A time-coded sequence of source pulses has been designed to allow for increased range to the target and reduced side-lobe energy with correlation. The method was designed for a new type of marine-combustion seismic source that can be fired repeatedly, but the method also can be applied to other acoustic-pulsed sources. The basic concept is that one can reduce autocorrelation side lobes by using semiperiodic sequences to concentrate much of the side-lobe noise into time intervals corresponding to the semiperiod. Signals can then be detected with better sensitivity in the time interval after the first autocorrelation peak and before the first periodic side lobe, because this interval contains only lower-level crosscorrelations between the sequence components. Compared with a source which fires pulses randomly, an 8-dB reduction of autocorrelation side lobes has been simulated for sequences of duration.