查看更多>>摘要：Using the magnetic method to detect soil pollution has attracted increasing interest. To study the influence of magnetic dustfall on urban environment, 105 soil samples (0-5 cm) were collected along three equidistant gradient profiles with respect to a major iron and steel industrial complex (ISIC) in the northern suburb of Shanghai, Southeast China. The soils and atmospheric particulate matter (APM) in the area were examined using magnetic and micromorphological analyses. As a result, magnetic susceptibility (xlf) of study soils is commonly significantly enhanced, with a mean of 284.8 x 10(-8) m(3) kg(-1). 68.6% of the soils are higher in xlf than 100 x 10(-8) m(3) kg(-1), more than three times the magnetic background. Moreover, it shows an increasing trend toward the ISIC. Frequency-dependent susceptibility (xfd%) of the soils is mostly less than 2%, and is negatively significantly correlated with xlf, suggesting that the enhancement of soil magnetism is due mainly to coarse anthropogenic magnetic particles (multi-domain (MD) or stable single domain (SSD)). A number of magnetic spheroids were observed in the soils and APM, which are believed to mainly come from the ISIC. The persistent deposition of anthropogenic Fe-borne magnetic dustfall has significantly influenced the urban environment, causing the synchronous enhancement of magnetic susceptibility and heavy metal contents in the soils. The study soils are moderately polluted by Zn and Pb, and the soils of Profile I, the nearest to the ISIC, are heavily polluted by Pb and moderately polluted by Cd and Zn. xlf can reflect the overall level of heavy metal pollution of the soils effectively. Moreover, it is positively significantly correlated with the content of Cd, Cu, Zn, Ni, Cr, Pb and Mn (p < 0.01) in the soils. However, it is difficult to measuring the content of one or more heavy metals in the soils using the magnetic method as locally quantitative correlations between xlf and heavy metal content cannot be established even within this small industrial area.
查看更多>>摘要：A fracture may be characterized by its normal and shear compliances that relate the discontinuity in displacement of the fracture faces to an applied traction. To assess the feasibility of using elastic waves for monitoring natural and hydraulic fractures in anisotropic shale reservoirs, an understanding of how these compliances relate to the properties of the background rock and any fracture infill material is required. Three models of vertical fractures in a VTI (transversely isotropic medium with vertical axis of rotational symmetry) background suitable for feasibility studies are discussed. The first two models are suitable for open vertical fractures and represent the fractures as aligned low aspect ratio elliptic cylindrical inclusions with long axis of the ellipse either horizontal or vertical. In these models the horizontal and vertical shear compliance differ in a way that depends on the anisotropy of the background medium and the orientation of the fractures. The third model is suitable for estimating the properties of hydraulic fractures containing proppant. In this model, proppant is represented as a random packing of identical elastic spheres subject to a uniaxial compression due to the normal stress acting on the fracture. The results are expected to help improve the reliability of fracture models used for assessing the feasibility of elastic waves for characterizing natural and hydraulic fractures.
查看更多>>摘要：Identifying the variability in and driving forces of relative sea-level (RSL) change in the Yangtze River Delta during 30-50 ka B.P. is vital for understanding global climatic and RSL changes. In this study, the environmental magnetism of sediments in the Yangtze River Delta was systematically studied for the period 30-50 ka B.P. based on the optically stimulated luminescence (OSL) dates. Detailed magnetic measurements, including hightemperature kappa-T, magnetic hysteresis loop, isothermal remanent magnetization acquisition, and demagnetization curve measurements illustrate that the predominant magnetic contribution of core YZ07 is from magnetite, with a small contribution from hematite. Detrital input may be the primary source of magnetic minerals according to the chi/chi(fd)% and chi(ARM)/chi correlations. In addition, the positive relationship between chi and the RSL curve indicates that chi is a valid indicator of RSL change. Additionally, comparing chi, RSL curves, and delta O-18 dates of the stalagmites demonstrated that climatic change influenced the RSL change, which mainly affected the concentration of magnetite. The results are conducive for understanding the response mechanism of magnetic indicators to relative sea-level change, and provide a scientific basis for encouraging for future work in understanding climate variability.
查看更多>>摘要：The processing of semi-airborne electromagnetic data (SAEM) mainly adopts1D or quasi-2D inversion, which is not conducive to fine detection. By targeting 2.5D inversion problem of SOTEM, a hybrid algorithm based on particle swarm optimization (PSO) and lateral constraint inversion (LCI) is optimized. PSO can remove dependency on the starting model and improve the accuracy of interpretation. LCI may overcome the disadvantages of PSO, such as low convergence rate and over-stochastic problem. The cosine probability function varying with the number of iterations is used to merge the two algorithms to arrive at a compromise between precision and speed. Compared with the particle swarm optimization algorithm, the convergence speed of the hybrid algorithm is significantly accelerated in the late stage and successfully suppresses the over-stochastic problem. Lastly, the new algorithm is applied to the inversion of the data measured by the coal mine target.
查看更多>>摘要：Time-lapse seismic is an important tool for reservoir management. As seismic data quality improves, there is a drive for moving towards quantitative interpretation, which demands data of higher repeatability. In deep water time-lapse projects, the changes in the speed of sound of the water layer between baseline and monitor acquisitions play a significant role in the data processing. In this work, we study the effects of these water velocity changes, in a scenario of ocean bottom nodes (OBN) acquisition. We perform 2D elastic seismic modeling with high-resolution property models for two distinct survey dates. In each date, different water velocity profiles, measured in field experiments in a Brazilian Pre-salt area, are considered. To incorporate source and receiver non-repeatability, some differences in the acquisition geometries are also tested for each vintage. The synthetic data are compared in terms of time-lapse amplitudes, and repeatability is quantified via the normalized root mean squared attribute (NRMS). For the parameters considered in our study, the deterioration of NRMS seems to be dominated by the non-repeatability of the receivers and by water velocity variations, with source non repeatability having a lower contribution. We believe the results presented in this work are a first step towards a more robust methodology for time-lapse feasibility studies, which incorporates imaging uncertainties and allows us to understand the contribution of each element that could affect the 4D seismic image quality.
查看更多>>摘要：Darakeh area is one of the major areas in terms of seismic hazards in Tehran. The North Tehran Fault has gone through the center of the studied area, thus creating fractures, displacements and folds of layers, forming anticlines and synclines. Considering the old residential texture understanding the tectonic characteristics and locating the hidden faults of the region can be quite useful in providing thematic maps and preventing damages caused by potential earthquakes. Nevertheless, no geophysical studies have been carried out on this area so far. To map the area, three GPR and two electric resistivity profiles were designed based on the identified tectonic lineations and previous seismic studies on the area. Each GPR profile, was 100 m in length and surveyed using a MALA GPR with an unshielded 50-MHz antenna. The geo-electric profiles were surveyed by WennerSchlumberger array. The results of the processed profiles demonstrate a severe drop in resistivity in some regions which could be due to the presence of fault zones and potential cavities in the region under study.
查看更多>>摘要：Using the cross-correlation or cross-coherence method to extract the traveltime information of surface waves between two stations from the ambient noise seismic data has become a popular method, which is widely utilized in shallow, regional, and global-scale geophysical surveys. Different from the common-used narrow-band filter (NBF), we adopt the continuous wavelet transform (CWT) as an alternative method to measure the phase velocity from the interstation cross-correlation function (CCF). On account of the distribution of urban noisy sources it is hard to satisfy the requirement of random, stable, uncorrelated, and uniform, which usually makes the measured phase-velocity uncertain. For those surveys with an ultra-short recording time of noise, the adverse effect would be more severe. Hence, we propose to adopt the plane-wave-based beamforming (PWBB) to scan the incoming direction of surface waves based on the hypothesis of plane-wave propagation. To suppress the incoherent components during the stack of CCFs, the phase-weighted stack (PWS) technique is also included in the proposed workflow. Several synthetic tests have verified the good performance of the new method. Especially when only a few seismic sensors are arranged in the research area, the advantages of our method would be more obvious. Then, the workflow is applied to the analysis of the field noise data, which were collected at Fengman hydropower station, Jilin province, China. The beamforming result indicates that the main component of the signals is produced by the effects of river scouring under the reservoir gate. Finally, the S-wave velocity tomogram of the research area and its geological interpretation are given. All data tests demonstrate that the proposed CWT with PWBB can achieve the stable dispersion measurement from the recorded seismic data of two station.
查看更多>>摘要：Ground-airborne transient electromagnetic (GATEM) methods combine the advantages of ground TEM and airborne TEM, which can simultaneously fulfill the requirements of detection depth and work efficiency. In this study, the GATEM responses of typical models were numerically modeled using the finite-difference time-domain (FDTD) approach. The traditional FD approach of TEM uses the upward-continuation boundary condition at the surface of the earth but does not model complex terrains with an air layer. In this study, the FDTD approach was used to directly mesh and calculate the air layer. Specifically, based on the quasi-static conditions of the electromagnetic field and the frequency spectral features of the transmitting current waveform, the choice method for obtaining the fictitious permittivity in the air layer was studied, and the stability problem in the calculation of the air layer was addressed. The convolutional perfectly matched layer boundary conditions were used at the air and underground boundaries to improve the accuracy of the solution in the late field. Moreover, the results of this approach were compared with the analytical solution and the results of other algorithms, and the accuracy and reliability of the approach were verified. Next, this approach was used to perform three-dimensional forward simulations on the GATEM responses of typical models such as the 3-D body model, rugged topography model, and low-resistivity overburden model. The modeling results show that the GATEM response is significantly affected by flight altitude in the early stage, whereas the late response is less affected by the flight altitude. The sensitivity of the GATEM response was smaller than that of the ground TEM. Signal reversal is observed in the partial differential Bz/ partial differential t response curve under the influence of a hill. However, the influence of the hill can be treated as a background field in the late period. The partial differential Bz/ partial differential t curve observed above the goaf is higher than that without the goaf, but the difference between them will be reduced due to the influence of the low-resistivity overburden and measurement noise.
NSTL
Elsevier