3-D Seismic Gridding

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ABSTRACT 3-D Seismic Gridding is a geophysical technique that converts arbitrarily oriented 2-D seismic data in an area into a dense, evenly spaced 3-D data volume. This technique makes use of apparent dips measured on the 2-D 1ines and reconstructs the three-dimensional structure so that trace interpolation performed along the structure thereby avoids aliasing. Amplitude of the interpolated trace is obtained from the input data but is weighted by the distance from the input data as well as the semblance of dip measurements. Unmigrated 2-D data sets are the input, so the regridded data can be 3-D migrated. Several examples of 3-D regridded data in the offshore Gulf of Mexico will be shown and their merits and limitations will be discussed. INTRODUCTION The benefits of 3-D seismic data are well known in exploration seismology. It is generally recognized that 2-D seismic processing and interpretation cannot be valid in an area where complex geology is involved; out of plane reflections, misties between inlines and crosslines as well as incorrect amplitude are problems often encountered in 2-D data. These problems arise because seismic events are reflections from 3-D geologic structure and can only be unraveled by 3-D data. From an imaging processing point of view. 3-D data allow 3-D migration in which the recorded wavefield is processed according to correct physical principles without making any spurious assumptions as in 2-D processing. Events after 3-D migration will move to their correct spatial positions and seismic amplitudes are properly imaged. The flexibility of 3-D data display and the use of a workstation for interpretation also allow fast and accurate evaluation of a large amount of data. Although 3-D seismic is an accurate exploration and exploitation tool, only a small number of 3-D data sets are collected each year by industry due to the high cost and time consuming procedures in acquiring and processing 3-D data. Futhermore in many areas where much 2-D coverage exists, an additional 3-0 program over the same areas may seem redundant or uneconomical. In this paper present a cost-effective way to utilize the existing network of 2-D lines and synthesize them into a 3-D data volume so that the benefits of 3-0 processing and interpretation can be realized. The technique involves regridding or trace interpolation from the existing 2-D data. 3-D seismic interpolation has been attempted in the past. Chilcoat et al (1983) discussed the approach of space-time filtering, techniques for regridding and interpolation. Recently, French et al (1987) proposed using a Common Reflection Point (CRP) stacking method for the 3-D interpolation process. The filtering technique has severe dip limitation due to spectral aliasing. On the other hand, the CRP stacking method makes use of structural dip information to compute the subsurface reflection point as the stacking bin. Our technique differs from the CRP method in how the structural dip information is utilized to reconstruct the three-dimensional structure. Dip as it appears on 2-D lines is the apparent dip and it varies as a function off azimuthal angle of seismic profile. However, the apparent dips are related to the true structure in a definite way.