(1. Guangzhou Marine Geological Survey Guangzhou 510760; 2. Key Laboratory of Seabed Mineral Resources, Ministry of Land and Resources, Guangzhou 5 10760)
Brief introduction of the first author: Zhai Jifeng (1982-), male, bachelor degree, assistant engineer, mainly engaged in marine earthquake investigation.
Seismic observation system is used to express the positional relationship among excitation point, receiving point and underground reflection point. The observation system determines the quality of seismic acquisition data, which directly affects the subsequent processing and interpretation results and accuracy, and is related to the success or failure of seismic exploration. Therefore, the importance of the observation system can be seen. Based on the basic theory of seismic observation system design, this paper discusses how to design a reasonable two-dimensional seismic observation system at sea from two aspects: basic principle and parameter selection.
Keywords observation system principle parameters
1 Introduction
The central problem of seismic data acquisition is to enhance effective waves, suppress interference waves, improve signal-to-noise ratio and obtain high-quality seismic records through various means and methods. The design of the observation system depends on the seismic exploration task, the seismic geological conditions of the work area and the exploration method. The general principle is to track the recorded underground interface continuously as far as possible to avoid the interference of effective waves and make the field construction simple. In the field construction of seismic exploration, the vertical survey line observation system is mainly adopted, that is, the excitation point and the receiving point are arranged on the same survey line. The system can get the reflection information of the interface directly below the survey line, and the obtained data is easy to interpret. The site construction scheme is simple and intuitive, and it is widely used in practical work.
2 Various parameters of observation system
Figure 1 is a common observation system for 240 cables of Treasure Hunt. For marine seismic survey, the seismic source, receiving cable and recording instrument used have fixed parameters. We mainly analyze the following ten parameters that can be changed.
2. 1 maximum cannon offset
The maximum offset is the distance from the center of the shot point to the center of the farthest straight line, which is indicated by X in Figure 2. The design should be based on the following factors:
1) time-distance curve, trying to approximate hyperbola. Proper offset can make the normal time difference large enough to distinguish primary reflection wave, multiple wave and other coherent noise; A relatively large offset will make the long-distance time-distance curve approximate to a high-order curve, so that the recorded in-phase axis does not meet the hyperbolic hypothesis. Seismic geological model of horizontal layered medium The time-distance curve of seismic reflection wave is as follows:
Figure 1 240-channel common observation system of treasure hunt ship
Figure 1 "Exploring Treasure" Shared Observation System for 240 Seismic Channels
Fig. 2 Schematic diagram of distance parameters
Fig. 2 Schematic diagram of distance parameters
Geological Research of South China Sea (20 14)
If seismic waves are received near the excitation point, the wave velocity of horizontal layered media can be simplified as root mean square velocity, and the time-distance curve equation of reflected waves can be simplified as:
Geological Research of South China Sea (20 14)
According to these two equations, when the maximum offset is 0.7 ~ 1.0 times the depth of exploration target, the time-distance curve of reflected wave is approximately hyperbolic.
2) Speed analysis to obtain higher accuracy. In horizontal layered media, it is generally considered that the ray velocity is an accurate velocity, which increases with the increase of offset. When the offset is fixed, the ray velocity is equal to the root mean square velocity, which means that the root mean square velocity at this time can be considered accurate, and the offset at this time is the maximum offset to be selected. According to the formula of ray velocity and offset, the maximum offset can be calculated as the buried depth of exploration target.
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3) dynamically correct the tensile deformation to make it smaller. The stretching degree of dynamic correction increases with the decrease of the ratio of the depth of the reflection interface to the offset, that is, the smaller the offset, the smaller the stretching degree, and the larger the offset, the greater the stretching degree.
Percentage dynamic correction stretching amount = (dynamic correction amount/bidirectional reflection time) × 100%
If the approximate formula is used to calculate the dynamic correction, when the maximum offset is 0.7 times of the buried depth of the target layer, the dynamic correction stretch is 6.12%; When the maximum offset is 1.0 times of the buried depth of the target layer, the dynamic correction stretch is 12.5%. Dynamic correction stretching will reduce the signal frequency, thus affecting the resolution.
4) reflection coefficient, and strive to make its change as small as possible. The reflection coefficient varies with the offset. If the offset is less than a certain value, the reflection coefficient hardly changes with the offset, so the offset should be selected. The reflection coefficient can be obtained by Zopritz equation.
5) High-frequency attenuation, and strive to minimize high-frequency attenuation in a long distance. The absorption and attenuation of seismic waves increase with the increase of propagation distance, which weakens the energy of high-frequency information and reduces the resolution.
From the above discussion, the maximum offset should be 0.7 ~ 1.0 times the depth of exploration target. If the maximum offset is too large, the long-distance reflection time-distance curve will approximate to a high-order curve, which is not in line with the assumption that the time-distance curve is regarded as a hyperbola in seismic exploration. If the offset is too large, the reflection coefficient of long distance changes greatly; Excessive offset will produce converted shear wave; If the offset is too large, the dynamic correction stretching will be more serious, and the high frequency information in the teleseismic signal will be attenuated more severely. If the maximum offset is too small, the whole array will be very short, which is not conducive to receiving the seismic reflection information in the middle and deep layers, and the time-distance curve will be too short to reflect the hyperbolic shape and obtain accurate velocity. In the process of data processing and stacking, the most important thing is the velocity parameter. Therefore, when selecting the maximum offset, the accuracy of velocity analysis of target layer should be considered.
2.2 Minimum offset
The minimum offset is the distance from the center of the shot point to the center of the first cable (short track), which is represented by Y in Figure 2 and should be less than the depth of the shallowest target layer. The minimum offset is large, which can effectively avoid the interference of some noise signals generated by the seismic source and the working ship, but it will lose useful shallow effective signals.
The selection of minimum offset should be considered from the following aspects:
1) Considering the relationship between offset and stacking characteristics, choose a smaller minimum offset.
2) According to the noise situation and seismic geological conditions of the operation ship, select the minimum offset that can better avoid the interference of some noise signals generated by the seismic source and the operation ship. Large offset distance is beneficial to avoid interference such as surface wave and ship noise.
3) In order to meet the needs of refraction static correction or tomographic inversion static correction at the first arrival of offset, a smaller minimum offset should be adopted.
4) In order to improve the resolution, a smaller minimum offset should be adopted.
With the increase of the number of migration traces, the passband width of the superimposed characteristic curve becomes narrower, and the range of the suppression band shifts to the left, while the amplitude of the cubic maximum of the characteristic curve becomes smaller within the suppression range. It shows that the increase of the number of migration traces can better suppress multiples with similar velocity to reflected waves, that is, the resolution can be improved. However, the increase of the number of offset tracks leads to the narrowing of the nip width and the increase of the amplitude of the quadratic maximum of the characteristic curve. Therefore, the multiple reflected waves, which are quite different from the reflected wave velocity, may enter the quadratic extreme value area, and good suppression effect cannot be obtained. It cannot be considered that the more migration traces, the better.
From the previous construction results, the minimum offset of 250m can effectively avoid the interference of noise signals generated by the seismic source and the working ship. However, in the study area, the water depth of some survey lines is less than 100m. If the minimum offset is too large, useful shallow effective signals will be lost, which makes the seabed difficult to track. At this time, the direct wave and the first reflection wave of the seabed arrive almost at the same time, which brings difficulties to remove the direct wave and track the seabed. In the past seismic data, there are also cases where the seabed identification is inaccurate. This is mainly related to shallow water depth and large minimum offset. Therefore, the minimum offset should also be tested in future field operations. Considering the accurate tracking of the seabed and the reduction of near-tracking noise, the appropriate minimum offset is determined by field processing results.
2.3 gun spacing
The shot spacing (Z in Figure 2) is the distance that the shot point moves, d is the distance that the shot point moves, m is the arrangement length, n is the coverage times, and δ x is the trajectory spacing. Order; υ is the number of trajectories of the shooting point. Then:; Single beat s is 1, and double beat s is 2.
Because the number of trajectories moved by the shot point is inversely proportional to the number of coverage times, the shorter the distance moved by the shot point, the higher the number of coverage times when the arrangement length and trajectory spacing are fixed. Shorten the moving distance of shot point and increase the coverage times, so as to improve the effect of multiple suppression, enhance the effective reflected wave energy and improve the signal-to-noise ratio of data.
2.4 geophone combination parameters
The arrangement and combination of geophones should give consideration to suppressing interference waves and prominent effective waves, and design reasonable arrangement and combination parameters by using the relationship between the apparent velocity, main period, time difference between tracks, random interference radius, the location of multiple groups of interference waves, intensity change characteristics and excitation conditions. The factors of geophone combination parameters include: the distance within the group, the combination base distance, the number and combination form of geophones in the combination. The apparent velocity is inversely proportional to the offset, that is, the time difference of each detector in the combination increases with the increase of offset. It is generally believed that the apparent velocity of the nearest channel is the largest and the apparent velocity of the farthest channel is the smallest, so the time difference between the first and second geophone points in the combination is the largest, and its low-frequency response is more serious. The longer the combination arrangement is, the larger the base distance is, and the more obvious this phenomenon is. In mid-deep seismic exploration, low-frequency response should be avoided when using geophone combination method to improve signal-to-noise ratio.
At present, the seal 24-bit cable used by the "Treasure Discovery" ship adopts the linear combination of 12 detectors as one. Due to the application of new technology, the detector has high linearity and sensitivity, and its resolution, hysteresis, repeatability, drift and stability have also been greatly improved.
2.5 Track spacing
Track spacing refers to the distance between two adjacent receiving points. The track spacing should be selected to ensure that the reflected waves between tracks can be compared. The time difference Δ t of the reflected wave arriving at two adjacent receiving points shall satisfy the following relationship: Δ t ≤ T */2, where t * is the apparent period of the reflected wave. Because the apparent velocity V* of the reflected wave is the ratio of trace distance Δ χ to time difference Δ t, that is, V*=. Then, in order to track the deep and shallow reflection waves reliably at the same time, the apparent wavelength λ * of the shallow reflection wave should be used to calculate the maximum suitable value δ χ of the tracking distance.
The size of trace spacing will directly affect the interpretation and lateral resolution of seismic data: if the trace spacing is too large, it will affect the reliability of effective wave tracking and identification in the same layer, and serious spatial aliasing will occur. The larger the trace spacing, the more serious the low-frequency response will be; Small gauge will greatly increase the field data, workload and cost. The choice of trace spacing should be based on the premise that the same phase of the same effective wave can be reliably identified in seismic records, which mainly depends on: (1) the repeatability of adjacent trace records; The vibration relationship among seismic effective wave, interference wave and random vibration background; Time difference of arrival of seismic waves in adjacent channels; Apparent period and lateral resolution of seismic waves.
From the frequency spectrum and velocity analysis of the data collected in the work area, it can be seen that the main distribution range of effective reflected wave video rate (calculated by -6dB) is 6-60 Hz, the shallow velocity is about1800-2400 m/s, and the trace distance is about1800/(2× 60). The results show that the track spacing of 12.5m fully meets the requirements of acquisition accuracy.
There are three 24-bit seismic acquisition and recording systems in our bureau, namely Seal, MSX and Hydroscience, and the cable track spacing is12.5m. From the results of previous seismic data acquisition, the same phase of the same effective wave can be clearly identified on the seismic records by using the track spacing of12.5m..
2.6 Coverage time
Coverage number is the tracking number of a point on the stratigraphic interface, n=S*N/2*r, where s represents a coefficient, generally taken as1; N represents the number of orbits; R represents the number of trajectories of the shooting point. If the covering times are increased, the left boundary of the pressure belt passing through the width of the belt and the superimposed characteristic curve will not change much. It shows that increasing coverage times will neither improve the deterioration of reflected wave superposition characteristics caused by inaccurate NMO correction speed, nor improve the ability to suppress multiples with similar reflected wave speed. However, if the coverage times increase, the width of the compression zone will increase, and the cubic maximum in the compression zone will decrease. The more times of superposition, that is, the more times of coverage, the smaller the average value of compression band and the better the compression effect, so increasing the number of coverage is beneficial to improve the signal-to-noise ratio. That is to say, the increase of coverage times is not only beneficial to suppress multiples, but also beneficial to suppress multiples with very different velocities from reflected waves. In a word, increasing the coverage times can improve the suppression effect and signal-to-noise ratio. The more times of coverage, the greater the improvement of SNR. Assuming that the signal-to-noise ratio after superposition is 1, the coverage times required by each target layer can be calculated by the following formula:
Geological Research of South China Sea (20 14)
Where is the source signal-to-noise ratio; TRA(i) indicates the energy loss of seismic wave caused by transflection, spherical diffusion and stratum absorption.
Choosing a larger number of coverage times can fully suppress the interference noise in high-frequency environment and increase the effective reflection energy of the target layer, thus improving the signal-to-noise ratio of the data and ensuring the imaging effect of the target layer. Therefore, it is necessary to select a large number of coverage times in the set.
2.7 Energy sources
Under the same conditions, the stronger the source energy, the higher the signal-to-noise ratio. However, the operation with large seismic source and high energy will not only receive stronger effective reflection signals, but also receive larger interference signals such as multiples, so the signal-to-noise ratio of data may not be improved. The middle and deep seismic exploration is concerned with the signal-to-noise ratio, not just the intensity of reflected signals.
At present, it is a better method to determine the source energy of conventional two-dimensional earthquakes by calculating the optimal source energy through computer simulation of seismic geological model and then comparing and verifying it through field source tests.
2.8 Combined sinking depth of seismic source cable
In marine seismic survey operation, we use hydrophones arranged in cables to record the pressure P. If the sinking depth of recording cables is λ and the incident angle of harmonics in seismic reflection signals is θ, the brief relationship is as follows:
Geological Research of South China Sea (20 14)
For the air gun source of marine earthquake, the seismic wave signal generated after excitation propagates underground together with the seismic wave signal reflected by the sea surface. Because the sinking depth of the air gun source is relatively small relative to the water depth and stratum thickness, it can be regarded as two superimposed signals propagating underground. The superposition effect of these two signals is controlled by the sinking depth of the air gun source, and the amplitude change of the superimposed signal is also controlled by the sinking depth of the air gun source, just like the seismic cable.
The theoretical analysis results show that the depth of the seismic source and the cable is the same, and the depth value is the z value calculated by the above formula, so that the pressure P is the maximum, where λ can be considered as the main frequency wavelength corresponding to the target layer.
In fact, the sinking depth of the combination of seismic source and cable, the diffusion and attenuation of seismic source excitation signal when it propagates in seawater and stratum, the reflection, refraction and scattering of various interfaces, the combined filtering effect caused by seawater and stratum absorption, and all kinds of noise interference have changed the signal received by hydrophone in cable, and the signal waveform and frequency spectrum received by cable are different from the original seismic source wave.
On the basis of theoretical values, through computer simulation and testing of the depth of the combination of the source and the cable in the work area, the best depth of the combination of the source and the cable can be found.
2.9 sampling rate
Appropriate sampling interval Δ t can avoid the shortcomings of discrete signal distortion and spectrum distortion due to too large interval, and can also avoid the shortcomings of increasing processing workload due to too dense sampling. According to the sampling theorem:
Geological Research of South China Sea (20 14)
δ t is the sampling interval, and fmax is the highest frequency of the destination layer to be protected. In a signal period, at least three samples (i.e. two sampling intervals (2δ t)) are needed to define a signal with a period.
The frequency spectrum analysis of the data collected in the study area shows that the frequency distribution range of effective reflected wave (calculated by -6dB) is 6 ~ 60hz. The calculation results show that the selection of 2ms sampling completely meets the requirements of acquisition accuracy. The sampling rate is 2ms, and the theoretical limit frequency of signals collected by seismograph is about 206Hz. The information in the middle and deep layers is mainly reflected in the low frequency, and the sampling rate has fully met the requirements.
2. 10 low-cut filtering
In recent years, in conventional seismic exploration, the determination of low cut-off filtering tends to keep the low cut-off frequency as low as possible and preserve the original acquisition signal as much as possible. In marine seismic survey, swells and so on will produce noise of tens of hertz, waterfowl hanging foreign objects will produce regular jitter on nearby roads, and low-frequency interference will affect the signal-to-noise ratio of data. When the low-frequency interference is too large, although it can be removed by filtering, the low-frequency effective signal is also lost. Therefore, it is useless to lower the threshold of low-cut filtering when the interference is too large. The frequency range and amplitude of low frequency interference can be obtained by noise analysis of field processing. Generally, the low-cut filtering value with good sea conditions is 3Hz. Of course, after the depth of seismic source and cable is deepened, the environmental noise such as surge is greatly reduced, and low-cut filtering can be omitted.
3 Conclusion
This paper mainly discusses the design principles of the parameters of marine two-dimensional seismic exploration and observation system, and introduces the functions and influences of each parameter in detail. Effective and reasonable observation system design is to select the geometry, maximum offset, minimum offset, shot spacing and trace spacing of the observation system under the constraints of some demonstration parameters, and the determination of these parameters is guided by the attribute analysis of the observation system. Under the established geophysical model, only by designing a reasonable observation system can we obtain the most suitable data for processing and interpretation with reasonable investment.
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Design principle of marine two-dimensional seismic observation system
Zhai Jifeng 1, 2, Wei Chenglong 1, 2, Zeng Xianjun 1, 2.
(1. Guangzhou Marine Geological Survey, Guangzhou, 510760;
2. State Key Laboratory of Marine Mineral Resources, Guangzhou, 5 10760)
Abstract: Seismic observation system is used to express the relationship among excitation point, receiving point and reflection point. The quality of collected data is determined by the observation system, which directly affects the quality and accuracy of subsequent processing and interpretation. Therefore, the observation system is very important for the success or failure of seismic exploration. Based on the basic theory of observation system, this paper discusses the basic principles and parameters of the design of 2D marine seismic observation system.
Keywords: observation system; Principle; parameter