No abstract

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.

No abstract

No abstract

ConocoPhillips acquired a production 3D surface seismic survey in 2010. The survey size was about 410 square miles recorded by 10-Hz geophones with INOVA 364 vibrators that are capable of sweeping from 1 to 150 Hz. In addition, we carried out a field experiment recording a swath of 3D surface seismic data with colocated 2-Hz and 10-Hz geophones. The cost of acquiring the 2-Hz geophone data was negligible when compared with the cost of the entire 3D survey. Because the vibrators can produce energy down to 2 Hz, the use of the 2-Hz geophone is crucial in capturing this energy. To our knowledge, this was the first field experiment employing the 2-Hz geophones and vibrators that could transmit enough low-frequency signals into the ground. The questions we investigate in this field experiment are: (1) how much low-frequency signal can be recorded using 2-Hz geophones, (2) how much degradation of low-frequency signal results from the 10-Hz geophones when compared to the 2-Hz geophones, and (3) the possibility of using the colocated data sets to enhance the low-frequency signal of 10-Hz geophone data that include both experimental and production data. The analyses of the 10-Hz and 2-Hz geophone data in prestack and poststack domains concluded that 2-Hz geophone data clearly exhibited more low-frequency signal than the 10-Hz geophone data. The 2-Hz geophone stack had low-frequency signal down to 2 Hz, and a spiking deconvolution further extended the amplitude spectrum down to 1 Hz. In addition, we develop a novel technique to derive a deterministic match filter from the colocated data sets. The application of this filter on the 10-Hz geophone data recovers the low-frequency signal below 10 Hz.