The magnetic field is strongest at the edge of a shallow manhole cover. Turn the Gain control to LO and you can easily trace the edge of a cover near the surface. Locating depth ranges up to 10 feet.
The great length of a well casing provides a strong field at the surface that makes it easy to locate casings buried up to 18 feet deep.
The MAC-51Bx receiver can be used to precisely locate the metal handles or reinforcing bats on septic tank covers at depths up to 4 feet.