@article{EiblMuellerWalteretal.2021, author = {Eibl, Eva P. S. and M{\"u}ller, Daniel and Walter, Thomas R. and Allahbakhshi, Masoud and Jousset, Philippe and Hersir, Gylfi P{\´a}ll and Dahm, Torsten}, title = {Eruptive cycle and bubble trap of Strokkur Geyser, Iceland}, series = {Journal of geophysical research : JGR. B: Solid earth}, volume = {126}, journal = {Journal of geophysical research : JGR. B: Solid earth}, number = {4}, publisher = {Wiley}, address = {Hoboken, NJ}, issn = {2169-9313}, doi = {10.1029/2020JB020769}, pages = {20}, year = {2021}, abstract = {The eruption frequency of geysers can be studied easily on the surface. However, details of the internal structure including possible water and gas filled chambers feeding eruptions and the driving mechanisms often remain elusive. We used a multidisciplinary network of seismometers, video cameras, water pressure sensors and one tiltmeter to study the eruptive cycle, internal structure, and mechanisms driving the eruptive cycle of Strokkur geyser in June 2018. An eruptive cycle at Strokkur always consists of four phases: (1) Eruption, (2) post-eruptive conduit refilling, (3) gas filling of the bubble trap, and (4) regular bubble collapse at shallow depth in the conduit. For a typical single eruption 19 +/- 4 bubble collapses occur in Phase 3 and 8 +/- 2 collapses in Phase 4 at a mean spacing of 1.52 +/- 0.29 and 24.5 +/- 5.9 s, respectively. These collapses release latent heat to the fluid in the bubble trap (Phase 3) and later to the fluid in the conduit (Phase 4). The latter eventually reaches thermodynamic conditions for an eruption. Single to sextuple eruptions have similar spacings between bubble collapses and are likely fed from the same bubble trap at 23.7 +/- 4.4 m depth, 13-23 m west of the conduit. However, the duration of the eruption and recharging phase linearly increases likely due to a larger water, gas and heat loss from the system. Our tremor data provides documented evidence for a bubble trap beneath a pool geyser.}, language = {en} } @article{NiemzDahmMilkereitetal.2021, author = {Niemz, Peter and Dahm, Torsten and Milkereit, Claus and Cesca, Simone and Petersen, Gesa Maria and Zang, Arno}, title = {Insights into hydraulic fracture growth gained from a joint analysis of seismometer-derived tilt signals and scoustic emissions}, series = {Journal of geophysical research : Solid earth}, volume = {126}, journal = {Journal of geophysical research : Solid earth}, number = {12}, publisher = {American Geophysical Union}, address = {Washington}, issn = {2169-9313}, doi = {10.1029/2021JB023057}, pages = {14}, year = {2021}, abstract = {Hydraulic fracturing is performed to enhance rock permeability, for example, in the frame of geothermal energy production or shale gas exploitation, and can potentially trigger induced seismicity. The tracking of increased permeabilities and the fracturing extent is often based on the microseismic event distribution within the stimulated rock volume, but it is debated whether the microseismic activity adequately depicts the fracture formation. We are able to record tilt signals that appear as long-period transients (<180 s) on two broadband seismometers installed close (17-72 m) to newly formed, meter-scale hydraulic fractures. With this observation, we can overcome the limitations of the microseismic monitoring alone and verify the fracture mapping. Our analysis for the first time combines a catalog of previously analyzed acoustic emissions ([AEs] durations of 20 ms), indirectly mapping the fractures, with unique tilt signals, that provide independent, direct insights into the deformation of the rock. The analysis allows to identify different phases of the fracturing process including the (re)opening, growth, and aftergrowth of fractures. Further, it helps to differentiate between the formation of complex fracture networks and single macrofractures, and it validates the AE fracture mapping. Our findings contribute to a better understanding of the fracturing processes, which may help to reduce fluid-injection-induced seismicity and validate efficient fracture formation.
Plain Language Summary Hydraulic fracturing (HF) describes the opening of fractures in rocks by injecting fluids under high pressure. The new fractures not only can facilitate the extraction of shale gas but can also be used to heat up water in the subsurface in enhanced geothermal systems, a corner stone of renewable energy production. The fracture formation is inherently accompanied by small, nonfelt earthquakes (microseismic events). Occasionally, larger events felt by the population can be induced by the subsurface operations. Avoiding such events is important for the acceptance of HF operations and requires a detailed knowledge about the fracture formation. We jointly analyze two very different data sets recorded during mine-scale HF experiments: (a) the tilting of the ground caused by the opening of the fractures, as recorded by broadband seismometers-usually deployed for earthquake monitoring-installed close to the experiments and (b) a catalog of acoustic emissions, seismic signals of few milliseconds emitted by tiny cracks around the forming hydraulic fracture. The novel joint analysis allows to characterize the fracturing processes in greater detail, contributing to the understanding of the physical processes, which may help to understand fluid-injection-induced seismicity and validate the formation of hydraulic fractures.}, language = {en} }