The automatic detection of temporal changes in sonar images is of high interest for monitoring critical areas as ports or channels used by submarines for instance. This problem is addressed here as incoherent change detection between two sonar passes. To be able to compare two surveys, sonar images have first to be registered to each other. Then two different methods have been proposed to detect changes in terms of contacts in the common area of the registered images. The first method is one of the well-known techniques used for synthetic aperture radar imagery: the log ratio of images. The second method is made of two stages: first a detector is applied so as to find small statistical anomalies in each image; secondly the difference of the processed images is performed. Advantages and drawbacks of the two proposed methods are discussed with results on a set of data collected by DRDC-Atlantic with a high-frequency sidescan sonar a month apart in the winter of 2008 in a port area.