Rapid 3-D Raytracing For Optimal Seismic Survey Design

Zhang, Jie; Lavely, Eugene; Toksoz, M. NafiA useful approach to optimal seismic survey design is to simulate the seismic response for a suite of a priori subsurface models and shot-receiver templates. The response can be used to evaluate many criteria such as subsurface coverage, target resolution, noise sensitivity, aquisition footprint, data redundancy, long-wavelength statics resolution, and others. A key requirement for practical implementation is the use of an accurate and rapid simulation method. For most cases survey optimization for a highly detailed 3D model would not be useful because (1) such information is often not available, (2) some of the conclusions may not be robust to small changes in the model, and (3) simulation of generally varying complex models would be prohibitively expensive. Instead, a more useful model class for survey design would be 3D models with constant velocity layers separated by arbitrary (and possibly complex) interfaces. The models may be from conjecture or previous seismic surveys. We present a rapid 3D raytracing method optimized for the computation of reflection and refraction wavefronts from a point source in this model class. We demonstrate that the method simulates wave phenomena such as diffraction and head wave propagation. The approach is extremely fast since it avoids traveltime expansion in the volume between interfaces, and solves a simple 2D problem on each interface. Other methods require local propagators (even in constant velocity regions), whereas our approach enables large jumps of wavefronts from interface to interface. The calculation of 3D reflection or refraction traveltimes for a model with an arbitrary interface from one source to any number of receivers requires less than 1 sec of CPU time on a DEC 3000/500 workstation. We briefly review how our new method can be used to facilitate survey resolution computations. We also develop a method for estimating an efficient source-receiver distribution for resolving an assumed 3D structure. To design the receiver distribution, we calculate continuous traveltime slices at the surface from a given source template and plot the RMS curvatures of the wavefronts. The spatial density of the receiver coverage should be in proportion to the locally-varying magnitude of the RMS curvature. Similarly, to determine the optimal source distribution, we sum the RMS curvatures of the wavefront traveltimes due to each source in the entire survey area. In the same way, the magnitude of the curvatures suggests the most important areas for source locations.Gas Research Institute (Contract 5096-210-3781)Massachusetts Institute of Technology. Earth Resources Laboratory. Reservoir Delineation Consortiu

Theoretical investigations in helioseismology

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1990.Includes bibliographical references.by Eugene M. Lavely.Ph.D

Study of surface nuclear magnetic resonance inverse problems

Motivated by the recent application of the Earth-field nuclear magnetic resonance (NMR) technique to the detection and mapping of subsurface groundwater (to depths of 100 m or so), and making use of a recently developed theory of the method, we consider in detail the resulting inverse problem, namely the inference of the subsurface water distribution from a given sequence of NMR voltage measurements. We consider the simplest case of horizontally stratified water distributions in a horizontally stratified conducting Earth. Inversion methods based both on the singular value decomposition (SVD) and Monte Carlo are used and compared. The effects of random measurement errors and of systematic errors in the assumed subsurface conductivity structure are studied. It is found that an incorrectly modeled highly conducting layer leads to a ‘‘screening effect’’ in which the water content of layers lying below it is severely underestimated. We investigate also the ability of different sourcereceiver loop geometries to provide complementary information that may improve a combined inversion. Finally, inversion of experimental data from Cherry Creek, CO, USA is performed. Since only the absolute magnitude of the NMR voltage i