The tracers are mounted on pistons which are received in cylinders and urged radially inwards by springs for retracting the tracers, the pistons being driven outwardly to displace {HEAD ~ the tracers} into {TAIL ~ working positions} abutting the well wall by supply of actuating fluid to the cylinders 
into
The tracers are mounted on pistons which are received in cylinders and urged radially inwards by springs for retracting the tracers , the pistons being driven outwardly to displace { HEAD ~ the tracers } into { TAIL ~ working positions } abutting the well wall by supply of actuating fluid to the cylinders
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH