Chinese Journal of Geophysics最新文献

Solution caves are important fluid reservoir space in carbonate reservoir, and researching FMI images' caves connected domain labeling and extracting their information are meaningful. A high resolution color image can be obtained after data processing of FMI. After a series of processes which include image graying, median filtering and threshold segmentation for the color image, a binary image will be obtained which can reflect the characteristic of solution caves on the wall of a well. And on the image, caves are black spots which are labeled by same number. The labeling algorithm for image connected domain based on equivalence pair processing has the advantages of fast and no-repeat labeling, which can eliminate equivalent pairs while labeling connected domain. The solution caves in the binary image can be marked from small to large number accurately by this arithmetic, in addition, the information of every connected domain including holes' size, grading factor, area of connected domains (areal porosity) and roundness can be extracted and processed. Using the labeled binary image can calculate porosity curve which reflects development degree of caves, and based on this curve the image can be divided into several layers. On this basis, the information distribution of areal porosity, holes' size, roundness and grading factor of every layer can be calculated easily. At last, all of these informations will be used to quantitatively evaluate the carbonate reservoir which has strong heterogeneity and lots of solution caves. And this work is also a helpful exploration for quantitative extracting of cave information from FMI images.

Based on the latest compiled 1/100 million aeromagnetic map data, the Curie point depth was estimated by power spectrum method. The calculation obtained 8004 Curie point depth values in Chinese continent. Eventually, we completely compiled the Chinese continental Curie point isotherm map. This map first fully shows the characteristics of Chinese continental Curie point depth. The study shows that Curie point isotherm is characterized by depression inside the stable blocks with the depth of 28∼45 km, such as the Tarim Basin, Junggar Basin, Qaidam Basin, Hoh Xil-Bayan Har Depression area, the Yangtze Basin area, North China basin area, Songliao Basin, Erlian Basin, Bayan-Wuwei-Chaoshui basin, Pearl River Estuary-southeast Hainan basin. The Curie depth of North China basin area is shallower than Tarim continent block and Yangtze continent block, which may be related to the fact that North China block suffered a complex post-transformation which resulted in the asthenosphere upwelling and lithospheric thinning. Hoh Xil-Bayan Har block is a NWW trending Curie point isotherm depression area in northern Tibetan Plateau, where developed a large area of Triassic sedimentary strata and experienced less magmatism. The stable blocks all have uplifted Moho and depressed Curie point isotherm. Conversely, the Curie point isotherm of active orogenic belt is characterized by uplift at the depth of 18∼26 km, such as the Mountain (Mtn.) area of northeast and northwest China, Qinling-Dabie area, West Kunlun-Tibet-Sanjiang-Kangdian area, the southeast coastal areas and so on. The uplift reflects the geothermal gradient difference caused by tectonic and magmatic activities. A collection of 816 heat flow data from published literatures are used to study the relationship between Curie point isotherm and heat flow. The results show that there is not a simply linear relationship between the depth and heat flow. When the depth is greater than 30 km, the heat flow values are less than 100 mW·m⁻². While the Curie point depth is less than 30 km, the heat flow values vary in a wide range. The high heat flow values are mostly found in the east coast of China, southern Tibet-Sanjiang area, Qinling-Dabie area, and the east of Liaoning province, which are all characterized by uplift of Curie point isotherm. These areas are important for future geothermal exploration.

This study investigated longitudinal differences in the thermospheric air mass density (ρ) during the solar minimum period from 2007 to 2009 at magnetic mid-latitudes. Both observations by CHAMP satellite and model simulations using the Global Ionosphere-Thermosphere Model were analyzed. It was found that ρ exhibited wave-1 structure in its longitudinal profile (i.e., one peak and one trough), which was almost 180° out of phase between the two hemispheres. The phase of the wave changed with the local time, exhibiting a clear diurnal variation. The value of ρ exhibited few longitudinal variations in the eastern part of the northern hemisphere, where the electron density showed some longitudinal differences. The reason for this was that the variation in the electron density was too slight to affect ρ in the Europe-Asia sector via ion drag. As confirmed by the simulation, the longitudinal difference in ρ was reduced by almost 40%∼50% in terms of the maximum value of the peak-trough difference when ion drag was negligible. Thus, both the model and the observations confirmed the important role of ion drag in the longitudinal structure of neutral density. This study further indicated the obvious longitudinal difference in the solar heating at magnetic mid-latitudes. The peak-to-trough difference in the solar zenith angle could reach 20°∼30°. The non-uniformity of solar irradiation heating could be another cause of the longitudinal structure of the air mass density.

The transient electromagnetic forward modeling of volcanogenic massive sulfide (VMS) ore deposits is to calculate the eddy currents electromagnetic response of a three dimensional body in the full space of deep-sea environment. We simulated the three dimensional transient electromagnetic response of the VMS deposits using full-domain vector finite element method. The ore body was discretized with brick rectangular elements. The finite element equation was deduced in frequency domain by employing Galerkin procedure, and the conversion to time domain was by inverse Fourier transform. We confirmed the validity of the full-domain vector finite element algorithm by comparing simulated results with analytical solutions of double half-space model. The results have a good agreement with each other, which indicates that the vector finite element method is capable of solving whole space problem. In order to demonstrate the ability of the numerical method in calculating the response of VMS deposits containing complex boundary conditions, we compared vector finite element solution of a three dimensional electrical model with physical experiment results according to electromagnetic physical scale modeling rules. The comparison suggests that for the complex electromagnetic boundary of seawater, ore body and country rocks, the transient electromagnetic response of VMS deposits calculated by full-domain vector finite element method has same features with the physical scale modeling result. The vector finite element method is simple and its results are precise with obvious and clear anomaly response.

During drilling, the mud column sustains a slightly higher pressure than the formation to maintain the stability of the well wall, which causes the mud filtrate to penetrate into formation pores and displace in-situ fluids. The invasion depth is affected by reservoir properties, especially the reservoir permeability. Therefore, it is possible to estimate the reservoir permeability if the invasion depth can be measured.

A numerical study was conducted to investigate the feasibility of evaluating reservoir permeability with array induction logging. A mud invasion model was built up by coupling mud cake growth with multiple-phase fluid flow, and an array induction logging model was established based on the Born geometric factor theory. Joint forward simulations of mud invasion and array induction logging indicated that the responses of array induction logging can reflect the effect of mud invasion on the formation resistivity. Inversion based on the damped least square method revealed that the invasion depth can be acquired from array induction logging data.

We investigated the association between reservoir permeability and invasion depth, and found that in a reservoir with a permeability of 1 to 100 mD (1 mD=0.987×10⁻³ μm²), the reservoir permeability governs the invasion depth, and thus the permeability can be evaluated according to invasion depth. A two-dimensional numerical simulation showed that the inversed invasion depth curve had a similar fluctuation to the permeability variation. For a layered formation, a series of interpretation charts can be produced to evaluate the permeability of every layer with tolerable errors. The numerical investigation proves the feasibility of estimating reservoir permeability with array induction logging.

The water in the overlying rock of seabed gold mine directly threats the exploitation safety. The strike drift at −135 m level of the Xinli mine district of Sanshandao gold mine in Shandong province, was selected to be the applicable site for high-density resistivity method, which was used to detect water-bearing structures in the surrounding rock 200 meters below a measuring line of 635 m long. Three DC devices including Wenner, Dipole-Dipole, and Schlumberger were implemented in the selected drift respectively, authenticated each other and collected good detections. The apparent resistivity inversion results of these devices give the consistent conclusion that three different resistive layers exist under the strike drift. The rock mass of 30 m thick under the level of −135 m is a high resistivity layer, which indicates no or little water in the rock mass between the levels of −135 m and −165 m. The rock mass ranging from 30 m to 60 m in depth under the level −135 m has low resistivity and bears fracture water. And a very high resistivity layer deeper than 60 m under the −135 m indicates that the aquosity of the rock mass under −200 m progressively decreases. These detection results show good agreement with the conclusions of field hydrogeology surveys and hydrological structure analyses and that high-density resistivity method is feasible for uncovering water-bearing structures in drift.

Understanding the physical mechanism of major earthquakes is very important for earthquake prediction and its disaster reduction. The mainstream hypotheses on earthquake mechanism are reviewed in this paper. It is pointed out that the elastic rebound and stick-slip hypotheses, both of which are usually used to explain the mechanism of shallow earthquakes, have some serious drawbacks, and that such hypotheses explaining the mechanism of intermediate and deep earthquakes as the dehydration embrittlement, phase transition instability, shear melting and anticrack-associated faulting, are inconsistent with some observation data. Thus, it is necessary to establish a new hypothesis or theory of earthquake mechanism. The brittle failure theory of multiple locked patches in a seismogenic fault system developed by us since 2010 is introduced in the present study. It is stated by the theory that the progressive failures of locked patch result in the occurrence of earthquakes due to the fault movement. Hereafter, the significant earthquakes occurred at its volume expansion and peak strength points are referred to as characteristic ones. The earthquake cases indicate that the seismogenic processes of shallow, intermediate and deep characteristic earthquakes can be well explained by the theory. We emphasize that both the elastic rebound and stick-slip hypotheses contain the same implicit assumption that there exist locked patches in the seismogenic faults, and that the source body of deep earthquakes is with appropriate environment conditions leading to brittle failures. Moreover, some controversial issues, such as seismic stress drop much less than that of rock failure in the laboratory test, heat flow paradox and Self-Organized Criticality (SOC), are discussed and can be reasonably explained by the theory. The present study shows that the physical mechanism of major earthquakes is attributed to the brittle failures of locked patches.

The Yishu segment of the Tanlu fault zone is the seismogenic structure of the 1668 Tancheng M8½ earthquake, and is also the research focus of the potential strong earthquake location in the future. Geological survey shows different activity degrees along the Shandong-Jiangsu-Anhui segment of the Tanlu fault zone, but few researches focus on the crustal velocity structure beneath this area, and the relationship of seismicity and the difference of crustal velocity structure. The 3D velocity structure beneath this segment of the Tanlu fault zone and adjacent areas (30°N–37°N, 113°E–122°E) was imaged by seismic tomography. Crustal velocity structure segmentation beneath the researched area, the velocity characters and its relationship with the geological structure segmentation and seismic levels are studied in this paper.

We select the earthquakes occurred from 1980 to 2011 in the research area, relocate these events through a relative relocation technique. Based on the traveltime data of Pg, Sg, Pm, Sm, Pn, and Sn waves of these earthquakes, using GABWIT (Genetic Algorithms in Body Wave Inversion of Traveltime) inversion method, the 3D velocity structure (inversion grid: 30 km×30 km) beneath the research area is imaged by seismic tomography.

The analysis of the velocity image of different depths for the research area suggests that the crustal velocity structure is segmented. For shallow layers, different velocity segments (north of 35.3°N, 34.5°N–35.3°N, and 33°N–34.5°N) are related to exposed strata, respectively corresponding to three rupture elements of the Tanlu fault zone. They are Anqiu segment, Juxian county-Tancheng segment, and Xinyi-Sihong segment, which have different earthquake activity patterns. It indicates that Xinyi-Sihong segment is a locked segment of Tanlu fault. The crustal velocity structure beneath the Shandong-Jiangsu-Anhui segment of the Tanlu fault zone and adjacent areas can be roughly divided into three different segments from top to bottom. They include the south segment (south of 32.5°N–33°N), middle segment (from 32.5°N–33°N to 35°N–35.3°N) and north segment (north of 35°N–35.3°N). The segmentation of upper crust is related to insertion from Sulu UHPM belt, the segmentation of middle and lower crust is related to detention of volcanics. The velocities in the west of the Tanlu fault zone is higher than that in the east. The differences of velocity image at different depths show different tectonic blocks, which have different evolution histories and compositions, also meaning that this fault extends down to the Moho.

Different velocity segments at different depths beneath the Shandong-Jiangsu-Anhui segment of the Tanlu fault zone are related to geology factors such as exposed strata, rupture units of the fault zone, Sulu UHPM belt, detention of volcanics or different tectonic blocks. Through analysis of the seismic activity of the different rupture units of the Tanlu fault zone in the research

Accurate location of micro-seismic source in underground coal mine is of great significance for monitoring and early warning of dynamic disaster. Micro-seismic source information is generally extracted through inversion of the data acquired by the underground sensors. The installation of the sensors is limited around underground roadway. The unreasonable arrangement of sensors along the roadway will greatly decrease the precision of the seismic location. As for the ill-posed problems caused by the inversion for source location with sensor-received information, high precision location algorithm of micro-seismic source based on the arrangement optimization of monitoring points is presented in this paper. Firstly, the ill-posed problems are judged by calculating the coefficient matrix condition numbers. The ill-posed matrix is then pretreated by centralization and row balance jointly. The regularization parameters of pretreated matrix A and b are calculated by the L-curve method. The seismic source coordinates regularization solution is obtained via Tikhonov regularization algorithm. The research shows that the matrix magnitude is decreased hugely with centralization method, ill-posed condition numbers are reduced with row balance pretreatment. The minimum seismic source coordinate error of Tikhonov regularization solution after the pretreatment is 3.09 m which has been greatly decreased compared to the error by Gaussian elimination solution before the pretreatment. High precision seismic source location is realized in limited space of underground roadway by the above optimization processing.

We proposed a new full waveform inversion (FWI) method, namely full waveform inversion of the second-order time integral wavefield, by enhancing low-frequency components of seismic data with a second-order time integration of seismic wavefield, which can efficiently reduce the initial model dependence of FWI. According to the propagation equation of scattering wavefield in scattering theory, we derived a propagation equation for the scattering wavefield with second-order time integral, and used the leading order Born approximation for the linearization of the propagation equation. Based on the propagation equation for the scattering wavefield with second-order time integral, using the scattering wavefield to invert for the distribution of scattering sources in subsurface, and using wavefield modeling to construct the incident wavefield, and according to the linear relationship between the scattering wavefield and the incident wavefield and velocity perturbation in the linear propagation equation for the scattering wavefield with second-order time integral, we applied a formula similar to the imaging formula of migration to obtain the estimation of velocity perturbation, and established an iterative inversion method for the full waveform inversion of second-order integral wavefield. Applying the inversion result of full waveform inversion of second-order integral wavefield as the initial velocity model for the conventional FWI can efficiently reduce the initial model dependence of FWI. Numerical tests using synthetic data of the Marmousi model demonstrated the validity and feasibility of the proposed method. The final results of the new method can deliver much improved results than the conventional FWI. Furthermore, to test the independence from the seismic frequency-band, we use a low-cut source wavelet (cut from 4Hz below) to generate the synthetic data. The inversion results by our new method show no appreciable difference from the full-band source results.