# Dictionary:ABC method

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A method of computing refractor depth based on refracted arrivals from sources near the surface. Especially used for determinations of weathering thickness from sources above the base of the weathering. See Figure A-1. The weathering time tW is sometimes multiplied by a ‘‘k-factor’’ to give the vertical weathering time:

${\displaystyle t_{v}=kt_{w}={\frac {(t_{w}V_{1})}{({\sqrt {V_{1}^{2}-V_{2}^{2}}})}}}$
FIG. A-1. ABC weathering method. Weathering time tW below B is tw=(tAB+tBCtAC)/2 where tAB=surface-to-surface time from A to B (obtained by adding the uphole time to the first-break time), etc.