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

Abstract

A B S T R A C TWe present an approach for analysing seismic reflections from faults in a crystalline hard rock environment. We analysed 3D seismic reflection data for geothermal reservoir characterization acquired in the Erzgebirge Region, Germany. The seismic image derived from this data set revealed two main features: a less pronounced reflector corresponding to a steeply dipping major fault zone Roter Kamm and a group of pronounced reflectors attributed to the existence of conjugate mineralized faults. We analysed these reflections in the pre-stack data to characterize the nature and origin of reflectivity. This was done by extracting the corresponding waveforms from the raw data and carefully pre-processing them, including amplitude correction for geometrical spreading and signal-to-noise enhancement. Reflection coefficients were derived from the pre-processed shot gathers by comparing the amplitudes of the reflected and direct waves. Synthetic waveform modelling using the reflectivity method has been performed for several model families consisting of one-dimensional velocity-depth functions with varying velocities, densities, and thicknesses of the layers. A comparison of the modelled and observed waveforms revealed that a reflection coefficient of 0.18 for the conjugate mineralized faults can be explained by single layers with high impedance contrast and a thickness between 30 m and 40 m, whereas the reflection from the Roter Kamm fault zone with a reflection coefficient of −0.23 requires a model consisting of several low-velocity layers with a total thickness of up to 100 m embedded in a high-velocity background model. These results are in accordance with the geological interpretation of these reflectors. However, the characteristics of these reflections vary significantly within the investigation area, both in terms of the reflection coefficient and the waveform, which is also in agreement with the general lateral variation of fault zone characteristics known from tectonic investigations such as geological mapping of outcrops and fabric analysis.

Sparse 3D Seismic Imaging in the Kylylahti Mine Area, Eastern Finland: Comparison of Time Versus Depth Approach

“…After some initial testing with the constant velocity model, we concluded that qualitatively best results are obtained from CBVFM among other approaches (e.g., FVM alone). This was also suggested by previous studies performed in crystalline environments (e.g., [12,13,16,30]). Therefore, here we focus on the results of the CBFVM approach.…”

“…Also, as discussed earlier, result depends on the power value "p" (Equation 4), and therefore we also tested two different values of p = 1 and p = 2 (Figure 8). was also suggested by previous studies performed in crystalline environments (e.g., [12,13,16,30]). Therefore, here we focus on the results of the CBFVM approach.…”

“…Therefore, they cannot produce accurate velocity models required for migration. Nonetheless, there are some recent successful case studies of PreSDM application to image crystalline rocks in 3D (e.g., [12,13]). Those examples employed a specialized variants of Kirchhoff PreSDM called Fresnel volume migration (FVM) [14,15] or coherency-based FVM (CBFVM) [16] to reduce artefacts related to the migration operator and consequently improve image quality.…”

Three‐dimensional focused seismic imaging for geothermal exploration in crystalline rock near Schneeberg, Germany

“…A detailed analysis of the reflection coefficients and the characteristics of both the Roter Kamm and the conjugate fault reflectors can be found in Schreiter et al . ().…”

Advanced seismic processing/imaging techniques and their potential for geothermal exploration

“…Recently, Hloušek et al (2015a) and Schreiter et al (2015) report on the processing and interpretation, respectively, of a 3D survey of approximate areal extent 10 × 13 km that was conducted in 2012 in western Erzgebirge near the city of Schneeberg, Germany (see also Ahmed et al, 2015;Lüschen et al, 2015). The aim of the project was to characterize the major fracture zones in the crystalline rock at approximately 4-5 km depth.…”