Processing method for seismic reflection data

Abstract

A method for using a reflection seismic survey to process received data according to the present invention involves: virtually transmitting a direct wave of a sound wave indicated in the received data in the progressing direction of the temporal axis by the time it takes for the sound wave to travel twice the distance of the depth underwater of an epicenter; calculating a first pseudo water-surface-reflected wave by correcting the amplitude thereof so as to approach the amplitude of a first water-surface-reflected wave of the sound wave indicated in the received data; and subtracting a component corresponding to the first pseudo water-surface-reflected wave from the received data.

Description

[Technical field]

[0001] The present disclosure relates to a method for processing received data from a seismic reflection survey using a seismic source placed underwater. [Background technology]

[0002] Seismic reflection exploration is known as one of the methods for exploring the bottom of the sea. In this method, a seismic source and a streamer cable towed by a seismic exploration vessel are dropped into the water. The seismic source generates sound waves in a specified band, and individual receivers (hydrophones) attached to the streamer cable receive the sound waves output from the seismic source. The received sound waves include those reflected by the strata below the bottom of the sea. In seismic reflection exploration, the positions of the seismic source and the streamer cable are intermittently changed and the arrival times of the reflected waves are measured, making it possible to identify the shape, density, and other physical properties of the strata (see Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2014-137320 A Summary of the Invention [Problem to be solved by the invention]

[0004] The target of underwater exploration is the strata beneath the bottom of the water. Therefore, the closer the epicenter is to the bottom of the water, the stronger the reflected waves from the strata can be. However, because sound waves propagate in all directions from the epicenter, the receiver receives not only the reflected waves of sound waves that reach the strata directly from the epicenter (for convenience, called direct waves), but also the reflected waves of sound waves that reach the strata after being reflected from the epicenter by the water surface (for convenience, called water surface reflected waves). In this case, the same strata appear at different depths in the cross-sectional view of the received data, making it difficult to properly evaluate the strata.

[0005] The present disclosure has been made in consideration of the above circumstances, and aims to provide a processing method capable of reducing the influence of waves reflected from the water surface on received data obtained by reflection seismic exploration. [Means for solving the problem]

[0006] A method according to one aspect of the present disclosure is a method for processing received data obtained by a seismic reflection survey, comprising: virtually propagating a direct wave of a sound wave shown in the received data in a direction along a time axis for a time period during which the sound wave travels a distance twice the depth of the epicenter in water; and further, calculating the amplitude of the direct wave according to the time period shown in the received data. The above Sonic No. 1 By correcting the amplitude of the reflected wave from the water surface, No. 1 A pseudo water surface reflected wave is calculated, and the received wave data is used to obtain the No. 1 The component corresponding to the pseudo water surface reflected wave is subtracted. The first water surface reflected wave is virtually propagated in the opposite direction to the direction of travel of the time axis for a time period corresponding to twice the water depth of the epicenter of the sound wave in water, and a first pseudo direct wave is calculated by inverting the phase of the first pseudo direct wave. Data indicating the first pseudo direct wave is added to the received data from which the component corresponding to the first pseudo water surface reflected wave has been subtracted. Here, the direct wave is a sound wave that is output from the seismic source and reflected by a reflecting surface without being reflected by the water surface, No. 1 The water surface reflected wave is output from the seismic source, reflected on the water surface, and then reflected again on the reflecting surface, and travels over a distance twice the water depth of the seismic source. Proceed with It is a time-delayed sound wave.

[0008] The method may further include virtually propagating the direct wave of the sound wave shown in the receiving data in the direction of travel on the time axis for the time it takes for the sound wave to travel a distance twice the water depth of the receiver of the sound wave, and further calculating a second pseudo water surface reflection wave by correcting the amplitude to approach the amplitude of a second water surface reflection wave of the sound wave shown in the receiving data; virtually propagating the direct wave of the sound wave shown in the receiving data in the direction of travel on the time axis for the sound wave to travel underwater a distance twice the water depth of the epicenter and a distance twice the water depth of the receiver of the sound wave, and further calculating a third pseudo water surface reflection wave by correcting the amplitude to approach the amplitude of a third water surface reflection wave of the sound wave shown in the receiving data; and subtracting a component corresponding to the second pseudo water surface reflection wave and a component corresponding to the third pseudo water surface reflection wave from the receiving data. Here, the second water surface reflected wave is output from the seismic source, reflected by the reflecting surface, and further reflected by the water surface, and thus reaches a distance twice the water depth of the receiver. Proceed with The third water surface reflected wave is a sound wave delayed by a time, and is output from the source, reflected on the water surface, reflected on the reflecting surface, and further reflected on the water surface, and reaches a distance twice the water depth of the source. Proceed with Time and twice the water depth of the receiver Proceed with It is a time-delayed sound wave.

[0009] The method further comprises detecting the first water surface reflected wave at a distance of twice the water depth of the seismic source in water. Proceed with The first pseudo direct wave is calculated by inverting the phase of the sound wave and calculating the second water surface reflected wave. Proceed with The second pseudo direct wave is calculated by inverting the phase of the second pseudo direct wave, and the third water surface reflected wave is calculated by calculating the third water surface reflected wave when the sound wave is reflected in water at a distance twice the water depth of the source. Proceed with Time and twice the water depth of the receiver Proceed withA third pseudo-direct wave may be calculated by virtually propagating the first pseudo-direct wave, the second pseudo-direct wave and the third pseudo-direct wave in a direction opposite to the direction of progression of the time axis for only a certain time, and data indicating the first pseudo-direct wave, the second pseudo-direct wave and the third pseudo-direct wave may be added to the received data from which components corresponding to the first pseudo-water surface reflected wave, the second pseudo-water surface reflected wave and the third pseudo-water surface reflected wave have been subtracted. Effect of the Invention

[0010] According to the present disclosure, it is possible to provide a processing method capable of reducing the influence of water surface reflected waves on received data obtained by reflection seismic exploration. [Brief description of the drawings]

[0011] [Figure 1] 1 is a schematic diagram showing a bottom exploration system. [Diagram 2] FIG. 1 is a block diagram of an underwater exploration system. [Diagram 3] 10 is a flowchart showing a method for processing received wave data. [Figure 4] 10 is a flowchart showing a method for processing received wave data. [Diagram 5] 1A to 1C are diagrams for explaining an example of a cross-sectional view obtained from received data and its processing. [Figure 6] 1A and 1B are diagrams for explaining the progress of sound waves reflected by reflecting surfaces at the same position. [Figure 7] 1 is a graph showing the relationship between the operational depth (water depth) of the seismic source and the signal strength. [Figure 8] 1 is a diagram showing a frequency spectrum of original received wave data and a frequency spectrum of received wave data obtained by performing processing according to the present embodiment on the original received wave data. [Figure 9] 1 is a schematic diagram showing a bottom exploration system. [Figure 10] FIG. 1 is a diagram for explaining a change in reception time of a sound wave (converted depth) due to a difference in path length. [Figure 11] 10 is a flowchart showing a method for processing received wave data. [Figure 12]10 is a flowchart showing a method for processing received wave data. [Figure 13] 1A to 1C are diagrams for explaining an example of a cross-sectional view obtained from received data and its processing. [Figure 14] 10 is a flowchart showing a method for processing received wave data. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] (First embodiment) Hereinafter, a first embodiment of the present disclosure will be described. Fig. 1 is a schematic configuration diagram showing an underwater exploration system 10 according to the present embodiment. Fig. 2 is a block diagram of the underwater exploration system 10 according to the present embodiment.

[0013] As shown in Figures 1 and 2, the underwater exploration system 10 includes a seismic source 11, a control device 12, and multiple receivers 13, and explores the structure below the bottom 3 using a reflection seismic exploration method. The underwater exploration system 10 of this embodiment is intended to operate on the ocean. However, the operating environment of the underwater exploration system 10 is not limited to the ocean, and the system can operate in environments where underwater exploration is possible, such as rivers and lakes.

[0014] The seismic source 11 is installed underwater at a depth D1 and outputs sound waves 5. The seismic source 11 is, for example, a low-frequency generating device having a known configuration, and includes a hydraulically driven diaphragm (not shown). The vibration of the diaphragm is controlled by a control device 12, thereby generating sound waves 5 of a desired frequency. The diaphragm may be driven by a piezoelectric element, or may be driven by other known driving mechanisms. Note that the seismic source 11 is not limited to the low-frequency generating device described above, and may be a sound wave generating device such as an air gun.

[0015] A plurality of receivers 13 are installed in the water and arranged in one direction. Each receiver 13 is a so-called hydrophone, and receives underwater sound waves 5. The receivers 13 are connected to each other or held at intervals by members 15 such as cables, forming a streamer cable 14.

[0016] The control device 12 is configured as a so-called computer equipped with a central processing unit (CPU), a memory unit, an auxiliary memory unit, etc., and is mounted on, for example, a seismic exploration vessel 20. The control device 12 controls the generation and frequency of sound waves 5 by the seismic source 11. The sound waves generated may be any of impulse waves, sweep waves, and pseudo-random waves, or may be sound waves with other well-known waveforms. In either case, the control device 12 records the sound waves 5 received by each receiver 13 as signal data.

[0017] The seismic survey vessel 20 is equipped with a lifting device 21. The lifting device 21 sends out and reels in a tow line 23 and a tow line 24. The seismic source 11 is connected to one end of the tow line 23, and the streamer cable 14 is connected to one end of the tow line 24. Therefore, according to the operation of the lifting device 21, the seismic source 11 and the streamer cable 14 are thrown into or retrieved from the water.

[0018] While the seismic source 11 and the geophone 13 of the streamer cable 14 are operating, the seismic survey vessel 20 tows them. The seismic survey vessel 20 has, for example, a lifting device 21 that supports a towing line 23 and a towing line 24 connected to the seismic source 11 and the geophone 13, respectively, so that they can be wound and unwound. Note that the facility on which the underwater survey system 10 is mounted is not limited to the seismic survey vessel 20, and may be any facility that can place the seismic source 11 and the streamer cable 14 underwater.

[0019] Next, a method for processing the received wave data acquired by the underwater exploration system 10 will be described. Figures 3 and 4 are flowcharts showing the method for processing the received wave data. Figure 5 is a diagram for explaining an example of a cross-sectional view obtained from the received wave data and its processing. Figure 6 is a diagram for explaining the progress of sound waves reflected by a reflecting surface at the same position.

[0020] First, basic processing in reflection seismic exploration such as cross-correlation processing, NMO correction, and common midpoint stacking is performed on the signal data recorded in the control device 12, etc., to create reception data showing a cross-section of the sub-bottom structure of the stratum, etc. (step S10). For convenience of explanation, it is assumed that the stratum A, stratum B, and stratum C (see FIG. 5(c)) are confirmed under the bottom of the water as the reflection surface A1 from the reception data obtained by the processing in step S10. The reflection surface A1 is the boundary surface between the water and the bottom of the water. The reflection surface of the sound wave 5 corresponding to the boundary surface between the stratum A and the stratum B is represented by B1, and the reflection surface of the sound wave 5 corresponding to the boundary surface between the stratum B and the stratum C is represented by C1. In other words, the reflection surface is the boundary surface between the water and the stratum (bottom of the water), or the boundary surface between two stratums with different geology.

[0021] 5(a) is a cross-sectional view 40A showing the structure beneath the reflecting surface A1 (bottom of the water) obtained by the process of step S10. In addition to the reflecting surface A1, which is the bottom of the water, and the two reflecting surfaces B1 and C1 beneath the bottom of the water, the cross-sectional view 40A shows the pseudo-reflecting surface A2, which is the pseudo-bottom of the water, and the two pseudo-reflecting surfaces B2 and C2 beneath the pseudo-bottom of the water.

[0022] The seismic source 11 used to obtain the reception data showing the cross-sectional view 40A is located at a water depth D1 (see FIG. 1). This water depth D1 is sufficiently deep with respect to the water surface 2 so that the influence of the water surface reflected waves described later cannot be ignored. In this case, as shown in FIG. 6(a), the receiver 13 receives the reflected waves (conveniently referred to as direct waves 6) of the sound waves that reach the stratum A (B, C) directly from the seismic source 11. Furthermore, as shown in FIG. 6(b), the receiver 13 also receives the reflected waves (conveniently referred to as water surface reflected waves 7) of the sound waves 5 that reach the stratum A (B, C) after being reflected from the seismic source 11 at the water surface 2 (i.e., delayed by a time equivalent to twice the distance of the water depth D1 of the seismic source 11). As a result, not only the reflection surfaces A1, B1, and C1 of the direct waves appear on the cross-sectional view 40A, but also the pseudo reflection surfaces A2, B2, and C2, which are the reflection surfaces of the water surface reflected waves. In other words, the received data contains components that appear at different depths and indicate the same stratum, and it is necessary to remove the effects of the water surface reflected waves.

[0023] Therefore, in step S20, the direct wave 6 is virtually propagated in the direction of travel on the time axis for the time q1 (see Figure 5(b)) it takes for the sound wave 5 to travel a distance twice the water depth D1 of the epicenter 11 underwater, and a pseudo water surface reflected wave is calculated by correcting the amplitude so that it approaches the amplitude of the water surface reflected wave 7 of the sound wave 5 shown in the received data.

[0024] The pseudo water surface reflected wave can be obtained, for example, by the following calculation. First, the wave field Q of the received wave data is subjected to a downward propagation process (i.e., in the direction of the time axis). For example, the following equation (1) is used:

number

[0025] Next, the water surface reflected wave component is removed from the received data (step S30). Specifically, the component corresponding to the pseudo water surface reflected wave obtained in step S20 is subtracted from the received data. For example, following the above-mentioned calculation, the water surface reflected wave 7 component is separated from the received data by the following formula (2) using the convolution matrix D' using the pseudo water surface reflected wave, and the received data P' containing only the direct wave 6 component is obtained.

number

number

[0026] FIG. 5(b) is a cross-sectional view 40B based on the pseudo water surface reflected wave component. As shown in this figure, by performing the process of step S20 from the received data, a cross-sectional view showing only the pseudo water surface reflected wave component corrected by the prediction filter is obtained. Furthermore, by performing the process of step S30, the components of the cross-sectional view shown in FIG. 5(b) are subtracted from the components of the cross-sectional view 40A shown in FIG. 5(a), and finally, a cross-sectional view 40C without the water surface reflected wave 7 component shown in FIG. 5(c) is obtained. That is, it is possible to obtain received data with reduced influence of the water surface reflected wave 7 and improved S / N ratio. The calculation method is not limited to the wave field extrapolation method described above, and any well-known calculation method that can arithmetically simulate the state of the wave field after t seconds can be applied. Well-known calculation methods include, for example, the above-mentioned 45° rule by Claerbout, the wave field extrapolation method by Gazdag (J. Gazdag, Modeling of the acoustic wave equation with transform methods, Geophysics, Vol. 46(6), pp. 854-859), the finite difference time domain method (FDTD method), or the finite element method.

[0027] In addition, the water surface reflected wave 7 shown in the original reception data is the sound wave 5 at a distance equivalent to twice the water depth D1 of the epicenter 11 underwater. (Proceed the distance) The pseudo-direct wave may be virtually propagated in the opposite direction to the time axis for a time q1, and then its phase may be inverted to calculate a pseudo-direct wave (step S40). Data representing the pseudo-direct wave may then be added to the received data from which the component corresponding to the pseudo water surface reflected wave has been subtracted (i.e., the received data that has been processed in step S30) (step S50).

[0028] That is, by the processes of steps S40 and S50, the water surface reflected wave 7 is utilized as a component showing a cross section in the same manner as a direct wave, instead of being treated as noise to be removed from the received data showing a cross section.

[0029] The reception data from the water surface reflected waves 7 can be regarded as reception data from a virtual epicenter 11 (hereinafter referred to as the mirror image epicenter 11V) that is located at a position that is a mirror image of the epicenter 11 with respect to the water surface 2 (see FIG. 6(b)). Therefore, the reception data from the mirror image epicenter 11V is added to the reception data from the direct wave 6. In other words, it is possible to obtain reception data equivalent to that obtained when performing seismic exploration using the actual epicenter 11 and the mirror image epicenter 11V with a single epicenter.

[0030] Specifically, the following calculations are carried out in the processes of steps S40 and S50. The received data from the water surface reflected waves 7, that is, the received data from the mirror image source 11V, can be calculated from equation (4) using the original received data d and the received data P' containing only the direct wave 6 component calculated in step S30.

number

[0031] The water surface reflected wave 7 is delayed by time q1 from the direct wave 6 (see FIG. 5(b)). Therefore, a propagation process is performed on the wave field R of the reception data due to the water surface reflected wave 7, by time q1 upward (i.e., in the opposite direction to the progression direction of the time axis). In other words, a correction is performed on the water surface reflected wave 7 due to the difference between the actual exploration results and the reception data obtained by the mirror image seismic source 11V, which arises from the difference in the propagation paths of both sound waves. In step S20, equation (1) is used to propagate the wave field Q downward. In contrast, in the upward propagation process, similar to the downward propagation process using equation (1) described above, the wave field R is propagated upward by using, for example, equation (5), to obtain a pseudo direct wave.

number

[0032] As the operational depth of the hypocenter 11 deepens, the mirror image hypocenter 11V moves upward away from the water surface 2, and the signal strength from the mirror image hypocenter 11V decreases. On the other hand, the signal strength from the actual hypocenter 11 increases.

[0033] Figure 7 is a graph showing the relationship between the operating depth (water depth) of the seismic source 11 and signal strength. The horizontal axis shows the operating depth of the seismic source 11, and the vertical axis shows the calculated signal strength of sound waves emitted from the seismic source at a water depth of 750 m. The solid line connecting the white circles shows the signal strength of the reflected waves obtained by the direct waves 6. The dotted line connecting the black circles shows the signal strength of the reflected waves obtained by the water surface reflected waves 7. The solid line connecting the black circles shows the sum of these signal strengths.

[0034] As can be seen from Figure 7, as the operating depth of the hypocenter 11 deepens, the signal strength (orange dotted line in the figure) from the water surface reflected wave 7, i.e., the mirror image hypocenter 11V, weakens as it moves away from the bottom of the water. Conversely, the signal strength from the actual hypocenter 11 strengthens as the hypocenter 11 approaches the bottom of the water 3. As evidence of this, Figure 7 shows that the signal strength increases as the operating depth of the hypocenter 11 deepens, and the effect of using the water surface reflected wave 7 is an increase in signal strength of about 5 dB when the operating depth of the hypocenter 11 is 50 m, and an increase in signal strength of about 2 dB when the operating depth is 225 m.

[0035] When a low-frequency generator equipped with a diaphragm or the like is used as the seismic source 11, the seismic source 11 outputs a sound wave with a relatively long output time, such as a sweep wave. Such a sound wave has a smaller sound pressure energy per unit time than an impulse wave obtained from an air gun. However, according to this embodiment, by deepening the operating depth of the seismic source 11, even a seismic source with a small sound pressure energy, such as a low-frequency generator, can obtain a sufficient signal strength, and the S / N ratio of the received data can be improved. In this case, since the sound pressure energy is reduced, it is possible to reduce the energy consumption of the underwater exploration system while also reducing adverse effects on the marine environment, such as the ecological environment of marine mammals.

[0036] Furthermore, the phase of the water surface reflected wave 7 is inverted with respect to the direct wave 6. Therefore, the water surface reflected wave 7 interferes with the direct wave 6 traveling to the bottom 3, causing a phenomenon (so-called notch) in which a specific frequency component and its integer multiple frequency component are weakened. This tendency becomes more pronounced as the operating depth of the seismic source 11 becomes deeper. However, according to this embodiment, the received data is separated into the component due to the water surface reflected wave 7 and the component due to the direct wave 6, and then the phases of both are matched and added, so that the frequency components weakened by the above-mentioned interference can be restored.

[0037] FIG. 8 is a diagram showing the frequency spectrum of the original reception data (dotted line) and the frequency spectrum of the reception data after processing according to this embodiment of the present invention (solid line). The operating depth of the seismic source 11 is 50 m. The arrows in the diagram indicate the frequency at which the effect of the notch described above appears. As shown in this diagram, according to this embodiment, the effect of the notch described above can be eliminated, and the S / N ratio of the reception data can be improved.

[0038] Second embodiment Next, a second embodiment of the present disclosure will be described. In the description of this embodiment, the same reference numerals are used for the same items as those in the first embodiment, and the description thereof will be omitted. Fig. 9 is a schematic diagram showing a bottom exploration system 10 according to this embodiment. Fig. 10 is a diagram for explaining the change in reception time of sound waves due to differences in path length. Figs. 11 and 12 are flowcharts showing a method for processing reception data.

[0039] As shown in FIG. 9, the configuration of the water-bottom exploration system 10 according to the second embodiment is the same as the configuration of the water-bottom exploration system 10 according to the first embodiment.

[0040] For convenience of explanation, the direct wave 6 described in the explanation of the first embodiment will be referred to as the direct wave DW, and the water surface reflected wave 7 described in the explanation of the first embodiment will be referred to as the first water surface reflected wave RW1, as shown in Fig. 9. As in the first embodiment, the receiver 13 receives the direct wave DW (direct wave 6) and the first water surface reflected wave RW1 (water surface reflected wave 7). As described above, the first water surface reflected wave RW1 is received with a delay of time q1, which corresponds to a distance twice the water depth D1 of the epicenter 11.

[0041] In the second embodiment, in addition to the direct wave DW and the first water surface reflected wave RW1, the second water surface reflected wave RW2 and the third water surface reflected wave RW3 that reach the receiver 13 are also used for the calculation processing described below. As shown in Fig. 9, the second water surface reflected wave RW2 is a sound wave that travels as the direct wave DW and is further reflected by the water surface 2, and the third water surface reflected wave RW3 is a sound wave that travels as the water surface reflected wave RW1 and is further reflected by the water surface 2.

[0042] The path length of the second water surface reflected wave RW2 is longer than the path length of the direct wave DW. Also, the path length of the third water surface reflected wave RW3 is longer than the path length of the second water surface reflected wave RW2. In other words, the sound waves are shifted from each other on the time axis of the vertical round trip.

[0043] Therefore, the sound wave signal data acquired by the receiver 13 independently shows each component of the second water surface reflected wave RW2 and the third water surface reflected wave RW3 in addition to each component of the direct wave DW (direct wave 6) and the first water surface reflected wave RW1 (water surface reflected wave 7).

[0044] For example, as shown in Fig. 10, assume that a direct wave DW appears at time q0. A first water surface reflected wave RW1 is a water surface reflected wave 7 in the first embodiment, which is a sound wave output from a seismic source 11, reflected by a water surface 2, and reflected by a reflecting surface, delayed by time q1 relative to the direct wave DW.

[0045] The second water surface reflected wave RW2 is output from the seismic source 11, reflected by the reflecting surface, and further reflected by the water surface 2, and reaches a distance corresponding to twice the water depth D2 of the receiver 13. (Proceed the distance) The second water surface reflected wave RW2 is a sound wave delayed by time q2. In other words, the second water surface reflected wave RW2 is a sound wave that travels as a direct wave DW and is then reflected by the water surface 2. Therefore, the second water surface reflected wave RW2 appears with a delay of time q2 relative to the direct wave DW.

[0046] The third water surface reflected wave RW3 is output from the epicenter 11, reflected by the water surface 2, reflected by the reflecting surface, and further reflected by the water surface 2, and reaches a distance corresponding to twice the water depth D1 of the epicenter 11. (Proceed the distance) The time q1 corresponds to twice the water depth D2 of the receiver 13. (Proceed the distance) The third water surface reflected wave RW3 is a sound wave delayed by time q2. In other words, the third water surface reflected wave RW3 is a sound wave that travels as the first water surface reflected wave RW1 and is then reflected by the water surface 2. Therefore, the third water surface reflected wave RW3 appears delayed by the sum of time q1 and time q2 with respect to the direct wave DW. The magnitude relationship between time q1 and time q2 follows the magnitude relationship between the water depth D1 of the source 11 and the water depth D2 of the receiver 13.

[0047] In this way, when the water depth D2 of the receiver 13 is a water depth that can detect four types of sound waves, namely, the direct wave DW, the first water surface reflection wave RW1, the second water surface reflection wave RW2, and the third water surface reflection wave RW3, these sound waves indicate the same reflection surface but are received at different times. Therefore, the received data contains components that appear at four different depths and indicate the same stratum, and it is necessary to remove the influence of each water surface reflection wave.

[0048] Therefore, in this embodiment, the same processes as in the first embodiment are performed from step S10 to step S30 using each water surface reflection wave (see FIG. 3). Specifically, reception data is created (step S10), and then the calculation process of step S20 is performed. In step S20, a pseudo water surface reflection wave (hereinafter referred to as a first pseudo water surface reflection wave) is calculated from the direct wave DW (direct wave 6) and the first water surface reflection wave RW1 (water surface reflection wave 7) (step S21). Furthermore, a second pseudo water surface reflection wave is calculated using the direct wave DW and the second water surface reflection wave RW2 (step S22), and a third pseudo water surface reflection wave is calculated using the direct wave DW and the third water surface reflection wave RW3 (step S23).

[0049] For example, when obtaining the first pseudo water surface reflection wave, the direct wave DW is virtually propagated in the direction of travel of the time axis for the time q1 that the sound wave 5 travels in water a distance twice the water depth D1 of the epicenter 11, and the first pseudo water surface reflection wave is calculated by correcting the amplitude to approach the amplitude of the first water surface reflection wave RW1 of the sound wave 5 shown in the reception data. The first pseudo water surface reflection wave is the pseudo water surface reflection wave described in the explanation of the first embodiment. Therefore, the first pseudo water surface reflection wave can be formed from the component of the direct wave DW by applying the above-mentioned formula (1) to the wave field Q of the reception data and propagating downward by Δz = q1 * v.

[0050] Similarly, to obtain the second pseudo water surface reflected wave, the direct wave DW is virtually propagated in the direction of travel on the time axis for the time q2 that it takes for the sound wave 5 to travel in water a distance twice the water depth D2 of the receiver 13, and the second pseudo water surface reflected wave is calculated by correcting the amplitude to approach the amplitude of the second water surface reflected wave RW2 of the sound wave 5 shown in the reception data. Therefore, the second pseudo water surface reflected wave can be formed from the component of the direct wave DW by applying the above formula (1) to the wave field Q of the reception data and propagating downward by Δz = q2 * v.

[0051] Similarly, to obtain the third pseudo water surface reflection wave, the direct wave DW is virtually propagated in the direction of travel on the time axis for the time q2 that it takes for the sound wave 5 to travel in water a distance twice the water depth D1 of the source 11 and twice the water depth D2 of the receiver 13, and the third pseudo water surface reflection wave is calculated by correcting the amplitude to approach the amplitude of the third water surface reflection wave RW3 of the sound wave 5 shown in the reception data. Therefore, the third pseudo water surface reflection wave can be formed from the component of the direct wave DW by applying the above formula (1) to the wave field Q of the reception data and propagating downward by Δz = (q1 + q2) * v.

[0052] Furthermore, as a calculation process in step S30, the components corresponding to the first pseudo water surface reflection wave, the second pseudo water surface reflection wave, and the third pseudo water surface reflection wave are removed from the reception data (steps S31 to S33). For example, following the above-mentioned calculation, the convolution matrix D' using the first pseudo water surface reflection wave is used to separate the first to third water surface reflection waves RW1, RW2, and RW3 from the reception data using the above-mentioned formula (2), and reception data P' containing only the direct wave DW component is obtained.

[0053] FIG. 13 is a diagram for explaining an example of a cross-sectional view obtained by the received data and its processing. FIG. 13(a) is a cross-sectional view 50A showing the structure under the reflecting surface A1 (bottom) obtained by the processing of step S10. The cross-sectional view 50A shows the first pseudo reflecting surface A2, the second pseudo reflecting surface A3, and the third pseudo reflecting surface A4, which are pseudo bottoms, in addition to the reflecting surface A1, which is the bottom of the water. The cross-sectional view 50A also shows the first pseudo reflecting surface B2, the second pseudo reflecting surface B3, and the third pseudo reflecting surface B4, which are pseudo bottoms, in addition to the two reflecting surfaces B1 under the bottom of the water. Furthermore, the cross-sectional view 50A also shows the first pseudo reflecting surface C2, the second pseudo reflecting surface C3, and the third pseudo reflecting surface C4, in addition to the two reflecting surfaces C1 under the bottom of the water. The reflecting surfaces A1, B1, and C1 are shown by the direct wave DW. The first pseudo-reflecting surfaces A2, B2, and C2 are indicated by the first water surface reflected wave RW1. The second pseudo-reflecting surfaces A3, B3, and C3 are indicated by the second water surface reflected wave RW2. The third pseudo-reflecting surfaces A4, B4, and C4 are indicated by the third water surface reflected wave RW3.

[0054] By performing the processing of steps S20 and S30 on the received data, the components of the first to third pseudo water surface reflected waves corrected by the prediction filter are subtracted from the components of the cross section 50A shown in Fig. 13(a), and finally, the cross section 50B shown in Fig. 13(b) without the components of the first to third water surface reflected waves RW1, RW2, RW3 is obtained. In other words, even if the receiver 13 is placed at the water depth D2 where the influence of the second and third water surface reflected waves RW2, RW3 appears, the influence of the first to third water surface reflected waves RW1, RW2, RW3 is reduced, and received data with an improved S / N ratio can be obtained.

[0055] Furthermore, in this embodiment, the receiver 13 (streamer cable 14) can be placed at a depth sufficiently far from the water surface 2. A typical streamer cable has a total length of several hundred meters to several kilometers. When such a long streamer cable is towed near the water surface, navigation of ships in the waters around the streamer cable is restricted, and there is a concern that the streamer cable itself may be damaged. However, according to this embodiment, the streamer cable 14 can be placed at a sufficiently deep depth. Therefore, it is possible to carry out estimation exploration while avoiding interference with ships.

[0056] Fig. 14 is a flow chart showing a method for processing received wave data. As in the arithmetic processing of the first embodiment shown in Fig. 4, in this embodiment, the first to third water surface reflected waves RW1, RW2, and RW3 may be used as components showing a cross section similar to a direct wave, instead of being treated as noise to be removed from the received wave data. That is, the processing of steps S41 to S43 is performed as the arithmetic processing of step S40, and the processing of step S51 is performed as the arithmetic processing of step S50.

[0057] First, the first water surface reflected wave RW1 is taken as the wave 5 at a distance corresponding to twice the water depth D1 of the hypocenter 11 in the water. (Proceed the distance) The first pseudo direct wave is virtually propagated in the opposite direction to the direction of travel of the time axis for a time q1, and the phase is inverted to calculate the first pseudo direct wave (step S41). This is the same as the calculation process for the water surface reflected wave 7 in the first embodiment.

[0058] Next, the second water surface reflected wave RW2 is taken as the wave 5 at a distance corresponding to twice the water depth D2 of the receiver 13 in the water. (Proceed the distance) The second pseudo direct wave is calculated by virtually propagating the wave in the opposite direction to the direction of travel of the time axis for a time q2 and inverting the phase (step S42). That is, in step S40 of the first embodiment, the water surface reflected wave 7 is replaced with the second water surface reflected wave RW2 and the time q1 is replaced with the time q2.

[0059] Furthermore, the third water surface reflected wave RW3 is taken as the wave 5 at a distance in the water that corresponds to twice the water depth D1 of the hypocenter 11. (Proceed the distance) The time q1 corresponds to twice the water depth D2 of the receiver 13. (Proceed the distance) A third pseudo direct wave is calculated by virtually propagating the wave in the opposite direction to the direction of travel of the time axis for only time q2 (step S43). That is, in step S40 of the first embodiment, the water surface reflected wave 7 is replaced with the third water surface reflected wave RW3, and time q1 is replaced with the sum of time q1 and time q2. However, unlike the first and second water surface reflected waves RW1 and RW2, the third water surface reflected wave RW3 has the same phase as the direct wave DW, so inversion processing is not performed.

[0060] Then, data indicating each of the first to third pseudo direct waves is added to the received data from which the components corresponding to each of the first to third pseudo water surface reflected waves have been subtracted (i.e., the received data that has been processed in steps S30 (S31 to S33)) (step S51).

[0061] By adding the first to third pseudo direct waves, the same effect as in the first embodiment can be obtained. That is, by deepening the operating depth of the seismic source 11, even if such sound pressure energy is small, sufficient signal strength can be obtained in seismic exploration. In addition, the effect of the above-mentioned notch can be reduced. This can improve the S / N ratio of the received data.

[0062] According to this disclosure, by suppressing acoustic pressure energy in underwater exploration, it is possible to reduce the adverse effects on the marine ecosystem, which can contribute, for example, to Goal 14 of the Sustainable Development Goals (SDGs) led by the United Nations, which states, "Conserve and sustainably use the oceans, seas and marine resources for sustainable development."

[0063] The present disclosure is not limited to the above-described embodiment, but is defined by the claims, and includes all modifications within the meaning and scope equivalent to the claims.

Claims

1. A method for processing received seismic data obtained by a seismic reflection survey using a seismic source that outputs sound waves underwater and a receiver that receives the sound waves, comprising the steps of: The direct wave of the sound wave shown in the receiving data is virtually propagated in the direction of travel on the time axis for the time it takes for the sound wave to travel a distance twice the depth of the epicenter in water, and a first pseudo water surface reflected wave is calculated by correcting the amplitude of the direct wave to be close to the amplitude of a first water surface reflected wave of the sound wave shown in the receiving data; A component corresponding to the first pseudo water surface reflected wave is subtracted from the received data, The first water surface reflected wave is virtually propagated in a direction opposite to the direction of travel of the time axis for a time period required for the sound wave to travel a distance twice the water depth of the seismic source in water, and a first pseudo direct wave is calculated by inverting the phase of the first water surface reflected wave; Data indicating the first pseudo direct wave is added to the received wave data from which a component corresponding to the first pseudo water surface reflected wave has been subtracted. Including, The direct wave is a sound wave output from the seismic source and reflected by a reflecting surface without being reflected by the water surface, The first water surface reflected wave is a sound wave output from the seismic source, reflected by the water surface, and then reflected by the reflecting surface, and delayed by the time it takes to travel a distance twice the water depth of the seismic source. How to process received data.

2. The direct wave of the sound wave shown in the receiving data is virtually propagated in the direction of travel on the time axis for a time required for the sound wave to travel a distance twice the water depth of the receiver, and a second pseudo water surface reflected wave is calculated by correcting the amplitude of the direct wave to be close to the amplitude of a second water surface reflected wave of the sound wave shown in the receiving data; The direct wave of the sound wave shown in the receiving data is virtually propagated in the direction of travel on the time axis for the time it takes for the sound wave to travel in water a distance twice the water depth of the source and twice the water depth of the receiver of the sound wave, and a third pseudo water surface reflected wave is calculated by correcting the amplitude of the direct wave to be close to the amplitude of a third water surface reflected wave of the sound wave shown in the receiving data; A component corresponding to the second pseudo water surface reflected wave and a component corresponding to the third pseudo water surface reflected wave are subtracted from the received data. It further includes: The second water surface reflected wave is a sound wave output from the seismic source, reflected by the reflecting surface, and further reflected by the water surface, and delayed by the time it takes to travel a distance twice the water depth of the receiver, The third water surface reflected wave is a sound wave that is output from the seismic source, reflected on the water surface, reflected on the reflecting surface, and further reflected on the water surface, and is delayed by the time it takes to travel a distance twice the water depth of the seismic source and the time it takes to travel a distance twice the water depth of the geophone. The method for processing received wave data according to claim 1.

3. The first water surface reflected wave is virtually propagated in a direction opposite to the direction of travel of the time axis for a time period required for the sound wave to travel a distance twice the water depth of the seismic source in water, and a first pseudo direct wave is calculated by inverting the phase of the first water surface reflected wave; The second water surface reflected wave is virtually propagated in a direction opposite to the direction of travel of the time axis by a distance twice the water depth of the receiver, and a second pseudo direct wave is calculated by inverting the phase of the second water surface reflected wave. The third water surface reflected wave is virtually propagated in a direction opposite to the direction of travel of the time axis for a time period during which the sound wave travels in water a distance twice the depth of the seismic source and a time period during which the sound wave travels in water a distance twice the depth of the seismic receiver, thereby calculating a third pseudo direct wave; Data indicating the first pseudo direct wave, the second pseudo direct wave, and the third pseudo direct wave are added to the received data from which components corresponding to the first pseudo water surface reflected wave, the second pseudo water surface reflected wave, and the third pseudo water surface reflected wave have been subtracted. The method for processing received data according to claim 2, further comprising:

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