Abstract: A method of monitoring the profile of an underwater pipeline extending between a pipelaying vessel and the seabed comprises deploying multi beam scanning sonar apparatus from the vessel; transmitting acoustic signals from the sonar, detecting return signals reflected from the pipeline; and processing the return signals to obtain a 3-dimensional model of the pipeline and to determine the pipeline touchdown point therefrom. In preferred embodiments, the sonar is located below the vessel on a flexible tether at a depth outside the thruster noise zone of the vessel. Additional sonar apparatus may be used for monitoring the range and bearing of the main sonar relative to the pipeline and its distance above the seabed.