
%0 Journal Article
%A Windhofer, M. 
%A Young, R. 
%T Error in estimation of arrival direction on offset VSP data
%J Exploration Geophysics
%D 1991/06/01
%V 22
%N 2
%P 451-456
%K 
%R https://doi.org/10.1071/EG991451
%U https://www.publish.csiro.au/paper/EG991451 
%X Recent advances in high resolution velocity estimation using first-arrivals on multicomponent offset VSP data are crucially dependent upon accurate determination of arrival direction (eg., Esmersoy, 1988). Such methods require post-acquisition orientation of 3-component data by coordinate rotation. Rotation into the direction of maximum P-wave energy in the horizontal plane at each depth of a VSP survey reorients the coordinate system. A second rotation in the vertical plane towards the source establishes the arrival direction and, in addition, separates the P-, SV-, and SH-transmitted waves. An error in the first rotation influences strongly a separation of the wavemodes. The desired rotation angles can be found from an expression which involves cross-products between measured components (DiSiena et al., 1981). The expression assumes there is no phase difference between these components. This will certainly be the case for an isolated P-wave arrival measured on a recording system with identical impulse responses for all three channels. However, non-identical channel responses or overlapping P-wave arrivals introduce, in turn, erroneous estimates of P-wave arrival direction. The present study quantifies the error in arrival direction as a function of phase difference between components and looks closer into the difference between an instantaneous and an average estimate when calculating the rotation angle. 
