Saturday, June 10, 2017

6/10/17 Report - How and Why To Map The Cone of Sensitivity For a Metal Detector Coil.

Written by the TreasureGuide for the exclusive use of

Area of Sensitivity Under Metal Detector Coil.

Sometimes I wonder if I am in an alternate universe.  Some of the things I read are so different from my personal experience that I marvel at how different people can reach such different conclusions.

I sometimes read that this or that detector is really good or bad or this or that feature is really good or bad, and it is 180 degrees from what I've found in my own experience.

Personally, I like testing things for myself.  I don't care what someone is says about this detector or that detector when I can test it for myself.

A lot of the time when you read a detector test, the user just took the detector out of the box and is supposedly doing a field test.  They don't do what I would consider the minimal basics before they begin.  They just don't know the detector well enough to give it a fair test.

I'm dumb, but one detector I now use took me a really long to master.  There is no way I could have given it a fair test the first day or week or even month.  

I haven't talked about this much lately, but I recommend that you do a lot of testing with your detector just to get to know it better.

I was just reading where someone hated a particular coil lock.  I have the same detector with the same coil lock and have used it for a couple of years now.  I often marveled at how well the thing works.  It amazed me to see other people complaining about it.

Maybe it is a matter of quality control.  Maybe some of them are better than others, but I don't think that is what is going on for the most part.

The detector is only part of the story.  The operating characteristics of the detectorist is as important as the operating characteristics of the metal detector.  That is something else I've talked about in the past.

To use your detector well, you have to know it well.  Do your air tests, field tests and learn the effect of each of the settings.

One of the things you should know about your detector is how sensitive the coil is to various targets under different parts of the coil.  This differs for different coils.

At the top of this post is a simple illustration showing that the area of sensitivity for some detectors is cone shaped.  You will detect targets much deeper  when the target is under the center of the coil.

That is one problem with air tests.  People usually pass the target under the coil's most sensitive spot, and they do that in a way that gives the best possible response even if that does not match at all how they move the coil in the field.  The results will therefore be misleading and not very relevant to how the detector will actually perform in the field.

My main point today is that you should know the cone of sensitivity for your detector and coil.  There are big and important differences.

You should map the cone of sensitivity for yourself.  This is one way that an air test can give helpful results.

Pass a sample target under the coil at the front edge of the coil to determine how far it can detect the target when it is passed under the edge.

Use the most relevant test target that you can.  I've written a lot about that in the past.

Next pass the target under the coil but an inch or so closer to the center of the coil.  Observe any change in depth. Repeat that multiple times going from the front edge to the back edge of the coil.  Observe the depth at each spot until you have a map of sensitivity like the one shown at the top of the post.

For example, when I mapped the area of sensitivity for a Garrett Ace 150 with the standard coil, I got sharp cone similar to the one shown above.  Targets passed close to the front or back edge of the coil were detected at only a couple of inches, while targets passed under the center of the coil were detected at least twice as far from the coil.

So what is the practical importance of that?  First, if you want to be sure to find all deeper targets, you will have to overlap your sweeps.  You will have less than half the depth when targets are near the front of back edge of the coil.

Depth is not always critical, especially if you are hunting near surface targets, but you should know the shape of your coil's cone of sensitivity so you will know how to use it for different situations.

The above illustration shows the cone of sensitivity and five overlapping passes for a coil having a sharp cone shaped area of sensitivity.

The first pass would hit the first target, but the deeper target would be missed between pass 1 and 2.

The triangles (cones of sensitivity) are slightly different in size.  That is actually realistic.  Things like ground mineralization changes as does the sweep speed, which is not always optimal, and coil height.

The second row illustrates four passes, but they are not overlapping.  Also the coil height is changed, as happens in real life.

Notice how little area is covered by the cone of sensitivity when the coil is high (passes 1 and 2).  A lot of ground and targets are missed..

That is one metal detector.  I also mapped another detector with a different type of coil.  Using the ATX with a 12 inch DD coil, you get a much more rounded cone of sensitivity.  In fact is not cone shaped at all.   That cone of sensitivity (if I continue to call it that) is very blunt and the edge of the coil detects nearly as deeply as the center of the coil.  Obviously that means you do not have to overlap your sweeps as much.

The edge of the coil on the ATX detected deeper than the center of the coil on the Ace., but my main point today is not to compare different detectors.  My point is that no matter what detector and coil you use, you should know the size and shape of the cone of sensitivity.  You can learn about that by mapping the cone of sensitivity with an air test.  All of my previous cautions and stated limitations of air tests apply.


Finally a day without rain.

There is no tropical activity in the Atlantic now, but you might want to start watching for that.

Here is a link.

Happy hunting,