Chinese Journal of Geophysics最新文献

With the global warming, the evolution of Asian summer monsoon (ASM) becomes more complicated, while the thermal effect of the Tibetan Plateau (TP) is an important forcing to the variability of weather and climate in ASM region. Regional climate model (RCM) is a useful tool in the regional climate change research and has higher resolutions that can represent topography and land surface processes more accurately comparing with atmospheric general circulation models (AGCMs). In this study, the impact of thermal forcing over different terrains of TP on ASM (including South and East ASM (SASM and EASM)) is investigated using the Weather Research and Forecasting (WRF) model. Results indicate that the local circulation and precipitation around the Himalayas (HIM) are significantly influenced by the surface heating over the HIM's southern slope, which is a dominant factor for the formation of the north branch of SASM. Meanwhile, the climbing moist airflow and precipitation over the southern slope of TP are mainly induced by HIM's thermal forcing. Due to HIM's sloping heating, the upper-level troposphere warm center is steadily located over the HIM area and the EASM is also intensified obviously (Characterized both by the enhanced low-level southwesterly over East China and the enhanced southward anomalous dry-cold northerly). As to the surface heating over TP's platform region (PL), although its influence on the summer monsoon circulation and precipitation is weaker than the HIM's, it induces a wider response of SASM and a stronger influence on the meridional Hadley circulation. The PL's heating is able to regulate low-level southwesterly over the remote tropical ocean. Furthermore, a comparison of multiple monsoon indices reveals that both HIM sloping heating and PL heating can intensify SASM or EASM, but the impacts of thermal forcing over different terrains are distinct on the two subsystems of ASM.

Traditional seismic oceanography researches dominantly pay attention to the oceanographic phenomena within the water column, such as internal wave, eddy, thermohaline fine structure, water mass boundaries, internal tide, thermohaline staircase, lee wave and so on, and could provide extra information quantitatively and qualitatively as compared with physical oceanography method. So far, very few researches try to study the water column near the seafloor, which is a significant boundary layer where water-sediment dynamic interaction, cold seepage, hydrothermal vents, biochemical activities and energy dissipation of many oceanic processes may occur. To be different from previous seismic oceanography researches, this paper mainly focuses on seismic reflections of seawater columns near the seafloor, by reprocessing a large amount of seismic sections acquired in the west and north of the South China Sea.

In this paper, conventional seismic facies analysis method is used to analyze, classify and summarize the external geometry, internal configuration, continuity, amplitude and apparent frequency of some complex seismic reflections of seawater column near the seafloor. Combined with the past research results of seismic oceanography, theory of bottom boundary layer and other various processes near the seafloor, this article not only classifies the reflection characteristics of meso-scale eddies, internal solitary wave and Lee wave, but also speculates that some possible processes could result in a few of the newly discovered abnormal seismic reflections. For example, sheet seismic facies unit may reflect turbulent bottom boundary layer; hair-like reflection configuration can be caused by the sediment resuspension, resulted from the interaction of bottom current and high frequency undulating seafloor, such as sand dunes; plume seismic facies unit indicates the characteristic of seep plume; and broom seismic facies unit could be associated with the upwelling currents and sediment resuspension in pockmarks. Results indicate that seismic oceanography can image not only processes of the ocean water column, such as internal wave and eddy, but also some complex processes near the seafloor, which greatly expands the research field of seismic oceanography and provides a new method and research perspective for the field observation of processes near the seafloor. Here, bottom boundary layer refers to the water column near the seafloor, and we call all the processes of this region “seafloor processes”.

Traditionally, for the tomography based on the correlation of seismic noise, primarily the phase information of ambient noise correlation function (NCF) is used to extract the seismic velocity and anisotropy of the earth by travel time inversion. Researchers recently utilize the amplitude of NCF to extract the earth's attenuation. According to the theory, NCF is proportional to the first kind of zero-order Bessel function in 2-D elastic case, which is directly extended to dissipative medium by introducing an exponential attenuation coefficient. The attenuation of the structure is then obtained by comparing the observed data from NCF to the Bessel function multiplied by a decaying exponential term. The NCF, however, is affected by the azimuth averaging of ambient noise source distribution in attenuating media. This empirical and simple extension may not be used to extract reliable decay coefficient. In this paper, we study the theoretical expressions of NCF in frequency domain between two stations under different coordinate systems and accordingly different source distributions which are composed by superposition of plane waves. We show that the coherency expressions in dissipative media vary with coordinate systems. The expressions are different for different normalizing factors. The attenuation coefficient obtained by fitting the coherency J₀(k₀r)e^−α(ω)r with the observed data is smaller than the real one.

Simulations of Typhoon (Tropical Cyclone) Fitow (2013) are used in this study to analyze the influence of the vertical wind shear (VWS) on its structure and intensity. A vertical wave-like distribution of VWS is revealed. This wave-like distribution varies at different stage of typhoon Fitow (2013), and exhibits bimodal structure in the mature stage. It is found that the VWS between the top and bottom of middle troposphere is the major part of the total VWS. The typhoon intensity obviously changes about 6 hours after the distribution mutation of VWS. The different configuration of the secondary circulation induced by bimodal VWS and the typhoon vertical circulation causes the asymmetric structure of the deep convection bands, and they tend to be symmetric gradually with the enhancement of cyclonic circulation. The diagnostic analysis also indicates that the wave-like distribution of VWS results in the inhomogeneous feature of vorticity forces in vertical direction. And the vorticity forces in the middle and low troposphere favor the development of convective instability. In accordance with the theoretical models, the maximum vertical velocity appears at the same altitude with the inflection of the vertical wind profile. Therefore, the bimodal VWS is crucial to the structure change of deep convection bands and the intensity maintenance in typhoon Fitow(2013). Furthermore, the wave-like distribution of VWS might be the trigger of instability accounting for the convective-rolls in typhoon.

The TRT technology is used to achieve recognition of the disaster body with the help of seismic wave reflection principle; it can locate disaster body using a confocal ellipsoid changing with time composed by seismic focus and the receiving focus. By comparing the abnormal image area formed by TRT technology with the tunnel disaster area caused by the earthquake, the TRT anomaly area is divided into four types, which is cluster abnormal area, single abnormal area, faulted abnormal area and interbedded abnormal area. Aiming at TRT technology's own problems which is that the calculation of direct wave velocity is not enough accurate and the setting of background wave velocity is unreasonable, a corresponding improvement method is proposed. On one hand, to get the actual direct wave velocity between seismic focus and the receiving focus, through calculating the minimum actual spreading track of the direct wave in the tunnel. On the other hand, to obtain 1-a confidence interval of actual direct wave velocity in the condition of Gaussian distribution, based on the distribution of direct wave velocity. To entry the upper and lower limits of the confidence interval as the background velocity into the TRT system, and gain two sets TRT's image, which gets rid of the singleness and blindness in setting the background wave velocity, and makes a certain confidence interval consideration for the improved geological hazard forecasting. The result of the present example shows that the effect of improved geological hazard forecasting is much better than the forecasting effect of setting the background wave velocity, in a certain degree, that avoids the possibility of omission in disaster prediction.

Current drought monitoring methods based on remote sensing technique generally are not of high precision. Seeking a new remote sensing drought monitoring technique is of great help to improve and develop the technique of drought monitoring and alert. The Bowen ratio, the ratio of sensible heat flux against latent heat flux, and reflecting surface hydro-thermal characteristics can be tentatively utilized for drought monitoring. Using EOS-MODIS satellite data and synchronized meteorological data, the Bowen ratio drought monitoring model was established based on surface energy balance. Then, the correlation between soil moisture against Bowen ratio index (β) and temperature-vegetation index (TVX) was analyzed. Finally, taking the clear-sky imagery (October 5, 2014) as an example, a drought level classification standard based on β was established and droughts in the study region were evaluated. Results show that β is highly negatively related to soil relative humidity, which has a better correlation than that between TVX and soil relative humidity in the depth range of 0∼20 cm. The precision of drought monitoring is significantly improved. The distribution of dry-wet condition based on the β drought classification standard is well consistent with the distribution of antecedent precipitation over the study region. Evaluation results show that generally no drought took place over the study region on October 5, 2014, which is consistent with precipitation anomaly percentage for the period of September, 2014. Our study suggests that the surface energy balance based Bowen ratio index can achieve excellent results when applied to drought monitoring and has a good application prospect.

We simultaneously relocate the M ≥ 1.0 earthquakes occurred from July 2005 to December 2014 in Xiaowan reservoir area, which is located in an active tectonic region in the middle and lower section of Lancang Jiang in western Yunnan province of China, using cross-correlation (CC) and double-difference (DD) method. The relocated epicenters are distinctly clustered in 8 subareas (subarea A to H), and the earthquakes located inside the Xiaowan reservoir area are generally shallower than that outside of the reservoir area. The relocated focal depths, the relation between the seismicity and water level, and the calculated b values in different subareas indicate that the characteristics of seismicity within Xiaowan reservoir area are quite complicated. The earthquakes occurred outside of the reservoir areas are likely to be tectonic earthquakes irrelevant to water filling of Xiaowan reservoir, and there are both reservoir-triggered seismicity (RTS) and tectonic earthquake activities in Xiaowan reservoir area. The reservoir-triggered earthquakes (RTE) are mainly located along the Heihui Jiang (subarea A) and Lancang Jiang (subarea B and C), and could be classified into rapid response RTS. In subarea A, a significant “migration” of hypocenters is observed in the first filling cycle along the Heihui valley and Qiaoxi-Weihou fault zone, which probably indicates the water permeation process in this area. According to the distribution of active faults, rock properties and relocated focal depths in the reservoir area, we suggest that the karst action and fluid permeation caused by water filling, and fault movements are the key factors of RTS in Xiaowan reservoir. Further studies are needed to better understand the physical process of RTS and tectonic seismicity in some subareas in the reservoir.

High-frequency (HF) sky-wave backscatter sounding, as a powerful tool for detecting the ionosphere and studying the characteristics of radio channels, can be used to monitor the ionosphere continuously at a remote distance and to acquire the ionospheric parameters in a large area. Backscatter ionograms show the relationship between operating frequency, group path, and echo amplitude. Since an ionogram carries the information of the ionospheric profile along the detection path, the ionospheric parameters can be evaluated through the inversion technique. An algorithm for backscatter sounding ionogram inversion is developed based on the restriction of solution space to reconstruct the horizontally inhomogeneous structures of the ionosphere. Furthermore, the Newton-Kontorovich method that generally treats nonlinear operator equations and the Tikhonov regularization method that generally deals with ill-posed problems are effectively combined to resolve the equations. The algorithm can get a stable and unique solution under the solution space limitation. The algorithm we have developed was tested against both model data and observation data, and compared with the method of Fridman and Fridman (1994). Test results prove that the method in this paper has reliable convergence, is insensitive to measurement errors, and has higher accuracy for the inversion of ionospheric structures than the method of Fridman and Fridman (1994). Our algorithm not only can inverse horizontal changes in electron density under quiet conditions (at night or during the daytime at mid-latitudes), but also can diagnose the horizontally inhomogeneous structures of the ionosphere during sunrise or sunset periods with high accuracy. This consequently demonstrates the application value of the proposed algorithm in the treatment of the complex and volatile measured backscatter ionograms.

2 years of four-component borehole strain data and well water level data are preprocessed to remove trends and outliers at Linxia Station from 2013 to 2014. The sum of observation strain values of two orthogonal measurement lines is equivalent to surface strain of borehole, and two thirds of near-ground surface strain is equal to volume strain. The volume strain can be obtained from four-component borehole strain observation, and the properties of the borehole system can be demonstrated from time and frequencies domain based on the comparison between volume strain and underground well water level observation. Results show that the volume strain and well water level is highly negatively correlated in time domain. The sensitivity of borehole system is −0.1620 mm/10⁻⁹. Enlarging the proportion of two coordinates of volume strain and well water level observation curves of selected two months in the two year period, we found that the peak and valley of water level curve corresponds to the valley and peak of volume strain one by one, and the phase lag is very small. In the frequency domain, the monthly tidal factor and the phase lag of diurnal and semidiurnal wave groups of volume strain and well water level were obtained with the Venedikov tidal harmonic analysis method, and the sensitivity and phase lag of the borehole strain system were calculated out subsequently. Results show that the sensitivities of most wave groups are not only close to each other, but also close to the sensitivity of annually periodic change obtained from the time domain, while the errors of phase lag are large. Considering that the phase lag obtained by arctangent is easily influenced by calculation error, the phase lag close to zero from time domain should be better in reliability. The analysis in time and frequency domain showed that the response of well water level to volume strain is substantially linear time-invariant. The borehole system of Linxia station basically meets the superposition, homogeneity and time invariance. Therefore, the borehole system at Linxia station is a linear time-invariant system basically.

P-wave and S-wave velocity will increase and the attenuation will vary when the concentration of gas hydrate increases. The analysis of velocity dispersion and attenuation for hydrate-bearing sediments (GHBS) would contribute to the estimate of gas hydrate concentration. Based on effective medium theory (EMT), we study the nonlinear variation feature of P-wave and S-wave velocity for marine unconsolidated hydrate-bearing sediments. Moreover, we use BISQ model to replace Gassmann equation in the EMT, and research the velocity dispersion and attenuation of hydrate-bearing sediments in the full frequency band. Based on this model, the velocity and attenuation always increase with the increasing amount of gas hydrate, and the rock porosity and clay content doesn't make any differences to the attenuation. After the numeral modeling, we apply the sonic logging (20 kHz) and VSP (100 Hz) data from Ocean drilling Program (ODP) leg 164 in obtaining the concentration of gas hydrate stability zone (GHSZ). In the application, the average hydrate concentration of GHSZ from site 995 in ODP leg 164 is about 5%∼7%, and corresponds to the pressure core sample (PCS) data, Helgerud et al.'s research conclusions and the prediction data from neural network (NN). Due to the velocity dispersion, the estimated hydrate concentration from VSP data would be lower than the estimated results from sonic logging data. The prediction results of three hydrate Sites (SH2, SH3 and SH7) from Shenhu area also coincide with the PCS, South China Sea. Moreover, based on the peak frequency method, the effective seismic quality factors (Q) of the BSR are estimated from the inline prestack seismic gathers. The Q-values suggest that the effective saturation of gas hydrate estimated by this model fluctuates between 15%∼30%. The results of numeral modeling and applications indicate the velocity dispersion and attenuation of GHBS always vary with the concentration of hydrate. The study of velocity dispersion and attenuation feature for GHBS in the full frequency band would contribute to the estimate of gas hydrate concentration.