scholarly journals Improved Seismic Monitoring with OBS Deployment in the Arctic: A Pilot Study from Offshore Western Svalbard

2021 ◽  
Author(s):  
Zeinab Jeddi ◽  
Lars Ottemöller ◽  
Mathilde Sørensen ◽  
Sara Rezaei ◽  
Steven Gibbons ◽  
...  
Keyword(s):  
Waveform Inversion ◽  
Arctic Region ◽  
The Arctic ◽  
Spreading Ridge ◽  
Ocean Bottom ◽  
Low Velocity ◽  
Arrival Times ◽  
Regional Seismicity ◽  
Velocity Anomaly ◽  
Significant Difference

The mid-ocean ridge system is the main source of earthquakes within the Arctic region. The earthquakes are recorded on the permanent land-based stations in the region, although smaller earthquakes remain undetected. In this study, we make use of three Ocean Bottom Seismographs (OBSs) that were deployed offshore western Svalbard, along the spreading ridges. The OBS arrival times were used to relocate the regional seismicity using a Bayesian approach, which resulted in a significant improvement with tighter clustering around the spreading ridge. We also extended the regional magnitude scales for the northern Atlantic region for OBSs by computing site correction terms. Besides location and magnitude improvement, the OBS network was able to detect hundreds of earthquakes, mostly with magnitude below Mw=3, including a swarm activity at the Molloy Deep. Our offshore observations provide further evidence of a low velocity anomaly offshore Svalbard, at the northern tip of Knipovich ridge, that was previously seen in full waveform inversion. We conclude that even a single permanent OBS near the ridge would make a significant difference to earthquake catalogs and their interpretation.

scholarly journals Improved Seismic Monitoring with OBS Deployment in the Arctic: A Pilot Study from Offshore Western Svalbard

2021 ◽  
Author(s):  
Zeinab Jeddi ◽  
Lars Ottemöller ◽  
Mathilde B. Sørensen ◽  
Sara Rezaei ◽  
Steven J. Gibbons ◽  
...  
Keyword(s):  
Waveform Inversion ◽  
Arctic Region ◽  
The Arctic ◽  
Spreading Ridge ◽  
Ocean Bottom ◽  
Low Velocity ◽  
Arrival Times ◽  
Regional Seismicity ◽  
Velocity Anomaly ◽  
Significant Difference

Abstract The mid-ocean ridge system is the main source of earthquakes within the Arctic region. The earthquakes are recorded on the permanent land-based stations in the region, although, smaller earthquakes remain undetected. In this study, we make use of three Ocean Bottom Seismographs (OBSs) that were deployed offshore western Svalbard, along the spreading ridges. The OBS arrival times were used to relocate the regional seismicity, using a Bayesian approach, which resulted in a significant improvement with tighter clustering around the spreading ridge. We also extended the regional magnitude scales for the northern Atlantic region for OBSs, by computing site correction terms. Besides location and magnitude improvement, the OBS network was able to detect hundreds of earthquakes, mostly with magnitude below Mw 3, including a swarm activity at the Molloy Deep. Our offshore observations provide further evidence of a low-velocity anomaly offshore Svalbard, at the northern tip of Knipovich ridge that was previously seen in full-waveform inversion. We conclude that even a single permanent OBS near the ridge would make a significant difference to earthquake catalogs and their interpretation.

Multiparameter full-waveform inversion of 3D OBC data from the Valhall field

Geophysics ◽  
2020 ◽  
pp. 1-122
Author(s):  
Nishant Kamath ◽  
Romain Brossier ◽  
Ludovic Métivier ◽  
Arnaud Pladys ◽  
Pengliang Yang
Keyword(s):  
Waveform Inversion ◽  
Oil Field ◽  
Full Waveform Inversion ◽  
Ocean Bottom ◽  
Low Velocity ◽  
Simultaneous Inversion ◽  
Velocity Anomaly ◽  
The North ◽  
Full Waveform ◽  
Quality Factor Q

Full waveform inversion (FWI) applications on 3D ocean bottom cable (OBC) data fromthe Valhall oil field in the North Sea have demonstrated the importance of appropriately ac-counting for attenuation. The Valhall field contains unconsolidated shallow sediments anda low velocity anomaly in its center - indicative of gas clouds - which have a significantattenuation imprint on the data. The challenge in which we are interested is to performtime-domain visco-acoustic 3D FWI, which requires more sophisticated tools than in thefrequency domain wherein attenuation can be incorporated in a straightforward manner.The benefit of employing a visco-acoustic, instead of a purely acoustic, modeling engineis illustrated. We show that, in the frequency band employed (2.5 - 7.0 Hz), it is betterto reconstruct velocity only keeping attenuation fixed, because simultaneous inversion ofvelocity and quality factor Q does not provide reliable Q-updates. We design an efficienttime-domain workflow combining a random source decimation algorithm, modeling usingstandard linear solid mechanisms, and wavefield preconditioning. Our results are similarto those obtained from state-of-the-art frequency-domain algorithms, at a lower computa-tional cost compared to conventional checkpointing techniques. We clearly illustrate theimprovement in terms of imaging and data fit achieved when accounting for attenuation.

scholarly journals Crustal velocity structure of the Apennines (Italy) from P-wave travel time tomography

1996 ◽  
Vol 39 (6) ◽  
Author(s):  
C. Chiarabba ◽  
A. Amato
Keyword(s):  
Velocity Structure ◽  
P Wave ◽  
Depth Interval ◽  
Low Velocity ◽  
Data Set ◽  
Arrival Times ◽  
Velocity Anomaly ◽  
Minimum Number ◽  
Velocity Images ◽  
Lower Crustal

In this paper we provide P-wave velocity images of the crust underneath the Apennines (Italy), focusing on the lower crustal structure and the Moho topography. We inverted P-wave arrival times of earthquakes which occurred from 1986 to 1993 within the Apenninic area. To overcome inversion instabilities due to noisy data (we used bulletin data) we decided to resolve a minimum number of velocity parameters, inverting for only two layers in the crust and one in the uppermost mantle underneath the Moho. A partial inversion of only 55% of the overall dataset yields velocity images similar to those obtained with the whole data set, indicating that the depicted tomograms are stable and fairly insensitive to the number of data used. We find a low-velocity anomaly in the lower crust extending underneath the whole Apenninic belt. This feature is segmented by a relative high-velocity zone in correspondence with the Ortona-Roccamonfina line, that separates the northern from the southern Apenninic arcs. The Moho has a variable depth in the study area, and is deeper (more than 37 km) in the Adriatic side of the Northern Apennines with respect to the Tyrrhenian side, where it is found in the depth interval 22-34 km.

Seismic detection of overpressuring and fracturing: An example from the Qaidam Basin, People’s Republic of China

Geophysics ◽  
1993 ◽  
Vol 58 (10) ◽  
pp. 1532-1543 ◽  
Author(s):  
Robert J. Paul
Keyword(s):  
Oil And Gas ◽  
Qaidam Basin ◽  
High Amplitude ◽  
Seismic Survey ◽  
Low Velocity ◽  
Velocity Anomaly ◽  
Deep Well ◽  
Significant Difference ◽  
Seismic Amplitudes ◽  
Definition Of

Shallow hydrocarbon reserves were discovered in 1959 in the Nan Yi Shan structure located near the western corner of the Qaidam Basin. The first successful deep well encountered an overpressured zone at 3000 m that resulted in a well blowout. To improve the structural definition of the field and delineate the overpressured layer a 3-D seismic survey was conducted. A region of anomalous seismic time sag associated with fracturing and small quantities of oil and gas was identified on the northwest plunging nose of the Nan Yi Shan anticline. The distribution of stacking (NMO) velocities in this region was regarded as abnormal; stacking velocities derived on the steeply dipping flanks adjacent to the sag anomaly were found to be slower than those on the shallower crest. Ray‐trace modeling of a buried low‐velocity anomaly provided a possible geometric solution to explain both the time variant nature of the sag and the unusual stacking velocity signature associated with it. A significant difference in seismic and sonic traveltimes was also observed for wells that penetrated the sag region and was attributed to localized fracturing. In a deeper interval, seismic amplitudes were used to identify gas‐saturated fracture porosity and to describe the spatial limits of overpressuring within a thin‐bed reservoir. Wells drilled through high‐amplitude anomalies encountered overpressuring, those drilled in a region of moderate seismic amplitude tested significant quantities of gas, and wells located outside the region of good coherent signal encountered poor or no hydrocarbon shows. These results demonstrate that with good quality seismic data and sufficient lateral and vertical resolution, thin fractured hydrocarbon‐bearing reservoirs can be delineated and overpressure zones identified.

Acoustic crosswell imaging using asymptotic waveforms

Geophysics ◽  
2000 ◽  
Vol 65 (5) ◽  
pp. 1569-1582 ◽  
Author(s):  
Henk Keers ◽  
Lane R. Johnson ◽  
Don W. Vasco
Keyword(s):  
Asymptotic Method ◽  
Waveform Inversion ◽  
Seismic Energy ◽  
Inversion Method ◽  
Inversion Algorithm ◽  
Low Velocity ◽  
Traveltime Inversion ◽  
Partial Derivatives ◽  
Velocity Anomaly ◽  
Sensitivity Functions

Seismic waveforms are inverted using an asymptotic method. The asymptotic method models amplitudes correctly at the caustics and takes nonstationary raypaths into account when computing the waveforms, and thus is an extension of geometrical ray theory. Using numerical differencing, partial derivatives of the data with respect to the model are computed. As expected, these partial derivatives (or sensitivity functions) are concentrated along, but not confined to, raypaths. The sensitivity functions enable the formulation of a waveform inversion algorithm, which is applied to a synthetic crosswell experiment and a laboratory crosswell experiment. The synthetic experiment shows the advantages of the waveform inversion method over conventional traveltime inversion methods. Boundaries of anomalies are better defined, and smearing is reduced. The waveform inversion produces a much lower misfit than the traveltime inversion. The goal of the laboratory experiment was the detection of a nonaqueous phase liquid (NAPL) in water saturated sand. The sand was imaged before and after injection of the NAPL. Using the waveform inversion method, low‐velocity anomalies were imaged that correlate well with post‐experiment determination of NAPL concentrations. The low‐velocity anomaly defocuses the seismic energy. However, the amplitude reduction due to the low‐velocity anomaly is not enough to explain the observed low amplitudes. We suggest that other mechanisms (such as multiple scattering, 3-D effects, or intrinsic attenuation) not included in the asymptotic waveform modeling play an important role in decreasing the amplitude.

scholarly journals Seismic Imaging and Seismicity Analysis in Beijing-Tianjin-Tangshan Region

2011 ◽  
Vol 2011 ◽  
pp. 1-13 ◽  
Author(s):  
Xiangwei Yu ◽  
Wenbo Zhang ◽  
Yun-tai Chen
Keyword(s):  
Focal Depth ◽  
Velocity Model ◽  
Double Difference ◽  
Low Velocity ◽  
Arrival Times ◽  
Velocity Anomaly ◽  
The North ◽  
Tomographic Method ◽  
Vertical Slice ◽  
Tomography Method

In this study a new tomographic method is applied to over 43,400 high-quality absolute direct P arrival times and 200,660 relative P arrival times to determine detailed 3D crustal velocity structures as well as the absolute and relative hypocenter parameters of 2809 seismic events under the Beijing-Tianjin-Tangshan region. The inferred velocity model of the upper crust correlates well with the surface geological and topographic features in the BTT region. In the North China Basin, the depression and uplift areas are imaged as slow and fast velocities, respectively. After relocation, the double-difference tomography method provides a sharp picture of the seismicity in the BTT region, which is concentrated along with the major faults. A broad low-velocity anomaly exists in Tangshan and surrounding area from 20 km down to 30 km depth. Our results suggest that the top boundary of low-velocity anomalies is at about 25.4 km depth. The event relocations inverted from double-difference tomography are clusted tightly along the Tangshan-Dacheng Fault and form three clusters on the vertical slice. The maximum focal depth after relocation is about 25 km depth in the Tangshan area.

scholarly journals Arctic Ocean Gas Hydrate Stability in a Changing Climate

2013 ◽  
Vol 2013 ◽  
pp. 1-10 ◽  
Author(s):  
Michela Giustiniani ◽  
Umberta Tinivella ◽  
Martin Jakobsson ◽  
Michele Rebesco
Keyword(s):  
Arctic Ocean ◽  
Sea Level ◽  
Gas Hydrates ◽  
Gas Hydrate ◽  
Arctic Region ◽  
Geothermal Gradient ◽  
The Arctic ◽  
Ocean Bottom ◽  
The Arctic Ocean ◽  
Hydrate Stability

Recent estimations suggest that vast amounts of methane are locked in the Arctic Ocean bottom sediments in various forms of gas hydrates. A potential feedback from a continued warming of the Arctic region is therefore the release of methane to the atmosphere. This study addresses the relationship between a warming of the Arctic ocean and gas hydrate stability. We apply a theoretical model that estimates the base of the gas hydrate stability zone in the Arctic Ocean considering different bottom water warming and sea level scenarios. We model the present day conditions adopting two different geothermal gradient values: 30 and 40°C/km. For each geothermal gradient value, we simulate a rise and a decrease in seafloor temperature equal to 2°C and in sea level equal to 10 m. The results show that shallow gas hydrates present in water depths less than 500 m would be strongly affected by a future rise in seafloor temperature potentially resulting in large amounts of gas released to the water column due to their dissociation. We estimate that the area, where there could be complete gas hydrate dissociation, is about 4% of the area where there are the conditions for gas hydrates stability.

scholarly journals ARKHANGELSK REGION

2019 ◽  
pp. 247-255
Author(s):  
Galina Antonovskaya ◽  
Yana Konechnaya ◽  
Natalya Vaganova ◽  
Alexey Morozov
Keyword(s):  
Barents Sea ◽  
Arctic Region ◽  
Seismic Network ◽  
The Arctic ◽  
Seismic Stations ◽  
East European ◽  
Regional Seismicity ◽  
Arkhangelsk Region ◽  
The North ◽  
New Association

An overview of regional seismicity registered by the Arkhangelsk seismic network is presented. In 2013 the Arkhangelsk seismic network included 9 digital seismic stations (ARH, KRR, PRG, TMC, PRO, LASH, SLV, AMD, and ZFI/ZFI2). A large number of technogenic events were recorded by the Arkhangelsk network in the north of the East European Platform (Arkhangelsk Region). Only 2 earthquakes were registered during the entire observation period: on October 22, 2005, at 17h46m with ML=2.9, and on March 28, 2013, at 07h02m with ML=3.4. The parameters of the hypocenter of the March 28, 2013 earthquake were evaluated using all currently available source data and bulletins of Russian and foreign seismic stations. The evaluation was done using the method of Generalized beamforming, in an improved form, implemented in the program NAS (New Association System). This earthquake is reprocessed using the same initial data (26 arrivals of seismic P- and S-phases from 13 seismic stations) by the NAS program. The epicentral distances ranged from 85 to 1800 km. The final parameters of the epicenter are as follows: 28.03.2013, t0=07:02:16.2, φ=63.95°N, λ=41.57°E. The epicenter is located 80 km south of Arkhangelsk in a sparsely populated area slightly west of the Severnaya Dvina River. The probable source depths lie in the interval from 2 to 33 km and the depth with the maximal value of the rating function is 19 km. A map of earthquakes in the Arctic region is provided. Earthquakes were registered by stations of the Arkhangelsk seismic network and processed together with the waveforms from other networks. The distribution of earthquakes correlates with the seismic zones of the Arctic – the Knipovich and Gakkel ridges, Spitsbergen. There are weak earthquakes on the shelf of the Barents Sea near the Frantz Josef land archipelago, as in 2012. They relate to the trench of Franz Victoria, the border of the continental slope, and the island of Bely. The coordinates of the epicenters of 290 earthquakes were determined in 2013. The representative level of the Euro-Arctic region's magnitude for 2013 according to the Arkhangelsk network was 2.9.

Mantle feeding sources of the Northern group of volcanoes in Kamchatka inferred from the tomographic inversion of travel time data of the KISS network

2020 ◽  
Author(s):  
Ivan Koulakov ◽  
Nikolay Shapiro ◽  
Evgeny I. Gordeev ◽  
Christoph Sens-Schoenfelder ◽  
Ilyas Abkadyrov ◽  
...  
Keyword(s):  
Vertical Direction ◽  
Quality Data ◽  
The South ◽  
Uppermost Mantle ◽  
Low Velocity ◽  
High Quality ◽  
Important Conclusion ◽  
Arrival Times ◽  
Velocity Anomaly ◽  
Northern Group

<p>The major part of the Northern group of volcanoes (NGV) in Kamchatka is occupied by the Klyuchevskoy group, which is a unique cluster of more than thirteen volcanos having exceptionally diverse eruption styles and compositions. The NGV also includes Shiveluch volcano to the north and Kizimen volcano to the south, both andesitic strongly explosive volcanoes. The crustal structure beneath the Klyuchevskoy group was previously explored using data of the permanent stations and several temporary networks; however, for studying the mantle structures, no high-quality data was available. To close this gap, a temporary seismic KISS network was installed throughout the NGV by an international consortium from August 2015 to July 2016. Together with 22 permanent stations, it included more than 100 simultaneously operating seismic stations. Based on the KISS data, we manually picked more than 43,000 arrival times of the P and S waves from 665 events (65 picks per event on average). Furthermore, this dataset was supplemented with the arrival times from the slab-related seismicity recorded by permanent stations during long-term observations. Several resolution tests have demonstrated that this dataset allows very high quality recoveries of the anomaly both laterally and in the vertical direction. The distributions of seismic anomalies in the uppermost mantle (50 km depth) show clear connection with the composition of the volcanoes. All the andesitic volcanoes (Kizimen, Udina, Zimina, Bezymyanny, Zarechny, Kharchenko and Shiveluch) are located above prominent low-velocity anomalies, whereas the basaltic volcanoes (Nikolka, Tolbachinsky Dol, Ostry and Plosky Tolbachik, Ushkovsky and numerous monogenic cones) are mostly associated with higher velocities in the mantle. This correlation might be explained by the effect of the mantle temperature to the rheological properties of the crust. Over the hot mantle, the crust becomes ductile, and it favors for forming intermediate crustal reservoirs, where magma is accumulated and separated for long time making it more felsic. Above the colder mantle, the crust is brittle and may be fractured by ascending mafic intrusions. In this case, mantle material quickly penetrates through the crust and reaches the surface producing fissure basaltic eruptions and shield volcanoes. Another important conclusion follows from the interpretation of the vertical section throughout the NGV from Kizimen to Shiveluch. Along this section, the only one deep low-velocity anomaly reaching depths of more than 100 km is located beneath Shiveluch, which perfectly coincides with the gap in the Pacific slab imaged by other studies. Further to the south, the low-velocity anomaly is observable in the uppermost mantle down to 60-70 km. This result shows that all the volcanoes of the NGV are fed from a single source associated with the ascent of the hot asthenosphere though the slab window beneath Shiveluch. Then the hot asthenospheric material spreads southward along the crust bottom. This flow heats the mantle wedge, which is highly contaminated with volatiles coming from the slab, and leads to active melting and forming magma sources. This may explain exceptional activity and diversity of the volcanoes in this zone.</p>

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