STUDY OF OUT-OF-PLANE EFFECTS IN THE INVERSION OF REFRACTION WIDE-ANGLE REFLECTION TRAVEL-TIMES

In the presence of three-dimensional (3D) inhomogeneous structure, the results of 2D traveltime inversion will be in error since the effects of 'out-of-plane' or 3D ray sampling are ignored. We have inverted sy nthetic data using 2D and 3D algorithms to examine the errors caused b y 3D inhomogeneities which produce significant out-of-plane ray bendin g. The results of inverting data from 2D experiments are compared with vertical :slices through 3D models obtained by inverting data using a number of recently employed 3D recording geometries, Our results show that, even for strong 3D inhomogeneities, out-of-plane effects are re latively small, with crustal velocity errors of less than 0.15 km/s, a nd intra-crustal boundary depth errors generally less than 2 km, These errors are approximately equal to the uncertainties commonly assigned to crustal models derived from traveltime inversion, The artifacts ar e also similar in magnitude to the lateral smearing that occurs in 3D models when using relatively coarse 3D geometries, Only for a dense ne twork of profiles will a 3D inversion using off-and in-line data provi de greater lateral resolution than a 2D independent or simultaneous in version of in-line data along each profile. 2D and 2.5D inversion of c rooked-line data in the presence of strong velocity variations produce s erroneous small-scale velocity structure. We conclude that most 3D c rustal experiments cannot be justified on the basis that the results f rom a 2D experiment or a network of 2D profiles will be significantly in error due to out-of-plane effects., 3D experiments can be justified when a dense grid of shots and receivers is used or if a volume image , as opposed to a cross-sectional image, is required, (C) 1998 Elsevie r Science B.V. All rights reserved.