How To Eliminate Responses To Hot Rocks
Hot Rocks are pebbles or rocks that contain more nonconductive minerals than the surrounding ground to which the metal detector is balanced.
When hot rocks are in the detector's electromagnetic field, they disturb the ground balance of metal detector so intensively that the machine reads them as metal targets, i.e. the hot rocks produce positive or false metallic responses which slightly vary in pitch tone from one area to another.
Hot rocks can contain either a mineral density (pebbles) or mineral quantity (large rocks), or both.
The today's advanced Motion Discriminate modes, such as the Full Band Spectrum (FBS) system, which is incorporated in the Minelab FBS metal detectors - Explorers, Safari, E-Trac and CTX 3030 (FBS 2), automatically adjust the ground balance every time it gets affected by a more concentrated change caused by the hot rocks or black sand deposits in nonconductive mineral intensity. But if the MANUAL SENSITIVITY level is high, and the SOUNDS setting is in FERROUS (pitch tone is based on the targets inductance), the detector's responses to the hot rocks can drive you crazy.
In most cases, the hot rock responses have a distinctive ("screaming") "silver-like" high-pitched tone but without any sound imperfections. The only difference between a coin-like signal (except the deep coin signals) and the hot rock response is that the latter sounds "smudged" - i.e. overextended with "blurry" edges, and does not sound as "solid" and "accentuated" as the coin-like response.
Another way to recognize the hot rock response is to pinpoint the detected "could-be-a-hot-rock" object. If you hardly get any consistent audio response or do not get any at all while pinpointing, there is indeed a hot rock under your search coil. Also, depending on a metal detector you use, when the hot rock is pinpointed, it may give a pulsating signal as if it is a deep target.
"Hot rocks" as well as the areas of highly nonconductive ground may "null" the threshold audio in an All Metal mode depending on their intensity. Nothing else can null threshold in the All Metal mode of detection.
To eliminate unwanted responses from hot rocks, you can do the following:
1) Reduce the maximum acceptable level of MANUAL SENSITIVITY or switch it from the MANUAL to AUTOMATIC SENSITIVITY mode which, for example, is available on the Minelab FBS metal detectors. In this mode, a metal detector continually measures the magnetic ground interferences and adjusts the level of Sensitivity to provide the most stable Target ID. However, automatic compensation of the hot rocks interference may reduce the responses of small and deep targets to inaudible levels (see details below).
2) If you do not want to reduce the reasonably high level of SENSITIVITY because your metal detector is stable, you can simply DISCRIMINATE hot rocks, but only if they do not register in the "coin spots" of the "silver" area on the 1- and 2-Dimensional Discrimination scales (SmartFind or SmartFind 2 windows).
NOTE: Muting the audio responses to the hot rocks (rejecting hot rocks) can be a "Catch-22" because if you accept hot rocks, you will receive many annoying high-pitched and false coin-like signals, but if you reject the hot rocks, you will miss some deepest coins positioned within detecting range. Often the deepest targets in magnetic ground emit weak electromagnetic fields and are registered by a metal detector as the hot rocks, i.e. these induced electromagnetic fields manifest Conductive or Ferrous-Conductive (FE-CO) properties that are similar to the properties of hot rocks.
For example, when I once metal detected using my usual Discrimination pattern, which rejected hot rocks, but in a different area, I missed a good coin. My treasure hunting buddy found the coin while walking behind me with his less advanced metal detector. I had to quickly adjust the Discrimination pattern on my FBS metal detector not to miss another valuable coin (a similar example with Minelab E-Trac is given on page 10 of My COINS 3-Level Search Program for E-Trac and CTX 3030). So, this option - discriminating the hot rocks, should be chosen and implemented carefully.
3) If your metal detector has only MANUAL Ground Balance, you can manually optimize the detector's ability to compensate the hot rock interferences. In this case, you ground balance your detector with a positive offset: when you lower the search coil to the ground, you hear a slight increase in audio threshold.
Some of the today's most advanced metal detectors that operate in SILENT SEARCH, such as XP Deus, allow to adjust the Ground Balance within a few seconds. And pumping the search coil is only optional and not required on the low-mineralized or neutral ground because constant indication of mineralization strength in real time is used as a reference. So, to eliminate responses to the hot rocks when they are encountered, one simply presses a touchpad a few times to increase a ground rejection level instead of increasing Discrimination (you may want to read my article "Less Discrimination Lets You Find More" on negative effects of ample Discrimination).
If you do not want to lower your detector's operational depth range by increasing the ground rejection level, you may distinguish responses to hot rocks by ear if you know their audio and VDI (visual Display Indicator) characteristics. As I stated above, usually the detector's responses to hot rocks are "smeared" - sort of extended, and their VDI values do not appear on a display. Audio responses to coins, on the contrary, are sharp and accentuated, and the VDI numbers are shown. And if you try to pinpoint a hot rock, you would not be able to do it in most cases.
In the Automatic Ground Balance mode (also called "tracking"), the metal detector automatically rebalances itself to changes in the ground mineral intensity without operator involvement, and, therefore, automatically mutes responses to the hot rocks. However, just like with the Automatic Sensitivity, automatic ground balancing can also muffle the responses to small and deep targets.
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