Super-deep drilling

The first super-deep drilling to 12.8 km in the Kola peninsula of Russia verified the effect of de-consolidation (the reverse of consolidation) in rocks at a depth >5 km (Dortman, 1992). This effect results from the increase of fracture content and porosity of rocks at depth. 

A mid-crustal boundary, the Conrad Discontinuity, is occasionally observed at a depth of about 10-20 km. Where present, it represents a discrete increase in seismic velocity with Vp increasing from about 6.1 km/sec to about 6.4-6.7 km/sec. It is often absent, or poorly constrained by seismic data, especially in shield areas. Sometimes it is gradational over several kilometers. Curiously, the super-deep drill holes have failed to identify many of the discontinuities that were based on interpretations of the geophysical evidence. 

For sites 417/418, the fluxes are calculated from the super composite, which is dominantly composed of materials from sites 417 and 418A and offer a grand average for these sites. In its upper 200 m, this composite includes about 20% of the oxidatively weathered materials from site 417A. Sites 417/418 and 735 are the only deep drill holes displaying average recovery rates >70%, allowing a quantitative reconstruction of... 

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