Simulation method for determining acquisition effect of different sinking depths of shallow cable

Abstract

The invention belongs to the technical field of marine seismic data acquisition, and particularly relates to a simulation method for determining the acquisition effect of different sinking depths of a shallow-section cable, which comprises the following steps of: acquiring ghost-free shallow-section data, and performing cable ghost reflection simulation on the ghost-free shallow-section data to obtain a synthetic seismic record; according to the scheme, the method is innovative in thought, a result comparable with an actual acquisition effect can be visually displayed, and the immersion depth of the acquired cable can be visually determined; compared with the problems that the same measuring line needs to be repeatedly experimented in traditional different cable sinking depth experiments and the cable sinking depth is difficult to determine, the method only needs to make a ghost reflection-free collecting measuring line once, is simple and convenient to implement, effectively reduces the labor working cost, is controllable in depth, is higher in simulation precision, and is suitable for large-scale popularization and application. The result is more accurate, actual production can be effectively guided, and the method has high actual application and popularization value.

Description

Technical Field

[0001] The invention belongs to the technical field of marine seismic data acquisition, and in particular relates to a simulation method for determining acquisition effects of shallow-profile cables at different sinking depths. Background Art

[0002] Shallow profiling plays an important role in marine engineering construction, including being used for geological stratification and sedimentary layer research, which can clearly divide the stratigraphic structure and analyze the sedimentary environment; being used for obstacle detection and project site selection, which can locate seabed obstacles and evaluate the pros and cons of site selection; and being able to monitor seabed erosion and siltation, provide data for channel dredging, and evaluate the effectiveness of coastal protection projects. In shallow profiling acquisition, different cable laying depths have a great impact on the acquisition effect. When the cable floats on the sea surface, it is more susceptible to environmental noise, which reduces the signal-to-noise ratio. When the cable is sunk below the sea surface, cable virtual reflections will appear and affect the subsequent interpretation.

[0003] The depth of shallow-profile cable sinking has always been a difficult problem in the shallow-profile collection process. In reality, field tests are generally used to determine the final collection parameters by comparing the collection effects of different cable sinking depths. The cable sinking depth is generally achieved by increasing or decreasing the counterweight (usually lead). This field experiment method is costly, and the shallow-profile profile corresponding to different cable sinking depths is sometimes difficult to determine. As a result, the relationship between the shallow-profile cable sinking depth and the actual collection effect is still unclear, that is, it is impossible to answer the best collection effect within the range of cable sinking depths.

[0004] Therefore, it is urgent to propose a simulation method to determine the shallow-section collection effect of shallow-section cables at different sinking depths, so as to clarify the simulation effect at different cable sinking depths, and to intuitively display the results comparable to the actual collection effect, which is conducive to intuitively determining the sinking depth of the collection cable. Summary of the invention

[0005] In order to solve the problems of high cost and unsatisfactory effect in the traditional field test method of determining acquisition parameters by comparing the acquisition effects at different cable laying depths, the present invention proposes a simulation method for determining the acquisition effects of shallow-section cables at different laying depths. By simulating cable ghost reflections on seismic data without ghost reflections, synthetic seismic records are obtained, thereby realizing the simulation of acquisition effects at different cable laying depths.

[0006] The present invention is implemented by adopting the following technical scheme: a simulation method for determining the acquisition effect of shallow-profile cables at different sinking depths, comprising the following steps:

[0007] Step 1, collecting shallow profile data without ghost reflection in the field, and preprocessing the data to obtain seismic records without ghost reflection;

[0008] Step 2: Combine the seismic record without ghost reflection obtained in step 1 to simulate the cable ghost reflection, synthesize the cable ghost reflection, and use M(t) to represent the synthetic seismic record, then:

[0009] M(t)=S(t)+RS(t-Δt)

[0010] Wherein, S(t) is the seismic record without ghost reflection, R is the reflection coefficient, Δt is the ghost reflection delay time, t is the reflection time, and S(t-Δt) is the seismic record without ghost reflection with S(t) delayed by Δt;

[0011] The reflection coefficient R is expressed as follows:

[0012]

[0013] Among them, ρ2 is the air density, ρ1 is the seawater density, v2 is the speed of sound wave propagation in air, v1 is the speed of sound wave propagation in seawater, and θ is the incident angle of virtual reflection;

[0014] Step 3: Determine the optimal cable laying depth based on the synthetic seismic record profile and monitor the laying depth during the acquisition process.

[0015] Furthermore, in the step 1, when collecting shallow profile data without ghost reflection in the field, no counterweight is added, and the cable is floated on the sea surface to obtain a shallow profile without ghost reflection of the cable.

[0016] Furthermore, in step 3, the preferred cable laying depth range is 30cm-70cm.

[0017] Compared with the prior art, the advantages and positive effects of the present invention are:

[0018] This scheme collects shallow profile data without ghost reflections and simulates the ghost reflections of cables on the ghost reflection-free seismic data to obtain synthetic seismic records to clarify the simulation effects at different cable sinking depths. This scheme is innovative in its method and can intuitively display results comparable to the actual acquisition effects, which is conducive to intuitively determining the sinking depth of the acquisition cable; compared with the traditional experiment of different cable sinking depths, which requires repeated experiments on the same survey line and the problem that the cable sinking depth is difficult to determine, this method only requires one ghost reflection-free acquisition survey line, which is simple and convenient to implement, and effectively reduces the labor cost. At the same time, the depth of the simulation method is controllable, the simulation accuracy is higher, and the results are more accurate, which can effectively guide actual production. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 This is a schematic diagram of the simulation method flow in an embodiment of the present invention;

[0020] Figure 2 It is a schematic diagram of a shallow cross-section simulated at different sinking depths according to an embodiment of the present invention. DETAILED DESCRIPTION

[0021] In order to more clearly understand the above-mentioned purpose, features and advantages of the present invention, the present invention is further described below in conjunction with the accompanying drawings and embodiments. In the following description, many specific details are set forth to facilitate a full understanding of the present invention, but the present invention can also be implemented in other ways different from those described herein, and therefore, the present invention is not limited to the specific embodiments disclosed below.

[0022] A simulation method for determining the acquisition effect of shallow-profile cables at different sinking depths, such as Figure 1 As shown, the following steps are included:

[0023] Step 1: Collect shallow profile data without false reflection in the field and perform data preprocessing;

[0024] When shallowly profiling the cable in the field, no weight is added. Instead, a float can be added to the front or tail of the cable to float the cable on the sea surface, thus obtaining a shallow profile without cable ghost reflections.

[0025] During data preprocessing, conventional shallow profile data processing methods are used, including filtering, spherical diffusion correction, surge correction, noise removal and other operations. The filtering range is 100-2000Hz. The speed of spherical diffusion can be the seawater speed, that is, 1500m / s. Surge correction only needs to be processed specifically when the sea surface fluctuates greatly. During processing, a simple arithmetic average is required according to the surge cycle. If the sea surface is relatively calm during acquisition, no processing is required. Noise removal is generally used to remove abnormally large amplitudes.

[0026] Step 2: Synthesize cable ghost reflection

[0027] Let M(t) represent the synthetic seismic record, then

[0028] M(t)=S(t)+RS(t-Δt) (1)

[0029] Among them, S(t) is the seismic record without ghost reflection, R is the reflection coefficient, and Δt is the ghost reflection delay time.

[0030] The calculation formula of reflection coefficient R is:

[0031]

[0032] Among them, ρ2 is the air density, ρ1 is the seawater density, v2 is the speed of sound wave propagation in air, v1 is the speed of sound wave propagation in seawater, and θ is the incident angle of virtual reflection. According to the general observation system setting of shallow profile acquisition, the gun offset is 5m. In this case, the incident angle of virtual reflection at different detection depths is shown in Table 1.

[0033] Table 1. Incident angle of ghost reflection at different detection depths

[0034] Detection depth (m) Virtual reflection incident angle (°) Detection depth (m) Virtual reflection incident angle (°) 5 26.56505 10 14.03624 15 9.462322 20 7.125016 25 5.710593 30 4.763642 40 3.576334 50 2.862405 60 2.385944 70 2.045408 80 1.789911 90 1.59114 100 1.432096 120 1.193489 140 1.02303 160 0.895174 180 0.795724 200 0.71616

[0035] When the maximum detection depth is 200m, the incident angle of the virtual reflection is 0.71616°.

[0036]

[0037] Its value is greater than 1, indicating that total reflection occurs at this time, and the reflection coefficient R = -1. When the detection depth becomes shallower, the incident angle of the virtual reflection increases, and total reflection still occurs, and the reflection coefficient R is still -1. Even if it is vertically incident, that is, the incident angle of the virtual reflection is 0°, the reflection coefficient R = -0.9994, which is close to total reflection. Therefore, for shallow profile surveys, it is feasible to use a reflection coefficient of -1. This is also the reason why when the cable is buried at a deep depth and the cable virtual reflection is separated from the effective wave, the cable virtual reflection energy observed during actual collection is basically the same as the effective wave energy.

[0038] Formula 1 is used to simulate the cable ghost reflection of the shallow profile data collected in step 1, and the synthetic record profiles at different sinking depths are simulated (Table 2). The simulation results are uniformly compared to obtain the optimal sinking depth according to the needs of the interpreters.

[0039] Table 2 Correspondence between different cable laying depths and delay times

[0040] Serial number Cable laying depth (cm) Simple delay time (ms) Offset(m) 1 15 0.2 5 2 30 0.4 5 3 45 0.6 5 4 60 0.8 5 5 75 1 5 6 150 2 5 7 225 3 5

[0041] Step 3: Determine the optimal cable laying depth and monitor the laying depth during the acquisition process

[0042] If the cable is laid too shallowly, it will be affected by wind and wave noise and affect the collection results. However, if the cable is laid too deep, the virtual reflection will reduce the resolution of the profile. According to the comparison of actual simulation results and field collection experience, the cable laying depth cannot be less than 15cm and not more than 1m. 30-70cm is the most suitable laying range for cable laying, and 50cm laying depth is preferred for the best overall effect.

[0043] Add weight to the shallow profile cable, and ensure that it is sunk to a depth of 50 cm during the acquisition process through experiments. Pay special attention to the changes in the acquisition effect, especially the changes in the seabed waveform, and compare the acquisition results with the simulated profile. If it is determined that the seabed waveform becomes wider, or even blurred and obvious virtual reflections appear, it indicates that the cable tension is often reduced due to downstream conditions, causing the cable to sink to a deeper position. At this time, it is necessary to increase the ship speed or reduce the cable weight to return the cable to a depth of 50 cm. If the seabed waveform becomes narrower and the noise becomes louder, it indicates that the cable tension is often increased due to upstream conditions, causing the cable to sink to a shallower position. At this time, it is necessary to reduce the ship speed or increase the cable weight to return the cable to a depth of 50 cm.

[0044] Actual Cases

[0045] In 2023, when collecting shallow seismic profiles in a certain sea area, the cable was floated on the sea surface using a buoy. At this time, a 2km profile without cable ghost reflection was obtained. The profile was filtered, spherical diffusion compensated, and noise removed. Figure 2 The original profile is shown. Using formula 1, the profiles of different sinking depths are simulated according to the requirements of Table 2. Figure 2 shown.

[0046] It can be seen that when the cable is sunk to a depth of 15 cm, it is affected by the cable ghost reflection, and its wavelet morphology is significantly deteriorated. When it is sunk to a depth of 30 cm-75 cm, it has little effect on the later interpretation as a whole, and the cable ghost reflection is superimposed on the original wavelet, which enhances its wavelet energy, thus facilitating seismic interpretation. When sunk to a depth of 150 cm, the cable ghost reflection has a significant destructive effect on the wavelet waveform, and the overall effect is poor. At 225 cm, the ghost reflection has been completely separated from the effective wave, and the stratigraphic sequence of the profile is difficult to accurately identify. The overall effect is poor and can no longer meet the needs of seismic interpretation. Therefore, it is reasonable to fix the cable sinking depth at 50 cm during acquisition. Its advantages are that it can avoid the influence of wind and waves, and the cable ghost reflection can enhance the reflection energy of the profile. At the same time, there is enough redundant space to basically maintain a good acquisition effect within the range of 20 cm up and down, and the sinking depth is not large, so it is relatively easy to implement.

[0047] The above description is only a preferred embodiment of the present invention and does not limit the present invention in other forms. Any technician familiar with the profession may use the technical content disclosed above to change or modify it into an equivalent embodiment with equivalent changes and apply it to other fields. However, any simple modification, equivalent change and modification made to the above embodiment based on the technical essence of the present invention without departing from the content of the technical solution of the present invention still falls within the protection scope of the technical solution of the present invention.

Claims

1. A simulation method for determining the acquisition effect of shallow-section cables at different sinking depths, characterized in that: The following steps are involved: Step 1, collecting field shallow profile data without ghost reflection, and preprocessing the data to obtain seismic records without ghost reflection; Step 2: Combine the seismic record without ghost reflection obtained in step 1 to simulate the cable ghost reflection, synthesize the cable ghost reflection, and use M(t) to represent the synthetic seismic record, then: M(t)=S(t)+RS(t-Δt) Wherein, S(t) is the seismic record without ghost reflection, R is the reflection coefficient, Δt is the ghost reflection delay time, t is the reflection time, and S(t-Δt) is the seismic record without ghost reflection with S(t) delayed by Δt; Step 3: Determine the optimal cable laying depth based on the synthetic seismic record profile and monitor the laying depth during the acquisition process.

2. The simulation method for determining the acquisition effect of shallow-section cables at different sinking depths according to claim 1 is characterized in that: In step 2, the reflection coefficient R is expressed as follows: Among them, ρ2 is the air density, ρ1 is the seawater density, v2 is the speed of sound wave propagation in air, v1 is the speed of sound wave propagation in seawater, and θ is the incident angle of virtual reflection.

3. The simulation method for determining the acquisition effect of shallow-section cables at different sinking depths according to claim 1 is characterized in that: The data preprocessing operations in step 1 include filtering, spherical diffusion correction, surge correction and noise removal.

4. The simulation method for determining the acquisition effect of shallow-section cables at different sinking depths according to claim 1 is characterized in that: In the step 1, when collecting shallow profile data without ghost reflection in the field, no counterweight is added, and the cable is floated on the sea surface to obtain a shallow profile without cable ghost reflection.

5. The simulation method for determining the acquisition effect of shallow-section cables at different sinking depths according to claim 1 is characterized in that: In step 3, the cable laying depth range is preferably 30 cm-70 cm.

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