Garrett Metal Detectors is a leader in the field of detector manufacturing.  They are constantly innovating and developing their technology and brand.  Garrett provides a variety of coils for most of their metal detectors.  Different size and shape of a metal detector coil can help you reach deeper into the ground and allows you to detect areas that may otherwise be difficult to metal detect.  

Small coils help you "snipe" or reach tight areas better than a large clunky coil will.  Although you may use a slight bit of depth, a smaller coil will help you hunt trashy areas by helping you avoid "iron masking" or detecting a desired target and an undesirable target simultaneously by detecting a smaller area with each sweep.

Want to go really deep?  Check out the Garrett GTI 2500 with the optional Eagle Eye Package that allows users to search several meters deep for large objects - perfect for looking for intentionally buried treasure caches.

A coil cover is a very important piece of equipment to use.  They protect one of the most valuable components of the metal detector - the search coil.  A search coil cover is meant to be put on the metal detector right out of the box and left on during operation.  They will not affect the depth, sensitivity, etc.