Martin Schoenball

New galactic open cluster candidates from DSS and 2MASS imagery

An inspection of the DSS and 2MASS images of selected Milky Way regions has led to the discovery of 66 stellar groupings whose morphologies, color-magnitude diagrams, and stellar density distributions suggest that the se objects are possible open clusters that do not yet appear to be listed in any catalogue. For 24 of these groupings, which we consider to be the most likely to be candidates, we provide extensive descriptions on the basis of 2MASS photometry and their visual impression on DSS and 2MASS. Of these cluster candidates, 9 have fundamental parameters determined by fitting the color- magnitude diagrams with solar metallicity Padova isochrones. An additional 10 cluster candidates have distance moduli and reddenings derived from K magnitudes and (J− K) color indices of helium-burning red clump stars. As an addendum, we also provide a list of a number of apparently unknown galactic and extragalactic objects that were also discovered during the survey.

Change of stress regime during geothermal reservoir stimulation

Earthquakes are induced by man-made changes of the stress field by injection or withdrawal of fluids in hydrocarbon production, geothermal exploitation, and wastewater disposal. However, the actual perturbation of the stress field and stress release by injection-induced seismicity remains largely unknown. We provide evidence for currently not understood hydromechanical processes after shut-in of the well. We invert earthquake focal mechanisms from a massive stimulation to invert for stress resolved in time and depth to obtain changes of the stress orientation and magnitude. Prior information about fracture orientations from well logs is taken into account. Comparison with independent stress measures reveals that stresses obtained from inversion of fluid-induced seismicity are highly perturbed and not representative of the initial stress field. The horizontal stresses change by tens of megapascals, turning the stress regime from transitional normal faulting/strike-slip faulting to pure normal faulting. The observed stress changes are attributed to large-scale aseismic deformation.

Differentiating induced and natural seismicity using space‐time‐magnitude statistics applied to the Coso Geothermal field

A remarkable characteristic of earthquakes is their clustering in time and space, displaying their self-similarity. It remains to be tested if natural and induced earthquakes share the same behavior. We study natural and induced earthquakes comparatively in the same tectonic setting at the Coso Geothermal Field. Covering the preproduction and coproduction periods from 1981 to 2013, we analyze interevent times, spatial dimension, and frequency-size distributions for natural and induced earthquakes. Individually, these distributions are statistically indistinguishable. Determining the distribution of nearest neighbor distances in a combined space-time-magnitude metric, lets us identify clear differences between both kinds of seismicity. Compared to natural earthquakes, induced earthquakes feature a larger population of background seismicity and nearest neighbors at large magnitude rescaled times and small magnitude rescaled distances. Local stress perturbations induced by field operations appear to be strong enough to drive local faults through several seismic cycles and reactivate them after time periods on the order of a year.

Quantifying the heterogeneity of the tectonic stress field using borehole data

The heterogeneity of the tectonic stress field is a fundamental property which influences earthquake dynamics and subsurface engineering. Self-similar scaling of stress heterogeneities is frequently assumed to explain characteristics of earthquakes such as the magnitude-frequency relation. However, observational evidence for such scaling of the stress field heterogeneity is scarce. We analyze the local stress orientations using image logs of two closely spaced boreholes in the Coso Geothermal Field with sub-vertical and deviated trajectories, respectively, each spanning about 2 km in depth. Both the mean and the standard deviation of stress orientation indicators (borehole breakouts, drilling-induced fractures and petal-centerline fractures) determined from each borehole agree to the limit of the resolution of our method although measurements at specific depths may not. We find that the standard deviation in these boreholes strongly depends on the interval length analyzed, generally increasing up to a wellbore log length of about 600 m and constant for longer intervals. We find the same behavior in global data from the World Stress Map. This suggests that the standard deviation of stress indicators characterizes the heterogeneity of the tectonic stress field rather than the quality of the stress measurement. A large standard deviation of a stress measurement might be an expression of strong crustal heterogeneity rather than of an unreliable stress determination. Robust characterization of stress heterogeneity requires logs that sample stress indicators along a representative sample volume of at least 1 km.

A Systematic Assessment of the Spatiotemporal Evolution of Fault Activation Through Induced Seismicity in Oklahoma and Southern Kansas: Induced Seismicity Evolution in Oklahoma

Author(s): Schoenball, M; Ellsworth, WL | Abstract: ©2017. American Geophysical Union. All Rights Reserved. Much of Oklahoma and southern Kansas has seen widespread seismic activity in the last decade that is attributed to large-scale wastewater disposal into the Arbuckle group. Using a waveform-relocated earthquake catalog, we perform a systematic study of the activity on several hundreds of identified faults. We use 93 sequences with at least 30 events for a detailed analysis of their spatiotemporal evolution. For most awakened faults, seismicity tends to initiate at shallower depth and migrates deeper along the faults as the sequence proceeds. No major sequence starts with the largest earthquake, and many sequences initiate months before they rise to peak activity. We study temporal clustering as a means to quantify earthquake interactions. Some sequences show no temporal clustering similar to Poissonian background seismicity but at much higher rate than the natural background. Other sequences exhibit strong temporal clustering akin to main shock-aftershock sequences. We conclude that once initiated by anthropogenic forcing, portions of the activated faults in the Oklahoma/Kansas area are close enough to failure to continue failing through earthquake interactions. In many sequences, including those with the largest earthquakes, seismicity continues within the previously activated region rather than by growing the activated area. Therefore, monitoring seismicity with a low magnitude threshold and high location precision has the potential to detect minor activity as it initiates failure on specific faults and thus provide time to take actions to mitigate the occurrence of potentially damaging earthquakes.

Changes of Coulomb Failure Stress during Stimulation

For the prediction of future reservoir performance, knowledge of the spatio-temporal evolution of the geothermal reservoir in response to stimulation and production is a key issue. During the project for developing an Enhanced Geothermal System at Soultz-

Effects of poroelastic coupling on microseismic signatures

SUMMARY We develop an approach to model microseismicity triggered by fluid injection on the basis of the theory of poroelasticity accounting for the external stress field. Consideration of the fully coupled poroelastic field equations enables us to apply a Coulomb failure criterion using pore fluid pressure and stress tensor as well as the coefficient of friction. The poroelastic fields are calculated with the finite element method simulating fluid injection with constant injection rate into a 2D domain. The influence of diffusivity, injection rate and stress field on the occurrence of microseismicity is analyzed and compared to simulations based on pore fluid pressure diffusion only. We find that the spatio-temporal evolution of microearthquakes is little sensitive to changes in diffusivity but strongly sensitive to changes of injection rate. Moreover, we show that an external stress field with unequal principal stresses causes elongated microseismic clouds. These clouds are indistinguishable from those generated in poroelastic solids with anisotropic diffusivity for equal principal stresses. This shows that microseismicity distributions are dependent on the coupling of pore fluid pressure and stress field.

Stress-induced Spatial Anisotropy of Microseismicity Distributions

We develop an approach to model microseismicity triggered by fluid injection on the basis of the theory of poroelasticity accounting for the external stress field. Consideration of the fully coupled poroelastic field equations enables us to apply a Coulom