Hot Rocks

 

How To Identify Hot Rocks and Reduce or Eliminate Their Signals

Hot Rocks are pebbles or rocks that contain more nonconductive or conductive minerals than the surrounding ground to which a metal detector is balanced automatically or manually.

This is why, when the hot rocks are in the detector's electromagnetic field, they disturb the ground balance of the metal detector so intensively that the machine reads them as metal targets, i.e. the hot rocks generate positive or false metallic audio signals 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. Most hot rocks contain high concentrations of the iron-bearing minerals which are predominately the iron oxides: magnetite, maghemite, hematite, limonite and lepidocrocite. Other hot rocks contain high concentrations of either nonmetallic but highly conductive carbon mineral (graphite) or nonferrous metals such as copper, aluminum, manganese, nickel, cobalt and even gold. Most common hot rocks in the US are basalt (magnetite-rich rock), magnetite and hematite.

 

 

The IRON-BEARING hot rocks are the most common and can be divided in two types:

Type I: NEGATIVE Hot Rocks

Negative hot rocks (also called "cold rocks") are more nonconductive than the surrounding ground matrix, and their Ground Balance values are higher than the Ground Balance value of soil they are in. This is why, the small, pebble-sized (mineral density) negative hot rocks generate negative signals in the All-Metal Motion Discriminate mode. However, large negative hot rocks (mineral quantity) at shallow depths cause a metal detector to respond with positive or false-metallic audio responses.

Negative hot rocks are highly nonconductive (magnetic) because they contain mostly high concentrations of Magnetite. They are dark in color, often heavy due to their high iron content, usually attracted to a magnet and, in some cases, stained with rust.

Magnetite (chemical formula Fe3O4) is an iron (II, III) oxide - one of the three common naturally occurring iron oxides, and the most magnetic (shows strong properties of Ferrimagnetism) of all the naturally occurring minerals on Earth. The sand-sized particles of Magnetite are called "Black Sand" (mineral sand or iron sand) which is sometimes found in large quantities in beach sand, streams and dry-washes in the gold-bearing regions. Usually a good indication of the gold-bearing area is an excessive presence of magnetite in the ground. This is why the negative hot rocks are abundant in many goldfields around the world. Magnetite also occurs in meteorites.

How To Identify False-Metallic Audio Responses To Negative Hot Rocks,
How To Use Positive Offset To Compensate Intense Non-conductive Ground

Besides the sound characteristics of audio responses, here are some additional "symptoms":

• The negative hot rocks will only sound off in one direction, i.e. their signals are not repeatable. In other words, a signal will be heard when moving a search coil in one direction (e.g. left-to-right or right-to-left) over the hot rock, but not in the reverse direction.
• The negative hot rocks are impossible to pinpoint. If you hardly get any consistent audio response or do not get any at all while pinpointing, there is indeed a negative hot rock under your search coil. Also, depending on a metal detector you use, when the negative hot rock is pinpointed, it may give a pulsating signal.
• In most cases, VDI values corresponding to the negative hot rocks do not appear on a display even when the generated signal is strong.

 

If your detector is operated in the Motion Discriminate search mode, and the detector's Operating Depth Range (Detection Depth) depends on the properly adjusted Manual Ground Balance (in All-Metal mode), entering an area with more intense non-conductive ground can negatively affect your detector's Depth Penetration ability resulting in reduction of the Detection Depth. Conventional Discrimination, if implemented, also contributes to missing deep valuable targets in the high-mineralized ground.

In such a situation, if you use the Non-Silent Search type metal detector, and you are metal detecting in the All-Metal mode of Discrimination, you will notice a "drop" in the Threshold tuning level and should re-balance your detector. Significant changes in the highly non-conductive mineral intensity along with deep negative hot rocks may "null" the Threshold audio in the All Metal mode depending on their intensity. Nothing else can null the Threshold and negatively affect the Ground Balance tuning in the All Metal mode of Discrimination.

To lessen this negative effect, readjust the Manual GB control until a slight increase in audio Threshold is heard while lowering the search coil toward the ground. This adjustment is called a Positive Offset to the Ground Balance. The Positive Offset helps compensate for any sudden decrease in Threshold level.

If you use the Silent-Search type metal detector, you will not notice any change in performance unless you encounter a large negative hot rock lying at moderate or shallow depth. Because the large and oversized negative hot rocks can cause drastic changes in non-conductive mineral intensity, they cause metal detectors of both the Non-Silent Search and Silent Search types to respond with the metallic-like audio responses (the greater the change, the louder the signal) described above.

 

NEGATIVE Hot Rocks

Usage of Automatic Ground Balance Mode
How To Reduce or Eliminate Audio Responses To Negative Hot Rocks

Downsides of Using Automatic Ground Balance and Semi-Automatic Sensitivity Modes

How To Reduce Audio False-Metallic Responses to Negative Hot Rocks

To reduce or eliminate annoying false-metallic signals generated by the negative hot rocks, you can try any of the following methods based upon the level of ground mineralization:

• 1) If you are using a VLF metal detector, a Double D search coil will be your best choice for dealing with the negative hot-rock falsing. The DD configuration, with its narrow "blade" of transmitted electromagnetic field, will substantially help reduce the negative effects of the positive hot rocks and ground noise. The Double D search coil always gives a better target separation in any target-clustered environment but does not detect targets as deep as a Concentric or Monoloop coil of the same size in areas with light concentrations of detectable targets.
• 2) Lower the maximum acceptable level of Manual Sensitivity a few units down. In most cases of detecting on the highly mineralized ground, lowering the Sensitivity level actually allows for greater Detection Depth while silencing the negative hot rocks positioned at depths.
• If your metal detector features Automatic or Semi-Automatic Sensitivity, you may want to use it because, 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 interference coming from the small or deep negative hot rocks may reduce audio signals generated by small and/or deep targets to inaudible levels.
• 3) If you do not want to reduce the Sensitivity level because your metal detector is quite stable, you can simply reject the negative hot rocks by Conventional Discrimination. But only if they do not register in the Conductivity ranges on the 1-Dimensional Discrimination scale, or the Fe-Co portions of the 2-Dimensional Discrimination pattern, that are occupied by VDI values of targets that you search for. If the negative hot rocks are scarce in your search area, a minimal amount of Conventional Discrimination will usually eliminate audio responses to the negative hot rocks without difficulty. But if the negative hot rocks are abundant in the high-mineralized ground, the discriminator may not quiet them all, and then you follow the rule of thumb: "leave the non-repeatable signals alone!"
• NOTE: Using Conventional Discrimination against negative hot rocks is a "catch-22" because if you mute the audio responses to them (reject them), you sure will not receive many annoying positive or false-metallic signals, but you will likely miss the deepest-to-detect coins positioned at the brim of the detector's Operating Depth Range, or the smallest, low-conductive valuable targets including gold chains and gold nuggets. Often the deepest or tiny targets in the highly magnetic ground emit electromagnetic fields that are suppressed by mineralization and received by the search coil as "weak". These weak electromagnetic fields are registered by the metal detector's Discriminator as the negative hot rocks. In other words, these induced electromagnetic fields do not manifest true Conductive or Ferrous-Conductive (FE-CO) properties of detected valuable targets. They rather manifest properties that are similar to the properties of the most concentrated magnetic (non-conductive) intensities. This is why caution should be applied in using even a low level of Conventional Discrimination.
• 4) If you do not want either to lower the Sensitivity or to use Conventional Discrimination, you can simply sweep the search coil a little higher above the ground than usual. This will cause a loss of some Detection Depth and Sensitivity, but also reduce the negative hot rock falsing. Not to miss many valuable targets, the Signal-Amplifying feature should be used, and your headphones' audio volume should be increased to the maximum acceptable level.
• 5) You can use the Manual Ground Balance control to optimize the detector's ability to compensate interferences caused by the concentrated non-conductive intensities including the negative hot rocks lying at depths. In this case, you ground balance your detector with a Positive Offset described on a previous page. Ground balancing must be done at the iron-free, mineral-deposit-free and hot-rock-free ground spot.
• 6) You can also manually Ground Balance your metal detector to the negative hot rocks, but you will most likely miss too many valuable targets because this trick may eliminate the negative hot-rock signals, but in turn increase the ground noise. It is best to Ground Balance your metal detector to the soil and deal with the hot rocks' false-metallic signals as they come. So, this method is not recommended. But if you decide to apply it while hunting on the lightly mineralized soil, you will have to implement the All-Metal mode of Conventional Discrimination in order to ensure detection of good targets partially masked by the negative hot rocks, and, therefore, exhibiting very low combined VDI values.
• 7) To ensure excellent Target Separation in cases of partial target-masking by the hot rocks, implement high levels of Reactivity (also called Speed of Response, Recovery Speed, Reset Speed, etc.). If you combine the high Reactivity setting with the sniper search coil, you will have a quite capable metal detector which will be able to detect more desirable targets than the low-Reactivity machines in the hot-rock-infested areas.

 

Type II: POSITIVE Hot Rocks

General Info, + Mineral Salts and How To Eliminate Their False-Metallic Signals

Positive hot rocks are more conductive than the surrounding ground matrix, and their Ground Balance values are lower than the Ground Balance value of soil they are in. This is why they cause a metal detector to sound off with positive, sharp audible responses sounding like signals generated by good metallic targets in Motion Discriminate mode. And even small positive hot rocks can often be detected up several inches deep.

An iron oxide Maghemite - product of natural oxidation of Magnetite, is the most common mineral-constituent of the iron-bearing positive hot rocks. Maghemite (Fe2O3, γ-Fe2O3) is a member of the family of iron oxides. It has the same structure as Magnetite, but exhibits weaker ferrimagnetic properties than Magnetite. That is why the iron-bearing positive hot rocks are not always drawn to a magnet. They are most often reddish in color, like brick-red, but can also be black, brown or even yellow. The maghemite rocks are often small, lie on the surface, and are common in many gold prospecting regions.

Another troublesome category of positive hot rocks comprises rocks containing sulfide minerals such as the flame-colored Pyrrhotite (also called magnetic Pyrite, Fe(1-x)S (x = 0 to 0.2)) and Bornite (also known as peacock ore, Cu5FeS4) which has a brown to copper-red color on fresh surfaces, that tarnishes to various iridescent shades of blue to purple in places, hence the name "peacock ore".

Bornite is important as an ore for its copper content of about 63 percent by mass. Pyrrhotite does not have specific applications, and is mined primarily because it is associated with another sulfide mineral - pentlandite, that can contain significant amounts of nickel and cobalt. Both sulfides are conductive, and can sound just like gold nuggets, but the gold is not found in the same type of rock as the sulfide ores in many geological settings. Therefore, the detector's audio responses to the sulfide hot rocks can be ignored once such a rock can be identified.

Other types of the positive hot rocks are not iron-bearing and contain high concentrations of copper ore, aluminum (bauxite - the raw form of aluminum), manganese, nickel, cobalt, gold and graphite - a highly conductive carbon mineral. On some coin shooting and relic hunting sites, the positive hot rocks may be represented by red-clay bricks and stones from a fireplace or a campfire.

The less challenging category comprises hot rocks containing the above-mentioned graphite or graphitic slate rock, which generate very broad signals, do not sound like gold, and can not be tuned out by ground balancing. If a suspected rock-find generates a positive audio response, is black in color, and leaves black streaks on paper like a pencil, it is a graphitic hot rock. In order to use a metal detector in the graphite-bearing area, you will have to reduce the Sensitivity level.

Because the metal detector's false-metallic audio responses to the positive hot rocks do not have a distinct character of sound (except maybe the graphitic hot rocks) like the audio responses to negative hot rocks, the positive hot-rock signals are hard to distinguish from audio responses to real non-ferrous targets, both high- and low-conductive, like coins and gold nuggets respectively. This is why the positive hot rocks have been considered a real nuisance to many detectorists, especially gold nugget and meteorite hunters, for years.

And in the hot-rock-infested areas with intensely high mineralization, the positive hot rocks can be a real challenge to deal with and extremely frustrating for those gold nugget hunters who do not use advanced metal detectors. The highly mineralized ground causes these detectors respond with so much noise that it is nearly impossible to discern gold-nugget signals among the signals of the hot rocks which may sound exactly like a gold nugget. Even users of the most advanced metal detectors have to practice a lot in order to learn how to "to separate the wheat from the chaff".

 

Mineral Salts and How To Eliminate Their False-Metallic Signals Using Negative Offset of Ground Balance

Besides the positive hot rocks, concentrations of mineral salts in moist soil also cause high levels of conductive mineralization, and, like the hot rocks, can often generate positive or false-metallic signals.

Common causes are the following:

• 1) spots where natural mineral salts have been concentrated by evaporation;
• 2) the spontaneously occurring "pockets" of mineral fertilizer (inorganic);
• 3) spots with organic fertilizer like "cow pies" and urine from livestock;
• 4) concentrations of residue from de-icing salts;
• These mineral-salt concentrations are "dormant" when completely dry, but can be easily "activated" by rains. Along with sea salt, mineral salts comprise another type of Conductive Mineralization - wet salts.

Just like in a case of dealing with "pockets" of sea salt in wet sand during coastal Beach Hunting, you can easily eliminate or reduce the detector's false-metallic audio responses to the wet mineral salts by applying a Negative Offset to your Non-Silent Search detector's Ground Balance. Ground balancing must be done at the iron-free, mineral-deposit-free and hot-rock-free ground spot.

Provided the mineral salts in a search area are within the Ground Balance range of your detector's ability to compensate them, adjust the Ground balance control until a slight null in audio Threshold is heard as the search coil is lowered to the ground. This will lessen the positive effect of the conductive ground, and muffle, if not eliminate, most false signals generated by the wet-salt concentrations.

 

POSITIVE Hot Rocks:

How To Reduce False-Metallic Signals Generated by Positive Hot Rocks

Here are a few methods that can help you lessen negative effects of the most troublesome hot rocks:

• 1) If you are running a VLF metal detector, a Double D (DD) search coil will be your best choice of the coil type for dealing with hot rocks. The DD configuration, with its narrow "blade" of transmitted electromagnetic field, will substantially help muffle some signals generated by the positive hot rocks, and reduce the ground noise. A small search coil, especially a "sniper" coil of 5", 5" x 3" or 6" x 4" in diameter, will work best in the hot-rock-infested search area. The Double D search coil always gives a better target separation in any target-clustered environment but does not detect targets as deep as a Concentric and Monoloop coils of the same size in areas with light concentrations of detectable targets and iron trash.
• 2) Implementing high levels of Reactivity (also called Speed of Response, Recovery Time or Recovery Speed) will ensure excellent Target Separation in the positive-hot-rock-clustered areas. If you combine the high Reactivity setting with the sniper search coil, you will have a quite capable metal detector, and be able to detect desirable targets that lie underneath the positive hot rocks, and therefore, are partially masked.
• 3) You may want to lower the maximum acceptable level of Manual Sensitivity a few units down. In most cases of detecting on the highly mineralized ground, lowering the Sensitivity level actually allows for greater Detection Depth while silencing the negative hot rocks positioned at depths.
• 4) If your detector incorporates adjustable Signal-amplifying and Transmit-Boost controls, reduce their setting and elevate a search coil slightly above the ground when searching. This will cause a loss of some Detection Depth and Sensitivity, but also will quiet many "zips" coming from the positive hot rocks.
• 5) Simply increasing the Conventional Discrimination level or amount will just not work as many desirable targets with VDI values below and above a selected rejection level will be silenced (see my note below). However, if the Notch Discrimination is available and potent enough (not cutting off adjacent VDI values of desirable targets), it may help eliminate the detector's audio responses to the most redundant positive hot rocks. Just make sure that conductivity ranges of desirable targets are not included in the rejecting notches, or "clipped" by them. Also keep in mind that the Notch Discrimination is a type of Conventional Discrimination and, therefore, can reduce the Detection Depth.
• Also the Two-Dimensional Dot-Discrimination of the Minelab FBS metal detectors may help mute the signals generated by the most redundant positive hot rocks. Just like in a case of the Notch Discrimination, make sure that the Fe-Co portions of the 2-Dimensional Discrimination pattern, that contain Fe-Co values of desirable targets, are not "clipped" by the rejecting "squares" in the Disc pattern. The Dot Discrimination is also a type of Conventional Discrimination and, therefore, can reduce the Detection Depth.
• NOTE: When setting a level or customizing a pattern of Conventional Discrimination to detect with less incoming audio noise, one should always keep in mind that often small and/or deep targets, when being detected in the highly mineralized ground, emit electromagnetic fields that get suppressed by mineralization and become very weak by the time they reach the search coil's receive windings. These weak signals do not manifest real Conductive or Ferrous-Conductive (FE-CO) properties (phase values) of detected targets, and are rather registered as electromagnetic fields corresponding to ferrous targets with certain conductive properties by the metal detector's Discriminator. In the worst case, if these conductive properties are within a rejecting range set by an operator, the valuable non-ferrous targets will be silenced. This is the best way to miss the deepest coins positioned within the detector's Operating Depth Range.
• 6) If you encounter and recover a few positive hot rocks, including the ones lying on surface, in the search area, and noticed that they are homogeneous, memorize what the rocks look like in this particular location. At least when you get an audio response to a hot rock lying on the surface, you will immediately recognize it and ignore. The hot rocks of both negative and positive types are fairly homogeneous in any given area, so you should only have to memorize a few different types. This simple practice will help you maintain a good time efficiency in the field.
• 7) If you attach a small Rare-Earth magnet to your pickax, you will be able to quickly identify most hot rocks which will be attracted to the magnet. However, for example, some meteorites are magnetic too and could be easily confused with the hot rocks if one does not know how to ID the meteorites. Fragments of iron junk and other ferrous objects will always be attracted to a magnet. Only gold will not be attracted to a magnet. But this is a different story...

Advanced Pulse Induction (PI) type metal detectors effectively ignore both types of hot rocks as well as the pockets of concentrated mineralization in the ground. Unfortunately these detectors do not have a Discriminate function, i.e. they can not reject targets, and therefore would be impractical to use at the trashy hunt sites.

The VLF metal detectors that are specifically designed for the gold-fields will respond to the positive hot rocks as well as all the common VLF machines. All but one - XP Deus, which is the only VLF metal detector that incorporates a revolutionary feature called Notch Ground. This feature easily takes care of all positive hot rocks as if they do not exist, and do not compromise the detector's Ground Balance, Depth Penetration ability and, therefore, the Detection Depth.

I hope that now you know how to deal with the hot rocks and improve your detector's performance. Please keep in mind that encountering both types of hot rocks in a search area happens quite often, and if to add the annoying effects of mineral salts to the "soup", it may take you some time to fine-tune your machine to these adverse metal detecting conditions, i.e. to determine an optimum combination of the search program settings. Make sure you know your metal detector well, and it is a capable machine. And always remain patient.

 

by Sergei Upstateny