GB1377812A - Method and apparatus for seismic processing - Google Patents

Abstract

1377812 Seismic signal processing PETTYRAY GEOPHYSICAL Inc 9 March 1972 [10 March 1971] 11018/72 Heading H4D Common depth point, seismic traces are processed by a method comprising the steps of (a) establishing a range of acceptable seismic interval velocities for reflection events on the traces, as a function of trace time, (b) picking trace events, (c) detecting and storing the average velocity and occurrence time of the first event which yields an interval velocity falling within the established range, (d) detecting and storing the average velocity and occurrence time of the next event which yields an interval velocity within the established range, and (e) repeating step d sequentially for succeeding events. In the Fig. 3 embodiment static and optional initial (assumed) normal movement (NMO) or other corrections are applied at 302 to a field record of common depth point (CDP) traces. The corrected traces at 304 are grouped in "files" according to depth point and significant reflection events are picked at 305 by known methods (details given) and are identified by discrete values which could be plotted as "dots" at the appropriate trace times. Values of the amplitudes, reflection times and dominant frequencies of these events may be stored and used as additional criteria for event picking. From misalignments of the dots of a trace file, residual NMO corrections are determined and, when added at 307 to the initial corrections give the actual NMO corrections needed for that file. Comparing these actual NMO values with a table of NMO values associated with given average velocities and source-detector offsets enables the event velocities to be determined. By multiplying the average velocity associated with an event by the one-way travel time associated with that event at 311 a value of "stacking depth" is derived. The stacking depth values for various events are plotted at 312 by a pen- or cathode-ray tube-recorder as a function of two-way travel time and enable primary reflections to be identified. At 134 a first event is identified as being a primary reflection if its average velocity is within prescribed limits. A second event is identified as a primary reflection if the interval velocity associated with both it and the first event is within prescribed limits, and so on. A series of interval velocities is thus computed at 314 and if they are within preset tolerances of predetermined nominal values they are used to derive a smooth stacking depth/ two-way time curve at 315. From this curve refined values of the actual NMO are computed at 316 then compared with the initial NMO values at 317 to determine residual NMO corrections which are applied at 318 to the trace file from 304 or 306. The corrected traces are then stacked at 319, e.g. by counting coincidences of dots in the traces from 306. The refined values of NMO may be used to update the initial values at 303, and the curve 315 may be used to update the interval velocity nominal values at 313.