Abstract: This invention relates to improvements in or relating to digital time-delay acoustic imaging. According to the invention there is provided an acoustic imaging method for marine survey and other purposes comprising the steps of: insonifying a target area by periodic acoustic pulses; detecting reflections of said pulses by an array of at least two receiver transducers, which transducers generate respective output signals in response thereto, wherein further the signals are: sampled in time at a predetermined rate during an interval between said pulses; converted from analog to digital form; processed so as to generate a composite acoustic signal for each pixel in the insonified target area required to be imaged; and match-filtered with optimised filters in the frequency domain with an estimated source pulse signature.