Full coverage of an area of interest is a common task for a robot in the underwater environment. Estimating the area explored by the robot is indeed essential for determining if path-planning algorithms lead to complete coverage. In this work, we propose a method for estimating the area explored by a Side-Scan Sonar. The proposed method is able to determine how many times each portion of the space has been sensed by the sonar using a novel approach based on the topological properties of the environment that has been scanned, and more precisely on an estimation of certain winding numbers. This property is useful for localization inside homogeneous environments, e.g. the underwater environment, and assessment for potential revisiting missions.