AN EXAMPLE OF LATERAL-INHOMOGENEITY EFFECTS ON SEISMIC TRAVEL-TIMES IN A ZERO-OFFSET VSP EXPERIMENT

2-D random media with ellipsoidal autocorrelation functions are used t o point out the effects of lateral inhomogeneities on zero-offset VSP data. The ellipsoidal autocorrelation functions allow the characteriza tion of lateral and vertical inhomogeneities by two independent scalin g factors a and b in the two Cartesian directions; a for lateral inhom ogeneities and b for vertical inhomogeneities. Thus we can describe 2- D media by taking non-zero finite values of a and b, or 1-D media by e xtending to infinity one of the two scaling factors. Here we consider two models: (i) a 2-D model where a and b are finite and non-zero, and (ii) a 1-D model with the same vertical realization of inhomogeneitie s (i.e. the same value for b) but with a tending to infinity. By compa ring zero-offset VSP synthetic seismograms obtained from these two mod els, we observe that the seismic pulse through the 2-D model arrives e arlier than the pulse through the 1-D model. The time difference betwe en these two arrivals increases linearly with the depth of receivers. We have reported time difference up to 10 ms per 1000 m. This time dif ference is also directly related to the scale of lateral inhomogeneiti es a.