Peiming Li

Constrained deformable layer tomostatics

Although first-arrival tomography provides an effective waytoestimatenear-surfacevelocitiesandstaticcorrections, the undulation of velocity interfaces such as the base of the weatheredzonemaynotbeeasilydeterminedbythismethod. The main reason is that first arrivals are insensitive to small geometric changes in velocity interfaces because their raypaths tend to traverse along those interfaces. To improve the solution of interface geometry, we developed a deformable layer tomostatics method that approximates the near-surface velocity field as several layers of constant velocity and variablethicknessthatcanbeinvertedforthegeometryofthevelocityinterfaces.Weuseamultiscalemodelparameterization in the inversion for interface geometry. Synthetic and field datatestsshowedthatthemethodcandeterminetheinterface geometry. Constraining the depth range of the basal boundary of the weathered zone increases the convergence rate of the iterative inversion process. Tests on field data showed greater reflection coherency in a stacked section based on constrainedstaticcorrectionsthaninonefromunconstrained static corrections. The method yielded a better match with statics computed from sand-dune curves than does a match obtained by using two commercial grid tomography packages.

Tomographic velocity model building of the near surface with velocity-inversion interfaces: A test using the Yilmaz model

First-arrival traveltime tomography is a popular approach to building the near-surface velocity models for oil and gas exploration, mining, geoengineering, and environmental studies. However, the presence of velocity-inversion interfaces (VIIs), across which the overlying velocity is higher than the underlying velocity, might corrupt the tomographic solutions. This is because most first-arrival raypaths will not traverse along any VII, such as the top of a low-velocity zone. We have examined the impact of VIIs on first-arrival tomographic velocity model building of the near surface using a synthetic near-surface velocity model. This examination confirms the severe impact of VIIs on first-arrival tomography. When the source-to-receiver offset is greater than the lateral extent of the VIIs, good near-surface velocity models can still be established using a multiscale deformable-layer tomography (DLT), which uses a layer-based model parameterization and a multiscale scheme as regularization. Compared with the results from a commercial grid-based tomography, the DLT delivers much better near-surface statics solutions and less error in the images of deep reflectors.

Deformable layer tomostatics: 2D examples in western China

Near-surface statics are corrections for traveltime variations caused by the extremely strong lateral variation of the seismic velocity field near the surface of the Earth. The statics act like a smear on a glass window, distorting the seismic image of deeper structures. Correction for near-surface statics is among the major challenges for onshore and near-shore seismic processing. One promising remedy is tomostatics, which determines the near-surface static contribution to reflection seismic data based on velocity models derived from traveltime tomography.

First‐break deformable‐layer tomostatics constrained by shallow reflections

Tomostatics using first breaks is a popular way to estimate near-surface statics due to the presence of low-velocity weathering zone. However, it is difficult to constrain the geometry of the base boundary of the weathering zone using first breaks, because the corresponding raypaths tend to be parallel with this boundary, traversing on top of the basement layer. We devise here a method of first-break deformable-layer tomostatics with constrains on the depth range of the base boundary of the weathering zone using reflections. Initial test of the method is conducted using a 2D field data set from western China. Comparison between the unconstrained and constrained tomographic models shows a similar geometry of the model layers, but the constrained portion of the base boundary of the weathering zone tends to have a sharper velocity contrast and laterally smoother than that of the unconstrained model. At many places greater than 10 ms difference exists in one-way vertical traveltimes over the weathering zone of the two models, meaning large difference in their static corrections.

2D Multi-scale Cell Tomography for Near Surface Velocities

Traveltime tomography has become an effective means for solving statics problems. However, traditional single-scale tomography (SST) does not obtain the best velocity values for model cells with low ray hit counts, making smoothing or interpolation of the velocity field after inversion a common practice even though it often reduces the accuracy and resolution of the solution model. Multi-scale tomography (MST) can address the above drawback by simultaneously inverting for velocity components of many overlapping SST models with different cell sizes. The final MST model is a superposition of inverted solutions of all overlapping cells. Here the MST method is demonstrated using 2D field datasets from western China.

3D Multi-scale Tomostatics using First Arrivals

Tomostatics is a technology to estimate a source of error in seismic imageries called near-surface statics, which are spurious timing variations of seismic reflections caused by the lateral variations in topography and the thickness of the weathering layer of extremely low seismic velocities. Most tomostatics methods use first arrivals of seismic data to invert for a near-surface velocity model and estimate the static corrections based on the model. In this paper we present a 3D tomostatics method based on deformable layer tomography (DLT) using first arrival data.

Tomographic velocity model building of near surface with reversed‐velocity interfaces

First-arrival tomography is a popular way to build the near-surface velocity models for oil and gas exploration, mining, geo-engineering, and environmental studies. However, the presence of reversed-velocity interfaces (RVI’s), across which the overlying velocity is higher than the underlying velocity, may corrupt the tomographic solutions. This is because that most first-arrival raypaths will not traverse along any RVI such as the top of a low-velocity zone. Here we examine the impact of the RVI’s on tomographic velocity model building of the near surface using first-arrival tomography and a near-surface velocity model from Oz Yilmaz. Our examination confirms the severe impact of the RVI’s on first-arrival tomography. Interestingly, good near-surface velocity models can still be established using a multi-scale deformable layer tomography (DLT). Comparing with the results from a commercial grid-based tomography, the DLT delivers much better near-surface statics solutions and less error in the images of deep reflectors.

2-D deformable-layer tomostatics with the joint use of first breaks and shallow reflections

We have developed a deformable-layer tomography (DLT) method with the joint use of first breaks and shallow reflections to calculate static corrections. The method is tested with a synthetic model and a field dataset from Tarim Basin, Western China. The synthetic test shows that our DLT method can provide high resolution for velocity interface geometries. The field test result indicates that the method can yield a geologically reasonable near-surface velocity model, and the corresponding stack section shows improvement in comparison with the sections from the DLT using only first breaks as well as those from commercial tomostatics software.

Application of 2‐D deformable‐layer tomostatics in western China

We applied a deformable-layer tomography (DLT) method to the tomostatics study. The advantage of the DLT is to directly invert for the geometry of velocity interfaces. In contrast, conventional tomostatics invert for the velocity values of a fixed-in-space framework of grids or cells, and the thickness of the layers can only be interpreted after inversion. The DLT approach allows an inversion for both the thickness and velocity of the weathering zone and basement layers. The method is tested with a field data from Chaidam Basin, western China. The result indicates that the DLT yielded a geologically reasonable near-surface velocity model, and the corresponding stack section shows significant improvement in comparison with the sections from commercial tomostatics software.

The Application of DSU1 High Density 3-D In Coal Field Exploration

The integrated coalfield exploitation and safety production require high precision knowledge of structural shapes and fault types in coal mine exploration, but the conventional 3D method is difficult to meet these needs. In order to improve the resolution, we use DSU1 high-density and wide-azimuth 3-D exploration methods to perform seismic exploration, which improves the precision of coalfield exploration and increases the ability to identify the structure undulation, small faults and thickness variations.