In shallow seismic exploration, signals from repeated impacts are recorded on a geophone. These records are synchronized and added to enhance the seismic signal component relative to the uncorrelated noise that impairs reception. Linear filtering is the technique proposed in this paper to improve seismic signal detection over and above that obtained by synchronous addition. To arrive at the practical filter which best (in mean square) approximates, at its output, the seismic signal component, when fed with the geophone record, requires at least two such repeated synchronized recordings. The filter network is composed of a set of band-pass sections with variable parameters which are automatically adjusted to optimum. The setting time for the self-adjusting procedure may be preselected; the longer the time to set, the greater the probable accuracy of the parameter settings. Once set to optimum, the filter is fed with the synchronized sum of geophone recordings. The resulting filter output is reversed in time and played back again through the same optimum filter to yield an output which has no phase distortion introduced by linear filtering. Such a filter is being incorporated into seismic and hydrosonde equipment.