The quality of SAR interferometric phase is significantly influenced by decorrelation noise, especially in densely vegetated regions. In this study, we developed a predictive model that incorporates normalized difference vegetation index (NDVI) and the spatiotemporal baselines of SAR image pairs, allowing for the a priori estimation of interferometric coherence. This predicted SAR coherence is then used to facilitate the selection of optimal interferograms for displacement time series analysis. We applied our method to a densely vegetated mining area in Yuncheng County, Shandong Province, China. The results indicate that our approach achieves high accuracy in predicting SAR coherence. Furthermore, the mean coherence of the selected interferograms is 15% higher than that obtained using the traditional method based on small spatiotemporal baseline criteria, resulting in approximately a 24% improvement in ground displacement measurement accuracy. This proposed method provides an effective strategy for selecting optimal interferometric pairs from extensive SAR datasets, thereby enhancing the practicality and efficiency of InSAR in mapping ground surface deformation.

The experimental detection of the changes in microwave radiation of rocks under pressure is key to identifying earthquake anomalies through satellite passive microwave remote sensing. However, such changes have not been comprehensively characterized due to the considerable differences in crustal lithology, weak microwave radiation signals, and strong environmental noise. Considering the intrinsic and significant diversity of different polarized microwave radiations of any materials, this study investigated the responses of different polarized microwave radiations during loading the rock materials. Thus, a synchronized detection system including multiple sensors was constructed at outdoor to reveal the stress-induced changes in C-band microwave brightness temperature (MBT) of diorite specimen. Experimental results show that both the horizontal and vertical MBT varied regularly with the changes of pressure; however, the trends of changes of MBT were greatly influenced by the polarization modes. Specifically, a positive correlation was illustrated between the change in vertical polarization MBT and cyclically varied pressure, during which the MBT changed with a rate of 0.033 K/MPa about. In contrast, the changes in horizontal polarization MBT exhibited a negative correlation with the varied pressure, and the MBT change rate was approximately −0.031 K/MPa. Based on the radiative transfer theory, it was found that the opposite MBT changes with respect to h- and v-polarizations are supposed to be caused by the dielectric anisotropy under uniaxial compression conditions. This study illustrates the significant and discernible MBT changes of diorite induced by the stress, which is helpful to identify the detectable microwave radiation anomalies before large earthquake occurrence.

The spatio-temporal difference in infrared radiation from partly loaded rock to fracturing was not investigated before, and the detailed mechanisms are unknown. With particularly produced battery-shaped rock specimen made of diorite being example, and a cooled long-wave infrared thermographic camera with high sensitivity and high frequency applying together with an acoustic emission (AE) system and an electrical potential signal acquisition system, the changes in infrared brightness temperature (IBT), AE and electrical potential of rock partly loaded to fracturing were detected simultaneously. The detected sequence of IBT, AE and electrical potential were carefully investigated and contrasted, and the characteristic variation of positive IBT anomalies were systematically classified and attributively analyzed. Results show that: 1) the IBT changes at different parts of the rock specimen behave various characteristics and different physical mechanisms; 2) The IBT of the loaded part changed a little before the macroscopic fracturing, which was attributed to the rise in physical temperature from the thermoelastic effect and the friction of shearing microfracture surface; while the IBT rose significantly at the moment of specimen failure, which was mainly attributed to the frictional heating on macroscopic fracture surface and the lift in rock surface equivalent emissivity; 3) The IBT of the free part rose also slightly in the pre-failure stage but not respond to stress increment in the initial loading stage, which could be attributed to the infrared radiation excitation effect of P-holes activated by pressure in the loaded part and transferred upward to the free part where they get recombination.

Pressure is a critical factor affecting the dielectric properties of rock materials. Previous studies have shown that the microwave dielectric properties of mineral rocks undergo regular changes under uniaxial pressure. In this study, diorite was selected as the specimen, and an open coaxial resonant microwave dielectric testing system was employed to investigate the microwave dielectric (2–18.3 GHz) properties of rock specimens under uniaxial and biaxial loading modes, respectively. Experimental results indicate that under uniaxial pressure, the dielectric constant at all frequency bands decreased gradually with the increase of compressive stress, with an average decrease amplitude of 4.5%; nevertheless, under the biaxial loading process, the change in dielectric constant exhibited a more complex pattern, which was closely related to the relationship between the axial stress and the lateral stress. When the axial stress was lower than the lateral stress, the dielectric constant increased significantly with the rise of axial stress; however, when the axial stress increased and got greater than the lateral stress, the dielectric constant began to decrease. The theoretical analysis based on dielectric physics indicates that both the changes in the number of polarization units and the polarization abilities of these polarization units influenced the dielectric properties of rocks under different loading conditions. This study is greatly important for revealing the change patterns and mechanisms of microwave dielectric properties of rocks, which implies that stress-induced disturbances are an important factor to be considered in applying radar investigation and microwave remote sensing for geological exploration and geohazard perception.

Although thermal infrared imaging has been developed as an important tool of remote sensing rock mechanics (RSRM) since the 1990s, the relationship between infrared emissivity and rock fragments, which is essential for interpreting positive infrared abnormity, has not been investigated. In this letter, the infrared brightness temperature (IBT) ($T_{b}$) of sandstone, marble, and granite, including an intact rock specimen and its fragments of six-level sizes, was experimentally detected with an infrared imaging system indoor. The relationship between the detected$T_{b}$and the measured size of rock fragments is investigated, and the equivalent emissivity$\varepsilon _{\!\!f}$of rock fragments of different sizes is obtained. It was discovered that$\varepsilon _{\!\!f}$of rock fragments rose up to 8.43% compared to that of intact rock and behaved the maximum as the fragment sizes get close to the mineral particle scale (MPS) of the rock.$T_{b}$is hence locally lifted by 6.16 K. This letter revealed that the rise of$\varepsilon _{\!\!f}$is a third remote sensing mechanism, besides the known thermoelastic and friction thermal effects, of local$T_{b}$enhancement in process of rock loaded to fracturing, which provides new experimental supports to infrared imaging detection on rock fracturing and IBT-based stability analysis in rock engineering and geostructures.

As a supplement to time-series interferometric synthetic aperture radar (TS-InSAR), time-series multiple-aperture InSAR (TS-MAI) can measure the spatiotemporal changes in SAR along-track surface deformation. TS-MAI is often applied with low-frequency SAR data (e.g., L-band data) due to its ability to retain high interferometric coherence. However, the low-frequency SAR signal is vulnerable to ionospheric delays, which can significantly degrade the measurement accuracy of TS-MAI. This letter presents an approach to correct the ionospheric errors in TS-MAI. A polynomial cubic model is employed to constrain the ground deformation, which is then incorporated into the observation model for effectively separating the deformation signal and the ionospheric delays. The proposed method is tested using the L-band ALOS-1 PALSAR-1 datasets covering the Tocopilla area in Chile between November 2007 and March 2011. The correction performance and accuracy of the proposed method are demonstrated by comparing the range split-spectrum interferometry (RSSI)-based method and the local GPS data, respectively. The root mean square error (RMSE) improvement rates between TS-MAI and GPS are 72.17% for the SRGD site and 84.51% for the VLZL site, and their correlation coefficients increase from 0.23 and 0.50 to 0.52 and 0.61 after the correction.

Ionospheric phase delay is a critical error source in Time Series Interferometric Synthetic Aperture Radar (TS-InSAR) for the purpose of monitoring ground surface deformation with SAR data obtained from low-frequency radar systems. Recently, the Range Split-Spectrum Interferometry (RSSI) method has been employed to estimate and rectify ionospheric errors in TS-InSAR. However, the performance of the RSSI method is largely restricted by the significant linear scale factors resulting from the current small SAR bandwidth. In this study, we propose a Reformulating RSSI (Re-RSSI)-based method for correcting the ionospheric error in TS-InSAR by optimizing the linear scale factors, with the aim of improving the accuracy of TS-InSAR measurements. We evaluate the Re-RSSI method using 121 ALOS-1 PALSAR images that cover two distinct regions: the low-latitude Lazufre volcano region and the high-latitude Anaktuvuk River tundra fire region. Our results demonstrate that the Re-RSSI method can effectively remove time series ionospheric errors at both test sites, where we detected ionospheric delays of approximately 2.5 cm/yr and 2.0 cm/yr, respectively. Using Global Navigation Satellite System (GNSS) measurements as ground truth, we achieved an 86.59% improvement rate in root mean square error (RMSE) with the Re-RSSI method, which is significantly higher than the 66.40% improvement rate achieved with the traditional RSSI method.

Microwave radiations from ground objects are capable of penetrating the shallow surface, vegetation, and atmosphere, but they are also contaminated by many natural factors. Microwave brightness temperature (MBT) data at different channels (frequencies and polarizations) have been used for studying seismic anomalies for decades. However, there lacks an in-depth research on how MBT responds to the natural but nonseismic (NbNS) factors, and the microwave channels used might not be optimal for earthquake study, which might bias the reliability of the retrieved MBT anomalies. In this study, the monthly mean MBT backgrounds (MMMBs) in and around the Qinghai-Tibet Plateau (QTP) were retrieved by using MBT data from the AMSR-2 sensors. The spatiotemporal characteristics of the MMMBs were carefully investigated and found to behave a significant spatiotemporal differentiation. The NbNS factors with profound influences on MMMBs were theoretically analyzed, and the sensitive microwave channels of these factors affecting MBT were revealed. Furthermore, the set of sensitive channels of different NbNS factors was used to determine reversely the optimal channels for searching weak seismic MBT anomalies in the QTP. The distribution map of the optimal channels has been then employed to explore MBT anomalies associated with ten large earthquakes (EQs) in the QTP, since 1997. The case studies demonstrated the good performances of the suggested channels and well verified their applicability. This study is of scientific meanings for understanding the MBT background in and around the QTP and has guiding significances for microwave channel selection of global seismic MBT anomaly study.

How to obtain millimeter-scale along-track deformations using phase measurements is still a pending question for InSAR community. Although Burst Overlap Interferometry (BOI) technique makes this question seem tractable, most applications of BOI still focus on extracting centimeter-scale deformations for co-seismic cases. To further improve measurement accuracy, here we propose a new time series BOI algorithm towards maximizing the performance of BOI and obtaining as high accuracy of the along-track deformations as possible. This algorithm has three major steps. The first step is to enhance BOI phase signal-to-noise ratio using our newly proposed phase estimator. In the second step, we apply a strain model-based method to further suppress the phase noise and rescue more data points. In the third step, a misregistration correction procedure which considers plate motion is applied to mitigate BOI time series bias. We tested our proposed algorithm over the Chaman fault. Although the derived millimeter-scale deformations demonstrate the effectiveness of our method, experimental results show that decorrelation and ionospheric disturbance are still two great challenges of BOI techniques.

Many studies have confirmed that the microwave radiation of rock materials will change with compressive stress and pore water. However, how the presence of pore water in rock volume influences the stress-induced change in microwave radiation ($\Delta T_{B}$) is not clear. In this study, both the air-dry and humid (partially saturated) sandstone specimens were loaded compressively and the C-band (6.9 GHz) microwave radiometer was used to investigate the change of$T_{B}$and the differences of$\Delta T_{B}$. The experimental results illustrated that the presence of pore water in sandstone promotes significantly the relationship between the positive$\Delta T_{B}$and the applied compressive load. On the basis of radiative transfer theory and effective dielectric constant model of multiple-phase media, we found that$\Delta T_{B}$of sandstone mainly results from the compressive stress distributed on mineral grains, which is able to reduce the dielectric constant of minerals and lead to the increase of microwave radiation. For dry sandstone, the distribution of compressive stress on minerals is mainly controlled by grain contact, while for humid sandstone, the pore water plays an important role in applying uniform pressure on grain surface and reducing the dielectric constant of grains; thus, the effective dielectric constant of rock volume is sensitive to the applied loads on partially saturated specimen. This study is important for understanding the positive$T_{B}$anomalies prior to large tectonic earthquakes in condition of ground surface rock partially saturated and is valuable to uncover potential$T_{B}$anomaly related to seismic activities.

Multiple aperture interferometric synthetic aperture radar (MAI) can measure ground displacements along SAR track. However, MAI measurements may suffer from severe ionospheric error, especially for long-wavelength SAR sensors. This study presents a new approach, azimuth split-spectrum interferometry (AziSSI), to correct ionospheric errors in MAI measurements. The proposed method can straightforwardly resolve ionospheric phase delay by exploiting two subband MAI interferograms with different centroid frequencies. We utilized two groups of ALOS-1 PALSAR images, which cover the 2008 Wenchuan earthquake containing strong coseismic ground deformation and a stable coast region in Chile, respectively, to test the proposed method. Experimental results show that the AziSSI method can successfully recover the coseismic displacements induced by the Wenchuan earthquake and circumvent the azimuth stripes for the Chile case’s MAI measurements. The maximum ionosphere-induced azimuth shifts are about 2.2 m for the Wenchuan case and 2.5 m for the Chile case. We validated the correction performance of the AziSSI method with the Wenchuan case by comparing the compensated azimuth displacements with the simulated deformation from a forward fault slip model and coseismic global positioning system (GPS) measurements. The validations show that the deformation patterns after the ionospheric correction are consistent with the simulations of the sinistral slip of the causative fault. The root-mean-square error between the GPS and MAI measurements is reduced from 0.77 to 0.28 m before and after the ionospheric correction, indicating that AziSSI can significantly compensate ionospheric errors for MAI.

The application of compressive stress on minerals and rocks can reduce dielectric constant ($\varepsilon '$) at microwave frequency, which is of considerable significance to geophysics, mineralogy, petrology, and microwave remote sensing. However, the stress near the surface (e.g., Earth surface) is generally on the order of several MPa only, and its increment is supposed to be excessively slight to influence rock$\varepsilon '$. The cubic- and conical-shaped diorite specimens were specially produced in this study to investigate experimentally the variations of rock$\varepsilon '$and tan$\boldsymbol {\delta }$at the stress-free end as the other end is subjected axially to compressive stress by using an open coaxial resonator probe working at 2.0–18.3 GHz. Surprisingly, the rock$\varepsilon '$at the free end decreased by approximately −13% until loading to fracturing, and rock tan$\boldsymbol {\delta }$demonstrated frequency-dependent variations. This particular phenomenon is supposed to be induced by stress-activated positive holes (h•) that are produced inside the loaded rock volume bearing with mineral peroxy defects, flown out to slightly stressed regions, and, finally, accumulated beneath the free rock surface. Consequently, these phenomena lead to the decrease in the electronic and ionic polarizability and the increase in the local field around dipoles, eventually changing the rock$\varepsilon '$and tan$\boldsymbol {\delta }$at the stress-free end. This study implies that the microwave dielectric property of the surface rock might decrease accordingly with deep stress enhancement, which is an important factor to be considered in the application of radar investigation and microwave remote sensing.

Two catastrophic earthquakes (EQs) with magnitudes of Mw 7.8 and 7.3 occurred in Gorkha and Dolakha, Nepal on April 25 and May 12, 2015, respectively. By employing the spatio-temporally weighted two-step method (STW-TSM), significant positive and negative microwave brightness temperature (MBT) anomalies preceding the two EQs were uncovered with satellite microwave data from FY-3B. Two strip-shaped positive MBT anomalies appeared in the Higher Himalayan 1 day before the Gorkha shock and the Dolakha shock, respectively, which were parallel to the thrust belt and whose intensity distributions exhibited good topographic consistency with the elevation profiles of the central Himalayas. Referring to the P-hole theory and seismogenic mechanism of the Nepal EQs, seismically activated mobile positive charges are presumed to have caused the surface positive MBT anomalies by reducing the superficial dielectric constant and increasing the local temperature of the Himalayan cliffs. A significant negative MBT anomaly occurred plausibly as early as March 2015 in the southern Gangetic Plain and developed into a stripe parallel to the thrust belt on the days of the Gorkha and Dolakha events. The negative MBT anomalies are suggested to result from soil moisture increases due to local synchronous precipitation. This work presents the first case study to use data from the FY-3B microwave radiation imager (MWRI) to investigate seismic anomalies, and discriminate both positive and negative MBT anomalies preceding an EQ for the first time.

Damaged building detection from remote sensing imagery helps to quickly and rapidly assess losses after an earthquake. In recent years, deep learning technology has become a favorable tool for remote sensing image information detection. Based on the characteristics of damaged buildings in remote sensing images, in this paper, a framework for damaged building detection that considers heterogeneity characteristics is proposed. First, a local-global context attention module is proposed to improve the feature detection ability of the network, which can extract the features of damaged buildings from different directions and effectively aggregate global and local features. In addition, the module takes the correlation between feature maps at different scales into account while extracting information. Second, a feature fusion module with self-attention is established to replace the simple connection between the encoding and decoding processes, which improves the detail feature recovery ability of the network during the upsampling process. Finally, to fully aggregate semantic and detail features at different scales, a multibranch auxiliary classifier is established by adding two separate branches in the prediction stage. The effectiveness of the proposed approach is verified based on data from the 2010 Haiti earthquake, and comparisons with 3 object-oriented methods and 16 existing excellent deep learning models are performed. The IOU increase of 0.03%-7.39% is achieved using the proposed approach compared with excellent deep learning models.

Passive microwave remote sensing is a feasible and promising way to reveal seismic thermal anomaly during the seismogenic phase. However, at present, the commonness study and mechanism exploration of MBT anomalies of different earthquake cases are still in urgent need of advancement. Based on the current microwave satellite observations with long temporal coverage, this research exhibited the MBT anomalies associated with some typical earthquakes by using STW-TSM approach. The results were classified as terrestrial anomalies and aquatic anomalies, and the respective principle model was proposed according to the characteristics of seismogenic environment, P-hole theory, and remote sensing physics.

Incidence angles ( θ) and polarization modes ( p) are the essential factors for detecting earthquake (EQ)-related microwave radiation anomalies. However, their influences are still not well studied. In this article, research was carried out to simulate the observation of satellite on EQ under different θ and p. The microwave observation experiments were performed on granite samples pressed with θ from 0° to 30°, as well as vertical (v) to horizontal (h) polarizations under weak radiation environment. It was found that the stress-induced changes ( ΔTB) in the samples' microwave brightness temperature ( TB) are anisotropic and polarized. It was observed that the trends of ΔTBmay be reversed under different θ ( p), with a probability of 57.14% (14.29%). The magnitude of ΔTBat h is also prone to greater changes than that at v, with a probability of 67.86%. Theoretical analysis was performed based on crystal optics, Fresnel formulas, and dielectric physics. This phenomenon may relate to the combined action of piezoelectric effect, extension and contraction of ionic bonds, and turning-direction polarization. The sensitivity of TBto the change in permittivity was analyzed by theoretical simulation in order to reveal the effects of modification in θ and p on the TBchange in stress of rock. For rocks with permittivity ε = 7 - i0.3, the most and least sensitive θ are near 52° at h polarization and 69° at v polarization, respectively. Finally, the significance of these results was discussed.

Generally, ground stress accumulates in the process of earthquake (EQ) preparation. The change trend of microwave brightness temperature (TB) of rock with stress change is an important factor for the understanding of the microwave anomalies associated with EQs. However, it is not yet clear whether the downtrend of rocks' TB is associated with increased stress. To confirm this, in this article, the instantaneous field of view of the microwave radiometer was identified first. Then, the microwave observation experiments were conducted on granite samples at 6.6 GHz under cyclic loading and outdoor conditions with weak background radiation. It was found that besides uptrend and fluctuation, the downtrend of granite samples' TB also correlates with stress, with an occurrence probability of 47.62% and a maximum rate of -0.038 K/MPa. The variation trends of TB with stress are not uniform across different areas of the same sample. To reveal the cause of this phenomenon, the permittivity of single-crystal quartz, one of the main mineral compositions of granite, was measured when it was under compassion loading at the direction perpendicular or parallel to the optical axis. For quartz, the real part of the permittivity rises (falls) when the two directions are perpendicular (parallel), causing the TB to fall (rise). The optic axes of minerals are randomly distributed in granite samples, which make the variation in the permittivity of minerals also random, thereby resulting in the nonuniformity of stress-induced TB variation in granite samples. Finally, the implications of these results were discussed.

Based on the spatiotemporally weighted two-step method (STW-TSM), the spatiotemporal characteristics of the residual microwave brightness temperature (MBT) with the Mw7.9 Wenchuan earthquake on May 12, 2008 are revealed by satellite data from the Advanced Microwave Scanning Radiometer - Earth Observing System (AMSR-E) sensor. Two significant MBT positive anomalies are found to be exactly in spatial accordance with two geological Quaternary zones, and the detailed geometric information of the MBT positive anomaly is found to be correlated with the microwave frequencies. After eliminating other possible influential factors, including surface temperature, vegetation index, land-surface roughness, and surface soil moisture under the conditions of space, time, and magnitude, and according to the microwave radiative transfer model, the dielectric variation in the ground surface is suggested to be the primary contributor of the MBT positive anomaly. The positive-hole (P-hole) theory is applied to interpret the geological preference of the MBT positive anomalies through a chain process: crustal stress enhancing-P-hole producing and flowing down stress gradients-surface P-hole accumulating-dielectric constant decreasing-and microwave radiation increasing. The stress-resulting effect of the dielectric decrease on MBT the increase provides a novel mechanism for microwave remote-sensing monitoring of crustal stress field alteration, earthquake preparation, and upcoming shocks. This research has a particular significance for searching potential georelations between the tectonic earthquake preparation and the abnormal satellite MBT.

The additional microwave radiation from underground rock mass (lithosphere) that is caused by tectonic activity and alternating crustal stress has been shown to be a detectable electromagnetic signal via satellite remote sensing. However, the emission of microwave radiation produced inside the lithosphere will be affected by its overburden matter, such as sand, soil, water, and vegetation, and this effect is unknown. In this article, we use a C-band microwave radiometer to detect the variations in the microwave brightness temperature (TB) of rock samples in the process of axial loading. For the rock samples, the surface was bare and covered with dry sand or humid sand. The experimental detection illustrates that the dry sand unexpectedly allows more stress-associated additional radiation (ATB) to be received by the radiometer, while humid sand shows significant extinction of ATB. Both the radiative transfer theory and the stress-activated positive hole hypothesis are applied to interpret the mechanisms of the effect of dry and humid sand layers on ATB. The outflow of stress-activated positive holes is from inside the loaded rock, and hence their accumulation beneath the surface of the sand layer is expected to reduce the local dielectric permittivity of sand, thereby making the detected ATB by the microwave radiometer to be larger than that extrapolated theoretically in accordance with radiative transfer theory. This article is valuable for understanding the diverse TB anomalies that have been observed prior to tectonic earthquakes, and it facilitates the evaluation of the potential application of ATB to seismic monitoring and earthquake prediction.

As one of the most important electrical properties, the dielectric permittivity of minerals and rocks has been studied with respect to its influencing factors, such as texture, density, moisture, frequency, pressure, etc., and has aroused great interest in geophysics, mineralogy, petrology, and microwave remote sensing. However, the studies on the effect of stress on the dielectric property of rock are limited to lower frequencies, and the related mechanism is not yet clear. Considering the limitations of traditional testing methods and the complexities of compositions and structures of rock specimens, in this article, we choose the minerals of the feldspar group as specimens to investigate the impact of compressive stress on their dielectric property at a higher frequency range from 2 to 18.3 GHz. For this purpose, an open coaxial resonator probe is applied to measuring the alteration of the dielectric permittivity of the feldspar specimen in the process of increasing compressive stress using a specially designed loading device. The dielectric constants of the feldspar specimen at all of the five high-frequency points demonstrate obviously decreasing trends with increasing compressive stress. The particular discovery is that the ionic and electronic polarizations are much sensitive to the increasing compressive stress in the microwave frequency band, where the minerals behave like ionic crystals. This article implies that the variation of the microwave dielectric permittivity of rock mass under altering crustal stress due to tectonic plate movements and engineering disturbances is an important factor to be considered in applying radar investigation and microwave remote sensing for mineral exploration, geological exploration, and geo-hazard perception.

At present, the earth surface stress monitoring mainly relies on the ground drilling method. Affected by test conditions and cost, the large-scale and regional stress measurement cannot be realized by traditional methods in the actual measurement. As the ground information at the large scale can be observation by satellite in all-weather condition, it is possible for detecting the stress information via the satellite remote sensing. In this paper, the experimental studies on the microwave and thermal infrared spectrum of loaded rock were carried out. Based on the experimental results, the possibility for detecting the earth surface stress condition via remote sensing was discussed. This study demonstrates the potential ability to use satellite remote sensing to observe ground stress condition and earthquakes.