Imaging subsurface structures

Major advances on imaging subsurface structures have been made in recent years thanks to 3D seismic exploration, tomography, numerical and forward modelling techniques. However, significant problems, and ambiguities in geological interpretation of subsurface data still remain. This session seeks contributions directly related to the practice of interpretation of the Earth's subsurface tectonic structure using subsurface imaging techniques but also combining geophysical geological observations with numerical modelling approaches. The session will address problems related to deep tectonic features, fault structure, fluid-rock interactions using time lapse imaging, storage structure and more generally basin and tectonic analysis. Contributions may include submissions that advance geophysical or geological concepts and principles of seismic interpretation; correlation with and calibration by geological and engineering data; case studies; algorithms for interpretation; image processing and forward modelling techniques. Contributions that describe interpretation methods and applications involving an integration of multiple datasets to quantify as well as visualize subsurface structure are strongly encouraged. Presentations that focus on the large scale structure , basin exploration , CO2 sequestration, and extraction of mineral resources, using seismic datasets, and coupled to geological observations with numerical experiments are welcomed.

Co-organized by SM5
Convener: David Iacopini | Co-conveners: Sian EvansECSECS, Francesco Emanuele Maesano, Karyna Rodriguez, Debora Presti
vPICO presentations
| Fri, 30 Apr, 15:30–17:00 (CEST)

vPICO presentations: Fri, 30 Apr

Chairpersons: David Iacopini, Francesco Emanuele Maesano, Sian Evans
Brydon Lowney, Lewis Whiting, Ivan Lokmer, Gareth O'Brien, and Christopher Bean

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, 2020.

Jorge Acevedo, Gabriela Fernández-Viejo, Sergio Llana-Fúnez, Luis Pando, Diego Pérez-Millán, Mario Ruiz, and Jordi Díaz

The Variscan belt was formed as a consequence of the collision of two major continents, Laurasia and Gondwana, in the late Paleozoic. Nowadays, it constitutes the basement of the Iberian peninsula (Iberian Massif) and a large part of western and central Europe. In the NW of Spain, the convergence between Iberia and Europe in the Cenozoic originated the uplift of the Cantabrian mountains (CM). In its central sector, the erosion of the Mesozoic sedimentary cover during orogenesis led to the exhumation of the underlying Variscan basement in their western sector. The section of the Variscan belt that is currently exposed in the CM illustrates the transition from the internal zones of an orogen, in the west, to the external ones, to the east.

In order to acquire new passive data from this region, a portable seismic network consisting of 13 three-component broadband stations was deployed (GEOCANTÁBRICA-COSTA, doi:10.7914/SN/YR_2019). The recorded ambient noise seismic signal was cross-correlated using the phase cross-correlation (PCC) processing technique and the resulting daily cross-correlograms were stacked to obtain the empirical Green’s function of the medium between each station pair. Since the vertical and the rotated horizontal components were processed, Rayleigh- and Love-wave group velocity dispersion curves were extracted. From these measurements, group velocity tomographic maps at periods between 2 – 14 s were calculated. Based on this set of tomographic maps, a final 3D S-wave velocity model (2 - 12 km) was derived from the joint inversion of the pseudo-dispersion curves created by extracting the Rayleigh and Love velocity values for each point of a dense grid.

Both the surface-wave and the S-wave velocity maps highlight essential elements of the surface geology of the area. The velocity pattern shows the boundary between two main geological domains: The Cantabrian Zone (CZ), to the east, which constitutes the foreland fold and thrust belt of the Variscan orogen; and the West Asturian-Leonese Zone (WALZ), to the west, the slate belt representing the low grade part of the internal zones. An E-W cross-section of the study area shows a high velocity unit to the west thrusting the lower velocity rocks of the CZ at the transition between the WALZ and the CZ.

How to cite: Acevedo, J., Fernández-Viejo, G., Llana-Fúnez, S., Pando, L., Pérez-Millán, D., Ruiz, M., and Díaz, J.: The Variscan structure of the western Cantabrian Mountains (NW Spain) from ambient noise interferometry, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-20, https://doi.org/10.5194/egusphere-egu21-20, 2020.

Kota Mukumoto and Takeshi Tsuji

In Japan, seismic velocity structures have been estimated by using first arrival tomography method. Many significant crustal structures such as the coordinate of the subducted Philippine Sea plate has been revealed by seismic tomographic images. In this study, we applied the adjoint tomography including full numerical simulation and finite frequency sensitivity kernels for the area of central Japan. The study area is characterized by the very heterogeneous geologic structures. We used 72 natural earthquakes in this study. Because the dominant phase used in our analysis is the surface wave, only S-wave velocity was inverted. We tried to minimize the time-frequency phase misfit between observed and calculated waveforms with the frequency of 0.033~0.1Hz. Based on the checker bord test, our inversion scenario resolved the upper and lower crust. From the results, we identified more heterogeneous structures compared to those from the first arrival tomography. The estimated S-wave velocity model clearly resolved the low velocity anomalies around the active volcanoes. Furthermore, the velocity boundaries agree with the main tectonic lines in the central Japan.

How to cite: Mukumoto, K. and Tsuji, T.: S-wave velocity structure in the central Japan derived from adjoint tomography., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14283, https://doi.org/10.5194/egusphere-egu21-14283, 2021.

Ziyi Xi, Min Chen, Tong Zhou, Baoshan Wang, and Younghee Kim

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.

Alex Hobé, Ari Tryggvason, Bjarne Almqvist, and Olafur Gudmundsson

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.

Cristina Totaro, Giancarlo Neri, Barbara Orecchio, Debora Presti, and Silvia Scolaro

By integrating data and constraints available in the literature, we defined a new “a-priori” 3D seismic velocity model imaging the lithospheric structure of Southern Italy, a highly complex area in the Mediterranean region where the Africa-Europe plate convergence and the residual rollback of the Ionian slab coexist. Involving the integration of multiple datasets and constraints (e.g. velocity patterns from seismic profiles and/or tomographies, moho depth estimates, subduction interface geometries) and following a procedure derived to the one already successfully applied in the area about a decade ago, we obtained the simplest 3D velocity structure consistent with all the available collected data. Studies and analyses performed in recent years allowed us to enlarge and improve the previous estimated model by adding further data and useful constraints. The so obtained "a-priori" velocity model has then been used as starting model for a new earthquake tomographic inversion of the study region. Dataset used for the velocity model computation has been selected from the Italian seismic database (www.ingv.it) and consists of ca. 10000 earthquakes with magnitude equal or greater than 2 and occurred in the time period 2000-2020 at depth less than 60 km and with at least 10 station readings. The obtained 3D velocity structure and the related hypocenter locations have been compared with other geophysical and geological observations and interpreted in the frame of the geodynamic models proposed for the region.

How to cite: Totaro, C., Neri, G., Orecchio, B., Presti, D., and Scolaro, S.: A new a-priori velocity model for seismic tomography investigation of the Southern Italy region, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12376, https://doi.org/10.5194/egusphere-egu21-12376, 2021.

Andrea D'Ambrosio, Eugenio Carminati, Carlo Doglioni, Lorenzo Lipparini, Mario Anselmi, Claudio Chiarabba, Teodoro Cassola, and Jan Federik Derks

The Central Apennines fold-and-thrust belt (Central Italy) is characterized by the presence of several active faults, potentially capable of generating damaging earthquakes. To support seismic hazard studies over the area, a new 3D velocity model was built, integrating a wide range of surface and subsurface data.

The tectonic framework of the area (from Sulmona plain to Maiella Mt), is still debated in literature, also due to the lack of both an adequate geophysical data set and a reliable velocity model at the crustal scale.

In addition, the low number of seismic stations available for the acquisition of Vp/Vs arrival times, and the very low seismicity detected in the study area (the Sulmona and Caramanico Apennine valleys are considered as “seismic gaps”), lead to a difficult interpretation of the subsurface tectonic structures.

3D velocity modelling could well represent an important tool to support these deep crustal reconstructions as well earthquake relocation studies and could enhance the definition of seismogenic faults deep geometries, hence supporting a better risk assessment over the area of these potential locked faults.

Using the knowledge developed within the oil&gas industry as well in gas/CO2 storage projects for the construction of 3D velocity models, extensively used to obtain subsurface imaging and define the geometry of the reservoirs and traps in the depth domain, a similar methodological approach was implemented over the study area.

The subsurface dataset was partially inherited by the past hydrocarbon exploration activities (e.g. seismic lines, exploration wells and sonic logs) and by the literature (e.g. time/depth regional models). Tomographic sections and relocated earthquake hypocentres were also integrated form geophysical studies. Geological maps (1:50.000 & 1:100.000 scale) represent the surface dataset that we used to create the surface interpretation of the regional geology.

As a first step, 18 2D balanced regional geological cross-sections, dip-oriented (W-E) across the Central Apennine, were built define the structural picture at regional scale. The cross-sections were built using MOVE (Petroleum Experts) and Petrel (Schlumberger) software. The following modelling step was the 3D model construction, in which the surface/subsurface data as well as all the geological sections were integrated in the final 3D structural and geological model.

The main geological layers reconstructed in the 3D model were than populated using the appropriated interval velocity values, building the final 3D velocity model in which the lateral velocity variation due to the presence of different facies/geological domains were considered.

As one of the results, we defined several 1D-velocity models coherent with the regional 3D velocity model, in which the key seismic stations and the earthquakes hypocentres dataset for the most potential seismogenic faults were included. 1D models were characterized by different degree of simplification, in order to test diverse approaches for the earthquake relocation. For this exercise, we used public dataset extracted by the analysis of microseismicity of the Sulmona basin.

We believe that the proposed approach can represents an effective method for combining geological and geophysical data to improve the subsurface and seismogenic faults interpretation, contributing to the seismic hazard assessment.

How to cite: D'Ambrosio, A., Carminati, E., Doglioni, C., Lipparini, L., Anselmi, M., Chiarabba, C., Cassola, T., and Derks, J. F.: A new 3D velocity model of Central Apennines: insights from 2D/3D geological modelling and earthquake relocation., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7118, https://doi.org/10.5194/egusphere-egu21-7118, 2021.

Amin Kahrizi, Matthias Delescluse, Mathieu Rodriguez, Pierre-Henri Roche, Anne Bécel, Mladen R Nedimovic, Donna Shillington, Manuel Pubellier, and Nicolas Chamot-Rooke

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.

Cheng-Nan Liu, Fan-Chi Lin, Hsin-Hua Huang, Yu Wang, Elizabeth M. Berg, and Cheng-Horng Lin

Taiwan located at the convergence margin of the Eurasian Plate (EP) and Philippine Sea Plate (PSP) is one of the most active orogenic belts around the world. Under vigorously tectonic activities, the northern Taiwan is composed of complicated geological features including rifting basins, fold-and-thrust systems, volcanoes, and hydrothermal activity. In this study, we apply the technique of Ambient Noise Tomography (ANT) to eight months of continuous waveforms from the Formosa Array and Broadband Array for Seismology in Taiwan (BATS), with 137 broadband stations and ~5km station spacing. We first calculate multi-components cross-correlation functions to extract the information of Rayleigh wave signals. We then invoke Eikonal tomography to calculate the phase velocity map through 3 to 10 second periods and estimate Rayleigh wave ellipticity at each station between 2 to 13 second periods. For each grid point, we jointly invert the two types of Rayleigh wave measurements through a Bayesian-based inversion method to obtain the local 1-D shear wave velocity model. All 1-D models are then combined to construct a comprehensive 3-D model. Our 3-D model reveals upper crustal structures that well correlate with surface geological features. Near the surface, the model delineates the low-velocity Taipei and Ilan basins from the adjacent fast-velocity mountainous areas, with basin geometries consistent with the results of previous geophysical exploration and geological studies. At greater depths, low velocity anomalies are observed associated with the Linkou tableland, Tatun volcano group, and a possible dyke intrusion beneath the southern Ilan basin. The model also provides new geometrical constraints on the major active fault systems in the area, which are important to understand the basin formation and orogeny dynamics. The new 3-D shear wave velocity model allows a comprehensive investigation of shallow geologic structures in northern Taiwan.

How to cite: Liu, C.-N., Lin, F.-C., Huang, H.-H., Wang, Y., Berg, E. M., and Lin, C.-H.: High Resolution 3-D Shear Wave Velocity Model of Northern Taiwan via Bayesian Joint Inversion of Rayleigh Wave Ellipticity and Phase Velocity with Formosa Array, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10658, https://doi.org/10.5194/egusphere-egu21-10658, 2021.

David Vargas, Ivan Vasconcelos, and Matteo Ravasi

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.

Ramon Carbonell, Yesenia Martinez, Irene de Felipe, Juan Alcalde, Imma Palomeras, Maurizio Ercoli, Puy Ayarza, and Edson Borin

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.

Nesrine Frifita, Mohamed Gharbi, and Kevin Mickus

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, 2020.

Anouk Beniest, Michael Schnabel, Anke Dannowski, Florian Schmid, Anna Jegen, and Heidrun Kopp

The northern Lau Basin in the southwest Pacific Ocean is one of the fastest opening back-arc basins on Earth, resulting in a mosaic of microplates, including the Niuafo’ou and Tongan microplates. The Fonualei Rift and Spreading Center (FRSC) is the eastern plate boundary that separates the Niuafo’ou from the Tongan microplate. The northern part of the FRSC is actively spreading, whereas the southern part is rifting. What is unclear, however, is how extension of the Lau Basin is accommodated north and south of the FRSC.

We present the results of six Multi-Channel Seismic profiles acquired during the ARCHIMEDES-I expedition and show an analogue lithosphere-scale model example of our proposed tectonic evolution. Profiles P1 (oriented NW-SE) and P2 (oriented W-E) cover the Mangatolu Triple Junction (MTJ) and the northern part of the FRSC. P3 and P4 (both oriented W-E) cover the southern Niuafo’ou microplate. P5 and P6 (both oriented W-E) cover the area south of the FRSC.

The northern profiles (P1 and P2) reveal a thick package of sediment towards the east, covering a heavily faulted basement over a wide area. Some indication for intrusive material is observed closer to the volcanic arc, but also further towards the western end of P2. Faults cross-cutting the basement but that do not reach the surface are considered inactive today. Faults reach the surface close to the MTJ and the northern tip of the FRSC and are considered active today. This leads to the interpretation that an earlier rift phase accommodated extension in a wide rift tectonic setting, whereas today, the extension is accommodated in a narrow rift or spreading tectonic setting. We will show an analogue model example that demonstrates this wide-to-narrow extensional tectonic evolution.

The profiles that cover the southern extent of the FRSC (P3, P4, P5 and P6), show that active faulting occurs towards the west, close to the Central Lau Spreading Center. Hidden faults that have deformed the basement, but do not affect the surface today anymore are observed in the abyssal parts of P3, P4, P5 and P6. Active faults that reach the surface are also observed towards the east. Recent volcanism is observed, both in the form of intrusive bodies, i.e. sills, as well as volcanoes that pierce through the stratigraphy. The observations lead to the conclusion that south of the FRSC an earlier (wide) rift system affected a larger area in the current abyssal parts of the profiles, whereas extension is currently accommodated through spreading in the CLSC, west of the southern tip of the FRSC.

How to cite: Beniest, A., Schnabel, M., Dannowski, A., Schmid, F., Jegen, A., and Kopp, H.: Tectonic activity of the northern and southern plate boundaries of the Niuafo’ou microplate, Lau Basin, southwest Pacific Ocean, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1557, https://doi.org/10.5194/egusphere-egu21-1557, 2021.

Laure Schenini, Alexandra Skrubej, Mireille Laigle, Alessandra Ribodetti, Laure Combe, Audrey Galve, Andreas Rietbrock, Philippe Charvis, Bernard Mercier de Lépinay, and Boris Marcaillou

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.

Karyna Rodriguez and Neil Hodgson

Seismic data has been and continues to be the main tool for hydrocarbon exploration. Storing very large quantities of seismic data, as well as making it easily accessible and with machine learning functionality, is the way forward to gain regional and local understanding of petroleum systems. Seismic data has been made available as a streamed service through a web-based platform allowing seismic data access on the spot, from large datasets stored in the cloud. A data lake can be defined as transformed data used for tasks such as reporting, visualization, advanced analytics and machine learning. The global library of data has been deconstructed from the rigid flat file format traditionally associated with seismic and transformed into a distributed, scalable, big data store. This allows for rapid access, complex queries, and efficient use of computer power – fundamental criteria for enabling Big Data technologies such as deep learning.  

This data lake concept is already changing the way we access seismic data, enhancing the efficiency of gaining insights into any hydrocarbon basin. Examples include the identification of potentially prolific mixed turbidite/contourite systems in the Trujillo Basin offshore Peru, together with important implications of BSR-derived geothermal gradients, which are much higher than expected in a fore arc setting, opening new exploration opportunities. Another example is de-risking and ranking of offshore Malvinas Basin blocks by gaining new insights into areas until very recently considered to be non-prospective. Further de-risking was achieved by carrying out an in-depth source rock analysis in the Malvinas and conjugate southern South Africa Basins. Additionally, the data lake enabled the development of machine learning algorithms for channel recognition which were successfully applied to data offshore Australia and Norway.

“On demand” regional seismic dataset access is proving invaluable in our efforts to make hydrocarbon exploration more efficient and successful. Machine learning algorithms are helping to automate the more mechanical tasks, leaving time for the more valuable task of analysing the results. The geological insights gained by combining these 2 aspects confirm the value of seismic data lakes.

How to cite: Rodriguez, K. and Hodgson, N.: Geological Insigths Gained from a Seismic Data Lake, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10281, https://doi.org/10.5194/egusphere-egu21-10281, 2021.

Shashank Verma, Dibakar Ghosal, Viaks Vats, Shudhanshu Pandey, Pratyush Anand, and Harshad Srivastava

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.

Wenpei Miao, John Cornthwaite, Alan Levander, Fenglin Niu, Michael Schmitz, German Prieto, and Viviana Dionicio

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 75o-65o west and 5o-12o 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.

Jennifer Cunningham, Wiktor Weibull, Nestor Cardozo, and David Iacopini

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.

Fernando Hutapea, Takeshi Tsuji, Masafumi Katou, and Eiichi Asakawa

The deep-towed Autonomous Continuous Seismic (ACS) is a deep-towed marine seismic acquisition method. The ACS utilizes high frequency seismic source (ranging from 700 Hz to 2300 Hz) and multi-channel receivers that both source and receivers can be located close the seafloor. Moreover, the ACS is suitable to obtain high-resolution image of shallow geological structures. Since ACS data acquisition can be operated near the seafloor, the ocean (strong) current makes the position of both receivers and sources irregular (unstable) and it is hard to measure the absolute depth of both receivers and sources. During data processing, the unstable depth of both sources and receivers not only make the recorded seismic reflection curve (hyperbolic curve) rugged, but also makes the velocity analysis process more difficult because the velocity semblance is not clear. In this study, we propose a processing scheme to solve the unstable source–receiver position problem and thus to construct an accurate final stack profile (Hutapea et al., 2020 doi:10.1016/j.jngse.2020.103573). We used deep-towed ACS data acquired in the Joetsu Basin in Niigata, Japan, where hydrocarbon features in the form of gas chimneys, gas hydrate, and free gas have been observed. Furthermore, sidelobes in the ACS source signature defocus the source wavelet and decrease the bandwidth frequency content. We designed a filter to focus the source signature. Our proposed approach considerably improved the quality of bandwidth frequency of the source signature and the final stacked profile. Even though depth information was not available for all receivers, the velocity semblance was well focused. Our seismic attribute analyses for the final stack section shows that free gas accumulations are characterized by low reflection amplitude and an unstable frequency component, and that hydrate close to the seafloor can be identified by its high reflection amplitude.

How to cite: Hutapea, F., Tsuji, T., Katou, M., and Asakawa, E.: Shallow Imaging of Gas and Hydrate Using the Deep-towed ACS Data in Joetsu Basin, Niigata, Japan, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14384, https://doi.org/10.5194/egusphere-egu21-14384, 2021.

Emma Michie, Mark Mulrooney, and Alvar Braathen

Significant uncertainties occur through varying methodologies when interpreting faults using seismic data.  These uncertainties are carried through to the interpretation of how faults may act as baffles/barriers or increase fluid flow.  Seismic line spacing chosen by the interpreter when picking fault segments, as well as the chosen surface generation algorithm used, will dictate how detailed or smoothed the surface is, and hence will impact any further interpretation such as fault seal, fault stability and fault growth analyses.

This contribution is a case study showing how picking strategies influence analysis of a bounding fault in terms of CO2 storage assessment.  This example utilizes data from the Smeaheia potential storage site within the Horda Platform, 20 km East of Troll East.  This is a fault bound prospect, known as the Alpha prospect, and hence the bounding fault is required to have a high seal potential and low chance of reactivation upon CO2 injection.

We can observe that an optimum spacing for fault interpretation for this case study is set at approximately 100 m.  It appears that any additional detail through interpretation with a line spacing of ≤50 m simply adds further complexities, associated with sensitivities by the individual interpreter.  Hence, interpreting at a finer scale may not necessarily improve the subsurface model and any related analysis, but in fact lead to the production of highly irregular surfaces, which impacts any further fault analysis.  Interpreting on spacing greater than 100 m often leads to overly smoothed fault surfaces that miss details that could be crucial, both for fault seal / stability as well as for fault growth models.

Uncertainty associated with the chosen seismic interpretation methodology will follow through to subsequent fault seal analysis, such as analysis of whether in situ stresses, combined with increased pore pressure through CO2 injection, will act to reactivate the faults, leading to up-fault fluid flow / seep.  We have shown that changing picking strategies significantly alters the interpreted stability of the fault, where picking with an increased line spacing has shown to increase the overall fault stability, and picking using every line leads to the interpretation of a critically stressed fault.  Alternatively, it is important to note that differences in picking strategy show little influence on the overall predicted fault membrane seal (i.e. shale gouge ratio) of the fault, used when interpreting the fault seal capacity for a fault bound CO2 storage site.

How to cite: Michie, E., Mulrooney, M., and Braathen, A.: How Chosen Seismic Fault Picking Strategy Influences Subsequent Fault Analyses: A Case Study from the Horda Platform, with Implications for CO2 storage, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1217, https://doi.org/10.5194/egusphere-egu21-1217, 2021.

Luis Pando, Carlos López-Fernández, Germán Flor-Blanco, and Sergio Llana-Fúnez

The detailed geological mapping in built-up areas presents challenges that arise mainly from the covering of outcrops, and the erase of natural geomorphological features during earthmoving works related to urban development. However, it also benefits from the existence of closely spaced site investigation data, including boreholes, not commonly available outside the cities.

This contribution explains the procedure carried out to improve the interpretation of faults below the city centre of an urban core located in NW Spain. Oviedo is placed on a basin formed by an alternation of sub-horizontal carbonate and siliciclastic formations of Cretaceous age, over which lies an unconformable cover of Paleogene fluvial-lacustrine deposits mostly composed by clays and marls. The paleorelief over which the Paleogene was deposited results in great lateral changes in the thickness of these sediments. Moreover, the basin was deformed during the Alpine convergence in northern Iberia developing an open syncline oriented East-West. During the shortening, a number of minor faults cutting across the gently dipping Cretaceous and Paleogene deposits affect moderately the cartographic pattern of lithostratigraphic units.

Therefore, this research was focused on the preferential use of information on the ground provided by hundreds of rotary boreholes managed through a GIS-type geotechnical database. The procedure of semiautomatic identification consisted essentially of investigating the spatial variations of the boundary between two Cretaceous formations, in order to find anomalies attributable to fault displacements. In using this boundary as a strain marker for post-depositional deformation, two scales were approached, one aimed at the identification of large faults, and another with greater detail based on trend-surface analysis for fractures of smaller size and local incidence (vertical offset less than 10 m).

The research has allowed to discuss faults deduced in previous geological maps, helping to interpret thickenings related to the paleorelief, and also to recognize the existence of structures not described in the regional literature. This study provides also better constrains to the analysis of the structural relationships between the faults affecting the Mesozoic-Palaeogene basin, and the Alpine reactivation of the underlying Palaeozoic basement.

How to cite: Pando, L., López-Fernández, C., Flor-Blanco, G., and Llana-Fúnez, S.: Mapping faults in an urban environment from borehole data (Oviedo, NW Spain), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5294, https://doi.org/10.5194/egusphere-egu21-5294, 2021.