A method for determining locations of impairment in the wall of a pipeline which contains a liquid, comprising: moving a moveable device (301; 351; 450) through the pipeline in contact with the liquid, generating a series of acoustic or seismic pulses at a pulse generator (101; 380; 501) and receiving the acoustic or seismic pulses at a receiver, at least one of said pulse generator and said receiver being on the moveable device, characterized in that the series of acoustic or seismic pulses are generated a) into the liquid by the pulse generator (380; 501); or b) into the wall by the pulse generator and are received by the receiver on the moveable device said receiver being in contact with the liquid in the pipeline, and in that the method further comprises: determining, when the device is at each of a plurality of locations along the pipeline, at least one of the velocity of travel of such pulses between the pulse generator and the receiver and the rate of change of such velocity as the device passes each such location, said pulses having at least one of (i) a frequency of from 20 Hz to 2000 Hz or (ii) a wavelength of from 1.5 to 75 times the diameter of the pipeline in which the method is performed; and identifying the location of impairment in the wall based on at least one of a deviation of the determined velocity of travel of said pulses from an average velocity in a length of the pipeline and a deviation in the rate of change of said velocity between such plurality of locations. moving a moveable device (301; 351; 450) through the pipeline in contact with the liquid, generating a series of acoustic or seismic pulses at a pulse generator (101; 380; 501) and receiving the acoustic or seismic pulses at a receiver, at least one of said pulse generator and said receiver being on the moveable device, characterized in that the series of acoustic or seismic pulses are generated a) into the liquid by the pulse generator (380; 501); or b) into the wall by the pulse generator and are received by the receiver on the moveable device said receiver being in contact with the liquid in the pipeline, and in that the method further comprises: determining, when the device is at each of a plurality of locations along the pipeline, at least one of the velocity of travel of such pulses between the pulse generator and the receiver and the rate of change of such velocity as the device passes each such location, said pulses having at least one of (i) a frequency of from 20 Hz to 2000 Hz or (ii) a wavelength of from 1.5 to 75 times the diameter of the pipeline in which the method is performed; and identifying the location of impairment in the wall based on at least one of a deviation of the determined velocity of travel of said pulses from an average velocity in a length of the pipeline and a deviation in the rate of change of said velocity between such plurality of locations. determining, when the device is at each of a plurality of locations along the pipeline, at least one of the velocity of travel of such pulses between the pulse generator and the receiver and the rate of change of such velocity as the device passes each such location, said pulses having at least one of (i) a frequency of from 20 Hz to 2000 Hz or (ii) a wavelength of from 1.5 to 75 times the diameter of the pipeline in which the method is performed; and (i) a frequency of from 20 Hz to 2000 Hz or (ii) a wavelength of from 1.5 to 75 times the diameter of the pipeline in which the method is performed; and identifying the location of impairment in the wall based on at least one of a deviation of the determined velocity of travel of said pulses from an average velocity in a length of the pipeline and a deviation in the rate of change of said velocity between such plurality of locations.