I'm not sure what you mean by measuring the mineralization. If you were asking for a Ground Phase comparison between the X-Terra coils, here is one I did this afternoon.
9-inch concentric at 7.5 kHz. -1 Noise Cancel, Sensitivity 20, GP reading 009755
9-inch concentric at 3 kHz. +2 Noise Cancel, Sensitivity 20, GP reading 009402
9-inch concentric at 18.75 kHz +2 Noise Cancel, Sensitivity 20, GP reading 009590
For the sake of comparison, I ran three consecutive GP tests with the 3 kHz coils, in the same exact location. Here are those results:
9-inch concentric at 3 kHz, NC at +2, GP reading 009402
9-inch concentric at 3 kHz, NC at 0, GP reading of 010305
9-inch concentric at 3 kHz, NC at +2, GP reading of 008429
And then I ran two consecutive tests with the 10.5-inch DD coil:
NC at -1, Sensitivity 20, GP reading of 005554
NC at 2, Sensitivity 20, GP reading of 005093
What do I make of this? All in all, it appears to me that the frequency selection has very little to do with the Ground Phase readings. I can't explain why the same coil would give different Auto NC settings at the same location, minutes apart. We can't see what type of interference is present at any one time. Notice that the DD coil application offers GP readings of approximately 50- 55% that of the concentric coils. DD coils are designed to better handle mineralzation. I also know that the windings in a DD coil are typically smaller in size, compared to similarly sized concentric coils of the same manufacturer. Smaller windings contribute to the DD coils having less sensitivity, generally speaking. Naturally, the DD coil has a much more narrow "detection pattern". This makes your sweep speed, when using a DD coil, more critical than with a concentric coil. On the other hand, the smaller windings used in a DD coil create less of an antennae effect, making DD coils less succeptible to noise.
As to detector frequency and how the various frequencies will respond to different targets..... If you had a detector that allowed you to change the operating frequency to any given kHz, you would find that ferrous targets will give the most negative response at the lowest frequencies. As you increase the frequency of operation, you will find that the higher you go, more ferrous targets will give a positive response similar to conductive targets. At some point in time, there is a "balance point" for every specific target. A point where each ferrous target will cross over from a negative response, to a positive response. In addition to the ferrous content, target size also plays a big role in where that "balance point" is. You might think that bigger ferrous targets are more easily discriminated than small ferrous targets. But that is not the case. The bigger the ferrous target, the more likely it will be to produce a positive response. On single frequency detectors, lower frequency detectors have better ferrous discrimination characteristics than higher frequency detectors. This is due, in part, to the Phase Shift (or timing) between the transmit frequency of the coil and the frequency of the target. The degree of phase shift is caused by the inductance and resistance of the target. Lower frequency detectors are better capable of giving a positive response on higher conductive targets like silver and copper coins. On higher frequency detectors, ferrous targets are not discriminated as well and you will likely have more ferrous targets appearing to be non-ferrous. On the same note, low conductive targets are more readily detected at higher frequencies, as represented by most gold specific detectors operating at what we consider to be high frequency. Although they will detect coins, you will not be discriminating out the ferrous targets as well as you would if you were operating at a lower frequency.
Bottom line for me.....if the mineralizatoin at the site I am hunting does not warrant the use of a DD coil, and a concentric of the same size is available, I will use the concentric. I believe a concentric coil will go deeper and have a wider "window of beep" than a like sized DD. And of the three concentrics available, I like the 3 kHz for coinshooting old silver and copper. It not only provides a more negative response to the ferrous targets. But it provides a more positive response to the higher conductive targets I am looking for. As I've said before, I hear a much more distinctive low tone on ferrous targets when using the 3 kHz coil. But as will all my posts, that is just my opinion. HH Randy
