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

Abstract

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.