Sunday, April 26, 2015

4/26/15 Report - Is It Size Or Surface Area That Most Affects A Metal Detector Signal? Dinosaur Eggs Discovered.

Written by the TreasureGuide for the exclusive use of

Dinosaur Eggs Discovered.
Source: ABC link below.
How the world has changed!

Social media has really changed how quickly we get news and how much information is available.  It wasn't that many years that you had to subscribe to magazines or buy books to get a lot of good information.  Now it is right in front of you on your computer screen.  You get fresh news from around the world daily.  Of course not everything you see on the internet is true, but you can use your own methods to seek out the best sources.  It sure is easier than depending upon a few magazines or going to the library every time you want to look something up.

The picture above shows fossil dinosaur eggs found at a construction site.  It just goes to show once again why archaeology needs and informed and cooperative public rather than an alienated tax base that only funds projects.

Here is the link for the dinosaur egg article.


Today I'll discuss that age old question, Does size matter.

The bigger the metal object, the stronger the signal.  Right?  Well, usually.  But not always.  It isn't the mass of the object, but rather the surface area that causes a strong signal.

Here is a section from page 38 and 39 of Charles Garrett's Successful Coin Hunting, 22nd printing, 2004.

Simply stated, the larger a metal target, the better and more deeply it can be detected.  Larger detection signals come from targets that produce more eddy currents.  An object with double the surface area of another will produce detection signals twice as strong as those of the smaller object but it will not necessarily be detected  from twice as far away.  it is true, however, that a large target will produced the same detection signal as a small target positioned closer to the searchcoil.

Generally speaking, modern metal detectors are surface area detectors.  How a detector "sees" a target will be determined to a large extent by the surface area of a metal target that is "looking at the bottom of the searchcoil.  You can prove for yourself that the actual volume or mass of a target has very little to do with most forms of detection.

With your detector operating move a large coin toward the searchcoil with the face of he coin 'looking at the bottom of the searchcoil.  Note the distance at which the coin is detected.  now move the coin back and rotate it so that the narrow edge "looks" at the searchcoil's bottom.  Bring the coin in  and you will notice that it must come far closer to the searchcoil to be detected.  The mass of metal itself did not change, only the surface area of the coin facing the searchcoil.

Most of what I learned over the years I learned by myself.  I read a lot, but I was a detailed observer and did a lot of testing on my own.

I tested how changing the presented surface area a coin affected the detector signal using three very different types of detectors.  I posted some observations on the shape of objects and sweep direction in a post some months ago.  See my 10/12/14 post for example.  Or take a look at the following linked YouTube video.

Using a clad dime and quarter an Ace 250, Excalibur and ATX, all with standard search coils, I first placed the coins flat on the ground and observed the distance at which the dime could be detected by all three detectors.  Of course there were differences in depth for the three detectors.  I won't get into  those details now.  I used both all metals and pinpoint modes with the Excalibur and the motion and non-motion modes with the ATX,but just the all-metals mode was used with the Ace.  You can see how the number of combinations and comparisons quickly increases.

I could give you depths for detecting the dimef or each detector but that would be fairly meaningless because it would be different in different environments.

After noting the depths at which the flat coins were detected by all three detectors, I then stuck the coins in the ground so that the detector was only "seeing" (to use Garrett's term) the edge of each coin when the coil went over it.  As predicted, the depth at which the coins were detected by all three detectors decreased dramatically.  Not surprising.

The depth decreased by roughly about one third to one half for the Excalibur and ATX when the coins were on edge.  Maybe a touch more for the ACE.

While it is unusual to find a coin on edge, there are times when you might have the flat surface of the coil over the edge of a coin.  One is when you are digging a hole and the coin gets stuck against the edge of the hole.   I've mentioned in the past how that can make the signal disappear.

Another time when you might be detecting the edge of a coin is if there is a cut and you run your detector coil up and down the face of the cut.  A coin laying flat in the sand would be presenting the edge of the coin to the face of the search coil in that case.

I took the test one step farther though.  I wanted to see the effect of sweep direction.  I've shown in the past that long elongated objects such as nails will cause a different type of signal depending upon the direction that you sweep your coil over it.  That provides one method of identifying objects like nails from the auditory signal.  When a coin is on edge it presents a long narrow surface to the coil, as shown in the above diagram. 

With the coin first laying flat, I swept the coil over it left to right and right to left, and then changed the sweep direction by ninety degrees with each detector.  There was no obvious change caused by changing the sweep direction over the flat round coins.  That is what you would expect.

Then I stuck the coins in the ground so the searchcoil would only "see" the top edge of the coin and swept the coil over the on-edge coin first in one direction and then the other direction.

My observation confirmed that for all three detectors there was a marked decrease in the depth when the coin was on edge.  The decrease in depth was very marked for all three detectors and for both sweep directions.  The coil had to be within a few inches of the on-edge coin to get a good signal no matter which direction the coil moved.

As with elongated objects, the signal was different when swinging the coil in one direction and then the other direction.

With the Excalibur in discrimination mode, for example, sweeping across the edge of the coin (direction 1 in diagram) caused a good strong defined signal (double blip) when the coil was close to the on-edge coin, however when sweeping in the other direction (direction 2) the signal was more of a single whisper, but good and strong when close.

A similar thing was found when sweeping the ATX coil across the coin.  When sweeping in direction 1, the signal was much stronger than when sweeping in direction 2.

When the coil was close enough to the on-edge coin, and sweeping in direction 1 relative to the on-edge coin, the auditory signal was very strong.  Strength of signal rapidly decreased with depth though.

For this test I thought the pinpoint and non-motion modes provided the best test of signal strength since it was less processed. 

Discrimination modes provide some unusual results in different situations.  I won't go into more detail than that now though, it can get very detailed and confusing.

One of the main points that is definitely true is that the surface area presented to the coil affects signal strength and detection depth.  It is not all about size (mass). 

I've also shown in the past how an on-edge ring can cause a much less strong signal than a ring laying flat.

Another finding is that the direction the coil is swept over a coin that is on-edge makes a difference.  Very much like an oblong object, one sweep direction will produce a different signal over an on-edge coin than a different sweep direction.

You might remember an old post in which I showed something similar with a rectangular copper tag that I dug.  It was bent and part of the surface of the tag was at an angle rather than flat or on edge.  (You can see that tag in the YouTube video mentioned above.)  It identified differently on an ID meter when the coil was swept in different directions.  Bends in the tag and being buried in the ground at different angles caused differences in the signal.

Ideally you want to have your detector coil parallel to the largest surface area of the buried object for maximum signal strength.  When detecting a flat beach, if your coil is tilted that has the same effect as tilting a flat object and therefore reducing the surface area being detected.  That is another reason to keep your coil flat front to back as well as throughout the swing.


I'd like to get some more emails concerning beach detecting etiquette and experiences.


On the Treasure Coast today (Sunday) the surf is small - about one foot.  The tides are small today too.

Happy hunting,