Written by the TreasureGuide for the exclusive use of treasurebeachesreport.blogspot.com.
Every once in a while I post information that you won't find anywhere else. Sometimes I get deeper into a topic, or sometimes I approach it a different way. Today I'll post something I bet you've never heard before, at least not in the same depth or the same way.
Before getting into the new stuff, I need to repeat a bit that I've posted before. Here goes.
There are several problems with air tests. An air test can be useful when you get a new detector and are just beginning to get to know a detector and how it responds to different types of targets. You can't, however, judge how deep a detector will detect in the field from an air test. I do recommend doing some air tests to begin with, but field tests are much better.
One of the problems with an air test is that people test targets directly under the center of the coil in order to get the maximum depth. In actual use in the field, the target will often not be under the exact center of the coil.
Also, when conducting an air test, people adjust the speed they move the target so they get the greatest possible depth. In the field, most people do not use the optimal sweep speed. That can make a big difference.
An air test can be useful for learning what type of signal, if any, a target will cause. It can also give you an idea of the relative depth that can be achieved on different targets in air.
A third use, and here is one that you probably haven't read or heard about before, is determining the shape of the area of target sensitivity under the coil.
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| Illustration Showing Coil and Two Different Areas Of Detection Sensitivity. |
The area between the green line and the coil would be the area of target sensitivity for a more sensitive coil.
The semi-conical shape would be a very common shape for the area of target sensitivity for many detectors.
Usually the greatest depth is achieved under the center of the coil. Less depth is achieved out closer to the edge of the coil. That is common knowledge.
The shape of the area of sensitivity is different for different detectors and different coils. The area of sensitivity is more narrow for some detectors and broader for others.
I've had some coils that are more sensitive than others, even on the same detector. One particular coil that I had was very good at picking up objects slightly outside of the edge of the coil. Some people might say that isn't possible, but I'm convinced that some coils/detectors/settings can do that.
Coils that are good at detecting good targets at the edge of the coil, or even beyond the edge, have what I call good peripheral sensitivity. You might think of that as being something like peripheral vision.
The Whites Surf PI, as one example, is much more sensitive near the center of the coil and not very sensitive out away from the center of the coil. The area of maximum sensitivity is narrow on that detector. That means that if you want to get good depth, you have to overlap your sweeps a lot. Otherwise much of the area you cover will only give you something like half the depth or less than what you will get under the center of the coil.
In the above illustration I have four targets. One would be detected by the less sensitive coil, one by the more sensitive coil and two missed by both coils.
Now here is an important point. Since the size and shape of the area of target sensitivity is different for different coils and detectors, it is useful to know the approximate size and shape of the area of target sensitivity for your coil and detector. One way to determine that is by air testing.
To determine the shape of the area of sensitivity for your coil for representative targets, do an air test, but instead of trying to find the maximum depth at the sweet spot, run your test target under the coil in the middle AND ALSO at different distances out from the center of the coil. You will then learn the approximate shape of the area of target sensitivity. How narrow or broad is the area of target sensitivity? How broad is the area of maximum sensitivity? How close to the edge can you get decent depth on the types of targets you are interested in?
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| Illustration Showing Three Sweeps, Two To The Rigth & One To The Left |
This above illustration isn't very good, but I tried to do it quickly and the tool I selected wasn't great for the purpose. I hope you can figure it out.
The circle labeled "coil" represents the starting location of the coil before the first sweep. The big red rectangle shows the area covered by the coil as you sweep to the right. The green rectangle shows the area covered by the coil on the sweep back to the left, and the blue rectangle shows the area covered by the sweep back to the right. In actual practice these would most likely be arcs instead of rectangles, but I hope you get the idea.
The yellow area in the middle of each larger rectangle represents the area of greatest depth and sensitivity near the center of the coil. Even with this tight sweep pattern some small area between the second and third sweep is not covered by the coil at all. There is some overlap at the end of the first sweep and beginning of the second and some overlap at the end of the second sweep and beginning of the third.
The area of maximum sensitivity and depth (yellow) is more narrow than the width of the coil. That means that even if you run a tight sweep pattern, the majority of the area covered by your coil is actually getting less than the maximum depth.
With detectors having a narrow area of maximum target sensitivity that is more serious. You would be getting much less than your maximum depth in most of the area you cover. That is why it is important to know the shape of the area of target sensitivity and the areas of good and poor sensitivity for your detector You can then adjust accordingly. If it has a narrow area of maximum sensitivity, you might want to overlap more. If your detector has a good broad area of sensitivity, you won't need to overlap as much to maintain good depth.
Different situations call or different strategies, but you have to know your detector to make the very best decisions. Knowing your coil and its response characteristics and strengths and weaknesses will help you make the best decisions.
