The DD coils are best for sifting through high trash areas. The stock coil that comes with the X-Terra detectors is a Concentric and this has a "cone-shaped" detection field (with the tip of the cone pointing down). The field gets wider as it gets closer to the concentric coil. The DD coils produce a "blade-shaped" emission field and therefore are better suited to areas with multiple targets or high trash areas. The overlapping swings from a DD coil are somewhat more thorough than the swings of a Concentric coil (which need to be tighter in order not to miss anything at depth).
The Elliptical 5x10" DD HF coil is the most commonly used as it is best suited to covering large areas quickly. (It's also water-resistant only) This is usually the first DD coil users will obtain and is incredibly lightweight. Designed especially for Gold Prospecting. It's sensitive to medium to large sized targets.
The smaller 6" DD HF coil is deeper penetrating and has an even narrower detection field blade, but it must be used very slowly and is best suited to smaller areas and especially creeks and rock pools (it's waterproof). It's super sensitive to incredibly small targets at depth. Can detect larger targets easily.
The larger 10.5" DD HF coil is the largest and heaviest DD coil. It's best for farmlands and open fields and penetrates slightly deeper than the Elliptical but is offset by considerably more weight. Many users will pick the Elliptical over this one due to the weight alone. But in the rural areas, this is a great coil with good results. No idea how well it performs on the beaches. Not quite as sensitive to smaller targets such as the 6" coil is... but then again, that coil is specialized.
Yes, the DD coils are best suited for sifting through multiple targets, but you'll need to pick the right coil for your needs.
No single coil does everything you want it to do (unfortunately).
Cheers,
Marco
