Conventional isotropic seismology has been the most spectacularly successful technology in the history of oil exploration. In recent years, its extension to 3D application, recognizing the inherent 3D nature of our world, has significantly enhanced our exploration. In recent years, its extension to 3D application, recognizing the inherent 3D nature of our world, has significantly enhanced our exploration effectiveness. Of equal importance have been the contemporaneous advances computing power and workstation-based data integration and visualization.
As we acquire longer spreads, and 3D bins, the vector nature of seismic wave propagation has become more apparent, for example in AVO analysis. As we attempt to deal with these vectors, we increasingly find it important to recognize the anisotropic nature of our world. By now, we have progressed beyond techniques to suppress and ignore the anisotropic signatures in our data, to realize that they represent valuable clues, positive aids to help us explore and exploit hydrocarbon more effectively.
- horizontal movement of reflected arrival yields a velocity which is inappropriate for converting time-to-depth, which requires a vertical movedown velocity. We used to account for the resulting seismic mis-ties via arbitrary shifting and stretching procedures, regarding the stretch parameters as fudge factors correcting errors, rather than (as we now know) as anisotropic lithology indicators.
- As we acquire longer spreads in order to achieve AVO leverage, we find that nonhyperbolic movement hampers our ability to pick amplitudes, since no velocity function will flatten the gathers. By correcting for this abnormal moveout, we can indeed flatten the gathers, in the process obtaining a measure of anisotropy which helps us to:
- measure the amplitudes, across the flattened gathers, and
- analyze this AVO, in terms of Vp/Vs ratios, fluids, and anisotropy.
- Shear wave anisotropy is usually greater than P-wave anisotropy. Hence consideration of anisotropic effects is greater for shear-wave and converted-wave surveys than for P-wave surveys. In case of waves converting at the reflection point, even the location of that point depends upon Vp/Vs, hence upon anisotropy.
If, as is common, there is an internal fabric (azimuthally aligned) in the rocks, such as sets of aligned fractures or sedimentary structures, then the seismic waves indicate that fabric, even if its elements have sizes far below seismic resolution, this is revealed by:
- Azimuthally variable AVO attributes
- Split shear waves (i.e. two different shear waves, with different polarizations and speeds)
- These effects encourage us to hope that, one day, we will be able to estimate fracture permeability via seismic techniques, despite the well-known difficulty in seismic estimates of matrix permeability. Then, we will be able to explore for fracture permeability, as well as for suitable structures, or hydrocarbon fluids directly. That day is not far off.
About the Author(s)
Dr. Leon Thomsen comes by his interest in geophysics naturally: his father, Erik Thomsen, was an early member of the SEG. During 1938-74, he found oil throughout the American southwest, and the family followed that search through thirty-five moves from Bakersfield to Tulsa to Odessa to Shreveport to Houston. The family was in Tyler when Leon graduated from high school, and (with the help of an SEG scholarship) went west to attend the California Institute of Technology, then and now a center of excellence in geophysics. In those days, the real excitement was in plate tectonics, planetary exploration, and the constitution of the deep interior, not in hydrocarbon exploration.
So, Leon followed those ideas to Columbia University in New York City. There he met and immediately married Purnima "Pat" Gulati. His Ph.D. thesis, in 1969, dealt with seismic rock properties, and represented a new way to physically interpret seismic data for clues to the composition and crystal structure of the deep interior of the earth.
In a post-doc position at the Centre Nationale de la Recherche Scientifique in Paris, another back at CalTech, a consulting position with IBM, a faculty position at the State University of New York at Binghamton, and a sabbatical appointment at the Australian National University in Canberra, he used relativistic quantum mechanics to improve and refine this physical interpretation. These particular tools of mathematics and physics might seem, to some, to be an inappropriate foundation for the task of finding oil and gas.
However, in 1980, during a period of high oil prices and rapid oil industry staff expansion, Leon joined Amoco's Research Center in Tulsa. Within two weeks of his arrival, he discovered that the mathematical tools and physical insight which he had acquired in his previous academic career uniquely equipped him to recognize, in exploration seismic data, the effects of azimuthal anisotropy, to interpret it, and to deal with it. For five years, unusually long in this business, he and others inside Amoco refined their understanding of these effects, unnoticed by the rest of the oil industry, despite the public discussion of some of the critical ideas by academics. In the 1986 SEG convention in Houston, they went public at a now-famous "Anisotrophy" technical session.
By now most oil and service companies are attempting to master these ideas, in order to more effectively explore and exploit hydrocarbon resources. Amoco had an early lead in developing these ideas because of the extraordinary intellectual environment created at Amoco's Research Center by leaders like Mike Waller, Gordon Greve and Sven Treitel (all now retired). Leon's contributions to this development were recognized by the SEG with the Reginald Fessenden Award of 1993.
In 1995, Leon transferred to the new Strategic Technologies Applications Team in Amoco's worldwide exploration division in Houston. This is a team of fifty technical experts, organized in self-directed teams, reporting to a single manager, applying Amoco's best proprietary technology in support of exploration teams worldwide. He had previously provided theoretical foundations for many of these techniques (multicomponent exploration, AVO, pore pressure prediction), so the assignment to aggressively apply these technologies with a natural one.
A group like STAT is uncommon in today's oil industry; and its Multicomponent Seismology sub-team is even less common. But, since seismic waves are vectors, it is inevitable that exploration will be more effective through utilization of all their components. These Distinguished Lectures are direct outgrowths of that insight.