Many refraction seismologists believe that a minimum of two reversed profiles, preferentially oriented at right angles to each other, are necessary to determine the attitude, velocity, and depth of a plane subsurface refractor. Russell et al. (1982) demonstrated that traveltimes recorded along three unreversed spreads can be analyzed to yield this same information. They state that similar measurements made on only two such profiles cannot define the three-dimensional (3-D) attitude of the dipping horizon. This statement, however, is incorrect. Russell et al. did not fully use the information contained in the intercept times of the traveltime curves. The purpose of the present note is to demonstrate that, in many cases, two refraction profiles are sufficient to define the 3-D attitude, true velocity, and depth of a plane refractor. Generally speaking, the main condition required is that the two lines provide independent traveltime information about the subsurface.

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