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 {TAIL ~ the tracers} into working positions abutting the well wall by {HEAD ~ supply} of actuating fluid to the cylinders 
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 { TAIL ~ the tracers } into working positions abutting the well wall by { HEAD ~ 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 TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH