1. A downhole tool for determination of the position and orientation of a bed boundary during drilling of a wellbore, comprising: (a) a transmitter for transmitting acoustic signals at at least one position in the wellbore during drilling of the wellbore, said acoustic signals having an acoustic velocity; (b) a plurality of spaced apart receivers for detecting signals transmitted by the transmitter that are reflected by the bed boundary; and (c) a processor determining semblance of the reflected signals in a slowness domain and determining from said semblance and said acoustic velocity a distance and orientation of the bed boundary relative to the tool.