Hot Rocks, page 3

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

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

Motion Discriminators of some advanced Single-, Dual- and Multi-Frequency (Full Band Spectrum (FBS and FBS 2)) metal detectors automatically adjust the Ground Balance to any significant change in the ground's non-conductive mineral intensity, that may be caused by the concentrated mineral "pockets", negative hot rocks or black sand deposits.

However, large negative hot rocks lying at shallow depths will still cause the metal detector to respond with positive or false-metallic signals.

In such cases, an operator simply identifies the hot-rock signals according to their characteristics (listed on a previous page), and ignores them.

However, there are two negative effects of operating a metal detector in the Automatic Ground Balance mode (also called "Tracking" and "Auto Tune") and/or the Automatic Sensitivity mode (also Semi-Automatic Sensitivity) on highly mineralized soils.

The first downside is a substantial reduction of the detector's Operating Depth Range resulting in muffling or silencing the audio responses to small and deep nonferrous targets. And if the Manual Sensitivity level is set high to compensate any loss in Detection Depth, the detector's responses to the negative hot rocks will drive a detectorist crazy. Same goes for the inappropriate high levels of Signal-Amplifying and Transmit-Boosting features if they are employed for detecting on the highly mineralized soils.

The second downside of the Automatic Ground Balancing is that it can also mute the audio responses to the nonferrous targets that may happen to be underneath the negative hot rocks, and therefore be masked by them. If the concentration of good targets seem to be high in a search area, special attention should be paid to any sort of audio "scraps" of the medium- or high-tone signals generated by the masked targets.

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 (see details in my article - Busting Myths About Metal Detector's Sensitivity, Discrimination and 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.