A signal extraction method is developed based on a prior knowledge on the propagation of seismic signal. To explore underground velocity structure based on OBS (Ocean Bottom Seismogram), it is necessary to detect reflection and refraction waves from the data contaminated with relatively large direct wave and its multiples. In this paper, we consider methods based on the time series and spatial-temporal decompositions of the data. In spatial-temporal decomposition, the difference of the travel time (moveout) corresponding to underground layer structure is utilized. The proposed methods are exemplified with a real OBS data.