Scattering ofPwaves under the Montana Lasa

The variations of amplitude and time delay of teleseismic P waves across the Montana Lasa (Large Aperture Seismic Array) were interpreted as being due to scattering by a random inhomogeneity in the earth's crust under the array. The prediction of the Chernov 1960 theory explains well the observed statistical properties of P waves with a frequency of 0.5 Hz. The inhomogeneity under Lasa has a correlation distance of about 10 km, with a fractional rms velocity fluctuation of 4% extending to a depth of about 60 km. The turbidity coefficient under Lasa at 0.5 Hz is 0.008 km−1, which is much greater than the values (10−3 to 10−4 km−1 at 5 to 10 Hz) observed for refracted waves in the crust and upper mantle in the USSR by Nikolayev and his colleagues. The scattering under Lasa is so strong that the condition for the Born approximation is violated for frequencies higher than 0.5 Hz. Accordingly, the observed statistical properties of 1-Hz waves show systematic departure from the prediction of the Chernov theory.