Seismic studies around the Kola Superdeep Borehole, Russia

The Kola Superdeep Borehole (SG-3) provided an ideal opportunity to test hypotheses on the origins of crustal reflections and on the presence and seismic expression of fluids in the upper crust. The alternative sources of crustal reflections include compositional changes, shear zones, fluids, and metamorphic facies changes, all of which are represented at the well. Both the 38-km-long CDP section and the borehole VSPs in the range 2.2-6.0 km demonstrate the presence of reflections from dipping compositional layering, shear zones, and fluid-filled zones. Subhorizontal reflectivity zones are interpreted as horizontal fluid-filled fracture-type reservoir rocks. Results suggest the presence of fluids down to a depth of at least 12 km in the upper crust; the presence of these fluids lowering seismic velocity causes estimates of upper crustal composition to be too felsic. (C) 1998 ELsevier Science B.V.