REFRACTOR IMAGING USING AN AUTOMATED WAVE-FRONT RECONSTRUCTION METHOD

The classical wavefront method for interpreting seismic refraction arrival times is implemented on a digital computer. Modern finite-difference propagation algorithms are used to downward continue recorded refraction arrival times through a near-surface heterogeneous velocity structure. Two such subsurface travel-time fields need to be reconstructed from the arrivals observed on a forward and reverse geophone spread. The locus of a shallow refracting horizon is then defined by a simple imaging condition involving the reciprocal time (the travel-time between source positions at either end of the spread). Refractor velocity is estimated in a subsequent step by calculating the directional derivative of the reconstructed subsurface wavefronts along the imaged interface. The principle limitation of the technique arises from imprecise knowledge of the overburden velocity distribution. This velocity information must be obtained from uphole times, direct and reflected arrivals, shallow refractions, and borehole data. Analysis of synthetic data examples indicates that the technique can accurately image both synclinal and anticlinal structures. Finally, the method is tested, apparently successfully, on a shallow refraction data-set acquired at an archeological site in western Crete.