JP3464200B2 - Method of exploring underground structure of seabed and apparatus for implementing the method - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and apparatus for enabling a method of estimating an underground structure by observing microtremors of the earth vibrating at a low frequency to be applied to an underground structure of a water bottom including a seabed. .

As a method for estimating underground structure, the inventors have investigated and measured physical signals from the underground and analyzed the physical signals to estimate the underground structure. A method is proposed (Japanese Patent Application No. 10-107147, etc.). Since the present invention is configured on the premise of this exploration method, first, an outline of this exploration method (hereinafter referred to as "premise technology") will be described. This "base technology"
Is based on the following scientific findings.

As a method of underground structure exploration, a direct exploration method for directly obtaining information on the underground structure by boring the ground and obtaining a sample, and a method for observing and measuring a physical signal from the underground and analyzing it. Indirect exploration methods for estimating underground structures have been used conventionally.

The indirect exploration method has advantages over the direct exploration method in that a relatively large area can be economically explored and that a wide range of exploration can be performed from shallow depth to large depth. This indirect exploration method is also called physical exploration method in the sense that it obtains information on the physical characteristics of the underground by a physical method, and a physical signal artificially given to the ground to capture the physical characteristics of the underground, For example, an active method of observing and measuring the response of the earth to vibrations and electromagnetic waves, and a physical signal generated from the underground condition as it is without giving such an artificial signal, such as tremor, natural potential, gravity, radiation. There are two passive methods of observing and measuring Noh.

Among the physical survey methods, the active method is premised on that some physical influence such as vibration is positively applied to the ground to be surveyed. For example, in the seismic exploration method called the reflection exploration method, since it is a prerequisite for the exploration to generate artificial vibration, there is a high possibility that the environment including the exploration target will be adversely affected.
It is necessary to use an exciter or an explosive as a source of this artificial vibration. Moreover, since the artificially generated vibration has a relatively high frequency, if a relatively high frequency vibration due to human activity is included in the reflected wave of this artificially generated vibration, it will be regarded as noise. It becomes difficult to remove it, and there is a possibility that the measurement accuracy is reduced.

In view of the above points, the passive method of observing and measuring a physical signal generated from a natural state without artificially modifying it does not select an estimated area,
In addition, since it is theoretically possible to estimate the crust from a relatively shallow depth to a large depth, it has the potential to be used more widely in the future.

Considering the vibration among the objects to be observed by the passive method, a weak vibration is always generated on the earth surface on a global scale. This weak vibration (hereinafter referred to as "fine movement") is caused by various human activities such as factory operation, transportation operation, construction work, or wind, pressure, river flow,
It is caused by natural phenomena such as tides and waves. Of these, vibration due to human activity is less than 1 second in cycle, that is, 1 Hz
The above components are prominent, and a clear diurnal change is observed in the amplitude and the cycle in response to changes in human activity.

On the other hand, it is considered that the fine movement due to a natural phenomenon is predominantly a component of 1 Hz or less, and the fine movement of 1.0 Hz or more is mostly due to human daily activities. From the viewpoint of vibration, microtremors caused by human activities and natural phenomena are a collection of body waves and surface waves, and appear to be irregular vibration phenomena that are extremely rich in time and space. However, for example, the time is about 1 hour, and the distance is about 2 to 3
When observed on the spatiotemporal scale of Km, which is expanded temporally and spatially, it is found that the micromotion is steadily composed of an almost stable spectrum. For this reason, it is possible to observe steady microtremors if properly observed.

The above-mentioned steady tremor includes information on the tremor source, information on the propagation path of the tremor, information on the underground structure at the observing location of the tremor, etc. in the form of body waves or surface waves. Therefore, if the observed microtremor is properly analyzed, it is possible to obtain information on the source of the microtremor, information on the propagation path of the microtremor, and information on the underground structure at the microtremor observation site.

Most sources of tremors are on the ground surface or the sea floor. Therefore, it is considered that the surface wave is predominant over the body wave (P wave, S wave) as the elastic wave during the slight movement. Therefore, it is advantageous to use this surface wave for the estimation of the fine movement.

As is well known, the surface wave has a frequency (period).
There is a property of dispersion in which the propagation velocity or the phase velocity changes depending on. Since the dispersion is closely related to the underground structure, it is possible to estimate the underground structure at the observation point if the dispersion of the observed surface waves is known. More specifically, the tremors are observed by a two-dimensional vibration observation network on the surface (observation points), and the surface wave dispersion that reflects the underground structure directly below the observing points is identified from the observing tremors and specified. It is based on estimating the underground structure that caused the dispersion from the inverse analysis of the generated dispersion.

In the above-mentioned "base technology", the array is constructed by arranging the observation element points for arranging the vibration observation equipment in a predetermined arrangement relation. By having a step of observing tremors by this array, a step of identifying the dispersion of surface waves from the tremor data observed by this array, and a step of estimating the underground structure for the surface wave phase velocity from the identified dispersion, It is configured to infer underground structure from observed microtremors.

[0013]

[Problems to be Solved by the Invention] The above "base technology"
Has attracted attention because it has almost no load on the environment and has the advantage of being able to estimate underground structures to large depths, and has received exploration requests from various fields at the time of filing of the present application, and The observation results are also satisfactory. However, the "base technology" is that the observation point is on the ground, and with this configuration as it is,
It is not possible to estimate the underground structure of the water floor. In addition,
Hereinafter, the term "seabed" shall be used regardless of seawater, freshwater, such as seabed or lakebed.

As a method of estimating the submarine structure of the seabed, an active indirect estimation method has been conventionally implemented. This method is carried out using a large ocean research vessel and a vibration source such as an air gun, but using such a vibration source would have a destructive effect on fish stocks. Moreover, the cost is also high, and there is a problem in widespread deployment in the future. .

From such a viewpoint, a sensor for observing microtremors is arranged on the seabed, and the microtremor observation data from this sensor is sent to a mother ship on the sea via a cable, whereby the "prerequisite technique" in the previous term is applied to the subsea underground structure. Can be applied to the estimation of.

However, in the above structure, the cable picks up noise such as wave noise by connecting the seabed and the mother ship with a cable, and the reliability of the microtremor observation data cannot be ensured. Due to the resistance of the cable, the position of the sensor placed on the seabed may be displaced, and even from this point it is difficult to ensure the reliability of the microtremor observation data.

[0017]

SUMMARY OF THE INVENTION The present invention has been constructed in view of the above problems, and provides an apparatus and a method for estimating the underground structure of the seabed. It is characterized in that it is possible to estimate the submarine underground structure by making it possible to carry out this microtremor observation even on the seabed, with the above-mentioned "base technology" for estimating

That is, according to the present invention, a plurality of underwater sensor units which are arranged at predetermined positions on the seabed and are arranged on the seabed for observing microtremors, and an analyzer for estimating the submarine underground structure based on the microtremor observation data of the underwater sensor units. Each underwater sensor unit is provided with a clock means, and is placed on the seabed independently of this analyzer during microtremor observation.
The time of the clock means is mutually synchronized in the underwater sensor unit, and the analyzing device analyzes the data of each underwater sensor unit by unifying the time axis based on the synchronized time data at the time of data analysis. The present invention relates to a method for exploring the underground structure of the sea floor, which is configured to estimate the crust structure of the sea floor where the unit is installed, and an apparatus for implementing the method.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION An underwater sensor unit hermetically seals a sensor section for detecting fine movements, a storage section for storing fine movement observation data and time data, a clock section for outputting a time signal, a power supply section for powering these, and the like. It is stored. Each underwater sensor unit is on the water (on the sea), the time is synchronized with other underwater sensor units and analysis means, and when the synchronization is completed, each underwater sensor unit is arranged at a predetermined position on the seabed.

Each of the underwater sensor units arranged on the seabed individually observes the micromotion, and the observation data is stored in the storage unit together with the clock signal output from the clock unit. When the preset time has elapsed, the acquisition of the fine movement data is stopped. Each underwater sensor unit that has finished capturing the micromotion data is picked up at sea (onboard) and outputs the micromotion data and time data to the analysis device.

The analyzer analyzes the micromotion data of the output data based on the time data of each underwater sensor unit while unifying the time axis, and estimates the submarine underground structure in which the underwater sensor unit is arranged.

Embodiments of the present invention will be described below with reference to the drawings. 1 and 2 show an underwater sensor unit. Code 1
Is an underwater sensor unit, and the underwater sensor unit 1 is opposed to a main body 1A having a storage space inside, a lid portion 1B for sealing an open end of a space portion of the main body 1A, and a mounting side end portion of the lid portion 1B. The insertion portion 1C is attached to the end of the main body 1A on the side of the insertion.

In the internal space of the main body 1A, there are housed a sensor 2 for detecting slight movements, a substrate section 3 having a memory, a clock circuit, an A / D conversion circuit, etc., and a power source 4 for supplying electric power to each of these sections. By attaching (screw-fitting) the lid 1B to the main body 1A, the main body 1A that accommodates these members
It is configured to completely seal the interior space of the.

On the other hand, an insertion portion 1C is attached to the other end of the main body 1A. This inserting portion 1C is for inserting into the earth and sand of the seabed and for fixing and fixing the entire underwater sensor unit 1 at a predetermined place on the seabed. Therefore, in order to facilitate the insertion, the insertion portion 1C is tapered as shown in the drawing. Is formed in a substantially cone shape.

The size of the underwater sensor unit 1 is, for example, about 60 mm in diameter of the main body 1A and about 600 to 700 in total height.
It is relatively small, about mm, and facilitates the installation or collection of the underwater sensor unit by underwater workers.
In addition to this, the small size makes it less susceptible to the influence of waves on the underwater sensor unit, and prevents the reliability of observation data from being deteriorated by wave noise.
As for the arrangement with respect to the seabed, as shown in the underwater sensor unit 1, the underwater sensor unit 1 is inserted as it is by pushing the insertion portion 1C against the seabed SB. Although depending on the state of the seabed SB, if about 90% of the underwater sensor unit 2 is inserted into the seabed as shown in the figure, the influence of waves becomes smaller and the observation of fine movement becomes easier. The lid 1B is provided with a ring-shaped mark attaching portion 1Ba, and if a mark such as a tape is attached to the mark attaching portion 1Ba in advance, the underwater sensor unit 1 can be easily found at the time of collection. .

FIG. 3 shows an example of formation of an arrangement shape (hereinafter referred to as "array") of the underwater sensor unit formed by the plurality of underwater sensor units 1 arranged on the seabed SB in this way. Both (A) and (B) in the figure show the positional relationship between a plurality of observation points (hereinafter referred to as “observation element points”) set at one observation point on the seabed. That is, since each underwater sensor unit 1 is arranged at each of these observation element points, each point shown in the figure indicates the arrangement relationship of the underwater sensor units 1 respectively arranged at these observation element points, that is, an array.

(A) is a triangular array in which the underwater sensor unit 1 is located at the apex of a triangle,
(B) shows a circular array in which each underwater sensor unit 1 is arranged on the circumference of a virtual circle and at the center thereof. In addition,
In both (A) and (B), the underwater sensor units 1 are connected by a solid line, but this is for clarifying the positional relationship of the underwater sensor units 1. Each underwater sensor unit 1 has a cable or the like. It does not mean that they are connected by physical connection means. The one shown in the figure is an example of an array, and various arrangements are possible according to the topography of the seabed.

In FIG. 4, reference numeral 5 indicates an analyzing device. A personal computer is used as the analysis device, and for example, a laptop computer is suitable in consideration of mobility. Due to the function of the analysis device 5, the control analysis means 5a and the time synchronization control means 5 are included.
b, GPS function unit 5c, and clock 5d.

Next, a procedure for estimating the submarine underground structure using the underwater sensor unit 1 will be described. First, in estimating the underground structure by microtremor observation, it is necessary to analyze the microtremor data observed by each sensor on the premise of strict time synchronization regardless of whether it is on the ground or in the water. As such time management, G
There is a method of using time data by PS radio waves. According to this method, precise time management of, for example, 5 μsec is possible, and time data of sufficiently high accuracy can be obtained for analysis of microtremor observation data.

However, since the underwater sensor units 1 are respectively placed on the seabed without contacting the analyzing device 5, it is not possible to take in time data by GPS radio waves at the time of micromotion observation. Therefore, it is necessary to precisely synchronize the clocks of the underwater sensor units arranged on the seabed with the analysis device 5 in advance. First, clock synchronization will be described with reference to FIGS.

The substrate unit 3 is arranged in the underwater sensor unit 1 as described above. The configuration of the substrate unit 3 is shown in the figure. As shown in FIG. 4, the fine movement observation data observed by the central processing unit 3a and the sensor 2 is shown. A storage unit 3b that stores information such as a clock, a clock 3c, an A / d conversion unit 3d, a startup circuit 3e, and a filter 3f.
Etc.

Prior to the microtremor observation, each underwater sensor unit is connected to the analysis device 5 on the ground or at the sea (on the ship) to synchronize the time. The analyzer 5 in a state of being connected to each of the underwater sensor units 1 to N receives the time data by the GPS radio wave received by the GPS function unit 5c via the time synchronization control means 5b.
To synchronize the time data of each underwater sensor unit 1. As the clock of the underwater sensor unit, a quartz clock (for example, 10 −7 precision) is suitable. In connection with this time synchronization, a code is assigned to each underwater sensor unit, and the underwater sensor unit is specified by this code data when collecting the micromotion observation data from each underwater sensor unit. .

When the synchronization of the time data of each underwater sensor unit 1 is completed, each underwater sensor unit is arranged at a predetermined position on the seabed so as to form an array on the seabed.
In this case, as shown in FIG. 2, most of the underwater sensor unit 1 is embedded to reduce the influence of various noises such as wave noise and ship navigation noise as much as possible. In addition, when the underwater sensor unit 1 is collected, the mark 6 is attached in order to facilitate the discovery of the underwater sensor unit.

In this way, each underwater sensor unit 1 observes a slight movement for a predetermined time (for example, about 30 minutes to about 3 hours), and the observation data is stored in the storage unit 3b together with the time data output from the clock 3c. The storage unit 3b of the underwater sensor unit 1 has, for example, a capacity of 4M.
A flash memory of about B or the like is suitable.

Next, a method of processing the micromotion observation data observed by each underwater sensor unit will be described with reference to FIG. The microtremor observation data observed by each underwater sensor unit 1 is taken into the analysis device 5 via the data input unit 7.
The captured data is the underwater sensor unit identification means 8
The code data is analyzed to identify which underwater sensor unit is the data. The fine movement observation data (waveform data) output from the specified underwater sensor unit is output to the waveform analysis unit 9 with the time axis being unified together with the time data of the underwater sensor unit output from the clock data detection unit 10. In this way, the waveform data of each underwater sensor unit is analyzed by the waveform analysis means 9 with the time axis unified according to the time data of each underwater sensor unit forming an array on the seabed.

The waveform data analyzed by the waveform analyzing means 9 is output to the underground structure estimating means 11, and the underground structure estimating means 11 estimates the underground structure immediately below the seabed forming the array. In this way, the estimated subsurface structure immediately below each array formed on the seabed is sequentially stored in the storage means 12 for each array, and finally, the comprehensive analysis means 13 is obtained.
A comprehensive analysis of the subsurface structure of the entire seabed where multiple arrays were formed in. The central processing unit 14 controls the above series of processing. Further, the processing content is displayed via the display unit 15.
The estimated underground structure, etc. will be displayed appropriately.

FIG. 7 shows a second embodiment of the present invention. Reference numeral 16 is a noise sensor that positively detects a wave motion other than the fine motion that is the observation target, that is, a wave motion that is noise for the observation. In the illustrated configuration, the noise sensor 16 is arranged in the internal space of the lid 1B. More specifically, the support plate 17
It is supported and arranged on the upper side through a vibration isolator 18 such as a coil spring.

Although it is not a serious problem on a quiet lake or the like, various noises are generated in the sea such as waves and waves caused by navigation of ships. When the underwater sensor unit is placed on the "sea floor" in the original sense of the sea,
It is practically impossible to completely eliminate waves (noise) other than slight movements from the wave data of the sensor 2.

In view of this point, in this embodiment, the noise sensor 16 positively detects the noise wave NW that interferes with the observation, and this noise data is also stored in the storage unit 3b (see FIG. 5) together with the time data. . At the time of analysis, the noise data is also output by unifying the time axis with the waveform data of the fine movement MW that is the observation target, and the waveform portion of the noise data of the sensor 2 and the noise data of the noise sensor 16 is superimposed. , This noise data is subtracted for analysis. In this case, the noise sensor 16 is configured to be supported by the vibration isolator 18 and not to detect a wave from the bottom of the sea so that the noise sensor 16 does not detect a minute movement as much as possible.

FIG. 8 shows a third embodiment. In this embodiment, the noise sensor is arranged independently of the underwater sensor unit 1. Reference numeral 19 in the figure is a noise sensor, which is housed in an airtight container. The noise sensor 19 is provided with a clock and storage means (neither shown), and the noise data detected by the noise sensor 19 is stored in the storage means together with the time data of this clock. The clock of the noise sensor is synchronized with the clock 3c of the underwater sensor unit 1.

The noise sensor 19 is constructed so as to generate buoyancy in water, and an anchor 21 fixed to the seabed SB via a vibration isolating connecting member 20 such as a rubber material or a coil spring.
It is supported by. Since the noise sensor 19 is independent of the seabed SB side by the vibration isolating connection member 20, almost only the noise wave NW is detected, and the wave data is stored in the storage means together with the time data of the clock having the built-in clock. Various types of noise sensors can be used including the above embodiment. For example, a noise sensor that receives noise by a crystal unit and detects the noise as a change in vibration frequency of the crystal unit is small and highly accurate. Therefore, it can be suitably used as the noise sensor.

In the analysis, the underwater sensor unit 1
The wave data of the sensor 2 and the wave data of the noise sensor 19 are unified on the time axis, and the noise data is subtracted as described above to analyze the fine movement.

The underwater sensor unit 1 according to the present invention has been described above.
Has a substantially "pile" -shaped configuration having a cylindrical main body and an insertion portion 1c connected to the main body. However, the configuration of the underwater sensor unit 1 is limited to this shape. It is not something that will be done. That is, the underwater sensor unit may have any shape as long as it is a shape that allows easy detection of tremors from the seabed and that noise caused by water flow or the like is small. In particular, the resistance to water flow is low, so the noise caused by turbulent flow of water is small, and even if turbulent flow occurs, the center of gravity of the unit is not eccentric so that the vibration due to this turbulent flow is not amplified. Can be obtained. As a shape satisfying such conditions, the entire shape of the underwater sensor unit 1 is formed into, for example, a spherical shape, a spindle shape, or a disk shape, or the insertion type underwater sensor unit shown in the drawing and exposed to water. Good results can be obtained by forming the portion having such a spherical shape, spindle shape, or disk shape.

Further, according to the present invention, when it is considered that the noise due to the cable does not interfere with the micromotion observation, such as when there is almost no water flow or when the array formation position by the underwater sensor unit is close to the analysis device. In addition to the time synchronization of each underwater sensor unit with a clock, or instead of this time synchronization, each underwater sensor unit and the analysis device are connected by a cable so that each underwater sensor unit and the analysis device are time synchronized. You may do it. In this case, it is not always necessary to set time synchronization with the clock 5d of each underwater sensor unit 1.

[0045]

As described in detail above, the present invention is as follows.
The present invention makes it possible for the first time to estimate the underground structure by microtremor observation that causes almost no environmental load on the seabed. For this reason, it is possible to perform exploration of the submarine underground structure over a wide range and with almost zero environmental load, without generating artificial vibrations or the like that cause great damage to fishery resources.

Further, since the fine movement detecting unit of the present invention is constructed as a small underwater sensor unit, the whole is small and inexpensive and is very economical.

[Brief description of drawings]

FIG. 1 is a cross-sectional view of an underwater sensor unit forming part of the present invention.

FIG. 2 is a side view of the underwater sensor unit showing a state in which the underwater sensor unit shown in FIG. 1 is arranged on the seabed.

3A and 3B are conceptual diagrams showing an example of an array shape formed on the seabed by an underwater sensor unit.

FIG. 4 is a block diagram showing an outline of an analysis device.

FIG. 5 is a block diagram showing a configuration example of a substrate portion of the underwater sensor unit.

FIG. 6 is a block diagram showing a configuration example of analysis means of the analysis device.

FIG. 7 is a partial sectional view of an underwater sensor unit showing a second embodiment of the present invention.

FIG. 8 is a diagram showing an arrangement state of an underwater sensor unit and a noise sensor formed separately from the underwater sensor unit.

[Explanation of symbols]

1 Underwater sensor unit 1A Main body (of underwater sensor unit) 1B (underwater sensor unit) lid 1C (Underwater sensor unit) insertion part 2 sensors 3 board part 3a Central processing unit 3b storage unit 3c clock 4 power supply 5 analyzer 5a Control analysis means 5b Time synchronization control means 5c GPS function unit 5d clock 6 landmarks 7 Data input section 8 Underwater sensor unit identification means 9 Waveform analysis means 10 Clock data detection means 11 Underground structure estimation means 12 storage means 13 Comprehensive analysis means 14 Central processing unit 15 Display means 16 (Internal storage type) Noise sensor 17 Support plate 18 Vibration isolation means 19 (independent type) noise sensor 20 Isolation connection material 21 anchor SB seabed NW noise wave MW tremor

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Claims (4)

(57) [Claims] 1. A plurality of underwater sensor units for observing microtremors are arranged on the seabed so as to form a predetermined array, and each of the microtremor observation data observed and stored by these underwater sensor units is time-axis after the observation is completed. In order to estimate the subsurface structure of the seafloor under the array by unifying the
With a noise sensor that positively detects noise waves,
Stores noise data around various underwater sensor units,
The noise data and the microtremor observation data are unified on the time axis.
Waves on which noise data is superimposed by analyzing
It is configured to subtract the noise data from the shape part.
And a method for exploring the underground structure of the seabed. 2. An underwater sensor unit arranged on the seabed, and an analysis device for estimating an underground structure of the seabed from micromotion observation data of the underwater sensor unit, wherein the underwater sensor unit is a sensor for detecting micromotion of the seabed. , A substrate unit and a power supply unit for supplying power to these units, and the substrate unit is provided with a storage unit for storing the fine movement data detected by the sensor and the time data output from the clock. The means for synchronizing the clocks of each underwater sensor unit, and the time axis are unified from the micromotion observation data and time data output from each underwater sensor unit to analyze the micromotion observation data of each underwater sensor unit and Seismic survey device smell seabed means for estimating the subsurface structure of the deployed seabed sensor units are provided
The underwater sensor unit has a noise detector that detects noise waves.
Noise sensor is provided, and noise detected by this noise sensor is detected.
Data is output from the clock together with the fine movement data.
The special feature is that the time data is superposed and stored.
A submarine underground structure exploration device. 3. A noise sensor is provided separately from the underwater sensor unit, and the noise sensor is provided with a means for storing the detected noise data and a clock for the noise sensor which is synchronized with the clock of the underwater sensor unit. The submarine underground structure exploration apparatus according to claim 2, wherein the noise data is stored in the storage means of the noise sensor together with the time data of the clock for the noise sensor. 4. The lid for sealing the main body of the underwater sensor unit is provided with a portion for attaching a mark for facilitating the discovery of the underwater sensor unit arranged on the seabed. Item 2. A submarine underground structure exploration device according to item 2 or 3 .