JP2006138706A - Method of estimating thickness of hydrate layer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of estimating a thickness of a hydrate layer, capable of obtaining a more accurate hydrate layer thickness than by conventional methods by performing a reflection survey by means of low and high frequencies and processing the survey results. <P>SOLUTION: The thickness-estimating method for estimating the thickness of a hydrate layer by means of a reflection survey method includes steps of estimating a pseudo-seabed reflecting surface appearing on a survey record obtained by exploring at a low frequencies in the range of 100 Hz to 300 Hz as the lower limit of the hydrate layer, and estimating the earliest reflected waves among those with short intervals, appearing on the survey record obtained by exploring at a high frequency in the range of 300 Hz to 1 kHz as the upper limit of the hydrate layer. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for estimating a layer thickness of a hydrate layer in a submarine formation, and more specifically, a layer thickness of a hydrate layer that estimates an upper limit and a lower limit of a hydrate layer using low-frequency and high-frequency sound waves. It relates to an estimation method.

  Reflection seismic exploration is one of the means of exploring resources such as seabed oil and methane hydrate. 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 from the bottom of the sea, and a certain amount of sound waves 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. 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. This method is called reflection seismic exploration because it uses the reflected sound wave for exploration. In seismic exploration records under the seafloor, the boundary between strata with different physical properties appears as black lines, and this boundary indicates the geological structure. 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, sound waves with a low frequency of about 100 Hz emitted from an air gun vibrate from an autonomous unmanned aerial vehicle that navigates near the sea floor because the resolution of the reflected wave is low and high-precision geological exploration is difficult. There has been proposed a submarine geological exploration system that generates waves and receives the vibration waves by a streamer cable towed by this autonomous unmanned vehicle (for example, see Patent Document 1).

  In particular, in the seismic reflection survey targeting the methane hydrate layer, the low-frequency (for example, 100 Hz) survey using an air gun or the like can confirm a submarine pseudo-reflecting surface that suggests the existence of a hydrate layer. There are cases where it cannot be clearly confirmed by high frequency (for example, 600 Hz) exploration using deep-sea towed bodies, and a blanking phenomenon with a small amplitude is confirmed directly above the seafloor pseudo-reflecting surface. ing.

  Note that this submarine pseudo-reflecting surface is a sonic boundary surface that extends ignoring the records indicating the surrounding boundary layers in order, and is substantially parallel to the recordings indicating the sea bottom, and has different reflection properties (phases). Means that the intensity of reflection is strong (it appears as a thick line in the record).

  In order to estimate the abundance of methane hydrate, etc. under the seabed, an accurate resource assessment method is required, but in the conventional assessment method, the submarine pseudo-reflecting surface, which is used as an indicator of methane hydrate presence, is required. Based on the distribution of seawater, the seafloor pseudo-reflecting surface appearing on seismic exploration records is set as the lower limit of hydrate, and the upper limit is estimated from logging data, interpretation of seismic exploration results (blanking, sandstone distribution), precision gravity exploration, etc. The thickness of the hydrate layer is estimated.

  However, since the logging data is obtained by excavation, it seems that the estimation of the upper limit of the hydrate layer by this is accurate. There is a problem that data cannot be obtained. In addition, there is a problem that the estimation accuracy by other precise methods such as gravitational exploration does not directly deal with the hydrate layer, and thus the estimation accuracy is low.

  In other words, with the conventional method, the lower surface of the methane hydrate layer can be identified based on the seafloor pseudo-reflecting surface, but the upper surface of the methane hydrate layer cannot be identified. Resource assessment methods are needed.

On the other hand, the hydrate layer was previously considered to be homogeneous, but the submarine pseudo-reflecting surface that appeared in reflection seismic surveys using low-frequency seismic sources was not It is known that it does not appear clearly, and could not be explained by the concept of homogeneous hydrate layer. The knowledge that the methane hydrate layer is not homogeneous, but is a thin layer of methane hydrate layer and non-methane hydrate layer, based on cores and logging data collected at the Nankai Trough, a basic borehole. Is obtained. As the constitution of this thin layer, for example, the entire mentor hydrate layer has a thickness of about 30 m, the methane hydrate layer has a thickness of about 0.25 m, the non-methane hydrate layer has a thickness of about 0.25 m. About one can be assumed.
Japanese Patent Laid-Open No. 2003-19999

  The present invention has been made to solve the above-mentioned problems, and its purpose is to perform reflection seismic surveys at low and high frequencies and process the results of the survey, thereby improving accuracy compared to conventional methods. An object of the present invention is to provide a method for estimating the thickness of a hydrate layer capable of obtaining a high hydrate layer thickness.

  The layer thickness estimation method of the hydrate layer of the present invention for achieving the above object is a layer thickness estimation method for estimating the layer thickness of the hydrate layer under the seafloor by the seismic reflection method, and is 100 Hz to 300 Hz. The seafloor pseudo-reflecting surface that appears on seismic records obtained by exploring at a low frequency of the lower limit of the hydrate layer, and at short intervals appearing on seismic records obtained by exploring at a high frequency of 300 Hz to 1 kHz The reflected wave of the earliest time in the reflected wave group is used as the upper limit of the hydrate layer.

  That is, as the first stage, the seafloor pseudo-reflecting surface is detected from the low-frequency exploration data, the lower limit of the hydrate layer is obtained, and as the second stage, the upper limit of the hydrate layer is obtained from the high-frequency exploration data. As a result, the hydrate layer thickness is obtained from the detected upper and lower limits.

  More specifically, when the wavelength of the epicenter is long with respect to the thin layer thickness (low frequency), the thin layer does not affect and shows the same reflection response as when it is homogeneous, confirming the submarine pseudo-reflecting surface be able to. The lower limit position of the hydrate layer is grasped from the position of the submarine pseudo-reflecting surface.

  Moreover, when the wavelength of the epicenter is close to the thin layer thickness (frequency is high), the reflected wave from the inside of the hydrate layer can be confirmed due to the influence of the thin layer. At this time, the reflected wave group at a short interval can be confirmed, but since the reflected wave at the earliest time is almost close to the upper limit of the hydride layer, the position of the reflected wave at the earliest time is determined. Know the upper limit position of the hydrate layer.

  From the obtained upper and lower limits, the hydrate layer thickness at that location can be estimated.

  And in the layer thickness estimation method of the hydrate layer, in the high frequency exploration, the frequency of the high frequency generated is changed within a range of 300 Hz to 1 kHz, and the exploration by a plurality of high frequencies is performed, By comparing the reflected wave group appearing between the low-frequency exploration and the seafloor pseudo-reflecting surface, the reflected wave group obtained from each high-frequency exploration is obtained by obtaining the reflected wave in the earliest time of the reflected group. If the configuration is such that the reflected wave of the earliest time closest to the seabed is the upper limit of the hydrate layer, the upper limit of the hydrate layer can be estimated more accurately.

  In other words, the reflection in the thin layers due to the high-frequency exploration changes depending on the relationship between the wavelength of the transmitted wave and the thickness of the thin layer. At a frequency of.

  According to the layer thickness estimation method of the hydrate layer of the present invention, the reflection method seismic survey at low and high frequencies is performed, and the result of the survey is processed, so that the hydrate layer thickness with higher accuracy than the conventional method can be obtained. Obtainable.

  Hereinafter, an embodiment of a layer thickness estimation method for a hydrate layer according to the present invention will be described with reference to the drawings, taking as an example the case of using an autonomous unmanned aerial vehicle.

  First, a reflection seismic exploration system for implementing this hydride layer thickness estimation method will be described.

  As shown in FIG. 1, this seismic reflection survey 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 vibration source 2 at the bottom and a transmission / reception device 12 at the stern, and tows a streamer cable 3 from the stern. Since the 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 vibration source 2 is composed of a small, lightweight, power-saving 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 does not transmit the vibration of the hydrophone that captures the reflected wave, the sensor module that detects the depth, the azimuth, and the temperature, the transmission module that transmits data, and the autonomous unmanned vehicle 1 to the streamer cable 3. The vibration control module and the stabilizer module for controlling the attitude of the streamer cable 3 are configured.

  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 transmitter / receiver 12 and the transmitter / receiver 14 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 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.

  In this example, a reflection seismic survey system using an autonomous unmanned vehicle is used as an example of a reflection seismic survey system for implementing this hydride layer thickness estimation method. Any system can be used as long as it can perform an earthquake exploration system and can perform exploration at a predetermined low frequency and high frequency.

  Next, a method for estimating the thickness of the hydride layer according to the present invention will be described with reference to FIG.

  In the present invention, a low-frequency sound wave and a high-frequency sound wave are used as the vibration wave P1 emitted from the vibration source 2.

  FIG. 3 is a diagram schematically showing a low-frequency exploration result and a high-frequency exploration result for a hydrate layer composed of alternating layers of a hydrate thin layer L1 and a non-hydrate thin layer L2. The hydrate layer (L1 + L2) has a first deposition layer L3 on the upper side and a second deposition layer L4 on the lower side, and the first deposition layer L3 is under the seawater W.

  The sound wave of this low frequency exploration has a low frequency of 100 Hz to 300 Hz, and the seafloor pseudo-reflecting surface Ls appearing on the seismic exploration record obtained by exploring at this low frequency is set as the lower limit of the hydrate layer (L1 + L2). In this case, since the wavelength of the epicenter is long (the frequency is low) with respect to the thickness of the hydrate thin layer L1 and the thickness of the non-hydrate thin layer L2, the thin layers L1 and L2 do not affect the hydrate. The reflection response similar to that in the case where the layer (L1 + L2) is homogeneous is shown, and the seabed pseudo-reflection surface Ls can be confirmed.

  Therefore, the lower limit position of the hydrate layer (L1 + L2) is obtained by setting the position of the submarine pseudo-reflecting surface Ls as the lower limit of the hydrate layer (L1 + L2). Ws is a portion indicating the seabed.

  This low-frequency exploration cannot reflect the case where the sonic velocities of the thin hydrated thin layer L1 and the non-hydrated thin layer L2 change finely, and corresponds to a gradual change in sonic velocity close to the average sonic velocity, A reflected wave corresponding to the entire hydride layer (L1 + L2) is obtained. As a result, in the low-frequency exploration, irregular reflection within the thin layers is unlikely to occur, and the seabed pseudo-reflection surface Ls can be determined.

  And about the frequency range used for this low frequency exploration, since there is almost no underwater seismic center whose center frequency is lower than 100 Hz, 100 Hz becomes a substantially lowest frequency. On the other hand, if the frequency is higher than 300 Hz, it is difficult to clearly determine the position of the submarine pseudo-reflecting surface because the thin layer has an influence and the reflected wave from within the thin alternate layer appears in the vicinity of the submarine pseudo-reflecting surface.

  Moreover, the sound wave of the high-frequency exploration has a high frequency of 300 Hz to 1 kHz, and the reflected wave at the earliest time among the reflected waves at short intervals appearing on the seismic exploration record obtained by exploring at this high frequency. Let Hs be the upper limit of the hydrate layer (L1 + L2). In this case, the hydrated thin layer L1 and the non-hydrated thin layer L2 are affected, and the reflected wave group from the inside of the hydrate layer (L1 + L2) at a short interval can be confirmed. The wave Hs is almost close to the upper limit of the hydride layer (L1 + L2). Therefore, the upper limit position of the hydrate layer is obtained by setting the earliest reflected wave Hs as the upper limit of the hydrate layer (L1 + L2). Ws is a portion indicating the seabed.

  In this high-frequency exploration, it is difficult to identify the seafloor pseudo-reflecting surface Ls due to multiple reflected waves generated in the thin layers. On the other hand, since it is possible to distinguish the reflection of the reflected wave Hs at the earliest time from the multiple reflected waves of thin layers, this is used.

  As for the frequency range used for this high-frequency exploration, when the frequency range is lower than 300 Hz, the thin layer does not affect, so the reflected wave does not appear from the inside of the thin layer, and the transmission capability decreases at a high frequency exceeding 1 kHz. It is difficult to determine the upper limit of the thin layers because the reflected waves from the layers cannot be obtained.

  In this high-frequency exploration, the reflection in the thin layer by high-frequency exploration varies depending on the relationship between the wavelength of the transmitted wave and the thickness of the thin layer. Unable to respond to changes. Therefore, the frequency of the generated high frequency is changed within the range of 300 Hz to 1 kHz, for example, the frequency is changed to 300 Hz, 700 Hz, and 1 kHz, and the search is performed with a plurality of high frequencies, and the reflected wave obtained from each high frequency search Of the reflected waves at the earliest time of the group, the wave closest to the seabed is set as the upper limit of the hydrate layer.

  The hydrate layer thickness LH at that location can be estimated from the obtained upper limit Hs and lower limit Ls.

  That is, as the first stage, the seafloor pseudo-reflecting surface Ls is detected from the low-frequency exploration data, the lower limit of the hydrate layer is obtained, and as the second stage, the upper limit Hs of the hydrate layer is obtained from the high-frequency exploration data, As three steps, a hydrate layer thickness LH is obtained from the detected upper limit Hs and lower limit Ls. Note that either the low-frequency exploration or the high-frequency exploration may be performed first, or may be performed simultaneously as long as the vibration source can generate vibration including both.

It is a figure for demonstrating the structure of a reflection method seismic exploration system. It is a figure for demonstrating the principle of the layer thickness estimation method of the hydrate layer concerning this invention. It is a figure for demonstrating embodiment of the layer thickness estimation method of the hydrate layer concerning this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Autonomous unmanned vehicle 2 Vibration source 3 Streamer cable 4 Transponder 5 Submarine 6 Mother ship 12 Transceiver 14 Transceiver L1 + L2 Hydrate layer L1 Hydrate thin layer L2 Non-hydrate thin layer L3 First sedimentary layer L4 Second Sediment layer Ls Submarine pseudo-reflecting surface (lower limit)
LH Thickness of hydrate layer Hs Reflected wave at the earliest time of multiple reflected waves (upper limit)
W Seawater Ws Portion showing the seabed

Claims (2)

  This is a layer thickness estimation method for estimating the thickness of the hydride layer under the seafloor by the seismic reflection method, and the seafloor pseudo-reflecting surface appearing on seismic records obtained by exploring at a low frequency of 100 Hz to 300 Hz is hidden. In addition to the lower limit of the rate layer, the upper limit of the hydrate layer is the reflected wave at the earliest time among the reflected waves at short intervals appearing on the seismic survey record obtained by exploring at a high frequency of 300 Hz to 1 kHz. A method for estimating a layer thickness of a hydrate layer. In the high-frequency exploration, the high-frequency generated is changed within a range of 300 Hz to 1 kHz, and a plurality of high-frequency explorations are performed. The reflected wave group that appears between the two is compared to obtain the reflected wave of the earliest time of the reflected group, and the reflected wave group of the earliest time of the reflected wave group obtained from each high frequency exploration is 2. The method for estimating a layer thickness of a hydrate layer according to claim 1, wherein the upper limit of the hydrate layer is set close to the hydrate layer.

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