A method of suppressing negative pressure airgun source wavelet sidelobes using a combination of airgun volume, firing depth, and firing time

Abstract

The application provides a method for suppressing negative pressure air gun source wavelet sidelobe by using air gun capacity, firing depth and ignition time combination. Based on the feature that the implosion time (forward main pulse forming time) of the negative pressure air gun is affected by the air gun firing chamber capacity, the application proposes a negative pressure air gun capacity combination method, and by adjusting the air gun firing depth and ignition time, the forward main pulses of different air guns are in-phase superimposed in the vertical direction, and then the negative sidelobe is suppressed. The method can effectively improve the resolution of the negative pressure air gun source wavelet and improve the smoothness of the source wavelet spectrum.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of marine seismic exploration sources, and particularly relates to a method for suppressing sidelobes of a negative pressure air gun source subwave by combining air gun capacity, firing depth and ignition time. BACKGROUND

[0002] A traditional high-pressure air gun source can excite a pulse-shaped source subwave by instantaneously releasing pressurized high-pressure gas (such as 2000 PSI ≈ 13.8 Mpa) into seawater. However, the performance of the air gun source is limited by factors such as the design of the source structure and the external excitation environment. For example: 1) the periodic oscillation of the residual bubble caused by the internal and external pressure changes during the expansion and contraction of the bubble formed after the gas is released into the water reduces the resolution of the subwave and causes low-frequency jitter in the frequency domain; 2) for safety reasons, the working pressure of the air gun is generally below 3000 PSI, and as the depth of the air gun increases, the hydrostatic pressure increases, and the energy that can be excited decreases sharply, so the depth of the air gun is usually less than 20 m; 3) the existence of the sea surface-air reflection interface produces a source ghost wave that trails the first wave, causing frequency domain notching and greatly compressing the effective frequency bandwidth of the subwave; 4) the consistency of the subwave excited by the traditional air gun, especially the ghost wave part, is low due to factors such as sea surface undulation, ocean current impact, ignition time delay and gas supply pressure variation.

[0003] In view of this, some researchers have proposed a negative pressure air gun source (CN202310520514.9). The negative pressure air gun source uses negative pressure gas supply, and "negative pressure" refers to the pressure of the gas in the air gun being lower than the surrounding water pressure at the excitation position, thereby forming a "negative pressure". When the marine negative pressure air gun is excited, the seawater will enter the cabin and squeeze the gas to form an implosion, generating a source subwave. The negative pressure air gun source can break through the conventional water depth limit and achieve deep water excitation, thereby reducing and delaying the interference of the source ghost wave and improving the imaging effect of the seabed structure (especially the shallow structure). Simulation and comparison show that the residual bubble oscillation of the negative pressure air gun source is significantly weaker than that of the traditional high-pressure air gun, and the resolution of the subwave is significantly improved.

[0004] However, in the process of squeezing the gas in the surrounding seawater, the negative pressure air gun source produces a negative pressure wave field, which becomes a sidelobe of the implosion shock pulse, reducing the resolution of the air gun source subwave, causing fluctuations in different frequency components in the frequency spectrum curve, and reducing the flatness of the frequency spectrum, thereby affecting the effect of subsequent deconvolution processing based on the source subwave. SUMMARY

[0005] To solve the problems in the prior art, the application provides a method for suppressing negative pressure air gun source wavelet sidelobes by combining air gun capacity, firing depth and firing time.

[0006] According to the purpose of the application, the application provides a method for suppressing negative pressure air gun source wavelet sidelobes by combining air gun capacity, firing depth and firing time, which comprises the following steps:

[0007] 1) The negative pressure air gun source is composed of multiple negative pressure air guns, the implosion time of different negative pressure air guns is calculated, and the relationship among the implosion time of the negative pressure air gun, the firing depth and the firing time is obtained according to the following formula (1):

[0008]

[0009] Wherein, T fire (i) is the firing time of the negative pressure air gun i, that is, the negative pressure air gun i is fired at T fire (i), H ave is the average value of the firing depths of all the negative pressure air guns, H(i) is the firing depth of the negative pressure air gun i, T implosion (i) is the implosion time of the negative pressure air gun i, C is the sound speed in water, and Constant is a given constant.

[0010] 2) The firing depth and the firing time of each negative pressure air gun are determined under the condition of satisfying formula (1); each negative pressure air gun is arranged according to the firing depth, each negative pressure air gun is fired according to the obtained firing time of each negative pressure air gun, the main pulses of the negative pressure air guns are in-phase superimposed along the vertical direction, and the negative pressure air gun source wavelet sidelobes are suppressed.

[0011] Wherein, the given constant Constant is a preset constant smaller than the shooting interval.

[0012] According to the preferred scheme of the application, the implosion time of the negative pressure air gun is the length of time from the air gun firing to the formation of the main pulse. The implosion time can be obtained by solving the bubble oscillation equation after the firing depth and the air gun capacity are given.

[0013] According to the preferred scheme of the application, in step 1), the implosion time of each negative pressure air gun can be adjusted by changing the capacity of each negative pressure air gun.

[0014] According to the preferred scheme of the application, the firing depth corresponding to the travel time difference is adjusted by changing the firing depth of each negative pressure air gun.

[0015] According to the preferred scheme of the present application, in step 2), the implosion time of each negative pressure air gun is simulated under the condition of a given negative pressure air gun capacity; the firing depth and ignition time of each negative pressure air gun are calculated under the condition of meeting formula (1) according to the construction environment and air gun spacing arrangement requirements; the negative pressure air guns are arranged according to the obtained firing depth; and each negative pressure air gun is fired according to the obtained ignition time of each negative pressure air gun, thereby suppressing the sidelobes of the negative pressure air gun wavelet.

[0016] According to the preferred scheme of the present application, in step 2), each negative pressure air gun is arranged in a horizontal plane at the same depth, that is, the firing depth is the same; the ignition time of each negative pressure air gun is calculated according to formula (1) through the difference in the implosion time of each negative pressure air gun obtained by simulation; and each negative pressure air gun is fired according to the obtained ignition time of each negative pressure air gun, thereby suppressing the sidelobes of the negative pressure air gun wavelet.

[0017] According to the preferred scheme of the present application, in step 2), each negative pressure air gun is set to be fired simultaneously, that is, the ignition time T fire (i) is the same; the firing depth of each negative pressure air gun is calculated according to formula (1) through the difference in the implosion time of each negative pressure air gun obtained by simulation; and each negative pressure air gun is arranged according to the firing depth and fired simultaneously, thereby suppressing the sidelobes of the negative pressure air gun wavelet.

[0018] According to the preferred scheme of the present application, a plurality of negative pressure air guns are selected; each negative pressure air gun is arranged along the vertical direction; the time delay caused by the difference in the implosion time and the firing depth of each negative pressure air gun is calculated; the ignition time of each negative pressure air gun is calculated according to formula (1); and each negative pressure air gun is fired according to the obtained ignition time of each negative pressure air gun, thereby suppressing the sidelobes of the negative pressure air gun wavelet.

[0019] The present application finds, through simulation analysis, that the implosion time (main pulse forming time) of a negative pressure air gun is related to the firing chamber capacity of the air gun. By utilizing the above-mentioned characteristics, the present application proposes a method of using air gun capacity combination and adjusting the firing depth and ignition time of the air gun to control the in-phase superposition of the main pulses of different air guns along the vertical direction, so that the negative sidelobes are relatively suppressed. This method will effectively improve the smoothness of the frequency spectrum of the negative pressure air gun and improve the resolution of the wavelet. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 The wavelet (left) and its frequency spectrum (right) obtained by simulation for a 100 cu.in negative pressure single gun at a depth of 1000 m;

[0021] Figure 2 The wavelet (left) and its frequency spectrum (right) obtained by simulation for different capacity negative pressure air guns at a depth of 1000 m;

[0022] Figure 3 Simulated wavelet (left) and its spectrum (right) of 100 cu.in negative pressure air gun at different depths

[0023] Figure 4 Variation of implosion time T after the negative pressure air gun is excited with the volume (left) and the excitation depth (middle, right). The excitation depth curve is calculated, and the negative pressure air gun is 100 cu.in.

[0024] Figure 5 Simulated wavelet and its spectrum results of four-bar air guns with different volumes at 1000 m using different firing times. (a) Original simulated wavelet results; (b) spectrum corresponding to (a); (c) normalized wavelet; (d) spectrum after aligning the main frequency.

[0025] Figure 6 Simulated wavelet and its spectrum of four-bar air guns at different depths using different firing times. (a) Original simulated wavelet results; (b) spectrum corresponding to (a); (c) normalized wavelet; (d) spectrum after aligning the main frequency. DETAILED DESCRIPTION

[0026] The present application will be further described and illustrated with specific embodiments. The embodiments are only exemplary and do not limit the scope. The technical features of each embodiment in the present application can be combined accordingly without conflict.

[0027] The negative pressure air gun source uses a method of supplying air below the surrounding hydrostatic pressure of the gun body, and uses the natural hydrostatic confining pressure to squeeze the air, thereby generating a negative pressure wave field. Negative pressure refers to the pressure of the gas in the air gun being less than the surrounding water pressure at the excitation position. According to the needs, the gas pressure in the air gun can also be below, equal to or slightly greater than atmospheric pressure as the gas pressure of the present application, but no matter what gas pressure is selected, it must meet the condition that the pressure is less than the surrounding water pressure at the excitation position. Different pressure differences will give different wavelet characteristics, and the size of the air gun pressure can be determined according to the required wavelet characteristics.

[0028] The negative pressure air gun in the following examples all uses the implosion type marine negative pressure air gun in CN202310520514.9, wherein the negative pressure air gun with different volumes refers to changing the size of the excitation chamber of the implosion type marine negative pressure air gun in CN202310520514.9. The present application simulates the source wavelet of the negative pressure air gun excited at different depths, as shown in Figures 1-3

[0029] As shown in Figure 1 ​The figure shows the simulated wavelet and its spectrum results obtained when a 100 cu.in negative pressure air gun is excited at a depth of 1000 m underwater. In the figure, T corresponds to the time from excitation to bubble implosion (main pulse formation). It can be seen that before the main pulse, there is a negative pressure field formed by the water compressing the air. Figure 1 (The left-middle arrow indicates the negative pressure sidelobe). Compared to the wavelet spectrum after removing the sidelobes, the negative pressure field causes the wavelet spectrum to exhibit fluctuating oscillations.

[0030] Figure 2 Wavelet and spectral curves of negative pressure air guns with different capacities simulated at a depth of 1000m are shown. Comparison reveals that as the capacity of the negative pressure air gun increases, the arrival time of the main pulse lengthens, and the corresponding spectral oscillation frequency increases. The spectra of air guns with different capacities exhibit varying fluctuations at different frequencies. Similar conclusions can be drawn from the wavelet and spectral results simulated at different depths. Figure 3 The comparison shows that the arrival time of the main pulse and the corresponding spectral oscillation frequency also change with different excitation depths.

[0031] Figure 4 The relationship between implosion time, volume, and firing depth was further compared. The analysis shows that varying the implosion time by changing the negative pressure air gun volume is easier to achieve in actual field operations than adjusting the firing depth. Figure 4 The implosion time of the right and middle parts varies very little within a depth range of 10m. Without changing the air gun capacity, it is difficult to effectively change the implosion time by combining depths within a small range.

[0032] Based on the above analysis, this invention proposes a method of combining multiple air guns for excitation. By combining ignition time and excitation depth, the main pulses of air guns with different capacities are superimposed in phase in the vertical direction. The specific scheme is as follows:

[0033] 1) The negative pressure air gun source consists of multiple negative pressure air guns. The implosion time of different negative pressure air guns is calculated; and the relationship between the implosion time, excitation depth, and ignition time of the negative pressure air gun is obtained according to the following formula (1):

[0034]

[0035] Among them, T fire (i) represents the ignition time of the negative pressure air gun i, i.e., at T fire (i) When the negative pressure air gun i is excited, H ave Let H(i) be the average depth of all negative pressure air guns, and T be the excitation depth of negative pressure air gun i. implosion(i) is the implosion time of the negative pressure air gun i (solved by bubble oscillation equation after given firing depth and air gun capacity), C is the sound speed in water, Constant is a given constant (a certain preset constant less than the shot interval);

[0036] 2) Determine the firing depth and ignition time of each negative pressure air gun under the condition of satisfying formula (1); arrange each negative pressure air gun according to the firing depth, and fire each negative pressure air gun according to the obtained ignition time of each negative pressure air gun, so that the main pulse formation time of each negative pressure air gun is consistent along the vertical direction, thereby realizing in-phase superposition and suppressing negative pressure air gun wavelet sidelobes.

[0037] Formula (1) shows that, for the different capacity air guns caused by the difference in air gun implosion and depth, the difference in main pulse arrival time can be eliminated by adjusting the ignition time, so as to realize the in-phase superposition of the main pulse.

[0038] The present application provides two embodiments. Assuming that there are four negative pressure air guns with capacities of 50 cu.in, 100 cu.in, 150 cu.in and 200 cu.in. Embodiment 1: All negative pressure air guns are in the same depth level, so there is no firing depth difference, and therefore the ignition time only needs to consider the difference in implosion time of air guns with different capacities. The implosion time of the four guns obtained by simulation is 0.875ms, 1.102ms, 1.261ms and 1.388ms respectively. Assuming that the artificial given constant Constant is 10ms, then the ignition time corresponds to 9.125ms, 8.898ms, 8.739ms and 8.612ms. That is, the air gun with large capacity is fired first, and the air gun with small capacity is fired later.

[0039] Figure 5 The source wave simulation results of four guns with different ignition times at the same depth. By comparison, it can be seen that by adjusting the ignition time of different air guns, the main pulse formation time is consistent, thereby realizing in-phase superposition, and the sidelobe is relatively suppressed, and the corresponding spectrum becomes smoother.

[0040] In the embodiment 2, the four-bar air gun is arranged along the vertical direction, and the firing depths are 1000 m (50 cu.in), 1001 m (100 cu.in), 1002 m (150 cu.in) and 1003 m (200 cu.in), and the average depth is 1001.5 m. According to the simulation results, the implosion times of the four-bar air gun are 0.875 ms, 1.101 ms, 1.260 ms and 1.386 ms, the time delays caused by the differences in the firing depths are 1.0345 ms, 0.3448 ms, -0.3448 ms and -1.0345 ms, and the ignition times are 8.0905 ms, 8.5542 ms, 9.0848 ms and 9.6485 ms respectively, given the constant of 10 ms. Figure 6 The simulation results of the source wavelet corresponding to the firing mode described above are obtained. Through comparison, by calculating the implosion times of different air guns and the differences in the travel times corresponding to the depths, the ignition times of different air guns are adjusted, the main pulse formation times along the vertical direction are consistent, the in-phase superposition is realized, the side lobes are relatively suppressed, and the corresponding spectrum becomes smoother.

[0041] By using the differences in the implosion times of air guns with different capacities, the main pulse in-phase superposition of different capacities under simultaneous firing can also be realized by adjusting the depth relationship between the air guns. At this time, the air gun with the largest capacity and the longest implosion time needs to be placed at the deepest position, and the air guns with small capacities are placed at relatively shallow positions for firing. Taking 50 cu.in, 100 cu.in, 150 cu.in and 200 cu.in as examples, it is assumed that the 200 cu.in air gun is located at the depth of 1000 m, the implosion times of the other air guns at different depths are simulated, and the travel times caused by the depth differences are combined to obtain the firing depths of the 150 cu.in, 100 cu.in and 50 cu.in air guns, which are 999.82 m, 999.59 m and 999.26 m respectively.

[0042] The above-described embodiments only express several embodiments of the present application, and the description is relatively specific and detailed, but it cannot be understood as a limitation on the scope of the patent of the present application. For ordinary skilled persons in the art, without departing from the concept of the present application, a number of modifications and improvements can be made, which are all within the protection scope of the present application.

Claims

1. A method of suppressing negative pressure airgun source wavelet sidelobes using a combination of airgun volume, firing depth, and firing time, characterized by The method comprises the following steps: 1) The negative pressure air gun source is composed of multiple negative pressure air guns with different capacities, the implosion time of the different negative pressure air guns is calculated, and the relationship among the implosion time, the firing depth and the ignition time of the negative pressure air gun is obtained according to formula (1) as follows: where T fire (i) is the firing time of the negative pressure air gun i, i.e. the time at which the negative pressure air gun i is fired, H fire (i) is the depth of the firing of the negative pressure air gun i, H ave is the average of the firing depths of all the negative pressure air guns, H(i) is the firing depth of the negative pressure air gun i, T implosion (i) is the implosion time of the negative pressure air gun i, C is the speed of sound in water, and Constant is a given constant; 2) In the case of a given negative pressure air gun capacity, the implosion time of each negative pressure air gun is simulated, the firing depth and the ignition time of each negative pressure air gun are determined according to the construction environment and the air gun spacing arrangement requirements under the condition of satisfying formula (1), each negative pressure air gun is arranged according to the firing depth, and each negative pressure air gun is fired according to the obtained ignition time, so that the main pulses of the negative pressure air guns are in-phase superimposed along the vertical direction to suppress the negative pressure air gun source wavelet sidelobes.

2. The method of suppressing negative pressure airgun source wavelet sidelobes using airgun volume, shot depth, and firing time combinations of claim 1, wherein, The given constant Constant is a preset constant smaller than the shooting interval.

3. The method of suppressing negative pressure airgun source wavelet sidelobes using airgun volume, shot depth, and firing time combinations of claim 1, wherein, The implosion time of the negative pressure air gun is the length of time from the air gun firing to the formation of the main pulse, and the implosion time is obtained by solving the bubble oscillation equation after the given firing depth and the air gun capacity.

4. The method of suppressing negative pressure airgun source wavelet sidelobes using airgun volume, shot depth, and firing time combinations of claim 1, wherein, In step 1), the implosion time of each negative pressure air gun can be adjusted by changing the capacity of each negative pressure air gun.

5. The method of suppressing negative pressure airgun source wavelet sidelobes using airgun volume, shot depth, and firing time combinations of claim 1, wherein, By changing the firing depth of each negative pressure air gun to adjust the corresponding travel time difference of the firing depth 6. The method of suppressing negative pressure airgun source wavelet sidelobes using airgun volume, shot depth, and firing time combinations of claim 1, wherein, In step 2), each negative pressure air gun is arranged in the horizontal plane at the same depth, that is, the firing depth is the same, the implosion time difference of each negative pressure air gun is obtained by simulation, the ignition time of each negative pressure air gun is calculated according to formula (1), and each negative pressure air gun is fired according to the obtained ignition time to suppress the negative pressure air gun source wavelet sidelobes.

7. The method of suppressing negative pressure airgun source wavelet sidelobes using airgun volume, shot depth, and firing time combinations of claim 1, wherein, In the step 2), each negative pressure air gun is set to be fired at the same time, that is, the firing time T fire (i) is the same; the difference in implosion time of each negative pressure air gun is obtained by simulation, the firing depth of each negative pressure air gun is calculated according to formula (1), each negative pressure air gun is arranged according to the firing depth, and each negative pressure air gun is fired at the same time to suppress the sidelobe of the negative pressure air gun wavelet.

8. The method of suppressing negative pressure airgun source wavelet sidelobes using airgun volume, shot depth, and firing time combinations of claim 1, wherein, Multiple negative pressure air guns are selected, each negative pressure air gun is arranged along the vertical direction, the time delay caused by the implosion time difference and the firing depth difference of each negative pressure air gun is calculated, the ignition time of each negative pressure air gun is calculated according to formula (1), and each negative pressure air gun is fired according to the obtained ignition time to suppress the negative pressure air gun source wavelet sidelobes.

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