A scientific paper written 39 years ago by UH research professor Leon Thomsen holds the current record for the most citations in geophysics. The paper, titled “Weak Seismic Anisotropy,” was published in 1986 in the journal Geophysics and has been cited 6,435 times in other scientific publications, according to Google Scholar data compiled in June 2025.
The paper developed a series of equations to describe weak anisotropy in materials, which was a state of bulk elastic media that had been recognized by previous workers but had not been simplified into a series of equations prior to this 1986 publication. In the early 1980s, most geophysicists understood the term “anisotropy” to mean polar anisotropy, because of the layered structure of sedimentary rocks. But the presence of oriented fractures in the subsurface removes the azimuthal symmetry and invalidates the assumption. Such fractured reservoirs may be detected from the surface using various seismic signatures of azimuthal anisotropy.
These observations were made by Thomsen and colleagues at Amoco in the early 1980s but not released to the public until the Thomsen publication was submitted in 1985 and published in 1986. Since that time, these ideas on weak anisotropy have become implemented throughout the oil industry, especially since wide-azimuth marine acquisition has become feasible. This concept of weak anisotropy remains central to research on shale gas prospects since the shales are seismically and hydraulically anisotropic.
“Leon’s 1986 paper is a milestone,” said Dr. Yingcai Zheng, a professor of geophysics in the Department of Earth and Atmospheric Sciences. “He discovered delta, or weak anisotropy, which lights up the darkness. It’s like F=ma—force equals mass times acceleration—for physicists. Many people use this concept without citing it. The real impact of the paper is much greater than the number of citations.”
Dr. Robert Stewart, director of the Allied Geophysical Lab in UH’s Department of Earth and Atmospheric Sciences, also praised Thomsen’s contributions. “Dr. Thomsen not only discovered a compelling approximation to a complex elastic equation—which describes a wide variety of rock types—he has tirelessly and clearly taught its theory and application to students and industry practitioners alike,” Stewart said.
Thomsen completed his BS in geophysics from Caltech in 1964 and his PhD at Columbia University in 1969. Following his PhD, he was a tenured professor of geophysics at State University of New York at Binghamton, followed by a senior research position with BP-Amoco from 1980 to 2008, where he completed the work on weak anisotropy in 1986. Since 2008, he has served as a Research Professor of Exploration Geophysics in the University of Houston’s Department of Earth and Atmospheric Sciences. His recent research focuses on anisotropy and poroelasticity, which refers to how a porous rock’s elastic properties depend on the compressibility of its pore fluid.