Abstract: A hybrid velocity model is used to image the subsalt reflector. A method for imaging subsalt reflectors includes receiving seismic data and depth migrating the raw data. Generated seismic P-waves are assigned a first P-wave velocity for travel outside a salt wedge. Prior to entering the salt wedge, the wave is treated as a compressional wave. Upon entering the salt wedge, the P-wave is treated as a converted S-wave and a corresponding S-wave velocity is assigned to the converted wave. When the converted S-wave exits the salt wedge, it is again converted, this time back to a P-wave. As this wave travels through the subsalt layer to a reflector interface, it is assigned a P-wave velocity. Thus, a velocity model which includes both P-wave and S-wave velocities is used to image reflectors below salt wedges.

Abstract: The present invention provides a method for rapidly and efficiently generating traveltime tables from complex velocity models for application in depth migration. Initially, a velocity model is received which may include a plurality of velocities with various subsurface reflectors or boundaries. The model is plotted with the boundaries defining closed area cells within the model, with the cells having given velocity structures. The outside of each cell, or the cell wall, is made up of a series of segments. In the first basic step, traveltimes are computed from a preselected source location to each sample point on each segment. In this step, the order of processing all the cells is important. Therefore, the present method proceeds from those cells that are closest to the source location first to those cells that are further away in an expanding wave front from the source location.

Abstract: A method for generating a three dimensional velocity model from a grid of two dimensional lines is presented. Each of the two dimensional lines is common offset depth migrated and the migrated common offset sections are sorted into common image point (CIP) gathers. The proposed method examines all the common image point gathers from all the two dimensional lines and extracts out of plane three dimensional information. An initial three dimensional model is constructed from the two dimensional depth sections by mapping the depth images of several reflectors from each two dimensional line. Ray tracing through this initial three dimensional model, the out of plane three dimensional information is used to compute a set of corrections to the boundaries in the three dimensional model. The three dimensional model boundaries are then iterated until all mismatches are minimized.

Abstract: The invention relates to a method for locating correct near vertical boundary or fault positions in a velocity model using common reflection point (CRP) gathers. Conventional migration velocity analysis examines the migrated image for hints to correct the velocity model. Vertical boundaries such as faults or salt flanks must be accurately determined to produce reliable images of deeper reflectors. Previous methods have placed faults by trial and error, since a depth image of a nearly vertical boundary is difficult to produce. The present method examines the sensitivity of CRP gathers to abrupt lateral changes across a nearly vertical fault to determine the location and the dip of a nearly vertical fault from a series of CRP gathers.

Abstract: A method for static correction includes a three step process which eliminates the problems of assuming hyperbolic moveout curves and forming base traces from common midpoint gathers while addressing long spatial wavelength variations. First, after calculating and applying an approximate high-frequency static correction to the individual shot and receivers (using traditional automatic methods), depth migration iterations are employed to coarsely tune the near-surface velocity layer. Second, after applying this correction, the remaining statics are incorporated into the model by tomography, which accounts for the velocity and thickness of the weathering layer and buried anomalies. Third, any residual misalignments are addressed using a multi-windowed statics adjustment within the common image point (CIP) gathers.

Abstract: The invention relates to a method of structural traveltime tomography which uses mismatched image information observed on common image point gathers to determine the precise location of near vertical boundaries or faults. The method can distinguish an error caused by a misplaced fault from velocity caused errors, then it reduces these errors by correcting the location of the fault. The result is a better depth image from better aligned events, and better correction to the velocities from a follow-up tomography using current velocity tomography.

Abstract: A method for improving the efficiency of pre-stack depth migration includes the steps of deleting a portion of the received data to obtain a representative remaining portion. All the shots are separated into several series, such that within each series the receiver locations are repeating. One or more series of shots is selected for migrating. With each series, receivers are selected from each shot such that their locations are repeating. This allows maximum re-use of the travel time tables and resulting in the best efficiency of the depth migration. The same principal can be applied to three dimensional prestack depth migration to improve its efficiency.

Abstract: A method for removing residual moveout includes sorting the results of common offset depth migration into common image point gathers. A subset of image point gathers are selected for analysis. Each common image point gather is separated vertically into windows, each of which centers on a strong event. For each window, all the offset traces are summed to produce a brute stack trace to be used as an anchor. All offsets are cross-correlated to the anchor to determine how much each trace window should be shifted to sum most constructively with the anchor. A set of dynamic shifts is produced, which when applied, will remove the residual moveouts and produce a truly flat image for stacking. These shifts vary with depth and offset and can be interpolated between the selected image point gathers.

Abstract: A method by which portions of a seismic data set are deleted since no meaningful data is contained in that portion contains the step where a line of data is examined, gather by gather, to determine the smallest useful arrival time for every offset. Data having an arrival time smaller than its useful arrival time is eliminated. In addition, the present method provides a manner of storing traces in pairs. In this method, the total number of traces is mathematically folded in half, storing the last trace with the first trace, etc. In an alternate embodiment, disk saving can be achieved by storing all the traces end to end in a single record and keeping track of the starting point and the length of each trace. The proposed method works for either SHOT or CDP gathers. Each line of data is examined, gather by gather, and the smallest useful arrival time is recorded for every offset.

Abstract: A method for rapidly and efficiently generating traveltime tables for application in depth migration initially includes receiving a velocity model which may include a plurality of velocities in multiple layers between various subsurface reflectors. The model is plotted on a two dimensional grid with the subsurface reflectors identified. A traveltime to the first reflector is determined. Traveltimes from the sources on each layer boundary to all grid points above the next reflector are determined. For the initial iteration, the layer boundary is the surface and the source is the actual source used in shooting the line. For layers below the surface there will be more than one source or secondary source. The actual determination of traveltimes for these lower layers may be done by comparing the traveltimes to all points on the first reflector. The minimum traveltime is selected as the true traveltime to the first reflector. Next, all points where the reflector intersects the grid are found.

Abstract: A method for calculation of raypaths and wavepaths from traveltime tables for the tomographic estimation of transmission velocities includes picking the earliest traveltime of waves transmitted between different source and receivers locations. A velocity model that describes transmission velocities in a region of interest is obtained. Traveltime from each source position to each point in the region of interest and traveltimes from each receiver loaction to each point in the region of interest are extrapolated. The extrapolated traveltimes from the source and receiver to each point in the region of interest are added together to quantify the path of a wave between one source and one receiver. All points in the region of interest whose total traveltimes are less than half a wavelength greater than the minimum traveltime are found. A representative raypath that passes through the center of the estimated wavepath region for each source and receiver location are saved.

Abstract: A method for quantitatively determining an accurate subsurface velocity prior to data migration includes steps whereby the accuracy of the velocity can be defined by measuring the deviation in depth as a function of offset in the common reflection point (CRP) gather. A point on reflector is selected and the CRP gather is formed. If the image is not flat, the velocity is adjusted until it is flat. The velocity is decreased and the far offset end of the image will be imaged to shallower depth than the near offset end. The velocity is increased and the image will tilt down at the far offset end. An error is defined which is the theoretical accuracy limit for the determination of velocity using the CRP method. A factor is defined that indicates the reliability of the image for a reflector.

Abstract: A method for performing velocity analysis while eliminating the effects on weak signals caused by strong signals includes migrating each event of the pre-stack trace to a single location instead of all possible locations. This correct location is determined by ray-tracing through a velocity model. The input trace is divided into many windows, and each window is migrated to a place determined by ray-tracing the center of the window through the model. If the velocity model is accurate, each event will be migrated to the proper location yielding an accurate depth section with no migration artifacts. As a by-product, if the model is not accurate, the post-migrated parts (PMP's) provide a clean velocity analysis.

Abstract: In applying residual movement correction to common offset depth migrated data, common offset depth migration is applied using the best available velocity/depth model. Post migrated parts, which are depth migrated common midpoint gathers, are saved. The post migrated parts are treated as if they were in time not depth. Normal movement based on a constant velocity is removed. Velocity functions time-velocity pairs, are derived for the post migrated parts with normal movement removed using a standard velocity analysis program. Normal moveout based on these velocity functions is applied. The events on the post migrated parts ae not imaged to the same depth. The corrected post migrated parts are then stacked and displayed.

Abstract: A method for derivation of interval velocities from post-migration parts first includes the step of determining the apparent depth and slope of an event. The apparatus depth of an event is measured. The travel time of the recorded reflection for a particular offset is determined by ray-tracing through the old model and recorded. A trial velocity is assigned to the layer between events in the new model. The depth of the reflector is varied up or down until the computed travel time agrees with the measured travel time, keeping the source/receiver separation constant. A new velocity for the layer between reflectors is selected for which the depths at each offset are the same.

Abstract: A method for generating a three dimensional velocity model makes use of two dimensional depth images, which are the result of two dimensional pre-stack depth migration, and corrects the out of plane distortion by ray tracing through a three dimensional model. The three dimensional model boundaries are iterated until the three dimensional effects are minimized. The final model can be used for a final three dimensional pre-stack depth migration, or as a three dimensional interpretation of all the two dimensional depth migration results.

Abstract: A method for interval velocity analysis and reflector position determination from before stack seismic data includes developing a constant travel-time curve for a source and receiver pair. The process is repeated with other source and receiver pairs in order to generate a set of constant travel-time curves, and a series of the travel-time curves define an envelope curve for the reflector which is sharply focused at the true interval velocity. An additional embodiment provides a method for quantifying the velocities and permitting selection of the best velocity. Use of this selection method greatly enhances the speed and the overall operation accuracy.