measurement sensors the said data is transmitted in real time in order to be used in the underreaming and drilling operations 56 or physically downloaded by removing said card when the tool is retrieved from the well the tool is provided with a built in link to the telemetry system 56 which also serves to monitor performance of the under reaming operation position of expansion blocks 62 and data recorded by the caliper for measuring the underreamed wellbore diameter 66 or 76 as well as data from the fluid properties sensor one or more acoustic sensors 64 or 76 are placed within the tool body 52 in order to emit a number of sound waves during a given time period which are reflected back by the wellbore wall fig3 22 and picked up by the receiver sensors 66 or 76 the processor 68 calculates the distance using transit time and calibrates transit time with data from the fluid properties sensors to establish the speed of return of the acoustic waves and wellbore diameter the processor compares the measured wellbore diameter to the programmed desired diameter if the two measurements match given user defined tolerances the tool continues to operate to the total depth of the wellbore section to be underreamed where the measurements do not match the processor automatically activates a series of logic steps to troubleshoot the fault as further shown in fig2 a keyway 74 provides a channel for wiring of the acoustic pulsers or transmitters 64 or 76 and the acoustic sensor receivers 66 or 76 to the processor 68 and.