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Firstbreak picking is an important step in the seismic data processing��and it is the important basis of speed calculation in tomographic imaging�� We present an automatic firstbreak picking method based on instanta- neous traveltime�� The calculation of the instantaneous traveltime consists of two separate but interrelated stages�� First��a trace is mapped onto the time- frequency domain�� Then the time- frequency representation is mapped back onto the time domain by an appropriate operation�� In this paper we chose S transform to map the seismic trace onto time- frequency domain��so as to realize automatic firstbreak picking�� Synthetic model and field seismic data show that the proposed method can effectively do the automatic firstbreak picking��