Apparatus and method for enhancement of the signal-to-noise ratio in seismic data

• US 4,208,732 A
• Filed: 02/11/1976
• Issued: 06/17/1980
• Est. Priority Date: 05/11/1972
• Status: Expired due to Term

• Current Assignee: Mobil Oil Corporation (Exxon Mobil Corporation)
• Original Assignee: Mobil Oil Corporation (Exxon Mobil Corporation)
• Inventors: Ruehle, William H.
• Primary Examiner(s): Moskowitz, Nelson
• Application Number:
  US05/657,326
• Time in Patent Office: 1,588 Days
• Field of Search: 340/15.5 DP, 340/15.5 CP, 340/15.5 GC, 340/15.5 AF, 444/924.4, 444/924.41, 364/421
• US Class Current: 367/42
• CPC Class Codes: G01V 1/366 by correlation of seismic signals
• CPC Class Codes: G01V 2210/30 Noise handling trace signal pre-filtering G01V2210/20
• CPC Class Codes: G01V 2210/322 Trace stacking
• CPC Class Codes: G01V 2210/52 Move-out correction

Abstract

In common depth point seismic exploration, seismic traces are corrected for normal moveout and stacked to form a composite trace. The composite trace is successively time shifted to provide a plurality of reference traces characterized by the normal moveout of the seismic traces. The reference traces are crosscorrelated with the seismic traces, and the resulting correlation functions are utilized to enhance the primary reflection signals of the seismic traces.

Claims

1. A system for enhancing primary seismic reflection signals recorded in the form of common depth point seismic traces, comprising:

(a) means for shifting each of said plurality of seismic traces along the time axis to compensate for time differentials in the occurrence of primary reflection signals between adjacent seismic traces caused by normal moveout,

(b) means for combining said plurality of seismic traces after correction for normal moveout to form a composite trace,

(c) means for generating from said composite trace a plurality of reference traces equal in number to the number of seismic traces, said composite trace being successively shifted along the time axis by the amounts of the time differentials caused by said normal moveout and recorded after each successive shift, thereby providing reference traces in which the primary reflection signals follow the same normal moveout that characterizes the seismic traces,

(d) means for crosscorrelating each of said plurality of reference traces with the corresponding one of said plurality of seismic traces at the point of zero time lag between the traces to provide a plurality of correlation signals, one correlation signal for each of said plurality of seismic traces, and

(e) means for utilizing said correlation signals to enhance the common primary reflection signals of the corresponding seismic traces.

2. The system of claim 1 wherein said means for utilizing said correlation signals comprises means for multiplying each of said plurality of seismic traces with the corresponding ones of said correlation signals.

3. The system as set forth in claim 1 wherein said means for utilizing said correlation signals comprises:

(a) means for generating half-wave rectified correlation signals,

(b) means for filtering said half-wave rectified correlation signals to eliminate random variations due to high-frequency components and thereby product a plurality of gain signals representing smoothed energy estimations, each of said gain signals being associated with one of said seismic traces, and

(c) means for multiplying each of said plurality of seismic traces with its associated gain signal.

4. The system of claim 3 wherein the half-wave rectified correlation signals are generated by the elimination of the negative-going portions of said correlation signals.

5. Apparatus for enhancing primary reflection signals from a plurality of seismic energy sources which have been recorded in the form of a plurality of common depth point seismic traces, comprising:

(a) a plurality of first time shifters to which said plurality of seismic traces are applied, each of said seismic traces being displaced along the time axis to compensate for time differentials in the occurrence of primary reflection signals between adjacent seismic traces caused by normal moveout,

(b) a stacker which combines said plurality of seismic traces after correction for normal moveout by said first time shifters to form a composite trace,

(c) a second time shifter to which said composite trace is applied, said composite trace being successively shifted along the time axis by the amounts of the time differentials caused by said normal moveout and recorded after each successive shift to produce a plurality of reference traces equal in number to said seismic traces whereby the primary reflection signals in the reference traces follow the same normal moveout that characterizes the seismic traces,

(d) a plurality of correlators to which said plurality of reference traces are applied along with the corresponding plurality of seismic traces at the point of zero time lag, said correlators crosscorrelating each of said reference traces with its corresponding seismic trace to produce a plurality of correlation signals,

(e) a plurality of half-wave rectifiers to which said correlation signals are applied, said rectifiers eliminating the negative portions of said correlation signals to produce rectified correlation signals,

(f) a plurality of smoothing filters to which said rectified correlation signals are applied, said filteres eliminating random variations in said rectified correlation signals due to high-frequency components to produce rectified and filtered correlation signals, and

(g) a plurality of multipliers which multiply each of said rectified and filtered correlation signals point by point with the corresponding ones of said seismic traces.

6. A machine implemented method of enhancing the signal-to-noise ratio of a plurality of seismic traces which have been corrected for normal moveout, comprising:

(a) summing said seismic traces to obtain a composite trace,

(b) generating a plurality of reference traces by shifting said composite trace along the time axis in accordance with the normal moveout of said seismic traces, said reference traces being equal in number to the number of seismic traces,

(c) crosscorrelating said reference traces with said seismic traces at the point of zero time lag between the reference and seismic traces to provide a plurality of correlation signals, each correlation signal corresponding with one of said seismic traces, and

(d) utilizing said correlation signals to enhance the signal-to-noise ratio in said seismic traces.

7. The method as set forth in claim 6 wherein the step of utilizing said correlation functions comprises the multiplication of said correlation signals with the corresponding seismic traces.

8. The method as set forth in claim 6 wherein the step of utilizing said correlation signals comprises:

(a) generating half-wave rectified correlation signals,

(b) smoothing said half-wave rectified correlation signals to provide a gain signal associated with each of said seismic traces, and

(c) multiplying each of said seismic traces by its associated gain signal.

9. A machine implemented method of processing a plurality of seismic traces which have been corrected for normal moveout, comprising:

(a) summing said seismic traces to obtain a composite trace,

(b) generating a plurality of reference traces by shifting said composite trace along the time axis in accordance with the normal moveout of said seismic traces, said reference traces being equal in number to the number of seismic traces,

(c) crosscorrelating said reference traces with said seismic traces at the point of zero time lag between the reference and seismic traces to provide a plurality of correlation signals, each correlation signal corresponding with one of said seismic traces,

(d) half-wave rectifying said correlation signals to obtain positive correlation signals, and

(e) utilizing said positive correlation signals as enhanced seismic signals.

10. The method of seismic exploration for the location of subsurface layers of the earth comprising:

generating an input of seismic energy,

recording seismic traces representing said energy reflected from subsurface interfaces of the earth received at geophones spaced along a line of exploration,

time shifting each of said seismic traces to compensate for time differentials in the occurrence of primary reflection signals between seismic traces from adjacent geophones caused by normal moveout,

stacking said seismic traces after correction for normal moveout to form a composite trace,

time shifting said composite trace by the time differentials of said normal moveout to produce a plurality of reference traces in which the primary reflection signals have the same normal moveout as the original seismic traces,

cross correlating each of said reference traces with the corresponding original seismic trace to produce a plurality of correlation signals,

filtering said correlation signals to eliminate high frequency components and to produce a plurality of gain signals, and

changing the gain of said original seismic traces under control of the associated gain signals to produce enhanced primary reflections which identify the subsurface layers of the earth.

11. The method recited in claim 10 wherein seismic energy is generated at each of a plurality of spaced points along said line of exploration and wherein said energy reflected from a subsurface interface at a common depth point is recorded at a plurality of spaced locations along said line of exploration to produce said seismic traces.

12. In seismic exploration for the location of subsurface layers of the earth wherein seismic energy is applied to the earth at a plurality of spaced locations along a line of exploration, reflected from a subsurface interface at a common depth point, and recorded on a storage medium at a plurality of spaced locations to produce seismic traces, the new use of computing apparatus to enhance the likelihood of identifying said subsurface interfaces from said seismic traces comprising:

time shifting each of said seismic traces to compensate for time differentials in the occurrence of primary reflection signals between seismic traces from adjacent geophones caused by normal moveout,

stacking said seismic traces after correction for normal moveout to form a composite trace,

time shifting said composite trace by the time differentials of said normal moveout to produce a plurality of reference traces in which the primary reflection signals have the same normal moveout as the original seismic traces,

cross correlating each of said reference traces with the corresponding original seismic trace to produce a plurality of correlation signals,

filtering said correlation signals to eliminate high frequency components and to produce a plurality of gain signals, and

changing the gain of said original seismic traces under control of the associated gain signals to produce enhanced primary reflections which identify the subsurface layers of the earth.