Recovery Speed & Target Masking

Metal detectors, decades ago, had very limited depth capability. Therefore, increasing detection depth was a prime focus for all detectorists. Metal detector advertising has always made “more depth” part of the selling point for most new detectors. This focus on “more depth” made sense for a long time but, more recently; it has been shown that more depth is only part of the equation…

Background

More depth does not always mean more good finds will be made. The assumption is that coins, jewelry, and relics all end up deeper in the ground over time, but that is not true in many areas. Drier less vegetated areas often do not have the conditions that cause items to be buried deeper. The only finds there are to be made are often relatively shallow, with nothing deeper down. There are also practical limitations on how deep a person can dig a hole in some locations. Metal detector depth is almost always tested on a single target buried in the ground, with no other distracting targets nearby. Yet, in many locations, it is not unusual to have more than a single target under the detector coil at one time. When this happens, how does a detector react? There are generally two possibilities that merge into each other:

Firstly, some detectors may sense various items under the coil and average them together into one target response. Combinations of ferrous items and non-ferrous items under the coil will often pull target ID numbers lower. The response from multiple non-ferrous items is often cumulative, with several items that have a lower target ID being reported as a single item with a higher target ID. These shifts in target ID can often cause desired items to be missed.

Secondly, a worse problem occurs when items under the coil are at varying depths. The shallower targets usually take precedence over deeper items. This can be quite stunning in reality, as a small item a few inches away from a larger, but deeper, item will cause the deeper item to be ignored entirely, with the detector only reporting the shallow item. The deeper item is hidden or ‘masked’ by the shallower item.

These two scenarios are both forms of ‘target masking’, in that desired items are missed due to close proximity to undesired items. After decades of detecting by many good deep-seeking detectors, it is actually quite likely today that more desirable items are left to be found, hidden at shallow depths by target masking, than can be found by having a detector that goes a little deeper. This is fortunate, in a way, because there are limits on detector depth imposed by physics, and we are already quite near those limits. This now means that many detectorists should consider changing their focus from going deeper and instead focus more on simply finding targets missed by other detectors. In many of the most target-rich environments the focus should be more on defeating target masking, rather than on achieving more absolute depth.

What is Recovery Speed?

Several terms define how well metal detectors can separate closely spaced items. It is sometimes described as recovery delay or reactivity, but perhaps a more common term in use has been recovery speed.

You could think of digital metal detectors reporting what is going on under the coil as taking a ‘snapshot’ and showing the result as a target ID number. Each snapshot can be of longer or shorter duration, but only one target ID number can be reported for each snapshot. To locate and identify two targets close together, the detector must be able to take two snapshots, one for each target. Recovery speed defines how quickly a detector can report several items that are close together.

Imagine you have three different coins in a row, three inches apart, nine inches total. All three coins can be under the coil at the same time. For this example assume it takes exactly one second to swing the coil nine inches over all the coins. Our imaginary detector has a recovery speed control calibrated from 1 to 3 with each setting representing faster recovery speeds. The setting of 1 means the detector can take one snapshot per second. A setting of 2 is two snapshots per second and 3 means three snapshots per second.

You set the detector at recovery speed 1 and make the one second swing over the coins. The first coin makes a nice one second beep with a target ID and the other two coins are either ignored or the target ID averaged together with the first coin to produce a less accurate target ID. Remember, ignoring items or producing inaccurate target ID numbers due to close adjacent items is referred to as target masking.

Setting the recovery speed to 2 means the detector can take two snapshots per second. Now the first coin makes a half second beep with target ID. The second coin is ignored/averaged (masked). The detector resets (recovers) and takes another half second snapshot, reporting the third coin with a clean signal and target ID.

Finally, a recovery speed setting of 3 produces three short but clean beeps with accurate target ID numbers on all three coins! Great – so why not just set the control at 3 and forget about it?

In the examples above, faster recovery speed translates directly into shorter audio reports. The problem is that when detecting for very deep targets a detector already is working with minimal information, so the deepest targets already tend to be weak or faint. A very fast recovery speed can take what is already a minimal target signal and reduce it even more to the point where it is no longer heard clearly, or not heard at all.

In summary, faster recovery speeds help separate adjacent targets and minimize target masking. Slower recovery speeds help produce fuller, more easily heard audio signals on deeper targets.

The EQUINOX Advantage

One of the main ways in which the new EQUINOX differs from previous Minelab multi-frequency detectors is that it has much faster recovery speed options. One of the few weaknesses in earlier BBS and FBS models is a tendency for target masking, especially in very target-rich environments. Minelab specifically attacked this issue with the new Multi-IQ technology, which can employ lightning fast recovery speeds. EQUINOX does this so well that it not only exceeds the speed of most single frequency detectors, it does so while operating at multiple frequencies!

In my opinion, one of the best things about the new EQUINOX models are the well-designed factory presets. Note: these are set for higher recovery speeds. One of the biggest mistakes I see new users making is assuming the presets are not well thought out and immediately trying to come up with what they think are better settings. In particular, some owners of previous Minelab's seem determined to immediately go to slower recovery speeds seeking “more depth”. Those doing so are ignoring one of EQUINOX's main advantages over other detectors.. This is truly a case where new owners should have a little faith in the presets and go with them initially.

Rather than try and make the EQUINOX into some other detector, leverage its new capabilities for better results. This means actually seeking out areas where trash is denser and target masking more likely to be hiding targets at relatively shallow depths. Fast recovery speeds also aid in depth in highly mineralized ground and anyone working in anything more than moderate mineralization should definitely stick to higher recovery speeds.

People who employ lots of target rejection or discrimination notching often are not even aware of how dense the trash is because it has been blocked out. I highly recommend that the threshold tone is used by anyone who rejects lots of target responses. A threshold tone will ‘null’ or ‘blank’ when going over rejected targets, giving you a better idea of how dense the trash is. An alternative is to not reject any items and go strictly by tones. With all items being heard, you get a better feel for how dense the target environment may be.

I hunt very mineralized ground and, for this reason alone, almost never drop my recovery speed below 5 on the EQUINOX 800 and, if I had an EQUINOX 600, would probably almost never use anything but the highest setting of 3, with 2 used for only the rarest occasions. Again, fast recovery speeds definitely aid in highly mineralized ground and should be the normal settings in such ground regardless of target density. I also like to employ full tones.

Here is a real-world scenario, for me, in a local park….

I might start in an open area with a recovery speed of 5 on my EQUINOX 800. As I near a picnic area, I notice (listening to the tones) that the target density is increasing. As soon as I start hearing more than one or two targets per swing I will bump the recovery speed up to 6. Then, as I get into multiple items per swing I will probably bump the recovery speed up again to 7. If I wander into a more open area again, I will lower the recovery speed. What I am doing is matching recovery speed to the location’s target density, which I can only know if I am hearing all targets. In areas with fewer targets I can enhance the response for deeper targets but, in areas with dense targets, focus more on eliminating target masking. 

One nice thing about the 800 versus the 600 is that the recovery speed setting has both a wider range AND a finer degree of control.

If you hunt less mineralized ground than I do, you have more options with the slower recovery speeds. In areas with few targets, I would be using the slower settings but, again, always go to faster settings, the denser the trash gets. The main way to benefit from the recovery speed control is to be aware, at all times, of the target density. This is best done by using either a threshold tone that nulls on rejected targets, or using full tones to alert you to all targets.

EQUINOX has quite a few strengths over previous Minelab BBS and FBS models, but you can only take advantage of those strengths if you know what they are and leverage them to your advantage. Fast recovery speed is one of the most potent tools in the EQUINOX tool box, so I hope you don’t throw that advantage away!

There are still vast numbers of good finds to be made almost anywhere trash targets are carpeting the ground. Take an EQUINOX, use fast recovery speeds, and go find them!