Methods and systems to detect changes in a region of space relative to baseline measurements, including to process data to detect potential objects, or contacts against a natural background environment, to characterize and geo-register the contacts, to compare and correlate the contacts with a database of known objects, and to report uncorrelated contacts as new objects in the space. Features disclosed herein may be implemented to process image data from a line-by-line image-generation system including, for example, side-looking sonar. Methods and system may be implemented with respect to multi-frequency band sonar data.