Abstract: A method of determination of fluid pressures in a subsurface region of the earth uses seismic velocities and calibrations relating the seismic velocities to the effective stress on the subsurface sediments. The seismic velocities may be keyed to defined seismic horizons and may be obtained from many methods, including velocity spectra, post-stack inversion, pre-stack inversion, VSP or tomography. Overburden stresses may be obtained from density logs, relations between density and velocity, or from inversion of potential fields data. The seismic data may be P-P, P-S, or S-S data. The calibrations may be predetermined or may be derived from well information including well logs and well pressure measurements. The calibrations may also include the effect of unloading. The determined pressures may be used in the analysis of fluid flow in reservoirs, basin and prospect modeling and in fault integrity analysis.

Abstract: The invention comprises a system for processing seismic data to estimate time shift resulting from velocity anisotropy in the earth's subsurface. A gather of seismic data traces is formed and selected seismic data traces included in said gather within selected time windows are cross-correlated to estimate the time shift in the seismic data traces included in said gather resulting from velocity anisotropy in the earth's subsurface.

Abstract: A method of migrating seismic event data in the presence of a vertically time-varying velocity field defined by a migration velocity function is provided. The method comprises approximating a migration velocity function by constant stepwise stage velocities. The seismic event data is time stretched and is then successively migrated using residual velocities. After the seismic event data is fully migrated, the effect of time stretching is removed by time unstretching the fully migrated seismic event data by a reverse operation to obtain a finally migrated seismic event data.

Abstract: A method and system for correcting seismic survey signals for separating and correcting for the effects of azimuthal anisotropy caused by the overburden, rather than the subsurface reflectivity, is disclosed. Following an initial estimate and normal moveout correction of the traces in a common midpoint gather, analytical traces are generated for coefficient series corresponding to zero-offset travel time response and to the azimuthal anisotropy, over the corrected gather. The quadrature-phase component of the azimuthal anisotropy coefficient series relative to the zero-offset travel time coefficient series indicates the effects of overburden azimuthal anisotropy, while the in-phase component indicates the effects of azimuthal anisotropy from subsurface reflective interfaces. Additional normal moveout correction to correct for the overburden azimuthal anisotropy may then be performed.

Abstract: An improved method for eliminating reflections from artificial model boundaries encountered in finite-difference acoustic wave propagation computations is based on applying two different boundary conditions at the artificial boundary. In this method one boundary condition generates reflections having the same polarity as the incident acoustic wave, and the other boundary condition generates reflections with opposite polarity. To apply the improved method simply compute both boundary conditions on a small narrow region adjacent the artificial boundary on a separate set of grids and average the two solutions.

Abstract: A method for determining a substantially optimal NMO velocity function for use in stacking a CMP gather of seismic data traces. The method begins with an initial estimate of the NMO velocity function for the CMP gather. This initial estimate is typically determined through conventional seismic velocity analysis. The method then picks a first velocity-traveltime pair falling on the initial estimate of the NMO velocity function and conducts a two-dimensional interpolative search of trial velocity-traveltime pairs in the neighborhood of the pick to find a substantially optimal velocity-traveltime pair. This substantially optimal velocity-traveltime pair is the trial velocity-traveltime pair having the highest stack response and is substituted for the pick in the NMO velocity function. The method then proceeds to find substantially optimal velocity-traveltime pairs to replace each of the other picks on the initial estimate of the NMO velocity function.

Abstract: The object of the method according to the invention is to synthesize zero-offset seismic data from data acquired within the scope of three-dimensional (or possibly two-dimensional) multiple-offset seismic reflection prospecting operations in a subsurface environment. It mainly comprises selection of a distribution model of the velocities in the environment, seismic data prestack migration (stacking of the multiple-offset seismic events is achieved in the depth domain), and synthesis of zero-offset seismic data in the time domain by applying to the results obtained with this depth domain stacking a known modelling technique, for example that referred to as explosive reflector technique.

Abstract: An array of output image points is distributed over a velocity model of a subsurface region of interest. A fan of rays is shot from each of a plurality of acoustic source points along the surface, the angular separation between the rays being equal to .pi./n, where n is the number of rays. A family of traveltime wavefronts is erected at selected time intervals, .DELTA.t, along the ray trajectories. The resulting wavefront map is superimposed over the array of output image points. Sub-arrays of output image points will lie within discrete traveltime cells, the widths of which are defined by the separation between adjacent rays paths and the heights are defined by the wavefront separation distance. Ray segments, wavefront segments and output image points are identified by a velocity layer index. A velocity interface that may cut across a traveltime cell, divides the cell into two parts, each having a different layer index.

Abstract: A method of analysing and processing seismic reflection data for the determination of a high resolution spatial hyperbolicity correction velocity field.The method is characterised in that:for the set of normal moveout-corrected gathers, the maximum values of the positive and negative residues of the NMO correction are determined: a time range analysis is determined located on either side of a time t.sub.0 and of which the width is equal to not more than twice the absolute value of the maximum residual moveout,a family of 2n+1 residual correction hyperbolas or parabolas are constructed, each having its apex centered on said time t.sub.0 and, at the value of the maximum offset, presenting a value of time that is equal to one of the 2n+1 equidistant time values predetermined on the analysis range, and including the value t.sub.0 and the extreme values of said analysis range,2n+1 sets of static corrections are determined for each of the offsets, defined by the time differences presented relative to said time t.

Abstract: A method and apparatus for adjusting the results of a seismic survey according to well log data obtained from wells within the survey region is disclosed. The disclosed method operates on one seismic horizon at a time, and first applies well log data, arranged by location of the wells in the survey region, to the seismic survey. Deviation values are calculated for each well location, as a signed difference between the actual depth data for the geological interface and a deduced horizon depth from the seismic survey. A difference function is evaluated for each survey point to be adjusted in the survey region (ranging from a single point to all survey points), preferably as a weighted average of the deviations at the wells, where the weighting is inversely related to the distance of the well from the survey point under adjustment.

Abstract: Disclosed is a method for efficiently and accurately determining subsurface velocities for use in migration of seismic data. The method calls for restricting the number of traces considered to those lying upon that portion of the Kirchhoff summation curve wherein the integrand for Kirchhoff migration is smooth. In the preferred embodiment, only a random sample of traces within this aperture are used in the calculations. Improvements in efficiency on the order of a factor of 1000 can be realized with the preferred embodiment.

Abstract: A method of prestack time migration based on the principles of prestack Kirchhoff time migration that can be applied to both 2-D and 3-D data. Common scatter point (CSP) gathers are created as an intermediate step for each output migrated trace. Normal moveout (NMO) and stacking of CSP gathers is performed to complete the prestack migration process. The method of the present invention allows the CSP gathers to be formed at any arbitrary location for velocity analysis, or to prestack migrate a 2-D line from a 3-D volume. The CSP gather is similar to a CMP gather as both contain offset traces, and both represent a vertical array of scatter points or reflectors. The CSP gather is formed from all the input traces within the migration aperture. Samples in the input traces are assigned an equivalent offset for each CSP location, then copied into the appropriate offset bin of the CSP gather in an efficient manner. The input time samples remains at the same time when copied to the CSP gather.

Abstract: An approximate method for imaging seismic data that originates from steeply dipping or overturned strata. The method provides a viable alternate approach to the generalized f-k migration that does not rely explicitly on some form of perturbation series expansion and thus avoids the steep-dip instability that is present in previously-known methods. This method is particularly useful in the presence of a moderate lateral velocity gradient, something that prior-art methods have trouble accommodating. An advantageous feature of the disclosed method is that the migration/imaging operators are composed of simple numerical coefficients.

Abstract: Disclosed is a technique for processing seismic data to enable the data to be migrated without stacking. Seismic data provided in common midpoint gathers are converted to a domain defined in terms of the travel time and the first power of velocity of reflected waves, and migration is performed in this domain. The migrated seismic data is converted back to the offset domain, wherein velocity analyses may be performed prior to stacking of the data.

Abstract: Seismic data is combined with well log data to generate a two-dimensional geopressure prediction display; this permits deviated and horizontal well planning plus lithology detection. Shale fraction analysis, compaction trend, and seismic velocity may be automatically or interactively generated on a computer work station with graphics displays to avoid anomalous results. Corrections to velocity predictions by check shots or VSP, and translation of trend curves for laterally offset areas increases accuracy of the geopressure predictions. Multiple wells' logs in a basin permits analysis fluid migrations.

Abstract: The mispositioning of reflectors in a velocity analysis enhanced common-offset time migrated image is corrected by carrying out inverse migration on the time image, followed by zero-offset depth migration.

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 and apparatus for deriving a three-dimensional seismic image from an irregular grid of two-dimensional seismic data is provided. The two-dimensional seismic data incldue a plurality of shot lines. For each of the shot lines, a two-dimensional migration is performed therealong (22) to create a plurality of migrated shot lines. For each of a plurality of three-dimensional image points, the following substeps are performed. A migrated trace (X) is selected from each of the migrated shot lines that is closest to the image point (I). A further migration (30) is performed on each selected migrated trace (28) with respect to the image point (I). A three-dimensional migrated image (30) comprising the image points is thereby obtained. The invention allows the use of historical, irregular two-dimensional grids of seismic data to be efficiently used for the three-dimensional imaging of subsurface features.

Abstract: A method for analyzing seismic shear wave data, using a layer stripping technique, to predict subsurface stress regimes is disclosed. Polarization directions of shear wave data, from either a vertical seismic profile or from surface reflection data, are analyzed, and time lags between fast and slow split shear wave are determined. Natural polarization directions of and time lags between the split shear waves in an upper layer are determined above the shallowest depth where data cues suggest polarization changes take place. Source and receiver axes of the data below the depth of polarization changes are rotated by an azimuth angle, to bring the axes into proper aligment. A static time shift is then applied to eliminate the time lag in the upper layer above the depth where polarization changes were indicated.

Abstract: A method for analyzing seismic shear wave data, using a layer stripping technique, to determine fault plane stress build-up is disclosed. Polarization directions of shear wave data, from either a vertical seismic profile or from surface reflection data, are analyzed, and time lags between fast and slow split shear wave are determined. Natural polarization directions of and time lags between the split shear waves in an upper layer are determined above the shallowest depth where data cues suggest polarization changes take place. Source and receiver axes of the data below the depth of polarization changes are rotated by an azimuth angle, to bring the axes into proper alignment. A static time shift is then applied to eliminate the time lag in the upper layer above the depth where polarization changes were indicated. Shear wave polarization azimuth angles, and time lags between the shear waves are determined for the depth of investigation, and are compared to the strike of a nearby fault.

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 process for tomographic development of seismic section data having known source and receiver positions. The process employs forward modeling to estimate travel times of the rays through the earth medium. The medium is assigned a cellular constitution and ray tracking proceeds through individual cell analysis within a two dimensional neighborhood to join a series of individual cell velocity values, e.g. the velocity values at the cell corners. Ray tracing proceeds utilizing derived expressions for position, orientation, and travel time of the ray in the cell as a function of cell dimensions, initial ray position, ray orientation, and the neighborhood velocity values. A travel time equation is then derived and used to construct a model parameter estimation and finalized estimated data output.

Abstract: Seismic energy is generated at a multiplicity of source offsets from a borehole and offset VSP data is collected by an acoustic detector at a multiplicity of geophone depths for each source offset. The recorded data is sorted into Zero Source-Receiver (ZSR), common reflection point bins. Data from each offset VSP, ZSR common reflection point gather are dynamically moveout corrected with data adaptive parameters including reduced direct arrival times, dip, borehole deviation and stacking velocities. The moveout corrected VSP data are then stacked.

Abstract: A method for migrating seismic data in the frequency-wave number domain is provided for migrating the seismic data having large vertical and lateral velocity variations in a fast, efficient and accurate manner. A root-mean-square (RMS) velocity function is derived from a set of seismic data samples comprising a body of seismic data. A best fit migration velocity is determined from the seismic data so as to provide a best migration. The seismic data may then be partitioned into a series of discreet frames and stretched according to a predetermined algorithm. Each partition is migrated in series by a portion of the RMS Velocity function until the data are fully migrated. After migration, the migrated seismic data are inversely stretched vertically followed by an inverse lateral stretch to remove any distortion instilled in the data by the initial data stretching.

Abstract: Method of analyzing the structure of a medium by means of waves reflected by inner interfaces, comprises: (a) generating multiple coverage data represented by traces corresponding to a plurality of source-receiver pairs distributed according to an asymetric polynomial distribution with respect to a predetermined central point with optimal parameters which provide that all reflected events of such traces correspond to the same common reflecting point on a reflecting interface; (b) correcting the recorded traces according to an optimal time delay correction which converts all reflected events to the same phase and which depends on the local parameters of wave propagation velocity Vo at the reference level, and of radius of curvature r.sub.o of the wavefront emitted by a fictitious source located at the common reflecting point and coming to the central point at an angle of entry .beta.

Abstract: The present invention provides methods for processing converted wave seismic data which includes, fractional point gathering of the data in a manner consistent with a selected velocity model, dynamic correction of the data using parameters measured from the data to account for the asymmetric travel path of the converted wave rays and stacking the dynamically corrected data. Methods are also provided for updating the velocity model.

Abstract: A moveout correction process and stacking velocity estimation process to permit stacking of vertical seismic profile (VSP) data is disclosed wherein the primary reflection time is determined by using the two-way travel time, the root mean square velocity of acoustic pulses in the formation and the first arrival time of direct path acoustic pulses.

Abstract: Method for the processing of recordings of signals resulting from a seismic exploration of a medium.It consists, after grouping recordings in pairs and crosscorrelation of the recordings of each pair, in grouping the elementary crosscorrelation functions thus obtained in at least one family in which the measuring spaces are included in a same predetermined reference space, in transforming the time variables of the said functions in order to bring the measuring space dimension to that of the reference, then in adding the transformed functions.Application in particular to acoustic logging seismic reflection or refraction.

Abstract: Disclosed is a method for constructing an optimal pilot trace from a gather of seismic traces, which pilot trace can be used to obtain statics estimates for time correction of the gathered traces prior to common depth point stacking. During construction of the optimal pilot trace, the statics estimates are inherently obtained.

Abstract: This invention relates to an improved system for employing seismic techniques for surveying the subsurface structures of the earth. The invention relates particularly to an improved system for generating, collecting and processing the data during the course of exploration with a group of trucks that travel over the surface of the earth and intermittently stop to perform the required operations. The processing aids in the performance of the generating and collecting step and in the further processing of the collected data. And the data resulting from processing in this invention are transferred to a computer center for further processing. In this system, use is made of two programmable computers to facilitate concurrent processing of seismic waves received in successive setups in different ways.

Abstract: Seismic signals from detectors forming split spreads are utilized to correct for weathering, elevation, and the like signals from detectors forming an expanded spread. The signals of the seismic section delineated by the expanded spread have applied thereto time changes under the control of a hyperbolic generator. There are generated as a result of the sweeping across the section and lengthwise thereof of the seismic signals the generation of functions utilized to provide normal moveout corrections of greater precision or accuracy than heretofore. With the normal moveout corrections of greater accuracy applied to the signals of the seismic section, additional corrections are made for dip and thereafter for delineation of the velocity characteristics of the earth along the stratigraphic column. There is included provision not only for identification of primary reflections and multiples but also for the elimination of multiples from the signals of the seismic section.