Inflating Shallow Plumbing System of Bezymianny Volcano, Kamchatka, Studied by InSAR and Seismicity Data Prior to the 20 December 2017 Eruption

Explosive eruptions at steep-sided volcanoes may develop with complex precursor activity occurring in a poorly-understood magma plumbing system so that timelines and possible interactions with the geologic surrounding are often unresolved. Here we investigate the episode prior to the energetic December 20, 2017 eruption at Bezymianny volcano, Kamchatka. We compare degassing activity inferred from time-lapse camera images, seismicity and real-time seismic amplitude (RSAM) data derived from a temporary station network, as well as high-resolution InSAR displacement maps. Results show that the first changes can be identified in low-frequency seismicity and degassing at least 90 days before the eruption, while the first volcano-tectonic (VT) seismicity occurred 50 days before the eruption. Coinciding with significant changes of the RSAM, surface displacements affect the volcanic flanks at least 9 days prior to the eruption. Inversion modeling of the pre-eruptive surface deformation as well as deflation-type, co-eruptive surface changes indicate the presence of a shallow and transient reservoir. We develop a conceptual model for Bezymianny volcano initiating with deep seismicity, followed by shallow events, rockfalls, steaming and an inflating reservoir. The eruption is then associated with subsidence, caused by deflation of the same reservoir. This sequence and conceivable causality of these observations are providing a valuable contribution to our understanding of the shallow magma plumbing system beneath Bezymianny and may have relevance for volcano monitoring and early warning strategies at similar volcanoes elsewhere.

Indications of Deep Marine Fans in the Early Miocene Foredeep of Lower Austria: A Potential New Play

The petroleum province in Lower Austria resulted from the Alpine collision and the subsequent formation of the Vienna Basin. OMV is active in this area since its foundation in 1956. Several plays have been successfully tested and produced in this complex geological region. The main exploration focus is currently on the deep plays. However, this paper proposes a so far unrecognized and therefore undrilled play in a shallower level to broaden OMV's portfolio in Austria. Seismic re-interpretations of reprocessed 3D seismic data and structural reconstructions were used to review some of the existing plays and get novel ideas from improved understanding of processes. In the frontal accretion zone of the Alpine wedge, the Waschberg-Ždánice zone discoveries are limited to the frontal thrust unit and associated structures. The more internal parts of the thrust belt have only sparsely been drilled and are perceived not to have high-quality reservoir rocks. The detailed structural interpretations indicated that the foredeep axis during the Early Miocene was positioned in the thrust sheet located directly in front of the advancing Alpine wedge (comprising the eroding Rhenodanubian Flysch in its frontal part). Seismic amplitude anomalies can be interpreted to represent Lower Miocene basin floor and slope fans. Nearby wells did not penetrate these fans but drilled instead shale-dominated lithologies. Thus, the presence of potential sand-rich fans in front of the advancing alpine wedge is considered a potential new play in Lower Austria. Analogues are found in Upper Austria some 250 km to the West, where several large gas fields in Lower Miocene deposits located in front of the advancing Alpine wedge have been discovered by another operator. In that area the fans are only partly involved in the fold-thrust belt. In Lower Austria, these fans are located within the rear thrust sheet(s), providing a structural component to a mixed structural-stratigraphic trap. Two potential charge mechanism can be considered: a) biogenic gas charge from the organic matter of surrounding shales (like the Upper Austria analogues) or b) oil charge via the thrust fault planes from the Jurassic Mikulov Formation (the proven main source rock in the broader area). Our results add to the understanding of the Miocene structural-stratigraphic evolution of the Alpine collision zone. The definition of a potential new play may add significant value to OMV's upstream efforts in a very mature hydrocarbon province.

Seismic Precursors to the Whakaari 2019 Phreatic Eruption detected in Five Other Volcanoes

Volcanic eruptions that occur without warning can be deadly in touristic and populated areas. Even with real-time geophysical monitoring, forecasting sudden eruptions is difficult because their precursors are hard to recognize and can vary between volcanoes. Here, we describe a general seismic precursor signal for gas-driven eruptions, identified through correlation analysis of 18 well-recorded eruptions in New Zealand, Alaska and Kamchatka. We show that the displacement seismic amplitude ratio, a ratio between high and medium frequency volcanic tremor, has a characteristic rise in the days prior to eruptions that likely indicates formation of a hydrothermal seal that enables rapid pressurization. Applying this model to the fatal 2019 eruption at Whakaari (New Zealand), we identify pressurization in the week before the eruption, and cascading seal failure in the 16 hours prior to the explosion. This method for identifying and proving generalizable eruption precursors can help improve short term forecasting systems.

A Multi-Frequency Acoustic Investigation of Seafloor Methane Seep Sites at Omakere Ridge, Hikurangi Margin, New Zealand

<p>Omakere Ridge is an anticlinal thrust ridge in water depths of 1100–1700mon the Hikurangi Margin, east of the North Island of New Zealand, and is an area of active seafloor methane seepage associated with an extensive gas hydrate province. Methane seep sites on the Hikurangi Margin are characterised by localised buildups of authigenic carbonate and chemosynthetic seep fauna that exist on a seafloor otherwise characterised by soft, muddy sediments and provide a unique window into the workings of the gas hydrate system. Seafloor methane seeps sites on Omakere Ridge have been successfully imaged using three newly-acquired acoustic datasets: a P-CableTM high-resolution 3D seismic reflection dataset (60 Hz); a multibeam sonar backscatter dataset (12 kHz); and a ParasoundTM subbottom profiler dataset (4 kHz). Seafloor seismic amplitude and similarity maps have been derived from a preliminary shipboard post-stack migrated data cube. A pronounced acquisition artifact is manifest in the seafloor horizon slice as high- and low-amplitude stripes that alternate periodically in the crossline direction. This artifact has been removed from the seafloor horizon slice using 2D spatial frequency filtering, followed by direct sampling and stochastic removal of the very-low-frequency components in the spatial domain. The seismic amplitude map has then been transformed into a calibrated seafloor reflection coefficient map. Sonar backscatter mosaics have been created after correcting for beam pattern effects and angular variation in backscatter after taking into account the bathymetry. Several backscatter mosaics were incorporated into a stacked mosaic over the study area to attenuate random noise. The ParasoundTM sub-bottom profiler data were processed to display instantaneous amplitude and separated into 43 lines over the study area. Comparison of 3D seismic attributes, multibeam backscatter intensity and shallow subsurface reflection characteristics provides new insights into the previously unknown extent of authigenic carbonate build-ups, methane migration pathways and seep initiation mechanisms at five seep sites on Omakere Ridge. Areas of high seafloor 3D seismic reflection coefficient and high multibeam backscatter intensity are interpreted as carbonate formations of at least 6–7 m thickness, while areas exhibiting low seismic reflection coefficient and moderate/high sonar backscatter intensity are interpreted as areas where the carbonates are less developed. Anomalous high-amplitude subsurface reflections beneath the seeps in the ParasoundTM data are interpreted as buried carbonates and may indicate a previously unknown earlier phase of seepage at Omakere Ridge, but could also be caused by gas or gas hydrates. The extent of authigenic carbonates is directly related to the duration of seepage and thus provides a new proxy for the chronology of seepage at Omakere Ridge, which has proved consistent with an existing hypothesis based on the abundance of deceased and live chemosynthetic fauna at the seep sites.</p>

A Multi-Frequency Acoustic Investigation of Seafloor Methane Seep Sites at Omakere Ridge, Hikurangi Margin, New Zealand

<p>Omakere Ridge is an anticlinal thrust ridge in water depths of 1100–1700mon the Hikurangi Margin, east of the North Island of New Zealand, and is an area of active seafloor methane seepage associated with an extensive gas hydrate province. Methane seep sites on the Hikurangi Margin are characterised by localised buildups of authigenic carbonate and chemosynthetic seep fauna that exist on a seafloor otherwise characterised by soft, muddy sediments and provide a unique window into the workings of the gas hydrate system. Seafloor methane seeps sites on Omakere Ridge have been successfully imaged using three newly-acquired acoustic datasets: a P-CableTM high-resolution 3D seismic reflection dataset (60 Hz); a multibeam sonar backscatter dataset (12 kHz); and a ParasoundTM subbottom profiler dataset (4 kHz). Seafloor seismic amplitude and similarity maps have been derived from a preliminary shipboard post-stack migrated data cube. A pronounced acquisition artifact is manifest in the seafloor horizon slice as high- and low-amplitude stripes that alternate periodically in the crossline direction. This artifact has been removed from the seafloor horizon slice using 2D spatial frequency filtering, followed by direct sampling and stochastic removal of the very-low-frequency components in the spatial domain. The seismic amplitude map has then been transformed into a calibrated seafloor reflection coefficient map. Sonar backscatter mosaics have been created after correcting for beam pattern effects and angular variation in backscatter after taking into account the bathymetry. Several backscatter mosaics were incorporated into a stacked mosaic over the study area to attenuate random noise. The ParasoundTM sub-bottom profiler data were processed to display instantaneous amplitude and separated into 43 lines over the study area. Comparison of 3D seismic attributes, multibeam backscatter intensity and shallow subsurface reflection characteristics provides new insights into the previously unknown extent of authigenic carbonate build-ups, methane migration pathways and seep initiation mechanisms at five seep sites on Omakere Ridge. Areas of high seafloor 3D seismic reflection coefficient and high multibeam backscatter intensity are interpreted as carbonate formations of at least 6–7 m thickness, while areas exhibiting low seismic reflection coefficient and moderate/high sonar backscatter intensity are interpreted as areas where the carbonates are less developed. Anomalous high-amplitude subsurface reflections beneath the seeps in the ParasoundTM data are interpreted as buried carbonates and may indicate a previously unknown earlier phase of seepage at Omakere Ridge, but could also be caused by gas or gas hydrates. The extent of authigenic carbonates is directly related to the duration of seepage and thus provides a new proxy for the chronology of seepage at Omakere Ridge, which has proved consistent with an existing hypothesis based on the abundance of deceased and live chemosynthetic fauna at the seep sites.</p>

Using relative geologic time to constrain CNN-based seismic interpretation and property estimation

Three-dimensional seismic interpretation and property estimation is essential to subsurface mapping and characterization, in which machine learning, particularly supervised convolutional neural network (CNN) has been extensively implemented for improved efficiency and accuracy in the past years. In most seismic applications, however, the amount of available expert annotations is often limited, which raises the risk of overfitting a CNN particularly when only seismic amplitudes are used for learning. In such a case, the trained CNN would have poor generalization capability, causing the interpretation and property results of obvious artifacts, limited lateral consistency and thus restricted application to following interpretation/modeling procedures. This study proposes addressing such an issue by using relative geologic time (RGT), which explicitly preserves the large-scale continuity of seismic patterns, to constrain a seismic interpretation and/or property estimation CNN. Such constrained learning is enforced in twofold: (1) from the perspective of input, the RGT is used as an additional feature channel besides seismic amplitude; and more innovatively (2) the CNN has two output branches, with one for matching the target interpretation or properties and the other for reconstructing the RGT. In addition is the use of multiplicative regularization to facilitate the simultaneous minimization of the target-matching loss and the RGT-reconstruction loss. The performance of such an RGT-constrained CNN is validated by two examples, including facies identification in the Parihaka dataset and property estimation in the F3 Netherlands dataset. Compared to those purely from seismic amplitudes, both the facies and property predictions with using the proposed RGT constraint demonstrate significantly reduced artifacts and improved lateral consistency throughout a seismic survey.

4D Seismic in Subsurface CO2 Plume Monitoring – Why It Matters?

CO2 sequestration in depleted carbonate reservoir stipulate incorporation of comprehensive and trailblazing monitoring technologies. 4D time-lapse seismic is sine qua non for Monitoring, Measurement and Verification (MMV) planning to demonstrate the migration of CO2 plume within geological storage. An ingenious, adaptive and site specific MMV plan for monitoring CO2 plume is paramount to minimize possible subsurface and project integrity risks. Integration of dynamic simulation with seismic forward modeling aggrandize the capabilities of 4D seismic in CO2 sequestration projects. Depleted carbonate reservoir has been thoroughly studied and its geomechanical and geochemical modeling results were coupled into dynamic simulation. Reservoir porosity and fluid properties along with CO2 saturation and injection pressure distribution within each reservoir level were generated. The dynamic simulation results were integrated with seismic forward modeling to demonstrate the CO2 plume migration and its impact on seismic amplitude. Fluid acoustic properties were computed for carbonate reservoir using FLAG method. Selection of wells was based on availability of superior quality acoustic logs as well as those representing the reservoir best. Gassmann fluid substitution exercise was carried using dry rock modeling. Several scenarios were generated, and results were analyzed to demonstrate the effect of CO2 saturation and pressure build-ups within reservoir on the seismic amplitude due to continuous CO2 injection. Synthetic seismic AVO gathers were generated for angles ranging from 5 to 50 degree. Near, Mid and Far seismic amplitude response at the top of carbonate reservoir were analyzed with respect to in-situ condition for each scenario. Results reveal that CO2 saturation as low as 25 - 30% in depleted carbonate reservoir can be distinguished from 4D time-lapse seismic. With continuous CO2 injection, the reservoir pressure increases and this in turn controls the properties of both in-situ and injected fluids. The gradual changes in fluid properties and their impact on bulk acoustic properties of reservoir were modeled to assess the feasibility of using 4D seismic as a predictive tool for detection of localized and provincial pressure build-ups. Modeling results show that although observed changes in amplitude on synthetic gathers were subtle, it is expected that 4D seismic with high signal-to-noise ratio possibly be able to image such localized pressure build-ups. To monitor CO2 plume migration as well as localized pressure build-ups, we recommend acquiring multi-azimuth (MAZ) surface seismic in combination with 3D DAS-VSP for superior subsurface imaging. The integrated modeling approach ensures that 4D Seismic in subsurface CO2 plume monitoring is robust. Monitoring pressure build-ups from MAZ surface seismic and 3D DAS-VSP will reduce the associated risks.