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ENSCO, INC., PORT ROYAL ROAD 5440, SPRINGFIELD, VIRGINIA 22151
Abstract
Incoherent beamforming and continuous frequency-wavenumber "polar-scan" analysis have been applied to study the characteristics of P coda and Lg waves recorded at the regional arrays NORESS and ARCESS from presumed explosions on the northwestern Russian platform and at test sites of Novaya Zemlya and eastern Kazakh. Also, recordings at the Graefenburg array, near the new GERESS regional array, of Novaya Zemlya events and an eastern European platform (White Sea) event have been compared with the Norwegian array recordings of the same events. The White Sea explosion, at distances of 14° and 22° from NORESS and Graefenburg, respectively, exhibits Lg rms amplitudes on incoherent beams that exceed those of Sn. However, for explosions at Novaya Zemlya at 20° from NORESS and eastern Kazakh at 30° from ARCESS, the Lg waves have much smaller amplitudes than Sn. In the case of Novaya Zemlya, the Lg wave is almost completely blocked at both NORESS and ARCESS, but is well recorded at Graefenburg. This reduction of the Lg amplitudes relative to those from events at longer distances cannot be accounted for by anelastic attenuation and spreading. Comparison of propagation paths suggests that the reduction in amplitudes may be due to blockage and scattering of Lg in the Barents Sea. Analysis of Sn coda waves on incoherent beams and continuous frequency-wavenumber analysis "polar scans" suggests that they can result from Lg to Sn scattering from the coastline between the Barents Sea and Scandinavia. Recent geologic studies have revealed significant variations in crustal structure in the Barents Sea basin, which may cause the blockage of Lg waves from Novaya Zemlya. In the case of eastern Kazakh explosions, ARCESS incoherent beams show much reduced Lg amplitudes relative to the P coda waves compared with NORESS, even though the distance to ARCESS (31°) is less than that to NORESS (38°). This reduction in Lg amplitude at both NORESS and ARCESS is accompanied by enhancement of P and Sn coda. The timing of the coda waves and the propagation paths suggest that Lg waves may be partially blocked and scattered by the Ural Mountains and that the P and Sn coda waves may result from Lg-to-Pn, Pn-to-Sn, Sn-to-Lg, and Lg-to-Sn conversions from lateral heterogeneities in the crust beneath the Urals. The blockage is greater for the Eastern Kazakh-to-ARCESS path because it crosses the middle part of the Urals whereas the Eastern Kazakh-to-NORESS path crosses the southern part of the Urals chain. These blockage effects seem to result from sudden lateral variations in shallow crustal structure, such as variations in sediment thicknesses in the sedimentary basins of the Russian platform and adjacent to the Urals. These blockage effects can be predicted from geologic studies and need to be accounted for when using Lg amplitudes to estimate absolute yields of explosions and for event identification.
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