To dig or not to dig, that is the question...

We all want that perfect signal. A clear tone, unbroken, with no grunt of iron. That is the sort of signal that you know you have to dig. However, you can wait a long time for a signal like that, and there is a good reason why. We metal detect where there has been human activity for a long time, and humans are messy creatures. There will often be things in the ground that mean a good signal is hard to pick out of the background noise. Iron is found everywhere, and depending on how you have your detector set up, some good targets can be rejected as iron. Sometimes we will have perfect signals that are as clear as a bell and turn out to be a large silver or copper coin—but if we only dig the perfect signals, we will miss a lot of good finds.

Consider the hammered silver coin, always a welcome find. Sometimes it will be a whole coin, sometimes a cut half, and sometimes a cut quarter. A cut quarter typically weighs about 0.3 grams, and the signal sounds a lot like a bit of foil. The VDI (Visual Display Indicator, or the target number) will also be about the same as it would be for a folded scrap of foil, and that makes sense. They are both tiny, thin, non-ferrous, and share a low-to-mid electrical conductivity. The edges of the cut quarter typically sound slightly cleaner as the foil is somewhat irregular, while the signal from the quarter may be a little more consistent - but they might not be, because targets are often at an odd angle, especially in ploughed soil.

If you are using an XP detector, the signal is often described as scratchy; the detector is frequently on the edge of rejecting it. Garrett machines typically assign mid-to-high conductivity ranges (often jumping in the mid-40s to low-50s VDI depending on the model) for small hammered bits. A cut farthing will give a quick, clean, high-toned blip. If it is deep or edge-on in the soil, the audio will soften into a faint, wispy whisper, but it should maintain a non-ferrous tone rather than a low iron grunt.

On a Nokta, such as the popular Legend, it will be a smooth, high-pitched non-ferrous squeak or clean bell tone when using multi-tone or pitch modes, but the VDI will be on the low end.

On an Equinox 600 or 800, it will be a brief, high-mid chirp if you are in 50-tone mode, but clearer and more bell-like in 5-tone mode, with a VDI in the 5–7 range. On a Manticore, the VDI might be a bit higher and you will typically see a nice, round blob on the 2D screen.

So, different machines on the same target give different results… and different coils and settings also change those results. Some machines, such as the ORX and Deus 1, are single-frequency machines in that you can only use one frequency at a time, and the available set of frequencies depends on your coil. A Deus 1 with the X35 coil can run at 4 kHz, which is good for large, deep targets but not at all good for tiny ones. With the HF white coil, it can go up to 81 kHz, which is great for tiny targets like the cut quarter, but not good for deeper, larger targets, and it gets overwhelmed by iron quickly.

Since this is complicated, how do you work out which signals to dig? The trick is to learn your machine. The best way to do this is to listen to the machine, read what the display is telling you, and then dig anything that you are not certain is iron. Will you dig a lot of trash? Yes, but you will also learn exactly what the sounds and numbers for your specific machine and coil are trying to tell you. Once you truly know your machine, you can afford to be much more selective in what you dig.

If you are using an XP Deus (I or II), the X-Y screen is a great help. If it gives a nice, straight diagonal line, you want to dig that. If you are using a Manticore and it gives you a nice round blob right on the center line, you want to dig that. However, some good targets don’t look like that. If there is trash near the target, it can look messy on the screen. If the find is an odd shape or resting at an angle relative to the coil, it can look messy. While the screen is an aid to understanding what is under the coil, your ears and your experience will always give you better guidance. If you don’t have that experience yet, dig the “iffy” signals and you soon will.

There are some common "gotchas" that can hurt your chances of making sense of a signal, causing you to miss out on something good. Here are a few that you may want to avoid:

Assuming that low numbers mean low value: As mentioned above, a cut quarter sounds a lot like foil. Furthermore, gold targets really don’t give high-VDI signals most of the time. A gold ring gives a great signal because it is a solid circle and current can flow around it, but a bit of delicate gold jewelry will give a low VDI because gold is not a massive conductor compared to silver or copper, and because jewelry targets are generally small.

To expand on that, lower frequencies (including lower channels on simultaneous multi-frequency machines such as the Deus 2, Vanquish, or Manticore) won’t get much of a signal from micro-targets. These targets are significantly smaller than the wavelength of the signal—a 14 kHz signal has a wavelength of roughly 21 km. A tiny target can only interact with a fraction of that energy. An 81 kHz signal has a shorter wavelength of 3.7 km; while still huge compared to the target, it hits the target's surface with much faster, tighter cycles, generating a more responsive signal on small targets. However, it also gets absorbed much more by ground mineralisation, so it won’t penetrate deeply. The return signal we get off these targets is incredibly small, and it requires great electrical engineering to make a decent detector pick it up.

Holes in a target will fool you: Modern detectors work by inducing eddy currents in a target. They send out an alternating magnetic field—and a metal does not have to be magnetic to respond to it. Because the magnetic field is alternating (changing direction thousands of times a second), the electrons don't just flow in a straight line. They form closed, swirling loops on the surface and inside the metal. These are your eddy currents, and it is these currents that the detector senses.

If there is a hole in the object (for example, an iron nut), the electrical currents are forced to travel in a large, circular loop around the hole because they cannot pass through the empty middle. Because the current is forced to flow in a wide, uninterrupted circle, it behaves like a much larger, highly conductive loop—almost like an antenna—and you get a lovely, strong signal that sounds non-ferrous.

If you have your iron audio turned off (which is tempting, as the detector chatters a lot with iron audio on), an iron nut will sound like a premium, repeatable target, which is why a lot of people end up digging deep iron. If you keep your iron audio on, you will hear a blended signal: an iron grunt mixed with a sweet tone. Is it iron with a hole in it, or a good coin resting next to an iron nail? Only digging will tell you for sure. However, if you lift the coil a few inches above the ground and the signal stays incredibly strong, it is probably a large piece of iron with a hole in it.

Conversely, a gold ring gives a lovely signal for this exact same reason, even though the same amount of gold melted into a solid bar would give a much weaker signal. If you ever run your coil over a buried length of iron chain, it will sound like a uniform line of excellent targets. Sometimes, a genuine find will be masking right next to a lump of iron, so it is always a good idea to approach an iffy target from multiple angles to see if there is a direction where the sweet tone cleanly breaks out in front of the iron grunt.

Pushing sensitivity into overload: If the ground seems impossibly noisy or you are getting a lot of chatter, it is very frustrating. You don’t want to turn down the sensitivity of the detector because you don’t want to lose depth on targets. That seems to make good sense, except that reducing the sensitivity reduces both the ground noise and the target signal. In practice, lowering it actually helps. The detector is constantly trying to analyse a tiny return wave, and if the processor is completely overwhelmed by a massive wall of signal (which is part target and part harsh ground mineralisation), it cannot do a good job of separating the two.

This is called receiver saturation or coil overload. You can demonstrate this to yourself easily: if you amplify audio too much, it starts to distort as the top and bottom of the sine waves get chopped off because they are too big for the available voltage to represent. This is a good thing if you are playing rock guitar, but a terrible thing if you are trying to pick an eddy current out of a lot of electromagnetic mush. You will get better stability with lower sensitivity in dirty ground, and often better depth. We want to be detecting in dirty ground, as there is usually nothing to find in clean ground.

Forgetting to re-check the hole: Finally, always check your hole for another target after recovering something. If you find an iron nail with your pinpointer, that may not be what your main detector saw. Just because you found a button doesn’t mean that there was only one metal object to find. It only takes a moment to be sure.

Hopefully, these tips will help you enjoy your next detecting trip and help you to find some history under your coil.