Chinese Journal of Geophysics最新文献

The application of multiple waves is an important content of marine exploration, and eliminating or utilizing the multiple waves is one of the significant topics in the processing of seismic data. However, little work is concerned with the multiple waves of Ocean Bottom Seismometer (OBS) wide-angle seismic survey and taking advantage of them to improve the ability of seismic imaging. This study attempts to understand the characteristics of the secondary Pg phases and analyze the applications of seismic imaging in OBS wide angle seismic survey.

We firstly identify and know the secondary Pg phases from synthetic seismogram sections and record waveforms then calculate and analyze particle motions of primary Pg and secondary Pg phases through the azimuth angle rotation. After understanding the secondary Pg phases, we get the propagation path by the theoretical model simulation and calculation of measured data with the P wave travel forward modeling method based on the RAYINVR. In addition, improving the seismic imaging is expected, so we used the theoretical model and the actual model of OBS2010 to show the work of the crustal structure imaging.

The secondary Pg phases roughly parallel and follow closely the primary Pg phase, and are characterized by continuous, clear phase and strong amplitude. An obvious vibration is observed behind the vibration of the primary Pg with stronger amplitude, which is supposed to be the secondary Pg phase. On the basis of particle motions, the secondary Pg phases belong to the P-wave seismic phase. The travel-time fitting of the possible propagation path based on the test data gave three different results: (a) the χ² value is 14.921 when the reflecting layer is water layer and sediment; (b) the χ² value is 193.264 when the reflecting layer is the single water layer; and (c) the χ² value is 1.786 when the reflecting layer is the single sediment. After theoretical investigation and data tests, we have the following conclusions: (1) the secondary Pg phases are characterized by P-wave; (2) the secondary Pg phases are mainly from the reflection between the sediments, which (3) greatly increase the constraint on the basement, and (4) improve the imaging resolution of the sediments and the upper crust.

To analyze the applicability of double rotation (DR), planar fit (PF) and fetch planar fit (FPF) over complicated terrain, turbulent data from the Semi-Arid Climate and Environment Observatory of Lanzhou University (SACOL) are used. A parameterization scheme is provided in integral turbulent characteristics (ITC) test for SACOL data. The overall quality after data processing and quality control including sonic temperature correction, coordinate rotations, WPL correction (correction for density fluctuations), stationarity test and ITC test shows that about 45%∼62% of the total data are of high quality for friction velocity (u_*), 66%∼68% for sensible heat flux, 62%∼65% for latent heat flux and 52%∼54% for CO₂ flux. The proportion of the high quality of u_* obtained by DR is 17% higher than PF, while the proportion of high quality data of the latter three kinds of fluxes obtained by PF is 2%∼3% higher than DR. The differences between PF and FPF are mainly in u_*. Comparing the three coordinate rotations in the dominant wind direction, DR still obtains the best quality of u_*. The use of DR is recommended in the complicated terrain for reducing calculation and improving the data quality.

The distribution, saturation, and hydrate indicator of free gas beneath gas hydrate stability zone have been the difficulty in the research of gas hydrate. Amplitude versus offset (AVO) and rock physics can be used to quantitatively interpret free gas saturation. In this paper, seismic data, acquired at K-G basin, India, are processed to preserve true amplitude information. Based on the logging data and horizon calibration, free gas saturation of NGHP01-10A is assessed with effective medium theory and AVO modeling. It is found that free gas saturation is related to its distribution patterns. Assessments of free gas saturation in homogeneous and patchy distribution are 0.3%∼0.4% and 3%∼4%, respectively. The results show good agreement with the crossplot of Poisson's ratio which are calculated from the measurement at NGHP01-10D. Free gas is further deduced to be homogeneous distribution on the base of dry frame Poisson's ratio with the saturation of 0.3%∼0.4%.

Edge detection plays an important role in the interpretation of potential field data. Many traditional methods are employed to outline the edges. However, some of them cannot equalize the amplitude of the edges of shallow and deep geological bodies simultaneously; some of them although can balance the anomaly amplitude, the identified edges include some additional false edge information, especially when the measured anomalies contain both positive and negative anomalies simultaneously. Some methods used to avoid this disadvantage are subjective to some extent. In order to solve these problems, this paper proposed a new method called enhanced analytic sinal tilt angle to identify the edges of geological bodies. The new method is tested with synthetic gravity anomaly, which shows that the new method can not only identify the edges of shallow and deep geological bodies clearly and precisely, also can avoid bringing some additional false edges. Finally, we apply the new method to real measured gravity data in Sichuan basin, China, obtaining good results.

Temperature is a key parameter of controlling hydrocarbon generation of source rocks. Elevated temperature resulted from tectonothermal events may significantly affect hydrocarbon generation. Based on geology, fission track analysis, apatite U-Th/He and volcanic rock isotopic age data, it is found that there existed Indosinian (T₃-J₂) and Yanshanian (J₃-K₁) tectonothermal events in Lower Yangtze area and the sedimentary basins reached their peak heat flow at about 130∼110 Ma. The maximum heat flows obtained from thermal history reconstruction based on vitrinite reflectance are ∼94 mW·m^–2 and ∼78 mW·m^–2 in Jurong and Taixing areas respectively, with an increasing trend from east to west. Hydrocarbon generation history reconstruction based on the EASY%Ro model shows that the dominating gas generation periods of the Cambrian source rock are the Early Permian-Late Triassic and the Late Triassic-Early Cretaceous in Changzhou and Jurong areas respectively. Because of the dual effects from the foreland sediments deposited in T₃-J_1–2 and the magmatism thermal event in the Early Cretaceous, the marine source rocks reached their maximum temperature at the end of Early Cretaceous. The temperature of the marine source rocks was no longer elevated owing to the decreased heat flow since the Late Cretaceous, though, in some parts of the Jurong, Taixing and Changzhou areas, subsidence occurred and burial depth increased during the K2-E period. Therefore, the secondary hydrocarbon generation of the marine source rocks is not a widespread process in central Lower Yangtze area.

Seismic inversion is generally implemented with certain optimization algorithm. However, the inverse operator estimation algorithm proposed in this study is to perform the inversion of data matrix directly under the hypothesis that the inverse mapping exists in the empirically constrained subspaces. The key point of the proposed approach is to search those subspaces instead of searching for the solution indirectly as optimization algorithms do and it's more efficient. AVO/AVA (amplitude variation with offset or angle) inversion is widely utilized in exploration geophysics, and the inversion process is restricted by the quality of seismic data. L1 norm is applied in the construction of the kernel function of inversion by combining the constraint from initial models, which is helpful in enhancing the efficiency and stability of the inversion. Model and field data examples indicate that the proposed AVO inversion algorithm based on inverse operator estimation is more accurate and reliable.

This study compares five methods for estimating the completeness magnitude M_c threshold of earthquake catalogs through applying them to synthetic catalogs generated from 3 different models. We have found that the Median-based analysis of the segment slope (MBASS) method is suitable for catalogs recorded by networks whose detection capability changes rapidly with magnitude and for those with spatiotemporal heterogeneity if the amount of earthquakes is large enough. The M_c by b value stability (MBS) method is optimal in dealing with catalogs recorded by networks whose detection ability improves slowly with magnitude, but it is time-consuming. The Maximum Curvature technique (MAXC) & Goodness-of-fit method (GFT) underestimate M_c and need an M_c criterion. The M_c from Entire Magnitude Range (EMR) method gives a stable and moderate M_c estimation. It is recommended when the amount of events is not large and the tolerance of missing events is relatively high. This study helps us to choose the optimal M_c estimation method in practice to cope with different earthquake catalogs and points out some potential problems caused by these methods.

In land acquisition, particularly in Western China, the near surface layer is often dominated by small-scale heterogeneities. Strong scatterings generated in this layer can seriously affect the quality of data acquisition and depth imaging. Although it has drawn wide attention from the industry and research community, there is lack of an effective way to investigate the relation between the shallow heterogeneities and the quality of the depth image. In this paper, we introduce the parameterized random velocity model to simulate the velocity heterogeneities in the shallow depth, thus statistical parameters such as the random spectra, correlation length, root mean square velocity perturbation can be used to describe the highly complicated small-scale heterogeneities. In the meantime, we introduce the point spreading function to investigate the image quality. The point spreading function and its amplitude and phase spectra in wavenumber domain provide useful information to characterize the quality of the image. This simplifies the descriptions for both complex shallow velocity models and distortions of depth image, making it possible to create a concise relationship between the two. We further use numerical calculations to correlate statistical parameters of random models with the characteristics of point spreading functions. In this way, the relationship between shallow heterogeneities and the depth image can be quantitatively investigated. The proposed method is validated by numerical examples. The results reveal that the thickness of the random layer, the root mean square velocity perturbation, and the correlation length all apparently affect the quality of the depth image.

Xisha block, as a micro-continental massif formed during the process of the formation of the South China Sea (SCS), has recorded important information about the evolutionary history of the SCS. The crustal structure, petrological properties and tectonic attributes of Xisha block are critical to the theory of formation and evolution of the SCS. Based on Ocean Bottom Seismometers (OBS) data acquired from Line OBS2013–3, the P-wave velocity model of Xisha block along this line is established employing the ray-tracing and forward travel-time modeling method. The velocity structure shows that the sedimentary layer is various with a velocity of 2.2∼3.2 km·s^–1 and a thickness of 0.8∼3.0 km. The basement interface is locally rugged and rough. The velocity values of the upper and lower crusts are 5.0∼6.4 km·s^–1 and 6.5∼6.9 km·s^–1, and the upper mantle is a layer with an velocity of 8.0 km·s^–1. The average crustal thickness of Xisha block is about 23 km. The depths of the upper and lower crusts are 9 km and 14 km. Moho interface lies at the depth of 23∼27 km. Xisha block is a thinned continental crust as compared with the South China continental margin; it has a size about 9.2×10⁵ km³ calculated from two mutually perpendicular seismic profiles crossing the block. Xisha block and Nansha block are distributed on the two sides of the fossil spreading ridge of Northwest Sub-basin. They are conjugate to each other since their velocity structures are very similar.

Low permeability sandstone has become an important target of exploration and development for increasing reserves and productions, while the complicated pore structure makes the reservoir and its effectiveness difficult to accurately identify. We used the physical property, mercury injection, and nuclear magnetic data to analyze the pore structure of low permeability sandstone of the Es₄ in the south slope of Dongying sag, and divided the pore structure into three different types. Nuclear magnetic T₂ spectrum and capillary pressure can reflect the distribution of pore throat to a certain extent. The pseudo capillary pressure curve reconstructed by T₂ spectrum with the routine method can be used to invert for the pore radius distribution, but there is a large error between the pore radius distribution and capillary pressure pore-throat radius distribution. The rock pore free fluid T₂ and mercury intrusion pore throat size distribution have a better corresponding relationship. Therefore, this relationship is used to construct the formula for different pore structure types and on different pore scales (large scale: the linear relationship; small scale: power function in different scale). By identifying the pore structure types along the wellbore profile, we can further determine the pore radius distribution using NML data without building the pseudo capillary curve. And this method not only supply a direct evidence for efficiency evaluation of low permeability sandstone reservoir, but also play an important role in the exploration of quantitative inversion of the pore radius distribution on a micro-scale.