Seismic Stack Research Articles

Transversely isotropic phase velocity analysis from slowness estimates

Near‐offset vertical seismic profiling (VSP) is an established technique to obtain interval velocities for lithology characterization, time‐to‐depth calibrations, and seismic data processing. Unfortunately, these velocities are often insufficient to distinguish lithology types and can disagree with surface seismic stacking velocities. Multiple‐source offset VSPs provide additional travel time information for calculating interval anisotropic velocities that can help discriminate lithology. In this paper, a technique is developed for computing transversely isotropic phase velocities over intervals less than the dominant seismic wavelength. Independent estimates of vertical and horizontal slowness are made for each receiver depth and source offset pair. These provide the magnitude and direction of phase velocity for calculating the in situ elastic properties of a transversely isotropic medium. This technique requires the assumption of lateral homogeneity because horizontal slowness components are related to surface properties between sources and not to rock properties between receivers at depth. Factors that violate this assumption such as source elevation differences and dipping layers are evaluated with synthetic data. Six impulsive source P wave offset VSPs in a 663‐m well in east Texas are analyzed and a near offset SH wave vibrator VSP provides vertical shear wave phase velocities. Strong transversely isotropic velocity variations observed in the Tertiary and Cretaceous sediments penetrated by this well correlate with lithology types, suggesting that this technique is sensitive to local properties at the receiver. These results may not indicate exact intrinsic elastic properties and could be due in part to dipping layers, vertical heterogeneities, and unknown local surface variations. Percent “phase velocity” anisotropy appears to increase from 6 to 20% for P wave and 10 to 50% for SV wave with increasing amounts of calcareous material and shale in the rock. (Percentage anisotropy is defined as the ratio of the horizontal to vertical velocity for P waves and the ratio of velocity at 45° to vertical velocity for SV waves.) Sandstones and chalk exhibit the least amount of anisotropy, while the shales and calcareous shales exhibit the most.

Method and apparatus for obtaining a composite field response to a variable source array using weighting coefficients

A multi-channeled digital seismic field stacking system is disclosed for obtaining the composite response to a variable source array. A variable source array is synthesized by applying preselected weighting coefficients to the seismic signals produced in response to a selected number of surface energy impulses. The seismic signals produced by these impulses are amplified, multiplexed and digitized to obtain digital samples to which are applied the weighting coefficients. A digital multiplier is used to multiply each digital sample by a weighting coefficient to obtain a weighted digital sample. Each digital sample is multiplied by two weighting coefficients, a partial and a final. The partial weighted samples from each of the impulses are summed together to form a partial stack, while the final weighted samples are summed together to form a final stack. The variable source array which is synthesized covers two station intervals. At each station, the partial stack represents the first half of a variable source array, while the final stack represents a complete source array covering the two previous station intervals. At each station, the final stack is recorded on magnetic tape as a seismic field record, and the partial stack is transferred to the final stack for summation with the final weighted samples for the last half of the next variable source array.

AN OPTIMIZED TECHNIQUE FOR DETERMINING STACKING VELOCITY FROM SEISMIC REFLECTION DATA*

AbstractA new method of estimating seismic stacking velocity from reflection seismograms is based on Fibonacci search technique and provides the highest rate of reduction of the interval of uncertainty of the stacking velocity. A review of the Fibonacci search strategy is presented, the application of the method is illustrated with synthetic and field examples.