Chinese Journal of Geophysics最新文献

The small-scale discontinuous and inhomogeneous geologies, such as tiny faults, cavities and fractures, play an important role in reservoir analysis. However, effectively extracting them from seismic imaging data is a challenging problem, as their seismic responses are much weaker than reflections' from large-scale structures. On the other hand, this small-scale information is easily contaminated with noises, which will make their analysis difficult to perform if there is no strategy adopted for improving the signal-to-noise ratio (S/N) of their images. By combing a non-linear filter and a sparsity constraint, a seismic sparse inversion method of imaging data is developed for detecting these small-scale discontinuous and inhomogeneous geologies.

The core of extracting discontinuous and inhomogeneous information lies in removing strong reflections and noises. The plane-wave destruction method uses a local plane-wave model for representing seismic structures and thus is appropriate for estimating reflections. Through subtracting the predicted reflections from seismic imaging data, the small-scale discontinuous and inhomogeneous information will be left into the seismic residual data. Considering the sparsity property of this small-scale information, a L₁ - L₂ norm model is built that uses a non-linear filter for promoting the S/N of the discontinuous and inhomogeneous inversion results. In order to guarantee the computation efficiency in solving this sparsity model, a L₁ norm approximation scheme and quasi-Newton algorithm is introduced.

Numerical experiment demonstrates the effectiveness of the proposed method in extracting the small-scale discontinuous and inhomogeneous geologies. This numerical model is composed of fractures, faults and cavities. The geological targets are four series of cavities in the shallow part and three series of cavities in the deep part. With the proposed seismic sparse inversion method, a profile with reflections eliminated and noises destroyed is obtained and the edges, faults, fractures and cavities are completely resolved. In field application, a carbonate reservoirs analysis is performed. The 3D prestack time migration profile can clearly display large-scale layers but fails in describing discontinuous and inhomogeneous geologies. Although coherency techniques can reveal discontinuous information, the small-scale tiny faults, fractures and cavities are beyond its detection. The proposed method succeeds in clarifying and locating the small-scale discontinuous and inhomogeneous geologies. The seismic attribute analysis based on seismic spares inversion data also provides valuable information about the planar distribution of the tiny faults, cavities and fractures.

Based on a sparsity-constraint model, a seismic sparse inversion method of imaging data is proposed for extracting small-scale discontinuous and inhomogeneous geologies. The method ca

Transient electromagnetic method (TEM) is a commonly-used geophysical method of measuring the secondary electromagnetic field induced by transient pulse sources. Because of the inherent mutual-induction between TX and RX coils, the measured field always contains the mutual-induction field, which becomes stronger when TX coil becomes smaller and closer to RX coil. It influences the near-surface TEM systems, which utilize small TX coils to effectively resolve the near surface explorations. The mixture of the mutual-induction field with the secondary field at early period leads to a blind region in the shallow surface in TEM exploration. To solve this problem, we applied two concentric and parallel current coils in the TEM system. They are opposing coils, which means that the two coils are physically same, but the currents in them are of equal value and in reverse directions. The RX coil is equidistant to the two opposing coils. By means of this special arrangement, the mutual-induction field received at the RX coil is zero, which means that the mutual induction effect is efficiently eliminated, and pure secondary field of the conductive underground will be measured. It is proved by theory and model calculation that opposing coils transient electromagnetic method has advantages in lateral resolution. It is quite suitable for small coil TEM system aiming at shallow subsurface detection.

Time-lapse, or 4D, seismic technology is a tool to monitor underground change for oil field maximum recovery or other purpose especially associated to human being activities. Repeatability is a key issue for time-lapse seismic and geometry repeatability is a fundamental element to essentially affect the repeatability. There were many practices to improve repeatability during data processing, but geometry repeatability needs to be acquired during monitor data acquisition phase. Theory and practice shows that geometry repeatability determined in acquisition cannot be thoroughly improved in processing. Thus, geometry repeatability analysis is important. Multi-trace geometry repeatability rises from practical situation. Multi-trace geometry repeatability is important for overall geometry repeatability evaluation. The difference or repeatability of time-lapse seismic data is evaluated by normalized RMS difference (NRMS). The definition of multi-trace repeatability can be derived from NRMS for time-lapse seismic/4D seismic data, showing that multi-trace repeatability is weighted RMS of all single traces. Noting that the uncertainty of the match between monitor data and baseline data and that probable data size difference between monitor and baseline, the repeatability of the best baseline-based match with imaginary data for mismatch was employed. Derived from previous researches, the linear model of relationship between seismic data repeatability and geometry repeatability was established, and the weighted RMS geometry repeatability of the best baseline-based match with extrapolation for mismatch was obtained as the equivalent of the repeatability of the best baseline-based match with imaginary data for mismatch. The weighting coefficient is determined on the basis of NMO and its stretch. Application study was also conducted based on real data to demonstrate that the new geometry repeatability can be utilized to valuate geometry repeatability during 4D monitor seismic data acquisition. The application study showed that one display of the multi-trace geometry repeatability upon best baseline-based match with extrapolation for mismatch can indicate repeatability and effect of fold of coverage simultaneously. The calculation in application was simplified for speed improvement, which is not yet the main point in this article.

It is difficult to calculate reservoir parameters of tight sand reservoirs using conventional interpretation methods, due to their complex lithology and variable pore structure. An optimization log interpretation method is able to take full advantage of the log data and geological information. Therefore, it is an effective method to evaluate tight sand reservoirs. In this study, in order to calculate the reservoir parameters of tight sand reservoirs, an appropriate interpretation model needed to be first established according to the reservoirs’ characteristics. Then, the interpretation parameters were chosen, and the specific form of the objective function was determined. Next, an optimization algorithm was adopted to search for the optimal solution. A bacterial foraging algorithm (BFA) is a newly developed algorithm which has strong global search capabilities. It simulates the behavior of the colon bacillus which swims with flagella for food in the human gut. However, since it slowly converges in the later part of the optimization, it was combined in this study with a complex algorithm (CM) for constituting a BFA-CM hybrid algorithm, in order to improve the precision and efficiency of the search process. Also in this study, the unknown reservoir parameters of the optimization log interpretation method were determined using a genetic algorithm (GA), particle swarm optimization (PSO), BFA algorithm, and BFA-CM hybrid algorithm, respectively. The calculation results showed that, when compared with the GA and PSO, the errors of the porosity and the component content calculated by the BFA were minimal. However, the calculation result curves were found to be inconsistent. Therefore, by combining a BFA algorithm with a CM algorithm to constitute a BFA-CM hybrid algorithm for calculating reservoir parameters, the accuracy was improved, and the curves became more stable. The results of the BFA-CM optimization log interpretation method verified that the objective function value was F ≈ 0. Also, the sonic, neutron, and density log theoretical value curves (AC0, CNL0, DEN0) fell within the confidence interval, which indicated that a system deviation influence did not exist, and that the optimization results were reasonable and credible. When compared with the other algorithms, the BFA-CM hybrid algorithm displayed unique advantages during the process of calculating the unknown parameters with the optimization log interpretation method. Its calculation results were of high accuracy and stability, and the efficiency was also improved. The experimental results showed that the BFA-CM optimization logging interpretation method was able to accurately calculate the tight sandstone reservoir parameters, and could therefore be applied to actual production practices.

With artificial heating term (Q_hf) added in the electron energy equation of SAMI2 model, the disturbance amplitude of electron temperature and density along field line is simulated. The effects of disturbances under different heating conditions are also compared. The results show that the terrestrial ionospheric plasma can be heated by powerful high-frequency radio waves, which can result in the rising of the temperature of electron in the whole field line, especially at the heated spot where electron temperature enhances more than 3 times; the pressure balance is broken as the increase in the electron temperature, which leads to plasma diffusion and then perturbation of electron density occurred; electron density gradient perpendicular to magnetic field line changes in accordance with the perturbation of electron density; the perturbation amplitude of electron temperature and density decreases during the heating time, which gradually becomes saturated. Electron temperature and density have a nonlinear relationship with the intensity of heating source.

The logging evaluations of tuffaceous sandstone reservoirs are always difficult problems. The existence of tuff results in the great variations in reservoir pore structures, and the associated physical properties, which greatly influence the formation parameters. Therefore, the effects of tuff on saturation models cannot be ignored. This study took the tuffaceous sandstone reservoirs in the X depression of the Hailar-Tamtsag Basin as an example. Based on the differences of the response characteristics between the tuff and shale, the component content was calculated using a method which combined a bacterial foraging algorithm and a particle swarm optimization algorithm. The experimental data of the cation exchange capacity (CEC) proved that the tuff had conductivity. Then, the resistivity of the tuff was obtained using the relationship between the CEC and the resistivity, which was then used to calculate the saturation. Finally, a new method was proposed to calculate the saturation of a tuffaceous sandstone reservoir, and was referred to as a CEC ratio method. The calculation results of this method were found to have a good application effect.

Coal elasticity is one of the important mechanical characteristics reflecting the material composition and structure of coal. Under the constraint of drilling and well logging, applying seismic exploration to predict elastic characteristics of the coal seams would be of important engineering significance for coal mining and CBM reservoir evaluation and development. In this regard, ultrasonic experiment of coal samples is the basis to realize the seismic inversion of physical properties of coal seam. In this paper, based on 30 pieces of six kinds of metamorphic raw coals collected from six mining areas of Yima, Fukang, Huainan, Pingdingshan, Hebi and Jiaozuo in China, laboratory ultrasonic measurements of these coal samples were respectively conducted in three directions of strike, dip and perpendicular to the beddings of coal seam under room temperature and pressure. The experimental results show: the samples’ velocities of both pressure wave (P-wave) and shear wave (S-wave) decrease in turn in the three directions; there are obvious anisotropies of velocities, and the average anisotropy of P-wave velocity is stronger than S-wave's. Moreover, larger differences exist among both quality factors and modules of elasticity in the three directions, and the S-wave quality factors are greater than P-wave's; with exception of Poisson's ratio, modules of elasticity of coal are less than those common sedimentary rocks. Through the experiment and analysis, it can be demonstrated that Gardener and Castagna formulas are not suitable to represent relations of coal elastic parameters in China's coal fields, and two empirical formulas with higher precision are statistically given.

For the 2001 M_S8.1 Kunlun earthquake, which was one of the largest events occurred around the Tibet plateau, a large controversy still exists about its rupture detail. In this paper, we invert the co-seismic GPS and InSAR data for a robust finite-fault model of the Kunlun earthquake based on a realistic fault geometry buried in a layered earth structure. The inversion is based on the constrained least-squares principle and realized using the steepest decent method (SDM). The different data sets are weighted according to their variance and spatial coverage. The results show that the slip maximum can reach up to ∼6.9 m and is located at 35.76° N and 93.40° E. The main rupture area is located at the shallow depth above 20 km. The inverted shallow slip agrees with the surface rupture observed by the field survey, and the whole slip pattern appears generally consistent with the results obtained from previous geological and seismic wave studies.

This study statistically investigates the temporal and spatial variations of plasma convection velocity (V_y) and thermospheric zonal wind velocity (U_y) in the range of –4 hour (h) and +2 h MLT relative to substorm onsets (MLT onset) with both CHAMP and DMSP (F13, F15) satellites observations from 2001 to 2005. It shows that V_y increase significantly in all MLT sectors and the latitudes of the peak values move equatorward in 1.5 h after substorm onsets. In the subsequent 1.5 h V_y decrease in intensity and the peaks retreat poleward, which shows that the onsets of substorm can enhance the plasma convection velocity dramatically. The relative increase of V_y is largest from –2 h to MLT onset, which indicates that the substorm mainly affects the thermosphere in the westward 0∼2 h MLT sector. U_y increases steadily in 3 hours after substorm onsets, with a 1.5 h delay relative to V_y in response to the substorm onsets.