Bruce Blake is a geophysical advisor with Repsol, currently working in Madrid, Spain. He began his career in the oil industry in 1977 after graduating from Texas A&M University with a BSc degree in geophysics. Working first for Houston Oil & Minerals and later for Hunt Oil, he joined Maxus in 1988. Through subsequent mergers, he is now working for Repsol. This has allowed an opportunity to work both exploration and development, onshore and offshore, Precambrian to Pleistocene, and extensional, compressional, and transpressional basins. Blake has been fortunate to be a key team member involved in exploration discoveries from southwest Texas to Bolivia to Morocco and Libya. Since 2008, he has been assigned to a major exploration campaign in the Murzuq Basin of Libya, focusing on both the problems of static solutions for time processing and predicting lithology from seismic.
Blake has presented papers at SEG and AAPG conferences as well as local exploration societies on diverse topics: interpretation of 3D seismic over a salt dome (1981), seismic exploration and interpretation techniques in fold-and-thrust belts (1998–1999), and methods to correctly solve statics in desert settings (2010–present). He is an active member of SEG, AAPG, and EAGE and currently serves on the SEG Global Affairs Committee and SEAM II committee.
2014 SEG Honorary Lecturer, Middle East and Africa
Improving seismic structural time-map accuracy with better near-surface measurements
Despite all the advances in both time and depth imaging algorithms, correct time statics remains the single largest factor affecting the interpreter's ability to make correct structural maps in many basins around the world. Static corrections (vertical time shift of the traces from elevation to datum) are simple in concept but remain problematic, especially the long-wavelength component in desert settings. Large (>100m) elevation changes within one cable length, slow velocities (~800 m/s), and mismatch between base of weathering and refractor picked on field records conspire to create large errors in the static estimate. Long-wavelength errors are impossible to detect by qualitatively examining stacks for event continuity. We regularly see differences on the order of tens of milliseconds between stacks processed by different contractors.
The problem of depth-velocity ambiguity in refraction static solutions has been detailed by earlier workers, but the magnitude of the problem is under-appreciated. Upholes, especially those drilled to the seismic reference datum, provide critical velocity and thickness data in modeling the near surface as well as measuring the total static. These uphole data points, perhaps because they are relatively expensive to gather, are widely scattered. Martin et al. (2009) have developed a method of recording near-surface reflections contemporaneous with production seismic data. Weathering reflection seismic (WRS) records provide an economical method of measuring time to base of weathering and average velocity of the weathered layer in a denser coverage than upholes. They provide measurements compatible with the seismic production shots but with finer-offset sampling containing critical information for the characterization of the weathered zone. These WRS data, combined with uphole data, make significant improvements in the time image.
Near-surface velocity and thickness data are critical to building near-surface models and estimating statics in many onshore areas. Also critical is understanding how the various methods work, including their strengths and weaknesses. For example, the "traditional" refraction algorithms require as input either the weathering velocity or the thickness of the weathered layer. They also assume that the refractor that is used to pick first-break times is consistent over the entire survey and is correctly identified as coming from the base of weathering. Obviously, getting any of these parameters wrong results in an error in the static estimation. However, close examination of the equations shows that the static solution is more sensitive to poor thickness estimates than poor velocity estimates. This has been confirmed in extensive testing of three different refraction solution methods.
The only way to measure the accuracy of an applied static solution in absolute terms is to compare the static solution to the statics measured in deep upholes. This can be viewed as a measure of time-map uncertainty or how well the processed time image matches the correct time value. Areas where the difference between the static applied in processing is different from the static measured in deep upholes should be considered as more risky than areas where the two agree.
This presentation will provide examples of the depth-velocity ambiguity in static solutions, will demonstrate the value of WRS in providing critical near-surface measurements, and will present a methodology to measure the accuracy of derived static solutions.