Abstract: An apparatus and method for detecting and locating a fluid leak in an underground pipe or the like in which the vibration produced by the leak is intercepted at selected spaced apart points by microphones or other transducers, in which the vibration intercepted at each of the two points is converted into an electrical signal, and in which the extent of correlation of the two resulting signals is varied by variably time delaying one signal relative to the other to determine the leak's location from the occurrence of maximum correlation between the signals. In one embodiment, the variable time delay of one signal relative to the other is accomplished by a variable length time delay line and a means for progressively varying the length of the delay line. In another embodiment the variable time delay is accomplished by a recirculating delay line analyzer that does more data age comparisons with the same delays in the same time than the variable length delay line.