Observation of P-wave Crustal Anistropy due to Material Tilt of Foliated Crustal Rocks in Moving-Source Seismic Profiling, KTB Borehole, Germany

Surface exposures of metamorphic terranes show planar structures such as slaty cleavage, schistosity, and foliation which have developed in response to penetrative flow and are often pervasive for tens to hundreds of kms. In such rocks, preferred orientations of minerals are widely prevalent and the crust is highly anisotropic to seismic waves. The physical processes needed to produce such preferred orientation of minerals are associated with deformation and metamorphism. In these cases, the orientations and amounts of anisotropy may serve as proxies for crustal deformation. The KTB deep borehole is located in the western margin of the Bohemian massif in southern Germany. This massif has complex and heterogeneous crystalline crust composed of granites and a tectonometamorphic unit containing paragneisses, orthogneisses, and amphibolites. These metamorphic rocks are in steeply dipping layers (60-90 degrees) with a penetrative foliation (50-80 degrees), all with regional uniform NW-SE strike and bulk tilt. A recent seismic experiment at the KTB borehole is "moving source" seismic profiling. This experiment was designed to look at azimuthal variation in the region immediately surrounding the borehole. The experiment consists of six 7.5-km surface profiles radiating from the borehole oriented approx. 45 azimuthal degrees apart. A seismic vibrator was used along each profile at 150 m spacing. The source energy was recorded at borehole depths of 0, 3.8, 7.8, and 8.5 km. From these MSP data an analysis of average upper crustal P-wave velocity was made using P arrival times and source-borehole geometries. Map contours of average Vp based on surface source position are to first order elliptical in shape with fast axis oriented NW-SE, as would be expected for this regional metamorphic block with vertical foliation. The elliptical contours are not symmetric about the borehole, however. This asymmetry can be explained by the regional steep tilt of the metamorphic textures. We show the MSP seismic data, the quantification of the asymmetric P-wave velocity contours, and Christoffel equations for tilted media to explain the asymmetric contours.