JP3259544B2 - Method and apparatus for exploring undersea buried objects - 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 an apparatus for searching for a buried object under the sea, for example, for searching a cable buried in the sea floor.

[0002]

2. Description of the Related Art Conventionally, as a technique for exploring a cable buried under the seabed, there have been conventionally used methods of (a) passing a signal current through a cable to measure a change and intensity of an electric field, and (b) generating a magnetic field with a coil. A method of measuring a change or intensity of an induced magnetic field generated in a cable as a conductor has been put to practical use. These methods can measure the distance between the sensor and the cable, but have the drawbacks that the distance measurement accuracy is poor at several tens of centimeters and that the burial depth (the amount of burrow) of the cable, which is important for cable protection, cannot be directly measured. Was. In (a), since it is necessary to send a signal through a cable, it is necessary to install a power supply and a signal source in advance on land. Further, there is a disadvantage that the exploration becomes impossible when the cable is broken. In (b), the exploration up to the burial depth of 1 m is the limit, and there is a disadvantage that the cable is lost at the current maximum burial depth of 2 m.

[0003] Since ultrasonic waves can be transmitted to a long distance in water, they are widely used for depth measurement of fish, detection of fish schools, exploration of undersea stratum, and the like. However, there are no reports that ultrasonic cables could detect cables buried under the seabed. Exploration technology using ultrasonic waves (elastic waves) has been used for the purpose of exploring the submarine stratum up to several km, but the frequency is as low as several kHz or less, and the distance resolution is only several meters or less for exploring deep underground. Absent. Therefore, a thin object such as a cable cannot be detected. In addition, since a low frequency is generated, the sound source and the receiver are large, ranging from several meters to several hundreds of meters, and are not only expensive but also inferior in mobility.

Although the underground radar method is useful as a technique for exploring buried objects on land, it cannot be used in the sea, which is an electric field, because microwaves are used.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and can directly measure the amount of soil covering of a cable having an outer diameter of about several cm buried at least about 2 m below the sea bottom. Another object of the present invention is to provide a method and an apparatus for exploring an undersea buried object that is small and has good mobility.

[0006]

In order to achieve the above object, the present invention uses an ultrasonic wave as a means for detecting an object buried under the seabed, and can simultaneously realize necessary and sufficient ground permeability and distance resolution. The frequency range is determined experimentally. As the underground permeability, the specific geology at which the cable can be buried is specified, and the conditions that allow the cable to penetrate to 2 m or more, which is the maximum burial depth of the cable, or to 5 m required for geological exploration are clarified. The distance resolution is set to about 10 cm in consideration of the fact that the burial depth of the cable is 2 m. In addition, the spatial resolution enough to separate the thin cable from the stratum and the parametric sound source method as a method for realizing a small and highly mobile sound source are used, and the necessary and sufficient detection distance to identify the cable is clarified. At the same time, a sound source is scanned in a plane orthogonal to the cable as a search unit for separating the cable from the stratum from the change in the reflected sound pressure. The problem is solved by simultaneously satisfying the above specific methods.

[0007]

According to the present invention, the use of a parametric sound source having a secondary frequency of 10 to 30 kHz allows the underground permeability (about 5 m below the seabed) and the distance resolution of 10 cm for soft geology.
The cable buried on the sea floor can be searched with a degree and a spatial resolution of about 10 cm.

[0008]

Embodiments of the present invention will be described below in detail with reference to the drawings.

FIG. 1 is a structural explanatory view showing an embodiment of the present invention. In FIG. 1, 1 is an underwater boat, 2 is a parametric sound source and a wave receiver, 3 is a parametric sound source scanning surface, 4 is a seabed, 5 is an embedded object (cable), A is a traveling direction of the underwater boat 1, and B is a vertical direction. The direction, L, is the distance (several meters) between the underwater boat 1 and the sea floor 4. That is, a parametric sound source and a receiver 2 are mounted on the underwater boat 1 so as to be submerged in the sea. The parametric sound source has a secondary frequency of 10
A sound of kHz to 30 kHz is emitted toward the sea floor 4.
The receiver receives the reflected sound pressure of the parametric sound source from the seabed direction. The parametric sound source is installed vertically in the sea so as to face the sea bottom 4, and the parametric sound source is moved along the sea bottom 4 along the sea bottom 4 while maintaining an altitude of several meters from the sea bottom 4, and is perpendicular to the moving direction. The surface is scanned, and the change in the reflected sound pressure is measured by the receiver.

In the above embodiment, the case where the parametric sound source and the receiver 2 are mounted on the underwater boat 1 has been described. However, the same applies when the parametric sound source and the receiver are mounted on the hull so as to be submerged in the sea. Can be implemented.

FIG. 2 shows an example of the relationship between the frequency of ultrasonic waves and the distance resolution. Underwater sonars have frequencies of tens of kHz or more and do not transmit below the sea floor. Underground exploration of offshore oil fields is below several kHz and penetrates deep underground but has poor resolution. In order to obtain a distance resolution of about 10 cm, it is considered that 15 kHz or more is necessary as indicated by a dotted line in the figure. A depth resolution of 10 cm is not strictly required for cable burial exploration.
It is understood that a frequency of about kHz or more is required. As the frequency increases, the resolution improves, but on the other hand, it becomes difficult to penetrate below the sea floor. Because the transmission characteristics depend on the geology,
An experiment was conducted to determine how much the frequency can be increased for specific geology. Since cable burial is usually carried out by digging the seabed with a high-pressure jet or a hoe, the geological layers that can be buried are naturally limited to soft ones. FIG. 6 shows the analysis results of the geology where the submarine cable can be buried, and the water depth of the sea area where the geology was sampled. It is necessary to clarify the maximum frequency that can pass 2 m or more in these geology.

On the other hand, in order to detect a submarine cable having an outer diameter of about 10 cm, a spatial resolution is also required. FIG. 3 shows the required conditions for the spatial resolution. That is, the relationship between the sensor diameter D of the sound source and the frequency for realizing a spatial resolution of 10 cm is shown. The distance Z between the sound source and the cable was assumed to be 3 m as the sum of the buried depth of 2 m and the distance of 1 m between the sea floor and the sensor. The calculation was performed using the equation of the radiation field from the vibrator shown in the figure.
As shown by the dotted line in the figure, even at 20 kHz, the sensor diameter of the sound source becomes 2 m or more, and it is understood that a small device cannot be realized. A parametric sound source is used to satisfy the spatial resolution and realize a small sensor.

FIG. 4 shows a conceptual diagram of a parametric sound source. That is, two strong primary waves with slightly different frequencies f1 and f2 are generated in water, and the frequency component of the difference between f1 and f2 among the secondary waves synthesized in water by the nonlinear effect is used for sensing. This is a method called a parametric sound source method. As shown in FIG. 3, if the primary wave is set to 100 kHz or more, an ultrasonic wave having a good directivity can be obtained with a small sensor, and the secondary wave is similar to the primary wave. It is known that a high directivity can be obtained. Further, since the secondary frequency can be changed by changing the frequency in the resonance region of the primary wave, a tunable sound source can be realized without changing the sensor structure. As a result of trial production of a sensor with a diameter of 40 cm that can change the primary frequency in the range of 100 to 130 kHz, it is possible to obtain an ultrasonic wave with a secondary frequency of 10 to 30 kHz and a very good directivity of a directional angle of 3 degrees, that is, a good spatial resolution. Was completed. As shown in FIG. 3, if the frequency is about 100 kHz, the sensor diameter is 40 cm and the spatial resolution is about 10 c.
It can be seen that m can be achieved. It is clear that if the distance between the sensor and the object increases, the spatial resolution deteriorates according to the equation in the figure. Therefore, it is necessary to maintain the distance to the object in order to maintain the spatial resolution. Specifically, it is necessary to mount the sensor on an underwater boat or the like to maintain the distance to the sea floor.

FIG. 7 shows the result of measuring the transmission loss of the ultrasonic wave in the geology of FIG. 6 using this sound source. In the experiment, an experimental ship in which a parametric sound source was installed just above the point where the geology was sampled in FIG. 6 was anchored, and the transmission characteristics at each frequency were measured. Transmission characteristics vary considerably with geology, but generally worse at higher frequencies. Whether or not the reflected wave of the object can be detected can be estimated by the following sonar equation.

EL = SL−W−C + TS (1) Here, EL is the reception level, SL is the transmission level, W is the underwater attenuation of the sound wave, C is the attenuation of the sound wave in the soil, and TS is the reflection level of the object. . Here, assuming that the underwater noise level is NL, it is a condition that the following equation can be satisfied.

EL> NL (2) Now, PZ which is a practically sufficient transmitting element in the equation (1)
The SL of the parametric sound source secondary wave when T is used is 1
The practical limit is about 70 dB (re μPa at 1 m). Here, “re μPa at 1 m” is a unit called “relative micropascal” at a distance of 1 m between the transducer of the microphone and the point to be measured. Also,
Assuming that the propagation distance is L (m), W is given by equation (3) regardless of the frequency. If L = 3m, W is about 10dB
Becomes

W = 20 log L (3) The TS differs depending on the material, shape, and size of the object. FIG. 8 shows the results of measuring the TS of an iron-coated cable for communication using a prototype parametric sound source. From this, it was found that TS was about -6 dB. The underwater EL is usually 100 dB. Using the above results, an allowable value of C that satisfies Equation (2) is obtained as follows: C <170-10-6-100 = 54 (dB) (4) On the other hand, soil attenuation is proportional to frequency and distance. It is considered to be larger, and is given at 0.15 to 0.45 dB / mkHz in the stratum where the cable can be embedded as shown in FIG. Therefore, in order to make the soil attenuation at a depth of 2 m, that is, 4 m round trip, satisfy the equation (4), the worst value of soil attenuation is 0.45, and the frequency is f.
From 0.45 <54, it can be seen that the frequency should be about 30 kHz or less. In addition, as an application of the present invention, ground exploration in harbor construction and cable laying route investigation can be considered in addition to the above-described cable exploration. In such a case, it is necessary to have a capability of detecting a soft stratum as shown in FIG. 6 to a depth of about 5 m.
It is understood from <54 that the frequency should be set to 12 kHz or less. As described above with reference to FIG.
Since f needs to be 10 kHz or more to obtain the order of 0 cm, the frequency range is 1 to satisfy both conditions.
It has been found that the frequency is limited to 0 to 30 kHz.

FIG. 5 shows a buried cable image (buried depth 1 m, water depth 26 m) by the prototype device. In other words, the geological No. in FIG. The result of exploring the 5 mm armored cable buried 1 m below the sea floor at the point 3 is shown. FIG. 5 shows the result at the secondary frequency of 10 kHz. In the experiment, the anchor was lowered in front of and behind the experimental ship in the water area at a depth of 26 m, and the reflection intensity was measured using a prototype parametric sound source fixed to the side of the experimental ship. In the drawing, the state shown directly above the cable indicates a state where the sensor is directly above the cable. At this time, a cable image was obtained 1 m underground. On the other hand, in FIG.
This shows a shifted state. At this time, the cable image has disappeared. When the sensor is returned directly above, the cable image is obtained again. By this experiment, it was confirmed that the cable image buried under the sea bottom was obtained by this prototype device.
Although the cable image is distributed in the range of 26.8 m to 27.3 m, locally, the cable image is within a range of about 20 cm. Strictly speaking, assuming that the sway of the hull at the time of the experiment is 10 cm, it can be understood that the distance resolution is about 10 cm.

The thin continuous lines set at water depths 26, 27, and 28 m are distance markers. In the experiment, the position of the ship and the position when the cable was buried were
The measurement was performed accurately using a transponder of m and the alignment was performed.

As in the above experiment, if a sensor can be held directly above even a thin embedded object such as a cable, a reflected wave can be received while the sensor is held. However, when the sensor is mounted on a submersible vehicle or the like, even if the sensor passes across the cable, it is difficult to determine the noise because only a cable image is obtained instantaneously. In the case of cables, it is necessary to continuously measure the burial depth along the burial route. To solve these problems, as shown in FIG. 1, the cable is continuously extracted from the stratum by scanning the parametric sound source and the receiver 2 in a vertical plane perpendicular to the traveling direction such as a cable route, A function to continuously measure the burial depth has been added. This function has solved the above-mentioned problem.

[0021]

As described above, conventionally, there has been no method for accurately detecting an object buried up to several meters below the seabed. However, according to the present invention, there is no method for sensing a cable buried under the seabed. It has become possible to detect a thin object without digging the sea floor and measure its depth with a distance resolution of the order of 10 cm. In addition, this method can also simultaneously search the stratum near the sea bottom surface with higher accuracy than before. Accordingly, undersea cable burial work, which conventionally required separate devices such as a ground exploration device and a cable detector, can be achieved by a single device using the method of the present invention. Savings.

Further, it is expected that this method will enable undiscovered detection of electromechanical and sunken ships left around harbors and ancient archeological sites, which were conventionally difficult to find.

[Brief description of the drawings]

FIG. 1 is a configuration explanatory view showing one embodiment of the present invention.

FIG. 2 is a characteristic diagram showing an example of a relationship between an ultrasonic frequency and a distance resolution according to the present invention.

FIG. 3 is a characteristic diagram showing an example of a relationship between a sound source sensor diameter and a frequency for achieving a spatial resolution according to the present invention.

FIG. 4 is a conceptual diagram of a parametric sound source according to the present invention.

FIG. 5 is a characteristic diagram showing an example of a buried cable image by the prototype device according to the present invention.

FIG. 6 is an explanatory diagram showing an example of an analysis result of geology in which a submarine cable according to the present invention can be buried.

FIG. 7 is an explanatory diagram showing an example of a result of measuring transmission characteristics of ultrasonic waves in the geology of FIG. 6;

FIG. 8 is an explanatory diagram showing an example of a result of measuring a TS of an iron-coated cable for communication using a prototype parametric sound source according to the present invention.

[Explanation of symbols]

1. Underwater boat, 2. Parametric sound source and receiver, 3.
... parametric sound source scanning plane, 4 ... sea bottom, 5 ... buried object (cable).

Continuation of the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) G01V 1/00 G01S 15/02 G01V 1/38 H03F 7/00

Claims (3)

(57) [Claims] (1) A primary frequency in a range of 100 to 130 kHz.
Varied enclosed, and installed so as to face the ocean floor vertically downward 30kHz parametric sound source from the secondary frequency 10kHz to the sea, the sound source while moving along the sea floor, is scanned a plane perpendicular to the direction of movement A method for exploring a buried undersea object, comprising measuring a change in reflected sound pressure by a receiver. 2. A parametric sound source mounted on an underwater boat,
2. The method according to claim 1, wherein the object is moved in the sea while maintaining an altitude of several meters from the sea floor. 3. A sound source and a receiver mounted on a flank or an underwater boat so as to be submerged in the sea, and have a primary frequency.
It is changed in the range of 100 to 130 kHz and the secondary frequency 1
The 30kHz of sound and departure towards the bottom of the sea from 0kHz, submarine
Scans in a plane perpendicular to the direction of movement while moving along the plane
Locator undersea buried objects characterized parametric sound source Ru is, that it is composed of a wave receiver for receiving a reflected sound pressure from the seabed direction of the parametric excitation.