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In the Po Valley (Italy), near Lodi, a depleted methane reservoir was recently converted into an underground gas storage (UGS) facility. We describe the new monitoring infrastructure that integrates seismic and ground deformation capabilities. We also present results obtained before and after the start of UGS operation, namely: (a) the so‐called "background," for seismicity and deformation estimated over several decades before the UGS; (b) the "seismic baseline," assessed using the new monitoring network over nearly 2 years before the gas injection began; and (c) the seismicity and deformation measured over the first 3 years of UGS operation. In practice, we observe the phenomena associated with the "birth" of a UGS with remarkable instrumental capabilities. Following three cycles of injection/extraction, about 30 events consistent with natural, tectonically related seismicity were located within 30 km of the UGS. Moreover, the observed uplift of about 2 cm is consistent with theoretical expectations of ground deformation. Our study confirms that UGS in depleted gas reservoirs, if well managed, can result in negligible, if any, human‐induced seismicity and limited ground surface deformation. It also shows the importance of measuring the undisturbed conditions prior to industrial activities over a sufficiently long period of time, to correctly interpret the phenomena observed later. Plain Language Summary: Just as power banks store energy or artificial lakes store water for later use, depleted hydrocarbon reservoirs are used to store gas for later exploitation. Underground gas storage (UGS) is needed to optimize the distribution system and balance near‐constant production/distribution flows with variable market demand. It also ensures gas supply in exceptional situations such as geopolitical instability or severe winters. Since UGS activities (essentially warm season gas injection and cold season gas extraction) interact with surrounding geological structures, geophysical monitoring is recommended. This article describes the new monitoring infrastructure established at the Cornegliano Stoccaggio UGS (Po Valley, Italy) and the results of 5 years of seismic and ground deformation measurements. The monitoring began 2 years before the start of UGS activities and covers the first 3 years of gas storage, allowing us to observe the phenomena associated with the "birth" of a UGS with remarkable observational capabilities. Our study confirms that UGS in depleted gas reservoirs, if well managed, can result in negligible, if any, human‐induced seismicity and limited ground surface deformation. It also demonstrates the importance of measuring undisturbed conditions prior to industrial activities for a sufficiently long period of time, to properly interpret the phenomena observed later. Key Points: The "birth" of a new underground gas storage in a depleted gas reservoir is observed by integrated seismic and ground deformation monitoringOur study confirms that gas storage in depleted gas reservoirs, if well managed, can result in negligible seismicityMeasuring undisturbed conditions prior to industrial activities is important to correctly interpret the phenomena observed afterward [ABSTRACT FROM AUTHOR]

ABSTRACTGround surface deformation must be monitored to understand and prevent geological hazards. Global navigation satellite system (GNSS) technology has been widely used in deformation monitoring; however, current GNSS station layout strategies are often subjective and lack theoretical guidance. Therefore, in this study, a GNSS deformation observation network optimization method was developed based on interferometric synthetic aperture radar (InSAR) prior to deformation assistance. This method considers the deformation distribution, cost, accuracy, topography, and actual monitoring requirements and uses kriging interpolation and multiscale iterative optimization to obtain the reference number, spatial distribution, and deformation capture accuracy of GNSS stations in areas experiencing deformation. Simulation experiments showed that the proposed method strengthened the spatial deformation monitoring ability of GNSS by 46%–68% compared with the traditional station layout methods, which ignore the distribution of the deformation. Then, based on the small baseline subset (SBAS) InSAR technology and applying the new method, we evaluated the number and location of the most suitable GNSS stations for a mining area and found that 89.6% of the error was less than 1.5 cm, which further illustrates the practicability and reliability of the proposed method. This method provides an effective supplement to station layouts for monitoring GNSS deformation.

Abstract In the Po Valley (Italy), near Lodi, a depleted methane reservoir was recently converted into an underground gas storage (UGS) facility. We describe the new monitoring infrastructure that integrates seismic and ground deformation capabilities. We also present results obtained before and after the start of UGS operation, namely: (a) the so‐called “background,” for seismicity and deformation estimated over several decades before the UGS; (b) the “seismic baseline,” assessed using the new monitoring network over nearly 2 years before the gas injection began; and (c) the seismicity and deformation measured over the first 3 years of UGS operation. In practice, we observe the phenomena associated with the “birth” of a UGS with remarkable instrumental capabilities. Following three cycles of injection/extraction, about 30 events consistent with natural, tectonically related seismicity were located within 30 km of the UGS. Moreover, the observed uplift of about 2 cm is consistent with theoretical expectations of ground deformation. Our study confirms that UGS in depleted gas reservoirs, if well managed, can result in negligible, if any, human‐induced seismicity and limited ground surface deformation. It also shows the importance of measuring the undisturbed conditions prior to industrial activities over a sufficiently long period of time, to correctly interpret the phenomena observed later.

Featured Application: This study emphasizes the importance of comprehensive monitoring, calibration, and optimization of storage strategies in a saline aquifer. It also highlights the need to manage geomechanical risks and uncertainties. By understanding these risks and employing suitable monitoring techniques, the integrity and safety of GCS can be ensured, contributing to the reduction of CO2 emissions. Geological Carbon Storage (GCS) involves storing CO2 emissions in geological formations, where safe containment is challenged by structural and stratigraphic trapping and caprock integrity. This study investigates flow and geomechanical responses to CO2 injection based on a Brazilian offshore reservoir model, highlighting the critical interplay between rock properties, injection rates, pressure changes, and ground displacements. The findings indicate centimeter-scale ground uplift and question the conventional selection of the wellhead as a monitoring site, as it might not be optimal due to the reservoir's complexity and the nature of the injection process. This study addresses the importance of comprehensive sensitivity analyses on geomechanical properties and injection rates for advancing GCS by improving monitoring strategies and risk management. Furthermore, this study explores the geomechanical effects of modeling flow in the caprock, highlighting the role of pressure dissipation within the caprock. These insights are vital for advancing the design of monitoring strategies, enhancing the predictive accuracy of models, and effectively managing geomechanical risks, thus ensuring the success of GCS initiatives. [ABSTRACT FROM AUTHOR]

With the help of interferometric synthetic aperture radar (InSAR) and global navigation satellite system (GNSS) technology, high spatial and temporal resolution surface deformation results can be generated, which can help to better understand the mechanism of surface deformation. The spatio-temporal Kalman-based InSAR and GNSS fusion method fully considers the spatio-temporal correlation of deformation and characterizes the potential spatio-temporal process of deformation through spatial modeling and Kalman filter or smooth. However, when focusing on deformation with typical subsidence spatial characteristics, existing studies did not fully consider the spatial distribution of deformations, resulting in the loss of spatial detail information. This article proposes an adaptive spatial modeling optimization method that takes into account the spatial distribution of deformation, which can capture the optimal spatial basis layout scheme under a limited number of spatial bases, establish a more accurate spatial model, and improve the accuracy of deformation fusion. The effectiveness and reliability of this method are verified through simulation experiments and the deformation monitoring results in Datong City, China. The results of the two experiments show that the proposed method can improve the accuracy of InSAR interpolation results by 27.7% and 11.5% on average, respectively.

Geological Carbon Storage (GCS) involves storing CO2 emissions in geological formations, where safe containment is challenged by structural and stratigraphic trapping and caprock integrity. This study investigates flow and geomechanical responses to CO2 injection based on a Brazilian offshore reservoir model, highlighting the critical interplay between rock properties, injection rates, pressure changes, and ground displacements. The findings indicate centimeter-scale ground uplift and question the conventional selection of the wellhead as a monitoring site, as it might not be optimal due to the reservoir’s complexity and the nature of the injection process. This study addresses the importance of comprehensive sensitivity analyses on geomechanical properties and injection rates for advancing GCS by improving monitoring strategies and risk management. Furthermore, this study explores the geomechanical effects of modeling flow in the caprock, highlighting the role of pressure dissipation within the caprock. These insights are vital for advancing the design of monitoring strategies, enhancing the predictive accuracy of models, and effectively managing geomechanical risks, thus ensuring the success of GCS initiatives.

With time-series PolSAR images, polarimetric persistent scatterer interferometry (PolPSI) algorithms can obtain optimized interferograms with overall better phase quality than any single-pol channel for ground deformation monitoring. Moreover, the open access of Sentinel-1 PolSAR data makes it possible for applications of PolPSI over large regions worldwide. However, the optimum scattering mechanism usually has to be searched in a high dimension solution space by PolPSI techniques from each pixel. This is with very high or even unacceptable computational costs if satisfactory results are expected. To this end, an efficient and effective PolPSI algorithm named as TP-ESM is proposed with Sentinel-1 data in this study, which optimizes pixels' interferograms by a weighted sum of their corresponding VV and VH channel interferograms. The effectiveness of TP-ESM is tested together with two other PolPSI techniques [i.e., TP-MSM and exhaustive search polarimetric optimization (ESPO)] over Beijing with 46 Sentinel-1 PolSAR images. The results show that TP-ESM can obtain similar optimized interferometric phases with ESPO over high quality pixels, and the ground deformation monitoring pixel density improvement achieved by TP-ESM and ESPO w.r.t. conventional PSI approach (with VV data) is 35% and 43%, respectively. On the other hand, the TP-MSM approach is found not applicable to Sentinel-1 data. Considering the negligible computational cost of TP-ESM w.r.t. ESPO, it presents a quite good performance on both interferograms' optimizations and ground deformation monitoring. Moreover, the proposed TP-ESM outperforms ESPO on distributed scatterers pxiels' optimization, and it is anticipated to have promising performances on other PolSAR images acquired by other sensors.

Human activities are increasingly altering subsurface conditions leading to unwanted ground deformations. Excessive ground deformations endanger buildings, linear systems, underground facilities, and, ultimately, people's safety. Thus, mapping and monitoring ground deformation are critical in managing disaster risk and mitigating socio-economic damage from possible geohazards. Accessible and archived satellite data and the continual development of advanced remote sensing technologies can now provide valuable information about the earth's surface for various applications. This study applied the descending Sentinel-1 Persistent Scatterer Interferometric Synthetic Aperture Radar (S1-PSInSAR) technique for long-term remote ground deformation monitoring over the coastal city of Naga in Cebu Island in the Philippines. The S1-PSInSAR technique provided relatively dense ground monitoring points, specifically over built-up areas. The maximum ground subsidence along the radar line-of-sight (LOS) estimated throughout the monitoring period from October 2014 to September 2018 exceeds-50 mm, whereas the LOS mean displacement velocity exceeds-15 mm/yr. The two-parameter hyperbolic model well fitted the S1-PSInSAR results to describe the ground deformation's non-uniform and nonlinear behavior. Ground deformation over the coastal city continues to evolve, and stabilization is yet to occur. This study suggests that the S1-PSInSAR technique and the hyperbolic function can provide technical support in mapping, monitoring, and (or) forecasting ground deformation over vast areas with less human labor and efficient cost. [ABSTRACT FROM AUTHOR]

The Sentinel-1A/B satellite launched by European Space Agency (ESA) in 2014 provides a huge amount of free Terrain Observation by Progressive Scans (TOPS) data with global coverage to the public. The TOPS data have a frame width of 250 km and have been widely used in surface deformation monitoring. However, traditional Multi-Temporal Interferometric Synthetic Aperture Radar (MT-InSAR) methods require large computer memory and time when processing full resolution data with large width and long strips. In addition, they hardly correct atmospheric delays and orbital errors accurately over a large area. In order to solve these problems, this study proposes a data partition strategy based on MT-InSAR methods. We first process the partitioned images over a large area by traditional MT-InSAR method, then stitch the deformation results into a complete deformation result by correcting the offsets of adjacent partitioned images. This strategy is validated in a flat urban area (Changzhou City in Jiangsu province, China), and a mountainous region (Qijiang in Chongqing City, China). Compared with traditional MT-InSAR methods, the precision of the results obtained by the new strategy is improved by about 5% for Changzhou city and about 15% for Qijiang because of its advantage in atmospheric delay correction. Furthermore, the proposed strategy needs much less memory and time than traditional methods. The total time needed by the traditional method is about 20 h, and by the proposed method, is about 8.7 h, when the number of parallel processing is 5 in the Changzhou city case. The time will be further reduced when the number of parallel processes increases. [ABSTRACT FROM AUTHOR]

Ground deformation has a certain impact on the safety and stability of engineering construction, and is of great significance to engineering construction planning and geological disaster prevention. This paper uses the continuous observation of the Global Navigation Satellite System (GNSS) of Continual Operating Reference Station (CORS) network to realize the monitoring of regional ground height, ground gravity and ground tilt changes. Through the time and space analysis with part of the geological disaster information, it is determined the factors and weights used in the analysis of ground stability changes, and then quantitative analysis of ground stability changes. This method has been tested and verified in the southeast region of Zhejiang province, and good results have been obtained. Continuous observation using the CORS network can quantitatively monitor the changes of the entire surface of ground deformation, analyze changes in regional ground stability, and can provide technical support and reference for regional planning and geological disaster prevention.

The Sentinel-1A/B satellite launched by European Space Agency (ESA) in 2014 provides a huge amount of free Terrain Observation by Progressive Scans (TOPS) data with global coverage to the public. The TOPS data have a frame width of 250 km and have been widely used in surface deformation monitoring. However, traditional Multi-Temporal Interferometric Synthetic Aperture Radar (MT-InSAR) methods require large computer memory and time when processing full resolution data with large width and long strips. In addition, they hardly correct atmospheric delays and orbital errors accurately over a large area. In order to solve these problems, this study proposes a data partition strategy based on MT-InSAR methods. We first process the partitioned images over a large area by traditional MT-InSAR method, then stitch the deformation results into a complete deformation result by correcting the offsets of adjacent partitioned images. This strategy is validated in a flat urban area (Changzhou City in Jiangsu province, China), and a mountainous region (Qijiang in Chongqing City, China). Compared with traditional MT-InSAR methods, the precision of the results obtained by the new strategy is improved by about 5% for Changzhou city and about 15% for Qijiang because of its advantage in atmospheric delay correction. Furthermore, the proposed strategy needs much less memory and time than traditional methods. The total time needed by the traditional method is about 20 h, and by the proposed method, is about 8.7 h, when the number of parallel processing is 5 in the Changzhou city case. The time will be further reduced when the number of parallel processes increases.

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With the launch of the Sentinel-1 satellites, it becomes easy to obtain long time-series dual-pol (i.e., VV and VH channels) SAR images over most areas of the world. By combining the information from both VV and VH channels, the polarimetric persistent scatterer interferometry (PolPSI) techniques is supposed to achieve better ground deformation monitoring results than conventional PSI techniques (using only VV channel) with Sentinel-1 data. According to the quality metric used for polarimetric optimizations, the most commonly used PolPSI techniques can be categorized into three main categories. They are PolPSI-ADI (amplitude dispersion index as the phase quality metric), PolPSI-COH (coherence as the phase quality metric), and PolPSI-AOS (taking adaptive optimization strategies). Different categories of PolPSI techniques are suitable for different study areas and with different performances. However, the study that simultaneously applies all the three types of PolPSI techniques on Sentinel-1 PolSAR images is rare. Moreover, there has been little discussion about different characteristics of the three types of PolPSI techniques and how to use them with Sentinel-1 data. To this end, in this study, three data sets in China have been used to evaluate the three types of PolPSI techniques’ performances. Based on results obtained, the different characteristics of PolPSI techniques have been discussed. The results show that all three PolPSI techniques can improve the phase quality of interferograms. Thus, more qualified pixels can be used for ground deformation estimation by PolPSI methods with respect to the PSI technique. Specifically, this pixel density improvement is 50%, 12%, and 348% for the PolPSI-ADI, PolPSI-COH, and POlPSI-AOS, respectively. PolPSI-ADI is the most efficient method, and it is the first choice for the area with abundant deterministic scatterers (e.g., urban areas). Benefitting from its adaptive optimization strategy, PolPSI-AOS has the best performances at the price of highest computation cost, which is suitable for rural area applications. On the other hand, limited by the medium resolution of Sentinel-1 PolSAR images, PolPSI-COH’s improvement with respect to conventional PSI is relatively insignificant.

The Neapolitan volcanic area includes three active and high-risk volcanoes: Campi Flegrei caldera, Somma–Vesuvius, and Ischia island. The Campi Flegrei volcanic area is a typical example of a resurgent caldera, characterized by intense uplift periods followed by subsidence phases (bradyseism). After about 21 years of subsidence following the 1982–1984 unrest, a new inflation period started in 2005 and, with increasing rates over time, is ongoing. The overall uplift from 2005 to December 2019 is about 65 cm. This paper provides the history of the recent Campi Flegrei caldera unrest and an overview of the ground deformation patterns of the Somma–Vesuvius and Ischia volcanoes from continuous GPS observations. In the 2000–2019 time span, the GPS time series allowed the continuous and accurate tracking of ground and seafloor deformation of the whole volcanic area. With the aim of improving the research on volcano dynamics and hazard assessment, the full dataset of the GPS time series from the Neapolitan volcanic area from January 2000 to December 2019 is presented and made available to the scientific community.

The rationale of polarimetric optimization techniques is to enhance the phase quality of the interferograms by combining adequately the different polarization channels available to produce an improved one. Different approaches have been proposed for polarimetric persistent scatterer interferometry (PolPSI). They range from the simple and computationally efficient BEST, where, for each pixel, the polarimetric channel with the best response in terms of phase quality is selected, to those with high-computational burden like the equal scattering mechanism (ESM) and the suboptimum scattering mechanism (SOM). BEST is fast and simple, but it does not fully exploit the potentials of polarimetry. On the other side, ESM explores all the space of solutions and finds the optimal one but with a very high-computational burden. A new PolPSI algorithm, named coherency matrix decomposition-based PolPSI (CMD-PolPSI), is proposed to achieve a compromise between phase optimization and computational cost. Its core idea is utilizing the polarimetric synthetic aperture radar (PolSAR) coherency matrix decomposition to determine the optimal polarization channel for each pixel. Three different PolSAR image sets of both full- (Barcelona) and dual-polarization (Murcia and Mexico City) are used to evaluate the performance of CMD-PolPSI. The results show that CMD-PolPSI presents better optimization results than the BEST method by using either $D_{\mathrm{ A}}$ or temporal mean coherence as phase quality metrics. Compared with the ESM algorithm, CMD-PolPSI is 255 times faster but its performance is not optimal. The influence of the number of available polarization channels and pixel’s resolutions on the CMD-PolPSI performance is also discussed. [ABSTRACT FROM AUTHOR]

Synthetic aperture radar (SAR) tomography has been strongly developed in the last years for the analysis at fine scale of data acquired by high-resolution interferometric SAR sensors as a technique alternative to classical persistent scatterer interferometry and able to resolve also multiple scatterers. SqueeSAR is a recently proposed solution which, in the context of SAR interferometry at the coarse scale analysis stage, allows taking advantage of the multilook operation to filter interferometic stacks by extracting, pixel by pixel, equivalent scattering mechanisms from the set of all available interferometric measurement collected in the data covariance matrix. In this paper, we investigate the possibilities to extend SqueeSAR by allowing the identification of multiple scattering mechanisms from the analysis of the covariance matrix. In particular, we present a new approach, named “Component extrAction and sElection SAR” algorithm, that allows taking advantage of the principal component analysis to filter interferograms relevant to the decorrelating scatterer, i.e., scatterers that may exhibit coherence losses depending on the spatial and temporal baseline distributions, and to detect and separate scattering mechanisms possibly interfering in the same pixel due to layover directly at the interferogram generation stage. The proposed module allows providing options useful for classical interferometric processing to monitor ground deformations at lower resolution (coarse scale), as well as for possibly aiding the data calibration preliminary for the subsequent full-resolution interferometric/tomographic (fine scale) analysis. Results achieved by processing high-resolution Cosmo-SkyMed data, characterized by the favorable features of a large baseline span, are presented to explain the advantages and validate this new interferometric processing solution. [ABSTRACT FROM PUBLISHER]

Monitoring ground deformation using SAR interferometry (InSAR) sometimes requires the introduction of coherent radar targets, especially in vegetated nonurbanized areas. Passive devices such as corner reflectors were used in such areas in the past. However, they suffer from drawbacks related to their large size and weight, conspicuousness, and loss of reliability because of geometric variations as well as material and maintenance-related degradation over several years of deployment. The viability of smaller, lighter, and less conspicuous radar transponders as an alternative is demonstrated via two field experiments: validation tests in a controlled environment, and operational performance for monitoring landslides in a heavily vegetated area. Comparison of 113 transponder-InSAR observations with independent validation measurements such as leveling and the global positioning system yields an empirical precision range of 1.8-4.6 mm, after outlier removal, for double-difference (spatial and temporal) transponder phase measurements in the radar line of sight, for Envisat and ERS-2. [ABSTRACT FROM PUBLISHER]

With the launch of the Sentinel-1 satellites, it becomes easy to obtain long time-series dual-pol (i.e., VV and VH channels) SAR images over most areas of the world. By combining the information from both VV and VH channels, the polarimetric persistent scatterer interferometry (PolPSI) techniques is supposed to achieve better ground deformation monitoring results than conventional PSI techniques (using only VV channel) with Sentinel-1 data. According to the quality metric used for polarimetric optimizations, the most commonly used PolPSI techniques can be categorized into three main categories. They are PolPSI-ADI (amplitude dispersion index as the phase quality metric), PolPSI-COH (coherence as the phase quality metric), and PolPSI-AOS (taking adaptive optimization strategies). Different categories of PolPSI techniques are suitable for different study areas and with different performances. However, the study that simultaneously applies all the three types of PolPSI techniques on Sentinel-1 PolSAR images is rare. Moreover, there has been little discussion about different characteristics of the three types of PolPSI techniques and how to use them with Sentinel-1 data. To this end, in this study, three data sets in China have been used to evaluate the three types of PolPSI techniques' performances. Based on results obtained, the different characteristics of PolPSI techniques have been discussed. The results show that all three PolPSI techniques can improve the phase quality of interferograms. Thus, more qualified pixels can be used for ground deformation estimation by PolPSI methods with respect to the PSI technique. Specifically, this pixel density improvement is 50%, 12%, and 348% for the PolPSI-ADI, PolPSI-COH, and POlPSI-AOS, respectively. PolPSI-ADI is the most efficient method, and it is the first choice for the area with abundant deterministic scatterers (e.g., urban areas). Benefitting from its adaptive optimization strategy, PolPSI-AOS has the best performances at the price of highest computation cost, which is suitable for rural area applications. On the other hand, limited by the medium resolution of Sentinel-1 PolSAR images, PolPSI-COH's improvement with respect to conventional PSI is relatively insignificant. [ABSTRACT FROM AUTHOR]

The Neapolitan volcanic area includes three active and high-risk volcanoes: Campi Flegrei caldera, Somma–Vesuvius, and Ischia island. The Campi Flegrei volcanic area is a typical example of a resurgent caldera, characterized by intense uplift periods followed by subsidence phases (bradyseism). After about 21 years of subsidence following the 1982–1984 unrest, a new inflation period started in 2005 and, with increasing rates over time, is ongoing. The overall uplift from 2005 to December 2019 is about 65 cm. This paper provides the history of the recent Campi Flegrei caldera unrest and an overview of the ground deformation patterns of the Somma–Vesuvius and Ischia volcanoes from continuous GPS observations. In the 2000–2019 time span, the GPS time series allowed the continuous and accurate tracking of ground and seafloor deformation of the whole volcanic area. With the aim of improving the research on volcano dynamics and hazard assessment, the full dataset of the GPS time series from the Neapolitan volcanic area from January 2000 to December 2019 is presented and made available to the scientific community. [ABSTRACT FROM AUTHOR]