Magnetic Toolface: It is the direction, in the horizontal plane, the bent sub scribe line is pointing with regard to the north reference (Grid, Mag, or True). Magnetic orientation is used when the inclination of the wellbore is less than 5° to 8°. When the inclination is below this amount, the survey instrument cannot accurately determine the highside of the instrument for orientation purposes. The toolface will be presented in azimuth or quadrant form, referenced to magnetic north. The magnetic toolface reading is whatever magnetic direction the toolface is pointed. If the orientation lug on the survey instrument is pointed towards S 20 E, then the magnetic toolface would be 160°.
Gravity Toolface : It is the angular distance that a bent sub scribe line is turned, about the tool axis, relative to the high side of the hole. If the inclination of the wellbore is above 5° to 8°, then the gravity toolface can be used. The toolface will be referenced to the highside of the survey instrument, no matter what the hole direction of the survey instrument is at the time. The toolface will be presented in a number of degrees either right or left of the highside. For example, a toolface pointed to the highside of the survey instrument would have a gravity toolface of 0°. A toolface pointed to the lowside of the survey instrument would have a gravity toolface of 180°. If the orientation lug was rotated to the right of highside, the gravity toolface would be 70° to the right.
Reference books: Directional Drilling Books