Metal Detector's Operational Depth Range, page 3

How To Avoid Reduction of Detecting Range Under Real Metal Detecting Conditions

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5) High concentration of trash can greatly reduce the operational depth range (detecting range) especially when undesirable and rejected targets are shallow and form a so-called "junk blanket". Some of the desirable targets inside the "blanket" are partially masked and the detector's responses to them are "broken" ("chirpy", "clipped", "ragged", etc.) and inconsistent. The older detectors may not even respond to most of these obscured targets as well as the good targets lying deeper underneath the "junk blanket". The "blanketed" targets are still off-limits to most today's metal detectors. The problem is that their users are not aware of it!

In general, the detector's transmitted electromagnetic field CAN NOT "pass" through a superficial junk target and reach a good target lying beneath it so that the detector would only respond to the deeper good target. Most detectors are not capable to "see" through the "blanket" because numerous junk targets dramatically distort the transmitted electromagnetic field as shown on a picture below (the picture does not depict a real "iron clutter").

Depicted below is a case of a concentric search coil with its electromagnetic field shaped like a cone and being attracted by nearby junk targets which diverted it away from the deep-lying coins. No matter if the search coil is moved left or right, it can not "grab" the coins due to reduction of the detector's operational depth range.

Coins Are Masked by Nearby Iron Objects

A Double-D search coil would perform better under such circumstances due to its narrow shaped transmitted electromagnetic field - a "blade", but just using the DD coil is not enough to solve the problem. The most advanced metal detectors incorporate a few quite effective features especially designed to improve target separation and maintain decent depth penetration when dealing with the iron- and/or mineral-contaminated ground. Such features include the 'FERROUS-COIN Target Separation' and graphic indication - 'TARGET TRACE' (Minelab CTX 3030), 'TRASH DENSITY-HIGH' and 'FAST-ON' (Minelab E-Trac), HIGH OPERATING FREQUENCY, 'REACTIVITY' and 'TX POWER' (XP Deus), just to name a few.

If you operate a less advanced metal detector, you can improve its performance on the iron-infested ground by 1) utilizing a smallest Double-D search coil of a "Sniper" type, 2) lowering the level of Manual Sensitivity to "below stability level" or, what is better - switching to Auto Sensitivity, 3) using NO DISCRIMINATION, i.e. operating your detector in ALL METAL mode, and 4) sweeping the search coil extremely slow and "wiggle" it in order to isolate responses to good targets from bad ones.

6) Strong electromagnetic interference (or EMI) can greatly desensitize a metal detector; thus, reducing its operational depth range for small and deeply buried targets. The electromagnetic interference affects the metal detector's circuitry, in particular Discriminate, Ground Balance and Visual Target ID circuits, due to either electromagnetic induction or electromagnetic radiation emitted from an external source such as electric power transmission lines, electric motors, thermostats, radio and TV stations, pager transmitters, high-tension electromagnetics, etc. The EMI source may be natural such as electrical storms (lightning), the Northern Lights or even the Sun (solar radiation and magnetic storms) that carry rapidly changing electrical currents.

If you metal detect in close proximity to the EMI external source, the electromagnetic disturbance may degrade or limit the effective performance of your metal detector. These effects can range from erroneous VDI readings (or large inconsistent fluctuations of a Target CrossHair) and distorted audio over desirable targets to a continuous stream of pulsating false signals that make the detector's operation impossible.

To avoid occasional EMI, you can slightly shift the operating frequency of your metal detector.

NOTE: Altering the operating frequency too far from the normal value can also reduce your metal detector's Operational Depth Range.

On the Minelab FBS metal detectors, there is a special feature, 'NOISE CANCEL', that is especially designed to overcome any EMI. As soon as you move away from the EMI source, and the external electromagnetic interference stops affecting your detector's performance, adjust the frequency back to the central setting which is more closely attuned to the emitter coil.

If the source of EMI is more than your detector is capable of handling, the Sensitivity level must be greatly reduced. If that does not help, increase Discrimination a little. Your last resort is to simply distance yourself from the EMI source.

A case of the EMI emitted by another metal detector operating at the identical frequency nearby (as it happens during competition hunts) is not covered in this article; however, the solution is the same - shift the operating frequency or change it.

7) Tall grass and thick surface vegetation would not allow you to sweep a search coil close and parallel to the ground surface. Failing to do so causes a substantial loss in detecting range. There are a few ways to overcome this problem, but describing them is out of scope of this article, so I will mention just the simplest one.

If the grass is thick and tall, you can simply short-pace backwards while tramping down the grass and scanning a narrow path that you have just created with the search coil. You will most likely pass up many valuable targets; however, you will get a notion of this hunt site: whether or not it would be worth of coming back and searching when the ground is bare in the early Spring. The thick ground vegetation can be treaded down with tires of your 4x4.

8) Parched ground makes a metal detector less sensitive to highly oxidized, low conductive coins and other non-ferrous targets. However, a capable detector can still respond to deep and small SILVER coins when the ground is "bone dry". This is possible because the silver coins are more conductive than the coins alloyed with copper and nickel.

It is better to wait until after the rain and metal detect when the soil is moist. This will ensure maximum sensitivity to all coins and other desirable targets, and allow easier target recovery and less damage to grass root structure when metal detecting on lawns.


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