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 obtaining two way travel times for source and receiver pairs includes the steps of determining a set of one way travel times for each source to a plurality of image points and a set of one way travel times for each receiver to the plurality of image points. Ray sets are generated for both the sources and receivers. Travel times from a surface position to image points are computed by two point interpolation using the ray sets. Two way travel time is computed by summing two sets, one set each for the source and receiver positions. A two way travel time set is obtained for a particular source and receiver combination for all imaging points.

Abstract: A method of translating a drawn or imagined model into a numerical format. Working from a paper sketch or interpreted seismic display, the geological horizons and faults are digitized in any order. If the horizon is unbroken, it is digitized continuously as one piece, if it is broken by faulting, it is digitized as a series of elements. Digitizing ceases if the horizon terminates within the model. During preconditioning the boundaries of the model are added as another element. A search is performed and if intersecting elements are found, the shortest limb is deleted. A second search locates elements which do not terminate at an intersection. If the element is from a horizon, it is projected until it intersects another element, if an unconnected fault element, the element is deleted back to its first intersection point. The first phase of cell construction identifies segments which connect only to themselves and produces single segment cells.

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 for improving velocity models so that constant-offset migrations estimate consistent positions for reflectors includes tomographic estimation of seismic transmission velocities from constant-offset depth migrations. A method of converting inconsistencies in reflector positioning from constant-offset migrations into equivalent errors in modeled travel times is introduced, so that conventional methods of traveltime tomography can improve the velocity model. An improved velocity model allows a more accurate migrated image of the subsurface. The estimated velocity model can also detect anomalous regions of geologic significance, such as low velocity gas accumulations and irregular near surface weathering. In an alternate embodiment the procedure can be iterative and allow alternating improvements in the positions of reflectors and in the velocity model.

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: 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 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 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.