Field experience in various locations has shown that an assumption of subsurface isotropy is not always satisfactory. For example, Dunoyer de Segonzac and Laherrere (1959) stated that "Computations using vertical velocities, as derived from well velocity surveys, seemed to yield insufficient depths. On the other hand, the use of horizontal velocity, as derived from refraction traveltime curves, gave far too great depths. Both assumptions were obviously incorrect and it appeared essential to know the actual velocity along refracted oblique raypaths." Northwood (1967) classified refraction interpretation errors into three main types: (1) those caused by incorrect reading of the data; (2) those caused by incorrect assumptions; and (3) those caused by incorrect geologic interpretation of the velocity layers. This note concerns type (2) errors. Its purpose is to show, in closed form, the errors made when computing the thickness of a layer with the refraction method, as a result of assigning an incorrect anisotropy to such a layer. In short, if a medium is composed of two layers separated by a horizontal interface at depth, and only the refraction traveltime graph is available, the computed depth depends upon the anisotropy assigned to the upper layer.

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