Results of off-line signal processing experiments are presented for long-period data obtained from the Large Aperture Seismic Array in eastern Montana. The signal-to-noise ratio gains are presented for body-wave as well as surface-wave phases. Several recommendations are made concerning optimum long-period array configurations and on-line or off-line processing methods. The usefulness of maximum likelihood processing in suppressing an interfering teleseism is demonstrated. The matched filtering of surface waves using chip waveforms is shown to be highly effective. A useful discriminant for distinguishing between natural seismic events and underground nuclear explosions, using both long-period and short-period data, was found to be the relation between surface-wave and body-wave magnitudes. Measurements of this discriminant made on events from four tectonic regions of the Earth are presented.