I owned the 15x12 for my GT and the depth in sand was outstanding. Better than my stock 10" Tornado, but in my high mineral land sites it's depth was not as good as the stock coil. About 9 or so inches on a coin. That's odd because my beaches are also mineralized like my land sites. The 15x12 would run at higher sensitivity settings than the stock 10" coil but that doesn't automatically translate into more depth. As with my prior Explorers, I often find that a much lower sensitivity setting with any coil will give best depth/ID, being easier to get proper ID/hold it/etc. Too high can null or degrade target quality. I attribute this to the coil taking in too much ground matrix with the target and thus deluting it's quality. I often calibrate by adjusting sensitivity on a buried silver dime at fringe depth for that particular site. The 15x12 was more stable and ran smoother over iron and such, and one can't argue with it's incredible coverage.
But I'm primarily a land hunter for old coins, and for that reason I traded the 15x12 for a 12x10. My theory is that the 12x10 would be taking in less ground matrix (even less than the stock 10" coil due to the SEF's tighter left/right detection field) and so allow higher sensitivity settings and deeper target signals without them stewing in as much ground matrix. I just got this coil a few weeks back so it's too early to tell if it's deeper than the 15x12 or stock 10" coil. Initial impressions are encouraging, because even in the dry soil conditions right now I've popped quite a few wheats deeper than the 15x12 or the stock 10" coil has been able to do in these dry conditions. Thus far in wet soil the stock 10" coil has got me easy target ID/audio of my deepest coins at around 11" deep, one being an Indian and the other being a V-nickle. In my higher mineral land sites that's pretty darn impressive. Of the three Explorers I used to own none of them could manage anything much beyond 8 or so inches on a coin sized target at most of my sites. In fact, thus far the 12x10 has got me deeper wheats in a small area I've completely gridded the p*ss out of with my prior 3 Explorers from every direction and in every possible combinations of settings you can possibly dream of. That tells me the GT with the stock coil as well as the 12x10 is getting deeper *in my ground* than my Explorers ever could. Just seems to run so much more stable in the kind of minerals I have.
If your primary targets are coin sized then I'd think carefully about going bigger than the 15x12. Anything beyond that size starts to lose coin sized target sensitivity. In fact, the 12x10 is a half inch deeper on a cut quarter penny (roughly smaller than a half dime) and yet matches the 15x12 in depth on any other coins. The only reason to own the 15x12 would be for more coverage, as it's not going to give you more depth on coin sized targets than the 12x10. The other important thing to consider is the amount of mineral content in your area. Yes, the 15x12 will run ultra smooth in that ground but for depth issues it may not show deeper targets than say a 10" coil. My theory is that the 12x10 has less of a foot print than say a Sunray 12" round coil or even a 10" round coil with it's much tighter left/right field, making it deeper in any kind of higher mineral content. The tighter left/right perspective of these SEF coils also makes them separate and pin point better than even smaller coils.
So if coverage is primary then consider that big 18x15. If coin sized max depth is primary then look towards the 15x12. If your minerals are medium to high and you still want max depth over max coverage then I'd lean towards the 12x10. The coverage of the 12x10 is still much better than the stock 10" coil on my GT. I can't even stand using the stock coil for land hunting now. Just feels too small. Think of it this way, with the length of these coils being longer than the width we all know the center line on a DD coil is the deepest spot (and where the main detection field is), so with that extra length you are getting deepest over a longer area than a round coil. Also, I believe the length determines depth on these SEF coils, while width determines separation/PP ability...And that's even better than say an 8 or 9" coil from my feel of it. I also believe the unique field dynamics of these coils get them deeper than the same size coil in terms of the SEF's length, and not just because of it stewing in less ground matrix. One other big perk to these coils is that the tight left/right field makes target outline much more distinct. Even the 15x12 was an improvement in separation and PPing over the stock coil, but the field generated by the 12x10 is ULTRA sharp. I swear it makes it feel like the clock speed on my GT has been sped up. Faster recovery, better separation, more crisp tones, very "reactive" with little coil movement to clear targets and investigate those next to them. Even the audio pitch on my GT has raised with the 12x10.
