Frictional drag means the air closest to the surface of the fuselage moves more slowly than the rest.

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Where the feed lies closest to the surface, as, for example, at the Suncor site, the bitumen is strip-mined and then separated.

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Where the rainbow fringes become most tightly spaced is where the fault break came closest to the surface, although the data indicates the fault is unlikely to have broken right through to the surface.

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