Abstract
The apparent body wave velocities υP of the oceanic mantle immediately below the Mohorovicic discontinuity (M), obtained from oceanic refraction shooting, can depend on azimuth ϕ; because of real mantle anisotropy or horizontal variations in an isotropic crust and mantle. A small anisotropy of the uppermost mantle leads to υP(ϕ)=∑n=04An cosn ϕ sin4-n ϕ=∑n=−22Bneinϕ The An determine 5 of the 21 elastic parameters of the uppermost mantle. Curvature in the M discontinuity has the same effect on velocities υP(ϕ), but the azimuth dependence found by Hess in Raitt's and Shor's velocities, if real, is too large to be due to curvature. A sloping M discontinuity adds to velocities υP(ϕ) a term a cos ϕ + b sin ϕ. In a refraction survey of anisotropy, only two lines need be reversed to find a and b. If the ϕ dependence of velocities υP is due to a small mantle anisotropy and a small slope of the M discontinuity, velocities υP(ϕ) is determined for all ϕ by shooting five different lines through a point and reversing two of them or simply by shooting seven different lines. The theory is compared with Raitt's and Shor's data as reported by Hess.
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