Abstract: A method and apparatus for detecting pipe penetration by the bore head of horizontal boring machine utilizes acoustic and seismic sensors. A sensitive acoustic microphone is placed in the bottom of the nearest sewer manhole to the location of the bore head to detect acoustic signals transmitted through the ground by the bore head for producing a first output signal. A seismic pickup device is placed in the vicinity of the bore head to detect vibrations of the ground caused by the bore head, as it moves through the ground, to produce a second output signal. These first and second signals produced, respectively, by the microphone and the seismic pick-up device, then are supplied to a two-channel strip chart recorder, which provides an output indicia permitting simultaneous comparison of the two signals. Particular patterns of these detected signals are indicative of the penetration of a sewer pipe.