The Kola Superdeep Drillhole and the nature of seismic boundaries

The Kola Superdeep Drillhole was drilled through 7 km of Proterozoic rocks of the Pechenga structure and 5 km of the Archean basement in the Baltic Shield to 12 km depth. The results of drilling revealed a significant heterogeneity of physical properties of the crystalline rocks and a surprisingly high porosity and water content throughout the section. Seismic wide-angle reflection data, obtained in the region of the hole, were used to delineate the volcanic-sedimentary layers of the Pechenga structure and the intersecting faults. These studies revealed also a subhorizontal boundary at a depth of 7 km of uncertain origin. Core sampling implies that this boundary is associated with a change in core pressure, with higher porosity and with velocity anisotropy. A change in rock fissuring and in the fluid regime correlates with a velocity inversion zone reached by the hole at a depth of 4.5 km.