Yike Liu

3-D tomographic static correction

A 3-D tomographic inversion approach based on a surface-consistent model for static corrections is presented in this paper. Direct, reflected, and refracted waves are used simultaneously to update the near-surface model. We analyze the characteristics of the first-break traveltime in complicated low-velocity layers. To improve the accuracy for the velocity model, the various first-break times from direct, reflected, and refracted waves are considered for model inversion. A fractal algorithm which overcomes the error caused by wavelet shape differences is applied to pick first breaks. It also overcomes the leg jump of refractions. The method can pick a large number of first breaks automatically. The raypaths and traveltimes are calculated with a 3-D ray tracer that does not increase computation time for complicated geological models. Our method can determine the raypath associated with minimum traveltimes regardless of wave mode (direct, refracted, or reflected). We use a least-squares approach in conjunction with a matrix decomposition to reconstruct a 3-D velocity model from the actual first-break times obtained from 3-D data. Finally, long- and short-wavelength static corrections are calculated concurrently, based on the reconstructed velocity profile. The method can be applied to wide-line profiles, crooked lines, and 2-D and 3-D seismic survey geometries. The results applied to a real 3-D data example indicate that the 3-D tomographic static corrections are effective for field data.

Three-Dimensional Velocity Images of the Crust and Upper Mantle beneath the North-South Zone in China

We have determined the three-dimensional P-wave velocity structure beneath the north-south tectonic belt between Tibet and Eastern China by simulta- neously inverting local, regional, and teleseismic data. Our data set is composed of 45,028 P-wave arrival times from 602 local and regional earthquakes and 985 travel times from 102 teleseismic events. With the LSQR (sparse linear equations and least squares) algorithm, the P-wave velocity perturbations were estimated by the simul- taneous inversion of hypocenters and medium parameters from the surface to a depth of 200 km. We tested the stability and the resolution of our inverted results with a checkerboard test and found that the models are well resolved up to a depth of about 50 km for most parts of the studied region. Results show that the north-south tectonic belt is characterized by a significant lateral heterogeneity in velocity both in the crust and in the upper mantle. Correlating these velocity images with the main tectonic features, we find that (1) the shallow velocity distribution above 3 km is consistent with the topographic features and the basin distribution; (2) the middle-lower crustal velocities from 20 km to 50 km characterize a mechanically weak north-south tec- tonic belt, because it bears a relatively lower-velocity perturbation over a large re- gion; (3) the upper mantle velocities from 85 km to 120 km delineate the eastern Tibetan boundary, but changes in some subzones may reflect the effects of several tectonic events, including paleorifting, the Cenozoic convergence between Tibet and southeastern China and other tectonic episodes.

Rock mass structure analysis based on seismic velocity and attenuation images

Seismic traveltime, amplitude and pulse-width can be used to reconstruct seismic velocity and attenuation coefficient images for a rock mass. This study discusses the appearance differences of velocity and attenuation coefficient in the ore vein and rock mass in the images. The location of the rock vein and the characteristic of the rock mass are discussed according to the difference responses of velocity and attenuation from the ore vein and mixed rock vein, fracture and rock body. The effect and limitation of the seismic tomography method for investigating ore body and rock mass is suggested on the basis of a field test result. The special observation method in this study gives a good reference for obtaining full position and directional observation in seismic tomography.

Lower Velocities beneath the Taihang Mountains, Northeastern China

We used regional P -wave arrival times to invert for 3D velocity structures beneath the Taihang Mountains, which are bordered by the intensely active zones of the Shanxi rift to the west and the eastern plain basin to the east in the North China Block. P -wave velocities show that low velocities (−1.0 to −3.0%) are focused at depths of ∼15–20 km beneath the Taihang Mountains, similar to those beneath the active zones of the Shanxi rift and the eastern plain basin, If the lower velocity is representative of weakness in the underlying crust, there our results indicate that the Taihang Mountains might be potentially active, because the structure patterns are similar to their bounding active zones. Future seismic research, therefore, should strongly focus on the Taihang Mountains.

Reverse Time Migration of Multiples

Multiple reflections have different wave propagation paths from primary reflections and thus can be used to complement the illumination where primary reflections from beneath the salt are not available. We propose to modify conventional reverse time migration (RTM) so that multiples can be used as constructive reflection energy for subsalt imaging. This new approach replaces the impulsive source wavelet with the recorded data containing both primaries and multiples and uses predicted multiples as the input data. In the RTM process, multiples recorded on the surface are extrapolated backward in time to each depth level, and the observed data with both primaries and multiples are extrapolated forward in time to the same depth level, followed by a cross-correlation imaging condition. A numerical test on the Sigsbee2B dataset shows that a wider coverage and a more balanced illumination of the subsurface area can be achieved by migration of multiples compared with conventional migration of primary reflections.

Vector Median Filter and its Applications in Geophysics

Summary This paper introduces the concept of the Vector Median Filter (VMF) and its applicability to noise reduction in seismic data processing. The VMF is a generalization of the well-known Scalar Median Filter (SMF) from scalar-valued to vector-valued functions. We shall also show that the conventional SMF is a special case of the VMF. Since vector-valued functions are often used in geophysics (e.g., three-component seismic data, measurements of electromagnetic field data, fracture orientations, calculated local dips of seismic events or windowed segments of seismic traces), we believe that the introduction of VMF to geophysics could have a broad impact by more effectively reducing noise than normal SMF. In deed, based on this paper, an important application for separating blended (or continuous) source acquisition had been developed by Huo et al. (2009). The effectiveness of VMF is demonstrated with a series of field datasets in this paper.

Tectonic framework of China and its relation with mineral resources——Cognition from geophysical data

TECTONIC and its evolution are theoretical basis of geology and summation in geosciences from investigation of the earths structure, composition, motion and history. Therefore, they are of great importance to the industrial activities. Various geological maps have been compiled in different scales after long-time observation of rock outcrops in mountains and torrents. These tectonic maps, which are obtained based upon the cognition and judgment of tectonic movements in different ages, are summarization regarding the regularity of geological phenomenon in terms of theory, and provide service of exploration and references for environmental protection and prevention and control of disaster. Using physical methods and technique, such as gravity, magnetism, electromagnetics, seismology, geotherrnics and radioactivity, geophysicists investigated the earth, made some measurements for special purposes, and provided basic data for interior structure and condition inside the earth. For example, the seismic tomography gives three-dimensional velocity image beneath the surface. Geochemistry, starting from the composition of chemical elements, analyzes and measures the stratigraphic rock, thus providing a new method to explore the earth. These studies, which open up and deepen the theory and practice of the earth studies, not only enrich the study greatly, but also provide data in a wide range. Thus, geoscience needs comprehensive studies of geology, geophysics, geochemistry and other disciplines to understand the earth. So does the foundamental scientific investigation and geoscience application investigation. The real specific study should be conducted under the guidance of.macroscopic cognition. Study on the tectonic framework of China has macroscopic and overall characteristics. Based on the work of predecessors, it should be extended and deepened and.include both the latest results of geology and geophysics, providing hypothesis for geosciences and initial conditions for solving the problems, and serving as the basis of further investigation.

Multiple subtraction using statistically estimated inverse wavelets

Surface-related multiple elimination (SRME) typically consists of two steps: The first step is prediction and the second step is subtraction. In subtraction, it is important to effectively attenuate multiple events and preserve primary events. When multiples cross with or overlap on primaries, least-square subtraction usually cannot subtract multiples effectively and may also damage the primaries. When multiples overlap with primaries, least-square subtraction cannot always subtract multiples accurately and often damages the primaries. To remedy this problem, we propose to statistically estimate the inverse source wavelet, correct for errors in the estimate of the inverse wavelet, and then use the corrected inverse wavelets for multiple subtraction. Synthetic tests and real data examples show that the proposed method can effectively attenuate multiples, while they also preserve the continuity of reflection events and successfully avoid amplitude distortion. The proposed method is characterized by low computational costs and ease of implementation.

Seismic Interferometry Using Non-Far-Field Sources and Removing the Spurious Arrival

Abstract Seismic interferometry using far-field correlation is a technique to obtain the Green’s function between two receivers using passive wave-field recordings, and often it is under the theoretical assumption that sources are in the far field when in fact they may not be. Using a heterogeneous medium enclosed by a closed boundary on which the sources are fired, we offer two views on the meaning of the far-field correlation. In the intrinsic view, the validity of the far-field correlation is based on the far-field approximation for wave scattering, and it is investigated by comparing various physical dimensions in the heterogeneous interior, regardless of the exterior. However, the extrinsic view centers on the medium properties in the exterior only without considering the interior. Previous studies showed that, with the correct scattering model and complete source coverage, no spurious arrivals should be generated if the illuminating sources are in the far field. We investigate the case of near-field cross correlation. This problem is considered in the context of two-dimensional space with a single embedded scatterer that is represented by a cylindrical inclusion with small radius. An analytical solution for the scattered wave is computed. However, if the sources are not in the far field, the cross-correlation kernel must be used to eliminate the spurious arrival, in addition to the correct scattering model and full source aperture. The kernel accounts for the near-field illumination of the region.

Characterizing Seismo-stratigraphic and Structural Framework of Late Cretaceous-Recent succession of offshore Indus Pakistan

Abstract Regional seismic reflection profiles and deep exploratory wells have been used to characterize the subsurface structural trends and seismo-stratigraphic architecture of the sedimentary successions in offshore Indus Pakistan. To improve the data quality, we have reprocessed the seismic data by applying signal processing scheme to enhance the reflection continuity for obtaining better results. Synthetic seismograms have been used to identify and tie the seismic reflections to the well data. The seismic data revealed tectonically controlled, distinct episodes of normal faulting representing rifting during Mesozoic and transpression at Late Eocene time. A SW-NE oriented anticlinal type push up structure is observed resulted from the basement reactivation and recent transpression along Indian Plate margin. The structural growth of this particular pushup geometry was computed. Six mappable seismic sequences have been identified on seismic records. In general, geological formations are at shallow depths towards northwest due to basement blocks uplift. A paleoshelf is also identified on seismic records overlain by Cretaceous sediments, which is indicative of Indian-African Plates rifting at Jurassic time. The seismic interpretation reveals that the structural styles and stratigraphy of the region were significantly affected by the northward drift of the Indian Plate, post-rifting, and sedimentation along its western margin during Middle Cenozoic. A considerable structural growth along the push up geometry indicates present day transpression in the margin sediments. The present comprehensive interpretation can help in understanding the complex structures in passive continental margins worldwide that display similar characteristics but are considered to be dominated by rifting and drifting tectonics.