Tien-when Lo

Fundamentals of Seismic Tomography

We define tomography as an imaging technique which generates a cross-sectional picture (a tomogram) of an object by utilizing the objects response to the nondestructive, probing energy of an external source. Seismic tomography makes use of sources that generate seismic waves which probe a geological target of interest. Figure 1(a) is an example configuration for crosswell seismic tomography. A seismic source is placed in one well and a seismic receiver system in a nearby well. Seismic waves generated at a source position (solid dot) probe a target containing a heavy oil reservoir situated between the two wells. The reservoirs response to the seismic energy is recorded by detectors (open circles) deployed at different depths in the receiver well. The reservoir is probed in many directions by recording seismic energy with the same receiver configuration for different source locations. Thus, we obtain a network of seismic raypaths which travel through the reservoir. The measured response of the reservoir to the seismic wave is called the projection data. Tomography image reconstruction methods operate on the projection data to create a tomogram such as the one in Figure 1(b). In this case we used projection data consisting of direct-arrival traveltimes and seismic ray tomography to obtain a P-wave velocity tomogram. Generally, different colors or shades of gray in a tomogram represent lithology with different properties. The high P-wave velocities (dark gray/black) in the tomogram in Figure 1(b) are associated with reservoir rock of high oil saturation. Seismic tomography has a solid theoretical foundation. Many seismic tomography techniques have close ties to more familiar seismic imaging methods such as traveltime inversion, Kirchhoff migration, and Born inversion. For example, seismic ray tomography used to determine lithologic velocity is essentially a form of traveltime inversion and seismic diffraction tomography is closely related to Born inversion and seismic migration. Thus, seismic tomography may actually be more familiar to you at this point than you might think since it is just another aspect of the subsurface imaging techniquesg eophysicistsh ave been using for years.

Minimum cross entropy seismic diffraction tomography

One difficulty in applying seismic tomography is that seismic sources and detectors cannot be deployed so that the imaging area is probed from all directions. This paper develops two methods to alleviate this difficulty. These methods combine the minimum cross entropy estimation algorithm with two tomographic image reconstruction algorithms: the direct transform reconstruction algorithm and the backpropagation reconstruction algorithm. These two methods were tested by numerical and scaled model ultrasonic laboratory experiments simulating seismic cross‐hole tomography. It was found that (1) the horizontal resolution, which is inherently poor for the cross‐hole geometry, can be improved, and (2) artifacts caused by the sparse sampling and noise can be reduced. Both methods work better when the targets are isolated impulses surrounded by a uniform background medium.

McKittrick Cross-Well Seismology Project: Part II. Tomographic Processing And Interpretation

A modified SIRT algorithm was used to invert crosswell seismic data acquired in the McKittrick Field, California. The resulting tomogram successfully imaged a complex fault system associated with the McKittrick Thrust. The fault-controlled reservoir was delineat,ed with a resolution of 40 ft [12.2 m] providing the geologists and production engineers with detailed geology and reservoir heterogeneity informa.tion previously unavailable. The McKittrick Thrust system consists of a main thrust and numerous subthrusts. The tomogram suggests that, a fault plane associated with one of the subthrusts is actually flatter than previously believed. We cross-checked the tomogram-derived interpretation for the McKittrick Thrust system with kinematic structural modeling (balanced cross section method) and found that, the McKittrick Thrust and its associated subthrusts can be classified as an imbricated fault-bend fold structure. Oil saturation for the reservoir was also estirnated from the seismic tornogram by exploiting an empirically derived relationship among oil saturation, temperature, and seismic p-wave velocity. The estimated oil saturation agreed with the average oil saturation values measured in the laboratory.

Imaging salt pods with born inversion

Seismic diffraction tomography, which can also be classified as a wavenumber domain Born inversion technique, can be applied to image offshore salt bodies. In this study, we use this technique to image two salt pods near the seafloor in a deep-water area. The larger pod is about 9700 m long and 1600 m thick along the 2-D seismic line. Tomographically derived interval velocities range from 3800 m/set to 4700 m/set within the salt (about a 20% velocity variation) and two distinct high velocity zones exist. The smaller pod is about 2700 m long and 2000 m thick and has similar interval velocity variation. For comparison, we also applied acoustic prestack depth migration to the seismic line, using velocity model derived by prestack focusing analysis. The focusing analysis derived velocity profile likewise suggests an inhomogeneous velocity distribution within the salt pods and the locations of the high velocity zones are consistent with the diffraction tomography results.