阅读说明：本技术 一种通过优化***空间分布减小空***震源子波方向性的方法 (method for reducing air gun seismic source wavelet directivity by optimizing air gun spatial distribution ) 是由 沈洪垒 陶春辉 王汉闯 周建平 丘磊 柳云龙 于 2019-09-09 设计创作，主要内容包括：本发明公开了一种通过优化气枪空间分布减小空气枪震源子波方向性的方法,属于气枪震源优化和设计领域。本发明研究了气枪空间分布对鬼波及干涉陷波的影响规律,在此基础上提出了沿测线方向气枪分布集中度的评价标准,通过对水平和深度方向的联合优化得到了有效频带带宽和有效入射角度明显优于常规水平震源的设计方案,可以减小空气枪震源子波方向性。(The invention discloses a method for reducing the directionality of air gun seismic source wavelets by optimizing air gun spatial distribution, and belongs to the field of optimization and design of air gun seismic sources. The invention researches the influence rule of air gun space distribution on ghost waves and interference trapped waves, provides an evaluation standard of air gun distribution concentration along the measuring line direction on the basis, obtains a design scheme that the effective frequency band bandwidth and the effective incident angle are obviously superior to those of a conventional horizontal seismic source through the combined optimization of the horizontal direction and the depth direction, and can reduce the wavelet directivity of the air gun seismic source.)

1. A method for reducing the directivity of a source wavelet of an air gun by optimizing the spatial distribution of the air gun, comprising the steps of:

1) For the air gun combined seismic source with the selected single gun capacity, the distance between two adjacent air guns and the horizontal distance between two adjacent air guns both need to meet the set range as the constraint condition, and the distribution concentration factor parameter F is used_conAs an evaluation criterion objective function;

Wherein x is_iThe position of the ith air gun along the survey line direction,Represents the average position of all air guns along the direction of the measuring line; n represents the total number of airguns;

In the same vertical plane, the distribution concentration parameter F is obtained by changing the horizontal and depth positions of each single gun_conThe combined seismic source with the largest horizontal-depth combination is used as an optimized seismic source;

2) For air guns with different depths in an optimized seismic source, a head wave synchronization excitation mode is adopted, namely, the excitation time is preset to ensure that head wave pulses obtained by excitation of all the air guns are superposed in the same phase in the vertical direction; the directivity of the air gun source wavelet can be reduced.

2. The method for reducing the directivity of the source wavelet of the air gun by optimizing the spatial distribution of the air guns according to claim 1, wherein the distance between two adjacent air guns needs to be controlled to be 3-5 m.

3. The method for reducing the directivity of the source wavelet of the air gun by optimizing the spatial distribution of the air guns according to claim 1, wherein the horizontal distance between two adjacent air guns is controlled to be 1-3 m.

4. The method for reducing the directionality of wavelet of an air gun source by optimizing the spatial distribution of air guns according to claim 1, wherein in step 1), when the combined air gun source to be optimized is a horizontal source with the selected single gun capacity and two gun spacing, the horizontal and depth positions of each single gun are changed and the distribution concentration parameter F is obtained by fixing the spacing between two adjacent guns in the same vertical plane_conThe combined source of the largest horizontal-depth combination serves as the optimization source.

Technical Field

The invention belongs to the field of air gun seismic source design and optimization, and particularly relates to a method for reducing the directivity of air gun seismic source wavelets by optimizing air gun spatial distribution.

Background

In modern marine seismic exploration, multi-volume air gun tuned compound sources have been widely used. The combined array has significant advantages over a single gun in terms of bubble ratio and absolute energy (penetration capacity). However, under the influence of seismic source ghost waves and array spatial arrangement, the seismic source wavelets obtained by the multi-gun combination mode have strong directivity, that is, the wavelets are obviously different along the incident angle, which brings challenges to subsequent processing and greatly reduces the imaging quality. A common method for canceling the directivity of source wavelets is to perform directional source signal deconvolution in the Tau-p/F-K domain along different directions of incidence and has been successfully applied to independent processing of 3D and single shot data. However, wavelet deconvolution performance is highly dependent on signal-to-noise ratio, and tends to be poor in frequency regions where signal-to-noise ratio is low, such as near ghost and interference notches. With the development of high-resolution seismic imaging technology and high-precision velocity modeling, higher requirements are put forward on effective frequency bandwidth and effective incidence angle. In particular, there is a clear requirement for the consistency of the seismic source signals in the positive and negative directions for the recently developed acquisition methods in which the seismic source is located above the streamer, such as the TopSeis acquisition technology of CGG corporation (Vinje, v., Lie, j.e, Danielsen, v., Dhelie, p.e., siiiqi, r., Nilsen, c., Hicks, e., and camamer, a.2017.shooting over the seismic spread.

Disclosure of Invention

the invention aims to solve the problems and provides a method for reducing the directivity of the air gun source wavelet by optimizing the air gun space distribution. Firstly, the influence rule of air gun space distribution on ghost waves and interference trapped waves is researched, and on the basis, an evaluation standard of air gun distribution concentration along a measuring line direction is provided. The design scheme that the effective frequency band bandwidth and the effective incidence angle are obviously superior to those of the conventional horizontal seismic source is obtained through the joint optimization of the horizontal direction and the depth direction.

The technical scheme of the invention is as follows:

A method for reducing the directivity of a source wavelet of an air gun by optimizing the spatial distribution of the air gun, comprising the steps of:

1) For the air gun combined seismic source with the selected single gun capacity, the distance between two adjacent air guns and the horizontal distance between two adjacent air guns both need to meet the set range as the constraint condition, and the distribution is concentrateddegree parameter F_conas an objective function of the evaluation criterion,

Wherein x is_iThe position of the ith air gun along the survey line direction,Represents the average position of all air guns along the direction of the measuring line; n represents the total number of airguns;

in the same vertical plane, the distribution concentration parameter F is obtained by changing the horizontal and depth positions of each single gun_conThe combined seismic source with the largest horizontal-depth combination is used as an optimized seismic source;

2) for air guns with different depths in an optimized seismic source, a head wave synchronization excitation mode is adopted, namely, the excitation time is preset to ensure that head wave pulses obtained by excitation of all the air guns are superposed in the same phase in the vertical direction; the directivity of the air gun source wavelet can be reduced.

As a preferable scheme of the invention, the distance between two adjacent air guns needs to be controlled to be 3-5m, so that strong interference between air gun wave fields is avoided, and the risk of air gun collision is reduced.

As the preferable scheme of the invention, the horizontal distance between two adjacent air guns is controlled to be 1-3m, so that the chain can be vertically mounted.

as a preferred scheme of the invention, in the step 1), when the air gun combination seismic source to be optimized is a horizontal seismic source with the selected single-gun capacity and the selected two-gun distance, the horizontal and depth positions of each single gun are changed in the same vertical plane by adopting a method for fixing the two-gun distance, and a distribution concentration parameter F is obtained_conThe combined source of the largest horizontal-depth combination serves as the optimization source.

The concentration parameter of the invention guides the optimization of the seismic source design, thereby weakening the directionality of the wavelet of the air gun seismic source and having rationality. The optimized air gun combined seismic source energy distribution is more uniform, and ghost wave trapped waves and interference trapped waves are effectively compensated. The optimized post-seismic source can realize that the effective incident angle can still be guaranteed to be +/-30 degrees when the frequency is up to 250Hz, and the method has important significance for wide incident angle acquisition, such as ocean bottom seismographs and the TopSeis technology of CGG company.

Drawings

FIG. 1 is a first non-zero ghost notch and interference notch influence factor relationship curve. (a) The change rule of ghost wave trapped wave along with the depth; (b) the change rule of the interference trapped wave along with the distance; (c) the law of the change of the interference trapped wave with the number of air guns.

FIG. 2 is F_conresults (left) and results of effective incidence angles for different patterns of alignment (right). When the effective angle of incidence is 90 deg., this means that the interference notch has not yet affected 150 Hz.

FIG. 3 shows the results of a directional analysis of a source combined with different airgun spacings. (a) Different combination modes of air guns; (b) f of different mode calculation_cona value; (c) spectrum curve under different combination modes of 150Hz frequency.

fig. 4 shows a tilted two gun combination. (a) The distance l between the two guns is unchanged; (b) the horizontal separation deltax between the two guns is constant.

FIG. 5 is a first order notch curve corresponding to the results of the two source designs of FIG. 6. (a) The distance between the air guns is kept unchanged at 10 m; (b) the air gun horizontal spacing was kept constant at 10 m.

FIG. 6 is a comparison of conventional and optimized seismic source combinations. (a) A conventional horizontal seismic source; (b) a horizontal-depth optimized combined source.

Fig. 7 shows the result of the energy distribution of the horizontal source combination pattern in fig. 6 a.

Fig. 8 shows the result of energy distribution for the horizontal-depth optimized combined source mode in fig. 6 b.

Detailed Description

The invention will be further described with reference to specific embodiments and the accompanying drawings.

The far-field wavelet of the air gun source can be regarded as the linear superposition of the imaginary source wavelet and the imaginary ghost source wavelet at the far-field position. Assuming that the source consists of n airguns, its far-field wavelet can be expressed as:

Wherein n is_iRepresenting the hypothetical source wavelet for the ith airgun. r is_iAnd r_i' represents the distance between the imaginary seismic source and the imaginary ghost source, respectively, and the far-field position, and R is the sea surface reflection coefficient, typically-1. t is t_delayFor delayed firing time of the airgun, it is commonly employed in the multi-depth synchronous firing mode. t is t_farIs the time, Δ t, required for the wavelet to propagate from the airgun to the far field_i(α) is the time lag of the imaginary ghost source wavelet relative to the imaginary source wavelet, which may be generally expressed as Δ t_i(α)＝2h_icos (alpha)/c, wherein c is the speed of the sound wave in water, and alpha is the incident angle.

Fourier transform is performed on the formula 1 to obtain a spectrum expression of the far-field wavelet:

Wherein N is_i(w) represents the result of Fourier transform of the ith airgun hypothetical source wavelet.

By derivation, we have found that for a conventional horizontal source consisting of multiple single guns, the first non-zero ghost notch and the first interference notch in the frequency domain are expressed as follows:

Wherein n represents the number of air guns and x represents the distance between the two guns. The first half of formula 3 represents ghost wave trapped waves introduced by a sea surface reflection interface, and the second half represents interference trapped waves caused by gun array spatial arrangement.

From equation 3, it can be seen that the ghost notch is controlled by the air gun depth, and in general, the first non-zero notch position determines the effective frequency bandwidth of the source wavelet, and the larger the depth, the smaller the notch frequency, as shown in fig. 1 a. The number and spacing of the air guns determines the frequency location of the interference notches. Unlike ghost notches, which mainly affect a large range of angles of incidence, the notches appear at progressively smaller angles as the frequency increases, as shown in fig. 1b, 1 c. The smaller the number of air guns and the smaller the spacing, the larger the effective angle of incidence.

the interference trapped wave is mainly controlled by the spatial arrangement of the gun array, such as the number of air guns and the distance between the air guns. In order to quantitatively describe the relation between the spatial arrangement of air guns and the effective incident angle of a seismic source wavelet, we propose to adopt an air gun distribution concentration parameter F along a survey line direction_conAs evaluation criteria for guiding the optimal design of the air gun array:

Wherein x_iThe position of the ith air gun along the survey line direction,The average position of all air guns along the line direction is shown.

FIG. 2 shows a different arrangement, F_conSimilar to the conclusions drawn in fig. 1 regarding the relationship between effective angles of incidence (150 Hz for example), a wider effective angle of incidence can be achieved with a smaller number of air guns and a smaller air gun spacing. Meanwhile, by comparing the left and right images of FIG. 2, the effective incident angle and concentration parameter F can be found_conThe method has strong positive correlation, and the rationality that the concentration parameter is used for guiding the optimization of the seismic source design so as to weaken the directionality of the wavelet of the air gun seismic source is proved.

For a conventional horizontal seismic source, the concentration parameter can be improved by adjusting the distance between air guns, and further the effective incident angle of a seismic source signal is widened. FIG. 3a shows how the calculation result of the concentration parameter is shown in FIG. 3b, which shows how three color points in the graph correspond to F respectively, by changing the possible combinations of the positions of the single guns in the gun array and keeping the length of the air gun array unchanged_conMaximum, minimum and mean (no change in airgun position) combinations of seismic sources. FIG. 3c shows the spectral response of 150Hz for different combinations visually, which can be seenThe effective frequency bandwidth of the wavelet frequency can be changed correspondingly by changing the relative position of the air gun, and the effective frequency bandwidth is changed along with the F_conExhibit a positive correlation, i.e. maximum F_concorresponding to the widest effective angle of incidence and vice versa. This again demonstrates the presence of F_conAnd the evaluation criterion is used as the rationality of the seismic source combination directivity evaluation criterion.

Conventional sources are limited by airgun wavefield interference and the potential risk of collision, the distance between airguns cannot be too close, which limits the space for further improvement of airgun directivity, and on the other hand, we have also found that optimization of the combination in the horizontal direction is not able to change the position of ghost wave traps. Ghost wave trapped waves are eliminated by adopting a multi-depth synchronous excitation mode and utilizing the diversity of the trapped waves changing along with the depth. Therefore, the idea of utilizing horizontal-depth combined optimization to realize ghost wave notch and interference notch synchronous compensation so as to reduce the directivity of the air gun seismic source is provided. Fig. 4 shows two combined modes of adjusting the air gun depth, namely keeping the air gun spacing constant and keeping the air gun horizontal spacing constant.

for air guns of different depths, we used a first-wave synchronized excitation mode (Siliqi r., Payen t., sabon r., and Desrues k.2013.synchronized multi-level source, a robustbroad band source solution.83^rdAnnual International Meeting, SEG, Expanded Abstracts,56-60.), that is, the excitation time is preset to ensure that the head wave pulses obtained by all air gun excitations are superposed in phase in the vertical direction (alpha is 0 °). The coherent notch can now be expressed as:

Where β is the angle of inclination shown in fig. 4.

FIG. 5 shows interference notch curves for two different combination patterns of tilted sources. It can be seen from the figure that when the distance between the two guns is fixed (fig. 5a), the effective angle of the wavelet can be widened towards the positive direction and the negative direction simultaneously by changing the depth of the air gun. When the air gun is vertically arranged (delta h is 10), the air gun concentration is highest, the corresponding seismic source wavelet directivity is also lowest, and the air gun concentration is higher than the seismic source wavelet directivityAt 150Hz, the effective angle of incidence is close to + -60 deg., far better than + -30 deg. for a conventional horizontal source. While changing the air gun depth does not significantly increase F when the horizontal separation of the two guns is fixed (FIG. 5b)_conaccordingly, the effective incident angle of the wavelet is widened toward the inclination direction of the air gun, but on the other hand, the effective angle is gradually decreased in the negative direction, so that the change of the overall effective angle is small, especially in a high frequency range, such as a part above 200 Hz.

As mentioned before, in practical construction, the distance between two adjacent air guns often needs to be controlled to 3-5m to avoid strong interference between air gun wavefields, and at the same time, to reduce the risk of air gun collision. Therefore, in the design process, on the premise of ensuring that the distance between two adjacent air guns is between 3 and 5m and the horizontal distance between two adjacent air guns is between 1 and 3m (ensuring that a chain can be vertically mounted), the air gun concentration F is adopted_conFor the objective function, the multi-depth seismic sources are optimized with 10cm as horizontal pitch step size, and the final optimized combination result as shown in fig. 6b is obtained. Comparing with the conventional horizontal seismic source shown in FIG. 6a, the optimized combination F_conThe improvement is obvious and is increased from 0.038 to 0.172.

Fig. 7 and 8 compare the results of the energy distribution of the two source combinations of fig. 6. It can be obviously seen that the energy distribution of the depth combination seismic source after optimization is more uniform, and ghost wave trapped waves and interference trapped waves are effectively compensated. The optimized post-seismic source can realize that the effective incident angle can still be guaranteed to be +/-30 degrees when the frequency is up to 250Hz, and the method has important significance for wide incident angle acquisition, such as ocean bottom seismographs and the TopSeis technology of CGG company.