Displays

Alpine lake sediments in the critical zone have proven their efficiency to record regional climate variability and geohazard history at several time-scales. However, the understanding of lake responses to external environmental factors depends on a precise knowledge of internal lake functioning. High resolution imaging of lake sedimentary infill is crucial to unveil internal and external factors impacting sedimentary processes. In memory of Walter Munk and to his considerable contribution to underwater geophysics, we present 29 high-resolution seismic reflection lines data (8 cm resolution/10 meters penetration) from Lake Altaussee (Walter Munk hometown lake in Austrian Alps), recently acquired using iXblue Echoes 10 000 sub-bottom profiler. Interpretations are supported by multibeam echosounder bathymetry and hydrochemical data.

Lake Altaussee is situated at 713 m a.s.l. in Northern Calcareous Alps (Salzkammergut, Austria). Lake depression is 2.6 km long, 1 km width and mean water depth is 53 m. Three main echofacies are observed: High/low intensity reflectors following the lake bed topography, Structureless weak amplitude layer on top of the bedrock, Massive and discontinuous structures at the eastern part of the lake.

First type suggests a great potential to reconstruct Late Holocene environmental and climatic events. Second type is probably associated to a landslide. Third type is located on top of holes and water resurgence (also visible in the bathymetry) and is attributed to carbonate sedimentation due to supersaturation and oxygenated conditions at the karstic system output. This hypothesis is supported by lower temperature and salinity measured at the karstic system output. Using Delph Seismic software, we constructed 3D modeling of the lake sediments by generating isopaches of main reflectors and estimated spatial distribution of sediment volume. Our model help at deciphering different sedimentary dynamics along the lake infill history and to suggest the deposition of historical earthquake, flood, etc, on top of the bedrock.

How to cite: Alban, B., Jouve, G., Leloup, D., Alain, P., and Chapron, E.: Imaging the Walter Munk lake: Sedimentary dynamics and water resurgence derived from high-resolution seismic reflection survey in Lake Altaussee (Salzkammergut, Austrian Alps), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18318, https://doi.org/10.5194/egusphere-egu2020-18318, 2020.

The oceanic crust in the vicinity of the Hawaiian Islands is of tectonic interest because it formed at a fast spreading mid-oceanic ridge during the Late Cretaceous (Turonian) and has been deformed since the Late Miocene by volcanic loads generated at a deep mantle hotspot. We have used legacy and recently acquired multichannel seismic reflection data to determine the character of oceanic crust and the Moho in a region south of the Hawaiian Islands where the Pacific plate has been flexed upwards partly by volcano loading and partly by the dynamics of the hotspot. The legacy data is based on Common Depth Point (CDP) and Constant Offset Profile (COP) data acquired onboard R/V Robert D. Conrad and R/V Kana Keoki during August/September 1982. Conrad was equipped with a 3.6 km long streamer and a 1864 cu. in. airgun array and Kana Keoki was equipped with a 1864 cu. in. array. During the COP experiment the two ships steamed on a similar heading and a separation distance of 3.6 km, yielding an effective offset for reflection data of 7.2 km. Original field data have been re-processed with ‘state-of-the-art’ seismic processing work flows using Shearwater REVEAL software. The recently acquired data was acquired during October 2018 with R/V Marcus G. Langseth, equipped with a 15 km long streamer and a 6600 cu. in. airgun array. Comparisons between the legacy and recently acquired reflection data have been informative, revealing new methods to process Conrad’s legacy of multichannel data acquired on 31 cruises during 1975 to 1989 and new insights on the structure and nature of the Moho in 95 Ma oceanic crust.

How to cite: Cilli, P., Watts, T., Boston, B., and Shillington, D.: A comparison of legacy and recently acquired multichannel seismic data on 95 Ma Pacific oceanic crust south of the Hawaiian Islands, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10612, https://doi.org/10.5194/egusphere-egu2020-10612, 2020.

Remotely sensed seismic images are our main window into sub-surface geology and are used extensively in industries that explore or exploit the sub-surface and it’s geo-resources. Seismic image data are made by bouncing sound waves off sub-surface rock layers to build a picture of the geology. Seismic imagery, has an inherent uncertainty derived from image resolution, processing and acquisition workflows; the interpreting geologist therefore has a crucial role in using information in the image to produce a realistic geological model on which multi-billion dollar and key environmental decisions can be made. Research has shown that experts look at such imagery differently to novices and that colour presentation and other factors influence response to an image. Here we investigate how such factors influence a geologists’ interpreting a seismic image, and consider if we can use our findings to improve future seismic image interpretation through expert learning and potential image enhancement.

Eye movements offer a valuable, unbiased, research method, to reveal not only an individual’s interests and priorities when viewing an image, but also their expertise. In other disciplines eye movements of experts have been used to improve skill and enhance training of novices.Here we use eye-tracking to capture where geologists’ look on a seismic image to investigate whether: 1) colour palette choice for image presentation can affect image perception and influence gaze and 2) if expert and novice gaze patterns are different. In our experiments we have considered multiple colour palettes for a range of seismic imagery containing structural and sedimentological features. We show that expert and novice gaze is different, particularly in the initial phase of image exposure and that colour palettes have a significant impact on gaze and attention of all participants. Ultimately the objective is to see if we can learn from expert gaze to help improve the seismic interpretation skills of novices through image enhancement, and ultimately image interpretation outcome.

How to cite: Bond, C. and Tatler, B.: Expert and novice gaze in seismic interpretation, implications for colour palette choice and learning. , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19067, https://doi.org/10.5194/egusphere-egu2020-19067, 2020.

Extension in rift zones and passive margins often occur by multiphase normal faulting which usually accommodates several episodes of lithosphere stretching by brittle deformation. In these settings, pre-existing normal faults may reactivate but also new-formed structures may nucleate, with multiple orientations and deformational styles. The various modes of fault growth and nucleation are strongly influenced by several parameters (including orientation and geometry of pre-existing discontinuities, stress orientation and magnitude, strain rates, confining pressure, etc..) with the lithostratigraphy controlling the brittle or ductile litho-mechanic behavior of each unit.

In this work, we interpreted and analyzed an industrial 3D seismic volume acquired in the Exmouth Plateau, (Northern Carnarvon Basin – offshore NW Australia), where pre-existing Mesozoic normal faults were reactivated during the Cenozoic and controlled the nucleation and growth of the new-formed overlying fault segments. The peculiarity of this system is that the two sets of faults are separated by a ductile interval of shales. The latter acted as decollement level and promoted the formation of prominent faulted anticlines in the overlying brittle sequence; these forced folds are poorly documented in other extensional settings while are common where salt layers are present. In this study, the high-resolution techniques adopted for seismic data interpretation aimed to understand the geometries of faults and their interactions in fine detail. The results of fault analysis suggest that the use of high-quality 3D seismic volumes is very useful to unravel the complex and subtle spatial variability and also the displacement pattern of faults with a limited amount of fault-throw.

How to cite: D'Intino, N.: High resolution imaging of fault reactivation in long-lived extensional setting: a case study from the Exmouth Plateau (NW Australia), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22009, https://doi.org/10.5194/egusphere-egu2020-22009, 2020.

The characteristics of oceanic crust are dependent on the spreading rate of a Mid-Ocean Ridge (MOR). Crustal structure near an extinct MOR, therefore, provide unique constraints on how the magma supply and the crustal accretion respond to the reduced and ultimately ceased spreading. We present the crustal structure beneath 11 OBS sites near the extinct MOR in the central sub-basin of the South China Sea (SCS). We use the Receiver Function (RF) method to reveal the thickness and the Vp/Vs ratio of the crust based on the passive-source OBS data collected in this sub-basin. The thickness of the crust varies systematically with the distance to the ridge.  The thinned crust near the ridge likely indicates that, in the late stage of spreading, the magma supply has diminished and the spreading rate has dropped to the ultra-slow range. While the Vp/Vs ratios at most sites fall into the normal range, there exist a few anomalously high Vp/Vs ratios (> 2.0) at sites very close to the ridge. These high Vp/Vs values can be explained by the serpentinization of the uppermost mantle beneath the sites. As the spreading rate and magma supply were reduced, fractures and fissures were easily developed at the frank of the crust accretion, allowing water enters the lowermost crust and serpentinizes the uppermost mantle.

How to cite: Yang, T., Hung, T. D., Le, B. M., and Xue, M.: Crustal structure across the extinct ridge in South China Sea from OBS receiver functions: insights into the spreading rate and the magma supply prior to the ridge cessation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6350, https://doi.org/10.5194/egusphere-egu2020-6350, 2020.

Northern Honshu, Japan, is a classic example of compressive island arc forming a trench-arc-backarc basin and was rifted away from the Asian continent in Miocene. The subduction of the Pacific plate generates megathrust earthquakes, such as the 2011 Tohoku-oki earthquake (M9) and an overriding plate deforms associated with M7-class reverse faulting. The amount of shortening is largest where along the Miocene failed rift basin, the Neogene post rift sediments form a fold-and-thrust belt. To understand the mechanisms of the deformation of overriding plate and generation of devastative earthquake, to reveal the detailed lithospheric structure of overriding plate is significant. In 2019, an onshore-offshore seismic reflection profiling was performed across the Japan trench to the axial part of the backarc basin. Here we focus on the crustal structure of the onshore section across the central part of northern Honshu.

The length of onshore seismic line is 160 km within the total 850-km-long seismic line. The air-gun shots in the forearc and backarc sides were recorded by onshore seismic line using 1616 fixed channels. Onshore seismic sources were four vibroseis trucks and dynamite shots. To obtain the deep crustal image, we used low-frequency signals. The produced sweep frequency was 3 to 40 Hz and seismic signals were recorded by 4.5 and 5 Hz geophones. Sets of 150 stationary vibroseis sweeps were performed at about 10 km interval along the seismic line. By conventional common-midpoint reflection methods and refraction tomography reveal the crustal structure down to 10 km. Together with the velocity structure obtained by earthquake tomography (Matsubara et al., 2019). Lithospheric structure is estimated by velocity structure obtained from active and passive sources, and geological data.

With seismic reflection profiles in the forearc (Miura et al., 2005) and backarc (No et al., 2014), the onshore seismic section portrays the first image of the seismic reflection profile across the Northern Honshu arc from the trench to the backarc basin. The basic structure of the over ridding plate were formed by the rifting of backarc opening stage. Most of the active faults inherited from the Miocene normal faults. The formation of backarc basins were achieved by the development of multi-rift systems. An axial part of failed rift within a continental crust is marked by a higher P-wave velocity lower crust, thick post-rift sediments underlaid by thick basalts. The failed rift is bounded by faults dipping to the outward of rift axis associated with mafic intrusion in a rift axis. The reverse faulting of the rift-bounding faults produced a fold-and-thrust belt in the post-rift Neogene basin fill. Judging from the tectonic geomorphological and geological features, these faulting and fault-related folding are active in late Quaternary. Detachment in this fold-and-thrust belt commonly accommodates in over pressured mudstone units in the rift basin. The major style of deformation of backarc is basement involved normal faults. Reactivation as reverse fault concentrated along the backarc continental failed rift.

How to cite: Sato, H., Ishiyama, T., Shimizu, H., Kato, N., Shinohara, M., Iwasaki, T., Ishige, H., Kawasaki, S., Abe, S., Matsubara, M., No, T., Kodaira, S., and Hirata, N.: Deep seismic reflection profiling across the central part of Northern Honshu arc, Japan, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20526, https://doi.org/10.5194/egusphere-egu2020-20526, 2020.

Modern seismic geomorphology techniques are implemented to infer modes of emplacement of a wide range of products derived by gravity-driven deformation in continental margins and to discuss the implications of these for exploration of georesources in offshore basins.
High-quality 3D seismic data from the Namibian margin (West Africa) show how margin-scale, extensional-contractional gravity-driven linked systems deformed at least 2 km of Cretaceous post-rift strata. The systems are laterally segmented into a series of scoop-shaped megaslides ~30 km long by ~20 km wide, which are formed by listric headwall fault systems with associated 3D rollover structures and thrust imbricates at their toes. Lateral segmentation occurs along sidewall fault systems that display oblique extensional motion and define horst structures up to 6 km wide between individual slides.
Key examples are shown of slope failures offshore North West Shelf of Australia that affected Jurassic rift strata as well as near seabed, Late Oligocene to Recent, post-rift sediments. The seabed collapse systems originated at water depths of ~1000 m and extended downdip to depths well in excess of 1500 m. These are shallow failures that exhibit width ranging 1-5 km and run out of ~15 km, with estimated volumes of sediments in excess of ~10 km3. A number of these failures are characterised by disrupted, slump-like facies which progressively pass downslope into packages of high amplitude, continuous reflections. This facies transition represents a downslope rheological transformation from slump to mass-flow as evidenced by prominent canyons that link the updip failures to well-developed, downdip fan systems several kilometres across.
In the rift section, slope failures affected domino extensional fault systems in the form of well-imaged footwall degradation complexes. These complexes exhibit overlapping, scoop-shaped scars up to 10 km in length that deteriorated the exposed footwall breakaways. Debris from footwall collapse was resedimented in the hanging wall basins, forming talus wedges up to 300 m thick that taper away from adjacent fault planes for distances of several kilometres. These deposits are characterised by sheeted to contorted seismic facies, indicating a variety of mass-wasting processes accompanying footwall collapse.

This research demonstrates that a broad spectrum of slope instability processes can ensue during the evolution of rift systems to passive margins. Margin-scale megaslides emplace through processes of complex lateral segmentation which can create a variety of trapping mechanisms in the post-rift section of unstable margins. Downslope transformation of deepwater slumps into sediment flows may explain the occurrence of sandy deposits in offshore basins, hundreds of kilometres away from coastlines and river inputs.
On the other hand, fault degradation complexes, which are relevant slope instability processes in rift systems, can redistribute and accumulate footwall reservoirs into hanging-wall basins, increasing the diversity of play types in rifted margins.

How to cite: Scarselli, N.: From megaslides to mass flows: using seismic geomorphology to unveil gravity-driven deformation at continental margins., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22035, https://doi.org/10.5194/egusphere-egu2020-22035, 2020.

The Hanö Bay basin was formed during Late Cretaceous transgression as a sedimentary trough on the NE margin of the Sorgenfrei-Tornquist Zone (STZ), a narrow NW-SE striking intraplate inversion zone within the Fennoscandian Border Zone. Sedimentation within the basin was primarily controlled by inversion tectonics, resulting in a coarse-grained syn-inversion clastic wedge forming adjacent to the basin-bounding fault in the Santonian-Maastrichtian. Previous studies have highlighted the deposition of contourite sediments associated with topographic relief of the chalk sea created by such local inversion-induced uplift. Imaged upper Cretaceous clinforms in the marginal trough show a NE-ward progadational character, that is, away from the uplifted and eroded inversion zone. These extend along the inversion axis all the way to NE of the Mid-Polish trough.

To gain detailed stratigraphic constraints and to better understand the interaction of these syn-sedimentary features that developed during inversion tectonics, we use a combination of high-resolution multichannel seismic data (MCS) from the 2019 AL526 cruise and a number of key profiles from reprocessed 70-80’s legacy industry MCS. Preliminary results suggest a drift-moat system developed during a stepwise uplift of the SW shoulder of the STZ, with the uplift driven by transpressional reactivation of basement faults. The resultant aggradational wedge formed a shelf-margin extending fairly far into the basin. The overlying clinoform depositional successions clearly demonstrate several depositional stages; including highstand-progradation, highstand-aggradation and distinct transgression-retrogradation, during which an overall landward migration of the paleo-shoreline position is revealed. The results constrain relative sea-level changes in this area that were primarily related to tectonic events during the Santonian-Campanian.

How to cite: Pan, Y., Seidel, E., Hübscher, C., Juhlin, C., and Sopher, D.: Tectonic controlled sedimentary features at the NE marigin of the Sorgenfrei-Tornquist Zone (STZ), southern Sweden, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3608, https://doi.org/10.5194/egusphere-egu2020-3608, 2020.

Geological structure and tectonics of the Phanerozoic sedimentary cover within the transition zone between the Precambrian and Paleozoic platform in the Polish sector of the Baltic Sea was imaged using new 2D high-resolution multi-channel seismic reflection data. The new seismic data were acquired in 2016 during the course of RV Maria S. Merian expedition MSM52 within the framework of the BALTEC project. Eight profiles (with the total length of ca. 850km) covered the tectonics blocks located within the Polish Exclusive Economic Zone, stretching from the East European Craton (EEC) to the Paleozoic platform across the Teisseyre-Torquist Zone (TTZ).

Our in-house seismic processing workflow focused on removing multiples contaminating this shallow-water data, both water bottom and interbed related. Various demultiple techniques such as SRME, TAU-P domain deconvolution, high resolution parabolic Radon demultiple and SWDM (Shallow water demultiple) have been tested. Combination of all those techniques at different stages of the processing with some modifications based on a particular seismic profile proved to be the most effective. Consequently, multiples obscuring seismic sections were efficiently reduced. Data were processed up to Kirchhoff pre-stack time migration.

The longest seismic profile (line BGR16-212, ca. 240 km long) crosses almost perpendicularly majority of Precambrian and Paleozoic fault systems bordering the tectonic blocks of the EEC basement, so fault systems could be easily interpreted. EEC Precambrian basement is characterized by a regional flexure towards the TTZ. Cambrian-Ordovician exhibits similar geometry and is characterized by a relatively constant thickness related to deposition on the Tornquist Ocean passive margin. Thick Silurian succession is characterized by a regional divergent pattern caused by deposition within the Caledonian foredeep basin. Structural pattern within the W part of the study area is much more complex as this area underwent Late Paleozoic extension/transtension, Variscan inversion, Permo-Mesozoic subsidence and Late Cretaceous inversion.

This study was funded by the Polish National Science Centre grant no UMO-2017/27/B/ST10/02316.

How to cite: Nguyen, Q., Malinowski, M., Krzywiec, P., and Huebscher, C.: Seismic processing and imaging of the new 2D marine reflection seismic data in the Polish sector of the Baltic Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7543, https://doi.org/10.5194/egusphere-egu2020-7543, 2020.

Post-orogenetic extensional/gravitational collapse events constitute a relatively poorly understood tectonic process, which is responsible for the quick and effective dismantling of the thickened crust and topographic bulge of fold-and-thrust belt edifices. These events are also responsible for the accumulation of very thick post-orogenetic successions and, in case of active extension, may trigger moderate to strong earthquakes resulting in obvious seismic hazards (e.g., the 1915 Mg 7.0 Fucino earthquake in Central Italy, which caused 30,000 victims)

Here, we combine seismic interpretation coupled with well analyses, basin modelling and a thorough literature review, in order to compare an ancient and a modern example of study areas subject to post-orogenetic collapse. The Devonian-age Old Red Sandstones of north-western Europe and ?Plio-Quaternary fill of the Fucino intramontane extensional basin in the central Apennines (Italy) share several stratigraphic, depositional and tectonic characteristics. Both are characterized by remarkably similar seismic-stratigraphic architecture (with syn-depositional half-grabens) and maximum thickness of >1,500 metres. In the Fucino, the border faults associated to the main tectonic depocentres achieved maximum throw rates of 1,000-1,400 mm/kyr.

Both units comprise thick continental siliciclastic successions, dominated by lacustrine and alluvial to fluvio-deltaic facies. The facies architecture reveals a progressive transition from localized, fault-bounded depocentres to transgressive lacustrine successions in wider basins that are less reliant on the sole fault-driven subsidence. The studied units were deposited due to high and quick tectonic subsidence which took place very shortly after the end (or during?) of crustal shortening processes (respectively Caledonian and Apenninic orogenesis) and in a post-orogenic collapse context.

In both study areas, the sedimentation of the thick continental units are intimately associated to a polyphase inversion tectonics, with pre-existing inherited deep-seated discontinuities affected, in places, first by a positive and subsequently by a negative reactivation during the extensional collapse. A further element common in the two study areas, is a strike-slip or oblique tectonics occurring during or immediately prior to the extensional collapse achieved by the normal faulting. This has been interpreted as a consequence of the gradual rotation of the stress vectors around their axes, culminating in the relaxation of the horizontal compressive stress and the onset of the post-orogenetic extensional/gravitational collapse process itself. For example, in the Fucino Basin, maximum Plio-Quaternary sediment thicknesses of >1700 m occur in two tectonic depocentres, situated respectively to the north and east of the basin. In contrast, the south-eastern striking dip-slip border faults bounding the eastern edge of the Fucino show maximum slip rates in the Lower-Middle Pleistocene, with evidence (e.g., Gioia dei Marsi) for a very recent activity, possibly linked with the 1915 seismic event.

The study of post-orogenic extensional collapse by comparison of ancient and recent basins suggest that in these settings poly-phase tectonic inversion commonly occurs and promote multiple reactivation of inherited zones of weakness. The comprehension of the common and dissimilar features, may be fundamental to better understand the mechanism and evolution of post-orogenic chain reworking and for natural resources and geological hazards assessment, including earthquakes. The coupled analysis of an ancient and recent example enables just that.

How to cite: Patruno, S. and Scisciani, V.: Inversion tectonics during post-orogenic extensional collapse: a comparison between ancient (North Sea, UK) and recent (Fucino Basin, central Apennines Apennines) intermontane systems, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19818, https://doi.org/10.5194/egusphere-egu2020-19818, 2020.

Nowadays depth imaging of seismic data, using different migration schemes (rays tracing or waves equation methods) and different techniques for velocity model building (i.e. grid or layer-based tomography, isotropic or anisotropic velocity field) is a standard approach for the earth’s subsurface characterization. When dealing with low fold vintage data, acquired with outdated technologies, modern processing algorithms may fail. On the other hand, the reprocessing of these old data with modern techniques may lead to an improvement of quality and resolution, allowing a more accurate interpretation of the investigated geological features. It is important to note that a lot of vintage data were acquired in areas with no recent surveys or currently subject to exploration restrictions. Therefore, available vintage data could be of great importance for all the stakeholders involved in geophysical exploration. We present a case study about the reprocessing of low fold marine seismic data that were acquired in 1971 in the Otranto Channel (Southern Adriatic Sea, Italy).

The first part of the work consists of a modern broadband sequence processing in the time domain, that allowed us to obtain a pre-stack time migrated seismic section; in the second part, depth imaging has been achieved through a pre-stack depth migration (PSDM). Reliable interval p-waves velocity model has been obtained using two tomographic approaches: grid tomography and layer-based tomography; for both, we carried out several iterations of the refinement loop, consisting of migration, ray tomography, residual velocity analysis, velocity model update.

The results show significant improvements compared to the original vintage section, in terms of resolution and signal to noise ratio. Moreover, depth imaging and velocity modeling added further information (e.g., reliable interval p-waves velocity model, real geometry and thickness of the main geological units). This study confirms that applying the up-to-date processing and imaging techniques to vintage data, their geophysical and geological value is enhanced and renewed at a relatively low cost.

How to cite: Forlin, E., Brancatelli, G., Bertone, N., Del Ben, A., and Geletti, R.: Time reprocessing and depth imaging of vintage seismic data: the Southern Adriatic Sea case study, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7164, https://doi.org/10.5194/egusphere-egu2020-7164, 2020.

In the frame of the geological characterization of the subsurface, the multidisciplinary approach is key to fully understand the geological and geophysical processes. Seismic data analysis and interpretation would result in a mere exercise without constraints provided by geological, geophysical and petrophysical data. These constraints may be provided by borehole data, surface geology or laboratory measurements on samples. In this work, to support geological understanding of foreland basins we integrate reprocessed seismic profiles and borehole data in the Central Adriatic Sea to investigate the velocity-depth trend of the Pliocene-Quaternary turbiditic siliciclastic deposits. These deposits play a key role in the reconstruction of the geodynamic and stratigraphic evolution of the foreland basin, as well as on the hydrocarbon exploration and gas storage in central Adriatic. Relying on independent approaches to map two way time (TWT) thickness of the PH deposits, we converge on testing linear and exponential functions to predict V_P depth trend. Results suggest that for large (> 1500 m) thicknesses of the PH deposits best fit is achieved by the exponential function V_P(z) = c z^(1-n) while for thinner deposits, a linear function like V_P(z) = V₀ + k z provides best fitting estimates. We also investigate anomalies in velocity trend with depth and suggest that velocity drops observed in deep (2500-3500 m) PH sequences may reflect overpressure of these deposits. An hypothesis supported by the high sedimentation rates in central Adriatic during Pliocene. Finally, we stress the importance of considering vertical-component phenomena and their time evolution when modelling foreland basins.

How to cite: Scisciani, V. and Mancinelli, P.: Seismic velocity-depth relation in foreland basins: the case study of the Central Adriatic Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22048, https://doi.org/10.5194/egusphere-egu2020-22048, 2020.

Acoustic full-waveform inversion (FWI), or waveform tomography, involves use of both phase and amplitude of the recorded compressional waves to obtain a high-resolution P-wave velocity model of the propagation medium. Recent theoretical and computing advances now allow the application of this highly non-linear technique to field data. This led to common use of the FWI for industrial purposes related to reservoir imaging, physical properties of rocks, and fluid flow. Application of FWI in the academic domain has, so far, been limited, mostly because of the lack of adequate seismic data. While refraction seismic datasets include large source-receiver offsets that are useful to find a suitable starting velocity model through traveltime tomography, these acquisitions rarely reach the high density of receivers necessary for waveform tomography. On the other hand, multichannel seismic (MCS) reflection data acquisition has a dense receiver spacing but only modern long-streamer data have offsets that, in some cases, enable constraining subsurface velocities at a significant enough depth to be useful for structural or tectonic purposes.

In this study, we show how FWI can help decipher the record of a fault activity through time at the Shumagin Gap in Alaska. The MCS data were acquired on RV Marcus G. Langseth during the ALEUT cruise in the summer of 2011 using two 8-km-long seismic streamers and a 6600 cu. in. tuned airgun array. One of the most noticeable reflection features imaged on two profiles is a large, landward-dipping normal fault in the overriding plate; a structural configuration making the area prone to generating both transoceanic and local tsunamis, including from landslides. This fault dips ~40°- 45°, cuts the entire crust and connects to the plate boundary fault at ~35 km depth, near the intersection of the megathrust with the forearc mantle wedge. The fault system reaches the surface at the shelf edge 75 km from the trench, forming the Sanak basin where the record of the recent activity of the fault is not clear. Indeed, contouritic currents tend to be trapped by the topography created by faults, even after they are no longer active.  Erosion surfaces and onlaps from contouritic processes as well as gravity collapses and mass transport deposits results in complex structures that make it challenging to evaluate the fault activity. The long streamers used facilitated recording of refraction arrivals in the target continental slope area, which permitted running streamer traveltime tomography followed by FWI to produce coincident detailed velocity profiles to complement the reflection sections. FWI imaging of the Sanak basin reveals low velocities of mass transport deposits and velocity inversions indicate mechanically weak layers linking some faults to gravity sliding on a décollement. These details question previous interpretation of a present-day active fault. Our goal is to further analyze the behavior of the fault system using the P-wave velocity models from FWI to quantitatively detect fluids and constrain sediment properties.

How to cite: Kahrizi, A., Delescluse, M., Rodriguez, M., Roche, P.-H., Becel, A., Nedimovic, M. R., and Shillington, D.: Using 2D long-streamer seismic data waveform tomography to decipher sedimentary record of fault activity, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9073, https://doi.org/10.5194/egusphere-egu2020-9073, 2020.

In marine seismic surveys, various acquisition systems are used depending on the survey purpose, target depth, survey environment, and conditions. A 3D survey of oil and/or gas exploration, for instance, require large-capacity air-gun arrays and six or more streamers with a minimum length of 6 km. In contrast, a high-resolution seismic survey for the shallow-water geological research and engineering needs a small capacity source such as air-gun, sparker, and boomer, deployed with a single-channel or multi-channel (24-channel) streamers. The main purpose of our seismic survey was to investigate the Quaternary geology and stratigraphy of offshore, Korea. Because the Quaternary is the most recent geological period, our target depth was very shallow at about 50 m below the sea-bottom. We used a high-frequency seismic source including a sparker of 2,000 J capacity or a 60 in³ mini GI-gun and an eight-channel streamer with a 3.125 m group interval or a single-channel streamer that included 96 elements. To compare the resolution of seismic data according to the seismic source, a boomer or sparker systems were used with the single-channel streamer on a small survey ship. The seismic data processing was performed at the Korea Institute of Geoscience and Mineral Resources (KIGAM) with ProMAX, and the data processing and resolution of each survey were compared based on their acquisition systems.

How to cite: Kim, Y. J., Cheong, S., Cukur, D., and Yoo, D.-G.: A comparative study of multi-scaled high-resolution seismic surveys in shallow marine environments: examples from three sites, offshore Korea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12869, https://doi.org/10.5194/egusphere-egu2020-12869, 2020.

In this work, we present full-waveform inversion (FWI) results of a typical Brazilian Pre-Salt model (Santos Basin) using new open-source tools. The large accumulations of oil with excellent quality and high commercial value discovered in the pre-salt carbonates of southeastern Brazil, especially in the Santos Basin, have made this province one of the most prospective in the world. Velocity model building in areas of highly complex geology (like the Santos Basin) remains a challenging step in seismic processing. FWI proved to be an efficient tool for the determination of high-resolution details in multiparameter models of complex subsurface structures, and it has been applied in different geophysical problem scales. However, since FWI is a computationally and mathematically challenging problem, many issues remain open, such as more efficient ways to deal with multiparameter inversion problems such crosstalk and different orders of magnitude in the seismic signal for different classes of parameters. Inversions for more than one class of parameters are of particular importance in the estimation of the physical properties of rocks (poroacoustic or poroelastic applications), for example, to monitoring oil and gas reservoirs and for monitoring the injection of carbon dioxide into geological structures. Also, programming complex numerical algorithms for each application is time-consuming and often evades the expertise of researchers from the geoscientific community. In this sense, a high-level computational tool for simulations and inversions would greatly improve the working time for researchers. Existing finite difference based FWI tools such as Devito, and finite elements based partial differential equations (PDE) solvers tools such as FEniCS and Firedrake are being explored and used for these purposes. In this work, we initially present an FWI acoustic isotropic inversion test (velocity inversion only), performed with the Devito software while a particular code is being developed in FEniCS and Firedrake computer programs. Devito is also a new and under development software and therefore must be tested under different conditions. Our first numerical results indicate the potential of using freely available computational programs in a real case scenario.

How to cite: Santos, H., Eikmeier, C., and Volpe, E.: New tools for 2D full-waveform inversion: applications on Brazilian Pre-Salt velocity model from Santos Basin, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21110, https://doi.org/10.5194/egusphere-egu2020-21110, 2020.

Northern Honshu, Japan, forms a classical example of the trench-arc-backarc basin system. Along the coast of the Sea of Japan, Miocene aborted rifts were developed filled with thick Neogene sediments and form an active fold-and-thrust belt. Devastative crustal earthquakes, such as the Shonai earthquake 1894 (M7), occurs historically. To reveal the relationship between active fault and fold structure with seismogenic source faults is significant for the evaluation of seismic hazards and possible risk. In the Shonai plain, northern Honshu, we performed 2D high-resolution seismic reflection profiling across the active faults. Seismic data was collected by 10 m shot and receiver interval using Enviro vib and Minivib (IVI) to obtain high-resolution image. Along some of the seismic lines, seismic reflection survey was recorded by fixed 800-1000 channels, producing high number of folds. The resultant seismic profiles provide the image of a fold-and-thrust belt developed in the Miocene volcanic rift basin. Former syn-rift faults reactivated as reverse faults and thin-skinned deformation prevails in the post rift sediments forming detachment in the Miocene over pressured mudstone units. Fault-related folds and wedge thrusting is common feature of the shortening deformation. There are two active thrust systems in the Shonai basin. One is known active fault system along the eastern margin of the Shonai plain and the other is an active-blind - thrust located in the central part of the basin. The late Quaternary tectonic movements along this fault was confirmed by the high-resolution seismic profiling. 

How to cite: Kato, N., Sato, H., and Ishiyama, T.: High-resolution seismic reflection profiling of the active fold-and-thrust systems in the Shonai backarc basin, northern Honshu, Japan, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12185, https://doi.org/10.5194/egusphere-egu2020-12185, 2020.