The character of the mantle-core boundary from observations of PcP waves

Differential PcP Travel Times and the Radius of the Core

“…Buchbinder (1965Buchbinder ( , 1968a and Buchbinder & Poupinet (1973) attribute examples of apparent dilatational PcP arrivals from explosions at A < 32" to a phase reversal. Other investigators (Chowdhury & Frasier 1973;Berzon, Kogan & Passechnik 1972;Kogan 1972) find no evidence of a phase reversal in phase correlated P and PcP waveforms from explosions and earthquakes in this distance range. Current opposing theories hold that either PcP phase reversals near 32" are produced by a thin high impedance layer with laterally varying properties at the core-mantle boundary (Buchbinder & Poupinet 1973) or that no phase reversal has occurred but that the compressional onset has been attenuated by a heterogeneous layer or irregular surface at the core-mantle boundary (Vinnik & Dashkov 1969) or by plate effects beneath the Amchitka source (Davies & Julian 1972).…”

Amplitudes of diffracted long‐period P and S waves and the velocities and Q structure at the base of the mantle

“…Previous amplitude studies yielded extremely different velocity models for the base of the mantle ranging between models with strong positive gradients to models with strong negative gradients (for extreme cases, see Mitchell and Helmberger [1973] and Miiller et al [1977]). In addition to this, models with a layered mantle base have also been suggested by, e.g., Ibrahim [1971Ibrahim [ , 1973, Buchbinder and Poupinet [1973], and Berzon et al [1972]. In order to obtain independent information on the velocities at the base of the mantle and substantiate the conclusions of the ray parameter study in paper I we decided to analyse the amplitudes of long-period P and S •Now at University of Khartoum, Geology Department, Khartoum, Sudan.…”