Intrinsic and Scattering Attenuation as Derived from Fluid Induced Microseismicity at the German Continental Deep Drilling Site

fielitz, d. ; wegler, u.;

Hydraulically induced microseismicity is used to study high-frequency attenuation properties (6–72 Hz) in an enhanced geothermal system. Intrinsic and scattering attenuation are separated by jointly inverting seismogram envelopes for structural parameters, source and site effects. Modelling of synthetic envelopes is based on radiative transfer theory. To speed up inversion, an analytical solution of the radiative transfer equation for a 3-D isotropic scattering medium is implemented. In order to compensate for the actual anisotropic scattering, a smoothing algorithm is applied to introduce envelope broadening and peak delay. The approach is tested with seismic data from four fluid-induced earthquakes (Mw ≤ 1) recorded by a temporary
seismic network at the German Continental Deep Drilling (KTB) site at epicentral distances of less than 20 km. Full S-wave envelopes are inverted in 12 overlapping frequency bands with centre frequencies between 1.5 and 72 Hz.With data sampling at 200 Hz and high-frequency Swave sources, attenuation estimates are obtained for the rarely probed frequency range between 30 and 70 Hz. From the inversion, we infer average values of the transport scattering coefficient g∗, and the intrinsic absorption parameter b, as well as corresponding quality factors Qs and Qi. By comparison with attenuation estimates from regions with different tectonic activities,
we see that both Qs and Qi for the investigated geothermal region fit best tomoderate scattering and intrinsic regimes as obtained in tectonically active regions. A comparison with a regional attenuation model for southern Germany proves that attenuation estimates are scale-dependent. To compare intrinsic and scattering attenuation in the KTB region the transport mean free path (TMFP) and the absorption length (la) are calculated. For both, we find a clear frequency dependence proportional to f −0.8 (TMFP) and f −0.3 (la). TMFP decreases from 340 km at 6 Hz to 60 km at 72 Hz, whereas absorption length drops from 40 to 20 km, respectively. Thus, intrinsic absorption dominates over scattering attenuation by at least one order of magnitude. The influence of scattering becomes more significant towards higher frequencies. Moreover, comparing the apparent attenuation (inverse sum of TMFP and la) to values estimated with the spectral ratio technique, achieves a good agreement with mean deviations in the order of 3–5 per cent. From the frequency dependence of TMFP, it can be inferred that a von Karman-type
of random medium with a Hurst exponent of κ = 0.11 is a good model for representing the stimulated reservoir at the KTB. The fractal distribution of scatterers agrees well with results derived from independent analysis of acoustic logs.