What are DD coils? A Double-D coil has two overlapping wire windings in the shape of two D’s. The benefits of a Double-D coil are stability (especially in heavily mineralized ground), good depth, sensitivity and a very thorough search pattern requiring less overlap. When used with GPX detectors, Double-D coils (unlike Mono loop coils) are able to discriminate between ferrous and non-ferrous targets when Iron Reject is activated. They are also more stable when used on wet salt beach sand and in electrically noisy environments. When used with X-TERRA detectors, Double-D coils perform quieter than Concentric coils in mineralised ground and are therefore well suited for gold prospecting.