Ultrathin hexagonal Co3O4 nanosheets with exposed reactive facets and porous architectures are synthesized by using a polyethyleneimine (PEI)‐mediated hydrothermal strategy. Characterization of the material indicates typical Co3O4 nanosheets of hexagonal shape that are approximately 100 nm in side length and 15 nm thick. PEI plays crucial roles in the synthesis of hexagonal Co3O4. The effects of the reaction conditions on the precursor morphologies and dimensions are investigated to understand the various roles of PEI. Electrochemical tests reveal the superior performance of ultrathin hexagonal Co3O4 nanosheets, including a remarkable specific capacity (e.g. 1007 mAh g−1 at 100 mA g−1 and 858 mAh g−1 at 500 mA g−1), excellent cyclability (e.g. capacity retention: 96–99 %), and a high rate capability. The improved performance is attributed to the combined effects from the high percentage reactive facets as well as the ultrathin and porous structures. This synthesis strategy can be extended to fabricate other anode materials for lithium‐ion batteries.
In the past few years, significant progress has been made on new velocity analysis algorithms. In the first part of this paper, we will briefly summarize recent advances on velocity analysis. Then we describe a new model-based globally-optimized residual curvature analysis algorithm we have just developed. Like conventional residual curvature analysis, the algorithm is based on the principle that after prestack migration with a correct velocity model, an image in the common image point (CIP) gather is aligned horizontally regardless of structure. Unlike conventional residual curvature analysis, this algorithm uses not only the interpreted CIP gathers, but also the interpreted migrated depth section as input. The algorithm is model-based, and uses modelbased CIP ray tracing to relate residual moveouts in CIP gathers to errors in the velocity model. Residual moveouts measured in CIP gathers are globally used in the optimization process for updating the whole velocity model. Also model-based normal incident ray tracing is used for updating the reflector boundaries.
REVIEW OF RECENT ADVANCES OF VELOCITY ANALYSISRecent advances of velocity analysis can be summarized into two categories: 1) traveltime inversion; 2) migration velocity analysis.
Traveltime inversionTraveltime Inversion (TI) (Bishop et al., 1985; Stork and Clayton, 1991) estimates a depth velocity model from traveltimes picked from prestack data. The main advantage of TI is that it is formulated as an optimization problem and therefore model updating is very effective and efficient. However, in areas of complex geological structure, picking prestack traveltimes in surface seismic data is almost unfeasible. Picking prestack traveltimes may have the following problems: 1) in the case of complex reflector geometry, seismic energies reflected from different parts of a reflector may arrive at the same receiver location; 2) reflection arrivals contaminated by diffraction energy; 3) low signal-to-noise ratio often associated with complex structure.In the past few years, a few researchers tried to solve the traveltime picking problem. IFP (Institut Francais du Petrole) developed a method called SMART (Sequential Migration Aided Reflection Tomography) (Delprat-Jannaud and Lailly, 1993) to solve the traveltime picking problem. The main idea of SMART is to use an approximate velocity model to migrate seismic data, then pick the imaged reflectors in the cube of migrated shot gathers, finally trace rays that propagate in the same velocity model as the one used for the migration and that are reflected on the picked imaged reflectors. They claimed that because the ray tracing undoes what the migration has done, even with an approximate velocity model they can recover traveltimes. More recently DATAID (1994) used a similar approach as IFP, but performed migration and raytracing in the common-offset gather instead of common-shot gather. The main point of these approaches is that instead of directly picking events in the time domain, picking is done after depth migration...
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