A method and apparatus for determining the topography of a seafloor includes providing a vessel with a multibeam sonar device which transmits first pulses of first signals, and a pair of sidescan sonar devices, which transmit second pulses of second signals, spaced apart rearwardly from the multibeam sonar device on the vessel. The first and second pulses produced by the multibeam sonar device and the sidescan sonar devices, respectively, are interleaved, and the time duration between each second pulse and the next following first pulse, and between each first pulse and the next following one of the first and second pulses is determined as a function of the maximum time-of-flight of the first signals of one of the previous first pulses from the time of transmission thereof to the time of reception of the backscattered first signals.