В работе использован коэффициент химической (петрофизической в применении к земной коре) неоднородности (η) по К.Е. Буллену и В.А. Магницкому для разделения эффектов, связанных как с трещиноватостью пород, так и с изменениями их вещественного состава с глубиной. Тестирование способа выполнено на образцах пород, подвергнутых всестороннему сжатию до 1500 МПа. Показано, что с ростом давления при разных вещественных составах значения η также различны, но закономерно уменьшаются, приближаясь к единице и коррелируя с закрытием трещин. Аналогичная стабилизация изменений η на глубине 15–20 км получена при обработке экспериментальных сейсмоплотностных данных. Обнаруженные отклонения от осредненного тренда трещиноватости образуют протяженные локальные аномалии изменений вещественного состава, прослеженные на глубину до 20 км. Они уверенно коррелируются с разломами по геологическим данным.

The upper-mantle structure was studied from first-arrival data along the Meteorite profile, run using underground nuclear explosions. Unlike the layered, slightly inhomogeneous models in the previous works, emphasis was laid on lateral inhomogeneity at the minimum possible number of abrupt seismic boundaries. We used forward ray tracing of the traveltimes of refracted and overcritical reflected waves. The model obtained is characterized by considerable velocity variations, from 7.7 km/s in the Baikal Rift Zone to 8.0–8.45 km/s beneath the Tunguska syneclise. A layer of increased velocity (up to 8.5–8.6 km/s), 30–80 km thick, is distinguished at the base of seismic lithosphere. The depth of the layer top varies from 120 km in the northern Siberian craton to 210 km in its southeastern framing. It has been shown that, with crustal density anomalies excluded, the reduced gravity field is consistent with the upper-mantle velocity model.

We investigate the upper mantle velocity structure through processing first arrival data from peaceful nuclear explosions. The reported 2D model has been obtained by ray tracing for a spherical Earth, unlike the classical plane-approximation approach with subsequent spherical symmetry corrections, which is not always applicable to a laterally heterogeneous subsurface. The upper mantle velocity highs and lows imaged to 200–220 km depths show obvious correlation with major structures of the craton basement. Namely, low-velocity zones are observed beneath basins, the largest (to 8.0–8.1 km/s) under the Vendian–Early Cambrian Sayan–Yenisei syneclise. A discontinuous high-velocity layer (8.6–8.7 km/s) at depths between 150 and 240 km is underlain by a zone of lower velocity (8.50–8.55 km/s) down to the 410 km discontinuity, where the velocity at the top of the transition zone is 9.4–9.5 km/s.

Structure of the deposits and basement beneath the South basin of Lake Baikal is investigated down to a depth of 14 km on a profile about 100 km long. Ray tracing modeling of refracted and reflected waves is applied. A discrepancy between the observed and calculated wave traveltimes does not exceed 0.05-0.1 s for the parameters of cross section. In addition, the reliability of the sedimentary cover structure is confirmed by similar synthetic and observed record sections. Four layers with individual characteristics are allocated, divided by seismic boundaries with rather sharp differences of velocity. Velocity changes with depth on the given profile are similar to results of seismic observations in the Selenga depression, in deposits of the West Siberian Plate, Siberian craton and Vilyui basin. It is possible to assume that the sedimentary cover up to 10-12 km thick hosts not only Cenozoic and Mesozoic but also Paleozoic rocks. Therefore it is possible that the age of the South basin of Lake Baikal is more ancient than the Cenozoic-Mesozoic.