Microseismic data contain a great deal of subsurface information. In this work, I show that the presence of fluids in hydraulic fracturing experiments, geothermal exploitation, or fluid pathways at natural fault systems, can result in regions of high reflectivity. Appropriate analysis of reflected seismic signals yields additional information about the subsurface. I present an approach for quantitative evaluation of reflections within microseismic waveforms, which allows us to better characterize the subsurface.
I present a theoretical model, which explains that the presence of fluid within a fracture can produce relatively high reflection coefficients. Furthermore, I give an analytical solution for the reflection coefficient as a function of the elastic parameters of the fracture itself and the surrounding rock matrix. I verify this theoretical solution through numerical modeling.
I demonstrate an approach for the extraction of reflection coefficients from microseismic waveform data, and proceed to apply this approach to three real data sets. In the first example I evaluate event clusters from the Basel Enhanced Geothermal System, which occur and are recorded in a homogeneous and isotropic granitic rock environment. In this relatively 'simple' acquisition geometry, I evaluate one example cluster. I extract an apparent reflection coefficient directly from the waveforms. I then image the reflected waveforms and locate the structure which is illuminated by the cluster. From the locations of the structure I calculate correction terms which account for the changes in amplitude due to different geometrical spreading and attenuation of the direct and reflected wave. I also account for changes in amplitude caused by the double couple radiation pattern of the event. Amplitudes from the event to the receiver are typically different than from the event to the reflector. By including these corrections I find the true reflection coefficient to be R=0.13. Through my theoretical analysis, this value yields an effective fracture width of 0.05m.
In the second real data example I apply the workflow to a single microseismic event recorded at the San Andreas Fault by a receiver array. In order to constrain the width of the imaged reflector, I suggest evaluation of additional events, using the full heterogeneous velocity model.
In the last real data example I present a pre-study with the aim to extract reflection coefficients from wave fields recorded in a heterogeneous anisotropic environment at the Horn River Basin. This study is required in order to interpret the complex wave field properly, and to identify reflections from hydraulic fractures.
This work shows that it is feasible to extract and interpret reflection coefficients at hydraulic fractures. The procedure outlined herein demonstrably works for simple cases, and is also applicable to more complex experiments.
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