Abstract: A method and apparatus are disclosed for carrying out the seismic exploration of the strata which surrounds a borehole during the drilling of that borehole. A compressional mode jar mechanism is disposed in a drill string above a drill bit. Sufficient weights to cock and actuate the jar are disposed both below and above the jar mechanism respectively. A telescoping joint above the jar prevents the impact on the drill bit from propagating up the drill string and multiple geophones at the surface are utilized to receive the seismic waves generated by the impact of the drill bit within the borehole. In a preferred mode of the present invention, an additional geophone is disposed within the drill string proximate to the drill bit to detect the "first break" of the impact thus generated.