Atlantic Richfield Company

E Stuart Nelan

Bok S Byun

David D Thompson

Chi Young

A method and computer system for correcting seismic signals in a seismic survey, sensed in horizontal and vertical directions at receiver locations, is disclosed. According to the disclosed method and system, the horizontal and vertical seismic signal traces are processed in prestack gathers, after normal move-out correction. For each reflection event indicated in the prestack gathers, a performance function is evaluated over a range of zero-offset directivity and directivity slope values, using the horizontal and vertical pressure wave components sensed at the receivers. The zero-offset directivity and directivity slope values that return the maximum performance function are stored in connection with the gather and the reflection event, as indicative of the zero-offset dip and offset-dependent directivity of the reflective surface corresponding to the reflected event. These zero-offset directivity and directivity slope values are then used to correct the traces, or generate a new trace, corresponding to the pressure wave signal that is in the true reflected wave ray direction, minimizing the component of the sensed pressure wave that is transverse to the longitudinal wave ray direction. Detection and analysis of pressure-to-shear wave conversion, as well as amplitude-versus-offset analysis, is then facilitated by the present invention.

Method and system for correcting for offset-dependent directivity

A method and computer system for deriving and applying normal moveout phase and time shift corrections to seismic survey traces is disclosed. Normal moveout time-shift corrections are first applied to traces in a common midpoint (CMP) or common depth point (CDP) gather, based upon stacking velocity analysis applied to envelopes of the traces. One offset in the survey, preferably the maximum offset, is selected as the reference point; the NMO time shift at this reference offset is then selected as a reference NMO time-shift. The traces in the gather are each phase-shifted based upon iterated reference phase dispersion values at the reference offset, times the ratio of the normal moveout time-shifts for the trace in the gather to that of the reference offset (raised to a power). Semblance analysis is performed to identify, for each point in normal incidence time and thus for each reflective event, the optimum reference phase dispersion value. Phase corrections, based upon the optimum reference phase dispersion value as a function of normal incidence time multiplied by the ratio of the normal moveout trace and reference time shifts, are then applied to the traces in the gather. Interpolation may be used to define the optimum reference phase dispersion for CMP gathers lying between the midpoints of analyzed gathers.

Method and system for applying dispersive normal moveout corrections to

A computer-implemented method and system of correcting seismic survey data for the effects of NMO stretch is disclosed. The method operates upon a CDP gather of seismic survey data after normal move-out correction (NMO) has been applied. A semblance analysis is used to derive a stretch coefficient .kappa. profile over the gather, as a function of time and offset. The values of the stretch coefficient profile .kappa. are used in producing a time-varying filter that is applied, preferably by way of a time-domain multiplication, to incremental windows in time for each trace, with the products added to one another to generate a corrected trace. After all of the traces in the gather are corrected, an event-weighted filter is applied to the traces by way of a correlation operation, to remove high-frequency artifacts in the corrected traces. The corrected and filtered traces are then ready for stacking and conventional processing.

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Dallas, TX, United States of America

E Stuart Nelan