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

Abstract

[1] 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 P g as a reference phase and models the amplitude ratios of the SE1 and EB reflections to P g . 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. P g 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/P g and EB/P g 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. P g amplitude modeling requires low Q p 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.

Superdeep vertical seismic profiling at the KTB deep drill hole (Germany): Seismic close‐up view of a major thrust zone down to 8.5 km depth

“…This is evident also from reflectivity coefficients for which maximum values of 0.35 ± 0.15 were observed for arrivals at 6 km depth [ Frank , 2002]. Zillmer et al [2002] derived reflection coefficients of the same order of magnitude for the SE1 structure by analyzing surface seismic data. These coefficients can only be explained by extreme velocity contrasts plus thin layer tuning effects such as those suggested by the velocity function derived from the sonic log [ Trela , 2003; C. Trela et al, Frequency dependence of PS reflections: Fine structure of the SE1–thrust fault system at the KTB superdeep drill hole, submitted to Journal of Geophysical Research , 2004].…”

Characterization of seismic reflections from faults in a crystalline environment, Schneeberg, Germany

“…In another case study, Zillmer, Müller, and Stiller () analysed a 3D seismic data set from the German Continental Deep Drilling site (KTB) in order to relate petrophysical parameters to the observed reflectors. By using the true amplitudes of the seismic data set, we followed the methodology of Zillmer et al .…”

“…By using the true amplitudes of the seismic data set, we followed the methodology of Zillmer et al . () to derive the approximate reflection coefficients and an internal fault zone structure. Laminated models with strong velocity contrast and maximum fault zone widths of 300 m produced the highest reflection coefficients of 0.1 – 0.2.…”

Pressure-dependent seismic reflection amplitude changes in crystalline crust: lessons learned at the Continental Deep Drilling Site (KTB)

“…Amplitude studies yielded lithological results with regard to reflection coefficients of the SE1 shear zone (e.g. Harjes & Janik 1994; Zillmer et al 2002). With respect to the pronounced SE1 shear zone, Zillmer et al (2002) identify reflection coefficients of up to 0.15.…”

“…Harjes & Janik 1994; Zillmer et al 2002). With respect to the pronounced SE1 shear zone, Zillmer et al (2002) identify reflection coefficients of up to 0.15. However, such relatively high values are limited to particular areas of the SE1 shear zone (Figs 2 and 3).…”