A method for obtaining far-field source wavelets using direct ocean earthquake waves in time domain

Abstract

The present invention relates to a method for obtaining far-field source wavelets by using direct ocean earthquake waves in a time domain. The method comprises the following steps: calculating the arrival time difference between the direct wave and the source ghost wave according to collected data; time-shifting the direct wave, wherein the time shift amount is the arrival time difference between the direct wave and the source ghost wave of an adjacent channel; adding the time-shifted direct ocean earthquake wave to the adjacent channel in the time domain to obtain a new offset direct ocean earthquake wave; obtaining the far-field wavelet by cumulative calculation, and repeating the time shifting and adding of the direct ocean earthquake waves of the remaining channels until the time difference of the combined direct ocean earthquake wave with a new offset meets the time difference condition at zero offset, thereby obtaining the far-field wavelet. The method obtains the far-field source wavelet by performing time-shifting and superposition on the direct waves of source ghost waves with different offsets in the time domain. The whole process does not involve the calculation of filtering factors and is simple and stable.

Description

Technical Field

[0001] The present invention relates to the field of marine geological monitoring, and in particular to a method for obtaining far-field source wavelets by utilizing direct ocean earthquake waves in the time domain. Background Art

[0002] Seismic source wavelets, often called "far-field wavelets," are essential input for seismic data signal processing. Accurate wavelets can easily achieve many signal processing goals. However, accurate source wavelets are often difficult to obtain, making the extraction and improvement of far-field wavelet estimation crucial for seismic signal processing.

[0003] In marine seismic exploration, direct waves are waveforms that propagate directly from the earthquake source to the receiving cable and are recorded. They carry no information about subsurface interfaces. Because they have the shortest propagation path, direct waves often appear as the first arrivals on most seismic traces in deep-ocean reflection exploration. They are unaffected by subsequent waves such as reflections, refractions, and surface waves, making them relatively easy to identify and crucial for extracting far-field wavelet information.

[0004] Direct waves in marine seismic exploration are also superimposed with source ghost waves. Unlike land-based seismic exploration, marine seismic exploration involves both the source and receiving cables being lowered at a certain depth below the water surface. After seismic waves are excited, in addition to the direct wave that propagates directly from the source to the hydrophone on the receiving cable, there are also waves that propagate upward from the source, reflect off the water surface, and then propagate to the hydrophone. These "source ghost waves" accompany the direct wave.

[0005] The depth of the source and receiving cables is relatively small relative to the total propagation path of the seismic waves. Therefore, the arrival time of the source ghost wave is very close to that of the direct wave. The source ghost wave reflects off the water surface, where the reflection coefficient of seismic waves is typically -1. Therefore, the source ghost wave is typically characterized on the seismic trace by appearing in the opposite direction of the sign a short time interval after the direct wave. The arrival time difference between the direct wave and the source ghost wave on traces at different offsets (the horizontal distance between the source and the hydrophone) is different, making it difficult to distinguish in the time domain.

[0006] Currently, the main methods for obtaining far-field wavelets are: direct on-site observation; theoretical simulation; and extraction from reflection seismic data. These methods all have shortcomings: seismic data acquisition is affected by factors such as the on-site environment and instrumentation, making direct observation of source wavelets difficult; theoretical wavelet simulation methods are limited by theoretical models, source type, and environmental parameters, resulting in differences between simulated and actual wavelets; and reflection seismic data contain stratigraphic information, which can affect far-field wavelet extraction results.

[0007] In 2019, Li Fuyuan et al. published an article in the Journal of Petroleum Geophysical Exploration titled "Extracting Source Wavelets from Direct Waves in Marine Seismic Data." Leveraging the advantage of direct waves, which contain no information about subsurface formations, they developed a method for extracting far-field wavelets from direct waves in seismic data. They derived a relationship between direct waves and source signals, and derived an analytical formula in the frequency domain for calculating the far-field wavelets of source signals from direct waves. However, this method has a complex theoretical basis, requiring consideration of the combined effects of the source and receiving systems. Determining the wavelets requires solving a system of equations, making the algorithm complex and difficult to implement in programming. Therefore, a simple method for extracting far-field wavelets from direct waves currently exists. Summary of the Invention

[0008] The purpose of the present invention is to address the above shortcomings and provide a method for obtaining far-field source wavelets using direct waves of marine earthquakes in the time domain. This method obtains far-field source wavelets by time-shifting and superimposing the direct waves of source ghost waves in different offset bands in the time domain.

[0009] The solution adopted by the present invention to solve the technical problem is a method for obtaining far-field source wavelets using direct ocean earthquake waves in the time domain, comprising the following steps:

[0010] S1: Based on the collected data, select at least two detection points, starting with the detection point with the smallest offset distance, and number the data collected at each detection point one by one in increasing order of offset distance, starting from the first track; each detection point collects and records a direct wave data and a source ghost wave data;

[0011] S2: Based on the collected data, take the direct wave and source ghost wave data at offset 0 and calculate the zero offset arrival time difference Δt0; calculate the arrival time difference of the direct wave and source ghost wave detected at the same detection point, Δt n represents the time difference of the nth track, where n is an integer not less than 1;

[0012] S3: Accumulate and calculate far-field wavelets;

[0013] Starting from the first direct wave, the second direct wave is time-shifted by Δt1 to form a first time-shifted direct wave. The first direct wave and the first time-shifted direct wave are added in the time domain to form a first combined direct wave with a smaller offset. The time difference corresponding to the combined direct wave is Δt1'=Δt2+Δt1. The third direct wave is time-shifted by Δt1' to form a second time-shifted direct wave. The first combined direct wave and the second time-shifted direct wave are added in the time domain to form a second combined direct wave with a smaller offset. The time difference corresponding to the combined direct wave is Δt'2=Δt3+Δt1'.

[0014] Similarly, the time difference between the direct wave and the ghost wave corresponding to the nth combined direct wave is Δt' n =Δtn+1 +Δt' n-1 , where Δt'0 is taken as Δt1;

[0015] After each accumulation forms a new combined direct wave, the time difference corresponding to the combined direct wave is compared with the arrival time difference of the direct wave and the source ghost wave at zero offset:

[0016] If the n-1th combined direct wave and the nth time-shifted direct wave are added in the time domain to form the nth combined direct wave with a smaller offset, then Δt' n ≈Δt0, it is considered that the far-field wavelet is obtained.

[0017] Furthermore, before step S1, the observation system parameters used to simulate marine seismic exploration need to be set, including the source sinking depth d s , the depth of the detection point d g , offset distance x, sound wave velocity in water v, water surface reflection coefficient r;

[0018] The arrival time of the direct wave signal that is excited by the earthquake source and propagates directly to the detection point is

[0019]

[0020] The arrival time of the signal of the source ghost wave after it is excited by the source and reflected by the water surface and propagates to the detection point is

[0021]

[0022] The arrival time difference between the direct wave and the source ghost wave is

[0023] Δt=t'-t.

[0024] A terminal includes a memory, a processor, and a computer program stored in the memory and executable on the processor. When the processor executes the computer program, a method for obtaining far-field source wavelets by utilizing direct ocean earthquake waves in the time domain is implemented.

[0025] Compared with the prior art, the present invention has the following beneficial effects: the far-field wavelet at zero offset (or near offset) can be regarded as a combination of a series of direct waves at different offsets, and the far-field source wavelet can be obtained by time-shifting and superimposing the direct waves of source ghost waves at different offsets in the time domain. The entire process does not involve the calculation of filtering factors and is simple and stable. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The present invention is further described below with reference to the accompanying drawings.

[0027] Figure 1Schematic diagram of the method for finding a new offset direct wave for two non-zero offset direct waves; (a) is the reflection coefficient sequence h1(t) and the synthetic seismic record x1(t) when the time difference between the direct wave and the source ghost wave is Δt1; (b) is the reflection coefficient sequence h2(t) and the synthetic seismic record x2(t) after the direct wave and the source ghost wave have a time difference of Δt2 and a time shift of Δt1; (c) h3(t) is the result of adding h1(t) and h2(t), and x3(t) is the result of adding x1(t) and x2(t); (d) is the reflection coefficient sequence h(t) and the synthetic seismic record x(t) when the time difference between the direct wave and the source ghost wave is Δt1+Δt2.

[0028] Figure 2 Flowchart for obtaining far-field wavelets using direct ocean earthquake waves in the time domain.

[0029] Figure 3 Schematic diagram of the propagation paths of direct waves and source ghost waves.

[0030] Figure 4 Synthesized direct-arrival wave records of marine earthquakes at different offsets.

[0031] Figure 5 It is the result of time-shifting and accumulating the direct wave.

[0032] Figure 6 It is the amplitude spectrum of the direct wave of the marine earthquake after time shift accumulation. DETAILED DESCRIPTION

[0033] The following will be combined with the accompanying drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. It should be understood that the specific embodiments described here are only used to explain the present invention and are not used to limit the present invention.

[0034] Example 1:

[0035] Example 1 provides a method for obtaining far-field source wavelets by using direct ocean earthquake waves in the inter-domain.

[0036] The principle of this method is as follows Figure 1 As shown in Figure 1, the direct wave record of an ocean earthquake contains source ghost waves and direct waves. The time difference between the two is referred to as "time difference" below. The water surface reflection coefficient is approximately -1, which is taken as -1 here.

[0037] Assume that the time difference of the direct arrival wave of the ocean earthquake at offset 1 is Δt1 (e.g. Figure 1 a), its reflection coefficient sequence is recorded as h1(t), and the direct wave convolved with the wavelet w(t) is recorded as x1(t);

[0038] Similarly, let the time difference of the direct wave of the ocean earthquake with an offset of offset2 be Δt2, and its reflection coefficient sequence is time-shifted by Δt1 and recorded as h2(t), and the direct wave convolved with the wavelet w(t) is recorded as x2(t) (as shown in Figure 1 b);

[0039] Will Figure 1 a and Figure 1 The reflection coefficient sequence and synthetic record of b are added in the time domain, and the direct wave reflection coefficient sequence h3(t) and synthetic record x3(t) with a time difference of Δt1+Δt2 can be obtained (e.g. Figure 1 c);

[0040] The synthetic record x(t) is obtained by directly convolving the reflection coefficient sequence h(t) with the time difference of Δt1+Δt2 and the seismic wavelet w(t). By comparison, it is found that the synthetic record x3(t) is basically consistent with x(t).

[0041] This shows that:

[0042] First, for any two non-zero offset direct ocean earthquake waves, one with a time difference of Δt and the other with a time shift of Δt, the two direct ocean earthquake waves can be added together in the time domain to form a direct ocean earthquake wave with a smaller offset.

[0043] Second, the zero-offset (or near-offset) direct wave can be regarded as a far-field wavelet, which can be composed of a series of direct waves with different offsets in the above manner.

[0044] The calculation steps of this method are as follows Figure 2 As shown, it includes the following steps:

[0045] A method for obtaining far-field source wavelets using direct ocean earthquake waves in the time domain comprises the following steps:

[0046] S1: Based on the collected data, select at least two detection points, starting with the detection point with the smallest offset distance, and number the data collected at each detection point one by one in increasing order of offset distance, starting from the first track; each detection point collects and records a direct wave data and a source ghost wave data;

[0047] S2: Based on the collected data, take the direct wave and source ghost wave data at offset 0 and calculate the zero offset arrival time difference Δt0; calculate the arrival time difference of the direct wave and source ghost wave detected at the same detection point, Δt n represents the time difference of the nth track, where n is an integer not less than 1;

[0048] S3: Accumulate and calculate far-field wavelets;

[0049] Starting from the first direct wave, the second direct wave is time-shifted by Δt1 to form a first time-shifted direct wave. The first direct wave and the first time-shifted direct wave are added in the time domain to form a first combined direct wave with a smaller offset. The time difference corresponding to the combined direct wave is Δt1'=Δt2+Δt1. The third direct wave is time-shifted by Δt1' to form a second time-shifted direct wave. The first combined direct wave and the second time-shifted direct wave are added in the time domain to form a second combined direct wave with a smaller offset. The time difference corresponding to the combined direct wave is Δt'2=Δt3+Δt1'.

[0050] Similarly, the time difference between the direct wave and the ghost wave corresponding to the nth combined direct wave is Δt' n =Δt n+1 +Δt' n-1 , where Δt'0 is taken as Δt1;

[0051] After each accumulation forms a new combined direct wave, the time difference corresponding to the combined direct wave is compared with the arrival time difference of the direct wave and the source ghost wave at zero offset:

[0052] If the n-1th combined direct wave and the nth time-shifted direct wave are added in the time domain to form the nth combined direct wave with a smaller offset, then Δt' n ≈Δt0, it is considered that the far-field wavelet is obtained.

[0053] Before step S1, the observation system parameters used to simulate marine seismic exploration need to be set, such as Figure 3 As shown in the figure, the reflected wave of the earthquake source energy on the water surface is called the earthquake source ghost wave, which can be regarded as a direct wave emitted from the virtual earthquake source (i.e., the mirror image point of the earthquake source point on the water surface). The parameters include the earthquake source sinking depth d s , the depth of the detection point d g , offset distance x, sound wave velocity in water v, water surface reflection coefficient r;

[0054] The arrival time of the direct wave signal that is excited by the earthquake source and propagates directly to the detection point is

[0055]

[0056] The arrival time of the signal of the source ghost wave after it is excited by the source and reflected by the water surface and propagates to the detection point is

[0057]

[0058] The arrival time difference between the direct wave and the source ghost wave is

[0059] Δt=t'-t.

[0060] In the above formula, the smaller the offset x, the larger the time difference Δt. Assuming there are n offsets (where n is an integer not less than 2), which are recorded as offset0, offset1, offset2, ..., offsetn respectively; the time differences of the direct arrival waves of the marine earthquake at all offsets (including zero offset) are calculated by the above formula and recorded as Δt0, Δt1, Δt2, ..., Δt n ;

[0061] Example 2:

[0062] To further verify the effectiveness of this method, a set of offset distances (offset distance is 175-338m, and the detection point spacing is 12-13m) is obtained from actual marine seismic data, and the time difference of each seismic trace is calculated.

[0063] The Ricker wavelet with a main frequency of 35 Hz was selected to synthesize the direct wave records of marine earthquakes with different offsets (such as Figure 4 ), where the zero-offset marine seismic direct wave ( Figure 4 Track 15) is used as an ideal far-field source wavelet for comparison and reference.

[0064] Then the synthesized direct ocean earthquake waves are time-shifted and superimposed to form new offset direct ocean earthquake waves.

[0065] like Figure 4 As shown, from the first direct wave of ocean earthquake ( Figure 4 Start with the first one in Figure 4 The second direct wave of the ocean earthquake is time-shifted by the time difference of the first wave. The first wave and the time-shifted second wave are added in the time domain to form a new offset direct wave. The result is Figure 5 The second earthquake record in .

[0066] On this basis, we will continue to Figure 4 The third direct ocean earthquake wave in the time shift is Figure 5 The time difference of the second track is calculated by comparing the time-shifted third track with Figure 5 The second trace in the time domain is added, and the result is Figure 5 The third direct ocean earthquake wave.

[0067] The far-field wavelet is calculated by accumulating and so on until all 14 seismic records are time-shifted and superimposed. The far-field source wavelet ( Figure 5 14) and the synthetic far-field source wavelet ( Figure 5 15) comparison, the difference between the two ( Figure 5 Track 16) is close to zero, and the waveform is consistent.

[0068] Final calculation Figure 4 The amplitude spectrum of each channel (such as Figure 6 ), and continue to examine the consistency between the two from the frequency domain.

[0069] It can be seen from the figure that the amplitude increases after each time-shift superposition, and the main frequency moves to the low frequency. The final superimposed far-field source wavelet ( Figure 6 14) and the synthetic far-field source wavelet amplitude spectrum ( Figure 6 15) are consistent, and the difference between the two ( Figure 6 Lane 16) is close to zero.

[0070] Example 3:

[0071] Example 3 provides a terminal comprising a memory, a processor, and a computer program stored in the memory and executable on the processor. When the processor executes the computer program, a method for obtaining far-field source sub-waves by using direct waves of ocean earthquakes in the time domain is implemented.

[0072] The preferred embodiments listed above further illustrate the objectives, technical solutions and advantages of the present invention in detail. It should be understood that the above are only preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for obtaining far-field source wavelets using direct ocean earthquake waves in the time domain, characterized in that: The following steps are involved: S1: Based on the collected data, select at least two detection points, starting with the detection point with the smallest offset distance, and number the data collected at each detection point one by one in increasing order of offset distance, starting from the first track; each detection point collects and records a direct wave data and a source ghost wave data; S2: Based on the collected data, take the direct wave and source ghost wave data at offset 0, calculate the arrival time difference Δt0 between the direct wave and the source ghost wave at zero offset; calculate the arrival time difference Δt0 between the direct wave and the source ghost wave detected at the same detection point. n represents the time difference of the nth track, where n is an integer not less than 1; S3: Accumulate and calculate far-field wavelets; Starting from the first direct wave, the second direct wave is time-shifted by Δt1 to form a first time-shifted direct wave. The first direct wave and the first time-shifted direct wave are added in the time domain to form a first combined direct wave with a smaller offset. The time difference corresponding to the combined direct wave is Δt1'=Δt2+Δt1. The third direct wave is time-shifted by Δt1' to form a second time-shifted direct wave. The first combined direct wave and the second time-shifted direct wave are added in the time domain to form a second combined direct wave with a smaller offset. The time difference corresponding to the combined direct wave is Δt'2=Δt3+Δt′1. Similarly, the time difference between the direct wave and the ghost wave corresponding to the nth combined direct wave is Δt' n =Δt n+1 +Δt' n-1 , where Δt'0 is taken as Δt1; After each accumulation forms a new combined direct wave, the time difference corresponding to the combined direct wave is compared with the arrival time difference of the direct wave and the source ghost wave at zero offset: If the n-1th combined direct wave and the nth time-shifted direct wave are added in the time domain to form the nth combined direct wave with a smaller offset, then Δt' n ≈Δt0, it is considered that the far-field wavelet is obtained.

2. The method for obtaining far-field source wavelets using direct ocean earthquake waves in the time domain according to claim 1, characterized in that: Before step S1, the observation system parameters used to simulate marine seismic exploration need to be set, including the source sinking depth d s , the depth of the detection point d g , offset distance x, sound wave velocity in water v, water surface reflection coefficient r; The arrival time of the direct wave signal that is excited by the earthquake source and propagates directly to the detection point is The arrival time of the signal of the source ghost wave after it is excited by the source and reflected by the water surface and propagates to the detection point is The arrival time difference between the direct wave and the source ghost wave is Δt=t'-t.

3. The method for obtaining far-field source wavelets using direct ocean earthquake waves in the time domain according to claim 1, characterized in that: In step S3, Δt' n The error range of ≈Δt0 is ±5%×Δt0.

4. A terminal comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein: When the processor executes the computer program, the method for obtaining far-field source wavelets by using direct ocean earthquake waves in the time domain as described in claim 1 or 2 is implemented.

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