Meijian An, Mei Feng, Marcelo S Assumpção, Marcelo B Bianchi, George S França, Marcelo P Rocha, Leda Sánchez Bettucci
Summary The crust of the South American platform recorded imprints of dynamic processes related with the opening of the Central and South Atlantic but has not been well measured. Crustal structure can be retrieved from teleseismic receiver functions using H-κ stacking, but nearly-parallel stripes of high stacking values existing in stacking images for seismic stations in sedimentary area cause difficulties in identifying solutions. We show that some seemingly spurious stripes that do not point to any layer solution are helpful in the identification of the solution position. With the aid of the auxiliary stripes, we retrieved thicknesses and Vp/Vs of sedimentary and crystalline crust for 65 permanent stations of the Brazilian Seismographic Network and 6 new portable seismic stations in Brazil and Uruguay. The resulted sedimentary thickness and Vp/Vs exhibit a good correlation with the Phanerozoic sediments in the South American basins. The crust of Paraná–Etendeka Large Igneous Province (LIP) had been expected to be more mafic since it had ever been penetrated by mantle magma in the Cretaceous related to the south Atlantic opening. However, we found very low Vp/Vs (1.67) in the crystalline crust beneath the LIP, implying a more felsic crust and that no significant mafic intruding/underplating has occurred in the region. The more felsic crust may be formed in a special evolution early than the magmatic event, or during the magmatic event by releasing crustal volatiles. The resulted sedimentary thickness and Vp/Vs ratios exhibit a good correlation with the Phanerozoic sediments in the South American basins, which implies that Triassic–Jurassic and Cretaceous magmatism did not cause significant metamorphism in sediments formed before the magmatic events.
{"title":"Influence of upwelling mantle magmas on cratonic crust implied from Vp/Vs beneath South America platform","authors":"Meijian An, Mei Feng, Marcelo S Assumpção, Marcelo B Bianchi, George S França, Marcelo P Rocha, Leda Sánchez Bettucci","doi":"10.1093/gji/ggae260","DOIUrl":"https://doi.org/10.1093/gji/ggae260","url":null,"abstract":"Summary The crust of the South American platform recorded imprints of dynamic processes related with the opening of the Central and South Atlantic but has not been well measured. Crustal structure can be retrieved from teleseismic receiver functions using H-κ stacking, but nearly-parallel stripes of high stacking values existing in stacking images for seismic stations in sedimentary area cause difficulties in identifying solutions. We show that some seemingly spurious stripes that do not point to any layer solution are helpful in the identification of the solution position. With the aid of the auxiliary stripes, we retrieved thicknesses and Vp/Vs of sedimentary and crystalline crust for 65 permanent stations of the Brazilian Seismographic Network and 6 new portable seismic stations in Brazil and Uruguay. The resulted sedimentary thickness and Vp/Vs exhibit a good correlation with the Phanerozoic sediments in the South American basins. The crust of Paraná–Etendeka Large Igneous Province (LIP) had been expected to be more mafic since it had ever been penetrated by mantle magma in the Cretaceous related to the south Atlantic opening. However, we found very low Vp/Vs (1.67) in the crystalline crust beneath the LIP, implying a more felsic crust and that no significant mafic intruding/underplating has occurred in the region. The more felsic crust may be formed in a special evolution early than the magmatic event, or during the magmatic event by releasing crustal volatiles. The resulted sedimentary thickness and Vp/Vs ratios exhibit a good correlation with the Phanerozoic sediments in the South American basins, which implies that Triassic–Jurassic and Cretaceous magmatism did not cause significant metamorphism in sediments formed before the magmatic events.","PeriodicalId":12519,"journal":{"name":"Geophysical Journal International","volume":"53 1","pages":""},"PeriodicalIF":2.8,"publicationDate":"2024-07-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141784287","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
A J Girard, J Shragge, M Danilouchkine, C Udengaard, S Gerritsen
Summary Large-scale ocean-bottom node (OBN) arrays of 1000s of multi-component instruments deployed over 1000s of square kilometers have been used successfully for active-source seismic exploration activities including full waveform inversion (FWI) at exploration frequencies above about 2.0 Hz. The analysis of concurrently recorded lower-frequency ambient wavefield data, though, is only just beginning. A key long-term objective of such ambient wavefield analyses is to exploit the sensitivity of sub-2.0 Hz energy to build long-wavelength initial elastic models, thus facilitating FWI applications. However, doing so requires a more detailed understanding of ambient wavefield information recorded on the seafloor, the types, frequency structure and effective source distribution of recorded surface-wave modes, the near-seafloor elastic model structure, and the sensitivity of recorded wave modes to subsurface model structure. To this end, we present a wavefield analysis of low- and ultra-low-frequency ambient data (defined as <1.0 Hz and <0.1 Hz, respectively) acquired on 2712 OBN stations in the Amendment Phase 1 survey covering 2750 km2 of the Gulf of Mexico. After applying ambient data conditioning prior to cross-correlation and seismic cross-coherence interferometry workflows, we demonstrate that the resulting virtual shot gather (VSG) volumes contain evidence for surface-wave and guided P-wave mode propagation between the 0.01-1.0 Hz that remains coherent to distances of at least 80 km. Evidence for surface-wave scattering from near-surface salt-body structure between 0.35-0.85 Hz is also present in a wide spatial distribution of VSG data. Finally, the interferometric VSG volumes clearly show waveform repetition at 20 s intervals in sub-0.3 Hz surface-wave arrivals, a periodicity consistent with the mean active-source shot interval. This suggests that the dominant contribution of surface-wave energy acquired in this VSG frequency band is likely predominantly related to air-gun excitation rather than by naturally occurring energy sources. Overall, these observations may have important consequences for the early stages of initial model building for elastic FWI analysis.
{"title":"Observations from the Seafloor: Ultra-low-frequency ambient ocean-bottom nodal seismology at the amendment field","authors":"A J Girard, J Shragge, M Danilouchkine, C Udengaard, S Gerritsen","doi":"10.1093/gji/ggae249","DOIUrl":"https://doi.org/10.1093/gji/ggae249","url":null,"abstract":"Summary Large-scale ocean-bottom node (OBN) arrays of 1000s of multi-component instruments deployed over 1000s of square kilometers have been used successfully for active-source seismic exploration activities including full waveform inversion (FWI) at exploration frequencies above about 2.0 Hz. The analysis of concurrently recorded lower-frequency ambient wavefield data, though, is only just beginning. A key long-term objective of such ambient wavefield analyses is to exploit the sensitivity of sub-2.0 Hz energy to build long-wavelength initial elastic models, thus facilitating FWI applications. However, doing so requires a more detailed understanding of ambient wavefield information recorded on the seafloor, the types, frequency structure and effective source distribution of recorded surface-wave modes, the near-seafloor elastic model structure, and the sensitivity of recorded wave modes to subsurface model structure. To this end, we present a wavefield analysis of low- and ultra-low-frequency ambient data (defined as &lt;1.0 Hz and &lt;0.1 Hz, respectively) acquired on 2712 OBN stations in the Amendment Phase 1 survey covering 2750 km2 of the Gulf of Mexico. After applying ambient data conditioning prior to cross-correlation and seismic cross-coherence interferometry workflows, we demonstrate that the resulting virtual shot gather (VSG) volumes contain evidence for surface-wave and guided P-wave mode propagation between the 0.01-1.0 Hz that remains coherent to distances of at least 80 km. Evidence for surface-wave scattering from near-surface salt-body structure between 0.35-0.85 Hz is also present in a wide spatial distribution of VSG data. Finally, the interferometric VSG volumes clearly show waveform repetition at 20 s intervals in sub-0.3 Hz surface-wave arrivals, a periodicity consistent with the mean active-source shot interval. This suggests that the dominant contribution of surface-wave energy acquired in this VSG frequency band is likely predominantly related to air-gun excitation rather than by naturally occurring energy sources. Overall, these observations may have important consequences for the early stages of initial model building for elastic FWI analysis.","PeriodicalId":12519,"journal":{"name":"Geophysical Journal International","volume":"46 1","pages":""},"PeriodicalIF":2.8,"publicationDate":"2024-07-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141784292","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
David Bercovici, Jennifer Girard, Elvira Mulyukova
Summary Earth’s plate tectonic behavior arises from lithospheric ductile weakening and shear-localization. The ubiquity of mylonites at lithospheric shear zones is evidence that localization is caused by mineral grain-size reduction. Most lithospheric mylonites are polymineralic, suggesting that the interaction between mineral phases by Zener pinning promotes grain-size reduction and weakening. Yet this interaction only occurs where mineral phases mix at the grain scale. Phase mixing and its effect on microstructure and strength have been shown in deformation experiments and natural field samples. Our theory for the interaction between phase mixing (treated as a stress driven diffusion) with two-phase grain damage has been compared to lab experiments. But using processes at the tiny grain-scale embedded within the small hand-sample and lab scales to model large-scale lithospheric processes, requires an upscaling scheme that captures the physics from micro- to macro-structures. For example, weakening from grain-damage in zones of mixing can lead to banded viscosity structure at the small scale that manifests as viscous anisotropy at the large scale. Here we provide a new framework for self-consistently upscaling from microscopic (grain) scales, to mesoscopic (petrological heterogeneity) scales to macroscopic (tectonic) scales. The first upscaling step models phase mixing and grain-size evolution in a small “mesoscopic” lab-scale volume or “patch”, which is equivalent to a point in the macroscopic space. Within this mesoscale patch, stress driven diffusive mixing is described by an analytical solution for mineral phase fraction, provided a minimalist Fourier representation of phase fraction, and a transformation to the patch frame of reference as well as to the principal stress directions at that point. The orientation and volume fraction of mixed-phase regions can then be extracted from the analytical solution for phase fraction. The grain-size and viscosity in the mixed bands are determined by two-phase grain-damage theory; the unmixed zone properties follow from mono-phase grain damage theory. The mesoscale banded viscosity field leads to a macroscale anisotropic viscosity at that point in space. But, the evolution of properties at each macroscale point involves tracking only a few quantities (phase fraction, grain sizes) rather than modeling each patch of mesoscale space as its own 2-D or 3-D system. For the final upscaling, the anisotropic viscosity field is used in a macroscale lithosphere flow model. We show an example of this scheme for a lithospheric Rayleigh-Taylor drip driven by ridge-push compressive stress, which can cause anisotropic weakening via grain mixing and damage that may help initiate subduction and passive margin collapse.
{"title":"Upscaling from Mineral Microstructures to Tectonic Macrostructures","authors":"David Bercovici, Jennifer Girard, Elvira Mulyukova","doi":"10.1093/gji/ggae263","DOIUrl":"https://doi.org/10.1093/gji/ggae263","url":null,"abstract":"Summary Earth’s plate tectonic behavior arises from lithospheric ductile weakening and shear-localization. The ubiquity of mylonites at lithospheric shear zones is evidence that localization is caused by mineral grain-size reduction. Most lithospheric mylonites are polymineralic, suggesting that the interaction between mineral phases by Zener pinning promotes grain-size reduction and weakening. Yet this interaction only occurs where mineral phases mix at the grain scale. Phase mixing and its effect on microstructure and strength have been shown in deformation experiments and natural field samples. Our theory for the interaction between phase mixing (treated as a stress driven diffusion) with two-phase grain damage has been compared to lab experiments. But using processes at the tiny grain-scale embedded within the small hand-sample and lab scales to model large-scale lithospheric processes, requires an upscaling scheme that captures the physics from micro- to macro-structures. For example, weakening from grain-damage in zones of mixing can lead to banded viscosity structure at the small scale that manifests as viscous anisotropy at the large scale. Here we provide a new framework for self-consistently upscaling from microscopic (grain) scales, to mesoscopic (petrological heterogeneity) scales to macroscopic (tectonic) scales. The first upscaling step models phase mixing and grain-size evolution in a small “mesoscopic” lab-scale volume or “patch”, which is equivalent to a point in the macroscopic space. Within this mesoscale patch, stress driven diffusive mixing is described by an analytical solution for mineral phase fraction, provided a minimalist Fourier representation of phase fraction, and a transformation to the patch frame of reference as well as to the principal stress directions at that point. The orientation and volume fraction of mixed-phase regions can then be extracted from the analytical solution for phase fraction. The grain-size and viscosity in the mixed bands are determined by two-phase grain-damage theory; the unmixed zone properties follow from mono-phase grain damage theory. The mesoscale banded viscosity field leads to a macroscale anisotropic viscosity at that point in space. But, the evolution of properties at each macroscale point involves tracking only a few quantities (phase fraction, grain sizes) rather than modeling each patch of mesoscale space as its own 2-D or 3-D system. For the final upscaling, the anisotropic viscosity field is used in a macroscale lithosphere flow model. We show an example of this scheme for a lithospheric Rayleigh-Taylor drip driven by ridge-push compressive stress, which can cause anisotropic weakening via grain mixing and damage that may help initiate subduction and passive margin collapse.","PeriodicalId":12519,"journal":{"name":"Geophysical Journal International","volume":"43 1","pages":""},"PeriodicalIF":2.8,"publicationDate":"2024-07-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141784288","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Summary Many intracontinental basins form as broad depressions through prolonged, slow subsidence of the continental lithosphere. Such long-lived basins can record lithospheric processes over hundreds of millions of years, serving as important archives of lithospheric evolution. Since continental amalgamation in the Mesoproterozoic, the lithosphere beneath the intracontinental Canning Basin has been subject to several tectonic events, with extensive crustal reworking evidenced through different upper crust datasets. However, knowledge of the structure of the subcontinental lithospheric mantle is lacking. As a consequence, understanding the coupled evolution between surface and deep lithospheric processes, crucial to resolving basin formation, development, and survival, remains problematic. Here, we combine geochemical, geophysical, and petrophysical data within a thermodynamic modelling framework to determine the thermochemical properties, rheology, density, and seismic structure of the lithospheric and sublithospheric mantle beneath the Canning Basin. The results indicate a thick, rigid lithosphere with a maximum thickness of 185 km and strength of ca. 1$times$1013 Pa m, and an anomalously Fe-enriched subcontinental lithospheric mantle with a Mg# of 88.6. This mantle structure is not consistent with pre-collisional fragments or a Precambrian collisional setting and may reflect magmatic refertilisation during high-volume mafic magmatic events. Potential candidate events are the ∼1070 Ma Warakurna, ∼825 Ma Gairdner, and ∼510 Ma Kalkarindji Large Igneous Provinces. The youngest of these is temporally and spatially correlated with and therefore interpreted to have influenced the Canning Basin formation. We propose that refertilisation caused a negatively buoyant subcontinental lithospheric mantle and prolonged subsidence and preservation of the basin, while the strong lithosphere ensured lithospheric stability and longevity.
摘要 许多大陆内盆地是通过大陆岩石圈长期、缓慢的下沉而形成的宽阔洼地。这种长寿盆地可以记录数亿年的岩石圈过程，是岩石圈演化的重要档案。自中新生代大陆合并以来，大陆内部坎宁盆地下的岩石圈经历了多次构造事件，不同的上地壳数据集证明了广泛的地壳再加工。然而，人们对次大陆岩石圈地幔的结构缺乏了解。因此，理解表层和深层岩石圈过程之间的耦合演化，对于解决盆地的形成、发展和生存至关重要，但仍然是个问题。在此，我们在热力学建模框架内结合地球化学、地球物理和岩石物理数据，确定了坎宁盆地下岩石圈和亚岩石圈地幔的热化学性质、流变学、密度和地震结构。结果表明，岩石圈厚而坚硬，最大厚度为 185 千米，强度约为 1 美元乘以 1013 帕米，亚大陆岩石圈地幔异常富集铁，镁含量为 88.6。这种地幔结构与碰撞前的碎片或前寒武纪碰撞环境不一致，可能反映了大体积岩浆事件中的岩浆再营养过程。潜在的候选事件有：1070 Ma ∼ Warakurna、825 Ma ∼ Gairdner 和 510 Ma ∼ Kalkarindji 大火成岩省。其中最年轻的一个在时间和空间上与坎宁盆地的形成相关，因此被解释为影响了坎宁盆地的形成。我们认为，再营养作用造成了负浮力的次大陆岩石圈地幔，延长了盆地的沉降和保存时间，而坚固的岩石圈则确保了岩石圈的稳定性和长久性。
{"title":"Anomalously fertile subcontinental lithospheric mantle beneath the intracontinental Canning Basin, Western Australia","authors":"P S Moro, A R A Aitken, F Kohanpour, M W Jessell","doi":"10.1093/gji/ggae258","DOIUrl":"https://doi.org/10.1093/gji/ggae258","url":null,"abstract":"Summary Many intracontinental basins form as broad depressions through prolonged, slow subsidence of the continental lithosphere. Such long-lived basins can record lithospheric processes over hundreds of millions of years, serving as important archives of lithospheric evolution. Since continental amalgamation in the Mesoproterozoic, the lithosphere beneath the intracontinental Canning Basin has been subject to several tectonic events, with extensive crustal reworking evidenced through different upper crust datasets. However, knowledge of the structure of the subcontinental lithospheric mantle is lacking. As a consequence, understanding the coupled evolution between surface and deep lithospheric processes, crucial to resolving basin formation, development, and survival, remains problematic. Here, we combine geochemical, geophysical, and petrophysical data within a thermodynamic modelling framework to determine the thermochemical properties, rheology, density, and seismic structure of the lithospheric and sublithospheric mantle beneath the Canning Basin. The results indicate a thick, rigid lithosphere with a maximum thickness of 185 km and strength of ca. 1$times$1013 Pa m, and an anomalously Fe-enriched subcontinental lithospheric mantle with a Mg# of 88.6. This mantle structure is not consistent with pre-collisional fragments or a Precambrian collisional setting and may reflect magmatic refertilisation during high-volume mafic magmatic events. Potential candidate events are the ∼1070 Ma Warakurna, ∼825 Ma Gairdner, and ∼510 Ma Kalkarindji Large Igneous Provinces. The youngest of these is temporally and spatially correlated with and therefore interpreted to have influenced the Canning Basin formation. We propose that refertilisation caused a negatively buoyant subcontinental lithospheric mantle and prolonged subsidence and preservation of the basin, while the strong lithosphere ensured lithospheric stability and longevity.","PeriodicalId":12519,"journal":{"name":"Geophysical Journal International","volume":"89 1","pages":""},"PeriodicalIF":2.8,"publicationDate":"2024-07-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141784293","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Ashar Muda Lubis, Rino Salman, Iwan Hermawan, Kyle Bradley, Lujia Feng, Qiang Qiu, Rio Sahputra, Danny H Natawidjaja, Kerry Sieh, Emma M Hill
Summary The Sumatran Fault Zone (SFZ) of the Indonesian island of Sumatra, which is broken up into 19 fault segments, accommodates much of the trench-parallel component of the oblique convergence between the Indo-Australian and Sunda plates. To understand the potential hazard of SFZ earthquakes to the local population, we investigate slip rates and locking depths of three SFZ segments in southern Sumatra using previously unpublished data from our Sumatran Fault Monitoring (SuMo) campaign Global Positioning System (GPS) network. We model the GPS data using a two-dimensional interseismic dislocation model optimized using a Bayesian approach. For the Musi segment of the SFZ, we find that slip rates ranging from 10 to 22 mm/year and locking depths from 1 to 20 km fit the data similarly well, suggesting a lack of resolution for the SuMo network in this segment. For the Manna and Kumering segments where the resolution is better, the estimated slip rates are 18 [12–22, 95 per cent confidence intervals] mm/year and 12 [9–15] mm/year, respectively, while the estimated locking depths are 29 [15–47] km and 5 [3–16] km, respectively. The deep locking depth estimated for the Manna segment can be explained by the large station gap in this segment. Considering the uncertainty, all the estimated slip rates from our study remains aligned with the SFZ's average slip rate of ∼15 mm/year, which was previously derived using updated geological slip rates and geodetic block modelling of the entire SFZ. Our results support the idea that the forearc sliver west of the SFZ behaves as a rigid microplate.
{"title":"Slip rates and locking depths of the southern Sumatran Fault Zone revealed by new campaign GPS observations","authors":"Ashar Muda Lubis, Rino Salman, Iwan Hermawan, Kyle Bradley, Lujia Feng, Qiang Qiu, Rio Sahputra, Danny H Natawidjaja, Kerry Sieh, Emma M Hill","doi":"10.1093/gji/ggae257","DOIUrl":"https://doi.org/10.1093/gji/ggae257","url":null,"abstract":"Summary The Sumatran Fault Zone (SFZ) of the Indonesian island of Sumatra, which is broken up into 19 fault segments, accommodates much of the trench-parallel component of the oblique convergence between the Indo-Australian and Sunda plates. To understand the potential hazard of SFZ earthquakes to the local population, we investigate slip rates and locking depths of three SFZ segments in southern Sumatra using previously unpublished data from our Sumatran Fault Monitoring (SuMo) campaign Global Positioning System (GPS) network. We model the GPS data using a two-dimensional interseismic dislocation model optimized using a Bayesian approach. For the Musi segment of the SFZ, we find that slip rates ranging from 10 to 22 mm/year and locking depths from 1 to 20 km fit the data similarly well, suggesting a lack of resolution for the SuMo network in this segment. For the Manna and Kumering segments where the resolution is better, the estimated slip rates are 18 [12–22, 95 per cent confidence intervals] mm/year and 12 [9–15] mm/year, respectively, while the estimated locking depths are 29 [15–47] km and 5 [3–16] km, respectively. The deep locking depth estimated for the Manna segment can be explained by the large station gap in this segment. Considering the uncertainty, all the estimated slip rates from our study remains aligned with the SFZ's average slip rate of ∼15 mm/year, which was previously derived using updated geological slip rates and geodetic block modelling of the entire SFZ. Our results support the idea that the forearc sliver west of the SFZ behaves as a rigid microplate.","PeriodicalId":12519,"journal":{"name":"Geophysical Journal International","volume":"82 1","pages":""},"PeriodicalIF":2.8,"publicationDate":"2024-07-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141784295","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Summary Moment tensor (MT) inversion is a classical geophysical inverse problem that infers a force-equivalent model of a seismic source from seismological observations. Like other inverse problems, the accuracy of the inversion depends on the reliability of the forward problem simulating waveforms from the source location through an Earth structural model. Apart from errors in data, the error in forward waveform simulation, also known as theory error, is a significant source of error contributing to the misfit function between the predicted and observed waveforms. Here, we set up numerical experiments to comprehensively probe the sensitivity of the linearized MT inversion to 3D regional Earth model errors, a known predominant factor of the theory error. Using the Monte Carlo method, we estimate the empirical structural covariance matrices to characterize the waveform mismatch due to the imperfect knowledge of Earth's structure. Firstly, although the inversion accuracy deteriorates with increasing model errors, incorporating the structural covariance matrices into the misfit function improves the accuracy of inversion results for all theorized error distributions. Secondly, we propose a slightly modified form of the structural covariance matrix, which further enhances the inversion outcome. Lastly, as the true structural errors are likely spatially correlated, we highlight the importance of adequately treating the correlation into the MT inversion because of its significant impact on inversion. Overall, as a preliminary effort in quantifying 3D structural errors on MT inversion, this study proves the computational feasibility by means of numerical experiments and will hopefully provide a way forward for future work on this topic.
{"title":"Towards a new standard for seismic moment tensor inversion containing 3D Earth structure uncertainty","authors":"T-S Phạm, H Tkalčić, J Hu, S Kim","doi":"10.1093/gji/ggae256","DOIUrl":"https://doi.org/10.1093/gji/ggae256","url":null,"abstract":"Summary Moment tensor (MT) inversion is a classical geophysical inverse problem that infers a force-equivalent model of a seismic source from seismological observations. Like other inverse problems, the accuracy of the inversion depends on the reliability of the forward problem simulating waveforms from the source location through an Earth structural model. Apart from errors in data, the error in forward waveform simulation, also known as theory error, is a significant source of error contributing to the misfit function between the predicted and observed waveforms. Here, we set up numerical experiments to comprehensively probe the sensitivity of the linearized MT inversion to 3D regional Earth model errors, a known predominant factor of the theory error. Using the Monte Carlo method, we estimate the empirical structural covariance matrices to characterize the waveform mismatch due to the imperfect knowledge of Earth's structure. Firstly, although the inversion accuracy deteriorates with increasing model errors, incorporating the structural covariance matrices into the misfit function improves the accuracy of inversion results for all theorized error distributions. Secondly, we propose a slightly modified form of the structural covariance matrix, which further enhances the inversion outcome. Lastly, as the true structural errors are likely spatially correlated, we highlight the importance of adequately treating the correlation into the MT inversion because of its significant impact on inversion. Overall, as a preliminary effort in quantifying 3D structural errors on MT inversion, this study proves the computational feasibility by means of numerical experiments and will hopefully provide a way forward for future work on this topic.","PeriodicalId":12519,"journal":{"name":"Geophysical Journal International","volume":"43 1","pages":""},"PeriodicalIF":2.8,"publicationDate":"2024-07-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141784289","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Eduardo Valero Cano, Andreas Fichtner, Daniel Peter, P Martin Mai
Summary Cross-correlations of seismic ambient noise are frequently used to image Earth structure. Usually, tomographic studies assume that noise sources are uniformly distributed and interpret noise correlations as empirical Green’s functions. However, previous research suggests that this assumption can introduce errors in the estimated models, especially when noise correlation waveforms are inverted. In this paper, we investigate changes in subsurface models inferred from noise correlation waveforms depending on whether the noise source distribution is considered to be uniform. To this end, we set up numerical experiments that mimic a tomographic study in Southern California exploiting ambient noise generated in the Pacific Ocean. Our results show that if the distribution of noise sources is deemed uniform instead of being numerically represented in the wave simulations, the misfit of the estimated models increases. In our experiments, the model misfit increase ranges between 5 % and 21 %, depending on the heterogeneity of the noise source distribution. This indicates that assuming uniform noise sources introduces source-dependent model errors. Since the location of noise sources may change over time, these errors are also time-dependent. In order to mitigate these errors, it is necessary to account for the noise source distribution. The spatial extent to which noise sources must be considered depends on the propagation distance of the ambient noise wavefield. If only sources near the study area are considered, model errors may arise.
{"title":"The impact of ambient noise sources in subsurface models estimated from noise correlation waveforms","authors":"Eduardo Valero Cano, Andreas Fichtner, Daniel Peter, P Martin Mai","doi":"10.1093/gji/ggae259","DOIUrl":"https://doi.org/10.1093/gji/ggae259","url":null,"abstract":"Summary Cross-correlations of seismic ambient noise are frequently used to image Earth structure. Usually, tomographic studies assume that noise sources are uniformly distributed and interpret noise correlations as empirical Green’s functions. However, previous research suggests that this assumption can introduce errors in the estimated models, especially when noise correlation waveforms are inverted. In this paper, we investigate changes in subsurface models inferred from noise correlation waveforms depending on whether the noise source distribution is considered to be uniform. To this end, we set up numerical experiments that mimic a tomographic study in Southern California exploiting ambient noise generated in the Pacific Ocean. Our results show that if the distribution of noise sources is deemed uniform instead of being numerically represented in the wave simulations, the misfit of the estimated models increases. In our experiments, the model misfit increase ranges between 5 % and 21 %, depending on the heterogeneity of the noise source distribution. This indicates that assuming uniform noise sources introduces source-dependent model errors. Since the location of noise sources may change over time, these errors are also time-dependent. In order to mitigate these errors, it is necessary to account for the noise source distribution. The spatial extent to which noise sources must be considered depends on the propagation distance of the ambient noise wavefield. If only sources near the study area are considered, model errors may arise.","PeriodicalId":12519,"journal":{"name":"Geophysical Journal International","volume":"73 1","pages":""},"PeriodicalIF":2.8,"publicationDate":"2024-07-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141784294","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Yu Shi, Jifeng Zhang, Xiran You, Ziben Ma, Jiachen Li
Summary The conventional transient electromagnetic inversion method has a low calculation speed and precision and is susceptible to falling into local minima, which does not meet the fine detection requirements of urban underground space. In this study, we proposed a novel inversion method based on convolutional bidirectional long short-term memory neural networks for shallow subsurface transient electromagnetic inversion. This network structure possessed strong spatial feature extraction capabilities and a proficient understanding of sequential data, thereby addressing the issues of slow conventional inversion computations and inadequate inversion accuracy. Utilizing the apparent resistivity from a three-layer model as the sample input and the real model as the target, the network was trained using batch normalization and dropout techniques to accelerate the convergence rate. The resulting model achieved real-time inversion speeds and high accuracy, with robust generalization capabilities and adaptability to new data. To assess the inversion performance, we used a novel one-dimensional inversion error calculation index, the correlation area loss error, for a more accurate measurement. Numerical simulation experiments showed that the proposed method required only 2.121 ss to invert data from 100 observation points. The inversion efficiency was significantly superior to the conventional methods, maintaining excellent accuracy while effectively discerning subsurface electrical stratification in geophysics. Applying convolutional bidirectional long short-term memory neural networks to multi-dimensional and field data yielded results superior to those of conventional inversion, demonstrating the promising applicability and generalization of this approach. This study offers an efficient solution for shallow subsurface transient electromagnetic exploration and holds potential for application in other areas.
{"title":"Transient electromagnetic inversion to image the shallow subsurface based on convolutional bidirectional Long Short-Term Memory neural networks","authors":"Yu Shi, Jifeng Zhang, Xiran You, Ziben Ma, Jiachen Li","doi":"10.1093/gji/ggae253","DOIUrl":"https://doi.org/10.1093/gji/ggae253","url":null,"abstract":"Summary The conventional transient electromagnetic inversion method has a low calculation speed and precision and is susceptible to falling into local minima, which does not meet the fine detection requirements of urban underground space. In this study, we proposed a novel inversion method based on convolutional bidirectional long short-term memory neural networks for shallow subsurface transient electromagnetic inversion. This network structure possessed strong spatial feature extraction capabilities and a proficient understanding of sequential data, thereby addressing the issues of slow conventional inversion computations and inadequate inversion accuracy. Utilizing the apparent resistivity from a three-layer model as the sample input and the real model as the target, the network was trained using batch normalization and dropout techniques to accelerate the convergence rate. The resulting model achieved real-time inversion speeds and high accuracy, with robust generalization capabilities and adaptability to new data. To assess the inversion performance, we used a novel one-dimensional inversion error calculation index, the correlation area loss error, for a more accurate measurement. Numerical simulation experiments showed that the proposed method required only 2.121 ss to invert data from 100 observation points. The inversion efficiency was significantly superior to the conventional methods, maintaining excellent accuracy while effectively discerning subsurface electrical stratification in geophysics. Applying convolutional bidirectional long short-term memory neural networks to multi-dimensional and field data yielded results superior to those of conventional inversion, demonstrating the promising applicability and generalization of this approach. This study offers an efficient solution for shallow subsurface transient electromagnetic exploration and holds potential for application in other areas.","PeriodicalId":12519,"journal":{"name":"Geophysical Journal International","volume":"62 1","pages":""},"PeriodicalIF":2.8,"publicationDate":"2024-07-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141784290","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Summary Magnetotelluric data are sometimes accompanied by ‘anomalous’ impedance phases ($phi $xy and $phi $yx) in the off-diagonal components deviating from the first (0º < $phi $xy < 90º) or third (−180º < $phi $yx < −90º) quadrant, especially in long-period bands. This phenomenon is called the phases out-of-quadrant (POQ). The POQ poses a challenge in Magnetotelluric modeling because simple one- or two-dimensional models cannot explain it. Previous studies have reported that strong inhomogeneity, anisotropy, or particular three-dimensional structures, such as the L-shaped or cross-shaped conductors, could explain the POQ. Aside from these models, we have discovered that a slanted columnar conductor also generates the POQ. Our systematic investigation through the synthetic forward modeling of an inclined conductive column with a varying geometry showed that the inclination angle and the column length may affect the POQ appearance. We investigated herein the behavior of the electric currents around the inclined conductive column embedded in a resistive half space. We found that the induced electric field in the region with the POQ tends to point in the opposite direction to the surrounding vectors. This result can reasonably explain the inverted phase in long-period bands. Furthermore, we confirmed that current is sucked into one end of the column, but discharged from the other end, suggesting that the column works as a current channel. The localized reverse vectors are associated with the current channeling along the inclined conductor, which generates the POQ. A volcanic conduit within a resistive host rock is one of the typical field examples of such an inclined channel. Our study suggests that the POQ is a helpful clue in imaging the geometry of a volcanic magma plumbing system through Magnetotelluric surveys.
{"title":"The inclined conductive column effect: A new simple model for magnetotelluric anomalous phases","authors":"Tomohiro Inoue, Takeshi Hashimoto","doi":"10.1093/gji/ggae252","DOIUrl":"https://doi.org/10.1093/gji/ggae252","url":null,"abstract":"Summary Magnetotelluric data are sometimes accompanied by ‘anomalous’ impedance phases ($phi $xy and $phi $yx) in the off-diagonal components deviating from the first (0º &lt; $phi $xy &lt; 90º) or third (−180º &lt; $phi $yx &lt; −90º) quadrant, especially in long-period bands. This phenomenon is called the phases out-of-quadrant (POQ). The POQ poses a challenge in Magnetotelluric modeling because simple one- or two-dimensional models cannot explain it. Previous studies have reported that strong inhomogeneity, anisotropy, or particular three-dimensional structures, such as the L-shaped or cross-shaped conductors, could explain the POQ. Aside from these models, we have discovered that a slanted columnar conductor also generates the POQ. Our systematic investigation through the synthetic forward modeling of an inclined conductive column with a varying geometry showed that the inclination angle and the column length may affect the POQ appearance. We investigated herein the behavior of the electric currents around the inclined conductive column embedded in a resistive half space. We found that the induced electric field in the region with the POQ tends to point in the opposite direction to the surrounding vectors. This result can reasonably explain the inverted phase in long-period bands. Furthermore, we confirmed that current is sucked into one end of the column, but discharged from the other end, suggesting that the column works as a current channel. The localized reverse vectors are associated with the current channeling along the inclined conductor, which generates the POQ. A volcanic conduit within a resistive host rock is one of the typical field examples of such an inclined channel. Our study suggests that the POQ is a helpful clue in imaging the geometry of a volcanic magma plumbing system through Magnetotelluric surveys.","PeriodicalId":12519,"journal":{"name":"Geophysical Journal International","volume":"27 1","pages":""},"PeriodicalIF":2.8,"publicationDate":"2024-07-23","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141784355","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Summary This study aims to expand on existing connections between magnetic minerals and hydrocarbons within petroleum systems. Previous studies have focussed on single-source petroleum systems whereas this study, for the first time, analyses a multi-source petroleum system to investigate potential correlations between different kerogen type source rocks and magnetic minerals. To do this, the study investigates the magnetic mineral characteristics of the Inner Moray Firth (IMF), UK North Sea, through room-, low-, and high-temperature techniques, and correlates this to published basin and petroleum systems modelling results that show a three-source hydrocarbon mix. Magnetic mineral analysis identifies extensive evidence for magnetite, goethite, and siderite, alongside more minor lepidocrocite and iron sulphides. Although we find that magnetite is ubiquitous within the IMF, its abundance is relatively low, and, in contrast, the relatively magnetically weak goethite is more likely the most abundant magnetic mineral throughout the IMF. In agreement with previous studies, we find magnetic enhancement at oil-water contacts (OWCs); however, here, we identify two different magnetic enhancement processes at OWCs in wells, which are dependent on the amount of sulphur available in the local environment. Wells with low levels of sulphur have increasing levels of magnetite towards the OWC, with the magnetic enhancement occurring at the top of the water-saturated section. Sulphur-rich environments display an increase in iron sulphides near the OWC at the bottom of the oil-saturated sediments. Additionally, we confirm the presence of siderite as indicator of upward vertical migration. Combining with petroleum system model predictions, we find direct links between iron hydroxide presence and Type I and II-III kerogen source rocks, and iron sulphide presence with Type II kerogen source rocks. This study shows the potential for further utilisation of magnetic mineral analysis within hydrocarbon exploration and petroleum system definition.
摘要 本研究旨在拓展石油系统中磁性矿物与碳氢化合物之间的现有联系。以往的研究侧重于单源石油系统，而本研究则首次分析了多源石油系统，以研究不同的角质源岩和磁性矿物之间的潜在关联。为此，该研究通过室温、低温和高温技术研究了英国北海内莫里峡湾（IMF）的磁性矿物特征，并将其与已公布的盆地和石油系统建模结果（显示了三源碳氢化合物组合）相关联。磁性矿物分析确定了磁铁矿、鹅铁矿和菱铁矿的大量证据，以及更少量的鳞铁矿和硫化铁。尽管我们发现磁铁矿在 IMF 中无处不在，但其丰度相对较低，相比之下，磁性相对较弱的鹅铁矿更有可能是整个 IMF 中最丰富的磁性矿物。与之前的研究一致，我们在油水接触点（OWCs）发现了磁性增强现象；不过，在这里，我们在油井的 OWCs 发现了两种不同的磁性增强过程，这取决于当地环境中的硫含量。硫含量低的油井中，磁铁矿的含量向 OWC 方向不断增加，磁性增强发生在水饱和段的顶部。硫含量丰富的环境显示，在石油饱和沉积物底部的 OWC 附近，硫化铁含量增加。此外，我们还证实菱铁矿的存在是向上垂直迁移的指标。结合石油系统模型的预测，我们发现氢氧化铁的存在与 I 类和 II-III 类角质源岩有直接联系，硫化铁的存在与 II 类角质源岩有直接联系。这项研究显示了在油气勘探和石油系统定义中进一步利用磁性矿物分析的潜力。
{"title":"Quantifying the mineral magnetic signature of petroleum systems and their source rocks: A study on the Inner Moray Firth, UK North Sea","authors":"J R Perkins, A R Muxworthy, A J Fraser, P Hu","doi":"10.1093/gji/ggae254","DOIUrl":"https://doi.org/10.1093/gji/ggae254","url":null,"abstract":"Summary This study aims to expand on existing connections between magnetic minerals and hydrocarbons within petroleum systems. Previous studies have focussed on single-source petroleum systems whereas this study, for the first time, analyses a multi-source petroleum system to investigate potential correlations between different kerogen type source rocks and magnetic minerals. To do this, the study investigates the magnetic mineral characteristics of the Inner Moray Firth (IMF), UK North Sea, through room-, low-, and high-temperature techniques, and correlates this to published basin and petroleum systems modelling results that show a three-source hydrocarbon mix. Magnetic mineral analysis identifies extensive evidence for magnetite, goethite, and siderite, alongside more minor lepidocrocite and iron sulphides. Although we find that magnetite is ubiquitous within the IMF, its abundance is relatively low, and, in contrast, the relatively magnetically weak goethite is more likely the most abundant magnetic mineral throughout the IMF. In agreement with previous studies, we find magnetic enhancement at oil-water contacts (OWCs); however, here, we identify two different magnetic enhancement processes at OWCs in wells, which are dependent on the amount of sulphur available in the local environment. Wells with low levels of sulphur have increasing levels of magnetite towards the OWC, with the magnetic enhancement occurring at the top of the water-saturated section. Sulphur-rich environments display an increase in iron sulphides near the OWC at the bottom of the oil-saturated sediments. Additionally, we confirm the presence of siderite as indicator of upward vertical migration. Combining with petroleum system model predictions, we find direct links between iron hydroxide presence and Type I and II-III kerogen source rocks, and iron sulphide presence with Type II kerogen source rocks. This study shows the potential for further utilisation of magnetic mineral analysis within hydrocarbon exploration and petroleum system definition.","PeriodicalId":12519,"journal":{"name":"Geophysical Journal International","volume":"48 1","pages":""},"PeriodicalIF":2.8,"publicationDate":"2024-07-23","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141784356","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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