Chinese Journal of Geophysics最新文献

The lithosphere-asthenosphere boundary (LAB) is a seismic discontinuity with negative velocity contrast in the upper mantle. Seismic detections of the LAB in subduction zone region are helpful to understand the interaction between the lithosphere and asthenosphere and the geodynamic process associated with the slab subduction. In this paper, the vertical broadband waveforms are collected from four deep earthquakes occurred from 2006 to 2012 beneath the central South America area. The seismic waveforms are processed with the linear slant stack method to get the vespagrams in the relative travel-time vs. slowness domain, and the sP precursors reflected at the bottom of the LAB (s_LABP) are successfully extracted. Based on the one-dimensional modified velocity model (IASP91-SA), the horizontal distribution for the six s_LABP reflecting points is obtained and divided into the western part (I) and the eastern part (II). In the part I, the LAB depth ranges between 60 km and 63 km, with the average depth of 61 km and the topography of 3 km; in the part II, the LAB depth ranges between 78 km and 82 km, with the average depth of 80 km and the topography of 4 km. Our results reveal the increasing LAB depths from west to east in the central South America area, and the trend may possibly represent the reformation differences of the continental lithosphere. We infer that near the trench, the continental lithosphere may be subjected to the stronger erosion for the higher degree of partial melting and more fertile melts in the asthenosphere; while far from the trench, the continental lithosphere may be subjected to the weaker erosion for the lower degree of partial melting and less fertile melts in the asthenosphere.

The WRF model was employed to simulate super typhoon Megi (2010) with the YSU and MYJ planetary boundary layer (PBL) schemes respectively. The sensitivity of the simulated tropical cyclone (TC) track to the PBL scheme was investigated, and the mechanism for the impact of the PBL scheme on TC track simulation was analyzed from the perspective of TC size. The results show that the description of the vertical mixing process with different schemes is the primary reason for the difference in the simulated lower-level vertical transport of water vapor. Compared to the MYJ scheme, which could well reproduce Megi's track, the simulation with the YSU scheme produced more active outer spiral rainbands and larger TC size. Accordingly, the increased pressure gradient and radial wind led to more air mass transport from the western Pacific subtropical high (WPSH) to TC, resulting in a weakened WPSH and the large scale steering flow was changed subsequently. As a result, the simulated TC with the YSU scheme turned northward earlier than observation.

The differences between the K indices derived with the FMI Method (V1.0) and the Kp indices were analyzed, four problems were found such as discrete K₁ and K₈ among different observatories, K indices about 0.5 larger than Kp indices, wrong S_R on several individual days, steps between S_R and its next day S_R. In order to resolve the above mentioned problems, we improved the FMI method of deriving K indices. The K indices derived with the improved FMI Method were compared with the Kp indices, the K indices derived with the original FMI Method and other methods as well, it was found that the K indices derived with the improved FMI Method were closer to the Kp indices and the K indices derived with other methods than the original FMI Method.

Influences of the western North Pacific (WNP) tropical cyclones (TCs) on their large-scale environment are investigated by lag regressing several physical variables, including 850 hPa relative vorticity, surface zonal wind, total column water vapor, outgoing longwave radiation (OLR), precipitation and sea surface temperature (SST), on an index of TC activity accumulated cyclone energy (ACE) on a monthly time scale. Differences between these variables on the monthly and weekly time scales are discussed. The results show that: (1) The zonal wind regressed against ACE on the monthly time scale is more significant than that on the weekly time scale for both its intensity and scope. The high-intensity and long-lifetime TCs near the equatorial region have a positive effect on triggering or strengthening surface westerly anomalies in the equatorial region. The long-lived and intense equatorial surface westerly anomalies can cause westerly wind bursts, which is believed to be a major factor during the occurrence and development of ENSO events. (2) More variables such as OLR, precipitation and SST exhibit a feature corresponding to El Niño on a monthly time scale compared to that on a weekly time scale, which suggests that TC activities on a monthly time scale are more closely associated with the ENSO cycle. (3) One to two months after the genesis of TCs, the column water vapor decreases while OLR increases at the TC genesis region. This could reduce the potential for TC genesis. Although the SST decrease caused by TCs only occurs at a small spatial scale, it can be spread to a large scale via oceanic and atmospheric propagation exhibiting feedback effect. In addition, the feedback does not occur instantaneously but with a certain lag, which explains why the signal of TC influences on large-scale environment is more pronounced on the monthly time scale.

The Three Gorges region is rich in water resources. With the added impact of impoundment and discharge of the Three Gorges Reservoir, the regional hydrodynamic environment is complex, and research on the impacts of this terrestrial water is still relatively scarce. Using the CORS network supplemented by a small amount of gravity station data, and using the load deformation and gravitational field spherical harmonic analysis method, the crustal deformation and gravity changes caused by the terrestrial water in the Three Gorges area are studied. We find that it is possible to use the CORS network to monitor the crustal deformation and ground-level gravity changes that are caused by the terrestrial water load. The impact of the terrestrial water load is up to the centimeter-level on the vertical deformation, less on the horizontal deformation, and up to hundreds of microgals in the ground level gravity variations. From January 2011 to June 2015, the vertical deformation caused by the terrestrial water load change tends to be balanced, and the influences of gravity changes are mainly concentrated in the Three Gorges Reservoir and the Jianghan Plain. Except for some areas, the crustal vertical deformations caused by the terrestrial water measured by CORS and GRACE are highly consistent in their distribution patterns and trends. In this paper, the research results can be used for monitoring the hydrodynamic environment and for the protection and utilization of water resources, which can also be used to dynamically maintain the elevation and gravity benchmarks.

We present a global weak form element free method (EFM) for simulation of direct current resistivity. EFM is a new numerical simulation method developed on the basis of the finite element method (FEM). The key point of this method is the absence of elements and the nodes free from the elemental restraint, which makes it very flexible and simple in pre-processing. It utilizes the nodes of local support domain to construct shape functions to achieve the accurate approximations of the local domain. Approximations of EFM are of high order and boundary conditions are enforced simply, because the radial point interpolation method (RPIM) is used to construct shape function. Therefore, EFM is more suitable to simulate complex models than FEM. First, the boundary value problem and the corresponding variational problem of direct current resistivity forward simulation are derived starting with the partial differential equation of current field. Second, the construction of RPIM shape function is introduced in details. Third, equations of the global weak form EFM for direct current resistivity are derived in details based on RPIM shape function. Then, a Fortran program is written according to the equations. By this program, a homogeneous half-space model was used to verify our element free approach. At the same time, we compared the solutions of EFM and FEM in details which shows that the solutions of EFM are more accurate. Furthermore, the solutions indicate the correctness and effectiveness of the EFM for direct current resistivity forward simulation. Finally, we improve the simulation accuracy successfully by refining nodes arbitrarily, and the solutions of EFM forward simulation for complex geoelectric models show that EFM has a high degree of flexibility.

CFC-11 is an important tool used to assess oceanic cycle models. The CFC-11 which is dissolved in seawater can be used to analyze the ventilation of the oceans. In this study, a tracer CFC-11 module was developed based on the global oceanic carbon cycle circulation model MOM4 L40, which was developed by the National Climate Center of China Meteorological Administration. Then, the model was employed to study the distribution of CFC-11 in the global oceans, and also to assess the model's ventilation capacity. The simulated parameters, such as the global sea surfaces' CFC-11 concentrations, inventory, vertical penetration depths, and concentration distributions, were verified against the actual observations. The results showed that the model reasonably simulated the surface and vertical distribution of the CFC-11. The main storage areas of the CFC-11 were determined to be located in the Northwest Atlantic Ocean, subtropical North Pacific Ocean, and the Southern Ocean. The distributions of the CFC-11 concentrations on the oceanic surfaces were found to be remarkably affected by the sea surfaces' temperatures. The distribution of the simulated CFC-11 was found to have a high agreement with that of the actual observations, and showed an opposite gradient to the sea surfaces' temperatures. When compared with the observations of five sections located in three oceans, the simulated results of the CFC-11 were in general agreement with the observations in the majority of the areas. In addition, the distribution of the simulated CFC-11 was found to be in agreement with meridional overturning circulation in the global oceans, and an improved simulation of the Southern Ocean and deep oceans, as well as the penetration depths, were achieved. However, there were some deviations observed between the observations and simulations. For example, in the North Atlantic Ocean, where a main storage of the CFC-11 was located, the model underestimated the absorption of the CFC-11, which was found to be related to the over-transport simulations from the high latitudes to the low latitudes. These results may have been influenced by the thermohaline circulation and forced data. Overall, it was determined that the MOM L40 reasonably simulated the absorption of the total CFC-11 in the oceans, and effectively reproduced the oceans' ventilation capabilities by simulating the passive tracer CFC-11.

As the clay film developed in low-permeability lithologic reservoirs absorbs oil, reservoirs become oil-wet, which results in abnormally high resistivity oil-water layers and water layers. Such layers bring great challenges to the identification of oil and water layers. In order to make clear the resistivity response characteristics and conductive mechanism in reservoirs with different wettability, cores were selected in Chang 8 formation, Yanchang group, Upper Triassic, the western Ordos basin of China. Experiments were conducted with these samples to simulate the process of oil displacing water, ageing and water displacing oil. In addition, the contact angles of thin slices after washing oil were also tested. The experimental results show that abnormally high resistivity cores are not completely water-wet after washing oil. However, the formation factors of abnormally high resistivity cores and normal resistivity cores have little difference. In the process of oil displacing water, the relationship between normal resistivity core resistivity index and water saturation is linear in a log-log plot, while that of abnormally high resistivity cores is convex. The resistivity of abnormally high resistivity cores remains unchanged during ageing process. Combining the analysis of nuclear magnetic resonance (NMR) T₂ spectra under different conditions, it can be inferred that wettability of abnormally high resistivity cores has become less water-wet after oil displacing water, whereas the ageing process has little effect on the wettability. In the process of water displacing oil, the relation between abnormally high resistivity core resistivity index and water saturation is almost linear, which shows that the conductive structure of rock is not changed. Based on the analyses and discussions of these experimental results, a hydrocarbon accumulation mode and the corresponding conductive mechanism are proposed for low-permeability reservoirs with complex wettability. It shows that abnormally high resistivity water layers are caused by the destruction of continuous conductive paths under oil-wet condition. The researches and experiments are of great importance for understanding the conductive mechanism, identifying oil and water layers and evaluating water flooded formations in low-permeability reservoirs with complex wettability.

Edge detection plays an important role in interpreting potential field data, and is widely used to delineate geologic boundaries and structures. Geologic boundaries can be determined by tracing the enhanced analytical signal, and many filters are developed to detect and enhance the edges. Horizontal and vertical derivatives are commonly used to enhance edge features, but they can only outline the edges of large-amplitude anomalies. In order to display large and small amplitude anomalies simultaneously, some balanced filters have been proposed. We define new filters based on the Theta map method, using 2^nd order horizontal and vertical directional derivatives, and display large and small amplitude edges simultaneously. These methods were tested on synthetic gravity data, and compared with other traditional filters; the results show that the new filters can achieve better results and reveal more details. The method has been applied to gravity-magnetic data acquired in the Luzong ore district in the Middle-Lower Yangtze River Valley Metallogenic Belt (MLYB, Eastern China). Based on the relations between lithology, density and magnetic susceptibility, the calculated results were analyzed. The edge detection results accurately depict the location of the Tanlu fault zone. A gravity boundary north of the Yangtze River is interpreted to be part of the Yangtze River fault. The identified boundaries from the magnetic data are consistent with the margins of the Luzong basin, and indicate that the bounding faults dip towards the basin. Some ring-like closed boundaries occur in the periphery of the Luzong basin. Recent exploration results confirm that there are buried intrusions beneath the gravity-magnetic anomaly bodies, where Fe-Cu skarn mineralization is present. The results of this study provide significant insights for regional deep-level Fe-Cu exploration.

Seismic wavelet estimation is an important part of seismic data processing and interpretation, whose reliability is directly related to the results of deconvolution and inversion. The methods for seismic wavelet estimation can be classified into two basic types: deterministic and statistical. By combining the deterministic spectral coherence method and the statistical skewness attribute method, the amplitude and phase of the time-varying wavelet are estimated separately. There is no assumption on the wavelet's time-independent nature or the phase characteristic. The advantage of this method is the ability to estimate time-varying phase. Phase-only corrections can then be applied by means of a time-varying phase rotation. Alternatively, amplitude and phase deconvolution can be achieved to enhance the resolution and support the fine reservoir prediction and description. We illustrate the method with both synthetic and real data examples. Synthetic examples certify its feasibility while real data example demonstrates the ability to estimate the time-varying characteristic of wavelets.