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{{#category_index:R|refractor}} A layer of higher velocity than overlying layers through which a head wave (q.v.) can travel. To be useful for mapping, refractors must be (a) sufficiently thick (greater than 1/10 of a wavelength) for a head wave to carry energy over an applicable distance, (b) sufficiently extensive that the same refractor is mappable over an appreciable area, (c) sufficiently distinctive in velocity that the head wave can be distinguished from the waves carried in other layers, and (d) not be ‘‘hidden’’ by a shallower refractor of higher velocity.