There are different types of coils that can be added to your detector. The Concentric, Double-D (DD) and the mono coil are the three types available. Be low are the differences between these types of coils.
Concentric Search Coil
This coil has 2 inner coils; send and receive. Magnetic waves are sent into the ground shaped as an inverted cone...detecting area diminishes with depth (diameter of search area decreases with depth). Easy to pinpoint with.
Double-D Search Coil
Sports two half-coils (Hence the “D” reference) that are joined in a straight strip down the middle. Sometimes called a “Butterfly” coil. Detection area does NOT shrink with depth! Challenging to pinpoint with if you started with concentric.
These coils are mainly used for finding gold. They are more sensitive than a DD coil and similar to a concentric coil. They do provide more depth than the DD and are great for finding small gold.