Abstract

Geological and geophysical interpretation of TEM data has revealed changes to the subsurface from the “Kraton-3” peaceful underground nuclear explosion (PUNE). The explosion was conducted on 24 August 1978 at a depth of 577 m in Middle Cambrian limestone on the eastern periphery of the Tunguska basin (Western Yakutia). The site is located in an area of 100 to 300 m thick permafrost and pressurized aquifers with Na-Ca-Cl brines (up to 400 g/l TDS) and cryopegs. The “Kraton-3” epicenter is only 160 m away from a fault emerging along the Markha River.

TEM responses collected at 22 stations along three profiles image a layered-earth background resistivity pattern. The highly resistive uppermost layer, ~150–200 m thick, consists of perennially frozen ice-rich rocks. Dry permafrost on watersheds of the Markha right side reaches 1200 ohm⋅m, while the hypsometrically lower frozen ground along the fault is 10 to 40 times less resistive. That is exactly the place of the PUNE epicenter, and the resistivity lows may record permafrost degradation and taliks (unfrozen layers).

The layers below are conductive and correspond to Upper Cambrian and Middle Cambrian (I) aquifers with brines. The top of the Upper Cambrian aquifer along the central profile is highly variable in depth, especially along the fault on the river left bank. The data indicate a local groundwater anomaly above the explosion: the Middle Cambrian I brines, which show up as a conductor in the resistivity pattern, become ~300 m shallower, most likely rising along the rubble chimney above the UNE containment cavity; the lateral extent of the anomaly reaches 400 m. There may exist paths for mass and heat transport maintained by pressurized brines in the system “containment cavity–rubble chimney–fault zone–ground surface”.

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