This paper introduces a technique for computing travel times in a blocky model based on optical principles. This technique is extremely fast compared to techniques working in gridded or smoothed models. It can handle almost any complexity in terms of structures and velocities.
Because of speed and easy availability of common image gathers common-azimuth migration is an attractive alternative to shot profile migration. Its limitations in the presence of strong reflection azimuth migration can be removed by using a narrow-azimuth approximation without sacrificing the speed advantage. Narrow-azimuth migration achieves the same image quality as full DSR or shot-profile migration. We demonstrate base salt and sub salt imaging improvements with narrow azimuth migration on the SEGEAGE C3 salt model data set.
We investigate the enhanced deconvolution of transmitted seismic waves from distant natural sources using autoregressive extrapolation. The amplitude spectrum of deconvolved seismograms is often restricted to a reduced frequency range from the use of a water-table for deconvolution using damped least squares (DLS). The propagation effects on the transmitted seismic waves from distant sources also reduce the frequency content of the data. After performing a standard DLS deconvolution, we first analyze the trend of the frequencies for the spectrum to differentiate between the domains of known and unknown spectral values. We then set up a prediction error filter to perform the autoregressive extrapolation for the unknown spectral values. This procedure is first tested on 1-D synthetics and for simplicity we just work with the power spectral values and the autocorrelation. Future tests will apply this approach to observed receiver function data from distant earthquake data.
An accurate estimation of the VSP first breaks is important for VSP processing and velocity calculation. The traditional approach for first breaks picking assumes manual picking first and then one-component automatic improvement using trace amplitudes. For large 3D VSP volumes, manual picking takes a lot of time and effort and achieves accuracy at about 1.0 ms. A new technique is described for first break automatic and accurate picking of large volume of three-component 3D VSP data. This approach includes four main steps. First we predict first break for the next source point (for each receiver separately ) using the previous one. After that we calculate the polarization vector and extract downgoing P wavelet from three components traces. On the third step, we use time shift calculation in the frequency domain to improve the predicted first break. Finally, we suppress upgoing wave to eliminate its influence on the downwave, which allows us to obtain more accurate first arrivals.
Miller, Richard D. (Kansas Geological Survey) | Steeples, Don W. (Department of Geology, University of Kansas) | Lambrecht, Jamie L. (Kansas Geological Survey) | Croxton, Neil (Kansas Department of Transportation)
Time-lapse seismic reflection imaging improved our understanding of the consistent, gradual surface subsidence ongoing at two sinkholes in the Gorham Oilfield discovered beneath a stretch of Interstate Highway 70 through Russell and Ellis Counties in Kansas in 1966. With subsidence occurring at a rate of around 10 cm per year since discovery, monitoring has been beneficial to ensure public safety and optimize maintenance. A miniSOSIE reflection survey conducted in 1980 delineated the affected subsurface and successfully predicted development of a third sinkhole at this site. In 2004 and 2005 a high-resolution vibroseis survey was completed to ascertain current conditions of the subsurface, rate and pattern of growth since 1980, and potential for continued growth. With time and improved understanding of the salt dissolution affected subsurface in this area it appears that these features represent little risk to the public from catastrophic failure. However, from an operational perspective the Kansas Department of Transportation should expect continued subsidence, with future increases in surface area likely at a slightly reduced vertical rate. Seismic characteristics appear empirically consistent with gradual earth material compaction/settling.
This case history demonstrates the impact of anisotropy on seismic imaging in the Canadian foothills. Results of recent drilling illustrate the large degree of mispositioning of subsurface reflectors as a result of ignoring anisotropy. This study showcases how pre-stack anisotropic depth migration (ADM) improved the accuracy of the structural image in the presence of dipping anisotropic overburden. A benefit of optimizing the location of the borehole on the structure is to encounter areas of increased fracturing at the leading edge, resulting in higher production rates. This analysis is based on drilling information gained from several wells in an area covered by a large 3-D seismic survey in the Alberta foothills.
For shot-profile migration, illumination compensation can be achieved if the imaging condition incorporates a deconvolution (division in the frequency domain) of the up-upgoing going wavefield by the down-going wavefield. To avoid division by zero, the deconvolution requires the selection of a damping parameter that turns out to be quite difficult to select. Consequently, a cross-correlation of the two wavefields is often selected. Alternatively, the zeros in the spectrum of the down-going wavefield can be filled with an average of the neighboring points. Therefore, instead of dividing by the wavefield, we can divide by a smoothed version of it. Smoothing is robust and easy to parameterize. It also corrects illumination problems in the migrated images.
2 D industry seismic data acquired in the Focsani Basin of Romania prove ideal for imaging deep crustal structures related to the tectonic evolution of one of the youngest yet deepest basins in the world. Fundamental plate tectonics research blends with oil exploration techniques to provide an image of the upper 60 km of the basin. The purpose of this study was to identify the best processing techniques that would provide deep information while preserving the shallow data.
Processing of 2D shallow (5s) seismic data is presently done routinely with software that was developed for this purpose. However no software is designed for the sole purpose of processing deep seismic data. Here we present processing steps undertaken for processing 2 seismic lines with recording time of 20s TWTT using Landmark ProMAX seismic processing package. The deep seismic data presented here were collected in the Focsani Basin of the SE Carpathian foreland as part of oil exploration activities.
The acquisition parameters are exploration-type, only the record length being increased to 20s. Some of the main problems of processing the data were related to signal depth penetration and its recovery, attenuation of multiples and velocity analysis. Standard industry offsets did not provide sufficient NMO and the thickness of the basin further complicated removal of multiples. The evolution of the basin is at odds, spatially and temporally with the formation of the Eastern Carpathians and to this effect the study of the deeper crustal structure was envisioned as a way of providing constraints on the main tectonic processes involved in the development of the SE Carpathian region.
The foreland basin in front of the SE Carpathians, Romania, formed during and after the Alpine continental collision bears significance due to its
(1) rich and extensively exploited oil fields,
(2) vicinity to the intermediate depth Vrancea Seismogenic Zone,
(3) thick sedimentary cover (~ 18 km Miocene-Quaternary),
(4) ongoing subsidence (~ 2mm/year),
(5) localized and unusually low topography,
(6) crustal scale faults oriented NNW-SSE,
(7) documented normal faults concentric to the Vrancea area and
(8) wide spread, low magnitude shallow seismicity.
One of the most debatable features associated with the SE Carpathians fold and thrust belt is the significant concentration of intermediate depth earthquakes (70-210 km) in an extremely confined and vertical volume, named Vrancea Seismogenic Zone. In the context of plate tectonics this seismicity was interpreted as being produced by the sinking of an oceanic slab. However, new interpretations envision an entirely different process, namely continental delamination of over thickened continental lithosphere (Knapp et al., 2005), also responsible for the active subsidence of the Focsani Basin. In a broader sense, this study examines the relationships between crustal foreland basin deformation and VSZ, foreland deformation at the crustal scale suggesting a geometric association with the Vrancea mantle source region and implying a mechanical coupling of the seismogenic body with the overlying crust. Part of the SE Carpathian foredeep, the Focsani Basin is made up of sands, shale and evaporites that thicken to the west.