Moment-Tensor Determination by Nonlinear Inversion of Amplitudes

International audienceWe propose an inversion method of the direct wave amplitudes P, SV, and SH to determine the general moment tensor (MT) and estimate the associated uncertainties. This method is a generalization of the one we developed to determine the double-couple (DC) or fault-plane solution from sparse observations in reservoir contexts (Godano et al., 2009). Like the previous one, it is based on a simulated annealing inversion algorithm. First, we test the reliability of this new method on synthetic data. The inversion allows retrieval of the expected moment tensor, but the analysis of the uncertainty associated with the solution shows that the resolution of the inversion is sensitive not only to the direct wave amplitudes but also to uncertainties in the velocity model and the event location. Next, we test the method on four induced microearthquakes recorded by the Soultz-sous-Forêts hot-dry-rock (HDR) reservoir network. The inversion correctly converges for three events, and the obtained MTs display a dominant DC part (70 to 90%). The uncertainty associated with the non-DC part is relatively high. This prevents us from determining if the small non-DC part is significant or an artifact related to noise in the data and/or uncertainties in the velocity model and at the event location

Fast Method for Amplitude Computation on High Contrast Velocity Models

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Assessment of focal mechanisms of microseismic events computed from two three-component receivers: application to the Arkema-Vauvert field (France)

International audienceWe have developed a method that enables computing double-couple focal mechanisms with only a few sensors. This method is based on a non-linear inversion of the P, Sv and Sh amplitudes of microseismic events recorded on a set of sensors. The information brought by the focal mechanism enables determining the geometry of the rupture on the associated geological structure. It also provides a better estimate of the conventional source parameters. Full analysis has been performed on a data set of 15 microseismic events recorded in the brine production field of Vauvert. The microseismic monitoring network consisted of two permanent tools and one temporary borehole string. The majority of the focal mechanisms computed from both permanent tools are similar to those computed from the whole network. This result indicates that the double-couple focal mechanism determination is reliable for both permanent 3C receivers in this field

Spatial and temporal evolution of a microseismic swarm induced by water injection in the Arkema-Vauvert salt field (southern France)

This study investigates a microseismic swarm induced by injection operations in the Arkema-Vauvert salt field. The seismic activity in this field is monitored only by two permanent 3-component stations deployed in two wells. This study focuses on a period of 21 months (2004 January2005 September) during which 1214 seismic events are located. The seismic activity is divided into three periods correlating with the water injection operations, highlighting a migration of the seismicity toward a thrust fault connecting the injection well and the production well. A waveform analysis reveals S-wave anisotropy, and focal mechanisms are computed using P, Sv and Sh amplitudes manually measured on anisotropy-corrected seismograms. First, synthetic resolution tests assess the reliability of the focal mechanisms determination from the two 3-component stations deployed in the field. Synthetic data are generated for 1056 earthquakes with various focal mechanisms and are perturbed with noise. The results indicate that the type of focal mechanism is correctly retrieved for 74 per cent of the synthetic earthquakes, but the uncertainties of the strike and rake are significant (from 15 to 45). Next, the focal mechanisms are computed for 532 real earthquakes. The solutions primarily correspond to a dip-slip/thrust fault type with subvertical NESW and subhorizontal NS to NWSE nodal planes. Correlations between the focal mechanisms and the spatio-temporal distribution of the seismic activity are noteworthy. The study shows it is possible to reliably retrieve doublecouple focal mechanisms for some faulting geometries with two 3-component seismological stations. However, the reliability of the focal mechanism retrieval depends on the station configuration. Therefore, the addition of further stations would improve the results