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This paper presents a novel approach for the internal inspection of sewers through the use of sonar techniques, generating enhanced 3D graphs which represent the inner sewer surfaces. These graphs not only show the inner contour of the pipe but also integrate the intensity of the received echoes. The enhanced profile is generated by superimposing the peak intensity from the returning echoes at the calculated x, y and z coordinates where they are reflected from the pipe wall. These coordinates are calculated by measuring the time of flight of the first reflections, which are extracted from consecutives B-mode images generated during the ultrasonic scanning of the pipe. The proposed method has been capable of showing anomalous conditions, inside pipes filled with liquid, with dimensions smaller than the theoretical lateral and axial resolution of the transducer, in contrast to traditional methods where these kinds of defects were not detected. The proposed inspection method and its capabilities were validated through the realization of simulations and experiments. The simulations were conducted to validate the proposed method and explore its limitations. The proposed approach was particularly developed with the aim of scanning internal sections of sewers or water pipes filled with liquid using rotary ultrasonic sonars where visual methods could not be employed. It is expected that this research could also be expanded to the inspection of other submerged structures, such as water tanks, or pressurized vessels.