JP2008014830A - Hydrate existence domain survey method and survey system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a hydrate existence domain survey method and a survey system capable of surveying a methane hydrate existence domain in a sea-bottom stratum where a sea-bottom pseudo reflection surface exists. <P>SOLUTION: A sea-bottom stratum structural drawing is drawn by using a low-frequency band from a measurement result of seismic survey using a seismic source having a high-frequency band, and a signal spectrum of a measurement result on the furthermore upside than the sea-bottom pseudo reflection surface is determined on a place of the sea-bottom pseudo reflection surface specified from the stratum structural drawing, and a portion wherein a spectrum is large over the whole frequency of the survey and the spectrum intensity in a high-frequency band is greater than the spectrum intensity in a low-frequency band is extracted. If an extracted portion does not exist, it is determined that the hydrate does not exist, and if an extracted portion exists, it is determined that the hydrate exists in the extracted portion. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  TECHNICAL FIELD The present invention relates to a method and system for exploring a hydrate existing area in a submarine stratum, and more specifically, a hydrate capable of detecting a hydrate existing area in a submarine stratum where a submarine pseudo-reflecting surface exists. The present invention relates to an existence area exploration method and an exploration system.

  There is a considerable amount of hydrate such as methane hydrate that can be an energy resource on the sea floor around Japan, and it is desired to make effective use of this. This requires exploration of these hydrate resources.

  One of the exploration means is reflection seismic exploration. In this seismic reflection survey, vibrations (sound waves) are generated by vibration sources (transmitters) such as air guns, piezoelectric elements, and giant magnetostrictive alloys that immediately release compressed air into the sea near the sea surface, in the sea, and at the bottom of the sea. The received sound waves are received by hydrophones (receivers) that are arranged at intervals in a cable called a streamer. In other words, the geological structure is analyzed by searching for the boundary surface between the strata with different physical properties that appear in the seismic survey record under the seafloor.

  This sound wave propagates through the sea, and the sound wave transmitted through the sea is reflected by the bottom of the sea, and a certain amount of sound returns to the receiver. However, there are sound waves that enter the formation below the ocean floor, and the sound waves that enter the formation are reflected at the boundary between the formations and return to the receiver. In the seismic survey record under the seafloor, the boundary surface between strata with different physical properties appears as a black line, and this boundary surface indicates the geological structure. This is called reflection seismic exploration because it uses the reflected sound waves for exploration.

  The depth of the sound wave that reaches under the seabed and returns to the receiver depends on the equipment used, but in seismic exploration for oil exploration, records up to about 10,000 m below the seabed. May take. In recent oil exploration seismic exploration, the streamer cable may be 6,000 m or more, or a plurality of streamer cables may be towed at the same time for investigation. When a plurality of streamer cables are used, a three-dimensional seismic survey record can be obtained.

  With regard to this seismic reflection survey, ocean exploration uses a low frequency of about 100 Hz because of the long distance and attenuation of sound waves, so the resolution of reflected waves is low and high-precision geological exploration is difficult. A submarine geological exploration system has been proposed in which vibration waves are generated from an autonomous unmanned vehicle to be sailed, and this vibration wave is received by a streamer cable towed by the autonomous unmanned vehicle (for example, (See Patent Document 1).

  In particular, in reflection seismic surveys targeting methane hydrate, the ocean floor pseudo-reflecting surface suggesting the existence of a hydrate layer can be confirmed by exploration at a low frequency (for example, 100 Hz) using an air gun or the like. There may be cases where it cannot be clearly confirmed by high-frequency exploration (e.g., 600 Hz) using a towed object, and a blanking phenomenon with a small amplitude is confirmed directly above the bottom simulated reflector (BSR). The characteristics of are known.

  In order to estimate the abundance of hydrates, etc. under the seabed, an accurate resource assessment method is required. However, in the conventional assessment method, hydrate is directly above the seafloor pseudo-reflecting surface that appears in seismic survey records. It is considered to exist, and the hydrate existence region is specified by detecting the seafloor pseudo-reflecting surface appearing in the seismic survey record. In other words, based on the distribution of the submarine pseudo-reflecting surface, which is regarded as an indicator of hydrate presence, the submarine pseudo-reflecting surface appearing on seismic exploration records is set as the lower limit of the hydrate, and the upper limit is the logging data, seismic exploration results The thickness of the hydrate layer is estimated from the interpretation (blanking, sandstone layer distribution), precise gravity survey, etc.

  In addition, in the Japanese Patent Application No. 2004-327728, the inventors of the present invention have the earliest time among reflected waves at short intervals appearing on the seismic survey record obtained by exploring at a high frequency of 300 Hz to 1 KHz. By making the reflected wave the upper limit of the hydrate layer, the upper surface of the hydrate layer that could not be explored by the prior art was determined, and a method for estimating the thickness of the hydrate layer was proposed.

  This hydrate layer thickness estimation method is intended for the alternate layer structure of a thin hydrate layer and a thin geological layer. In the case of a thin alternate layer structure, reflected waves from the boundary surface are clearly observed in the high frequency band. Therefore, it is extremely effective, but there is a problem that the applicable range is limited to a thin-layered alternating layer structure.

  However, there are many cases where clear reflected waves from the upper surface of the hydrate layer are not observed in seismic exploration records, and when excavation was made at multiple points where the seafloor pseudo-reflection surface exists, the seafloor pseudo-reflection surface exists. However, it has become clear that the hydrate layer may not exist. For this reason, it is desired to develop a more general hydrate existence region search method that can be applied to such a case.

  At present, the structure of the hydrate layer has not been elucidated, and an effective exploration method has not been established. However, from the results of seismic exploration for the hydrate layer, a low-frequency seismic survey shows a seafloor pseudo-reflecting surface, but a high-frequency seismic survey does not observe a submarine pseudo-reflecting surface. It has been found that the upper surface of the hydrate layer cannot be specified even in frequency.

  From these results, the present inventor made the following consideration. If the hydrate layer is homogeneous, reflected waves are generated from the top surface of the hydrate layer because the sound velocity in the formation and the hydrate layer is different, but the top surface of the hydrate layer cannot be detected from the reflected wave. Is not considered homogeneous.

  In addition, the low-frequency seismic waves can be detected from the submarine pseudo-reflecting surface, which is a reflected wave from free gas or water existing on the lower surface of the hydrate layer, so the low-frequency seismic waves are greatly affected by the hydrate layer. Not originally considered. In addition, it is considered that high-frequency seismic waves do not reach the free gas layer, or reflected waves from the free gas layer do not reach the reception point, and thus are greatly affected by the hydrate layer.

  Assuming that the hydrate layer is composed of a large number of scattered small hydrate blocks, low-frequency seismic waves are not easily reflected by these hydrate blocks, and high-frequency seismic waves are reflected by these hydrate blocks. As a result, it is easy to reach the free gas layer with low-frequency seismic waves, and you can observe the seafloor pseudo-reflecting surface. However, with high-frequency seismic waves, you cannot reach the free gas layer and cannot observe the seafloor pseudo-reflecting surface. Can be explained.

In the case of this hydrate layer configuration, even if high-frequency seismic waves are used, reflection from individual hydrate blocks is very small and cannot be observed as clear reflected waves. It is considered very difficult to specify.
Japanese Patent Laid-Open No. 2003-19999

  The present invention has been made in order to solve the above-mentioned problems based on the above consideration, and an object of the present invention is to detect a hydrate-existing region in a submarine formation where a submarine pseudo-reflecting surface exists. It is an object of the present invention to provide an exploration method and exploration system for an existing area.

  The hydrate existence region exploration method of the present invention for achieving the above object is a hydrate existence region exploration method for exploring hydrate under the seafloor by a reflection seismic exploration method, and has a wide frequency band. Performing a seismic exploration using an epicenter, creating a submarine geological structure map using a low frequency band from the seismic exploration measurement results, and identifying a submarine pseudo-reflecting surface existing area from the geological structure map; Obtaining a spectrum in a wide frequency band where the submarine pseudo-reflecting surface exists, and in the spectrum, the spectrum is large over the entire frequency of exploration and the spectrum intensity in the high frequency band is in the low frequency band. Extracting a site greater than the spectral intensity, and if there is no such site, methane hydrate does not exist. And a step of, characterized by comprising a determining that said extraction region methane hydrate at the site extraction if any are present.

  This wide frequency band refers to a frequency band having a frequency width in the range of about 100 Hz to 800 Hz, preferably about 100 Hz to 1000 Hz, and a low frequency band is a frequency in the range of about 100 Hz to 200 Hz. Say bandwidth. The high frequency band means a frequency band higher than the low frequency band.

  The seafloor pseudo-reflecting surface is also called BSR (Bottom Simulated Reflector), which is a sound wave boundary surface that extends ignoring the records showing the surrounding boundary layers in order, and is almost parallel to the records showing the seabed. The reflection properties are different (the phase is reversed), and the reflection intensity is high (appears as a thick line for recording).

  This spectrum can be obtained by applying an arbitrary frequency analysis method such as a fast Fourier transform (FFT) method, a maximum entropy (MEM) method, or a wavelet (WT). This spectrum is calculated using data in a frequency band as wide as possible, preferably data in all measured frequency bands, where the seafloor pseudo-reflecting surface exists.

  Further, the fact that the spectrum is large over the entire frequency of exploration is determined by comparison with a spectrum in a region where there is no surrounding hydrate. Since the temperature range and pressure range in which the hydrate does not exist are known in the region where the hydrate does not exist, it can be determined with high accuracy that the hydrate does not exist.

  Further, in the above-described hydrate presence region search method, the calculation accuracy can be improved by using the common reflection point polymerization method when obtaining the spectrum in the wide frequency band. This common reflection point superposition method is also called CDP (Common Depth Point) superposition method, and the reflected wave with different path is converted (NMO: Normal Move out correction) during vertical travel at the common reflection point (CDP) position. This is a method of emphasizing the reflected wave by superimposing (adding), correcting and adding the arrival times of the reflection records having different distances where the reflection points are common.

  Further, in the hydrate existence region exploration method, the step of obtaining for each analysis window for moving the spectrum in the depth direction, and setting the depth at which the analysis window starts to change as the upper surface of the hydrate layer And a step of setting the seafloor pseudo-reflecting surface as the lower surface of the hydrate layer, the upper and lower surfaces of the hydrate layer can be specified. The analysis window which is the cut-out part is a window relating to the depth, in other words, the time for the reflected wave to reach the receiver, and may be overlapped, continuous, or spaced apart. .

  A hydrate existence area exploration system for achieving the above object is a hydrate existence area exploration system for exploring hydrate under the seafloor by a reflection seismic exploration method, and a sound wave having a wide frequency band. And a receiver for receiving the reflected wave from the formation of the sound wave transmitted by the transmitter, and an analysis means for analyzing the signal of the reflected wave received by the receiver. Then, the analysis means specifies the existence area of the seafloor pseudo-reflecting surface from the submarine stratum structure map created using the low frequency band of the seismic survey measurement result, and in the place where the seabed pseudo-reflecting surface exists, The spectrum obtained in the wide frequency band of the seismic survey results is large over the entire frequency of the survey, and the spectrum intensity in the high frequency band is low and the spectrum intensity in the low frequency band Ri extracting large portions, if not the extraction site, hydrate determines that there is no configured to determine a hydrate is present in the extraction region if the extraction site.

  In the hydrate layer existence region exploration system, the analysis unit is configured to use a common reflection point polymerization method when obtaining the spectrum.

  Furthermore, in the hydrate layer existence region exploration system, the analysis means obtains each analysis window for moving the spectrum in the depth direction, and determines the depth at which the extraction window starts to appear in the spectrum. The upper surface of the hydrate layer is used, and the submarine pseudo-reflecting surface is used as the lower surface of the hydrate layer.

  According to these hydrate existence area search systems, the above-described hydrate existence area search method can be implemented, and the same effects can be achieved.

According to hydrate existence region search method and search system of the present invention, since the existing area of the hydrate becomes possible estimated by submarine seismic exploration, it is not necessary to perform a high drilling cost, economically hydrate It can be used.

  Embodiments of a hydrate existing region exploration method and exploration system according to the present invention will be described below with reference to the drawings, taking as an example the case of searching for methane hydrate using an autonomous unmanned vehicle.

  First, a hydride existence region exploration system for carrying out this hydride existence region exploration method will be described. As shown in FIG. 1, this exploration system includes an autonomous unmanned vehicle 1, a mother ship 6, a transponder 4 installed on the seabed 5, and the like.

  The autonomous unmanned aerial vehicle 1 includes a variable frequency vibration source (transmitter) 2 at the bottom and a transmission / reception device 7 at the stern, and tows a streamer cable 3 from the stern. Since the variable frequency vibration source 2 cannot use an air gun due to the relationship between the scale of the autonomous unmanned vehicle 1 and the power source, the variable frequency vibration source 2 is composed of a small and light-weight piezoelectric element, a giant magnetostrictive alloy, or the like. These have the merit that they can be transmitted continuously and the waveform can be processed electrically. The streamer cable 3 is a hydrophone (receiver) that captures reflected waves, a sensor module that detects depth, direction, temperature, and the like, a transmission module that transmits data, and vibrations of the autonomous unmanned vehicle 1 A vibration control module that prevents transmission to the cable 3 and a stabilizer module that controls the attitude of the streamer cable 3 are provided.

  The autonomous unmanned aerial vehicle 1 is transported to the exploration sea area by the mother ship 6, then lowered into the sea and receiving signals transmitted from a plurality of transponders 4 previously installed on the seabed 5. The stern transceiver 7 and the transmitter / receiver 8 of the mother ship 6 travel in a predetermined sea area at a predetermined depth while communicating and detecting a relative position. For example, it travels while maintaining a height of several meters to several hundred meters from the sea floor.

  During this autonomous traveling, a vibration wave P <b> 1 is emitted from the variable frequency vibration source 2 at the bottom of the autonomous unmanned traveling body 1 toward the seabed 5. The reflected wave of the vibration wave P1 is received by the hydrophone of the streamer cable 3 being towed by the autonomous unmanned vehicle 1 and recorded in the recording device. In this case, even if the vibration wave P1 has a relatively high frequency, it is reflected from the hydrophone of the streamer cable 3 that is emitted from the vicinity of the seabed 5 and is reflected by the streamer cable 3 that is towed near the seabed. Can receive waves. Therefore, high resolution can be obtained.

  After this seismic exploration, the autonomous unmanned vehicle 1 is collected in the mother ship 6 and the data recorded in the recording device is analyzed to analyze the stratum structure and the like. Thereby, the seismic exploration by the vibration wave P1 is performed.

  Here, as an example of a hydride existence area exploration system for implementing this hydride existence area exploration method, a reflection seismic exploration system using an autonomous unmanned vehicle is taken as an example. Any system can be used as long as it can be a seismic exploration system and can conduct exploration at a predetermined low frequency and high frequency.

  The analysis device (analyzing means) of this hydrate existence region exploration system is composed of a data collection unit and a data processing unit. The data collection unit is a part that records data received by the hydrophone in a recording device, and the data processing unit is composed of an earthquake exploration processing unit and a hydrate existing region exploration unit. This seismic exploration processing unit draws the structure of the stratum and detects the pseudo-bottom reflection surface that is estimated to be the lower surface of the methane hydrate layer. The hydrate existence region exploration unit determines whether or not methane hydrate exists, The upper surface of the hydrate layer is estimated, and processing for specifying the hydrate existing region is performed.

  This hydrate existence region exploration method uses seismic waves in a wide frequency band of about 100 to 800 Hz, preferably about 100 to 1000 Hz in the first S1 step, as shown in the flow of FIG. 2 in exploring methane hydrate. Wide frequency seismic survey.

  In the next step S2, in the data of the measurement result, the seafloor pseudo-reflecting surface (BSR) is detected from the response data for a low frequency that falls within the range of 100 to 200 Hz, and the lower surface of the methane hydrate layer is obtained. In other words, using the low-frequency component near 100 Hz of the response waveform, draw a submarine stratum structure map by normal seismic exploration processing, search the submarine pseudo-reflecting surface from this stratum structure map, and detect the detected submarine pseudo-reflecting surface. The lower surface of the methane hydrate layer.

  In the next step S3, a reflected wave having a different path is generated by using a polymerization method such as a common reflection point (CDP) polymerization method in the range immediately above the ocean bottom pseudo-reflection surface for the region where the ocean bottom pseudo-reflection surface exists. Conversion and addition (addition) are performed during vertical travel at the common reflection point position, and frequency analysis is performed on this overlapped waveform. In other words, in the upper part of the portion where the seafloor pseudo-reflecting surface is detected, a superposition waveform is calculated using a wide range, preferably all the measured frequency bands, and a frequency analysis of the superposition waveform is performed. .

  In this frequency analysis, as shown in FIG. 3, an analysis window (time window) W is provided, and the frequency analysis of the signal in the analysis window is performed to obtain a kind of sonagram. The analysis window W is moved in the depth direction of the vertical axis (time direction of the time axis) to obtain a spectrum at each depth. The analysis windows W may overlap, may be continuous, or may be spaced apart from each other. In short, it is only necessary to obtain information on changes in the spectrum in the depth direction. In this frequency analysis, any frequency analysis method such as Fast Fourier Transform (FFT), Maximum Entropy Method (MEM), Wavelet (WT), etc. can be applied. Alternatively, a spectrum that is an energy distribution is obtained.

  In the next step S4, the spectrum is checked, and the state of the spectrum obtained sequentially according to the movement of the analysis window W is inspected. As a result, the spectrum is large or the high frequency component is higher than the low frequency component. Determine if it is strong. Note that if only the presence or absence of methane hydrate is determined, the spectrum may be checked with a single large analysis window covering a range where methane hydrate above the seafloor pseudo-reflecting surface is likely to exist.

  If it is determined in step S4 that the spectrum is not large compared to normal or the spectrum value of the high frequency component is not large compared to the low frequency component as schematically shown in FIG. Go to, and determine that there is no methane hydrate. As a result, the exploration is terminated, and exploration in the next area is performed as necessary.

  Further, in the determination of step S4, when the spectrum is large compared to the normal spectrum and the spectrum value of the high frequency component is large compared to the low frequency component as schematically shown in FIG. Go to step S6 and determine that methane hydrate is present.

  This normal spectrum is a spectrum in a region considerably above the seafloor pseudo-reflecting surface or a region where there is no surrounding methane hydrate. Whether or not this methane hydrate can exist can be determined with high accuracy by using these data together because the temperature range and pressure range for the methane hydrate to exist are known. Further, in practical use, the determination is made by comparison with a spectrum in a region considerably above the seafloor pseudo-reflecting surface or a region where the surrounding methane hydrate layer does not exist.

  Considering the results of the simulation using the following geological model, the spectrum shown in FIGS. 4 and 5 is typically obtained in a region where methane hydrate does not exist and a region where methane hydrate exists. However, in reality, the spectrum is curved due to the frequency characteristics of the epicenter and other influences, and it is thought that its shape changes depending on the situation.

  Going to the next step S7, the spectra for each analysis window W moved in the depth direction are compared, and the depth at which the spectrum starts to change is defined as the upper surface of the methane hydrate layer. The region from the depth at which it is first determined that methane hydrate exists to the ocean floor pseudo-reflecting surface is defined as the methane hydrate existing region. As a result, the exploration is terminated, and exploration in the next area is performed as necessary.

  Next, the results of the seismic response to a geological model with a methane hydrate layer composed of many scattered methane hydrate clumps are shown by simulation calculation. FIG. 6 shows the formation model 10 having the methane hydrate layer 15, and FIG. 7 shows details of the methane hydrate blob group. For comparison, the seismic response to the geological model from which the methane hydrate block was removed was also obtained. FIG. 8 shows a formation model 10A from which the methane hydrate lump group 15 has been removed.

  In these formation models 10 and 10 </ b> A, a first formation 12, a free gas formation 13, and a second formation 14 are provided under the seawater 11. In the formation model 10, a methane gas hydrate mass group is formed on the free gas formation 13. 15 was placed. This geological model 10 has a full size, a width of 390 m, a total depth of 440 m including a seawater portion of a depth of 320 m, the first geological layer 12 is 55 m thick, and the free gas layer 13 is 5 m thick. The second formation 14 has a thickness of 60 m. The lumps 15a of the methane hydrate lump group 15 have dimensions corresponding to a trapezoidal shape having an upper base length of 1 m, a lower base length of 3 m, and a height of 1 m in actual dimensions. And the physical property value of each layer is given to each layer. The sound velocity in the methane hydrate was 3000 m / s (longitudinal wave) and 1175 m / s (lateral wave).

  In this seismic response test, a sound wave was transmitted from a transducer (transmitter) 16 at a transmission point at a water depth of 280 m, and a reflected wave was received by a hydrophone (receiver) 17 at a reception point at a water depth of 280 m. The reflected wave was received while moving the transmission / reception point and changing the distance D between the transmission point and the reception point from 50 m to 90 m.

  In the formation models 10 and 10A, assuming that the longitudinal wave velocity of the formation is 1800 m / s, 1 m corresponds to a wavelength of 1800 Hz. The wavelength of 100 Hz is 18 m, the wavelength of 500 Hz is 3.6 m, and the wavelength of 1000 Hz is 1.8 m.

  9 to 12 show seismic responses at 100 Hz, 200 Hz, 400 Hz, and 500 Hz, respectively. (A) of the figure shows the case where there is a methane hydrate lump 15a, and (b) of the figure shows the case where there is no methane hydrate lump 15a. In these figures, the horizontal axis represents the distance (offset) D between the transmission and reception points, and the vertical axis represents time t, which corresponds to the depth. At this time t, the time when the transducer 16 which is the epicenter generates a sound wave is displayed as zero.

  9 to 12, mainly two reflected waves appear. The reflected wave indicated by A in the figure is a reflected wave from the sea bottom, and the reflected wave indicated by B is reflected from the free gas layer. It is a wave and corresponds to the ocean floor pseudo-reflecting surface. FIG. 9 shows that when the frequency is 100 Hz, the influence of the methane hydrate lump 15a is small, and there is almost no difference depending on the presence or absence of the methane hydrate lump 15a. On the other hand, as shown in FIGS. 10 to 12, when the frequency is increased, the reflection corresponding to the seafloor pseudo-reflecting surface is reduced, and a minute reflected wave is generated immediately before the seafloor pseudo-reflecting surface. It was also found that the change increases as the frequency increases. These earthquake response results are in good agreement with conventional knowledge.

  Next, the results of performing common reflection point (CDP) polymerization on the measurement data of FIGS. 9 to 12 are shown in FIGS. In FIG. 13, there is almost no difference in the polymerization waveform depending on the presence or absence of the methane hydrate lump 15a, but in FIG. 14, the strength of the seafloor pseudo-reflecting surface is slightly weakened, and as shown by the arrow C, the seafloor pseudo-reflecting is shown. A minute reflection is seen in front of the surface. This tendency is strong in FIGS. 15 and 16.

  From the examination of the seismic response waveforms shown in FIGS. 9 to 12 and the polymerization waveforms shown in FIGS. 13 to 16, the methane hydrate layer 15 is composed of a large number of scattered methane hydrate blobs 15a. It is confirmed that the geological model 10 is effective for exploring the methane hydrate layer 15, and the present invention is applied to the methane hydrate layer 15 composed of a large number of scattered methane hydrates 15a. It proved to be an effective method for exploring the existence area of hydrate.

It is a figure for demonstrating the structure of a reflection method seismic exploration system. It is a figure for demonstrating the flow of the presence area | region search method of the hydrate layer concerning this invention. It is a figure for demonstrating embodiment of the methane hydrate layer presence area | region search method concerning this invention. It is a figure which shows typically a spectrum when a methane hydrate layer exists. It is a figure which shows typically a spectrum when a methane hydrate layer does not exist. It is a figure which shows the geological model which has a methane hydrate layer. It is a figure which shows the methane hydrate layer which consists of a blob group of methane hydrate. It is a figure which shows the geological model without a methane hydrate layer. It is a figure which shows the earthquake response waveform of frequency 100Hz, (a) shows the case with a methane hydrate layer, (b) shows the case without a methane hydrate layer. It is a figure which shows the earthquake response waveform of frequency 200Hz, (a) shows the case with a methane hydrate layer, (b) shows the case without a methane hydrate layer. It is a figure which shows the earthquake response waveform of frequency 400Hz, (a) shows the case with a methane hydrate layer, (b) shows the case without a methane hydrate layer. It is a figure which shows the seismic response waveform of frequency 500Hz, (a) shows the case with a methane hydrate layer, (b) shows the case without a methane hydrate layer. It is a figure which shows the superposition | polymerization waveform of frequency 100Hz, (a) shows the case with a methane hydrate layer, (b) shows the case without a methane hydrate layer. It is a figure which shows the superposition | polymerization waveform of frequency 200Hz, (a) shows the case with a methane hydrate layer, (b) shows the case without a methane hydrate layer. It is a figure which shows the superposition | polymerization waveform of frequency 400Hz, (a) shows the case with a methane hydrate layer, (b) shows the case without a methane hydrate layer. It is a figure which shows the superposition | polymerization waveform of frequency 500Hz, (a) shows the case with a methane hydrate layer, (b) shows the case without a methane hydrate layer.

Explanation of symbols

1 Autonomous unmanned vehicle 2 Vibration source (transmitter)
DESCRIPTION OF SYMBOLS 3 Streamer cable 4 Transponder 5 Submarine 6 Mother ship 7 Transmitter / receiver 8 Transmitter / receiver 10 Formation model with methane hydrate layer 10A Formation model without methane hydrate layer 11 Seawater 12 First formation 13 Free gas layer 14 Second formation 15 Methane hydrate Rate layer 15a Methane hydrate blob 15b Formation 16 Transducer (transmitter)
17 Hydrophone (receiver)

Claims (6)

  This is a hydrate existence region exploration method for exploring hydrate under the seafloor by the reflection seismic exploration method. The seismic exploration using a seismic source having a wide frequency band, and the seismic exploration measurement results in a low frequency Creating a submarine stratum structure map using the band, identifying the submarine pseudo-reflecting surface existing area from the stratum structure map, and obtaining a spectrum in a wide frequency band at the location where the submarine pseudo-reflecting surface exists Extracting a portion of the spectrum having a large spectrum over the entire frequency of the search and having a spectrum intensity in the high frequency band larger than the spectrum intensity in the low frequency band; and if there is no extraction portion, hydrate And if there is the extraction part, hydrate exists in the extraction part Existing area exploration method hydrate characterized by comprising and a determining.   2. The hydrate presence region search method according to claim 1, wherein a common reflection point polymerization method is used when obtaining the spectrum.   Obtaining each analysis window for moving the spectrum in the depth direction, and setting the depth of the analysis window where the extraction site begins to occur in the spectrum as a top surface of the hydrate layer; and The hydrate existence region exploration method according to claim 1, further comprising a step of forming a lower surface.   This is a hydrate existence region exploration system for exploring hydrate under the seafloor by a reflection seismic exploration method, a transmitter serving as an epicenter for transmitting a sound wave having a wide frequency band, and a sound wave transmitted from the transmitter A receiver for receiving the reflected wave from the formation of the earth, and an analyzing means for analyzing the signal of the reflected wave received by the receiver, the analyzing means using the low frequency band of the seismic survey measurement result The existence area of the submarine pseudo-reflecting surface is specified from the created submarine geological structure map, and the spectrum obtained in the wide frequency band of the seismic survey results at the place where the submarine pseudo-reflecting surface exists is the total frequency of the exploration. If a region that is large over the range and the spectrum intensity in the high frequency band is larger than the spectrum intensity in the low frequency band is extracted, and there is no such extraction region, there is no hydrate. Judgment, hydrates existing area exploration system, characterized by determining the hydrate extraction site if the extraction site is present.   5. The hydrate presence region exploration system according to claim 4, wherein the analysis means uses a common reflection point polymerization method when obtaining the spectrum. The analysis means obtains each analysis window for moving the spectrum in the depth direction, and the depth at which the analysis window where the extraction site begins to occur in the spectrum is defined as the top surface of the hydrate layer, and the submarine pseudo-reflecting surface is the hydrate. 6. The hydrate existence region exploration system according to claim 4 or 5, wherein the hydrate presence region exploration system is a lower surface of the layer.

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