Autonomous underwater vehicles (AUV) perform inspection missions and intervention in known and unknown environments, where it is necessary to ensure their safety. The AUV must have the ability to detect and avoid obstacles in the path of navigation. This article, an obstacle detection system for experimental Guanay II AUV is proposed, using a mechanical scanning SONAR Tritech Micron MK3. Since Guanay II operates autonomously, we have designed software that allows adjustment and control of the parameters of SONAR, and the acquisition and processing of the signals obtained. Experimental tests at sea have allowed to verify the correct operation of software designed, as well as, experimental tests in a controlled environment have allowed to determine the optimal values of the basic parameters of SONAR.