Seismic reflections from the crystalline crust below the Continental Deep Drilling Site KTB-modeling and inference on reflector properties

The three-dimensional (3-D) reflection-seismic data set ISO-89 3D was recorded near the deep borehole KTB in southeastern Germany. Reflections from the SE1 reflector and from the top of the Erbendorf body (EB) in the upper crystalline crust can be identified in 5–10% of the single-shot sections. The reflectors have been first identified in previous studies of stacked and migrated seismic data. In this paper the velocity and density variations of these two structures are estimated in a new way using true amplitude single-shot (vibroseis) data. The method uses the direct wave Pg as a reference phase and models the amplitude ratios of the SE1 and EB reflections to Pg. Modeling in this paper uses a combination of ray theory and the reflectivity method, and the SE1 and the top of the EB are assumed to be obliquely oriented 1-D structures. Pg modeling shows that a depth-dependent velocity function within the uppermost crystalline basement explains the amplitudes and travel times of this phase with sufficient accuracy. The largest observed amplitude ratios SE1/Pg and EB/Pg are explained by laminated models with strong velocity contrasts and with reflection coefficients of magnitude 0.1–0.2 (SE1) and 0.05–0.15 (EB). The total thickness of the reflecting zones is less than ∼300 m. Pg amplitude modeling requires low Qp factors (<100) to a depth of ∼1 km, whereas at larger depths, values of several hundred are necessary to keep the SE1 and EB velocity contrasts in realistic ranges. Both reflectors can be interpreted as cataclastic zones. For the SE1 this interpretation agrees with the view that it is a steeply dipping thrust fault which continues the tectonic Franconian Lineament into the upper crust. We assume that the EB is the fractured top of a high-velocity zone at depths below ∼10 km, known from earlier wide-angle measurements. Both reflectors have large weakly reflecting or nonreflecting parts. The SE1 is nonreflecting at the intersection with the KTB borehole