Optimum designs for 2-D and 3-D seismic surveys via modeling and reverse-time migration: Pierce Junction Salt Dome, Texas

Abstract

Establishing effective seismic survey parameters in complex structural areas, such as salt domes, is of vital importance for accurate imaging. Shot and group intervals, maximum offset, recording time and profile length, to image the subsurface structures, are critical 2-D parameters. Geometry, aspect ratio of a recording patch and number of in-line and cross-lines are key additional parameters for 3-D seismic design. This study provides a workflow for determining optimal 2-D and 3-D seismic survey parameters as exemplified by a Texas salt dome case. The Pierce Junction oil field, in proximity to the salt dome, is located in Houston, Texas, and has been one of the most prolific hydrocarbon producers in the region. Engineered caverns in the salt dome itself are now used for fluid storage. Design parameters for the future seismic surveys are partially informed by previous seismic data shot over the structure (where the top of the salt is at about 290 m depth and overlying cap rock, 210 m). Existing 2-D seismic data, crossing the salt dome, are processed to extract the velocities of the salt, cap rock, and near-surface sediments. In the following step, 2-D and 3-D velocity models of the study area are constructed using legacy well data as well as gravity measurements which were acquired as part of this study. Synthetic shot gathers are next modeled with a finite difference method using the acoustic wave equation. To generate images of the constructed model, reverse time migration (RTM) is applied to the synthetic data. By assessing the coverage and continuity of the imaged salt boundaries using a variety of decimated input data sets, the optimal survey parameters are determined. In this case, 20 m group and 40 m shot interval, 3000 m maximum offset, and 8 km profile length with a 4 s recording time are found to be most favorable 2-D acquisition parameters. Using similar coverage and continuity criteria, group and shot intervals of a 3-D seismic survey are determined as 25 m and 50 m, respectively. The receiver and shot line intervals are chosen as 250 m in an orthogonal geometry with 33 in-lines and 33 cross-lines distributed in the 8 × 8 km² survey area. The aspect ratio of any patch with 13 in-lines and 13 cross-lines is accepted as 1:1. This forward modeling and migration procedure, using a range of decimated data sets, can inform decisions on the final field parameters.