Crossline Direction Research Articles

MAKE FLAT SHEET USING CONTRABEND ROLLS

Most plastic sheet produced today is made using a typical three-roll sheet stack. Material is passed through a preset gap between the rolls to form the final thickness of the sheet. The variation in this gap is related to the bending of the chill roll, which is a major factor in the gage variation of the sheet. Producing “flat sheet” has always been the goal of the sheet producer. Until recently there has been little advancement in cooling roll construction as it relates to bending in the cross line direction. The ContrabendTM Reverse Bending Roll has been developed to greatly reduce the effect of roll deflection on sheet thickness variation. This work will explore the Contrabend concept and compare the bending to standard chill rolls.

A two‐pass approximation to 3-D prestack migration

A two‐pass approximation to 3-D Kirchhoff migration simplifies the migration procedure by reducing it to a succession of 2-D operations. This approach has proven very successful in the zero‐offset case. A two‐pass approximation to 3-D migration is described here for the prestack case. Compared to the one‐pass approach, the scheme presented here provides significant reduction in computation time and a relatively simple data manipulation scheme. The two‐pass method was designed using velocity independent prestack time migration (DMO‐PSI) applied in the crossline direction, followed by conventional prestack depth migration in the inline direction. Velocity analysis, an important part of prestack migration, is also included in the two‐pass scheme. It is carried out as a 2-D procedure after 3-D effects are removed from the data volume. The procedure presented here is a practical full volume 3-D prestack migration. One of its main benefits is a realistic and efficient iterative velocity analysis procedure in three dimensions. The algorithm was designed in the frequency domain and the computational scheme was optimized by processing individual frequency slices independently. Irregular trace distribution, a feature that characterizes most 3-D seismic surveys, is implicitly accounted for within the two‐pass algorithm. A numerical example tests the performance of the two‐pass 3-D prestack migration program in the presence of a vertical velocity gradient. A 3-D land survey from a fold and thrust belt region was used to demonstrate the algorithm in a complex geological setting. The results were compared with images from other 2-D and 3-D migration schemes and show improved resolution and higher signal content.

Hammer-induced Seismic Investigations in an Area of Observed Anisotropy

A seismic experiment was undertaken in order to investigate the effect of an observed (outcropping) fracture set on shear-wave propagation, and more specifically whether the observed fracture set induced shear-wave birefringence. The source for the experiment was a hammer swung to impact horizontally on a source block coupled to the ground by two large metal pins. Eight three-component geophones recorded arrivals up to 40m offset from the hammer source. Impacts of opposite polarity in the in-line and cross-line direction were differenced in order to maximise S-wave, and minimise P-wave energy. After vector gain, filtering and rotation of the recorded data into in-line and cross-line directions, Alford rotation was undertaken on selected windowed events in order to detect shear-wave birefringence. In-line and cross-line energy is maximised by a co-ordinate rotation to the north?south direction, parallel to the observed fracture direction, suggesting that the observed fracture direction results in an anisotropy that induces shear-wave birefringence.

3-D transmission tomography using wide aperture data for velocity estimation for irregular salt bodies

Reliable 3-D velocity estimation is a prerequisite to 3-D migration in laterally varying environments, but is difficult with data from conventional seismic surveys because of limited aperture, especially in the crossline direction. A possible solution to this problem is tomographic estimation of the 3-D velocity distribution using transmitted (turned) rays recorded in wide areal apertures. Using this approach, the shape and position of salt structures, including overhangs, subsalt features, and deformation of surrounding sediments, are accurately estimated from synthetic test data. Model enhancement based on a priori geologic constraints gives 3-D macro velocity distributions for models of three actual salt structures: a salt tongue in the Gulf Coast, and the Hainesville and Mount Sylvan domes in the East Texas Basin. The geometries required are feasible with current land acquisition systems. The main practical limitations are the requirements for nonstandard (wide aperture) recording arrays and for accurate time picking of the transmitted arrivals.

Depth‐imaging resolution of 3-D seismic recording patterns

The prestack imaging resolution capabilities of various 3-D seismic recording patterns are revealed when a Born‐approximate theory is applied to backscattered waves. The 3-D imaging resolution of such patterns is compactly studied using a constant‐velocity model with a point scatterer or a line scatterer at depth. By use of a well‐known acoustic imaging formula, it is shown that the resolution of a point scatterer in the X and Y directions is not significantly different for various realistic layouts of sources and receivers in land‐type recording, even though the recording patterns appear to be greatly dissimilar, provided the aperture remains constant. For marine‐type survey patterns, the image is almost as well‐resolved in the crossline direction as the inline direction. This is true whether the scattering object is a point or a line. In marine surveying, symmetric resolution of subsurface features can be achieved by conducting two orthogonal surveys at half the density of one unidirectional survey.

Seismic Solutions in the Val Verde Basin

The Val Verde basin has been an underexplored basin, despite the discovery of world-class gas fields on the northern edge of the basin in the 1950s and the 1960s, because the basin was opaque to conventional seismic reflection methods. Permian Exploration was approached in 1988 to design an acquisition geometry and processing sequence that would produce data that could be used with confidence. A field test was designed to test the hypothesis that the high Q limestones at the surface of the mesas were reflecting source generated noise into receiver lines from cross-line directions. That test crossed both mesa and valley terrains and looked for noise arriving from all directions. Analysis of the test data showed that cross-line coherent noise was a significant problem and that unconventional techniques were necessary to record the reflected signal. Acquisiton of data using multiline techniques, coupled with new high-channel recording systems, exhaustive statics analysis, and powerful noise cancellation algorithms has given data the interpreter can use with confidence.

Three‐dimensional migration of swath surveys

A three‐dimensional (3-D) wave‐equation migration program is used to migrate swath data (swath data are here defined as a very narrow 3-D survey consisting of approximately ten seismic lines and having a width of about 500 m). Three‐dimensionally migrated swath data give an accurate 3-D image of the subsurface and have a higher signal‐to‐noise ratio than 2-D data. These advantages are gained at the expense of lateral resolution in the crossline direction and less extensive 3-D subsurface coverage. Since swaths are on the order of one‐tenth the size of a normal 3-D survey, the costs of gathering and processing swath data are about 10 percent those of a conventional 3-D survey. Thus 3-D migrated swath surveys are a practical means of producing 3-D images in areas where the expense of conventional 3-D surveys is not justified. Following 3-D wave‐equation migration, inlines from the swath data are geologically interpretable. However, crosslines from the swath data are dominated by migration artifacts (often called migration smiles), making them uninterpretable. Relatively high‐amplitude portions of these migration smiles indicate where the data were accurately migrated. These facts lead to a two‐step method of interpreting 3-D migrated swath data. First the inlines are interpreted geologically, then the accuracy of that interpretation is verified by tying it to the crossline data.

Synthetic Shear VSP’s in Azimuthally Anisotropic Media

The azimuthal anisotropy due to a system of aligned vertical fractures, even when very weak, is sufficient to enable the strike of the fractures to be determined from shear VSP data. Synthetic VSP data were computed for several offsets using two perpendicular horizontal point forces into pairs of perpendicular horizontal receivers. The medium consisted of an anisotropic layer of orthorhombic symmetry (with the degree of azimuthal anisotropy due to the fractures an order of magnitude less than the degree of non-azimuthal transverse isotropy due to horizontal layering) over an isotropic halfspace. The line of sources from the well was at a fixed angle to the strike of the fractures. Source and receiver pairs were rotated numerically in small increments from the in-line to cross-line direction. When sources and receivers were aligned with the directions normal to, and parallel to, the fractures, the downgoing signal from a source to the receivers orthogonal to it decayed rapidly with depth. At zero offset this signal vanished identically at all depths.

Evaluation of two‐pass three‐dimensional migration

The theoretically correct way to perform a three‐dimensional (3-D) migration of seismic data requires large amounts of data manipulation on the computer. In order to alleviate this problem, a true, one‐pass 3-D migration is commonly replaced with an approximate technique in which a series of two‐dimensional (2-D) migrations is performed in orthogonal directions. This two‐pass algorithm produces the correct answer when the velocity is constant, both horizontally and vertically. Here I analyze the error due to this algorithm when the velocities vary vertically. The analysis has two parts: first, a theoretical analysis is performed in which a formula for the error is derived; and second, a field data comparison between one‐pass and two‐pass migrations is shown. My conclusion is that two‐pass 3-D migration is, in general, a very good approximation. Its errors are usually small, the exceptions being when both the reflector dip is large (in practice this typically means greater than about 25 to 40 degrees) and the orientation of the reflector is in neither the inline nor the crossline direction. Even then the error is the same order of magnitude as that due to the uncertainty in the migration velocities. These conclusions are still valid when there is lateral velocity variation, as long as this variation is accounted for by trace stretching. The analysis presented here deals with time migration; no claims are made regarding depth migration.

Poststack estimation of three‐dimensional crossline statics

In three‐dimensional land data acquisition, the crossline dimension of the receiver spread is often much smaller than the inline dimension, typically for reasons of economy. Because of fundamental wavenumber limitations in the estimation of residual statics by prestack surface consistent methods, unresolved static errors will persist through processing to the stack data, particularly in the crossline direction. The present method involves an analysis of poststack 3-D data through the creation of a correlation time surface from crosscorrelations of adjacent stack data traces. This time surface is decomposed in the wavenumber domain to isolate and correct some of the spectral components of residual static errors which are beyond the resolution of prestack approaches. Assumptions are implicit within the method regarding the expected 2-D wavenumber nature of the wave field response of true geologic structure. The method is proposed as an additional procedure to the standard 3-D processing sequence for land data.

A post-stack method for 3-D cross-line statics estimation

"A post-stack method for 3-D cross-line statics estimation." Exploration Geophysics, 14(3-4), p. 193

Torpedoes

The general crustal structure of the Youanmi Terrane – a tectonic unit within the Neoarchean Yilgarn Craton – and its relationship to ore deposits are of major interest in Western Australia. To image geological structures, the Geological Survey of Western Australia in collaboration with Geoscience Australia acquired three deep crustal 2D seismic profiles over the region in 2010. In this study, we have reprocessed a 120 km long section of one of the seismic profiles, line 10GA-YU1. We reprocessed the data with focus on shallow reflections down to 2 s (about 6 km depth). A cross-dip analysis was performed to investigate the dip of the reflections in cross-line direction. The cross-dip analysis showed that there is no significant cross-dip influence, confirming that a 2D interpretation is a valid assumption for this dataset. The seismic image of the reflections was improved after applying dip-moveout correction. Finally, the reflections were migrated to their “true” locations in the subsurface. To better understand the physical properties of the subsurface, we also tried First Arrival Travel-Time Tomography. The tomography results show that low velocities correlate with shear zones and that high velocity zones are common where mineral deposits are located close to the surface.