Prospecting is the hunt for minerals economically worth exploiting. The simplest technique is direct observation of local surface features characteristically associated with specific mineral deposits. This is often done by prospectors on the ground, but increasingly by aerial photography has been employed. Other techniques include examining the seismic waves caused by explosions – these supply information about the structures through which they have passed; testing local magnetic fields to detect magnetic metals or metallic gangues associated with nonmetallic minerals; and, especially for metallic sulfides, testing electric conductivity.