Chinese Journal of Geophysics最新文献

An improved interferometric synthetic aperture radar (InSAR) performance evaluation method based on prominent scatterers (PS) is proposed to precisely assess the accuracies of the absolute/relative interferometric phase and the absolute/relative height. The critical technical defect of the conventional PS-based method is analysed, where overvalued performance indexes are caused by higher coherence on the PS than on the ordinary ground scene. Virtual prominent scatterers (VPS) around the actual PS are introduced and treated as statistical samples when the indexes are computed. The pixel location of the VPS is estimated indirectly by utilizing the high signal to clutter ratio of the VPS and the accurate relative location between the PS and the VPS. A spaceborne InSAR signal simulation experiment is carried out to validate the new VPS-based method. The developed method can support performance analysis, error isolation and troubleshooting in InSAR systems.

Rayleigh wave ellipticity (or ZH ratio) is a function of frequency and is particularly sensitive to shallow crustal structure beneath the seismograph station. Since the depth sensitivity kernels of ZH ratios are different from those of dispersion data, the ZH ratio provides good complementary information for the dispersion-based inversion method. Therefore, we can combine the ZH ratio and dispersion data of Rayleigh wave fundamental mode to better invert for the velocity structure under a specific seismograph station. In this paper, we propose a joint inversion method using the dispersion and ZH ratio data based on the Neighborhood Algorithm. We conduct synthetic tests based on a theoretical model and prove the robustness of the joint inversion method, which can better constrain the shallow crustal structure. Compared to traditional inversion methods that only use dispersion data, the joint inversion can provide a more accurate crustal V_s model as well as V_p/V_s ratios for the layered crust. Finally, we apply the joint inversion technique to real measurements and obtain a more accurate crust shear velocity and V_p/V_s model beneath the station at Kunming (KMI) in southwest China.

Processing and analysis of ocean bottom seismometer (OBS) data are of great importance to obtain the deep crustal structure. The deep seismic profile OBS2006-2 was carried out in 2006 along the extinct spreading ridge of the northwest sub-basin of the South China Sea. This survey is successful to provide high quality data whose seismic signals can be observed at the offset up to 120 km, but 2 OBSs’ recording data (OBS03 and OBS06) cannot be read correctly and were not used in later structure modeling. However, OBS data are very precious due to the high cost and arduous work. The price is more expensive when encountering severe weather during a survey. This paper is focused on reprocessing of these 2 OBSs’ data by use of the methods of checking data format, comparing signals with adjacent OBSs and resampling the data. Finally we acquire these 2 OBSs’ seismic record sections in which abundant seismic phases are clearly seen. We also obtain the seismic record section of OBS03 along the profile OBS973-3 in the Nansha Island using the same methods above. The instrument OBS06 along the profile OBS2006-2 and the instrument OBS03 along the profile OBS973-3 are the same instrument confirmed by checking their logs recorded in 2006 and 2011, respectively. It demonstrates that the processing method for abnormal OBS data is reliable and effective. Then the ray-tracing and travel-time simulation were carried out for OBS06 and OBS03 using the interactive trial-and-error 2D ray-tracing method based on the previous P-wave velocity model of OBS2006-2. The input of new travel-time picks should provide stricter constraints for the deep structure beneath the profile OBS2006-2. This research on abnormal data reprocessing does not only improve the reliability and resolution of the crustal structure, but also provide valuable experiences for OBS processing for other study areas in the future.

The Nantinghe fault zone (NFZ), which divides the southwest Yunnan bock (SYB) into two sub-blocks, is the longest one of NE striking left-lateral strike-slip faults in the SYB with a total length of ∼380 km. Although it is very active in the late Quaternary, only an M ∼ 7 earthquake occurred on its southern segment in 1941 and since then there has been no earthquake of M > 5 recorded on the other segments of the NFZ. Therefore, the NFZ might be in the stage of stress accumulation for a large earthquake. The 2014 Jinggu M_s 6.6 earthquake is ∼94 km southeast away from the NFZ, the long axes of aftershock distribution and seismic intensity contours strike in northwest and point to the NFZ. This study attempts to reveal its influence on the static Coulomb stress on surrounding faults, especially the NFZ, and to analyze the seismic risk of the NFZ.

To solve the problems above, this work combined the paleoseismological trenching and numerical simulation. The paleoseismological trench excavation revealed the paleoearthquake history in Holocene as well as the status of the NFZ. On the other hand, utilizing the coseismic slip distribution models of the Jinggu earthquake, the static Coulomb stress perturbations on surrounding faults, especially the NFZ, were calculated. Integrating these studies, the seismic potential of the NFZ was further analyzed.

Through the paleoseismological trench on the northern segment of the NFZ, we determined a paleoearthquake that generated surface ruptures in Holocene with a magnitude no less than 7. Using radiocarbon dating, the age of this event is constrained to be between 900 AD and 1480 AD, indicating an elapsed time of 535–1115 years. Utilizing two coseismic slip distribution models, our results show that the static Coulomb stress perturbations triggered by the Jinggu earthquake on neighboring faults, such as the Lancangjiang fault and the Jinggu fault, are obviously positive, up to ∼90 kPa. While on farther faults, such as the NFZ, the Longling-Lancang fault zone, and the Wuliangshan fault zone, the increased values are less than 10 kPa. The distribution of the stress perturbation on the fault plane of the NFZ, further computed using different fault parameters, illustrates that the maximum change of stress occurs near the surface at 24.15°N and decreases along the strike and depth. The maximum change of static Coulomb stress on the west branch of the NFZ is 0.89 kPa and the value on the east branch is 1.18 kPa.

Combing the slip rate and elapsed time since the latest paleoseismic event, it is estimated that the slip accumulated on the northern segment of the NFZ could be 2.8+1.5/–1.0 m. Further using the empirical scaling laws between magnitude and displacement, an earthquake with magnitude of 7.5+0.1/–0.2 would be generated by the accumulated slip. Although the change of static Coulomb stress triggered by the Jinggu earthquake indicates that the earthquake does not

During the data processing and interpretation of mine transient electromagnetic method (MTEM), in order to improve the identification of the geo-electrical interface between low resistivity abnormal body and surrounding rock in advanced detection, transient electromagnetic field was transformed to pseudo-seismic wave-field based on the function relationship between spreading electromagnetic field and seismic wave-field in whole-space. Meanwhile, the signal of pseudo-seismic after being transformed was processed with correlative stack to strengthen the amplitude and improve signal-noise-ratio (SNR) using the synthetic aperture imaging (SAI). The data of different detection directions on one surveying point in MTEM were imaged as aperture data, which would highlight weak abnormality and improve SNR. In addition, the application effect of SAI was testified with the in-site advanced detection in roadway using MTEM. The results showed that the SAI of the MTEM could improve SNR of the electrical interface information and highlight the geometrical resolution to increase exploration accuracy. The technique is significant theoretically and practically in accurate advanced detection of aquifer structure by MTEM.

Estimation of source depth plays an important role in quantitative interpretation of magnetic or gravity data. Various methods have been developed to conduct this estimation, especially for magnetic source depth. They include slope, Naudy, Werner deconvolution, Euler deconvolution, analytical signal, source parameter imaging (SPI), the continuous wavelet transform (CWT) and tilt-depth approaches.

We present a new method to estimate the depth of a field source, which is based on equivalent source technology and potential field inversion. A single layer of 2.5D cuboids model is established as an equivalent source with initial physical property parameters. The single equivalent source layer moves from shallow to deep at certain interval and is used as the initial model to invert the data. Then we estimate the field source depth by inversion fitting error. From shallow to deep, the inversion fitting error usually becomes smaller. The minimum inversion fitting error matches the corresponding field source depth. Because only one equivalent source layer is necessary to invert, the inversion is faster than traditional inversion methods and does not require depth weighting.

Calculation of theoretical model data shows that this method can obtain accurate depth of the field source. The data processing of a thin plate with an aspect ratio of 7.5 shows that the depth calculation error is about one measured point (25 m). The data processing of a thick plate with an aspect ratio from 0.5 to 1.5 shows that the depth calculation error is less than one measured point (25 m). Processing of measured aeromagnetic gradient data indicates that the central depth of the magnetic source is between 200 m to 250 m. Drilling data show that such anomalies are caused by the diorite at depth from 200 m to 300 m, in agreement well with estimation. These tests demonstrate that the depth estimation method suggested in this paper is applicable to both isolated anomalies and combined anomalies.

The formula of migration of seismic data is re-derived by using the forward propagation equations of seismic waves, in which the migration of seismic data is viewed as an approximate solution to the linear waveform inverse problem, a scattering migration method suitable for scattering seismic data and a reflection migration method suitable for reflection seismic data are proposed. Based on the scattering theory of seismic wave propagation, firstly we study and establish the migration theory for scattering seismic data through the linear equation describing the forward propagation of primary scattering wave. A subsurface reflectivity function is derived by applying the high frequency approximation to the spatial variation of velocity perturbation function generating the scattering field, and a forward propagation equation of reflection wave using the reflectivity function is derived from the propagation equation of scattering wave, then we study and establish the migration theory for reflection seismic data through the linear equation describing the forward propagation of primary reflection wave. We pointed out and corrected the shortcomings in Claerbout's migration method. The migration method of seismic data introduced in this paper is an improvement to current migration technique and theory, and establishes a solid theoretical base of mathematical physics for the migration of reflection seismic data. The migration results from the new methods have correct phase, accurate position and improved resolution.

Experiments of dynamic high pressure using shock wave are very effective to study physical properties of material under super-pressure. A shock wave experimental study on Damaping olivine with pressure from 10 to 45 GPa is reported in this paper. Combining previous works about isothermal equation of state for olivine, the temperature in experimental process is determined. The temperatures range from dozens of degrees to 800 °C when the pressures of our experiments are between 10 and 30 GPa. The density variation with pressure is obtained from our experiments and ranges from 3.627 to 4.009 g·cm⁻³. According to recover experiment of the sample and estimated temperatures under experimental pressures, it is derived that the phase transition during our experimental process under pressures of 30GP did not occur. Meanwhile, the parameters for equation of state are determined. Finally, the geodynamic implication of the experimental results to interior material movement in mantle is discussed, i.e. cold slab with metastable olivine is easily to sink into the mantle's transitional zone.

Coulomb stress change calculation has been playing an important part in investigating fault interactions and earthquake triggering. However, the results of most workers showed that Coulomb stress changes (or earthquake triggering effect) would become larger and larger with the increase of apparent frictional coefficients. This phenomenon is clearly in contradiction with our common knowledge in which frictional stress always resists fault slips and inhibits earthquakes under any circumstances. By analyzing the formula for calculating Coulomb stress changes (ΔCFS), we found that previous researchers did not take into account the additional ΔCFS which are only resulted from the variations of frictional coefficients. Suppose the depth of typical receiver fault is 15 km, the value of combined ΔCFS will be as large as ∼39.2 MPa when the variation of apparent friction coefficient is 0.1 (e.g., from 0.3 to 0.4), whereas traditional ΔCFS is only 0.8 MPa. If we incorporated the additional ΔCFS in calculation, the above contradiction will disappear completely. Therefore, it is suggested that we should consider changes of combined ΔCFS due to the variation of friction coefficient, especially when we compare different Coulomb stress models with different apparent frictional coefficients.

Magnetotail plasma sheet is the most active area in Earth's magnetosphere. It has been found that the features of plasma sheet are controlled by the conditions of solar wind and Interplanetary Magnetic Field (IMF). Some previous statistical studies have found that some parameters of the plasma sheet are dawn-dusk asymmetric. But the thickness of the magnetotail plasma sheet in the near and mid-tail region and how they are influenced by the IMF is still unclear. In this paper, the probability of the Cluster-C1 satellite encountering the plasma sheet is examined statistically by utilizing the proton flux and ß data from the CODIF and FGM equipment on board the Cluster-C1. Using data from July to November of year 2001–2004, the distributions of the probability of satellite in the plasma sheet are mapped on the Y-Dz plane (Dz denotes the distance between the satellite and the neutral sheet) during the southward and northward IMF periods, respectively. By comparison, we found that the plasma sheet is thinner during southward IMF periods than that during northward IMF periods. It is more obvious in the flank regions of the plasma sheet. We also found that the plasma sheet in the dusk side is thinner than that in the dawn side.