Saturday, February 4, 2017

2/4/17 Report - Neat 18th Century Map Depicting A Shipwreck. The Metal Detector Signal/Noise Issue: Technological and Psychological.

Written by the TreasureGuide for the exclusive use of

By Pieter van der Aa, published 1706 in Leiden, Netherlands Map of a voyage from Jamaica to Panuco showing settlements along the coast of the Gulf of Mexico, Cuba, and Jamaica 
Here is a neat old map published in 1706.  Notice that in the middle of the gulf is a picture of a shipwreck with people and horses swimming to the island.   

Here is the link.


There is a lot more to a metal detector signal than you might think.

A detector signal always exists in a field of noise.  The amount of noise will vary from situation to situation.

The first and perhaps the most obvious kind of noise is environmental.  Maybe you've been on a beach on a day when there was so much wind that you could barely hear your detector signal.  A good pair of ear phone with a volume control will help in that situation.  But that is only one type of noise.

Another type of noise is internal.  Maybe you have tinnitus (ringing in your ears), that masks sounds of a certain frequency.  A tone control can help with that.  Some detectors have a light or other visual signal if you have auditory problems.  

Visual indicators are also subject to noise, such as bright sunlight or glare.

I should mention that spending long hours over the years listening to the same tone can damage your hearing, especially if the volume is high.  Hearing normally changes some with age.

Your concentration is a factor too, especially for the subtle signals produced by deep targets.  

Concentration is also especially important when there are lot of junk targets in the area or false signals caused by the environment or poor operating technique.  You cane easily begin to ignore good or borderline signals when there is a lot of this kind of noise.  It can be difficult to tell the difference between these types of signals without listening carefully and without experience.

Some of the auditory or visual signals can be what you might call false signals.  Electrical interference from electrical lines, lightening, other metal detectors or other sources can be a problem.  One common solution for some of those situations is to turn down your sensitivity.  Some detectors allow you to switch frequencies and some have a frequency check that will switch you to what the detector determines to be the best frequency.

Underground cables and nearby transmitters can also cause a lot of noise.

I'm assuming that there is no real problem with your detector.  Bad cable connections and other technical issues can cause problems.  

You can cut down on the amount of some kinds of noise by using good technique.  Don't jerk your detector when changing coil direction.  Keep the coil flat and at a constant height off the ground. 

Jerking or lifting the coil at the end of each swing can cause a lot of false signals.  People tend to learn to ignore a lot of false signals when there are a lot of them, but some of those signals can be good signals.

Black sand, salt and other mineralization can also cause false signals and amplify and falsing caused by poor technique. 

A lot of people have trouble with black sand.   Some detectors handle black sand better than others.
No matter what detector I used, black sand never bothered me too much.  With many detectors you can learn to distinguish between most signals caused by the black sand and targets in or under the black sand.  I often also found pinpoint mode to be better than discrimination mode when working in black sand.

One way you can learn to better understand what your detector is telling you is by watching the ground carefully when detecting.  You can see the subtle line made by the last wave and if it causes your detector to react.  You can see what sound is caused by ripples in the sand or transitions in the black sand.  You can learn how those types of things will cause your detector to react, and you can learn what they sound like.

There are various adjustments that you can make, and there are trade-offs associated with each adjustment.  It is always a trade-off between two types of errors: false positives and false negatives.

False positives are when you think you have a signal for a good target but don't, and false negatives are good targets that you miss.  When you decrease one, you generally increase the other.

It is not that simple though.  If you increase sensitivity to get the more subtle signals that indicate deep or smaller targets,  you also might get more false signals, but that isn't all.  It isn't only about technology.  There are psychological effects.  Will you hear and attend to the smaller signals the same when there are more loud signals?   I say the answer is no. I've talked about that some before.
That is just one of many example.

Noise is not only a matter of technology.   It is also a complex psychosensory phenomena.

I've noticed, for example, when running high sensitivity, perhaps higher than optimal, I'll get more falsing and as a result begin to ignore more signals and that causes me to miss some good signals.  You can pretty well predict how a change to your detector settings will affect your signals.  That is the easy part.  But how does the operator react to the change?  That is more complex and just as important.

The metal detector is just one part of the system.  The other part is human.