Hejun Zhu

Elastic imaging and time-lapse migration based on adjoint methods

We have drawn connections between imaging in exploration seismology, adjoint methods, and emerging finite-frequency tomography. All of these techniques rely on spatial and temporal constructive interference between observed and simulated waveforms to map locations of structural anomalies. Modern numerical methods and computers have facilitated the accurate and efficient simulation of 3D acoustic, (an)elastic, and poroelastic wave propagation. Using a 2D cross section of the SEG/EAGE salt model, we have determined how such waveform simulations might be harnessed to improve onshore and offshore seismic imaging strategies and capabilities. We have found that the density sensitivity kernel in adjoint tomography is related closely to the imaging principle in exploration seismology, and that in elastic modeling the impedance kernel actually is a better diagnostic tool for reflector identification. The shear- and compressional-wave speed sensitivity kernels in adjoint tomography are related closely to finite-frequency banana-doughnut kernels, and these kernels are well suited for mapping larger-scale structure, i.e., for transmission tomography. These ideas have been substantiated by addressing problems in subsalt time-lapse migration.

Mapping Tectonic Deformation in the Crust and Upper Mantle Beneath Europe and the North Atlantic Ocean

European History at Depth The preferential alignment of mineral grains as a result of flow direction can polarize seismic waves traveling through Earths interior, based on velocity and direction. Zhu and Tromp (p. 871, published online 8 August) mapped this anisotropy beneath the European continent and the North Atlantic Ocean—which are products of several tectonic events across varying temporal and spatial scales—using seismic tomography. The model generated correlates well with plate motion in the region, and, compared to geodetic measurements, much of the anisotropic features are consistent with modern strain rates. Some features below Eastern Europe, however, appear to be remnants of tectonic events that occurred over 350 million years ago. Anisotropy of the crust and mantle under Europe is a relict of the continent’s formation. We constructed a three-dimensional azimuthally anisotropic model of Europe and the North Atlantic Ocean based on adjoint seismic tomography. Several features are well correlated with historical tectonic events in this region, such as extension along the North Atlantic Ridge, trench retreat in the Mediterranean, and counterclockwise rotation of the Anatolian Plate. Beneath northeastern Europe, the direction of the fast anisotropic axis follows trends of ancient rift systems older than 350 million years, suggesting “frozen-in” anisotropy related to the formation of the craton. Local anisotropic strength profiles identify the brittle-ductile transitions in lithospheric strength. In continental regions, these profiles also identify the lower crust, characterized by ductile flow. The observed anisotropic fabric is generally consistent with the current surface strain rate measured by geodetic surveys.

Seismic modeling and imaging based upon spectral-element and adjoint methods

The overarching goal of exploration geophysics is to map or “image” geological structures. Seismic migration, which attempts to map reflected and/or refracted signals to their actual spatial origin, plays a central role in imaging. Migration techniques are frequently based upon approximations to the seismic wave equation (e.g., ray-theory-based Kirchhoff migration or acoustic/first-arrival wavefield continuation methods).