(1104) Introduction Velocity-depth modeling is one of the crucial exercises for successful completion of a PSDM project. To get a focused and positioned image we need to account for heterogeneity and anisotropy of the sub-surface in the velocity-depth model. Determination of these parameters, if only P-wave data is available, is a challenge and requires well data along with good seismic data (S/N ratio and sufficient offset-depth ratio). Apart from this velocity-depth ambiguity [Archer, (1982), Toldi, (1985, 1989), Bickel (1990), Stork (1992) ], is a serious problem and again well data is required to minimize its effects. In our area of study, sub-surface is characterize by alternation of sand and shale sequences Fig.1 (Stratigraphy) deposited in fluvial environment. The clay in shallow section and Girujan clay (Oligocene) are expected to show anisotropy and heterogeneity and may affect the imaging. The S/N ratio of the data is good and offset-depth ratio is more than 2 at these shallower levels, and sufficient no of wells are presen in the area, therefore we can determine the anisotropy due to these formations if it is significant . Sonic logs are not present in the shallow section therefore gradient can not be determined from the well data. Integrating Well Data with Velocity-Depth Modeling: A Case Study from Upper Assam Basin S P Singh*, B K Medhi, Deepak Sareen, Kunal Niyogi Processing Divn., GEOPIC, ONGC, Dehradun. E-mail : singh_sp@ongc.co.in Summary In the nineties, the Pre-stack Depth Migration (PSDM) technology was limited mostly to imaging below salt in the Gulf of Mexico, where time imaging could not produce any meaningful image below salt. With the more and more availability of cheaper computing resources this technology is now progressively being employed in relatively simpler geology with rewarding results in providing focused image with more accurate positioning of image in the sub-surface. To achieve this, we need to build data driven Velocity-Depth model taking into account the heterogeneity and anisotropy of the sub-surface. The other challenge is the velocity-depth ambiguity, which results due to limited band width and aperture during data acquisition. Using well data helps to minimize this ambiguity at seismic scale. In the present paper, we show a case study from Upper Assam basin, India where anisotropic effect is not significant from imaging point of view and isotropic velocity depth modeling using seismic data and proper integration of well data has brought out focused and accurately positioned image in the sub-surface, circumventing partially the problem of velocity- depth ambiguity. We have also shown a method to extract compaction gradient from seism data during velocity depth modeling, in the portion where well data is not available. Fig. 1: Generalized stratigraphy of the area