For accurate modeling of groundwater flow and transport processes within an aquifer, precise knowledge about hydraulic conductivity K and its small-scale heterogeneities is fundamental. Methods based on pumping tests, such as hydraulic tomography (HT), allow for retrieving reliable K-estimates, but are limited in their ability to image structural features with high resolution, since the data from time-consuming hydraulic tests are commonly sparse. In contrast, geophysical methods like induced polarization (IP) can potentially yield structural images of much higher resolution, but depend on empirical petrophysical laws that may introduce significant uncertainties to the K-estimation. Therefore, this paper presents a joint inversion procedure for both HT and IP data, which allows for combining the complementary abilities of both methods. Within this approach, a travel time inversion is applied to the HT data, while the IP inversion is based on a full-decay time-domain forward response, as well as a re-parameterization of the Cole-Cole model to invert for K directly. The joint inversion is tested on a synthetic model mimicking horizontally layered sediments, and the results are compared with the individual HT and IP inversions. It is shown that jointly inverting both data sets consistently improves the results by combining the complementary sensitivities of the two methods, and that the inversion is more robust against changes in the experimental setups. Furthermore, we illustrate how a joint inversion approach can correct biases within the petrophysical laws by including reliable K-information from hydraulic tests and still preserving the high-resolution structural information from IP. The different inversion results are compared based on the structural similarity index (SSIM), which underlines the robustness of the joint inversion compared to using the data individually. Hence, the combined application of HT and IP within field surveys and a subsequent joint inversion of both data sets may improve our understanding of hydraulically relevant subsurface structures, and thus the reliability of groundwater modeling results.
要对含水层内的地下水流动和传输过程进行精确建模，就必须精确了解水力传导率 K 及其小尺度异质性。基于抽水试验的方法，如水力层析成像法（HT），可以获得可靠的 K 估计值，但由于耗时的水力试验数据通常比较稀少，因此在对结构特征进行高分辨率成像方面能力有限。相比之下，诱导极化（IP）等地球物理方法有可能生成分辨率更高的构造图像，但这种方法依赖于经验岩石物理定律，可能会给 K 值估算带来很大的不确定性。因此，本文提出了 HT 和 IP 数据的联合反演程序，将两种方法的互补能力结合起来。在这种方法中，旅行时间反演应用于 HT 数据，而 IP 反演则基于全衰减时域前向响应，以及对 Cole-Cole 模型的重新参数化，以直接反演 K。联合反演在模拟水平分层沉积物的合成模型上进行了测试，并将结果与单独的 HT 和 IP 反演进行了比较。结果表明，通过结合两种方法的互补敏感性，对两个数据集进行联合反演可以持续改进结果，而且反演对实验设置的变化具有更强的鲁棒性。此外，我们还说明了联合反演方法如何通过纳入水力测试中可靠的 K 信息，同时保留 IP 中的高分辨率结构信息，来纠正岩石物理定律中的偏差。根据结构相似性指数（SSIM）对不同的反演结果进行了比较，结果表明，与单独使用数据相比，联合反演具有稳健性。因此，在野外勘测中结合应用 HT 和 IP，并随后对两套数据进行联合反演，可以提高我们对与水文相关的地下结构的理解，从而提高地下水建模结果的可靠性。
{"title":"Joint inversion of induced polarization and hydraulic tomography data for hydraulic conductivity imaging","authors":"Lukas Römhild, G. Fiandaca, Peter Bayer","doi":"10.1093/gji/ggae197","DOIUrl":"https://doi.org/10.1093/gji/ggae197","url":null,"abstract":"\u0000 For accurate modeling of groundwater flow and transport processes within an aquifer, precise knowledge about hydraulic conductivity K and its small-scale heterogeneities is fundamental. Methods based on pumping tests, such as hydraulic tomography (HT), allow for retrieving reliable K-estimates, but are limited in their ability to image structural features with high resolution, since the data from time-consuming hydraulic tests are commonly sparse. In contrast, geophysical methods like induced polarization (IP) can potentially yield structural images of much higher resolution, but depend on empirical petrophysical laws that may introduce significant uncertainties to the K-estimation. Therefore, this paper presents a joint inversion procedure for both HT and IP data, which allows for combining the complementary abilities of both methods. Within this approach, a travel time inversion is applied to the HT data, while the IP inversion is based on a full-decay time-domain forward response, as well as a re-parameterization of the Cole-Cole model to invert for K directly. The joint inversion is tested on a synthetic model mimicking horizontally layered sediments, and the results are compared with the individual HT and IP inversions. It is shown that jointly inverting both data sets consistently improves the results by combining the complementary sensitivities of the two methods, and that the inversion is more robust against changes in the experimental setups. Furthermore, we illustrate how a joint inversion approach can correct biases within the petrophysical laws by including reliable K-information from hydraulic tests and still preserving the high-resolution structural information from IP. The different inversion results are compared based on the structural similarity index (SSIM), which underlines the robustness of the joint inversion compared to using the data individually. Hence, the combined application of HT and IP within field surveys and a subsequent joint inversion of both data sets may improve our understanding of hydraulically relevant subsurface structures, and thus the reliability of groundwater modeling results.","PeriodicalId":502458,"journal":{"name":"Geophysical Journal International","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2024-06-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141385041","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
We define double (S1S2) and triple (PS1S2) singularity points and their degeneracy classes in triclinic anisotropic media. We derive equations for the group velocity image for all these cases. The degeneracy classes are defined by factorization of quadratic (double singularity point) and cubic (triple singularity point) forms with three variables.
{"title":"Singularity points and their degeneracies in anisotropic media","authors":"Alexey Stovas, Y. Roganov, V. Roganov","doi":"10.1093/gji/ggae191","DOIUrl":"https://doi.org/10.1093/gji/ggae191","url":null,"abstract":"\u0000 We define double (S1S2) and triple (PS1S2) singularity points and their degeneracy classes in triclinic anisotropic media. We derive equations for the group velocity image for all these cases. The degeneracy classes are defined by factorization of quadratic (double singularity point) and cubic (triple singularity point) forms with three variables.","PeriodicalId":502458,"journal":{"name":"Geophysical Journal International","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2024-06-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141266065","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The successful recovery of hydrogeophysical parameters through surface-NMR measurements depends on the quality of the signal, which can be significantly degraded by harmonics from multiple noise sources with different fundamental frequencies in urban areas. Accurate estimation of the fundamental frequencies of harmonics is the main step in harmonic noise cancellation-based methods. The existing 1D and 2D model-based approaches involve a computationally expensive process that sets limits for processing of large surface-NMR data sets. In addition, the classical Nyman, Gaiser, and Saucier estimation (NGSE) algorithm, despite its fast implementation, may not accurately recover harmonic components when there is no prior knowledge of the expected value of the frequency offset between the true fundamental frequencies and their nominal values. This lack of knowledge can make it difficult to accurately estimate the maximum number of harmonics and, consequently, result in an incorrect recovery of the fundamental frequency. To surmount these limitations, we propose an enhanced version of the NGSE approach based on an efficient maximum number of harmonics search approach to process surface-NMR signals corrupted by powerline harmonics with both single and multiple frequency content. We verify the efficiency of our algorithm on a synthetic dataset embedded in simulated powerline harmonic signals, and real electromagnetic noise recordings, as well as a real surface-NMR data set. Our numerical experiments confirm that the proposed algorithm can retrieve the multiple fundamental frequencies simultaneously with a significant speedup ranging from 4 to 87 times, depending on whether the signal has single, dual, or triple frequency content, in the overall computation time compared to the model-based methods.
{"title":"An Efficient Algorithm to Retrieve Multiple Fundamental Frequencies of Harmonic Interference in Surface-NMR Measurements","authors":"R. Ghanati, Trevor Irons, Mohammad Reza Hatami","doi":"10.1093/gji/ggae184","DOIUrl":"https://doi.org/10.1093/gji/ggae184","url":null,"abstract":"\u0000 The successful recovery of hydrogeophysical parameters through surface-NMR measurements depends on the quality of the signal, which can be significantly degraded by harmonics from multiple noise sources with different fundamental frequencies in urban areas. Accurate estimation of the fundamental frequencies of harmonics is the main step in harmonic noise cancellation-based methods. The existing 1D and 2D model-based approaches involve a computationally expensive process that sets limits for processing of large surface-NMR data sets. In addition, the classical Nyman, Gaiser, and Saucier estimation (NGSE) algorithm, despite its fast implementation, may not accurately recover harmonic components when there is no prior knowledge of the expected value of the frequency offset between the true fundamental frequencies and their nominal values. This lack of knowledge can make it difficult to accurately estimate the maximum number of harmonics and, consequently, result in an incorrect recovery of the fundamental frequency. To surmount these limitations, we propose an enhanced version of the NGSE approach based on an efficient maximum number of harmonics search approach to process surface-NMR signals corrupted by powerline harmonics with both single and multiple frequency content. We verify the efficiency of our algorithm on a synthetic dataset embedded in simulated powerline harmonic signals, and real electromagnetic noise recordings, as well as a real surface-NMR data set. Our numerical experiments confirm that the proposed algorithm can retrieve the multiple fundamental frequencies simultaneously with a significant speedup ranging from 4 to 87 times, depending on whether the signal has single, dual, or triple frequency content, in the overall computation time compared to the model-based methods.","PeriodicalId":502458,"journal":{"name":"Geophysical Journal International","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2024-05-23","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141103094","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Greenland’s tectonic history is complex, and the resulting lithospheric structure is, although extensively researched, not well constrained. In this study, we model the lithospheric structure of Greenland in a consistent, integrated framework with three steps. First, we build a lithospheric background model by forward modelling, adjusted to gravity gradient data and shear wave velocities from a regional tomography model. Subsequently, we jointly invert for the upper crustal density and susceptibility structure by minimizing the gravity residuals and magnetic total field anomaly misfit. The last modelling step searches for upper crustal thermal parameters to fit our model to the most recent geothermal heat flow predictions for Greenland. Finally, we present 3D models of the density, temperature and velocity structure for the lithosphere as well as thermal parameters and susceptibilities for the upper crust. Our model also includes the depth of the Moho and LAB in Greenland. A comparison between inverted crustal parameters and surface geology shows a clear correlation. The novelty of our model is that all these results are consistent with each other and simultaneously explain a wide range of observed data.
{"title":"The lithospheric structure of Greenland from a stepwise forward and inverse modelling approach","authors":"A. Wansing, J. Ebbing, M. Moorkamp","doi":"10.1093/gji/ggae183","DOIUrl":"https://doi.org/10.1093/gji/ggae183","url":null,"abstract":"\u0000 Greenland’s tectonic history is complex, and the resulting lithospheric structure is, although extensively researched, not well constrained. In this study, we model the lithospheric structure of Greenland in a consistent, integrated framework with three steps. First, we build a lithospheric background model by forward modelling, adjusted to gravity gradient data and shear wave velocities from a regional tomography model. Subsequently, we jointly invert for the upper crustal density and susceptibility structure by minimizing the gravity residuals and magnetic total field anomaly misfit. The last modelling step searches for upper crustal thermal parameters to fit our model to the most recent geothermal heat flow predictions for Greenland. Finally, we present 3D models of the density, temperature and velocity structure for the lithosphere as well as thermal parameters and susceptibilities for the upper crust. Our model also includes the depth of the Moho and LAB in Greenland. A comparison between inverted crustal parameters and surface geology shows a clear correlation. The novelty of our model is that all these results are consistent with each other and simultaneously explain a wide range of observed data.","PeriodicalId":502458,"journal":{"name":"Geophysical Journal International","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2024-05-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141110458","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Charlie Peach, S. Nippress, David N Green, Kevin Mayeda, James M Wookey, M. J. Werner
The United Kingdom (UK) experiences low-to-moderate levels of seismicity; only 12 onshore earthquakes with local magnitude (ML) ≥ 4.0 have been recorded in the past 20 years. It is therefore difficult to estimate moment magnitude (MW) using conventional moment tensor inversion for the majority of UK seismicity, resulting in limited reliable estimates of MW. To address this, we calibrated coda envelopes at 16 broadband seismic stations distributed across the UK, to produce a MW catalogue for 100 events with MW≥2.13 that occurred since 2006. This was achieved using the open-source Coda Calibration Tool, which requires independent source parameter estimates for calibration. For 13 UK events between 2006 and 2022, we used spectral modelling to estimate apparent stress (0.32 to 1.74 MPa), and moment tensor inversion to estimate MW (3.35 to 4.52). These independent source parameters formed a subset of the inputs into the final calibration, which used seismic data from 33 events with coda-derived values of 2.57$le $Mw$le $4.49. The resultant coda calibration parameters were applied to 67 further events (MW≥2.13). The coda envelopes exhibit slow seismic coda decay across the UK, with significant energy up to 20 Hz, consistent with other regions of low tectonic activity. This MW catalogue, and the application of the calibration to future UK seismic events, will be useful for both assessing seismic hazard and event characterisation.
{"title":"A UK MW catalogue derived from coda envelopes","authors":"Charlie Peach, S. Nippress, David N Green, Kevin Mayeda, James M Wookey, M. J. Werner","doi":"10.1093/gji/ggae180","DOIUrl":"https://doi.org/10.1093/gji/ggae180","url":null,"abstract":"\u0000 The United Kingdom (UK) experiences low-to-moderate levels of seismicity; only 12 onshore earthquakes with local magnitude (ML) ≥ 4.0 have been recorded in the past 20 years. It is therefore difficult to estimate moment magnitude (MW) using conventional moment tensor inversion for the majority of UK seismicity, resulting in limited reliable estimates of MW. To address this, we calibrated coda envelopes at 16 broadband seismic stations distributed across the UK, to produce a MW catalogue for 100 events with MW≥2.13 that occurred since 2006. This was achieved using the open-source Coda Calibration Tool, which requires independent source parameter estimates for calibration. For 13 UK events between 2006 and 2022, we used spectral modelling to estimate apparent stress (0.32 to 1.74 MPa), and moment tensor inversion to estimate MW (3.35 to 4.52). These independent source parameters formed a subset of the inputs into the final calibration, which used seismic data from 33 events with coda-derived values of 2.57$le $Mw$le $4.49. The resultant coda calibration parameters were applied to 67 further events (MW≥2.13). The coda envelopes exhibit slow seismic coda decay across the UK, with significant energy up to 20 Hz, consistent with other regions of low tectonic activity. This MW catalogue, and the application of the calibration to future UK seismic events, will be useful for both assessing seismic hazard and event characterisation.","PeriodicalId":502458,"journal":{"name":"Geophysical Journal International","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2024-05-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141112840","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
A major challenge in seismic tomography consists in quantifying and representing model resolution and uncertainty, particularly at global scales. This information is crucial for interpretations of tomographic images and their technical application in geodynamics. However, due to large computational costs, there have been only few attempts so far to coherently analyse the spatially varying resolving power for a complete set of model parameters. Here, we present a concept for an effective evaluation and global representation of the 3-D resolution information contained in a full set of averaging kernels. In our case, these kernels are constructed using the ‘Subtractive Optimally Localized Averages’ (SOLA) method, a variant of classic Backus-Gilbert inversion suitable for global tomography. Our assessment strategy incorporates the following steps: 1) a 3-D Gaussian function is fitted to each averaging kernel to measure resolution lengths in different directions; 2) we define a classification scheme for the quality of the averaging kernels based on their focus with respect to the estimated 3-D Gaussian, allowing us to reliably identify whether the inferred resolution lengths are robust. This strategy is not restricted to SOLA inversions, but can, for example, be applied in all cases where point-spread functions are computed in other tomographic frameworks. Together with model uncertainty estimates that are derived from error propagation in the SOLA method, our concept reveals at which locations, resolution lengths and interpretations of model values are actually meaningful. We finally illustrate how the complete information from our analysis can be used to calibrate the SOLA inversion parameters —locally tunable target resolution kernels and trade-off parameters— without the need for visual inspection of the individual resulting averaging kernels. Instead, our global representations provide a tool for designing tomographic models with specific resolution-uncertainty properties that are useful in geodynamic applications, especially for linking seismic inversions to models of mantle flow.
地震层析成像的一个主要挑战是量化和表示模型的分辨率和不确定性，特别是在全球尺度上。这一信息对于层析成像的解释及其在地球动力学中的技术应用至关重要。然而，由于计算成本高昂，迄今为止只有少数几次尝试对整套模型参数的空间变化分辨力进行连贯分析。在这里，我们提出了一个概念，用于有效评估和全局表示全套平均核中包含的三维分辨率信息。在我们的案例中，这些核使用 "减法优化局部平均"（SOLA）方法构建，该方法是经典 Backus-Gilbert 反演的一种变体，适用于全局层析成像。我们的评估策略包括以下步骤：1）将三维高斯函数拟合到每个平均核上，以测量不同方向上的分辨率长度；2）我们根据平均核相对于估计的三维高斯的焦点，为平均核的质量定义了一个分类方案，使我们能够可靠地识别推断出的分辨率长度是否稳健。这一策略并不局限于 SOLA 反演，也可应用于其他层析成像框架计算点展宽函数的所有情况。结合从 SOLA 方法的误差传播中得出的模型不确定性估计值，我们的概念揭示了哪些位置、分辨率长度和模型值的解释实际上是有意义的。最后，我们说明了如何利用分析得出的完整信息来校准 SOLA 反演参数--本地可调目标分辨率核和权衡参数--而无需目测各个平均核。相反，我们的全局表示为设计具有特定分辨率-不确定性属性的层析成像模型提供了工具，这些属性在地球动力学应用中非常有用，特别是将地震反演与地幔流动模型联系起来。
{"title":"A concept for the global assessment of tomographic resolution and uncertainty","authors":"Roman Freissler, B. Schuberth, Christophe Zaroli","doi":"10.1093/gji/ggae178","DOIUrl":"https://doi.org/10.1093/gji/ggae178","url":null,"abstract":"\u0000 A major challenge in seismic tomography consists in quantifying and representing model resolution and uncertainty, particularly at global scales. This information is crucial for interpretations of tomographic images and their technical application in geodynamics. However, due to large computational costs, there have been only few attempts so far to coherently analyse the spatially varying resolving power for a complete set of model parameters. Here, we present a concept for an effective evaluation and global representation of the 3-D resolution information contained in a full set of averaging kernels. In our case, these kernels are constructed using the ‘Subtractive Optimally Localized Averages’ (SOLA) method, a variant of classic Backus-Gilbert inversion suitable for global tomography. Our assessment strategy incorporates the following steps: 1) a 3-D Gaussian function is fitted to each averaging kernel to measure resolution lengths in different directions; 2) we define a classification scheme for the quality of the averaging kernels based on their focus with respect to the estimated 3-D Gaussian, allowing us to reliably identify whether the inferred resolution lengths are robust. This strategy is not restricted to SOLA inversions, but can, for example, be applied in all cases where point-spread functions are computed in other tomographic frameworks.\u0000 Together with model uncertainty estimates that are derived from error propagation in the SOLA method, our concept reveals at which locations, resolution lengths and interpretations of model values are actually meaningful. We finally illustrate how the complete information from our analysis can be used to calibrate the SOLA inversion parameters —locally tunable target resolution kernels and trade-off parameters— without the need for visual inspection of the individual resulting averaging kernels. Instead, our global representations provide a tool for designing tomographic models with specific resolution-uncertainty properties that are useful in geodynamic applications, especially for linking seismic inversions to models of mantle flow.","PeriodicalId":502458,"journal":{"name":"Geophysical Journal International","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2024-05-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141112568","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
This study examines the topographic effect on the body-wave polarization and, subsequently, on near-surface wave speed estimation. We first derive 3D P- and S-wave polarization angles in the presence of ground tilt, where the angles are functions of the ground tilt orientation, the near-surface wave speeds, and the incident wave direction. We find that S-wave polarization angle varies considerably (e.g., more than 100 %) when the incident angle is close to the critical angle. The counter-intuitive phenomenon for flat surface, that is, P-wave polarization being only sensitive to S- but not P-wave speeds, breaks down in the presence of ground tilt, i.e., P-wave polarization becomes sensitive to both P- and S-wave speeds. Examining the differences in the inferred wave speeds with and without the flat-surface assumption reveals that bias in wave speed estimates is, in general, higher for smaller incident angles, e.g., about 50 % or higher for a 15○ ground tilt and near-vertical (<5○) incidence. The effect on P-wave speed estimates is also significant when the S-wave incident angle approaches the critical angle. In order to investigate the topographic effect on wave speed estimates inferred using teleseismic polarization data, we revisit the near-surface wave speeds estimates at Hi-net stations from Park and Ishii (2018). Based on the ground tilt and strike angles measured at 300-m scale for each Hi-net site, we constrain P- and S-wave speeds utilizing the P-wave polarization data. We find that P-wave polarization data alone can effectively constrain not only S- but also P-wave speeds, especially when the ground tilt is sufficiently large (e.g., >5○). Furthermore, our additional test suggests that including S-wave polarization data with the tilt consideration will improve the near-surface wave speeds estimates significantly compared to when the tilt effect is ignored.
本研究探讨了地形对体波极化的影响，以及随后对近表面波速度估算的影响。我们首先推导了地面倾斜情况下的三维 P 波和 S 波极化角，其中极化角是地面倾斜方向、近地表波速和入射波方向的函数。我们发现，当入射角接近临界角时，S 波极化角变化很大（例如，超过 100%）。在地表平坦的情况下，P 波极化只对 S 波速度敏感，而对 P 波速度不敏感，这种与直觉相反的现象在地面倾斜的情况下被打破，即 P 波极化对 P 波和 S 波速度都变得敏感。通过研究使用和不使用平坦表面假设时推断波速的差异，可以发现，一般来说，入射角度越小，波速估计值的偏差就越大，例如，地面倾斜度为 15○和接近垂直（5○）时，偏差约为 50%或更高。此外，我们的附加测试表明，与忽略倾斜效应时相比，将 S 波极化数据与倾斜考虑在一起将显著提高近地表波速估计值。
{"title":"Topographic Effect on Body-Wave Polarization and Near-Surface Wave Speed Estimation","authors":"Yuanshen Li, Sunyoung Park","doi":"10.1093/gji/ggae181","DOIUrl":"https://doi.org/10.1093/gji/ggae181","url":null,"abstract":"\u0000 This study examines the topographic effect on the body-wave polarization and, subsequently, on near-surface wave speed estimation. We first derive 3D P- and S-wave polarization angles in the presence of ground tilt, where the angles are functions of the ground tilt orientation, the near-surface wave speeds, and the incident wave direction. We find that S-wave polarization angle varies considerably (e.g., more than 100 %) when the incident angle is close to the critical angle. The counter-intuitive phenomenon for flat surface, that is, P-wave polarization being only sensitive to S- but not P-wave speeds, breaks down in the presence of ground tilt, i.e., P-wave polarization becomes sensitive to both P- and S-wave speeds. Examining the differences in the inferred wave speeds with and without the flat-surface assumption reveals that bias in wave speed estimates is, in general, higher for smaller incident angles, e.g., about 50 % or higher for a 15○ ground tilt and near-vertical (<5○) incidence. The effect on P-wave speed estimates is also significant when the S-wave incident angle approaches the critical angle. In order to investigate the topographic effect on wave speed estimates inferred using teleseismic polarization data, we revisit the near-surface wave speeds estimates at Hi-net stations from Park and Ishii (2018). Based on the ground tilt and strike angles measured at 300-m scale for each Hi-net site, we constrain P- and S-wave speeds utilizing the P-wave polarization data. We find that P-wave polarization data alone can effectively constrain not only S- but also P-wave speeds, especially when the ground tilt is sufficiently large (e.g., >5○). Furthermore, our additional test suggests that including S-wave polarization data with the tilt consideration will improve the near-surface wave speeds estimates significantly compared to when the tilt effect is ignored.","PeriodicalId":502458,"journal":{"name":"Geophysical Journal International","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2024-05-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141118256","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Bedrock temperature contains effective information about changes in the crustal stress. A new method of Detecting crustal Stress Change by bedrock Temperature (DSCT) had been proposed. Understanding the stress-induced temperature response characteristics of loaded rocks is fundamental for applying DSCT. In this study, temperature observation experiments of different rocks and water-saturated sandstones subjected to tiered cyclic loading were conducted to investigate the temperature-stress relationship throughout the whole rock deformation and failure process. Through experiments, some valuable results are obtained: (a) temperature changes synchronously with stress and has a very strong linear correlation with it; (b) the magnitude of Temperature Response to Stress (TRS) is approximately 1 mK/MPa, ranging from strong to weak are sandstone, marble, diorite and basalt, respectively. The differences in TRS of various rocks are determined by their major rock-forming minerals, textures and structures; (c) the evolution of TRS experiences three stages: the TRS rises rapidly in the compression stage, slowly in the acoustic emission quiet period and a significant increase in TRS before rock failure is observed on marble, sandstone, and basalt, consistent with the abnormal bedrock temperature rise preceding earthquakes; (d) the TRS of water-saturated sandstones is higher compared to the dry ones, and the abnormal sharp increase in TRS before rock failure is also more significant in the former. These findings mentioned above promote the understanding of thermal anomalies preceding earthquakes.
{"title":"Experimental study on the thermal response of rocks to stress change and its significance","authors":"Wenfang Liu, Shanjun Liu, Lianhuan Wei, Xin Han, Ankui Zhu","doi":"10.1093/gji/ggae177","DOIUrl":"https://doi.org/10.1093/gji/ggae177","url":null,"abstract":"\u0000 Bedrock temperature contains effective information about changes in the crustal stress. A new method of Detecting crustal Stress Change by bedrock Temperature (DSCT) had been proposed. Understanding the stress-induced temperature response characteristics of loaded rocks is fundamental for applying DSCT. In this study, temperature observation experiments of different rocks and water-saturated sandstones subjected to tiered cyclic loading were conducted to investigate the temperature-stress relationship throughout the whole rock deformation and failure process. Through experiments, some valuable results are obtained: (a) temperature changes synchronously with stress and has a very strong linear correlation with it; (b) the magnitude of Temperature Response to Stress (TRS) is approximately 1 mK/MPa, ranging from strong to weak are sandstone, marble, diorite and basalt, respectively. The differences in TRS of various rocks are determined by their major rock-forming minerals, textures and structures; (c) the evolution of TRS experiences three stages: the TRS rises rapidly in the compression stage, slowly in the acoustic emission quiet period and a significant increase in TRS before rock failure is observed on marble, sandstone, and basalt, consistent with the abnormal bedrock temperature rise preceding earthquakes; (d) the TRS of water-saturated sandstones is higher compared to the dry ones, and the abnormal sharp increase in TRS before rock failure is also more significant in the former. These findings mentioned above promote the understanding of thermal anomalies preceding earthquakes.","PeriodicalId":502458,"journal":{"name":"Geophysical Journal International","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2024-05-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141115576","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
A. Muxworthy, M. Riishuus, R. Supakulopas, C. M. Niocaill, D. Barfod, A. Døssing, Kathryn Turner, Brendan Cych
The Geocentric Axial Dipole (GAD) hypothesis is key to many palaeomagnetic applications, e.g., plate-tectonic reconstructions; however, the validity of this hypothesis at high latitudes is not fully resolved. To address this, in this paper we conducted a determine the palaeomagnetic directional data of 156 lava units in Eyjafjarðardalur, Iceland, with the aim of determining the validity of the GAD hypothesis at high-latitudes using time-averaged field (TAF) analysis. In addition to the palaeomagnetic directional data, we constructed an age model for the sequences using new 40Ar/39Ar dates, magnetostratigraphy and field data. The sequence age range is 2.6 to 8.0 Ma. We show that the mean virtual geomagnetic pole (VGP) for our data, does not agree with GAD theory at 95% confidence, when only the standard tilt and tectonic corrections are made, however, when inclination-shallowing processes are accounted for, e.g., TRM anisotropy and refraction effects, the mean VGP can align with GAD at 95% confidence. These inclination-shallowing processes are shown to reduce the inclination by up to 14° for some of the basaltic units. Applying the inclination shallowing correction also reduces VGP dispersion to levels which agree with global model predictions. We propose that much of the scatter within the palaeomagnetic directional databases are due to inclination-shallowing processes effects, which become more important as the natural remanent magnetisation (NRM) intensity is high, e.g., > 2 A/m. We propose that inclination-shallowing processes can be identified and corrected for by examining the NRM intensity and dispersion.
{"title":"The palaeomagnetic field recorded in Eyjafjarðardalur basalts (2.6-8.0 Ma), Iceland: Are inclination-shallowing corrections necessary in Time-Average Field analysis?","authors":"A. Muxworthy, M. Riishuus, R. Supakulopas, C. M. Niocaill, D. Barfod, A. Døssing, Kathryn Turner, Brendan Cych","doi":"10.1093/gji/ggae182","DOIUrl":"https://doi.org/10.1093/gji/ggae182","url":null,"abstract":"\u0000 The Geocentric Axial Dipole (GAD) hypothesis is key to many palaeomagnetic applications, e.g., plate-tectonic reconstructions; however, the validity of this hypothesis at high latitudes is not fully resolved. To address this, in this paper we conducted a determine the palaeomagnetic directional data of 156 lava units in Eyjafjarðardalur, Iceland, with the aim of determining the validity of the GAD hypothesis at high-latitudes using time-averaged field (TAF) analysis. In addition to the palaeomagnetic directional data, we constructed an age model for the sequences using new 40Ar/39Ar dates, magnetostratigraphy and field data. The sequence age range is 2.6 to 8.0 Ma. We show that the mean virtual geomagnetic pole (VGP) for our data, does not agree with GAD theory at 95% confidence, when only the standard tilt and tectonic corrections are made, however, when inclination-shallowing processes are accounted for, e.g., TRM anisotropy and refraction effects, the mean VGP can align with GAD at 95% confidence. These inclination-shallowing processes are shown to reduce the inclination by up to 14° for some of the basaltic units. Applying the inclination shallowing correction also reduces VGP dispersion to levels which agree with global model predictions. We propose that much of the scatter within the palaeomagnetic directional databases are due to inclination-shallowing processes effects, which become more important as the natural remanent magnetisation (NRM) intensity is high, e.g., > 2 A/m. We propose that inclination-shallowing processes can be identified and corrected for by examining the NRM intensity and dispersion.","PeriodicalId":502458,"journal":{"name":"Geophysical Journal International","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2024-05-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141114294","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Reconciling rock unit boundary geometry is crucial for geological and geophysical studies aiming to achieve a comprehensive 3D subsurface model. To create a unified 3D parametrization suitable for both geological modeling and geophysical inversion, an integrated approach utilizing implicit modeling is essential. However, a key challenge lies in encapsulating all pertinent information within the 3D model, ensuring compatibility with the utilized datasets and existing constraints. In this study, we present a workflow that enables the generation of an integrated 3D subsurface model primarily using gravity and reflection seismic datasets. Our approach involves a cooperative geophysical inversion workflow, which incorporates the inverted model from the reflection seismic data while leveraging sparse petrophysical information. Despite advances in integrated modelling, the incorporation of implicit modelling approaches in cooperative inversion workflows remains unexplored. In our gravity inversion process, we employ a generalized level set method to refine the boundaries of rock units in the prior model. We integrate the inverted model, derived from seismic and other sparse petrophysical datasets, to create a comprehensive 3D prior model. To enhance the integration of reflection seismic datasets in the level set inversion, we introduce a weighting uncertainty matrix containing constraint terms. This step refines the model's accuracy and ensures greater consistency. Finally, we search for any missing rock units within inverted model through nucleation investigations. The introduced methodology has undergone successful testing in the Boulia region (Southern Mount Isa, Queensland), utilizing two 2D reflection seismic profiles and regional gravity datasets. This study primarily aims to reconstruct the geometry of major structures within the basement units and the basin at a regional scale. By combining seismic profiles and gravity datasets with constraining information, we are able to create a 3D model of the area that accurately represents distinct rock units and their boundary geometries. Additionally, relevant legacy datasets and prior modeling results from the region have been incorporated and refined, ensuring that the final model aligns with all available knowledge about the area.
{"title":"Cooperative geophysical inversion integrated with 3D geological modelling in the Boulia region, QLD","authors":"M. Rashidifard, J. Giraud, M. Lindsay, M. Jessell","doi":"10.1093/gji/ggae179","DOIUrl":"https://doi.org/10.1093/gji/ggae179","url":null,"abstract":"\u0000 Reconciling rock unit boundary geometry is crucial for geological and geophysical studies aiming to achieve a comprehensive 3D subsurface model. To create a unified 3D parametrization suitable for both geological modeling and geophysical inversion, an integrated approach utilizing implicit modeling is essential. However, a key challenge lies in encapsulating all pertinent information within the 3D model, ensuring compatibility with the utilized datasets and existing constraints. In this study, we present a workflow that enables the generation of an integrated 3D subsurface model primarily using gravity and reflection seismic datasets. Our approach involves a cooperative geophysical inversion workflow, which incorporates the inverted model from the reflection seismic data while leveraging sparse petrophysical information. Despite advances in integrated modelling, the incorporation of implicit modelling approaches in cooperative inversion workflows remains unexplored.\u0000 In our gravity inversion process, we employ a generalized level set method to refine the boundaries of rock units in the prior model. We integrate the inverted model, derived from seismic and other sparse petrophysical datasets, to create a comprehensive 3D prior model. To enhance the integration of reflection seismic datasets in the level set inversion, we introduce a weighting uncertainty matrix containing constraint terms. This step refines the model's accuracy and ensures greater consistency. Finally, we search for any missing rock units within inverted model through nucleation investigations.\u0000 The introduced methodology has undergone successful testing in the Boulia region (Southern Mount Isa, Queensland), utilizing two 2D reflection seismic profiles and regional gravity datasets. This study primarily aims to reconstruct the geometry of major structures within the basement units and the basin at a regional scale. By combining seismic profiles and gravity datasets with constraining information, we are able to create a 3D model of the area that accurately represents distinct rock units and their boundary geometries. Additionally, relevant legacy datasets and prior modeling results from the region have been incorporated and refined, ensuring that the final model aligns with all available knowledge about the area.","PeriodicalId":502458,"journal":{"name":"Geophysical Journal International","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2024-05-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141118370","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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