Written by the TreasureGuide for the exclusive use of the Treasure Beaches Report.
Everybody wants a metal detector that detects deep. That is what detectorists talk about a lot. They compare detectors talking about inches in depth. But what many don't realize is how much they miss that is right up near the surface.
Yesterday I talked about masking. I showed how much space a large junk target can mask. The junk doesn't have to be right on top of the good target to mask it. A junk target can mask an area much greater than the ground it covers. A three foot by one inch (36 sq. inches) bar was shown to mask several square feet of area.
Masking is one big reason I recommend removing all junk targets when that is feasible. Some times you might want to gamble with the probabilities that you won't lose much by discriminating. And there are times when you might want to cherry pick, which in my mind is ok if you make a calculated strategic decision taking various factors into account.
While people are talking about depth, they should be thinking also about coverage. What difference does it make if the detector will detect a coin at ten inches if it can't get past the first inch? A person might carefully grid an area, but not be aware that they are still missing many square feet. If you are discriminating junk, you might not be aware of how much area is being masked.
Today I'll get into more details about how junk items can mask good targets such as coins.
I'll use again the Garrett Ace and Minelab Equinox metal detectors again. This time I'll use a railroad spike as my junk item for my informal experiments.
I placed a railroad spike on the ground and a modern clad dime next to the spike. First the dime was placed a foot away from the spike. Below is a simplified illustration showing the spike and the dime at one foot and 1.5 feet away from the spike. I actually also tried the experiment with the dime six inches from the spike. The illustration is obviously not drawn to scale.
For this part of the experiment, the coil was swept left to right and right to left over the center of the spike and dime. I kept my sweep speed fairly normal, but perhaps a bit slower than you normally see used in the field.
First, I used the Ace in all metals mode. The Ace did not detect the coin unless the coin was about 1.5 feet from the spike. In all metals mode, all I heard was the signal from the spike unless the coin was moved about 1.5 feet away from the spike. When the coin was 1.5 feet or more away, I heard both the spike and the coin. The Ace then gave a correct readings for both items and showed on the meter that a dime was being detected. The dime was not under the spike, or even real close to it when it was masked. If I was using the Ace in the coin mode in the field, I would miss both the spike and the dime unless the dime was a good distance from the spike.
If I swept the coil from right to left and ended the sweep before the detector coil got too close to the spike, and then swept back the other direction over the coin, I got a good signal from the coin, which you would expect. However, it was not necessary to go directly over the spike to get some masking effect on the coin. There was some masking effect from the spike even if the coil did not pass over the spike, but came very close to it.
Did you ever get a signal when sweeping in one direction, right to left for example, but got no signal over the same spot when sweeping left to right? Maybe you repeated that several times and each time got the same result - a signal when sweeping one direction but not the other? That can happen using targets like I used in this experiment. In fact I got exactly that when using the Ace in coin mode. I got it in this experiment and also working in the field.
With the dime at about one foot from the spike, I swept right to left slowly enough so that the coil passed over the coin and produced a signal, but then after the coil passed over the spike and returned there was no signal when the coil, moving left to right, passed over the dime. Signal over the dime going from right to left towards the spike, but no signal moving from the spike left to right and over the dime. That occurred when the dime was less than 1.5 feet away from the spike. When passing over the coin first, there was a distinct dime signal, but on the return sweep, the dime was in the shadow of the spike.
So when you get a signal sweeping in one direction, but not the other, there is a good chance you have a junk item that is being discriminated on the side of the target that is not producing a signal. The target is far enough away from the discriminated item so that it is not completely masked, but not far enough away from the junk item for complete target separation.
Using the Equinox produced very similar results to the Ace. I therefore won't take time to describe everything for the Equinox. I found the masking and target separation to be about the same for both detectors.
For the Equinox I used Field 1 mode, multi-frequency and the factory default settings. The sweep speed was again perhaps a bit slower than what you normally see in the field.
You can get slightly better target separation with a slower sweep, but fi you are discriminating out all indications of junk targets, you won't know that you might be dealing with masking.
With the dime one foot or less from the spike and swinging the Equinox the same way as the Ace was used, no signal was obtained from the dime. The spike read 16/17 on the meter.
Just a couple additional observations.
If the spike was turned ninety degrees. so that the spike was inline with the dime, the spike produced a double signal, and what I've called the shadow, or masking area, was just a touch bigger.
As I explained before, when you swing your detector over an elongated object such as a nail or spike, if you sweep your coil over the length of the object, you will get two signals. The signals will be over the two ends of the elongated object. I explained that in detail in the past.
If you sweep in one direction over a spike over the width of the spike you'll get a single signal, but when you sweep over the entire length of the spike you'll get a double signal - a signal over each end of the spike. If you are aware of that, you can easily identify elongated buried objects from the signal.
Let's move the spike and coin now. Place the coin to one side of the spike like shown below. Swing the detector back and forth from left to right and right to left. Will you get any masking effect from the spike? What do you think?
I wont' get any more detailed than that for now. There are a lot of variations you can try.
The main point to get today is that masking can be a big effect and can cover a lot of area.
While most detectorists seem to be concerned about depth, they might actually lose more targets to masking.
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Happy hunting,
TreasureGuide@comcast.net