Chinese Journal of Geophysics最新文献

With the datasets of Global Land Data Assimilation System (GLDAS) NOAH land surface model, GPCC monthly mean rainfall, and NCAR/NCEP global monthly mean reanalysis from 1948 to 2010, by using methods of filtering, composite and linear regression and correlation, the characteristics of Eurasian snow depth anomalies in El Niño mature winter, its influences on soil moisture after snow melting, and finally on East Asian summer monsoon are investigated, and the main conclusions are as follows: In El Niño mature winter, snow depth in regions of the Iranian Plateau, the northeast of Lake Balkhash and the southern Tibetan Plateau increases remarkably, so are the related snow melting and soil moisture. The above-mentioned three regions are identified as the key regions for snow depth to store and extend the El Niño signals. In spring, the snow begins to melt, and the soil moisture increases correspondingly, thus the El Niño signals are transmitted from winter snow depth to soil moisture in spring. As a result, sensible heat flux decreases and latent heat flux increases, and the atmospheric circulations are greatly influenced. The anomalous soil moisture in the Iranian plateau is most important for the East Asian summer monsoon in El Niño decaying summer, since it has similar impact pattern on the anomalous summer precipitation as the El Niño composite. The spring and summer soil moisture in both the southern Tibetan plateau and the northeast of Lake Balkhash increase simultaneously, which significantly contribute to the increased precipitation in North China. Therefore, to investigate and predict the East Asian summer monsoon variability by using El Niño signal, the roles of snow depth in storing and modulating El Niño impacts in those key regions should be considered.

The lack of low-frequency components and the subsurface strong variations of velocity can lead to severe cycle-skipping phenomenon, which is a big challenge to full waveform inversion of seismic data. Through the applications of the time-damping and the time integration for decreasing the dominant-frequency of seismic wavefield, a time-damping full waveform inversion of multi-dominant-frequency wavefields is proposed. This method can efficiently eliminate the cycle-skipping phenomenon. Velocity errors from shallow to deep can lead to misfits of travel-time and their accumulations. The accurate inversion of shallow velocity can efficiently reduce the misfits of travel-time and cycle-skipping phenomenon in the inversion of later waveform. Applying the time-damping approach to seismic data can obtain time-damped data. The inversion of these time-damped data with different damping values can produce the results from shallow to deep depth. The cycle-skipping phenomenon is weak for the wavefields with lower dominant-frequency compared to higher dominant-frequency wavefields. The time integration of different orders of the seismic wavefield can produce wavefields with different dominant frequencies. The inversion results of low dominant-frequency wavefields are used as the starting models for the full waveform inversion of high dominant-frequency wavefields. Numerical tests using synthetic data lacking low-frequency components below 4Hz of the 2D salt-dome model have demonstrated the validity and feasibility of the proposed method. The final results show that the time-damping full waveform inversion of multi-dominant-frequency wavefields has proper flexibility for seismic data lacking low-frequency components and in case of strong subsurface velocity variations.

A decreasing radius iterative method in spatial domain is presented for regional-residual separation of potential field data. A new eight-point circumference average formula is derived by arithmetical average of potential field values at eight points along the circumference of a circle of given radius, which can be seen as a filter for calculating regional anomaly from gravity or magnetic data. The transfer function of the filter has a main lobe and multiple side lobes. When the radius becomes large, the number of the side lobes increases, and the filter characteristics become bad. The product of the transfer functions for various values of the radius from large to small is constructed, which is defined as decreasing radius iterative transfer function herein, with the largest radius as its parameter. The decreasing radius iterative transfer function is similar to the low-pass filter, and the cut-off wave number is inversely proportional to the largest radius. Based on the decreasing radius iterative transfer function, the decreasing radius linear iterative method in spatial domain is presented for separating regional anomaly, and the residual anomaly is obtained by subtracting the regional anomaly from the gravity or magnetic data. Furthermore, by constructing the nonlinear correction coefficient, the linear iterative formula of the decreasing radius linear iterative method is transformed into the nonlinear iterative formula, and the decreasing radius nonlinear iterative method in spatial domain is proposed. The decreasing radius nonlinear iterative method is tested with synthetic data from model and a field data set from the Nihe iron deposit in Anhui Province. The results show that the proposed method effectively suppresses false anomaly and high frequency interference, reduces anomaly distortion, and separates regional anomaly and residual anomaly from the gravity and magnetic data.

This paper presents a new algorithm for the three-dimensional marine controlled source electromagnetic method modeling in arbitrarily anisotropic medium based on unstructured grids and coulomb gauge. In order to avoid the effect of source singularity, we apply the secondary potential formulation for the quasi-static of Maxwell's equation. Next, solving the finite element equations with IDR(s) iterative algorithm method combined with the incomplete LU decomposition technology. Once the secondary vector and scalar potential is solved, we can use the moving least square method to calculate their spatial derivative and compute the secondary electric and magnetic field. In order to test the finite element method for numerical modeling of 3D controlled source electromagnetic data in an arbitrarily anisotropic conductive medium of this paper, we applied the developed algorithm to compute the typical CSEM response of two 3D models. The modeling results show that the proposed algorithm is feasible and effective; the IDR(s) is competitive with or superior to ILU-QMR, ILU-BICGSTAB methods; the proposed algorithm is general and can be applied to EM modeling in borehole and environment geophysics in arbitrarily anisotropic medium.

According to the response formulas of the scalar CSAMT (controlled source audio-frequency magnetotelluric) method, its programs can be designed in several manners with different sources such as a vertical double dipole source and a three-dipole source of varied geometries. Through numerical simulation, this study obtains the value distributions of X- and Y-direction components of the resultant electromagnetic (EM) field, which permit to further analyze the effective observational area. The results show that the distribution of crosslike source exhibits scattered weak responses, representing a smaller effective observational zone. Meanwhile, its requirement to the terrain condition of the working site is very harsh, thus cannot work well in mountainous areas with high topographic relief. Based on the principle of the rotating three-dipole device, we propose a new device composed of multiple dipole sources for the CSAMT which can facilitate integrated measurement of tensors of the EM field. Compared with the cross-like source, this approach can realize measurement throughout the whole area without weak-value zones. Moreover, this new device is easy to deploy in various terrains. The results of this study provide a technical basis for further engineering production of tensor measurement.

The formation of gas hydrate reservoir in marine sediments is mainly controlled by methane supply and sedimentary burial. Based on the mass conservation of methane in gas hydrate system, a numerical model of gas hydrate formation was established considering the methane supplied by dissolved methane diffusion, pore water advection, and in situ methanogenesis. A case study of ODP site 1247 at the Hydrate Ridge, offshore Oregon shows that dissolved methane transported by molecular diffusion and pore water advection is the major supply of methane for gas hydrate formation, while in situ methanogenesis contributes little to the gas hydrate reservoir. The gas hydrate reservoir was also evaluated considering the changes of sedimentation rate since 1.67 Ma. Our model results show that the variations of sedimentation rate lead to little change in the size of gas hydrate reservoir at ODP 1247. The calculated hydrate saturation amounts to ∼0%∼3%, which is consistent with the measured values using pressure coring.

Gas hydrate system is generally indicated by seismic indicators like bottom simulating reflection (BSR) and underlying high amplitudes (often caused by free gas zones: FGZs), but not all BSRs and underlying high amplitudes are caused by hydrates or free gas. In this study, we choose apparent polarity attribute to help identify hydrate-related BSR and FGZs, both of which are characterized by negative apparent polarity and high amplitude. In the study area, the gas hydrate system is mainly distributed in areas with gravitational structures including growth faults and related rollover anticlines. Faults, chimneys, unconformity and permeable sandstone beds can act as fluid migration conduits, supplying gas to the shallower gas hydrate system. When gas hydrates dissociate or the FGZs are critically overpressured, these conduits can also play a role in gas recycling between the FGZs and hydrate-bearing strata with some of the free gas escaping to the seabed and even to the atmosphere. Generally, apparent polarity is an effective attribute to identify hydrate-related BSR and FGZs, and fluid migration pathways are of great importance for the formation of gas hydrate system in this study area.

Stacking velocity analysis is a routine procedure in seismic data processing, and is also a classical method for initial velocity model building. Usually, stacking velocity analysis is divided into two steps: calculating stacking velocity spectra and picking spectra maximums. Until now, many researchers are trying to improve the stacking velocity spectra by computing a better semblance, considering the AVO effect or improving the anti-noise ability of algorithm. However, it is seldom discussed on how to calculate the stacking velocity automatically. In this paper, we try to solve this problem by combining the velocity spectra calculation and picking procedure into a model parameter estimation under the framework of sparse inversion. Therefore, it is possible to invert the stacking velocity automatically and shorten the turn-around time of initial velocity model building and reduce human costs considerably. To solve this problem, first we give the definition of forward problem, which is the prediction model for CMP gather using stacking velocity and t₀ time as model parameters. Then, the inverse problem is defined as finding the sparse model parameters with the given CMP gather. Using the sparsity of model parameters as model constraint, we reformulate the conventional stacking velocity analysis problem as a new sparse inverse problem, and present an adaptive matching pursuit (MP) algorithm to solve it. The proposed method is quite promising for automatic initial model building, and can provide a good initial model for subsequent high-resolution velocity inversion methods. Numerical and field data tests demonstrate the effectiveness of the proposed method.

The Synchrosqueezing S-transform (SSST) is a novel approach for time-frequency (T-F) representation of non-stationary signals. By “squeezing” T-F spectrum of the S-transform (ST) of a signal, the SSST can obtain a high-resolution T-F spectrum. However, when the phase of a signal varies non-linearly with time, the instantaneous frequency (IF) of the signal calculated by the SSST would be inaccurate, causing reduction of the resolution of the T-F spectrum. In order to improve its performance in imaging such signals, we modify the formula of IF with the help of the second-order partial derivatives of time and frequency, and propose a second-order Synchrosqueezing S-transform (SSST_2nd). Synthetic examples show that the SSST_2nd not only has obviously higher resolution than commonly used T-F transforms, but also images the signals of which the IFs vary quadratically or sinusoidally with time better than the SSST. We apply the SSST2nd to perform spectral decomposition of seismic data for natural gas exploration. The results illustrate that the SSST2nd can be used to well detect frequency spectral anomalies correlated with natural gas accumulations. Therefore, it can be concluded that the SSST2nd is a good potential technique for seismic interpretation.

Based on the geomagnetic field model (NGDC-720) established by the U.S. National Geophysical Data Center, the spatial variations of crustal magnetic anomalies in the Yunnan region were studied, including the distribution of magnetic anomalies and their gradients, attenuation of magnetic anomalies with altitude, and the contributions of components with different wavelengths to overall magnetic anomalies. The magnetic anomalies were compared with other geophysical data such as gravity anomalies, terrestrial heat flow, and seismic activities to reveal similarities and differences, and the relationship between crustal magnetic anomalies and geological structure was studied. The research results show that the magnetic anomalies are relatively weak in the Sanjiang (Three Rivers) and western Yunnan orogenic belts to the west of the Lijiang-Xiaojinhe and Red River fault zones, and relatively strong in the rhombic block. The crustal magnetic anomalies in the study region were mainly generated by the shallow magnetic bodies superposed on the background of weak magnetism basement. The satellite magnetic anomalies show that the middle Yunnan depression area has clear dipole field characteristics. The positive and negative magnetic anomaly zones distributed along the Red River fault zone have consistent strikes with that of the faults. The Lijiang-Xiaojinhe fault, the Red River fault, the Kangding-Yiliang-Shuicheng fault and the Mile-Shizong-Shuicheng fault zones surrounding the rhombic block are transition belts between strong and weak magnetic anomalies. The places with active seismicity and the areas with high terrestrial heat flow values have negative or relatively weak crustal magnetic anomalies.