change in frequency of oscillator 72 that is indicative of the passing of the top releasing plug 20 the preferred rate of the first frequency is approximately one hertz and the preferred rate of the second frequency is approximately three hertz a preferred acoustic transducer 31 is an accelerometer commercially available from a number of sources such as columbia research lab inc of woodlyn penn 19094 such an accelerometer includes a piezoelectric wafer loaded by weight acoustic energy traveling up the drill string 10 from the acoustic transmitter apparatus 30 stresses the piezoelectric wafer to transmit an electrical signal over conductor 32 to the indicating means 33 the indicating means 33 includes a band pass filter passing a band of approximately 4 000 hertz to assist in filtering out mechanical noises such as machinery etc imparted to the drill string 10 the indicating means 33 additionally includes known amplifier circuits to light the bulb 34 and to produce a sound over the head set 35 responsive to the filtered electrical signal from acoustic transducer 31 illustrated heretofore is an embodiment of the invention whereby the arrival at a predetermined location in a well bore of a top releasing plug may be sensed and acoustically transmitted to a surface monitoring apparatus the invention disclosed could well be used in connection with other well equipment such as pressure gauges logging tools and other specialized oil well equipment as illustrated the transmitting apparatus is located on the seafloor and an acoustic signal indicating the arrival of a well tool at the seafloor is.