Near-surface Media Research Articles

Description of Random Heterogeneity and Characteristic Analysis of Wave Field in Near-surface Medium

Random heterogeneity of near surface medium is caused by weathering layer, desert and gravel layer etc. For seismic exploration in this complex area, it is helpful to improve the exploration effect that detailed description of this heterogeneity and in-depth analysis the characteristics of seismic wave field in this medium. In order to figure out the above-mentioned issue, firstly, a regional multi-scale disturbance random medium modeling method in complex topographical conditions is proposed to describe the random heterogeneity of near-surface medium. Secondly, in the condition of undulating earth’s surface, the finite difference method (FDM) of wave equation numerical simulation is used to simulate the seismic wave field. Finally, some seismic wave characteristics in near-surface random heterogeneity are obtained by analyzing seismic snapshots and gathers.

Prestack time migration from 3D irregular surfaces with near-surface-related deabsorption

We have developed a 3D prestack time migration (PSTM) approach that can directly migrate nonplanar data with near-surface-related deabsorption using three effective parameters. The proposed scheme improves the so-called topography PSTM approach by adding a near-surface effective [Formula: see text] parameter that compensates for the absorption and dispersion of waves propagating through near-surface media. The two effective velocity parameters above and below the datum can be estimated by flattening events in imaging gathers, and the additional near-surface effective [Formula: see text] parameter can be obtained using scanning technology. Hence, no knowledge with respect to near-surface media is needed in advance for implementing the proposed scheme. The proposed topography-deabsorption PSTM method can be applied to seismic data recorded on a 3D irregular surface without statics corrections. Consequently, traveltimes are obtained with improved accuracy because the raypath bends away from the vertical in the presence of high near-surface velocities, and the absorption and dispersion caused by strong intrinsic attenuation in near-surface media are correctly compensated. Moreover, we attenuated the migrated noise by smearing each time sample only along the Fresnel zone rather than along the entire migration aperture. As a result, an image with a higher resolution and superior signal-to-noise ratio is achieved. The performance of the proposed topography-deabsorption PSTM scheme has been verified using synthetic and field data sets.

Metal-Bearing Nanoparticles Observed in Soils and Fault Gouges over the Shenjiayao Gold Deposit and Their Significance

Mineral deposits concealed by thick cover sequences present special problems for geochemical exploration. A variety of penetrating geochemical methods have been developed in the last few decades to explore for buried deposits. The theoretical basis of the mechanism by which metals migrate upward from buried deposits through the cover to the surface is still not fully understood. One hypothesis is that metal particles or metal elements could be carried onto bubbles or micro-flow of geogas and migrate upward to the surface. After years of study, nano-scale metal-bearing particles have been widely observed in geogas samples from different kinds of concealed deposits. However, the occurrence of these metal-bearing particles carried by geogases in near-surface media, such as soil, has not been studied in detail. In this study, metal-bearing nanoparticles were observed in samples from soils and fault gouges over the Shenjiayao gold deposit. The results indicate that (1) the ore-forming elements in soils can only come from deep-seated ore bodies and they occur in nanoparticles in the study area; (2) there is an obvious relationship between metal nanoparticles in fault gouges and soils; (3) the metallic nanoparticles in fault gouges represent a transitional phase along the whole vertical migration process. In addition, the observation results show that the metal-bearing nanoparticles tend to be adsorbed on the surface of clay minerals, which provide theoretical support for using fine fraction soils as sampling media to carry out geochemical exploration in sediment-covered terrains. Based on the results and discussion, a simple migration model was built in this paper.

High-frequency Rayleigh-Wave method

High-frequency (≥2 Hz) Rayleigh-wave data acquired with a multichannel recording system have been utilized to determine shear (S)-wave velocities in near-surface geophysics since the early 1980s. This overview article discusses the main research results of high-frequency surface-wave techniques achieved by research groups at the Kansas Geological Survey and China University of Geosciences in the last 15 years. The multichannel analysis of surface wave (MASW) method is a non-invasive acoustic approach to estimate near-surface S-wave velocity. The differences between MASW results and direct borehole measurements are approximately 15% or less and random. Studies show that simultaneous inversion with higher modes and the fundamental mode can increase model resolution and an investigation depth. The other important seismic property, quality factor (Q), can also be estimated with the MASW method by inverting attenuation coefficients of Rayleigh waves. An inverted model (S-wave velocity or Q) obtained using a damped least-squares method can be assessed by an optimal damping vector in a vicinity of the inverted model determined by an objective function, which is the trace of a weighted sum of model-resolution and model-covariance matrices. Current developments include modeling high-frequency Rayleigh-waves in near-surface media, which builds a foundation for shallow seismic or Rayleigh-wave inversion in the time-offset domain; imaging dispersive energy with high resolution in the frequency-velocity domain and possibly with data in an arbitrary acquisition geometry, which opens a door for 3D surface-wave techniques; and successfully separating surface-wave modes, which provides a valuable tool to perform S-wave velocity profiling with high-horizontal resolution.

Nanohole arrays in metal films as optofluidic elements: progress and potential

Subwavelength holes in metal films exhibit coupled optical phenomena specific to structure geometry, incident light and properties of the near-surface medium. As optofluidic components, nanohole arrays in metal films present several opportunities. This review provides an overview of the unique optical characteristics of such arrays, with emphasis on their application in the micro and nano-fluidic environment. The majority of contributions in this area have focused on sensor applications, and the results of nanohole array based chemical and biomolecular sensors are reviewed here. Also relevant to on-chip analysis, various field and spectroscopic enhancements achieved with nanohole arrays are discussed. The general benefits and limitations of nanohole arrays for analytical applications are discussed in the context of existing tools. Beyond sensing, particle trapping and other potential optofluidic applications of nanohole arrays are discussed.