Recently, the method called HSP (Horizontal Seismic Profiling) was used for the forward imaging of tunnel face. In HSP, the reconstruction of structure is the most important problem. In the present paper, the algorithm of depth transform by use of equi-traveltime planes developed for cross-well and VSP survey was applied to the forward prediction of geological structure of tunnel face. At first, in order to reconstruct the forward structure, the equation of equi-traveltime planes of waves which travel from source to receiver through reflection point on reflector is derived. The derived equation shows the equation of ellipsoid of which foci are source and receiver. This means that a reflection point on reflector is located on ellipsoid. Ellipsoids are drawn for the data of each pair of source and receiver. Consequently, reflector is determined by drawing a common tangent plane to these ellipsoids. Next, this algorithm was applied to the field data that was observed in a tunnel. The result of prediction of tunnel face by the proposed algorithm showed a good agreement with the actual geological observation report after the completion of tunnel. So, it is concluded that the proposed algorithm is useful for the on-site check of geological condition using a personal computer with small computer time.


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