As the search coil is moved around, the capacitance between it and the ground or other objects changes. This changing capacitance "pulls" the oscillator frequency and can completely swamp-out the small change in inductance we are looking for. The coil can be screened from this capacitance effect by using a FARADAY SHIELD around the coil. This consistsof a ring of tubing, or a wrapping of aluminium foil around the coil but broken at one point, so it does not make a shorted turn. This shield is then connected to the common supply rail (0V) of the circuit.
Ref: Electronics Australia International Mar 1980 pg 33 et seq