Higher Order Moveout Research Articles

Prestack depth imaging of multi-azimuth seismic data in the presence of orthorhombic anisotropy

The presence of orthorhombic anisotropy can severely affect the imaging of multi- and wide-azimuth data which is rapidly developing with the benefit of better illumination, better imaging, and better multiple elimination. Analysis of multi-azimuth (MAZ) data often reveals noticeable fluctuations in moveout between different acquisition directions, preventing constructive summation of MAZ images. Vertical transverse isotropy (VTI) effects can also co-exist causing well misties and higher order moveout. We have developed an approach for imaging in the presence of orthorhombic anisotropy. In this paper, we first describe our approach, including a newly developed orthorhombic imaging method and a newly developed practical method for orthorhombic anisotropy model building. We then demonstrate with both synthetic and real data from offshore Australia that our approach can successfully take into account the coexisting HTI/VTI effects, reduce the structural discrepancies between seismic images built for different azimuths, thereby producing constructive summation of MAZ datasets and resolving well misties to match with geology. The combined effect of these improvements is a step-change in the final seismic image quality.

EXTRACTING SUB-SURFACE INFORMATION FROM LONG OFFSET P-WAVE DATA—A CHEAPER ALTERNATIVE TO MULTICOMPONENT OBC FOR EXPLORATION?

Seismic energy that has been mode converted from pwave to s-wave in the sub-surface may be recorded by multi-component surveys to obtain information about the elastic properties of the earth. Since the energy converted to s-wave is missing from the p-wave an alternative to recording OBC multi-component data is to examine p-wave data for the missing energy. Since pwave velocities are generally faster than s-wave velocities, then for a given reflection point the converted s-wave signal reaches the surface at a shorter offset than the equivalent p-wave information. Thus, it is necessary to record longer offsets for p-wave data than for multicomponent data in order to measure the same information.A non-linear, wide-angle (including post critical) AVO inversion has been developed that allows relative changes in p-wave velocities, s-wave velocities and density to be extracted from long offset p-wave data. To extract amplitudes at long offsets for this inversion it is necessary to image the data correctly, including correcting for higher order moveout and possibly anisotropy if it is present.The higher order moveout may itself be inverted to yield additional information about the anisotropy of the sub-surface.

Long Offset Towed Streamer Recording - A Cheaper Alternative to Multi-Component OBC for Exploration?

Multi-component OBC data relies on mode conversion from p- wave to s-wave in the subsurface to obtain information about the elastic properties of the earth. Since the energy converted to s- wave is now missing from the p-wave, an alternative to recording OBC multi-component data is to examine p-wave data for the missing energy. The conversion is dependent upon the incident angle and occurs most at wider angles of incidence; thus, estimation of shear wave properties can be achieved by wide angle or long offset AVO analysis. Note that since s-wave velocities are typically less than p-wave velocities, then for a given conversion point, the p-wave energy reaches the surface at a longer offset than the corresponding s-wave. Consequently, if we are to obtain the same information from wide angle AVO as from multi-component recording, we need longer offsets for pure p-wave than for multi- component recording.A non-linear, wide-angle (including post critical) AVO inversion has been developed that allows elastic properties to be extracted from long offset p-wave data. In order to extract amplitudes at long offsets for this inversion it is necessary to image the data correctly, including correcting for higher order moveout.In addition, the higher order moveout may itself be inverted to yield additional information about the anisotropy of the sub-surface.

Analysis of Higher Order Moveout in Terms of Vertical Velocity Variation and VTI Anisotropy

The standard NMO equation for a seismic reflection from a flat interface within a homogeneous, isotropic media is exact. Whenthe subsurface is layered or has TI anisotropy, this is no longer true. The accuracy of the NMO equation decreases with increasingoffset, preventing gathers from being flattened at long offset. Theaddition of a fourth order term to the NMO equation provides abetter physical model for reflection moveout and is a necessaryrequirement to flatten gathers at long offsets. Flat events arerequired for a good stack response, multiple suppression, amplitudeversus offset analysis, etc.Accurate measurement of both the moveout velocity and fourthorder moveout coefficients is time consuming and prone to errors; any measurement of moveout velocity is sensitive to the fourthorder coefficient and vice versa. A semi-automated method is usedto obtain both coefficients simultaneously from an initial estimate. This is more accurate than attempting to measure or refine bothcoefficients separately.Once known, the reflection moveout can be inverted forproperties of the subsurface media. The NMO equation iscommonly used to obtain velocity information. Both the verticalvelocity variation and the TI anisotropy make a contribution to thefourth order moveout coefficient. These two effects can beunwrapped using the moveout velocity. Practical analysis ofsynthetic and real seismic data exhibiting fourth order moveouteffects has been undertaken. This has allowed estimates for the TIanisotropy parameters to be made. This inversion for anisotropy issensitive to uncertainties in both the second and fourth ordermoveout coefficients, but accurate measurement and somesmoothing of velocity information can allow a good solution.