The effect of spall on Lg waves

Introduction The Lg phase plays a central role in nuclear test detection, discrimination, and yield estima-tion because of its stability in propagation, but the excitation mechanisms of Lg phase are not fully understood. Regional Lg is a short-period guided wave composed mainly of a sequence of multi-ply reflected post-critical S waves trapped in the crustal wave guide. Several studies carried out recently (Gupta et al, 1992; Patton and Taylor, 1995; Gupta et al, 1997; Wallace, 1991) have …

The effect of spall on Lg waves

The generation mechanism of Lg wave from underground nuclear explosion is still not clear at present. The gen-eral viewpoint is that the S wave generated by the near-source scattering of explosion-generated Rg appears to be the primary contributor to the low-frequency Lg (0.2~2.0 Hz) from nuclear explosions. The viewpoint is supported by the analysis of regional data from several Yucca Flats, NTS explosions by Patton and Taylor (1995), who fur-ther indicated that the prominent low-frequency spectral null in Lg is due to Rg from a compensated linear vector dipole (CLVD) source. In the paper, the data from Kazakstan Test Sites are investigated by a spectral ratio method. We have found that the spectral ratio of Lg waves is characterized by a spectral scalloping and a pronounced null, and the spectral null does not shift with spall dwell times, showing a strong dependence on shot depth and a very good agreement with those expected from Rg due to a CLVD source.