Accuracy Verification and Correction of D-InSAR and SBAS-InSAR in Monitoring Mining Surface Subsidence

The accuracy of InSAR in monitoring mining surface subsidence is always a matter of concern for surveyors. Taking a mining area in Shandong Province, China, as the study area, D-InSAR and SBAS-InSAR were used to obtain the cumulative subsidence of a mining area over a multi-period, which was compared with the mining progress of working faces. Then dividing the mining area into regions with different magnitudes of subsidence according to the actual mining situation, the D-InSAR-, SBAS-InSAR- and leveling-monitored results of different subsidence magnitudes were compared and the Pearson correlation coefficients between them were calculated. The results show that InSAR can accurately detect the location, range, spatial change trend, and basin edge information of the mining subsidence. However, InSAR has insufficient capability to detect the subsidence center, having high displacement rates, and its monitored results are quite different from those of leveling. To solve this problem, the distance from each leveling point to the subsidence center was calculated according to the layout of the rock movement observation line. Besides, the InSAR-monitored error at each leveling point was also calculated. Then, according to the internal relationship between these distances and corresponding InSAR-monitored errors, a correction model of InSAR-monitored results was established. Using this relationship to correct the InSAR-monitored results, results consistent with the actual situation were obtained. This method effectively makes up for the deficiency of InSAR in monitoring the subsidence center of a mining area.

Imaging an Underwater Basin and its Resonance Modes using Optical Fiber Distributed Acoustic Sensing

Distributed acoustic sensing is an ideal tool for ambient noise tomography owing to the dense spatial measurements and the ability to continuously record in harsh environments, such as underwater. We demonstrate the ability to image a complex underwater basin using ambient noise recorded on a fiber deployed offshore Greece. A two-dimensional shear-wave velocity model was derived by analyzing Scholte-wave dispersion. In addition, extremely detailed frequency-dependent resonance and wave propagation characteristics were revealed by computing power spectral densities (PSD) and auto-correlations (AC), respectively. These observations provide crucial information on lateral and vertical wave propagation, and were used to further constrain the velocity model. The analysis reveals significant lateral variations across the short 2.5 km long fiber segment, including basin edge effects and scattered waves. Waveform simulations further support the obtained model. Our results demonstrate the advantages of incorporating PSD and AC observations into ambient noise-based imaging.

Physical Protection in Aggregates and Organo-Mineral Associations Contribute to Carbon Stabilization at the Transition Zone of Seasonally Saturated Wetlands

Wetlands store significant soil organic carbon (SOC) globally due to anoxic conditions that suppress SOC loss, yet this SOC is sensitive to climate and land use change. Seasonally saturated wetlands experience fluctuating hydrologic conditions that may also promote mechanisms known to control SOC stabilization in upland soils; these wetlands are therefore likely to be important for SOC storage at the landscape-scale. We investigated the role of physicochemical mechanisms of SOC stabilization in five seasonally saturated wetlands to test the hypothesis that these mechanisms are present, particularly in the transition between wetland and upland where soil saturation is most variable. At each wetland, we monitored water level and collected soil samples at five points along a transect from frequently saturated basin edge to rarely saturated upland. We quantified physical protection of SOC in aggregates and organo-mineral associations in mineral horizons to 0.5 m depth. As expected, SOC decreased from basin edge to upland. In the basin edge and transition zone, the majority of SOC was physically protected in macroaggregates. By contrast, overall organo-mineral associations were low, with the highest Fe concentrations (5 mg Fe g -1 soil) in the transition zone. While both stabilization mechanisms were present in the transition zone, physical protection is more likely to influence SOC stabilization during dry periods in seasonally saturated wetlands. As future climate scenarios predict changes in wetland wet and dry cycles, understanding the mechanisms by which SOC is stabilized in wetland soils is critical for predicting the vulnerability of SOC to future change.

Physical protection in aggregates and organo-mineral associations contribute to carbon stabilization at the transition zone of seasonally saturated wetlands

Wetlands store significant soil organic carbon (SOC) globally, yet this SOC is sensitive to climate and land use change. Seasonally saturated wetlands experience fluctuating hydrologic conditions that may promote the physicochemical mechanisms known to control SOC stabilization in upland soils; these wetlands are therefore likely to be important for SOC storage at the landscape-scale. We investigated the role of physicochemical mechanisms of SOC stabilization in five seasonally saturated wetlands to test the hypothesis that these mechanisms are present, particularly at the transition zone between wetland and upland where saturation in the upper soil profile is most variable. At each wetland, we monitored water level and collected soil samples at five points along a transect from frequently saturated basin edge to rarely saturated upland. We quantified physical protection of SOC in aggregates and organo-mineral associations in mineral horizons to 0.5 m depth. As expected, SOC decreased from basin edge to upland. In the basin edge and transition zone, the majority of SOC was physically protected in macroaggregates. By contrast, overall organo-mineral associations were low, with the highest Fe concentrations (5 mg Fe g− 1 soil) in the transition zone. While both stabilization mechanisms were present in the transition zone, physical protection is more likely to be a dominant mechanism of SOC stabilization in seasonally saturated wetlands. As future climate scenarios predict changes in wetland wet and dry cycles, understanding the mechanisms by which SOC is stabilized in wetland soils is critical for predicting the vulnerability of SOC to future change.

Observations of Rayleigh waves in Mexico City Valley during the 19 September 2017 Puebla–Morelos, Mexico earthquake

This article discusses the principal features of Rayleigh surface waves generated by basin-edge effects in Mexico City during the Mw7.1 19 September 2017 Puebla–Morelos, Mexico earthquake. Rayleigh waves were extracted from ground motions recorded at 12 stations in Mexico City. We used a recently proposed method for extracting surface waves, where the earthquake record is filtered based on the normalized inner product of the Stockwell transform of the three-component earthquake recordings. Results of this study reveal that basin-edge effects produced strong Rayleigh waves, particularly at certain stations, with frequencies that are mainly between 0.2 and 0.9 Hz, which is consistent with previous frequency ranges reported in the literature. Evidence of higher-mode Rayleigh waves was found at all stations located on soft soil sites, even at stations that are more than 1 km away from the basin edges. It was also observed that peak acceleration spectral ordinates of the retrograde component of the extracted Rayleigh waves at two stations exceeded the design spectral ordinates of the 1976 and 2004 editions of the Mexico City Seismic Provisions.

Timing of the martian dynamo: New constraints for a core field 4.5 and 3.7 Ga ago

The absence of crustal magnetic fields above the martian basins Hellas, Argyre, and Isidis is often interpreted as proof of an early, before 4.1 billion years (Ga) ago, or late, after 3.9 Ga ago, dynamo. We revisit these interpretations using new MAVEN magnetic field data. Weak fields are present over the 4.5-Ga old Borealis basin, with the transition to strong fields correlated with the basin edge. Magnetic fields, confined to a near-surface layer, are also detected above the 3.7-Ga old Lucus Planum. We conclude that a dynamo was present both before and after the formation of the basins Hellas, Utopia, Argyre, and Isidis. A long-lived, Earth-like dynamo is consistent with the absence of magnetization within large basins if the impacts excavated large portions of strongly magnetic crust and exposed deeper material with lower concentrations of magnetic minerals.

Extension of the Basin Rayleigh-Wave Amplification Theory to Include Basin-Edge Effects

ABSTRACT The presence of sediments near the Earth’s surface can significantly amplify the strength of shaking during earthquakes. Such basin or site amplification effects have been well documented in numerous regions, yet the complex and often situational dependence of competing reasons for this amplification makes it hard to quantify in a general sense or to determine the most significant contributions. Simple 1D seismic profiles can be used to estimate the amplitude differences between a basin site and a hard-rock reference site, but this ignores any reflections or conversions at the basin edge or a resonance effect depending on the basin’s geometry. In this article, we explore an analytic model based on coupling coefficients for surface Rayleigh waves to account for the lateral discontinuities at a basin’s edge (Datta 2018). We use this simple tool to explore the relationship between the basin’s Rayleigh-wave amplification spectrum and various parameters such as basin depth, edge slope angle, and impedance contrast. The step-by-step construction of the model allows us to quantify the contributions from various wave propagation effects with the goal of identifying situations under which various basin-edge effects must be considered in addition to purely 1D estimates. For the most velocity contrasts (less than a factor of 5), the error made by the 1D theory in predicting maximum Rayleigh-wave basin amplification is under 35% for both the horizontal and the vertical components. For simple basins, the vertical amplification dominates at larger high frequencies and the horizontal at lower frequencies. Finally, we demonstrate from comparisons with spectral-element wavefield simulations that realistic velocity structures can be reduced to a simpler “box” shape for the semi-analytic formulation used here with reasonable results. For the purposes of estimating site-amplification or microzonation, an improved model that accounts for basin-edge effects can be implemented without high-computational cost.

Basin Amplification Effects in the Puget Lowland, Washington, from Strong-Motion Recordings and 3D Simulations

ABSTRACT Sedimentary basins in the Puget Sound region, Washington State, increase ground-motion intensity and duration of shaking during local earthquakes. We analyze Pacific Northwest Seismic Network and U.S. Geological Survey strong-motion recordings of five local earthquakes (M 3.9–6.8), including the 2001 Nisqually earthquake, to characterize sedimentary basin effects within the Seattle and Tacoma basins. We observe basin-edge generated surface waves at sites within the Seattle basin for most ray paths that cross the Seattle fault zone. We also note previously undocumented basin-edge surface waves in the Tacoma basin during one of the local earthquakes. To place quantitative constraints on basin amplification, we determine amplification factors by computing the spectral ratios of inside-basin sites to outside-basin sites at 1, 2, 3, and 5 s periods. Ground shaking is amplified in the Seattle basin for all the earthquakes analyzed and for a subset of events in the Tacoma basin. We find that the largest amplification factors in the Seattle basin are produced by a shallow earthquake located to the southwest of the basin. Our observation suggests that future shallow crustal and megathrust earthquakes rupturing west of the Puget Lowland will produce greater amplification within the Seattle basin than has been seen for intraslab events. We also perform ground-motion simulations using a finite-difference method to validate a 3D Cascadia velocity model (CVM) by comparing properties of observed and synthetic waveforms up to a frequency of 1 Hz. Basin-edge effects are well reproduced in the Seattle basin, but are less well resolved in the Tacoma basin. Continued study of basin effects in the Tacoma basin would improve the CVM.

Chapter 6: The Supergiant, High-Grade, Paleoproterozoic Metasedimentary Rock- and Shear Vein-Hosted Obuasi (Ashanti) Gold Deposit, Ghana, West Africa

Obuasi, with a total mineral resource plus past production of 70 Moz, is the largest gold deposit in West Africa, and one of the largest in the world. It is hosted by ~2135 Ma siliciclastic rocks of the Eburnean Kumasi Basin, which were obliquely shortened along an inverted boundary with the older Eoeburnean Ashanti belt to the east. Greenschist facies metamorphism was coeval with mineralization and related alteration at ~2095 Ma. The steeply dipping, ENE-plunging lodes extend over an 8-km strike length and to depths of >2.5 km. They include paragenetically complex gold-rich quartz veins surrounded by refractory auriferous arsenopyrite and closely associated carbonate-muscovite alteration halos in deformed carbonaceous phyllites and subordinate metaigneous host rocks. Gold and arsenic were initially precipitated during deformation-assisted interaction with reduced host rocks at ~350°C and 100 to 200 MPa. The mineralizing fluids were derived primarily from deeper, As-rich metasedimentary sources by basinal fluid expulsion and metamorphic devolatilization triggered by inversion and shortening, followed by transpression. Continued fluid injection during and after the metamorphic peak produced changes in gold fineness, sulfide assemblages, repeated dissolution (stylolites) and reprecipitation of mineralized veins, and a change from early deformed shear-related, sulfide-rich lodes to later quartz-rich lodes that plunge down or across the axes of younger transpressional folds. Channelized fluid flow due to reactivation of basin-edge transfer structures, and/or irregularly distributed gold source rocks, may explain the variation in gold endowment along the former basin boundary.