GB2134259A

GB2134259A - Seismic data processing

Info

Publication number: GB2134259A
Application number: GB08401729A
Authority: GB
Inventors: Muhammed Shafiqul Alam
Original assignee: Individual
Current assignee: Individual
Priority date: 1983-01-25
Filing date: 1984-01-23
Publication date: 1984-08-08
Also published as: GB2134259B; GB8401729D0

Abstract

Conventionally processed seismic data help discover the large hydrocarbon traps but they are not always sufficiently useful to delineate minor but subtle and important geologic features and also details of discovered fields. The invention discloses, as a solution, a pre-stack process applied to the CDP gather records of a seismic line after NMO and static corrections. Within specified time gates including the reflections and/or within any zone of interest the undesirable trace-segments are zeroed and others multiplied to make up, if necessary, for the zeroing of the trace-segments and to retain full fold multiplicity. Each gate is slightly wider than the main lobe of the reflection refined. Its width is varied, if necessary along the line. The width and time of each gate are submitted at a few points of the line. At other points they are interpolated.

Description

SPECIFICATION Seismic signal refinement This discovery relates to a special method of processing of seismic reflection data collected for exploration for oil and gas in any area.

The major subsurface geological structures in any area which are possible oil and/or gas fields can be discovered by processed common depth point (CDP) stack sections. But subtle geologic details like horizontal and vertical changes of lithology and facies, unconformities, on-laps, pinch-outs, minor faults, etc. cannot always be accurately identified on the conventional data, whereas they are important for stratigraphic trap detection, reservoir development, and even for detailing the discovered fields. One reason why a seismic reflection section processed by the existing methods lacks in sufficient resolution and fails to display the important geologic details distinctly is because the pure and strong signal of the inside and central traces of the CDP gather records is not properly used in processing.

Generally, the CDP gather records have clean and strong signal on the inside traces only i.e. the short offset traces. In some areas the reflection from deep horizons may be the best on the central traces. On the outside traces i.e. the far offset traces the signal is weak, deshaped and noise-ridden. The surface noise, the refractions, and their repetitions are dominant on the outside traces and it is not always possible to eliminate them completely by muting which is also known in the industry as ramping. If severe ramping is applied, CDP multiplicity is sacrificed and signal over noise enhancement is lost whereas with liberal ramping the noise-free signal of the inside and central traces is distorted and important geologic details are obscured.

In fact, in CDP stacking the outside traces do not contribute much to signal enhancement. Their main use is in the zones outside the signal zones and when stacked with the other traces they contribute to the attenuation of random noise as well as the multiples.

Ideally, the signal on the best inside trace and/or the central trace of the CDP gather record of any depth point of a line is better both qualitatively and quantitatively then the signal of the corresponding stack trace, although on the latter the signal stands out better. This is because the background noise on the stack trace is suppressed by stacking and also the signal over noise is enhanced by the post-stack processes applied.

In order to overcome these problems a special method of processing is suggested here. According to the method, a process of gated multiplication and editing is applied to the CDP gather records to remove the undesirable far offset trace-segments and to compensate at the same time for these edited tracesegments, retaining thereby full-fold stacking mutliplicity within the gates. Within a time gate chosen for the enhancement of a particu lar primary signal the undesirable trace-seg ments are multiplied with zero and each of the other trace-segments with a factor, N v (N - m) where N is the full-fold multiplicity of the data and m is the number of tracesegments zeroed.

The multiplication with the factor, N + (N - m) of each of the trace-segments that are not zeroed is applied only if in the stacking process that is applied to the data subsequently, all traces including the zeroed ones instead of only the live traces i.e. the non-zeroed traces are averaged at any reflection time within the time gate of signal refinement. However, if in the stack process, only the live trace-segments are averaged, excluding the zeroed trace-segments, then according to this discovery, the trace-segments that are not zeroed i.e. the live trace-segments are multiplied with unity instead of the factor, N v (N - m).

The process is applied after statics and normal move-out corrections applied to the data and generally it is the last of all the prestack processes applied. It is applied only to selected trace-segments to include the primary reflections that the user chooses to refine. A number of gates may be used on a line of data, some within and some outside the muted zone, each gate including only one event i.e. a reflection, and if necessary, one or more wide gates may also be used, also within or outside the muted zone, for enhancing the data of special zones of interest. The interpreter of a seismic section may wish to apply the process to a particular time gate where he thinks a primary signal and/or some important geologic detail need improvement.

The width of each gate when it includes a reflection and not a zone of interest of the section of the seismic line is substantially equal to or slightly larger than the period i.e.

the main lobe of the event multiplied. Also the width of a gate may be varied along a line of data. The width and the centre or start or end time of each gate are submitted at some points only of the line, and they are interpolated at the intermediate points. The gates are distinctly different and do not overlap in time at any point of the line. For geological pinchouts, on-laps etc., two or more gates may be very close or consecutive to each other with little or no gaps between them.

The reflections and/or the special zones to which this process is applied are selected from raw (initial) stack sections. For reprocessing or special processing of data however, the unmigrated final stacks are used to choose the gates.

The CDP gather records are examined to choose the multiplication factors to apply to the different trace-segments of different gates.

For shallow events outside the muted zone, the corresponding segments of the outside traces which are weak and noisy are zeroed and each of the other corresponding tracesegments is multiplied either with the factor, N . (N - m) or with 1, depending on whether all trace-segments including the zeroed ones are averaged in stack or only the non-zeroed ones are averaged. For deep events outside the muted zone, the energy may be the best on some central traces of the CDP gather records in which case these central traces are each multiplied either with N + (N - m) or with 1, as necesssary, and the others zeroed.

In the case of one or more events within the muted zone, the signal on all the unmuted trace-segments within the gate of each of these events may not be good, and additional editing (zeroing) may be necessary. In this case each of the good and finally live i.e. nonzeroed trace-segments corresponding to each of these events is multiplied either with n + (n - m) or with f, as necessary. Here n is the original number of the unmuted traces at the reflection time of the event and m is the number of the trace-segments additionally zeroed in this special process of signal refinement.

The gated events on the final section of this process are stronger and noise free than those of the conventional section. Outside the gates, the sections are essentially the same, although some of the post-stack processes produce better results on the section of this process. The final section by this method is also better with high multiplicity data. The effects of statics and normal move-out errors are less on the gated primary signal, and outside the gates, the noise (both random and systematic) is better reduced by high fold stacking.

Claims

1. A method of processing seismic reflection data in which some trace-segments of the CDP gather records of a seismic line to which normal move-out and static corrections have already been applied are edited i.e. zeroed and other trace-segments multiplied with a multiplication factor, only within one or more distinctly different i.e. not overlapping reflection time gateseach gate including either one reflection i.e. an event or a zone of interest of the seismic section of the line and being either inside or outside the muted zone of the section, the width of each gate when it includes a reflection instead of a zone of interest being substantially equal to or slightly larger than the period i.e. the main lobe of the reflection included, the width of any gate being varied, if necessary, along the seismic line, the zeroing being applied to the noisy and weak trace-segments both inside and outside the muted zone and the multiplication factor with which each of the other tracesegments, whether inside or outside the muted zone, is multiplied being either unity if in the stack process that is applied after the process of this discovery, only the non-zeroed i.e. the live trace-segments are used excluding the zeroed and muted (if in the muted zone) ones or a number other than unity such that the zeroing and muting of the trace-segments are compensated for and full fold multiplicity is retained by multiplying each of the live trace-segments with this number:

2.A method as claimed in claim 1 in which the process of this discovery is generally the last of all the pre-stack processes applied to the data.

3. A method as claimed in claims 1 and 2 in which the width and the centre or start or end time of each gate are submitted at some points only of the seismic line and at the intermediate points they are interpolated.

4. A method as claimed in claims 1 to 3 whereby the reflections and/or the special zones of interest to which the process of this discovery is applied are selected from the raw (initial) stack sections of the seismic line, and for reprocessing or special processing of the line the unmigrated stack section of the line is used to choose the gates.

5. A method as claimed in claims 1 to 4 in which the CDP gather records of the seismic line are examined to choose the multiplication factors to apply to the different tracesegments of the different gates.