CN111323810A - Marine seismic detection system with seismic source below towing cable and method thereof - Google Patents

Abstract

The invention discloses a marine seismic detection system with a seismic source located below a streamer and a method thereof, belonging to the field of marine seismic exploration. The system includes a mother ship, a seismic source, an optoelectronic composite cable and an independent combined towing cable device; the mother ship is connected to the seismic source through the optoelectronic composite cable; the independent combined towing cable device is fixed on the optoelectronic composite cable through a hoop; the independent combined towing cable device includes power supply and acquisition unit, multi-channel streamer and resistance device; firstly, lay the seismic source according to the exploration target, then fix the depth of the multi-channel streamer by adjusting the position of the hoop on the photoelectric composite cable, and finally adjust the relationship between the hoop and the power supply and acquisition unit. The distance between them is such that the source is located directly below the middle of the multiple streamers. On the one hand, the present invention can record the near (zero) offset which is crucial to the imaging of the shallow structure of the seabed, and on the other hand, it can obtain the high-resolution seismic data of the shallow layer which is not affected by the ghost wave, so that the shallow seabed can be effectively improved. Imaging effect of layer structure.

Description

A marine seismic detection system and method with the source located under the streamer

technical field

The invention belongs to the field of marine seismic exploration, and in particular relates to a marine seismic detection system and a method with a seismic source located below a streamer.

Background technique

The existing offshore multi-channel seismic detection system usually uses an air gun as the seismic source, and the streamer is located at a certain distance behind the source. The source is usually placed in the depth range of 5-20m underwater, and a single-depth or multi-depth combination mode can be used. The streamer is generally placed between 5-50m underwater, in a horizontal or inclined posture. This acquisition method can ensure the acquisition of mid-deep reflection signals, but it also results in the lack of near (zero) offset data, which reduces the imaging accuracy of shallow structures on the seafloor. At the same time, the existence of the strong seawater-air reflection interface introduces the ghost wave at the hypocenter and the ghost wave at the end of the hydrophone into the seismic records, resulting in the notch effect in the frequency domain and reducing the effective frequency bandwidth and stratigraphic resolution of the seismic data.

The seismic data acquired by the multi-channel seismic acquisition system needs to be dynamically corrected for the common reflection point gathers before stacking. The dynamic correction of different offset data can be expressed as follows:

Among them, t ₀ is the self-excitation and self-closing time (zero offset distance), x is the offset distance, and v _NMO is the dynamic correction speed.

The dynamic calibration process will cause different degrees of dynamic calibration stretch to the data, which is usually represented by frequency distortion:

where f is the dominant frequency and Δf is the frequency distortion amount.

The dynamic calibration stretch increases with the increase of water depth and offset distance. When stacking common reflection point gathers, it is necessary to excise the gathers with excessive stretch to ensure the stratigraphic resolution. Due to the lack of small offset data in the conventional acquisition method, the shallow seabed data, especially in the shallow water area, has too few effective stacking channels, thus affecting the subsequent imaging results.

In addition, the conventional air gun source and streamer are placed in shallow water depth. The source wavelet excited by the air gun is reflected by the sea surface to form a hypocenter ghost wave that follows the first wave. The first wave and the ghost wave are reflected by the underground reflection horizon and propagate upward to the streamer. , recorded by the hydrophone, the wave field continues to propagate upwards and is reflected again by the sea surface before reaching the streamer, forming a ghost wave at the end of the hydrophone. The existence of ghost waves complicates the originally simple source wavelets, and causes notches in the frequency domain, which seriously affects the spectral integrity of the wavelets and reduces the effective bandwidth of seismic data. Therefore, by reasonably designing the marine seismic acquisition system, increasing the number of effective stacking traces in shallow water areas, and obtaining shallow high-resolution seismic data that are not affected by ghost waves, it is of great significance for the imaging of shallow seabed structures.

SUMMARY OF THE INVENTION

The purpose of the present invention is to overcome the deficiencies of the prior art, in order to record the near (zero) offset which is crucial to the imaging of the shallow structure of the seabed, and at the same time obtain the shallow high-resolution seismic data that is not affected by ghost waves, To improve the imaging effect of the shallow structure of the seabed, the present invention provides a marine seismic detection system and a method with the seismic source located under the streamer.

The technical scheme of the present invention is as follows:

A marine seismic detection system with a seismic source located below a streamer, comprising a mother ship, a seismic source, a photoelectric composite cable and an independent combined streamer device; the mother ship is connected with the seismic source through the photoelectric composite cable; the independent composite streamer device It is fixed on the photoelectric composite cable through a hoop; the independent combined streamer device is located above the seismic source, and includes a power supply and collection unit, multiple streamers and a resistance device; one end of the power supply and collection unit can be adjusted through the distance The device is connected with the hoop, and the other end is connected with the multi-channel towing cables. The tail of the multi-channel towing cables is connected with a resistance device to help straighten the towing cables. The depth of the cable in the ocean.

Preferably, the independent combined streamer device further includes an auxiliary float, and the auxiliary float is arranged between the power supply and collection unit and the multiple streamers.

Preferably, the seismic source is located just below the middle position of the multiple streamers.

Preferably, a plurality of hydrophones are evenly arranged on the multi-channel streamers, and each hydrophone is connected to the power supply and collection unit through a data line.

Preferably, the multiple streamers present a horizontal attitude.

反之，调整多道拖缆深度

其中h是水深，d₂为震源和拖缆的深度差，c为水中声速。Preferably, when the two-way seismic travel time of the exploration target downward from the seabed is greater than or equal to (hd ₂ )/c, the seismic source is sunk closer to the seabed, and the depth of the multi-channel streamer is adjusted.

Conversely, adjust the depth of multiple streamers

where h is the water depth, d ₂ is the depth difference between the source and the streamer, and c is the speed of sound in water.

其中Δt_ext为地震子波延续时间长度，d₃为震源离海底的深度。Preferably, the arrangement of the seismic sources satisfies the

where Δt _ext is the duration of the seismic wavelet, and d ₃ is the depth of the source from the seafloor.

Preferably, the shock source is a transducer or an electric spark shock source.

The invention also discloses a marine seismic detection method of the system, comprising the following steps:

1) According to the exploration target, lay the seismic source and multiple streamers:

其中Δt_ext为地震子波延续时间长度， d₂为震源与多道拖缆深度差，d₃为震源离海底的深度，c为水中声速；First, make sure that the focal depth meets the

where Δt _ext is the duration of the seismic wavelet, d ₂ is the depth difference between the source and multiple streamers, d ₃ is the depth of the source from the seabed, and c is the speed of sound in water;

反之，调整多道拖缆深度

其中h是水深；通过调节抱箍在光电复合缆上的位置，固定多道拖缆的深度；最后调整抱箍与供电及采集单元之间的距离，使得震源位于多道拖缆中间位置的正下方；When the two-way seismic travel time of the exploration target from the bottom of the sea is ≥(hd ₂ )/c, the seismic source is sunk closer to the bottom of the sea, and the depth of the multi-channel streamer is adjusted.

Conversely, adjust the depth of multiple streamers

where h is the water depth; by adjusting the position of the hoop on the photoelectric composite cable, the depth of the multiple streamers is fixed; finally, the distance between the hoop and the power supply and acquisition unit is adjusted so that the source is located in the middle of the multiple streamers. below;

2) The source is excited, and after the seismic wavelet is reflected at the reflection horizon where the exploration target area is located, it propagates upward to the multi-channel streamers, and the hydrophones on the multi-channel streamers collect seismic signals; Power is supplied and the seismic data collected by each hydrophone is recorded.

The beneficial effects possessed by the present invention are:

1) In the marine seismic detection system of the present invention, the source of the earthquake is located below the streamer, and the streamer is powered by the power supply and acquisition unit and the seismic data is recorded. At the same time, the distance between the power supply and acquisition unit and the photoelectric composite cable can be adjusted. Under the premise of not changing the depth of the multi-channel streamer and the source depth, the source is located just below the middle position of the streamer. After the source is excited, the streamer can record the near (zero) offset that is crucial for imaging the shallow structure of the seabed. shift distance;

2) The traditional hypocenter and streamer are placed in shallow water depth, and due to the existence of seawater-air strong reflection interface, it is easy to introduce hypocenter ghost wave and hydrophone end ghost wave in the seismic record; By studying the relationship between the arrival time of different wave fields when the source and streamer are placed at different water depths, the depth of the source and streamer can be arranged according to the detection target, which has strong adaptability to detection targets at different depths below the seabed. ability; the present invention can acquire shallow high-resolution seismic data that is not affected by ghost waves, thereby effectively improving the imaging effect of the shallow structure of the seabed;

3) Compared with the traditional air gun as a seismic sound source, the present invention adopts a transducer or an electric spark source, and its main frequency is higher than that of the air gun, and can realize deep-sea operations, which is more suitable for high-resolution exploration, especially suitable for seabed. shallow structure detection;

4) The marine seismic detection system of the present invention has the advantages of low cost, simple structure, and convenient operation of the detection method.

Description of drawings

Figure 1 shows the relationship between the dynamic calibration stretch with different water depths (up to 50%);

Figure 2 shows the influence of the source ghost wave and the hydrophone ghost wave on the first wave signal in the time domain and frequency domain by taking the pulse wavelet as an example; among them, (a) time domain, (b) frequency domain;

3 is a schematic structural diagram of a marine seismic detection system with a seismic source located below a streamer according to an embodiment of the present invention;

Figure 4 is a comparison diagram of the arrival time (b) of different wave fields under the same depth difference (a) between the source and multiple streamers;

Fig. 5 is the curve of the variation law of the length of the stratum reflection time with the depth of the focal point that is not affected by the ghost wave;

Figure 6 is a comparison diagram of the arrival time (b) of different wave fields under different depth differences (a) between the source and multiple streamers;

In the picture: 1 seismic source, 2 photoelectric composite cables, 3 mother ship, 4 hoop, 5 power supply and acquisition unit, 6 auxiliary floats, 7 multi-channel towing cables, 8 resistance devices, d ₁ is the depth of multi-channel towing cables, d ₂ is the depth difference between the epicenter and the multi-channel streamer, and _d3 is the depth of the epicenter from the seabed.

Detailed ways

The present invention will be further described below with reference to the accompanying drawings.

Figure 1 shows the percentage of frequency distortion (up to 50% distortion) for different offsets at different water depths. Through comparison, it can be found that the dynamic correction stretching increases with the increase of water depth and offset distance. When stacking common reflection point gathers, it is necessary to excise the gathers with excessive stretching to ensure the stratigraphic resolution. . Due to the lack of small offset data in the conventional acquisition method, the shallow seabed data, especially in the shallow water area, has too few effective stacking channels, thus affecting the subsequent imaging results.

Figure 2 takes the pulse wavelet as an example to show the influence of the source ghost wave and the hydrophone ghost wave on the first wave signal in the time domain and frequency domain. By comparison, it can be found that the existence of ghost waves complicates the originally simple source wavelets, and causes notch waves in the frequency domain, which seriously affects the frequency integrity of the wavelets, thereby reducing the effective frequency bandwidth of seismic data.

3 is a schematic structural diagram of a marine seismic detection system with a seismic source located below a streamer according to the present invention, including a mother ship 3, a seismic source 1, a photoelectric composite cable 2 and an independent combined streamer device; the mother ship 3 passes through the photoelectric composite cable 2. It is connected with the seismic source 1 ; the independent combined streamer device is fixed on the photoelectric composite cable 2 through the hoop 4 .

In a specific implementation of the present invention, the source part adopts a transducer or an electric spark source that can work underwater for thousands of meters, and is connected to the mother ship through a photoelectric composite cable to realize signal transmission and power supply; the transducer and the electric spark source are used. The main frequency of the air gun is higher than that of the air gun, so it is more suitable for high-resolution exploration. In the acquisition part, in order to facilitate the adjustment of the depth, the present invention adopts an independent combined streamer, and the independent combined streamer device is located above the earthquake source, including a power supply and acquisition unit 5, multiple streamers 7, and a resistance device 8; the One end of the power supply and acquisition unit 5 is connected to the hoop 4 through a distance adjustable device, and the other end is connected to the multi-channel streamer 7, and the power supply and acquisition unit 5 is used to supply power to the multi-channel streamer 7 and record seismic data; the described The independent combined streamer device also includes an auxiliary float 6. The auxiliary float 6 is arranged between the power supply and collection unit 5 and the multi-channel streamers 7 to assist in adjusting the depth of the front end of the multi-channel streamers 7; in actual operation , Multi-channel towing cables must ensure the horizontal attitude, multi-channel towing cables can achieve equal buoyancy through material selection, and the tail of multi-channel towing cables 7 is provided with resistance device 8, which can be an umbrella-shaped device, or it can use equal-floating material , the main purpose is to assist in straightening the multi-channel streamer 7; a number of hydrophones are evenly arranged on the multi-channel streamer 7, and each hydrophone is connected to the power supply and collection unit 5 through a data line, and the power supply and collection unit 5 On the one hand, it is used to power multiple streamers 7, and on the other hand, it is used to record the seismic data collected by each hydrophone.

The invention adjusts the depth of the multi-channel streamers in the ocean by moving the position of the hoop on the photoelectric composite cable; and adjusts the distance between the power supply and acquisition unit and the photoelectric composite cable, so that the seismic source is located in the middle of the multi-channel streamers. directly below. In this way, after the source is excited, multiple streamers can record near (zero) offset data, thereby ensuring accurate imaging of shallow structures. The distance adjustable device can directly use Kevlar rope, and the distance between the power supply and acquisition unit and the photoelectric composite cable can be changed by adjusting the length of the Kevlar rope.

On the other hand, the source is located below the multi-channel streamer, which provides the possibility to obtain high signal-to-noise ratio data completely free from ghost waves. Figure 4 shows the time-of-arrival curves of different wave fields when the source and multiple streamers are placed at different water depths. In a specific implementation of the present invention, the water depth is assumed to be 2000m, and the depth interval between the source and multiple streamers is fixed at 200m, as shown in Figure 4(a), where combination 1: focal depth 500m; combination 2: focal depth 700m ; Combination 3: focal depth 900m; Combination 4: focal depth 1100m; Combination 5: focal depth 1300m; Combination 6: focal depth 1500m; Combination 7: focal depth 1700m. The formation reflection in Fig. 4(b) corresponds to the target data, and the time length of the high signal-to-noise ratio data depends on the time of the subsequent arrival of the wave field. Through analysis, it can be found that there is a relationship between the arrival time of the source ghost wave and the arrival time of the formation reflection. When the source depth is shallow (the source-streamer average water depth does not exceed half of the water depth), the length of the data without the interference of ghost waves is mainly Depends on the arrival time between the formation reflection and the ghost wave at the hydrophone end, and as the source water depth increases, the effective time length increases, as in combinations 1-4. In combination 5-7, the source-streamer average water depth is more than half of the water depth. At this time, the source ghost wave direct wave begins to affect the formation reflection. As the source depth continues to increase, the effective time is extended again.

震源距离水底越近，相同偏移距的动校拉伸越大，有效叠加道数越小。因此，在设计观测系统时，需要根据探测目标有针对性的选择。一般而言，当勘探目标位于海底以下较深处时(勘探目标自海底向下的双程地震走时≥(h-d₂)/c)，可以选择将震源下沉到离海底较近处，这样可以获取较长的有效地层反射时窗，从而尽量保证目标地层不受鬼波影响，此时拖缆深度应满足

其中h是水深，d₂为震源与多道拖缆深度差。反之，则选择将震源-拖缆置于平均深度为一半水深处，此时拖缆深度

可以保证尽可能多的有效叠加道。Fig. 5 shows the relationship between the time length of the formation reflection without the influence of ghost waves and the depth of the epicenter in the model of Fig. 4. Consistent with the analysis results of Fig. 4, the time length first increases linearly, and then when the ghost waves directly reach the epicenter After the arrival time of the wave is later than the formation reflection, the time window length decreases sharply, and then increases linearly again with the increase of the source depth. The closer the source is to the water bottom, the longer the effective formation reflection time window length can be obtained without the interference of ghost waves. However, according to the formula:

The closer the epicenter is to the water bottom, the greater the dynamic correction stretch at the same offset distance, and the smaller the effective stacking traces. Therefore, when designing the observation system, it is necessary to make a targeted selection according to the detection target. Generally speaking, when the exploration target is located deep below the seabed (the two-way seismic travel time of the exploration target from the seabed down ≥(hd ₂ )/c), the seismic source can be chosen to sink closer to the seabed, which can Obtain a longer effective time window of formation reflection, so as to ensure that the target formation is not affected by ghost waves as much as possible. At this time, the depth of the streamer should meet the

where h is the water depth, and d ₂ is the depth difference between the source and multiple streamers. On the contrary, choose to place the source-streamer at the average depth of half the water depth, at this time the streamer depth

As many effective stacking tracks as possible can be guaranteed.

其中Δt_ext为地震子波延续时间长度。Figure 6 shows the time-of-arrival curves of different wave fields when the source and multiple streamers are placed at different water depths. In a specific implementation of the present invention, the water depth is assumed to be 2000m, as shown in Figure 4(a), wherein combination mode 1: focal depth 950m, streamer depth 1050m; combination mode 2: focal depth 900m, streamer depth 1100m; Combination mode 3: the focal depth is 850m, and the streamer depth is 1150m. From the data comparison in Figure (b), it can be found that the smaller d ₂ is, the longer the effective time window length is. However, since seismic wavelets are usually not ideal pulse wavelets, they often have a certain duration. In order to ensure that the effective formation reflection is not affected by the wavelet continuation, the system design needs to meet:

where Δt _ext is the duration of seismic wavelet.

To sum up, the marine seismic detection method of the present invention can be obtained, comprising the following steps:

1) According to the exploration target, lay the seismic source and multiple streamers:

其中Δt_ext为地震子波延续时间长度， d₂为震源与多道拖缆深度差，d₃为震源离海底的深度，c为水中声速；First, make sure that the focal depth meets the

where Δt _ext is the duration of the seismic wavelet, d ₂ is the depth difference between the source and multiple streamers, d ₃ is the depth of the source from the seabed, and c is the speed of sound in water;

反之，拖缆深度

其中h是水深，d₂为震源与多道拖缆的深度差；通过调节抱箍在光电复合缆上的位置，固定多道拖缆的深度；最后调整抱箍与供电及采集单元之间的距离，使得震源位于多道拖缆的正下方；When the exploration target is located deep below the seabed (the two-way seismic travel time of the exploration target downward from the seabed ≥(hd ₂ )/c), the seismic source is sunk closer to the seabed, and the depth of the streamer

Conversely, the streamer depth

Where h is the water depth, d ₂ is the depth difference between the source and the multi-channel streamer; by adjusting the position of the hoop on the photoelectric composite cable, the depth of the multi-channel streamer is fixed; finally, adjust the hoop and the power supply and acquisition unit. distance, so that the source is located directly below the multiple streamers;

2) Exciting the source 1, after the seismic wavelet is reflected at the reflection horizon where the exploration target area is located, it propagates upward to the multi-channel streamers 7, and the seismic signals are collected by the hydrophones on the multi-channel streamers 7; power supply and acquisition unit 5 pairs Multiple streamers 7 provide power and record the seismic data collected by each hydrophone.

Through the above steps, on the one hand, the near (zero) offset, which is crucial for imaging the shallow structure of the seabed, can be recorded, and on the other hand, shallow high-resolution seismic data that is not affected by ghost waves can be obtained, thereby effectively improving the seafloor. Superficial structure imaging effect.

The above-mentioned embodiment is only a preferred solution of the present invention, but it is not intended to limit the present invention. Various changes and modifications can also be made by those of ordinary skill in the relevant technical field without departing from the spirit and scope of the present invention. Therefore, all technical solutions obtained by means of equivalent replacement or equivalent transformation fall within the protection scope of the present invention.

Claims (9)

1. a marine seismic detection system with a seismic source located below a streamer, is characterized in that comprising a mother ship (3), a seismic source (1), a photoelectric composite cable (2) and an independent combined streamer device; the mother ship (3) The photoelectric composite cable (2) is connected to the seismic source (1); the independent composite streamer device is fixed on the photoelectric composite cable (2) by a hoop (4); The independent combined streamer device is located above the seismic source (1), and includes a power supply and acquisition unit (5), a multi-channel streamer (7) and a resistance device (8); the power supply and acquisition unit (5) has a One end is connected with the hoop (4) through the distance adjustable device, and the other end is connected with the multi-channel towing cables (7), and the rear of the multi-channel towing cables (7) is connected with a resistance device (8) that assists in straightening the towing cables, and The depth of the multiple streamers (7) in the ocean can be adjusted by moving the position of the hoop (4) on the photoelectric composite cable (2). 2. The marine seismic detection system with the seismic source located below the streamer according to claim 1, wherein the independent combined streamer device further comprises an auxiliary float (6), and the auxiliary float (6) is arranged on the Between the power supply and collection unit (5) and the multiple streamers (7). 3. The marine seismic detection system according to claim 1, wherein the seismic source (1) is located just below the middle position of the multiple streamers (7). 4. The marine seismic detection system according to claim 1, wherein a plurality of hydrophones are evenly arranged on the multi-channel streamers (7), and each hydrophone passes data The line is connected with the power supply and acquisition unit (5). 5. The marine seismic detection system according to claim 1, wherein the multi-channel streamer (7) presents a horizontal attitude. 6. The marine seismic detection system with a seismic source located below a streamer according to claim 5, characterized in that, when the two-way seismic travel time of the exploration target downward from the seabed ≥ (hd ₂ )/c, the multi-channel streamers are adjusted depth

Conversely, adjust the depth of multiple streamers

where h is the water depth, d ₂ is the depth difference between the source and multiple streamers, and c is the speed of sound in water. 7. The marine seismic detection system with the source located below the streamer according to claim 1, characterized in that the arrangement of the source and the streamer satisfies

where Δt _ext is the duration of the seismic wavelet, d ₂ is the depth difference between the source and the multi-channel streamer, d ₃ is the depth of the source from the seabed, and c is the speed of sound in water. 8 . The marine seismic detection system according to claim 1 , wherein the seismic source is a transducer or an electric spark seismic source. 9 . 9. A marine seismic detection method based on the system of claim 5, characterized in that it comprises the steps of: 1) According to the exploration target, lay the seismic source and multiple streamers: First, make sure that the focal depth meets the

where Δt _ext is the duration of the seismic wavelet, d ₂ is the depth difference between the source and multiple streamers, d ₃ is the depth of the source from the seabed, and c is the speed of sound in water; When the two-way seismic travel time from the bottom of the exploration target is ≥(hd ₂ )/c, the seismic source is sunk and the depth of the multi-channel streamer is adjusted to meet the

Conversely, adjust the depth of multiple streamers

where h is the water depth; by adjusting the position of the hoop on the photoelectric composite cable, the depth of the multiple streamers is fixed; finally, the distance between the hoop and the power supply and acquisition unit is adjusted so that the source is located in the middle of the multiple streamers. below; 2) The source is excited, and after the seismic wavelet is reflected at the reflection horizon where the exploration target area is located, it propagates upward to the multi-channel streamers, and the hydrophones on the multi-channel streamers collect seismic signals; Power is supplied and the seismic data collected by each hydrophone is recorded.

Images