An underwater seismic source positioning method based on direct wave and ghost wave joint travel time inversion

Abstract

The application discloses an underwater seismic source positioning method based on direct wave and ghost wave joint travel time inversion and belongs to the field of marine seismic detection. A far-field hydrophone is arranged at a near-seabed position to record the arrival time of direct waves and sea surface reflection ghost waves of an underwater seismic source and to extract the measured travel time difference of the direct waves and the ghost waves; based on initial seismic source positioning information, a seismic source optional position set is constructed; the arrival time of the direct waves and the ghost waves corresponding to different candidate seismic source positions in the seismic source optional position set is extracted by ray tracing, the travel time difference of the direct waves and the ghost waves is extracted, and least square error evaluation is carried out on the measured travel time difference, and the candidate seismic source position corresponding to the minimum error is the best seismic source position. The application adopts the mode of additionally arranging a far-field hydrophone, determines the accurate seismic source position by monitoring the travel time difference of the direct waves and the ghost waves, and makes up for the positioning error of the existing USBL system.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of marine seismic exploration, and in particular to an underwater source positioning method based on joint travel time inversion of direct wave and ghost wave. BACKGROUND

[0002] Marine seismic(acoustic) exploration is an important means of seabed structure exploration. With the gradual increase of the water depth of the exploration sea area, deep-towed sources are gradually applied, and their underwater accurate positioning becomes the key to subsequent data processing. The current underwater acoustic positioning system mainly includes a long baseline positioning system (LBL), a short baseline positioning system (SBL), an ultra-short baseline positioning system (USBL), and an ultra-short baseline and long baseline positioning combination system.

[0003] Among them, the ultra-short baseline positioning system has the characteristics of small base array size and easy to arrange, and is the most widely used. The ultra-short baseline positioning system is composed of a source, a transponder, and a receiving array. The source is generally towed or carried by a ship body, and the receiving array is installed on the ship. After the source emits a sound wave pulse, the transponder fixed to the underwater source receives the sound emitted by the source and transmits it to the receiving array through the return pulse. The distance from the underwater device to the receiving array is calculated by measuring the phase difference in the horizontal and depth directions, and the planar coordinate position and depth of the underwater probe are calculated. However, due to the complex and changeable marine environment, the USBL positioning result often has a certain deviation. The USBL inversion distance is mainly obtained by the geometric model constructed by time and sound velocity. However, because the temperature, salinity, and pressure in the ocean are always in a state of change, the sound velocity is also always changing, which leads to errors in distance measurement.

[0004] At the same time, based on the observation mode of the USBL system, as the observation depth gradually deepens, the measurement error gradually increases. In addition, GPS positioning error, attitude and azimuth error, etc. also have a certain influence on the positioning result. SUMMARY

[0005] To solve the existing ultra-short baseline positioning error problem, the present application proposes a method for positioning the source by using the arrival time difference of direct wave and ghost wave. By arranging far-field hydrophones at the near-seabed position, the arrival times of direct wave and sea surface reflected ghost wave of the underwater source are recorded, and the travel time difference T obs is extracted as the matching target. Using the initial source positioning information provided by the USBL, a possible source position information library is constructed, the direct wave and ghost wave travel time information corresponding to different source positions is inverted by ray tracing, and the least squares error is evaluated with T obs , and the minimum error corresponds to the best source position.

[0006] The technical scheme adopted by the present application is as follows:

[0007] A method for positioning an underwater seismic source based on joint travel time inversion of direct wave and ghost wave, comprising the following steps:

[0008] (1) A far-field hydrophone is arranged at a near-seabed position to record the arrival time of direct wave and sea surface reflected ghost wave of the underwater seismic source, and the measured travel time difference T obs of the two is extracted;

[0009] (2) Based on the initial seismic source positioning information, a set of optional seismic source positions is constructed;

[0010] (3) The arrival time of direct wave and ghost wave corresponding to different candidate seismic source positions in the set of optional seismic source positions is extracted by ray tracing, the travel time difference of the two is extracted, and the least square error is evaluated with T obs , the candidate seismic source position corresponding to the minimum error is the best seismic source position, which is represented as:

[0011] min{E=[T rt (S)-T obs ] 2}

[0012] Wherein, T obs is the measured travel time difference of the arrival time of direct wave and sea surface reflected ghost wave of the underwater seismic source, T rt (.) is the ray tracing inversion result, E is the square error, and S is the set of optional seismic source positions.

[0013] Further, the initial seismic source positioning information is measured by an ultra-short baseline positioning system.

[0014] Further, the construction method of the set of optional seismic source positions is:

[0015] The connecting line from the receiving array of the ultra-short baseline positioning system carried by the ship body to the initial seismic source position is offset by a certain angle around the receiving array, the perpendicular distance from the initial seismic source position to the offset line is taken as the radius, and a sphere is constructed with the initial seismic source position as the center, and the range surrounded by the sphere is taken as the range of the set of optional seismic source positions;

[0016] Based on the range surrounded by the sphere, the set of optional seismic source positions is initialized.

[0017] Further, the offset angle is 0.75°-1.25°, preferably 1°.

[0018] Further, the candidate seismic source positions in the set of optional seismic source positions are uniformly distributed.

[0019] Further, the ray tracing is realized based on finite element simulation.

[0020] Further, the far-field hydrophone adopts an omnidirectional hydrophone.

[0021] Further, the far-field hydrophone is connected with a weight at the bottom and a buoy at the top, during operation, the far-field hydrophone is sunk to the near-sea-bottom position through the weight at the bottom, after the operation is completed, the pulling line connected with the weight is cut, and the far-field hydrophone is floated up under the action of the buoy and is recycled.

[0022] The present application has the beneficial effects of:

[0023] The present application adopts the mode of additionally arranging the far-field hydrophone, the accurate source position is determined through monitoring the travel-time difference of the direct wave and the ghost wave, and the positioning error of the existing USBL system is made up. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 is a schematic diagram of a far-field acquisition observation system;

[0025] Figure 2 is a raw shot gather after peak alignment (a) and zoom display of near-peak area (b);

[0026] Figure 3 is a contrastive diagram of sound velocity profile (a) and propagation path based on variable depth and constant velocity model (b);

[0027] Figure 4 is a source position after inversion based on ray tracing of travel-time of direct wave and ghost wave, wherein (a) is a plane source position; (b) is a source depth; (c) is a contrast of measured travel-time difference and inverted travel-time difference. DETAILED DESCRIPTION

[0028] In order to make the purpose, technical scheme and advantages of the present application more clear, the present application is further described in detail below in combination with the drawings and examples. It should be understood that the specific examples described herein are only used to explain the present application, and are not used to limit the present application.

[0029] The present application records the arrival time of the direct wave of the underwater source and the ghost wave reflected by the sea surface by arranging the far-field hydrophone at the near-sea-bottom position, extracts the travel-time difference T obs of the two as the matching target. The initial source positioning information provided by the USBL is used to construct the information database of possible source positions, the travel-time information of the direct wave and the ghost wave corresponding to different source positions is extracted by ray tracing, and the least square error evaluation is performed on T obs , and the minimum error corresponds to the best source position. The specific mathematical process can be expressed as follows:

[0030] min{E=[T rt (S)-T obs ] 2} (1)

[0031] Wherein, Tobs is the measured travel time difference, T rt is the ray tracing inversion result, E is the square error between them, and S is the set of optional source locations.

[0032] In one specific implementation of the present application, a test verification was carried out in the southwest Indian Ocean, and the observation system was as shown in Figure 1 , in which the source was simulated by a Janus-Helmholtz transducer, towed by a traction cable on a ship, located underwater, with a preset depth H1 of 1000 meters. The far-field hydrophone was an omnidirectional hydrophone, about 280 meters from the seabed H2, fixed on the seabed by concrete blocks, and straightened by a buoy. Figure 2 (a) is the direct wave and ghost wave profile recorded by the far-field hydrophone, Figure 2 (b) is an enlarged display, and the signal near the center point is less affected by seabed reflection, and by extracting the travel time of the direct wave and the ghost wave, the travel time difference between the two can be calculated.

[0033] For ray tracing inversion, it is critical to obtain the sound speed in water. The sound speed in water depends on various factors (temperature, salinity, pressure, etc.), and usually changes with depth, and the change in sound speed due to depth change can cause the propagation path of the direct wave and the ghost wave to bend, so it is crucial to build an accurate velocity model. In this embodiment, the CTD (Conductivity, Temperature, Depth) instrument is used to measure the accurate sound speed profile in water Figure 3 , which provides a basis for subsequent ray tracing. Figure 3 (a) is the sound speed profile measured according to the CTD, which shows that except for the far observation points near 100 meters and 1450 meters, it basically shows a decreasing trend from shallow to deep; Figure 3 (b) is a model based on the change in depth that causes the direct wave and the ghost wave to bend during propagation, which obtains the sound speed information of the direct wave and the ghost wave at different depths for subsequent ray tracing inversion.

[0034] Another key point of inversion is the assignment of the initial source position. In this embodiment, the position of the hydrophone is located based on the USBL ultra-short baseline system, and the precise position of the hydrophone is obtained after a large amount of data is collected by adjusting the position of the ship body carrying the USBL ultra-short baseline system multiple times, and it is assumed that the position of the hydrophone does not change in the subsequent observation process. The initial source position is given by the USBL ultra-short baseline system, and based on the initial source position given by the USBL ultra-short baseline system, the initial source position is offset by 1° downward from the connecting line of the ship body receiving array to the initial source position, the vertical distance of the initial source position from the offset line is taken as the radius, and the initial source position is taken as the center of the sphere to construct a source position selection library. The range of the source position selection library is the range enclosed by the sphere, and the interval between the candidate source positions can be set according to the actual situation. The smaller the interval, the denser the candidate source points in the sphere range, the larger the calculation amount but the higher the accuracy; the larger the interval, the sparser the candidate source points in the sphere range, the smaller the calculation amount but the lower the accuracy; no matter how the interval is set, the present application expands an initial source position into a source position selection library in the sphere range, improves the accuracy of the source position obtained by the final positioning, and makes up for the positioning error of the existing USBL system.

[0035] According to the source selectable position set and the ray tracing technology, the travel time difference of the direct wave and the sea surface reflection ghost wave corresponding to each selectable position is obtained. The working principle of the ray tracing technology is that the wave can be modeled as a large number of very narrow beams (rays), and is considered to be straight on a very small finite element. The light tracer pushes the light to this distance, and then uses the local derivative of the medium to calculate the new direction of the light. A new light is emitted from this position, and the process is repeated until a complete path is generated. The ray tracing is realized based on finite element simulation, which is well known in the art, and will not be described here.

[0036] Figure 4 The joint inversion results based on ray tracing are shown. By comparison, it can be found that the distance in the X and Y directions tends to decrease after inversion, and the source depth is slightly deepened (Figs. Figure 4 a and 4b). This shows that the source is adjusted to be closer to the hydrophone during inversion rather than the initial position. The ghost delay time related to the ray tracing inversion is very matched with the observed curve (Fig. Figure 4 c).

[0037] The above only lists specific embodiments of the present application. Obviously, the present application is not limited to the above embodiments, and there can be many variations. All variations that can be directly derived or thought of by those skilled in the art from the disclosure of the present application should be considered as falling within the scope of protection of the present application.

Claims

1. A method for locating underwater seismic sources based on joint travel-time inversion of direct waves and ghost waves, characterized in that, Includes the following steps: (1) Deploy far-field hydrophones near the seabed to record the arrival times of the direct wave from the underwater seismic source and the ghost wave reflected from the sea surface, and extract the measured travel time difference T between the two. obs ; (2) Based on the initial source location information, construct a set of possible source locations; (3) Using ray tracing, the arrival times of the direct wave and ghost wave corresponding to different candidate source locations in the set of selectable source locations are extracted, and their travel time difference is extracted and compared with T. obs The least squares error assessment is performed, and the candidate source location corresponding to the minimum error is the optimal source location, expressed as: min{E=[T rt (S)-T obs ] 2 } Among them, T obs It is the measured travel time difference between the arrival times of the direct wave from the underwater seismic source and the ghost wave reflected from the sea surface, T. rt (.) represents the ray tracing inversion result, E is the squared error, and S is the set of possible source locations.

2. The underwater seismic source localization method based on joint travel-time inversion of direct wave and ghost wave as described in claim 1, characterized in that, The initial seismic source location information was obtained through an ultra-short baseline positioning system.

3. The underwater seismic source localization method based on joint travel-time inversion of direct wave and ghost wave according to claim 2, wherein the method for constructing the set of selectable seismic source locations is as follows: The line connecting the receiving array of the ultra-short baseline positioning system on the ship to the initial seismic source position is offset around the receiving array by a certain angle. The vertical distance between the initial seismic source position and the offset line is used as the radius, and the initial seismic source position is used as the center of the sphere. The range enclosed by the sphere is used as the range of the set of possible seismic source positions. Initialize the set of possible earthquake source locations based on the area enclosed by the sphere.

4. The underwater seismic source location method based on joint travel time inversion of direct wave and ghost wave according to claim 3, wherein the offset angle is 0.75°-1.25°.

5. The underwater seismic source location method based on joint travel-time inversion of direct wave and ghost wave according to claim 3, wherein the candidate seismic source locations within the set of selectable seismic source locations are uniformly distributed.

6. The underwater seismic source localization method based on joint travel-time inversion of direct wave and ghost wave as described in claim 1, wherein the ray tracing is implemented based on finite element simulation.

7. The underwater seismic source localization method based on joint travel time inversion of direct wave and ghost wave as described in claim 1, wherein the far-field hydrophone is an omnidirectional hydrophone.

8. The underwater seismic source localization method based on the joint travel time inversion of direct wave and ghost wave as described in claim 1 or 7, wherein the far-field hydrophone is connected to a weight at the bottom and a buoy at the top. During operation, the far-field hydrophone is lowered to a position near the seabed by the weight at the bottom. After operation is completed, the traction line connecting the weight is cut, and the far-field hydrophone floats up and is retrieved under the action of the buoy.

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