Communications – electrical: acoustic wave systems and devices – Seismic prospecting – Land-reflection type
Reexamination Certificate
2001-08-24
2002-12-31
Toatley, Jr., Gregory J. (Department: 2838)
Communications, electrical: acoustic wave systems and devices
Seismic prospecting
Land-reflection type
C702S014000
Reexamination Certificate
active
06501703
ABSTRACT:
This application is based upon and claims the benefit of priority from Chinese Patent Application No. 01107066.8, filed Jan. 21, 2001, the entire contents of this application are incorporated herein by reference.
FIELD OF THE INVENTION
The present invention relates to a static correction method for exploration seismic data using the first arrivals of seismic waves recorded by receiver gathers on the earth surface. More particularly, the present invention relates to a shorter spatial wavelength static correction method for smoothing the time values of the first arrivals such as refracted seismic waves which are picked up from seismic gather records after the field static correction, to obtain shorter spatial wavelength static correction.
BACKGROUND OF THE ART
Seismic exploration method is the most widely used and effective geophysical technique in oil and gas exploration for locating the drilling sites. The seismic waves generated by artificial sources travel into the earth and return to the surface after reflection from interfaces between formations having acoustic impedance contrasts. The reflection and/or refraction of the seismic waves generated on the earth surface by respective shot points, which are arranged and moved in a predetermined regular manner, are recorded by receivers for detecting the generated seismic waves, which are laid along the ground at distances from the shot points in a predetermined manner. Variations in the reflection times from place to place indicate structural features in the strata underground. The seismic structure figures those come from seismic data processing and interpretation are the most important information to locate the drilling sites. Therefore, the detected arrival times and waveforms of the reflection generated by the interfaces between the geology formations on the seismic waves are processed and analyzed, and the status and location of the geology formations can be acquired and determined.
Due to the differences between various geology formations underground in composition, density, and uniformity of distribution etc., propagation velocities of the seismic waves generated by the shot points are correspondingly different therein, and the respective arrival times and waveforms of the reflection and/or refraction waves, which are generated by the interfaces between the formations having velocity and acoustic impedance contrasts and return to the surface of the earth, detected by the receivers are different from each other. Among these data, the first arrivals such as refracted waves, which are earliest detected and most significant and valuable for static corrections during processing, are the returned seismic waves after refracted by the lower boundary of the weathered low velocity layer (LVL) covering the land surface, such as desert and loess plateau, and mountainous area.
Usually the land surface is covered with a weathered layer of low velocity. The topography is never flat, the lower boundary of LVL is never planar. Variation in thickness and velocity of the upper layer can cause travel time delay or priority for waves to the surface. The reflections are diverged from the normal role—the hyperbola relationship between the arrival times and offsets(receiver—shot distances). It can cause a dramatic deterioration in the quality of seismic data. The reflection energy can not be focused in horizontal stack and the images of reflectors in seismic section become ambiguous. So static corrections for eliminating the divergence of wave arrivals are very important and become a key step of seismic data processing in exploration regions with complex LVL.
Investigations show that the particular geology formations of certain LVL have same influence on the divergence of the arrival time of the reflection waves from deeper strata as that for the first arrival waves refracted from the LVLs. By performing seismic static correction to the time values of the first arrivals detected by the respective receivers iteratively and in various manners, to eliminate the divergence of the arrival time of the first arrival waves refracted in the LVLs, as a result, the divergence of the reflected waves from the formations of deeper strata can be also eliminated, so that the formation of the deeper strata can be focused in the seismic data and the image thereof can be more clear.
So far, the most powerful techniques for receiver and shot point static corrections are refraction statics. They are used after field static correction and before residual static correction. The waves refracted by the interfaces between the LVL and consolidated rocks below are transmitted back to surface before the direct arrival and reflection arrivals, they are known as the headwaves and become conventional first arrivals in seismic records.
The conventional method of static corrections for receiver and shot point includes steps of: performing coarse field static correction to the original exploration seismic data recorded by each receiver using height and surface measurement; performing refraction static correction to the first arrivals, i.e., obtaining the LVL thickness and velocity by means of inversion of the structure of the LVL, then performing static correction by calculating the time difference; and performing residual static correction as well as other processes to the seismic data. However, because of the complexity of LVL model design and the non-uniqueness of inversion, the accuracy of the conventional refraction static usually is rather low for shorter spatial wavelength statics and they can not be satisfied in complex LVL regions in practice.
On the other hand, there are two modes in current seismic exploration: 2-D seismic survey and 3-D seismic survey. In 2-D seismic survey usually the shot points and receivers are laid along an exploration line, and the shot points are moved along the exploration line after each shot to get the exploration data, until the detection of one line is completed. After acquiring the data of one exploration line, data for other exploration lines can be acquired in the same manner. For 3-D seismic survey, the receivers of a shot are laid in several lines (for example, q) covered a region, and the shot points are also distributed in several lines (for example, p). Usually shot lines and receiver lines are in the same direction but not in same places, so that a two dimensional p×q exploration array is formed. During the data acquiring process of 3-D seismic survey, after each of the shot points in an array is activated once respectively to record the exploration waves, the whole exploration array is moved forward to activate all the shot points once again, until the detection of the whole area is completed.
Furthermore, the static correction for 3-D seismic exploration, especially the shorter spatial wavelength static correction using the first arrivals is much more complex than that for 2-D seismic exploration, and there is no effective method to convert 3-D seismic exploration data into 2-D seismic exploration data in static processing. Therefore, the shorter wave static correction for exploration seismic data using the first arrivals of refraction waves are mainly used in 2-D seismic data processing. It is difficult and thus rarely to apply shorter spatial wavelength static correction to 3-D seismic data processing.
SUMMARY OF THE INVENTION
In view of the above, an object of the present invention is to provide a novel shorter spatial wave static correction method for exploration seismic data using the first arrivals, which is simple in operation, and has better correction results. With the method of this invention, the static correction value can be obtained directly, and the curves of first arrivals can be smoothed, so that the reflection waves can be better focused and have more clear images.
Another object of the invention is to provide a shorter spatial wave static correction method, which is applicable to 2-D ad 3-D exploration seismic data both, especially applicable to the static correction of 3-D seismic data using first arri
Zhong Benshan
Zhou Xixiang
Banner & Witcoff , Ltd.
Toatley , Jr. Gregory J.
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