you will find very little difference in the detection depth or the ability to detect coins, or coin sized targets, with the three various frequencies. I say this because the difference between 3 kHz to 7.5 kHz to 18.75 kHz is not very much. I have convinced myself that the 3 kHz coil will allow me to hear the low tone created by iron, when adjacent to a coin, better than the other two frequencies. And, I have convinced myself that the higher 18.75 kHz coils are more sensitive than the other two. When I hunt with the 18.75, I have to run my sensitivity a little lower than with the other two frequencies, or it will chatter. I can't see any change in sensitivity settings between the 3 and the 7.5. I'm not saying this with any scientific proof. Just my findings based on having used all three of them, coin shooting at sites similar to what you describe.
Theoretically, the lower the frequency, the deeper it will detect and the more responsive it will be to larger and/or higher conductive targets. And, the although the higher frequency will not hunt as deeply, it will be more responsive to lower conductive targets and/or smaller targets. VLF detectors generally operate at a frequency of 3kHz to 50kHz. The Sovereign series operates at frequencies of 1.5 kHz to 25.5 kHz. And, the Explorer series operates at 1.5 kHz to 100 kHz. The DFX has 100 frequencies, but operates at 3 kHz and 15 kHz. The XLT operates at 6.59 kHz. The Tejon operates at 17.2 - 17.6 kHz. Minelab says their Eureka Gold detector operates at 6.4kHz for maximum depth, 20kHz for general detecting and the super sensitive 60kHz to find the smallest gold nuggets. So, compared to many other makes and models, the frequency range of 3 kHz to 18.75 kHz available on the X-Terra is not very big. That is why I don't think, as a coin shooter or relic hunter, you will see much difference between the 3 kHz , 7.5 kHz and 18.75 kHz.
Now, the DD vs Concentric configuration is a different story. I am convinced that DD designed coils will perform better in moderate to highly mineralized soil. However, as in your part of the State, I don't worry about highly mineralized soil conditions. Frankly, I can't make any distinction in performance stability between DD and concentric. For me,, it just requires a little different sweeping action and pinpointing technique. In my opinion, I find the concentrics will hunt a bit deeper, due to the narrower electro-magnetic field introduced into the soil.
As to whether to add the 10.5-inch DD coil at 7.5 kHz, I don't think you will see much difference in detection depth, compared to the 9-inch concentric at the same frequency. I think that any additional depth that you might realize due to the larger coil size will be offset by the additional depth achieved by the concentric design. If I had to chose between the 10.5 DD and the stock 9-inch concentric, I would chose the stock coil. I ain't trying to put a damper on Minelab's coil sales. But, if they end up with the same depth, the lighter weight and better balanced concentric coil (compared to the 10.5 DD coil) I chose the stock coil.
With that said, I can see a use for the elliptical coil design. But, at the current 18.75 kHz, I recognize I will have to set the sensitivity a bit lower than with the 7.5 or 3 kHz coils. And to me, setting the sensitivity lower, to compensate for a coil that is more sensitive, don't really accomplish much for depth of detection. But as you said, it does allow you to cover a bit more ground, and is easier to work into some places that are difficult for the 9-inch round concentrics.
In a perfect world, I would like to see Minelab introduce the elliptical coil in a 7.5 kHz operating frequency. Right after, of course, a 6-inch coil operating at 7.5 kHz. Who knows? Maybe 2007 will be the year for getting a whole arsenal of new coils!!!
HH
Randy
