Underground carbon dioxide storage monitoring system using permanent source fixed source and subsea optical cable

Abstract

The present invention relates to an underground carbon dioxide monitoring system using a submarine optical cable. The monitoring system according to the present invention comprises: an optical cable laid on the seabed of a carbon dioxide submarine storage facility; a fixed continuous vibration source that generates a sweep signal by applying continuous vibration for a certain period of time at a fixed location of the underground storage facility; a receiver installed close to the fixed continuous vibration source to directly receive the sweep signal that is not reflected from the ground layer; a light source that irradiates a laser light onto the optical cable; an interrogator including a photodetector that receives a backscattered light signal returning from each point of the optical cable from the laser light irradiated from the light source; and a signal processing unit that processes the backscattered light; and a signal conversion unit that converts the backscattered light signal received from the interrogator into a waveform form explored by an impulse-type elastic wave transmitter and a receiver.

Description

Technology Field

[0001] The present invention relates to underground carbon dioxide storage technology, and more specifically, to a technology for continuously monitoring the injection status and behavior of carbon dioxide in a carbon dioxide subsea storage facility using distributed acoustic measurement technology. Background Technology

[0003] Research is actively underway on Carbon Capture and Storage (CCS) technology, which captures carbon dioxide emitted from industrial sites and stores it underground, particularly in seabed reservoirs.

[0004] In offshore CCS technology, carbon dioxide is injected using developed gas fields or aquifers as reservoirs; therefore, continuous monitoring of the storage status and behavior of carbon dioxide within the reservoir is required before, during, and after injection.

[0005] Previously, traditional seismic exploration techniques were utilized in marine CCS exploration and monitoring. Specifically, a streamer equipped with multiple seismic receivers (hydrophones) mounted in a line at regular intervals was towed by a research vessel at sea; the vessel transmitted seismic waves, and the hydrophones received the reflected waves returning from the seabed strata. Marine seismic exploration was conducted regularly at regular intervals, both before and after the injection of carbon dioxide, to monitor changes in the reservoir conditions.

[0006] However, existing marine seismic exploration methods have many limitations.

[0007] First, in conventional ocean seismic exploration, the exploration system consisting of a probe, air gun, and hydrophone streamer is operated, but there is a problem in that the monitoring process and operations are not only very complex but also very costly.

[0008] Therefore, periodic monitoring at short intervals is difficult. For example, the Norwegian Sleipner project conducts monitoring only about twice a year. However, continuous monitoring is required before, during, and after carbon dioxide injection. In particular, since the condition of the reservoir changes in real time during carbon dioxide injection, it is desirable to perform monitoring in real time as well. Furthermore, if monitoring is conducted by operating a spacecraft at intervals of several months, there are limitations in examining patterns of change by performing monitoring under identical conditions and environments, as climate, weather conditions, and even the conditions under which the air gun is detonated on the spacecraft all change.

[0009] In addition, conventional ocean seismic exploration uses high-energy explosive transmitters, such as air guns, which can have a negative impact on marine ecosystems, particularly marine mammals. The problem to be solved

[0012] The present invention aims to solve the aforementioned problems by installing a distributed sensor using optical fibers on the seabed and a vibration source also being fixedly installed to monitor the behavior of carbon dioxide in the reservoir in real time, and to provide an environmentally friendly and highly economical underground carbon dioxide monitoring system.

[0013] Other unspecified objectives of the present invention will be further considered to the extent that they can be easily inferred from the following detailed description and effects. means of solving the problem

[0015] The underground carbon dioxide monitoring system using a submarine optical cable according to the present invention is characterized by comprising: an optical cable laid on the seabed of a submarine carbon dioxide storage facility; a fixed continuous vibration source that generates a sweep signal by applying continuous vibration for a certain period of time at a fixed location of the underground storage facility; a receiver installed close to the fixed continuous vibration source to directly receive the sweep signal that is not reflected from the ground layer; an interrogator comprising a light source that irradiates a laser light onto the optical cable, a photodetector that receives a backscattered light signal returning from each point of the optical cable from the laser light irradiated from the light source, and a signal processing unit that processes the backscattered light; and a signal conversion unit that converts the backscattered light signal received from the interrogator into a waveform form explored by an impulse-type elastic wave transmitter and a receiver.

[0016] According to the present invention, the signal conversion unit generates the backscattered light signal into a converted signal in the form of an impulse response received using an impulse-type elastic wave transmitter and a receiver through sweep deconvolution processing that compares the backscattered light signal processed by the signal processing unit of the interrogator with the sweep signal received by the receiver.

[0017] In one example of the present invention, it is preferable that a strength reinforcing wire capable of withstanding tension is attached together so that the optical cable does not break when the optical cable is laid on the seabed from the sea.

[0018] In one example of the present invention, it is preferable to further provide a plurality of weights coupled to the strength reinforcing wire or optical cable so that the optical cable can be settled on the seabed.

[0019] In one example of the present invention, a signal enhancement unit is further provided to increase the signal-to-noise ratio by performing time-delayed monitoring on the optical cable to generate a plurality of shock wave reflection signals, and then combining the plurality of shock wave reflection signals.

[0020] In one example of the present invention, a time-series analysis unit is further provided to generate the shock wave conversion signal at different points in time before and after carbon dioxide injection or during carbon dioxide injection, respectively, and then perform a time-lapse analysis to derive the difference of the shock wave conversion signal at each point in time.

[0021] In one example of the present invention, the time series analysis unit separates the reflected wave signal from the shock wave shape conversion signal at a different time point to identify changes in the reservoir layer.

[0022] In one example of the present invention, a borehole is formed in the reservoir, and a vertical optical cable connected to an interrogator is installed and positioned vertically on the wall of the borehole, and the time series analysis unit separates the upward-sloping signal from the shock wave conversion signal acquired and converted from the vertical optical cable at different time points to identify changes in the reservoir.

[0023] In one example of the present invention, the fixed continuous vibration source is an offshore wind turbine, and the optical cable may be a communication optical cable already installed on the seabed. Effects of the invention

[0025] In this invention, the behavior of carbon dioxide in a seabed carbon dioxide reservoir is monitored using a DAS system.

[0026] In this invention, not only is the position of the vibration source fixed, but the position of the optical cable is also fixed. Unlike conventional marine seismic exploration, monitoring can always be performed under constant conditions and environments, thereby improving the reliability of the analysis of monitoring results.

[0027] In addition, by performing monitoring using the DAS system, monitoring is not only very simple but also economical compared to existing monitoring systems using exploration vessels and streamers. This allows for monitoring to be performed in short cycles, and furthermore, continuous real-time monitoring is possible during carbon dioxide injection, making it advantageous for understanding carbon dioxide behavior in the reservoir.

[0028] In one example of the present invention, an offshore wind turbine is used as a fixed continuous vibration source, and by using a communication optical cable already installed on a carbon dioxide storage offshore platform as a DAS system optical cable, there is an advantage in that the investment facilities for the monitoring system can be minimized.

[0029] When laying optical cables separately on the seabed, there is an advantage in that the cable installation is easy because a structure has been developed that allows the cable to settle accurately on the seabed without breaking during the installation process.

[0030] Meanwhile, it is added that even if an effect is not explicitly mentioned herein, the effects described in the following specification and the provisional effects expected by the technical features of the present invention shall be treated as described in the specification of the present invention. Brief explanation of the drawing

[0032] FIG. 1 is a schematic diagram of an underground carbon dioxide monitoring system using a submarine optical cable according to an example of the present invention. Figure 2 is a diagram showing the state in which an optical cable is laid on the seabed and the state in which vibration is applied to the optical cable from a fixed continuous vibration source. Figure 3 compares the shapes of signals received from a vertical optical cable (a) and a horizontal optical cable (b). Figure 4 is intended to explain the process of laying an optical cable on the seabed. Figure 5 is a diagram illustrating sweep deconvolution processing. Figure 6 shows DAS data. Figure 7 shows the result of sweep deconvolution processing of DAS data. Figure 8 is a diagram illustrating time series analysis. Figure 9 shows the result of Kirchhoff migration. ※ It should be noted that the attached drawings are provided as examples for reference to help understand the technical concept of the present invention, and the scope of the rights of the present invention is not limited by them. Specific details for implementing the invention

[0033] In describing the present invention, detailed descriptions of related known functions are omitted if they are deemed obvious to a person skilled in the art and could unnecessarily obscure the essence of the invention.

[0034] The present invention is intended to monitor the injection status and behavior of carbon dioxide when storing carbon dioxide, a greenhouse gas, in a seabed reservoir. It monitors the reservoir status before and after the injection of carbon dioxide. Additionally, it monitors the behavior of carbon dioxide during the injection.

[0035] The present invention has the advantage of enabling real-time monitoring during the carbon dioxide injection process, and since the monitoring method is simple and economical, reservoir monitoring can be performed at very frequent intervals. This is because the present invention utilizes Distributed Acoustic Sensing (DAS) technology using fiber optic cables as a monitoring means.

[0036] Hereinafter, an underground carbon dioxide monitoring system using a submarine optical cable according to an example of the present invention will be described in detail with reference to the attached drawings.

[0037] FIG. 1 is a schematic diagram of an underground carbon dioxide monitoring system using a submarine optical cable according to an example of the present invention, FIG. 2 is a diagram showing the state in which an optical cable is laid on the seabed and vibrations from a fixed continuous vibration source are applied to the optical cable, and FIG. 3 is a comparison of the shapes of signals received from a vertical optical cable (a) and a horizontal optical cable (b).

[0038] Referring to the drawings, an underground carbon dioxide monitoring system using a submarine optical cable according to an example of the present invention (hereinafter referred to as the "carbon dioxide monitoring system") comprises an optical cable (10, 20), a continuous vibration source (30), a receiver (35), and an interrogator (40).

[0039] In the present invention, the optical cable basically uses a horizontal optical cable (10). The horizontal optical cable (10) is laid on the seabed of the seabed reservoir. As shown in FIG. 2, it is preferable to lay it in a matrix form, but it may also be laid in a straight line form even if it is not in a matrix form. If a separate seabed optical cable is laid for carbon dioxide monitoring, it is laid in a matrix form as shown in FIG. 2. However, there are cases where a communication optical cable is installed near the carbon dioxide storage facility. In addition, since the communication optical cable has spare lines that are not used for communication, the existing communication optical cable can be utilized for carbon dioxide monitoring purposes. There is a very significant economic advantage to using the existing communication optical cable.

[0040] Meanwhile, a ship is used when laying a horizontal optical cable (10) on the seabed. The optical cable is wound onto a winch on the ship, and as the ship moves along a predetermined path, the optical cable is unwound to settle on the seabed. The problem is that since the optical cable must be installed over a long distance of several kilometers, the optical cable may break or be damaged due to tension during the laying process. In order to solve this, the present invention attaches a strength reinforcing wire (12) parallel to the optical cable (10) as shown in FIG. 4. In this example, the strength reinforcing wire (12) uses a steel wire made of a steel material capable of withstanding tension. The strength reinforcing wire and the optical cable are attached by a separate coupling means (not shown). In addition, weights (13) are attached to the optical cable and the strength reinforcing wire at regular intervals. When the optical cable adheres to the seabed by the weights (13), vibrations transmitted underground are accurately applied, thereby improving signal quality.

[0041] Once the laying of the optical cable (10) is complete, the location information and path of the laid optical cable can be checked using a side-scan sonar (8) on a GPS-equipped vessel (7), and in shallow areas, a diver can be used as an auxiliary.

[0042] In addition, in one example of the present invention, a vertical optical cable (20) may be additionally used in addition to the horizontal optical cable (10) laid on the seabed. That is, the vertical optical cable (20) is installed by being buried vertically along the wall of the borehole (5) for injecting carbon dioxide into the seabed reservoir (2).

[0043] The fixed continuous vibration source (30) is intended to apply continuous vibration to the reservoir (2) for a certain period of time at a fixed location. Generally, in marine seismic exploration, it is common to apply shock wave-type vibration using an air gun, etc., but air guns, etc. are difficult to install in a fixed position and there is also the problem of disturbing the marine ecosystem as mentioned above. Accordingly, the present invention generates a stable and continuous sweep signal in a frequency range of several Hz to several hundred Hz. The vibration propagates along the seabed and seabed strata.

[0044] In this example, an eccentric motor equipped with waterproof and pressure-resistant facilities can be placed on the seabed and used as a vibration source (30). Although the eccentric motor may be operated by a battery, as shown in FIG. 4, it is common for a carbon dioxide storage facility to have an offshore platform (9), so a power cable is connected to the eccentric motor for use. However, even if a vibration source such as an eccentric motor is not installed separately, if there is an offshore wind turbine (not shown) near the reservoir, the vibration generated by the wind turbine can be utilized. Since the wind turbine is also fixed in position and generates a stable and continuous signal with low energy, it can function as a fixed continuous vibration source of the present invention. When using a wind turbine as a vibration source, there is an advantage that the system can be operated very economically, just as the communication optical cable was utilized as a fiber optic sensor as described above.

[0045] The receiver (35) is installed in close proximity to a fixed continuous vibration source. The receiver (35) receives the sweep signal generated from the vibration source (30) in the form of a direct wave transmitted through the ground, rather than a reflected wave reflected from the ground layer. The direct wave received by the receiver (35) preserves the original sweep signal form and is used as a reference waveform in the sweep deconvolution processing described later. Various forms of receivers may be used, but in this example, a geophone is used.

[0046] The interrogator (40) is installed on a marine platform and is connected to an optical cable (10, 20) laid on the seabed to implement a distributed acoustic sensing system.

[0047] The interrogator (40) is equipped with a light source (41). Laser light emitted from the light source is converted into a light pulse through a light modulator (42) and transmitted to the optical cable (10, 20) through a light circulator (43). The optical signal travels in the forward direction along the optical cable (10, 20), but there is backscattered light that is reflected back due to light scattering at each point (continuous) of the optical cable through which the light passes. That is, the light propagating through the optical cable collides with particles smaller than the wavelength of light due to the non-uniformity of the optical cable's density, causing scattering and generating a reflected signal (backscattered light) in the reverse direction. The backscattered light returning from the optical cable is guided to the optical amplifier (44) through the light circulator (43). The optical amplifier amplifies the backscattered light and transmits it to the photodetector (45), and the photodetector (45) converts the backscattered light into an electrical signal. Finally, the signal conversion unit (46) converts the electrical signal (analog signal) into a digital signal.

[0048] The components of the interrogator (40) include various elements such as an ASE bandpass filter in addition to the elements described above, but since this is a widely known technology in the field, a detailed explanation will be omitted.

[0049] Backscattered light originates at every point along the optical cable and returns; the backscattered light generated at the beginning of the cable arrives first, while that generated at the end arrives last. If the backscattered light returning after a single optical pulse transmission is arranged in chronological order, it corresponds to the order in which it returns from each point along the cable, from the beginning to the end. In other words, the arrival time of the backscattered light is ultimately identical to the point (distance) where the backscattered light originated within the optical cable. If a specific part of a signal detected continuously in chronological order (or distance) exhibits an abnormal pattern different from the existing one, it can be confirmed that external factors, such as vibration or temperature changes, have acted on the optical cable at that point.

[0050] In other words, in a steady state where there is no external action on the object, the reflected signal exhibits a constant pattern. However, when external influences such as vibration act on the object or optical cable, the scattering magnitude, frequency, and phase of the light change, causing the reflected signal to display a pattern different from that of the steady state. By detecting this reflected signal pattern, which differs from the steady state, it is possible to infer that an event has occurred on the object or optical cable. Furthermore, as mentioned earlier, since backscattered light is detected in chronological order, if an abnormal pattern signal arrives at a certain point after the normal pattern signal has arrived first, it is possible to identify the specific point in the optical cable where this abnormal signal originated.

[0051] As explained above, using the DAS system allows for obtaining backscattered light signals that are scattered and return from all points of the optical cable, and since the optical cable sensor has a spatial resolution of at least 0.25m, it produces the same effect as installing existing vibration sensors, such as geophones or hydrophones, at 0.25m intervals.

[0052] Meanwhile, when the fixed continuous vibration source (30) vibrates continuously for a certain period of time (e.g., 20 minutes), the vibration is transmitted to the optical cable in various forms through the ground layer. That is, as indicated by a and c in FIG. 2, the vibration can be transmitted directly to the optical cable (10, 20) through the ground layer. In addition, when the medium changes, such as at the ground layer boundary or the carbon dioxide reservoir (3), the vibration is reflected and transmitted to the optical cable (10, 20) in the form of a reflected wave, as indicated by b and d in FIG. 2.

[0053] The left and right sides of FIG. 3 represent DAS signals obtained from a vertical optical cable (20) installed in a borehole and a horizontal optical cable (10) installed on the seabed, respectively. The signal (a) transmitted directly from the vibration source (30) in the vertical optical cable (20) shows a downward signal shape as the signal transmission time (T) becomes slower as the depth (h) of the borehole increases, whereas the reflected wave (b) signal reflected from the seabed layer shows an upward signal shape as the signal transmission time (T) becomes faster as the depth of the optical cable (20) increases. In the horizontal optical cable (10), the signal (c) transmitted directly from the vibration source (30, point V) shows a signal transmission time (T) becoming slower as the distance (d) from the vibration source increases. Since the reflected wave (d) propagates upward in a concentric circle shape from the point of reflection, the signal shape in the horizontal optical cable (10) installed on the seabed shows a hyperbolic signal shape.

[0054] The digital signal generated by the interrogator (40) is converted into an impulse response form by the signal conversion unit (50) using an impulse-type seismic wave transmitter and receiver. In conventional marine seismic exploration, when seismic waves are transmitted using an impulse-type transmitter, such as an air gun, the reflected waves reflected from each stratum boundary of the seabed are received in the form of pulse waves. When received in the form of pulse waves, it is easy to identify the boundaries of the strata. However, when a low-energy continuous sweep signal, such as from an eccentric motor, is transmitted and reflected, the signal is spread out over a long distance, making signal interpretation difficult. Accordingly, in the present invention, the sweep signal is converted into a pulse wave form using a sweep deconvolution technique in the signal conversion unit (50). Hereinafter, the converted signal is referred to as the 'impulse wave type converted signal'.

[0055] Figure 6 shows data received by the DAS system after transmitting a sweep signal from an eccentric motor, and Figure 7 shows the result of sweep deconvolution processing of the DAS data. In Figures 6 and 7, the X-axis is the depth axis, with the center being the location of the vibration source; the depth decreases as it moves to the left and increases as it moves to the right. The Y-axis is the time at which the signal was recorded. In Figure 6, the DAS signal, which records a low-energy continuous sweep signal, is difficult to interpret because the signal is spread out and not distinct, but in the shock wave reflection signal after sweep deconvolution, the signal is clearly distinguishable and easy to interpret.

[0056] Sweep deconvolution is briefly explained with reference to Figure 5 to explain the concept of sweep deconvolution processing.

[0057] In FIG. 5, the Pilot Sweep on the left is a sweep signal generated from a continuous vibration source (30). The vibration source vibrates for, for example, a few minutes to generate a sweep signal, and the sweep signal is transmitted directly through the ground to a receiver (35) installed close to the vibration source (30) and received. The entire sweep signal for a few minutes is recorded at the receiver. Then, the sweep signal propagates through the ground layer and is reflected at a point where the medium changes, for example, the boundary of the ground layer or a reservoir where carbon dioxide is stored, and is applied to an optical cable to be recorded as a DAS signal.

[0058] In Fig. 5, the right side of the Pilot Sweep signal shows the reflected waves (Reflection 1, 2, 3) reflected by the geological layer or the carbon dioxide reservoir. The Field Recording to the right of the reflected waves represents the actual signal received by the DAS interrogator. The actual signal received by the DAS interrogator appears as a form in which the aforementioned reflected waves A, B, and C partially or entirely overlap over time. For example, if the boundary line of geological layer A, the boundary line of geological layer B, and the carbon dioxide reservoir are arranged sequentially in the depth direction from the seafloor, the sweep signal generated from the vibration source for 5 minutes is reflected and recorded at the boundary line of geological layer A, the boundary line of geological layer B, and the boundary line of the carbon dioxide reservoir, respectively. In the sweep deconvolution processing, as shown in the OUTPUT of Fig. 5, the point where the reflected wave was generated can be detected by comparing the signal waveforms of the sweep signal and the reflected wave recorded by the receiver. In addition, when there is not a significant difference in depth between the boundary lines of layer A, layer B, and the carbon dioxide reservoir layer above, reflected wave signals may overlap, but each reflected wave can be identified through signal processing. Looking at the output of Figure 5, the point where the reflected wave is generated can be identified because an impulse wave-shaped reflected signal appears at the point where the reflected wave is generated as a result of sweep deconvolution.

[0059] In the present invention, vibration is generated for several minutes using the method described above, and when a signal is recorded in the DAS system, it undergoes signal processing and conversion to be recorded as an impulse waveform conversion signal as shown in the OUTPUT of Fig. 5. Then, this sensing operation is repeated several times with a time interval. In the signal enhancement unit (60), multiple impulse waveform conversion signals are combined to improve the signal-to-noise ratio. When signals are combined multiple times, random noises cancel each other out and do not increase, but signals overlap and increase, so the signal can be clearly distinguished from the random noise.

[0060] The signal passing through the signal enhancement unit (60) is analyzed by the time series analysis unit (70). The time series analysis unit (70) detects changes in the state of the reservoir using the results of monitoring at time intervals, that is, before and after the injection of carbon dioxide. Alternatively, it detects differences from the results of monitoring performed at different times during the injection of carbon dioxide.

[0061] Figure 8 is a diagram illustrating time-lapse analysis. Referring to Figure 8, (A) is the result of monitoring before injecting carbon dioxide, and (B) is the result after injecting carbon dioxide. The difference between before and after injection is shown in (C). Looking at the arrow in (C), a thick line is drawn upward to the right; this indicates a state that was not recorded before injection and changed after injection. In time-lapse analysis, as illustrated in Figure 3, time-lapse changes can be detected by deriving an upward wave in the form of a straight line sloping upward and a reflected wave in the form of a hyperbola.

[0062] As described above, once the time series analysis is completed, the Kirchhoff migration technique is used to display the geological layers including the reservoir on a 2D or 3D screen as shown in Fig. 9 to improve visibility. When the signal recording form of Fig. 8 is subjected to Kirchhoff migration, it becomes the form shown in Fig. 9. In Fig. 9, (A) represents the time before carbon dioxide injection, (B) represents the time after injection, and (C) represents the difference; the X-axis represents the distance from the borehole, and the Y-axis represents the depth (or time). Looking at Fig. 9, from the perspective of carbon dioxide monitoring, it is possible to identify the pool filled with carbon dioxide in the reservoir before and after the injection of carbon dioxide.

[0063] For reference, the signal conversion, signal enhancement, time series analysis, and Kirchhoff migration transformation described above, which are located after the interrogator, are processed by executing software on a computer.

[0064] As described above, the present invention monitors the behavior of carbon dioxide in a seabed carbon dioxide reservoir using a DAS system. In this invention, since the position of the vibration source is fixed as well as the position of the optical cable, monitoring can always be performed under constant conditions and environments, unlike conventional offshore exploration, thereby improving the reliability of the analysis of monitoring results. Furthermore, by performing monitoring using a DAS system, the monitoring operation is not only very simple but also economical compared to existing monitoring systems using exploration vessels and streamers. Consequently, not only is very frequent monitoring possible, but continuous real-time monitoring is also possible during carbon dioxide injection, which is advantageous for understanding the behavior of carbon dioxide.

[0065] In one example of the present invention, an offshore wind turbine is used as a fixed continuous vibration source, and by using a communication optical cable already installed on a carbon dioxide storage offshore platform as a DAS system optical cable, there is an advantage in that the investment facilities for the monitoring system can be minimized.

[0066] When laying optical cables separately on the seabed, there is an advantage in that the cable installation is easy because a structure has been developed that allows the cable to settle accurately on the seabed without breaking during the installation process.

[0068] The scope of protection of the present invention is not limited to the description and expression of the embodiments explicitly described above. Furthermore, it is added once again that the scope of protection of the present invention cannot be limited by obvious changes or substitutions in the technical field to which the present invention belongs. Explanation of the symbols

[0069] 10 ... horizontal optical cable, 20 ... vertical optical cable 30 ... vibration source, 35 ... receiver 40 ... interrogator, 50 ... signal converter 60 ... Signal Enhancement Unit, 70 ... Time Series Analysis Unit

Claims

Claim 1 An underground carbon dioxide monitoring system using a submarine optical cable, characterized by comprising: an optical cable laid on the seabed of a carbon dioxide submarine storage facility; a fixed continuous vibration source that generates a sweep signal by applying continuous vibration for a certain period of time at a fixed location of the underground storage facility; a receiver installed in close proximity to the fixed continuous vibration source to directly receive the sweep signal that is not reflected from the ground layer; an interrogator comprising a light source that irradiates a laser light onto the optical cable, a photodetector that receives a backscattered light signal returning from each point of the optical cable from the laser light irradiated from the light source, and a signal processing unit that processes the backscattered light; and a signal conversion unit that converts the backscattered light signal received from the interrogator into a waveform form explored by an impulse-type elastic wave transmitter and a receiver. Claim 2 A subsurface carbon dioxide monitoring system using a submarine optical cable, characterized in that, in the first paragraph, the signal conversion unit generates the backscattered light signal into a converted signal in the form of an impulse response received using an impulse-type elastic wave transmitter and a receiver through sweep deconvolution processing that compares the backscattered light signal processed by the signal processing unit of the interrogator with the sweep signal received by the receiver. Claim 3 An underground carbon dioxide monitoring system using a submarine optical cable according to claim 1, characterized in that a strength reinforcing wire capable of withstanding tension is attached together so that the optical cable does not break when the optical cable is laid on the seabed from the sea. Claim 4 A subsurface carbon dioxide monitoring system using a submarine optical cable, characterized in that, in paragraph 3, it further comprises a plurality of weights coupled to the strength reinforcing wire or the optical cable so that the optical cable can be settled on the seabed. Claim 5 An underground carbon dioxide monitoring system using a submarine optical cable according to claim 1, further comprising a signal enhancement unit that increases the signal-to-noise ratio by performing monitoring on the optical cable at a time interval to generate a plurality of shock wave reflection signals and then combining the plurality of shock wave reflection signals. Claim 6 An underground carbon dioxide monitoring system using a submarine optical cable, characterized in that, in claim 1, it further comprises a time-lapse analysis unit that generates the shock wave conversion signal at different points in time before and after carbon dioxide injection or during carbon dioxide injection, and then performs a time-lapse analysis to derive the difference of the shock wave conversion signal at each point in time. Claim 7 A subsurface carbon dioxide monitoring system using a submarine optical cable, characterized in that, in claim 6, the time series analysis unit separates a signal in the form of a reflected wave from a shock wave-shaped converted signal at a different time point to identify changes in the reservoir layer. Claim 8 A subsurface carbon dioxide monitoring system using a submarine optical cable according to claim 6, wherein a borehole is formed in the reservoir, a vertical optical cable connected to an interrogator is installed and arranged vertically on the wall of the borehole, and the time series analysis unit separates an upward-facing reflected wave signal from the shock wave type conversion signal acquired and converted from the vertical optical cable at different time points to identify changes in the reservoir. Claim 9 A subsurface carbon dioxide monitoring system using a submarine optical cable, characterized in that, in claim 1, the fixed continuous vibration source is an offshore wind turbine. Claim 10 An underground carbon dioxide monitoring system using a submarine optical cable, characterized in that, in claim 1, the optical cable is a communication optical cable already installed on the seabed.

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