idea of the double D design is to try and shape the footprint so it is in a narrow long shape which is the hot strip. This also cuts down on stray RF noise but a double D does not get quiet the depth of a concentric. The concentric however has much more of a cone shape to the electromagnetic footprint so at maximum distance from the coil we only have a detection field of a few inches. This can be good however in picking through trash at depth.

I have believed for years that the best size coil for coin hunting is a double D or concentric, I prefer a concentric, at 8". A 5" is a great coil and even down to 3 and 4 inches but it does require a lot of time to work the smaller ones to cover much ground.

If we compare the size of a metal target we can detect with a coil the smaller the coil the smaller the targets we can detect. We can detect anything with the 8" as we can with the stock coil on the Explorer. I have not found any depth difference because I can adjust for the size with settings. I can detect targets with the 8" that I cannot with my Explorer with the stock coil. The larger after market coils is shooting for those ultra deep targets that are out of range of our machines with these coils and I understand there are very good reasons for them but not in the ground I search. We have some pretty rocky rubble type of soil which is very different than the soil I searched when I lived in Nebraska in the seventies and later sixties.

There use to be some really good PR material by Garrett and Whites that illustrated the overlap in size of targets and depth of coils.

 

[cspene]

n/t