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A Comparison of Depth-Conversion Methods in the Gulf of Mexico
Melvan D. Carter, Gary R. Hyatt
The conversion of two-way seismic times into a depth picture has been done using two versions of three different methods for a proprietary study area in the Gulf of Mexico. From the simplest constant-function approach to the "layer-cake" method to an average-velocity technique based upon seismic velocities, each method has advantages and disadvantages. Four time horizons were converted using each method and calibrated at nine well locations. The shallowest and flattest marker has a variable mean error (from 0 to 93 feet) but a relatively stable standard deviation (from 22 to 43 feet). The deepest reflection has a highly variable mean error (from 0 to 474 feet) and an almost equally variable standard deviation (from 75 to 280 feet). This mistie study examines the depth-conversion error as a function of layer, depth and space. While the results cannot be generalized blindly, the seismic average-velocity technique calibrated to the wells was the best method for the four horizons. It is noteworthy that the "layer-cake" result using a constant depth normalized interval velocity was slightly better than a full-blown seismic-velocity technique for the deepest horizon. Because the conversion of time into depth is done with some measure of uncertainty, it is vital that the final depth map be quantified as to accuracy. For the explorationist, it is imperative that the best technique for depth conversion be used as dictated by the exploration objectives and the available time and resources.
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