Hey Bubber,
In a nutshell, yes. The 10x5" DD HF coil is less responsive to the smaller hotrocks. I've only found two hotrocks with this larger coil. They were larger hotrocks too... one was too large to lift. Using the smaller 6" DD HF coil is a different experience altogether but don't get me wrong, these two coils are very impressive when used in the right order or in the right location. I personally wouldn't part with either of them.
The 6" HF Coil is smaller so the field that it emits is denser and more sensitive to very small targets. I sometimes use mine for a studfinder at home before hanging a picture.
The larger the coil is, the more immune it is likely to be to smaller targets (and usually ground noise to a degree).... but the deeper it tends to go.
So the small 6" DD HF coil for the X-Terras is a fantastic coil for poking around in the Goldfields when looking for small specimens. It's also waterproof so you can immerse it in streams and creeks. It's popular in rockpools on beaches and even in parks for coins because it's emission field is so narrow that it's very blade-like. This enables the user to work in trashy areas with close-proximity targets.
Since the 6" DD HF coil is super sensitive, it's very good for finding very small Gold Specimens - so small that most other detectors and coils can't see them at all. It will easily respond to targets in the subgram category. If the target is touching the 6" DD HF coil when the detector is set to default settings, it will respond to a target just 0.02 of a gram. That's smaller than a pin head. This means it's too sensitive to use as a general search coil when you're out and about. If you attempt to cover large areas of ground with this smaller coil, you'll be stopping every two feet or so if there's fragments of iron or tiny pebbles from the hotrock family. The smaller coils also tend to be more sensitive to Ground Noise as well so this means that it can be hard to wander around in mineralized areas without being annoyed by a barrage of false signals and wavering sounds.
As noted above by David, the VLF (Very Low Frequency) detectors are not particularly immune to ground noise. They respond more readily to hotrocks and ground noise caused by mineralization. Pulse Induction (PI) detectors tend to handle those soils much better but don't discriminate nearly as well as a VLF detector does. Trashy areas are often good locations for a VLF. Most VLF detectors tend to find targets as deep as 9 to12 inches on average... and sometimes more if the target is larger. PI detectors will go much deeper but tend to cost considerably more. PI detectors (as noted above) are also less reactive to hotrocks.
Something VLF users with detectors like the X-Terra 70 (and the new X-Terra 705) can do is use a hotrock to re-balance the detector on. You can do this to rebalance the detector ...to assist in ignoring similar rocks in the immediate vicinity. It may require turning off the Ground Tracking feature though in order to work. Don't make the mistake I once made and attempt to cover an entire mountain with the 6" DD Coil because you'll be stopping every few feet in order to dig for plankton sized bits of ironstone etc.
The strength of the 6" DD HF coil is it's amazing versatility in creeks and waterways (both on the shore and submerged) although it's an excellent "sniper coil" for use on patches and on old tailings and mulloch heaps. It goes quite deep for a smaller coil but its sensitivity is extraordinary.
Cheers,
Marco
