Hengchang Dai

Anisotropic Migration And Model Building For 4C Seismic Data: A Case Study From Alba

Assuming a scatter point located beneath a stack of layers with vertical transverse isotropy (VTI), we derive an accurate double-square-root (DSR) converted-wave (Cwave) diffraction equation, and incorporate the equation into the Kirchhoff prestack time migration. The DSR is controlled by five parameters: Pand S-wave stacking velocities VP2 and VS2, vertical velocity ratio g0, and anisotropic parameters heff and zeff, which define the anisotropic velocity model for migration. We demonstrate using real data how to build the anisotropic model from reflection moveout analysis, and evaluate the merit of prestack time migration. The DSR equation has a similar form to its isotropic counterpart, which allows an efficient implementation of prestack time migration. Applications to real data show that the C-wave imaging obtained by the new approach is more focused and coherent than the imaging by isotropic methods.

Compensating for the effects of gas clouds by prestack migration: A case study from Valhall

In this paper, we develope a practical scheme for compensating for the effects of gas clouds using the 4C dataset from Valhall. The Valhall C-wave data suffer from severe diodic effects (variations with the source-receiver direction) due to gas clouds. These include diodic illumination effect, diodic velocity (Vc) effect, and diodic velocity ratio (γeff ) effect. To compensate for these effects, the dataset is separated into positive and negative offset data volumes and a Kirchhoff prestack migration method that can handle both vertical and lateral velocity variations is applied to the positive and negative datasets separately. This allows the determination of the migration velocity and velocity ratio by imaging optimization and focusing analysis. This approach yields an improved C-wave imaging in Valhall.

Azimuthal AVO And Seismic Scattering Attenuation In 3D Fractured Media: a Numerical Simulation Study On the Discrete Fracture Model

In this study we build three numerical models based on the Discrete Fracture Model (DFM) method with the finite difference (FD) scheme for two purposes as follow: the azimuthal AVO analysis on the PP reflection of the top of the fractured layer, and the estimation of scattering attenuation of the reflection from the bottom of the fractured layer. These models include a vertical fracture model, an orthogonal fracture model, and an isotropic model, which is for comparison (referred as VFM, OFM and IM respectively). First we observe that the distribution pattern of coda waves from either wavefield snaps or shot sections, due to the presence of the fractures, changes with the azimuth and the spacial structure of the fractures in the two fractured models. Azimuthal AVO variation reveals that the compliant fractures lower the magnitude of the PP reflection amplitudes in the two fractured models compared with the IM, and in the VFM we find the magnitude of the reflection amplitudes is smaller in the normal direction of the fracture strike than in other direction, which may indicate the fracture strike. In the case of two or more vertical fracture sets, the magnitude is even smaller. Frequency analysis of the reflection from the bottom of the fractured layer shows that the dominant frequency shifts to lower frequency with the increase of the offset. However, the estimation of the scattering attenuation is very small in the two fractured models.

Estimating anisotropic parameters from PS converted-wave data: a case study

Anisotropic parameters are estimated from PS converted-wave (C-wave) data. The data were acquired by digital MEMS (micro-electro-mechanical system) sensors over a volcanic gas reservoir in Northeast China with a mixed sand and shale sequence in the overburden. This gives rise to both shear-wave splitting and anisotropy moveout effects due to presence of vertical transverse isotropy (VTI). We use a four-parameter theory to evaluate the effects of non-hyperbolic moveout due to an asymmetric raypath and VTI. These four parameters include the PS converted wave stacking velocity (VC2), the vertical velocity ratio (?0), the effective velocity ratio (?eff), and the anisotropy parameter (?eff). This four-parameter theory leads to an improvement in imaging quality and correlation between the P-waves and converted-waves. After compensating for the VTI effects, we analyze the shear-wave splitting in the post stacked data. We find that the amount of splitting determined from the data can be correlated to the known gas reservoirs, and is also consistent with other amplitude and spectral anomalies associated with the gas reservoirs, revealing a potential for using shear-wave splitting to delineate gas reservoirs in volcanic rocks.

Characterizing Volcanic Gas Reservoirs Using PP And PS Seismic Data: Case Study

Nowadays multicomponent seismic has gained more and more acceptance from the hydrocarbon industry than was the case several years ago. Nevertheless, the anticipated breakthrough into a mainstream technology is still to be realized. One of the bottlenecks is the lack of convincing data examples that demonstrate the full benefit of the technology. This is particularly true for land multicomponent seismic. As a step towards filling this gap, we present a case study of using land PP and PS convertedwaves for characterizing volcanic gas reservoirs in Daqing Oilfield in Northeast China.

Accuracy of a Simplified Moveout Formula For PS Converted-waves In Multi-layered Media

We examined the accuracy of a simplified moveout formula of PS converted (C-) waves in multi-layer media. This formula uses only two parameters (stacking velocity and anisotropic coefficient) to control the reflection moveout of C-waves. The results from synthetic models show that this simplified formula has the same order of accuracy as the other formulae with four parameters for Cwaves. This confirms that only two parameters can be reliably estimated from C-waves alone. C-wave moveout can be corrected without knowing the velocity ratios. The processing of C-wave data may then be simplified.

3D pre‐stack Kirchhoff time migration of PS‐waves and migration velocity model building

We present a practical approach to perform 3D pre-stack Kirchhoff time migration in a PC cluster and to build the migration velocity model. This approach is successfully applied to a 3D-4C OBC dataset acquired in the North Sea to use its PS-waves to image the structure under a gas chimney. The results show that neither VTI nor HTI anisotropy can be observed in this dataset. An isotropic, Common Image Point (CIP)-consistent velocity model is sufficient for imaging processing. The 3D migrated results are encouraging. The structure under the gas chimney is clearly imaged using PS-waves. The faults in the target area can be clearly identified. These structures are confirmed by the drilling results.

Model Based Registration of PP and PS Waves in the Depth Domain

We introduce an approach to perform a model based PP and PS registration in depth domain which involves converting the RMS time model to the depth model and performing PSDM. We have developed a GUI tool for depth model building and a PSDM tool which is based on the ray-tracing method and Kirchhoff summation. This approach is applied to a real 2D dataset. Time processing is applied to the PP and PS waves to obtain time images and RMS velocity models separately. Then the depth models are obtained for the PP and PS waves. For PS waves, we build the depth model as close to the P wave depth model as possible. Finally, we apply the prestack depth migration to produce migrated depth images. The results show that the events in PP and PS depth images are well correlated. The model building in this approach is straightforward and this approach is practical.

Compensation for the Azimuthal Anisotropy of PS-converted Waves in HTI Media

This paper studies the azimuthal anisotropy of PS-converted waves recorded in the horizontal radial and transverse components in HTI media. Due to converted wave splitting, the horizontal radial and transverse components contain two types of converted wave: the fast P-SV1 and the slow P-SV2 that propagate along the directions parallel and perpendicular to the fracture strike, respectively. The NMO velocity variations of both the P-SV1 and P-SV2 waves are close to two different ellipses. Therefore it is necessary to analyse and compensate individually for the azimuthal anisotropy of each of the P-SV1 and P-SV2 waves. By applying two NMO-velocity ellipses to the synthetic and field data, the NMO correction results are improved.

Delineating the Gas Reservoir With the C-wave Diodic Effect Correction In Qaidam Basin, Northwest China

The C-wave imaging may be degraded by the diodic effect introduced by the gas cloud though C-waves have been commonly used to image through gas cloud. This diodic effect may be compensated with a velocity perturbation method which decouples the diodic moveout into two parts: the base velocity and the velocity perturbation. In this paper, the velocity perturbation is written with base velocity and the PS velocity of positive and negative offset. And the relation between velocity perturbation and gas reservoir is discussed. A land 2D3C seismic dataset is analyzed from the Sanhu area in the Qaidam basin, northwest China and significant diodic effects are observed in the data. The perturbation approach is applied to this data and the resultant C-wave imaging and the details of the reservoir structure are significantly improved. The characterization of the reservoir boundary using velocity perturbation is well consistent with the real reservoir.