With the costs involved in offshore seabed mapping being very large and dominated principally by the expense of survey vessel time, any improvement in the planning and execution of seabed mapping, improvement in the quality of the resulting data, and especially reduction in survey costs are of significant interest to the marine and offshore community. This paper presents a terrain adaptive seabed mapping system, which simultaneously operates co-located high-resolution multibeam and sidescan sonar of similar frequency. The automated system is based on the interleaving of sonar transmission-reception cycles to avoid interference. The proposed system optimises the execution of the seabed mapping survey, improves the quality of the data and reduces survey costs. © 2007 IEEE.