Geophysical Journal International

SummaryDistributed Acoustic Sensing (DAS), a photonic technology that converts a fibre-optic cable into a long (tens of kilometres) high-linear-density (every few metres) array of seismo-acoustic sensors, can provide high-density, high-resolution strain measurements along the entire cable. The potential of such a distributed measurement has gained increasing attention in the seismology community for a wide range of applications. It has been shown that DAS has a sub-wavelength sensitivity to heterogeneities near the fibre-optic cable. This sensitivity is linked to the fact that the DAS measures deformation, as opposed to the displacements that seismometers measure. However, this sensitivity can create difficulties for many DAS applications, such as source location or distant imaging. Regardless, it can be advantageous in obtaining information about the subsurface near the cable. Here we present a method to locate small heterogeneities near the fibre-optic cable by inverting an indicator of the small-scale heterogeneities: the homogenised first-order corrector. We show that this first-order corrector can be used to locate heterogeneities near the fibre-optic cable at the gauge length precision, independent of the wavelength.

AbstractWe present an earthquake catalog in northeastern Tibetan plateau between September 2013 to April 2016 during the ChinArray-II deployment. Using continuous records from 676 transportable ChinArray-II stations and 172 permanent stations, the P/S phases are obtained using one deep learning phase picker. After associating these phases, the events are identified and located to establish the ChinArray-II Regional Earthquake Database (CARED-II). Benefiting from both improved station coverage and sensitive phase picker, CARED-II catalog has 156 057 events (around 3 million picks), about tenfold more than the manual routine catalog (15 967 events) using the permanent stations. The improved event catalog delineates the fault structures clearly. The deep structure of south-dipping north Qilian thrust faults is revealed, consisting with previous geology studies. The hidden faults and fault connectivity are revealed by improved seismicity, especially in the Alxa Block with sparse permanent stations and severe environments restricting geological field work. Moreover, small anthropogenic events are identified and related to highway tunnel construction across Qinling Mountain, forming a straight event cluster. The results demonstrate the high event detection ability of our procedure and reliability of the automatic catalog. Our array-based CARED-II catalog provides improved seismicity images in northeastern Tibet and could be used for further seismology and geotectonic studies.

SummaryThe global prevalence of organic pollutants presents a significant environmental challenge, necessitating sustainable remediation strategies. In situ biodegradation emerges as a cost-effective and eco-friendly solution. However, the real-time monitoring of in situ bacterial activities, particularly biodegradation processes, remains a challenge due to the limitations of traditional intrusive methods, including issues of representativeness, reproducibility, and high associated costs. Spectral induced polarization (SIP) has shown sensitivity to surface changes in subsurface environments, especially for biogeochemical reactivity monitoring including those associated with biodegradation. Despite this potential, advances have to be made to quantitatively link SIP parameters to in situ biodegradation processes. This study addresses this gap by conducting controlled biogeophysical experiments on a sand-packed column undergoing biodegradation facilitated by Rhodococcus wratislaviensis IFP 2006. SIP measurements were paired with bacterial growth kinetics to develop a quantitative model estimating bacterial growth. The results demonstrate that SIP, coupled with routine laboratory measurements, can effectively and quantitatively assess bacterial growth and the biodegradation of organic pollutants. These findings highlight the potential of SIP as a non-intrusive and reliable method for monitoring biodegradation in contaminated subsurface environments.

AbstractThe Surface Water and Ocean Topography mission (SWOT), equipped with the Ka-band Radar Interferometer (KaRIn), provides groundbreaking two-dimensional sea surface heights (SSHs), bringing new potential for optimizing the deflection of the vertical (DOVs). However, conventional DOV modeling—combining along- and cross-track geoid gradients with equal weights—fail to fully exploit the potential of SWOT/KaRIn observations and overlook the spatial variability in precision. We present a tailored method for optimizing DOVs estimation. The method combines geoid gradients in the along-track, cross-track, diagonal (forward and backward) directions with adaptive weighting. The refined weights are employed to exploit the potential of each geoid gradient based on the relationship between the standard deviation of SSHs and the significant wave height. To mitigate data gaps, prior and locally averaged geoid gradients are incorporated in the gaps and overlapping regions. SWOT/KaRIn-derived DOVs and gravity anomalies from the science-phase observations are validated against shipborne gravity in the Philippine Sea. Results indicate that the DOV model derived by the tailored method—particularly by combining triple-directional (along, cross, and diagonally forward) geoid gradients with refined weights—achieves a 7.3% improvement in accuracy over the conventional method. The supplement of additional geoid gradients is critical for mitigating leakage errors caused by missing or reduced observations in the gap regions. Furthermore, the gravity anomaly model recovered from DOVs by stacking 17-cycle observations achieved an accuracy of 2.97 mGal, representing a 7.2% improvement over single-cycle observations. The clear advantages of SWOT/KaRIn observations are gradually emerging in marine gravity recovery.

SummaryThe estimation of topographic gravity field models has attracted significant interest in recent years due to its growing relevance in Earth sciences. In this study, we present a robust methodology for the computation and comprehensive validation of global, complete spherical Bouguer and isostatic gravity anomalies that are essential for accurately interpreting subsurface mass distributions therefore geological structures. We synthesize these crucial gravitational functionals by leveraging spherical harmonic coefficients from high-resolution global gravity field models and various topographic/topographic-isostatic gravity field models. Our findings underscore the critical role of comprehensive terrain corrections in deriving physically meaningful, complete Bouguer gravity fields. The calculated global anomalies demonstrate strong coherence with established benchmark datasets, such as the World Gravity Map 2012. Residual differences are primarily attributed to variations in input Digital Terrain Models. Comparisons with regional Bouguer datasets reveal systematic biases that are largely explained by differing terrain correction methodologies. After removing this effect, there is a high level of consistency between the calculated global and published regional datasets, highlighting the utility of our global solutions, particularly in regions with sparse terrestrial data. Furthermore, the globally computed isostatic gravity anomalies exhibit significant agreement with both external global and diverse regional datasets, notably without the large systematic biases observed in Bouguer comparisons. This agreement reflects the effectiveness of the combined topographic and isostatic corrections in capturing Earth’s mass balance. This research provides valuable tools for new studies in the geoscience community by offering globally consistent and complete Bouguer and isostatic gravity field anomalies that have been rigorously validated for the ICGEM service.