an analogue pressure signal generated by the transducer 240 passes to an electronics module 241 in which it is digitized and serially encoded for transmission by a carrier frequency suitably of 1 hz 10 khz preferably 1 khz 10 khz utilizing an fsk modulation technique the resulting bursts of carrier are applied to a magnetostrictive transducer 242 comprising a coil formed around a core not shown whose ends are rigidly fixed to the well bore casing not shown at spaced apart locations the digitally coded data is thus transformed into a longitudinal sonic wave the transmitter electronics module 241 in the present embodiment comprises a signal conditioning circuit 244 a digitizing and encoding circuit 245 and a current driver 246 the details of these circuits may be varied and other suitable circuitry may be used the transducer is connected to the current driver 246 and formed round a core 247 suitably the core 247 is a laminated rod of nickel of about 25 mm diameter the length of the rod is chosen to suit the desired sonic frequency fig3 shows a receiving portion 360 of the safety mechanism a receiving portion 361 comprises a filter 362 and a transducer 363 connected to an electronics module powered by a battery not shown the filter 362 is a.