TS12.1

Diffraction imaging is the technique of separating diffraction energy from the source wavefield and processing it independently. As diffractions are formed from objects and discontinuities, or diffractors, which are small in comparison to the wavelength, if the diffraction energy is imaged, so too are the diffractors. These diffractors take many forms such as faults, fractures, and pinch-out points, and are therefore geologically significant. Diffraction imaging has been applied here to the Porcupine Basin; a hyperextended basin located 200km to the southwest of Ireland with a rich geological history. The basin has seen interest both academically and industrially as a study on hyperextension and a potential source of hydrocarbons. The data is characterised by two distinct, basin-wide, fractured carbonates nestled between faulted sandstones and mudstones. Additionally, there are both mass-transport deposits and fans present throughout the data, which pose a further challenge for diffraction imaging. Here, we propose the usage of diffraction imaging to better image structures both within the carbonate, such as fractures, and below.

To perform diffraction imaging, we have utilised a trained Generative Adversarial Network (GAN) which automatically locates and separates the diffraction energy on pre-migrated seismic data. The data has then been migrated to create a diffraction image. This image is used in conjunction with the conventional image as an attribute, akin to coherency or semblance, to identify diffractors which may be geologically significant. Using this technique, we highlight the fracture network of a large Cretaceous chalk body present in the Porcupine, the internal structure of mass-transport deposits, potential fan edges, and additional faults within the data which may affect fluid flow pathways.

How to cite: Lowney, B., Whiting, L., Lokmer, I., O'Brien, G., and Bean, C.: Diffraction imaging of sedimentary basins: An example from the Porcupine Basin, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-21, https://doi.org/10.5194/egusphere-egu21-21, 2021.

We present a three-dimensional transversely isotropic velocity model of the crust and upper mantle beneath the East Asia Continent and Western Pacific subduction zone based on the full waveform inversion (FWI). It minimizes waveform shape misfit between the synthetics and the observations from a large dataset, with 142 earthquakes recorded by about 2,000 broadband stations in East Asia. Compared to the previous models, the new FWI model, East Asia Radial Anisotropy Model 2020 (EARA2020) after 20 iterations shows much stronger wave speed perturbations within the imaged slabs including the Japan, Kuril, Izu-Bonin, Ryuku slabs. The high wave speed anomalies of the subducted slabs have a maximum perturbation of 8% for Vp and 13% for Vs. We have also systematically analyzed the slabs’ morphology, the intraplate tectonic structures including the North China Craton, the Sichuan Basin, and the Songliao Basin. and investigated the origin of four typical intra-continent volcanos in the continent of China including the Hainan volcano, Changbaishan volcano, Tengchong volcano, and Datong volcano.

How to cite: Xi, Z., Chen, M., Zhou, T., Wang, B., and Kim, Y.: Full waveform inversion of the crust and upper mantle beneath the East Asia Continent and Western Pacific Subduction Zone, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13849, https://doi.org/10.5194/egusphere-egu21-13849, 2021.

Time-varying processes present both challenges and opportunities when interpreting tomographic models. The challenges mainly arise from differences between a method’s temporal and spatial resolution, and the size and timing of the physical processes. A mismatch here will produce a weighted average of the subsurface changes in models using time-dependent tomography. Even when there is such a mismatch, there are clear benefits from studying time-varying processes.

Many different variables are packed into geophysical properties. Seismic velocities, for example, change due to porosity, fluid presence, fluid phase, fractures, and fracture properties, among other factors. This makes interpretation non-unique. Tomographic changes due to time-varying processes help reduce the possibilities for interpretation, especially when integrating multiple geophysical methods. Because tomographic methods can also produce artificial changes between models (Hobé et al., 2021), an important question in time-dependent tomography becomes:
 
What is the theoretical magnitude of changes in geophysical properties in a time-varying area?

As an initial answer, we present an overview of empirical data from the IMAGE project, which investigated the Reykjanes Peninsula, Iceland. This overview shows the inherent temporal variability of this volcano-tectonic region. In the lab, geophysical properties changed as much as 30% due to, e.g., fracturing, fracture healing, fluid-phase changes, and changes in fluid saturation. All these phenomena are common occurrences in this area, which hosts several volcanic systems with hydrothermal and seismic activity. These phenomena also have a secondary impact on how the empirical data should be interpreted, which is usually underestimated or overlooked in tomographic interpretations: These time-varying processes can strongly affect effective pressures, which is one of the main variables in the empirical data. To show the magnitude of this oversight, we present examples where effective pressure is affected, along with the theoretical changes in geophysical properties. Lastly, we show how the inclusion of effective pressures in interpretation can aid in the identification of time-varying processes.

Hobé, A., Gudmundsson, O., Tryggvason, A., and the SIL seismological group (2021): Imaging the 2010-2011 inflationary source at Krýsuvík, SW Iceland, using time-dependent Vp/Vs tomography, in Proceedings World Geothermal Congress 2020, Forthcoming

How to cite: Hobé, A., Tryggvason, A., Almqvist, B., and Gudmundsson, O.: Interpreting Time-Varying Processes using Empirical Data, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9509, https://doi.org/10.5194/egusphere-egu21-9509, 2021.

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. Modern multichannel seismic (MCS) reflection data acquisition now  have long offsets which, in some cases, enable constraining FWI-derived 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 R/V Marcus G. Langseth during the 2011 ALEUT cruise 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 and forms the ~6-km deep Sanak basin. However, the record of the recent fault activity remains unclear as 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 result in a complex sedimentary record that make it challenging to evaluate the fault activity using conventional MCS imaging alone. The long streamers used facilitated recording of refraction arrivals in the targeted continental slope area, which permitted running streamer traveltime tomography followed by FWI to produce coincident detailed velocity profiles to complement the reflection sections. We performed FWI imaging on two 40-km-long sections of the ALEUT lines crossing the Sanak basin. The images reveal low velocities of mass transport deposits as well as velocity inversions that may indicate mechanically weak layers linking some faults to gravity sliding on a décollement. One section also shows a velocity inversion in continuity to a bottom simulating reflector (BSR) only partially visible in the reflection image. The BSR velocity anomaly abruptly disappears across the main normal fault suggesting either an impermeable barrier or a lack of trapped fluids/gas in the hanging wall.

How to cite: Kahrizi, A., Delescluse, M., Rodriguez, M., Roche, P.-H., Bécel, A., Nedimovic, M. R., Shillington, D., Pubellier, M., and Chamot-Rooke, N.: Using 2D long-streamer seismic waveform tomography to decipher sedimentary processes surrounding a forearc fault offshore Alaska, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13056, https://doi.org/10.5194/egusphere-egu21-13056, 2021.

Structural imaging beneath complex overburdens, such as sub-salt or sub-basalt, typically characterized by high-impedance contrasts represents a major challenge for state-of-the-art seismic methods. Reconstructing complex geological structures in the vicinity of and below salt bodies is challenging not only due to uneven, single-sided illumination of the target area but also because of the imperfect removal of surface and internal multiples from the recorded data, as required by traditional migration algorithms. In such tectonic setups, most of the downgoing seismic wavefield is reflected toward the surface when interacting with the overburden's top layer. Similarly, the sub-salt upcoming energy is backscattered at the salt's base. Consequently, the actual energy illuminating the sub-salt reflectors, recorded at the surface, is around the noise level. In diapiric trap systems, conventional seismic extrapolation techniques do not guarantee sufficient quality to reduce exploration and production risks; likewise, seismic-based reservoir characterization and monitoring are also compromised. In this regard, accurate wavefield extrapolation techniques based on the Marchenko method may open up new ways to exploit seismic data.

The Marchenko redatuming technique retrieves reliable full-wavefield information in the presence of geologic intrusions, which can be subsequently used to produce artefact-free images by naturally including all orders of multiples present in seismic reflection data. To achieve such a goal, the method relies on the estimation of focusing operators allowing the synthesis of virtual surveys at a given depth level. Still, current Marchenko implementations do not fully incorporate available subsurface models with sharp contrasts, due to the requirements regarding the initialization of the focusing functions. Most importantly, in complex media, even a fairly accurate estimation of a direct wave as a proxy for the required initial focusing functions may not be enough to guarantee sufficiently accurate wavefield reconstruction.

In this talk, we will discuss a scattering-based Marchenko redatuming framework which improves the redatuming of seismic surface data in highly complex media when compared to other Marchenko-based schemes. This extended version is designed to accommodate for band-limited, multi-component, and possibly unevenly sampled seismic data, which contain both free-surface and internal multiples, whilst requiring minimum pre-processing steps. The performance of our scattering Marchenko method will be evaluated using a comprehensive set of numerical tests on a complex 2D subsalt model.

How to cite: Vargas, D., Vasconcelos, I., and Ravasi, M.: Band-limited scattered wavefield reconstruction beneath complex overburdens using the Marchenko method, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12557, https://doi.org/10.5194/egusphere-egu21-12557, 2021.

Hardware and software innovations taking place since the commercial development of seismic reflection imaging in the 60’s and early 70’s have resulted in various improved powerful seismic imaging solutions. Overall, these have been very successful in contrasting geological environments pursuing a wide variety of different targets. The innovative advances in seismic processing may constitute critical tools when analyzing seismic data acquired in highly heterogeneous geologic environments as they can efficiently increase the resolution power. In addition, they can become relevant when using modern acquisition instrumentation and strategies. Furthermore, these new developments significantly increase the value of legacy seismic reflection data. Currently, reassessing controlled source seismic data is becoming a critical issue mostly due to the increasing difficulties for acquiring new profiles posed by environmental regulations and high prices. However, the knowledge of the subsurface is an asset for our society, for example: land-use planning and management; natural risk assessments; or exploration and exploitation for geo-resources. Here we present examples of analysis schemes such as seismic attribute analysis and Common Reflection Surface stacking applied on a number of old seismic reflection profiles (Deep lithospheric transects as well as high resolution profiles) in an effort to bring up their validity. Results indicate how these leading edge methods contribute to significantly improve the quality of vintage seismic data, significantly reducing reflector uncertainties and easing their interpretation.

This research is supported by: Generalitat de Catalunya (AGAUR) grant 2017SGR1022 (GREG); EU (H2020) 871121 (EPOS-SP); EIT-RaewMaterias 17024 (SIT4ME).

How to cite: Carbonell, R., Martinez, Y., de Felipe, I., Alcalde, J., Palomeras, I., Ercoli, M., Ayarza, P., and Borin, E.: Reassessing legacy seismic reflection data. Extracting new information through advanced seismic processing schemes., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8441, https://doi.org/10.5194/egusphere-egu21-8441, 2021.

The nature of the basement beneath the Southern Atlassic front of Tunisia is relatively unknown. To study the basement, a geophysical study was undertaken using gravity, seismic reflection and seismicity data. Additionally, these data were used to determine the relationship and the tectonic environment between the known seismicity and basement structures under the Chotts fold belt and the surrounding basins. Based on 2.5D gravity modeling, 2D seismic reflection profiles and known geological mapping, the geometry of the basement was modeled as consisting of horsts,grabens and half-grabens. Specifically, the Sidi Mansour and El-Fejej basins are located on basement uplifts. The variations in the depths of the known earthquakes reveal that the deepest events occurred on basement faults beneath the Metlaoui and Sidi Mansour basins. While the surrounding anticlines within the northern Chotts range are probably inverted into graben and half-graben structures by both thin- and thick-skinned tectonic events. The geophysical findings indicate that the geometry of the basement to consist of a series of uplifted and downdropped regions, where the depth to basement increases from south to north and from east to west. This basement structure can explain the concentration of earthquakes in the northwestern portion of the study area by linking a reactivation of pre-existing east trending fault systems that formed during Alpine Orogeny. The results provide a coherent model showed a mixed thick and thin-skinned tectonic style was active within the study area. 

How to cite: Frifita, N., Gharbi, M., and Mickus, K.: Conceptual model of basement deformation beneath the Southern Atlassic front of Tunisia from gravity modeling, seismic reflection profiles and seismotectonic data, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-216, https://doi.org/10.5194/egusphere-egu21-216, 2021.

Offshore 2D-Multichannel seismic (MCS)-reflection profiles were acquired in northern Ecuador during the HIPER survey (March/April 2020, R/V L’Atalante) together with one 2D-OBS-seismic-refraction profile (presented in a joint abstract by A. Skrubej). This project (presented in a joint abstract by A. Galve) aims at deciphering the role of lower plate structural heterogeneities and fluids on subduction zone seismogenesis processes within the 2016 Pedernales rupture segment, which is characterized by contrasting slip behaviors. We put a particular emphasis on the segment located at the northern termination of the subducting Carnegie Ridge which was devoid of previous seismic investigations. Three lines of 315-km-long in total, one North of the 2016 Pedernales rupture zone sampling an area experiencing aseismic slip and two lines parallel to the trench, were recorded using an airgun source of 4990 in³ and a 6-km-long streamer. In this study, we present in detail the seismic processing workflow used to produce an enhanced imaging of the Ecuadorian margin, a prerequisite for tackling the project’s objectives.

We performed routine MCS data processing onboard to produce post-stack time migrated sections using Geovation^® CGG’s software. The dip-line collected across the northern Atacames seamounts area provides a detailed image through the whole Nazca oceanic crust down to the Moho, showing a normal crust thickness, at least on the oceanward portion, up to 15 km to the west of the trench. At the trench, we image a horst-like basement topographic high, which outcrops at sea-bottom, offsets the deformation front arcwards, with the outcropping frontal decollement reflector topping this oceanic basement high. Its nature, fluid content potential and lateral extent need to be determined, but its observation at the shallow portion of the interplate megathrust contribute to expand the inventory of subducting rough structures possibly impacting the megathrust frictional slip behavior.

Further advanced processing include noise attenuation, 2D-SRME multiple attenuation, Kirchhoff pre-stack time migration and preserved amplitude pre-stack depth migration (PSDM) performed in the angle domain. The megathrust fault located at the top of the subducting oceanic crust is imaged down to 7 km depth at a distance of 28 km from the trench which will contribute to complement the high-resolution version of the slab’s top topography close to the trench. A joint analyze of this MCS line and the coincident 2D-OBS-refraction Vp model, reveal that variations in moho acoustic features at 15 km distance to the west of the trench correlates with a 30 km wide and >10-km-thick low Vp anomaly. Nearby previous experiment SISTEUR seismic lines are being reprocessed using the same workflow, in order to further investigate the deep crustal seismic structures over the Pedernales 2016 rupture zone.

How to cite: Schenini, L., Skrubej, A., Laigle, M., Ribodetti, A., Combe, L., Galve, A., Rietbrock, A., Charvis, P., Mercier de Lépinay, B., and Marcaillou, B.: Multichannel Seismic Imaging of the Northern Andean subduction margin in Ecuador: preliminary seismic processing results from HIPER campaign., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9551, https://doi.org/10.5194/egusphere-egu21-9551, 2021.

The Himalayan fold-thrust belt has been developing due to the northward convergence of the Indian plate against the Eurasian plate since ~55 Ma. Three major thrust systems: Main Central Thrust (MCT), Main Boundary Thrust (MBT), and Himalayan Frontal Thrust (HFT) are distinctly observed in the Himalayan orogeny from north to south indicating southward propagation of active deformation. These active thrust systems produced several devastating earthquakes in the past such as 1905 Kangra (Mw 7.8), 1934 Nepal-Bihar (Mw 8), and 1950 Assam (Mw 8.6) earthquakes. Presently HFT is found to be the tectonically very active zone that accommodates a strain rate of ~10-15 mm/year and is a zone for great threats in near future to the societies residing over the Himalayan foothills. The present study carried out in the lower Siwalik Himalaya near Pawalgarh in Nainital District of Uttarakhand, India with an objective to estimate the velocity model across HFT in the locality. To accomplish the objective, seismic data were acquired along three profiles of a cumulative length of ~13 km using a seismic thumper as a source and 96 vertical component geophones with the natural frequency of 5 Hz and Remote Acquisition Unites (RAUs) as sensors and data loggers, respectively, and with a group and shot interval of 20 m and near offset of 100 m. Highly uneven Himalayan terrain causes large static errors. In order to overcome this challenge, we used Real Time Kinematics (RTK) to estimate more precise source and receiver surface elevation. In the pre-processing phase of acquired seismic data, three different shots taken at the same location are vertically stacked to eliminate random non-coherent noises and improve the SNR of the data. We then applied a low-frequency array filter (LFAF) to suppress the ground roll using velocity estimates from the ambient noise tomography (ANT). We process the data by implementing conventional seismic processing techniques including normal move-out (NMO) correction, velocity analysis followed by stacking. In the stack section, we observe a northward dipping reflector extending from the surface to ~ 1- 1.25 s TWT indicating evidence of HFT. Another reflector observed at ~3-4 s TWT demarcating the extent of overlying sedimentary deposits on the top of the under-thrusting lithosphere. Rocks of the Siwalik Himalaya mainly composed of sedimentary deposits of sandstone mudstone, and alluvial deposits. Average velocity obtained from the refraction tomography ~ 2900 m/s matches well with rock type in the region. Thus, the high-resolution crustal structure across the highly active HFT can be crucial to understand the earthquake mechanism in the locality and for a better hazard assessment.

How to cite: Verma, S., Ghosal, D., Vats, V., Pandey, S., Anand, P., and Srivastava, H.: Crustal structure across Himalayan Frontal Thrust (HFT) using high-resolution seismic datasets, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11139, https://doi.org/10.5194/egusphere-egu21-11139, 2021.

The Caribbean plate (CAR) collided with and initiated subduction beneath northwestern South America (SA) at about 60-55 Ma. Since the onset of subduction, it has formed the Lara nappes and subsequently the Laramide-style uplifts of the Merida Andes, Sierra de la Perija and Santa Marta ranges, with maximum elevations > 5km. The triangular Maracaibo block, bounded by the Santa Marta-Bucaramanga, Bocono and Oca-Ancon Faults, is currently escaping to the north relative to SA over both the subducting and nonsubducting elements of the CAR plate.

Although many petroleum related seismic studies have been done in this area, the details of the subduction geometry of the CAR plate beneath the Maracaibo block remain unclear. The few deeper seismic investigations are either very large scale, very local, or only peripheral to this area. Previous geodetic studies have suggested that this region has potential for a great (M~8+) earthquake (Bilham and Mencin, 2013). To investigate this complex region we fielded a 65 element broadband seismic array to complement the 48 existing stations of the Colombian and Venezuelan national seismic networks. The array is collectively referred to as the CARMArray.

In this study, we jointly inverted ambient noise Rayleigh wave Z/H ratios, phase velocities in the 8-30s band and ballistic Rayleigh wave phase velocities in 30-80s band to construct a 3D S-wave velocity model in the area from 75^o-65^o west and 5^o-12^o north. Rayleigh wave Z/H ratios are sensitive to the shallow sedimentary structure, while the phase velocity data have good resolution of the crust and upper mantle. The Vs model shows strong low-velocity anomalies beneath the Barinas-Apure and Maracaibo Basins, and the Paraguana Peninsula that are well correlated with surface geology. Sediment thickness beneath the Maracaibo basin reaches up to ~9 km depth, consistent with previous studies (Kellogg & Bonini, 1982). Crustal thickness beneath the Santa Marta uplift is 27-30 km, shallow for its nearly 4km elevation. From the trench to the southeast, Moho depth increases from 25-30 km near the coast to 40-45 km beneath the Maracaibo Basin, with the thickest crust, ~50 km, lying under the Merida Andes beneath the Bocono Fault. Crustal thickness decreases under the Venezeulan interior to ~45 km. From 50km to 150km depth, the CAR plate shows ~2% high Vs anomalies beneath the Santa Marta uplift and the Serrania de Perija range. Our slab image matches local slab seismicity very well (Cornthwaite et al., EGU 2021 GD7.1), and is consistent with and complements images from teleseismic P-wave tomography (Cornthwaite et al, 2021, submitted).

How to cite: Miao, W., Cornthwaite, J., Levander, A., Niu, F., Schmitz, M., Prieto, G., and Dionicio, V.: 3D shear velocity structure of the northwestern South America-Caribbean Subduction Zone from ambient noise and ballistic Rayleigh wave tomography, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13807, https://doi.org/10.5194/egusphere-egu21-13807, 2021.

PS seismic data from the Snøhvit field are compared with forward seismic modelling to understand the effect of azimuthal separation and incidence angle on the imaging of faults. Two faults, one oriented oblique to the survey and one approximately parallel to the survey were chosen. Azimuthally separated W (source is W relative to receivers) and E (source E relative to receivers) data demonstrate that fault imaging is more affected by azimuth when the faults are oblique to the survey orientation, and W data image the faults better. Partial stack data show that with increasing incidence angle there is a systematic improvement in the quality of fault imaging in both the E and W data. In addition, the frequency content of seismic waves back-scattered from within and around fault zones is analysed in the Snøhvit data. Low-medium frequencies are dominant within fault zones, compared with higher frequencies in adjacent areas and haloes of medium frequencies surrounding the faults. Two synthetic experiments support the azimuth, incidence angle and frequency observations. In the first experiment, the fault is modelled as a planar discontinuity and the data were processed in the same way as the Snøhvit data (into separate azimuths and incidence angle stacks). The first experiment confirms a strengthening in the seismic signal from faults in the W data. This is due to the interaction of specular waves and diffractions which are more abundant in the W data. The second experiment had three parts modelling the fault zone with different layering complexity. It proved that frequencies in the fault and adjacent areas increase with fault zone complexity, and that the internal architecture of faults can impact the frequencies in the data adjacent to faults. 

How to cite: Cunningham, J., Weibull, W., Cardozo, N., and Iacopini, D.: Investigating the seismic imaging of faults using PS data from the Snøhvit field, Barents Sea, and forward seismic modelling, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16322, https://doi.org/10.5194/egusphere-egu21-16322, 2021.