What are the hot rocks in metal detection world

What are the hot rocks in metal detection world

Hot rocks (sometimes known as “cold rocks”) are rocks, pebbles, or sediments that contain higher or lower amounts of conductive or nonconductive minerals relative to the ground around them; Specifically, with respect to what your metal detector is manually or automatically balancing.

 

Types of hot rocks found during metal detection

There are two main forms in which “Hot Rocks” can be categorized. First, hot cationic rocks, which contain greater amounts of conductive material. Second, hot passive rocks, also known as “cold rocks” which contain larger amounts of non-conductive material.

 

• Hot negative rock – magnetite (magnetite)

Hot passive rocks are highly non-conductive because they (usually) contain high concentrations of magnetite. Magnetite is an iron oxide (Fe3O4 to be exact) and often causes rocks or sediments to turn into a dark black colour. In addition, it will cause the rock or sediment to become heavy due to the high atomic weight of the iron oxide molecule. One of the most common ways to encounter this type of hot rock in North America is “black sand”, dark or black sands that are available in magnetite.

However, magnetite can be used for more than just disrupting the ground balancing systems of metal detectors. In places where magnetite is prevalent, it is not uncommon for the soil to also be gold-bearing. This is especially noticeable in streams and dry washes of gold-bearing areas because the magnetite accumulates in a more concentrated area.

 

Tips for identifying and dealing with negative interference in hot rocks

Passive hot rocks, at shallow depths, will display a pseudo-metallic acoustic response. This false metal detector acoustic response will sound less specific and more general than the responses of “real” targets. This is often accompanied by delayed gain and then nullification of the audio response when the file is moved away from the area and back to the target location. Some other characteristics of the Negative Hot Rock acoustic response are: the response is non-repetitive or only repeatable when the coil is swung in one direction, the signal is completely incapable of locating it, and in cases where the metal detector displays a signal on the screen. Signals will not be present on the screen of these devices.

If you are using a manual ground balance, besides hearing these false-metal acoustic signals, you will also have a lower depth of detection in highly non-conductive sediments. One way to solve this problem is to use the silent search type with All-Metal discrimination mode, and then rebalance the metal detector. You will notice a decrease in the threshold setting level before rebalancing is an indication that this process is necessary.

If there are significant inconsistencies in nonconductive hot rocks or passive hot rocks at deeper depths, the threshold adjustment level may become completely null. In this case, when using all-metal discrimination, nothing can be done to mitigate the effect on the metal detector. Instead, in this case you will need to reset the manual floor balancing until a slight increase in the sound threshold is observed when the coil is lowered to the ground. This is an adjustment known as the “positive offset” of the floor scale. This helps your metal detector compensate for any sudden drop that might be caused by inconsistencies or deeper signals. If you are using Silent-Search, you will not notice any change in performance unless you find a large negative rock at a shallow or medium depth level. In this case, the metal detector will display the previously described pseudo-metallic sound signal that can be ignored once you feel proficient in recognizing the signal.

To reduce these false, or non-acoustic, metallic responses, the best thing you can do is use the discrimination features on your metal detector to eliminate ferrous responses. This, of course, is only available in very low frequency metal detectors. Furthermore, the use of these discrimination features may have an effect on the coil’s sensitivity or responses to metal objects displaying a false ferrous metal signal. In the event you are using a pulse induction metal detector, hot rocks of any type (besides non-graphite cationic hot rocks) will not affect the efficiency/responses of the metal detectors.

 

• Cationic Hot Rocks – Think Maghemite

Hot cationic rocks are highly conductive because they (usually) contain high concentrations of Maghemite Magmatite is an iron oxide and often causes rocks or sediments to turn red, reddish-orange, or yellow. Although Maghemite is an iron oxide, just like magnetite, it exhibits lower ferromagnetic properties (aka higher magnetic properties) than magnetite.

However, it is also possible for hot cationic rocks to be highly conductive because they contain high amounts of sulfide minerals. These minerals are Pyrrhotite (also known as magnetic Pyrite Fe(1-x)S (x = 0 until 0.2)) and Bornite (also known as Peacock ore, CuFeS4). Peridotite will be the same color as Maghemite but Bornite will be copper-red or brown in color unless stained. In this case, it will turn out to be a combination of blue and violet shades.

While these are the most common iron-bearing positive hot rocks, they are a difficult subsection of non-iron-bearing positive hot rocks. It can be any rocky or sedimentary material that contains high concentrations of copper ore, bauxite (aluminum), manganese, gold, nickel, or common graphite. Graphite is a highly conductive material for carbon which you may know from the fact that most pencil leads are now made of graphite.

 

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