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| Titel |
Upper-crustal scattering parameters as derived from induced micro-seismicity and acoustic log data |
| VerfasserIn |
Daniel Fielitz, Ulrich Wegler |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250081742
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| Zusammenfassung |
| In deterministic seismology it is assumed, that each inhomogeneity within the traversed
medium causes a travel time shift and/or special phase in the observed seismogram. This
information may then be used to analyze the properties of the inhomogeneity (e.g. location,
size). However, this method is only successful for inhomogeneities larger than the station
separation, the Fresnel zone and the wavelength. In the Earth’s curst there exist many
small-scale heterogeneities. Stochastic models can be used to determine the most important
statistical parameters of the small-scale inhomogeneities neglecting their exact
locations. In high-frequency seismograms (> 1 Hz) information on heterogeneity and
seismic absorption is reflected by wave trains following the direct wave featuring
decreasing amplitude with increasing lapse time, know as Coda waves. Since seismic
wave propagation through a heterogeneous and absorbing medium is an extremely
complex process, it has become common practice to use seismogram envelopes instead
of complete waveforms to gain insight in the attenuation properties. Besides the
manifestation in high-frequency seismograms information on heterogeneity can be
extracted from well-logs. Borehole measurements provide detailed 1D information on
the distribution of elastic properties within the upper crust at scales from about
one meter to several kilometers. Strong random fluctuations in seismic velocity
having short wavelengths superposed on a step-like structure represent here the
deterministic and stochastic components of the crustal structure. These observations
suggest a description of the crust as a random medium with a broad spectrum of
heterogeneity.
In the framework of developing techniques for the estimation of attenuation properties in
geothermal reservoirs, as part of the German research program Geothermal Energy and
High-performance Drilling (gebo), seismogram envelope inversion and statistical analysis of
acoustic logs have been applied to data from the German Continental Deep Drilling (KTB)
project. In the present research a passive seismic data set is considered which was acquired
during a long-term hydraulic fracturing treatment at the KTB in 2000. Induced seismicity was
recorded with a temporal seismic network, consisting of 40 stations, at epicentral distances
less than 20 km. Processed seismic events have magnitudes Ml -¤ 1.0. Acoustic log data
comprise the P- and S-wave velocity distribution logged in two boreholes. In the pilot
borehole continuous data reach from the surface (28 m) to a depth of approx. 4000 m,
while for the main borehole coherent logs are available between 285 m and 7160
m.
Scattering and intrinsic attenuation, derived from micro-seismic events at the KTB,
reasonably match regional attenuation models for Southern Germany. In contrast,
scattering strength estimated from acoustic log data exceeds the regional attenuation
models by one order of magnitude. The scattering coefficient shows weak but almost
identical frequency dependence for both types of analysis that is best-described by
a power-law form. From the frequency dependence it can be inferred that a von
Kármán-type of random medium is a good model for representing the fractured
geothermal reservoir at the KTB. The estimated Hurst exponent, related to the scattering
coefficient, is also in good agreement with reference values derived for the upper
crust. |
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