A.H. Balch

A dynamic programming approach to first arrival traveltime computation in media with arbitrarily distributed velocities

Curved‐ray tomographic traveltime inversion, reverse‐time migration and various other seismic modeling applications require the calculation of traveltime and raypath information throughout a two‐ or three‐dimensional medium. When arbitrary velocity distributions and curved rays are involved, traditional ray shooting or bending procedures can be time consuming and error prone. A two‐dimensional dynamic programming traveltime computation technique, based upon Fermat’s principle, uses simple calculus techniques and a systematic mapping scheme to determine first‐arrival times on a uniform grid, given an arbitrary, discrete velocity distribution. It accurately handles large contrast, discontinuous velocity distributions, including those that generate caustics. First arrival seismic energy can travel either as transmitted waves, diffracted waves, or headwaves, and this technique models all types. The traveltime computations begin with starting values computed near the source location. Then, the mapping systemat...

Multimode reverse time VSP imaging over complex structures at Yucca Mountain, Nye County, Nevada

Structural interpretation of vertical seismic profiles (VSP’s) is always a challenge because conventional plots of the reflected wavefield itself, observed along a borehole, do not bear much resemblance to the reflectors responsible for the reflected events. Wyatt and Wyatt (1981) and Dillon and Thomson (1983) pioneered this problem by mapping or migrating the reflected events from space-time into spacespace domains. A more elaborate migration of an extensive multiple offset VSP was demostrated by Mons, et al (1985).

Separation of P And S-Waves On Multicomponent Cross-Borehole And VSP Data

In order to process multi-component seismic data recorded on crossborehole and VSP configurations, it is often convenient to separate events into P and S modes. In this paper, a new wave-mode separation technique is described. Our technique performs decomposition of Pand S-waves on multi-component data in time-space domain, Ranzinger (1990). The technique was tested on both cross-borehole and VSP elastic physical model data sets. The results are compared with those obtained using Dankbaars (1985,1987) mode separation technique which decomposes tne multicomponent data in frequency-wavenumber (f-k) domain. Both methods give comparable results. However, our method does better when variable velocity media are involved.

Seismic Crosshole Imaging of 2-D And 3-D Physical Elastic Models Using Prestack Reverse Time Migration

Crosshole reflection imaging using reverse time migration has been attempted on data sets acquired from twoand three-dimensional physical elastic models. Many crosshole common shot gathers were collected for the models using ultrasonic transducers and a digitizing oscilloscope in the laboratory. Horizontal and vertical crosshole gathers were collected for P-wave source for a 2-D Plexiglas model, and out-of-line horizontal (SH) component common shot gathers ware collected for a SH-wave source for a 3-D concrete model. The SH-SH combination showed the best reflections for the 3-D model because the polarization of the SH-waves was parallel to the reflecting surfaces. Prestack images have been created by finite difference reverse time migration. Final images have been created by stacking many prestack images using different source locations. Post migration stacking of prestack images was helpful in enhancing image quality. The final images of both models clearly showed all reflecting edges and diff ractors.

Diffraction Imaging of a Producing Horizon Using Cross-Borehole Seismics A: Physical Elastic Model Study

A physical elastic seismic model, based on pinch outs in the reservoir in the Peoria oil field was constructed and used to produce multi-mode cross-borehole model seismic data. Data processing included mode separation, signature deconvolution, reverse time migration of common source gathers, and stacking. The resultant multi-mode image showed the pinch outs with remarkable clarity and detail. It is felt that the technique will yield images of sufficient quality to influence production decisions, especially well location, well spacing, and production rates.

Seismic Cross-borehole Imaging of the Near Surface Using Tomography And Prestack Migration In Elastic Physical Models

The problem of using seismic cross-borehole measurements to image the subsurface is an important one. There are applications to the analysis of abandoned mine workings, mapping of petroleum reservoirs and other valuable mineral deposits, delineation of fracture zones relevant to hazardous waste disposal, and military applications related to the detection of hostile tunnels. Raw cross-borehole data are complex. Energy is reflected and scattered in all directions. All possible reflected modes are usually present. Scattered events reach the seismic detectors from all directions. For these and other reasons, original, or “raw”, cross-borehole data are difficult to interpret. By using prestack migration, and/or cross-medium tomography, these raw data can be converted into images of the subsurface that indicate the location, size, and approximate shape of seismic reflectors. These images can often be interpreted geologically, and these can lead to an accurate geologic cross-section of the earth near the boreholes. Physical elastic models are useful to study crossborehole imaging. They yield very realistic 2and 3component data containing all reflection modes, dispersion, amplitude loss due to spreading and attenuation, and both random and coherent noise. Arbitrary reflecting boundary shapes are easily accommodated. Our results suggest that good quality subsurface images can be obtained with cross-borehole data using both migration and/or cross-medium tomography.