Xianhuai Zhu

Recent applications of turning-ray tomography

Recent applications of 2D and 3D turning-ray tomography show that near-surface velocities are important for structural imaging and reservoir characterization. For structural imaging, we used turning-ray tomography to estimate the near-surface velocities for static corrections followed by prestack time migration and the near-surface velocities for prestack depth migration. Two-dimensional acoustic finite-difference modeling illustrates that wave-equation prestack depth migration is very sensitive to the near-surface velocities. Field data demonstrate that turning-ray tomography followed by prestack time migration helps to produce superior images in complex geologic settings. When the near-surface velocity model is integrated into a background velocity model for prestack depth migration, we find that wave propagation is very sensitive to the velocities immediately below the topography. For shallow-reservoir characterization, we developed and applied azimuthal turning-ray tomography to investigate observed apparent azimuthal-traveltime variations, using a wide-azimuth land seismic survey from a heavy-oil field at Surmont, Canada. We found that the apparent azimuthal velocity variations are not necessarily related to azimuthal anisotropy, or horizontal transverse isotropy (HTI), induced by the stress field or fractures. Near-surface heterogeneity and the acquisition footprint also could result in apparent azimuthal variations.

Load Cell System Test Experience: Measuring the Vibrator Ground Force On Land Seismic Acquisition

Many methods have been developed in an attempt to expand the recorded bandwidth associated with Vibroseis surveys, particularly to boost the signal at high frequencies for improving seismic image resolution. However, the dominant frequency band still remains in the range of 10– 60Hz and no significant improvement has been achieved at high frequencies in conventional 3D seismic surveys. Ground attenuation is one of possible limitations, and another is that during the course of the seismic acquisition, the vibrator may not actually be generating as much energy at high frequencies as expected. In this paper the letter possibility is addressed by using a Load Cell System to measure the force actually being generated by the servo hydraulic vibrator. We find that these measurements are not consistent with the drive signal from the vibrator electronic controller (weighted-sum ground force estimate) therefore the conventional weighted-sum ground force signal is deemed questionable.

Application of multiscale waveform tomography for high-resolution velocity estimation in complex geologic environments: Canadian Foothills synthetic data example

Seismic imaging in compressional belts such as the Canadian Foothills is very challenging due to complex geological structures, rugged surface topography, and highly variable near-surface conditions. Seismic sections across the Canadian Foothills are usually progressively more distorted when approaching the Canadian Foothills region. Figure 1 shows the degree of structural complexity and topographic variations which are in part responsible for the deteriorated imaging in the thrust belt. Accurate velocity models of subsurface structures are critical for improving seismic images of thrust belts in both the time domain (e.g., tomostatics) and the depth domain (e.g., prestack depth migration).

Geologic-to-seismic Modelling for Eldfisk SOA Reservoir Characterization - An Integrated Study

Seismic imaging for the Eldfisk SOA requires a good understanding of overburden geology and detailed reservoir structure. In Eldfisk, we found that, based on well logs and seismic amplitude attributes, gas zones represented as pancake-shaped geobodies in the overburden, produce significant time sags in modeled seismic sections that are similar to sags observed on field datasets. Amplitude decays observed on the acquired seismic data also suggest that the pancake-shaped geobodies are heterogeneous, causing scattering and energy loss. The main contribution of this paper is to develop a workflow to convert geologic modeling property (e.g. porosity) to seismic velocity for 3D seismic modeling and RTM imaging. High-resolution seismic modeling and imaging are important for well planning and field development at Eldfisk. The integrated velocity model contain both geobodies constructed from the overburden and detailed geologic features such as small faults in the reservoirs. Results from the full 3D RTM and decimation studies have illustrated that receiver spacing greater than 300x300 m would produce significant artifacts in the reservoirs. Target-oriented visibility studies based on the integrated velocity model suggested that dominant energy around ~5-10km may provide imaging uplift to reservoir structures in the Eldfisk SOA.

Modelling and Imaging Through a Gas-Obscured Zone in Malaysia

Summary We demonstrate using viscoelastic modeling followed by dual velocity RTM imaging that ocean bottom node (OBN) survey is needed in a gas-obscured zone in Malaysia. Scattering caused by “geobodies” filled or partially filled with gas will distort the P-wave propagation. S-wave, on the other hand, is less sensitive to the geobodies, and therefore has a good chance for imaging through gasobscured zones. However, when intrinsic Q (or attenuation) is present in the overburden, the ability of PSconverted wave imaging will be degraded. The reliability and resolution of PS-converted wave imaging also depends on the accuracy of velocity models used for RTM imaging.

Near-Surface Complexity Could Masquerade As Anisotropy

Near-surface velocities could vary with azimuth, impacting seismic data processing and interpretation. In this study, we developed a methodology to investigate the variations of near-surface velocities with azimuth, using 3D turning-ray tomography. The input data are the first arrivals selected from pre-defined azimuth sectors in terms of shot-receiverpair directions. The output velocities from tomography correspond to the selected azimuth sectors. A near-surface tomography study based on seismic data from a shallow heavy-oil reservoir in Canada has suggested that the observed azimuthal traveltime variations are not necessarily related to azimuthal (HTI) anisotropy induced by the stress field or fractures. It could also be caused by the nearsurface heterogeneity or acquisition footprint. Near-surface complexity could masquerade as anisotropy. Potentially this can influence statics and prestack imaging.