as exemplified in the foregoing paragraphs in practice an operator may cause the drill pipe movement to result in interrogation of the logging tool status the operator then may wait a predetermined time for propagation of the borehole fluid pulse to be detectable at a surface location if the pulse is detected within the predetermined time the operator may conclude that the logging tool 27 is in an operational status if the pulse is not detected in a timely fashion or a fluid pulse indicating a failure is received the operator may conclude that the logging tool 27 is not operational the operator may in the case of an undue delay or of receipt of a signal failure pulse initiate such remedial steps as are required these may include the sending of further commands to the logging tool 27 the dispatch of further tools such as drop balls darts or various logging tool retrieval devices to the downhole location of the logging tool 27 or the removal of drill pipe stands in order to bring the logging tool 27 to the surface e g for repair the method and apparatus of the invention allow decisions on whether to initiate steps such as the foregoing to be made in less.