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<div role="document"> <div id="Item.MessagePartBody" class="_rp_T4"> <div id="Item.MessageUniqueBody" class="_rp_U4 ms-font-weight-regular ms-font-color-neutralDark rpHighlightAllClass rpHighlightBodyClass"> <div> <div> <div> <div>We report on a Bayesian (i.e., probabilistic) inversion for the shear-wave velocity structure of the Reykjanes peninsula, SW Iceland. Travel times of Rayleigh waves traversing the peninsula served as input to the probabilistic algorithm. These Rayleigh waves were retrieved through the application of seismic interferometry to yearlong recordings of ambient seismic noise. The Reykjanes peninsula is well placed for this technique because it is surrounded by ocean, which implies a relatively uniform seismic noise illumination; the latter being a condition for accurate interferometric surface wave retrieval. The Bayesian algorithm uses a variable model parametrization by employing Voronoi cells in conjunction with a reversible jump Markov chain Monte Carlo sampler. The algorithm is entirely data-driven, meaning that, contrary to conventional deterministic tomographic inversions, the user does not need to define any regularization or parameterization parameters to solve the inverse problem.</div> <div> </div> <div>The geology in the area of interest is characterized by four NE-SW trending volcanic systems, orientated oblique to the divergent plate boundary cutting across the Reykjanes Peninsula. These are from west to east; Reykjanes, Svartsengi, Fagradalsfjall and Krýsuvík, of which all except Fagradalsfjall host a known high-temperature geothermal field. We observe relatively high shear wave velocity patches close to the Earth’s surface (top two kilometers) at the location of these known high-temperature fields. These high velocity anomalies invert to relatively low shear wave velocities (in comparison to shear wave velocities in the same horizontal plane) at depths greater than 3 km. The latter low-velocity anomalies are relatively small below Reykjanes and Svartsengi. At depths of 5 to 8 km, a low-velocity anomaly extends horizontally below Reykjanes and Svartsengi, correlating relatively well with the inferred brittle-ductile transition below the high-temperature fields at 4-5 km depth. The low-velocity anomaly below Krýsuvík is much larger and coincides with a deep-seated low electrical resistivity anomaly. Horizontally, it coincides with the center of an inflation source at 4–5 km depth. For example, in 2010 this resulted in an uplift exceeding 50 mm/year, but several periods of alternating uplift and subsidence associated with increased seismicity have been observed in Krýsuvík since 2009. Our results both confirm and add details to previous models obtained in the area. Our study demonstrates the potential of Bayesian surface wave inversion as a complementary geophysical tool for geothermal exploration.</div> </div> </div> </div> </div> </div> </div> <div class="_rp_65"> <div class="_rp_75 ms-bg-color-neutralLighter"> </div> </div>

A geomechanics program for wellbore stability analysis has been developed consisting of two modules: an analytical-based solution and a numerical-based solution. In the first part, input data are imported, including petrophysical well logs, pressure data, formation well tops, and a well path. Lithology intervals are set with proper prediction equations to calculate rock mechanical properties based on laboratory tests. In-situ stress and pore pressure are determined using different methods, including the poroelastic plane strain model and stress polygon. From the theory of plane strain, new equations are solved to determine horizontal tectonic strains (ε h , ε H ) from drilling events such as total mud loss and breakout during drilling. Safe mud weight bounds are calculated through depth and in different azimuths and inclinations applying the Mohr-Coulomb and the Mogi-Coulomb failure criteria. The latter underestimated the minimum mud weight to prevent wellbore breakout. The transversely vertical isotropy of shale formation is programmed with multiple stress transformations via the weak-plane method. In the second module, a 3D model around the wellbore is discretized with hexahedral eight-point elements and programmed using the finite-element (FE) method. Rock mechanical property and displacement boundary conditions are applied to solve FE equations. Stress from the numerical model matched to the Kirsch model and results show that maximum stress concentration around the wellbore corresponds to the wellbore breakout, which has analytically been established. A new well plan across the 3D model was examined to obtain the safe mud weight bounds and results were in agreement with the analytical calculations.

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