Probing crustal anisotropy by receiver functions at the deep continental drilling site KTB in Southern Germany

Abstract

Seismic anisotropy is a unique observational tool for remotely studying deformation and stress within the Earth. Effects of anisotropy can be seen in seismic data; they are due to mineral alignment, fractures or layering. Seismic anisotropy is linked to local stress and strain, allowing modern geophysics to derive geomechanical properties from seismic data for supporting well planning and fracking. For unravelling anisotropic properties of the crust, the teleseismic receiver functions methodology has started to be widely applied recently due to its ability in retrieving the three‐dimensional characteristics of the media sampled by the waves. The applicability of this technique is tested here by a field test carried out around the Kontinental Tiefbohrung site in southeastern Germany. We compare our results to previous investigations of the metamorphic rock pile of the Zone Erbendorf‐Vohenstrauss, drilled down to 9 km depth, which sampled an alternating sequence of paragneiss and amphibolite, in which a strong foliation has been produced by ductile deformation. The application of the receiver functions reveals the presence of two distinct anisotropic layers within the metamorphic rock pile at 0–4 km and below 6 km depth, with up to 8% anisotropy; the depth of these two layers corresponds to the location of mica‐rich paragneiss which show intense foliation, and finally proves the relation between the signal in the receiver functions, rock texture and presence of cracks. We have now the capability of providing insights from passive seismic data on geomechanical properties of the rocks, useful for geological exploration and engineering purposes, which will help influencing expensive drilling decisions thanks to future application of this seismic technique.

Deformation Microstructures of Phyllite in Gunsan, Korea, and Implications for Seismic Anisotropy in Continental Crust

“…Because the mineral assemblages of the phyllite samples analyzed in this study correspond to typical greenschist-facies minerals of a pelitic protolith (biotite zone [67]), it is suggested that the calculated seismic properties reflect a part of the observed seismic anisotropies in the highly deformed middle crustal zone. Tectonic processes during compressive or extensional regimes in the middle and lower crust result in ductilely deformed planar structures aligned subnormal to compressive or extensional stress, inducing strong seismic anisotropies when the rocks are rich in mica and/or amphibole [5,18,20,22,60,109]. It is particularly important below the depth of microcrack closure (approximately 200-250 MPa [15,16,18]), where the plastically deformed shear fabrics and aligned minerals play an important role in controlling the seismic properties.…”

“…Tectonic processes during compressive or extensional regimes in the middle and lower crust result in ductilely deformed planar structures aligned subnormal to compressive or extensional stress, inducing strong seismic anisotropies when the rocks are rich in mica and/or amphibole [5,18,20,22,60,109]. It is particularly important below the depth of microcrack closure (approximately 200-250 MPa [15,16,18]), where the plastically deformed shear fabrics and aligned minerals play an important role in controlling the seismic properties.…”

“…Many studies using the receiver function technique [9,18], acoustic wave velocity measurements in laboratory settings [15,16,19], and fabric analysis on naturally [5,[19][20][21][22][23] and experimentally [24,25] deformed rock samples have suggested that the layering and lattice preferred orientation (LPO) of anisotropic minerals is one of the important factors controlling the seismic anisotropy in the middle crust below the depth of microcrack closure (P ≈ 150-250 MPa [15,16,18,26]). Mica and amphibole groups are major constituent minerals in continental crust, which are elastically anisotropic [27][28][29].…”

Depth‐Dependent Anisotropy Along Northwest Segment of the North Anatolian Fault Zone: Evidence for Paleo‐Tectonic Structures Contributing to Overall Complexity

“…Our aim was to select the best-fitting model by minimizing the misfit of all estimated parameters. To assess reliability, we calculated the standard deviation of model parameters from an ensemble of models with misfits less than 1.1 times of the misfit of the best-fit model (Bianchi & Bokelmann, 2019).…”

“…Local S-wave polarization properties along northwestern segments of the NAFZ suggests that both stress-and structure induced mechanisms cause seismic anisotropy in the upper 8-10 km of the crust (Eken, Bohnhoff, et al, 2013;Hurd & Bohnhoff, 2012). The joint interpretation of receiver function analyses (e.g., harmonic decomposition, and NA-based receiver functions modeling), for instance, can reveal the actual orientation and geometry of anisotropic structures at various depths of the crust and uppermost mantle (Bianchi & Bokelmann, 2019;Frederiksen & Bostock, 2000;Licciardi & Piana-Agostinetti, 2016). Licciardi et al (2018) reported an asymmetry in the variation of crustal anisotropy parameters across the central NAFZ using a harmonic decomposition approach that exploits directional dependence of receiver functions.…”