6/27/17 Report - Getting To Know Your Detector/Coil Combination. A 250-Year-Old Lock. Surveillance.

Written by the TreaureGuide for the exclusive use of treasurebeachesreport.blogspot.com.

A few days ago (/10/17) I talked about the cone of sensitivity for different metal detector coils.  I showed that for some coils, the area of sensitivity is more cone shaped.  For a cone-shaped coil, the maximum depth is obtained when a target is under the center of the coil.  You get less depth when a target is out near the edge of the coil.

The illustration above shows the theoretical cone of sensitivity for two different types of coils - a concentric coil and a DD coil.  According to what you will typically read or hear, the area of sensitivity under a DD coil will not be as sharply cone-shaped as it will be for a concentric coil.  When looking at the area of sensitivity from the side you can see that it is more blunt from a side view.  The area of sensitivity is supposed to be narrow for a DD coil when viewed from the front or back.

The depth you will get with a DD coil for a target under the center of the coil will be similar to the depth that you get when the target is near the edge of the coil.  The importance of that is that you will be getting greater depth under a wider span of the coil, making overlapping of sweeps less important because you are getting good depth under the entire area of the coil (front to back).  When using a concentric coil, failure to overlap sweeps would mean missing more deeper targets that fall closer to the front or back edge of the coil where you get less depth.

I am not one to go by theory or what everybody says or even by what the manual says.  I test things out.

In my 6/10 post, I encouraged you to use a test object to map out the area of sensitivity for your coil and detector.  Doing an air test, pass a test object under the center of the coil and see how deep the object can be detected.  (That is how most air tests are conducted.)  Then pass the object out towards the edge of the coil and see how deep the object is detected, and then do the same thing with the object farther out near the edge of the coil.  You will get a good idea of how much depth you will get under the center of the coil and farther away from the edge of the coil.

Mostly we talk about the area of a coil's sensitivity from front to back.  DD coils are often said to have a narrower area of side to side.  That is supposed to result in better target separation.  You should be able to distinguish between two targets that are beside each other.

With my tests I definitely found that one detector/coil combination had a much sharper cone shaped area of sensitivity than another.  As expected the concentric coil had a sharp cone shaped area of sensitivity, and the DD coil had a more blunt shaped area of sensitivity, as shown in the illustration.  I did not, however, find that the side to side area of sensitivity was narrow.  It seemed about as blunt as the front to back sensitivity.  So much for what the manual says.  Although that test did not correspond to what the manualsaid, my tests corresponded very well with what I experienced in the field.  This particular detector/coil combination does not provide good target separation even though it involves a DD coil.  Targets sound big.  And a bunch of small close targets sound very much like one big target.  How much of that is electrical engineering and physics and how much of it is psychosensory, I don't know, but that isn't important to the detectorist.  The important thing is how you perceive it in the field.

My main point is to experiment so that you really get to know your detector and how it responds.  Get to know both its strong points and its weaknesses.  Read the manual, but test everything out for yourself.  You might find that what you have been reading or hearing does not seem to be true for your situation.

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Source: See link below.

MACKINAW CITY, MI - A 250-year-old lock was discovered at Colonial Michilimackinac earlier this week.

Staff at the historic fort and trading post in Mackinaw City discovered the intact, 2.75-inch long and 2.25-inch wide brass artifact while excavating a fur trader's home on the site.

Lynn Evans, curator of archaeology at Mackinac State Historic Parks, said the piece was likely used to lock a small trunk or chest sometime between 1760 and 1770. The lock is a rare find, even for a place full of hidden treasures like Fort Michilimackinac...

Here is the link for the original article.

http://www.mlive.com/news/grand-rapids/index.ssf/2017/06/revolutionary_war-era_lock_une.html

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Some of you will consider this off-topic.  Thats ok.

BAE, a British company that has been widely criticized for sharing communications surveillance software to countries with human rights abuses.  The software is capable of tracking virtually ALL computer communications.

I saw a TED talk, which I wish I could find again, on Evident, a software package about which it has been said, "You'd be able to intercept any internet traffic," a former employee told the BBC. "If you wanted to do a whole country, you could. You could pin-point people's locations based on cellular data. You could follow people around. They were quite far ahead with voice recognition. They were capable of decrypting stuff as well."

I just think a computer user should be aware of the digital environment they inhabit and more generally the world they live in.  

Treasure hunters used to be very much interested in treasure maps, codes, covert activities and the like.  Doesn't seem to be the case so much anymore.

Watching the hearings on TV, one thing that impressed me is the number of things that they feel they have to keep secret from the citizens of the country.  Very much like archaeology, but a byproduct of terrorism instead of looting.  

Here is one link if you are interested in this type of thing.

http://www.bbc.com/news/world-middle-east-40276568

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On the Treasure Coast we have more smooth surf for days or weeks.  The tides are pretty good now, with some nice low tides.

Happy hunting,
TreasureGuide@comcast.net

4/3/15 Report - Concentric and DD Metal Detector Search Coil Test Comparisons. Ace 250 & ATX. Easter Surf.

Written by the TreasureGuide for the exclusive use of treasurebeachesreport.BlogSpot.com.

The few days ago I talked about determining the shape and size of the area of sensitivity of detector coils.  Someone asked me about DD coils.

DD search coils are said to provide better performance in highly mineralized soil, provide better target ID separation, better accuracy or pinpointing and greater depth in hot soil.

As an example, Garrett's DD coil design has a broad area of target sensitivity - what they call "detection field." 

In two recent posts I used diagrams showing a detection field like what you would expect from a concentric coil.  The detection field for a concentric search coil would have a shape something like that shown in the left diagram in yellow, extending from the edge of the coil and narrowing as you get farther away from the coil.



Illustration of detection fields for concentric and DD coil.
Yellow represents the detection field for a
concentric search coil and green, that of a DD search coil.
 

I found this illustration showing the detection field for a Garrett DD coil as compared to the detection field for a concentric search coil.  The Garrett DD is said to produce a narrow field in the center of the coil that runs from the front of the coil to the back of the coil.  That is represented by the green area in the diagram on the left.

Looking from the side (illustration on the right), the detection field of the DD extends almost straight down from the edge of the coil and then has a more rounded bottom.  It therefore would be expected to detect targets deeper near the edge of the coil.  Using the term I introduced a few days ago, it has more peripheral sensitivity.

Notice the shape of the detection field for the concentric coil and how it is more narrow.

The DD field as shown in this illustration is more narrow when viewed from the front, but broader than the field for the concentric coil when viewed from the side.

I already had some general impressions of the DD as compared to a concentric coil but did a few tests before writing this. 

I used the Ace 250 with the standard concentric coil (7 by 9 inch) and the standard ATX  DD search coil (10 by 12 inch) for these tests.

I often use these two detectors for tests of various types simply because they are so different.  Ideally to test coils, everything else would be the same except for the coil, but even that would require many tests to compare the two coils while using various settings and different conditions.

First I used the Ace on four different coins, including a penny, nickel, and two foreign coins, one of which was magnetic.  I put them in a line with each just a couple of inches from each other.  The Ace didn't respond with four distinct separate IDs when the coins were that close.  I then moved the coins farther apart.  At about six to eight inches apart and using a relatively slow sweep speed, the Ace then could give a correct ID for each of the coins separately on a single slow sweep.  The magnetic coin was identified as iron and the others were correctly identified too.

The ATX in motion mode (with the 10 inch DD) coil did not provide separate signals for the different coins even at eight inches or more apart.  I did not get the improved target separation that would be expected from the narrower front to back detection field illustrated in the above diagram.   In fact it was much worse. 

As I pointed out above, I can not blame the coil exclusively since many other factors were different in this test, yet the results were not just different from what I would expect from seeing the diagram, but remarkably different.  I wondered if the DD coil diagram shown above might have been for a different type of DD coil than what I had.  I guess that is possible.

I also have to acknowledge the hugely different types of detectors being used.

In non-motion mode the same ATX did not give a good auditory distinction between the four different coins even when they were a foot apart, however I could see that there were four different peaks in target strength indicating four different targets by watching the visual target strength display.  If I was depending upon the auditory signal alone, I would not have noticed any indication of target separation.   Going on auditory signal alone, I could not identify and indication of target separation with the DD coil even when the coins were a good distance apart.  Maybe I'm not good at auditory discrimination or maybe I should lower the threshold or something.

I did another test using a piece of iron shipwreck spike and a nickel (see picture at left).

The Ace showed more target separation on those  objects too.

The ATX with the DD coil that I used in these tests, has a large detection field left to right.  That large detection field is even larger when the target is an iron object.  In fact if you watch the ATX coil as it moves towards, over and away from the targets, the signal begins before the edge of the coil is over the target and doesn't end until the trailing edge is from under the coil.

That is something that you might actually like.  The detection field is bigger than the coil.  However, it makes pinpointing more difficult.  And it does not provide what I would call good target separation.  You can, however, go into pinpoint mode to do your pinpointing.

These simple tests confirmed what I previously noticed in actual field use.  The ATX standard DD coil has a broad area of peripheral sensitivity.  That can be a good thing because you cover more ground, but it does not make for easy pinpointing. 

There are times when you might want good target separation and times when that is not so important.

When working in non-motion mode, there were times when I could not sense an auditory indication of multiple targets but could get an indication of multiple targets from the visual display.  The signal strength would show a peak over each of the four coins on the visual display.  That visual indication can be improved for shallow targets by lifting the coil to diminish overall signal strength.  That is a trick that I just learned that I will adopt in the field when I want to see if a large area giving off a signal might be a cluster of smaller targets.

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There will be a little bump in the Treasure Coast surf on Easter - up to three to five feet, and an additional foot or so on Monday.

The weather couldn't be more beautiful.

Happy hunting,
TreasureGuide@Comcast.net