Sidewall coring:

A coring technique in which core samples are obtained from the hole wall in a zone that has already been drilled.

A hollow bullet is fired into the formation wall to capture the core and then retrieved on a flexible steel cable.

Core samples of this type usually range from 3/4 to 1-3/16 inches (20 to 30 millimeters) in diameter and from 3/4 to 4 inches (20 to 100 millimeters) in length.

This method is especially useful in soft-rock areas.