the logging instrument 12 is suitably supported the arms 16 are then extended against the wall of the wellbore and the ft tool 14 is induced to make its measurements of the earth formation 2 the measurement data may be stored in the electronic memory and part of the logging data may be transmitted by the ft tool 14 in the form of electrical signals to the pressure pulse device 18 which signals induce controlled variations of the variable flow restriction simultaneously with operating the logging instrument 12 drilling fluid is pumped by pump 28 from the fluid reservoir 26 into the drill string 9 via inlet 30 and outlet 32 the controlled variations of the variable flow restriction induce corresponding pressure pulses in the body of drilling fluid present in the drill string 9 which pressure pulses are monitored by the control system 34 in this manner the system operator can monitor the well logging operation and can take corrective action if necessary for example incorrect deployment of the arms 16 of the rft tool can be detected in this manner at an early stage after the logging run has been completed the logging instrument 12 may retrieved through the drill string 9 to surface by wireline connected to latch 20 optionally the drill string 9.