CN215005683U - Deep sea optical cable fault detection device - Google Patents

Abstract

The invention provides a deep sea optical cable fault detection device which comprises a first electrode sensor and a second electrode sensor which are connected in series, wherein the first electrode sensor and the second electrode sensor are arranged in water and fixedly arranged on the same cable; the shore signal output equipment provides constant current for the optical cable positioned at the sea bottom; the first electrode sensor and the second electrode sensor feed back the positions of the first electrode sensor and the second electrode sensor and the potential difference between the first electrode sensor and the second electrode sensor to the receiving device in real time, and the receiving device judges the fault point of the submarine cable according to the positions and the potential difference of the first electrode sensor and the second electrode sensor. The invention can quickly and accurately detect the position of the fault point of the optical cable.

Description

Deep sea optical cable fault detection device

Technical Field

The invention relates to the technical field of submarine optical cables, in particular to a deep-sea optical cable fault detection device.

Background

At least one metal wire is usually arranged in a cable core in the deep sea optical cable, and once the submarine cable breaks down, the insulation of the metal wire is usually damaged, and the conductor is naturally grounded. Direct current or low-frequency current signals are injected into the shore end through the conductor, current flows into seawater at the submarine cable fault position, and therefore a corresponding electric field exists around the fault point.

Disclosure of Invention

The invention aims to provide a deep-sea optical cable fault detection device aiming at the defects of the prior art, which can quickly and accurately detect the position of an optical cable fault point.

The technical scheme adopted by the invention is as follows: a deep sea optical cable fault detection device is characterized by comprising a first electrode sensor and a second electrode sensor which are connected in series, wherein the first electrode sensor and the second electrode sensor are arranged in water and fixedly arranged on the same cable; the shore signal output equipment provides constant current for the optical cable positioned at the sea bottom; the first electrode sensor and the second electrode sensor feed back the positions of the first electrode sensor and the second electrode sensor and the potential difference between the first electrode sensor and the second electrode sensor to the receiving device in real time, and the receiving device judges the fault point of the submarine cable according to the positions and the potential difference of the first electrode sensor and the second electrode sensor.

In the above technical solution, the second electrode sensor is located below the first electrode sensor, and the second electrode sensor is farther from the receiving device than the first electrode sensor in horizontal distance; when the receiving device measures that the potential difference between the first electrode sensor and the second electrode sensor is the maximum value in the process of reciprocating movement on the sea surface of the area where the fault point of the submarine cable to be positioned is located, the receiving device judges that the fault point of the submarine cable is located right below the position where the second electrode sensor is located.

In the technical scheme, the first electrode sensor and the second electrode sensor have the same structure and respectively comprise an electrode, a motor base, a cavity, a signal conditioning circuit, a shell and a multi-core watertight connector; one end of the electrode is exposed outside the cavity, and the other end of the electrode is arranged in the cavity; the electrode seat is arranged on the outer wall of the cavity, one end of the electrode penetrates through the electrode seat and is arranged in the cavity, and sealant is arranged at the joint of the electrode and the outer surface of the electrode seat; the multi-core watertight connector is arranged on the cavity shell, the signal conditioning circuit is arranged in the cavity, and one end of the electrode, which is positioned in the cavity, is electrically connected with the cable through the signal conditioning circuit and the multi-core watertight connector in sequence; the electrodes and the cavity are fixedly arranged in the shell, and the shell is a honeycomb spherical porous cover.

In the above technical scheme, the housing and the cavity are both made of ABS material.

In the above technical scheme, the receiving device is provided with a power supply, the power supply supplies power to the first electrode sensor and the second electrode sensor through cables, and grounding points of the first electrode sensor and the second electrode sensor are both power grounding ends.

In the above technical solution, the weight of the second electrode sensor is greater than that of the first electrode sensor.

In the technical scheme, the signal conditioning circuit comprises an instrument amplifier and a filter which are connected in series, wherein a power supply access end of the instrument amplifier is connected with a filter circuit in series; the filter adopts a passive low-pass filter.

In the technical scheme, the electrode adopts an all-solid-state Ag/AgCl electrode, and the technical parameters are as follows: the electrode range potential is within plus or minus 1.0mv, the 12-hour range stability is better than 0.05mv, the electrode self-noise is less than 1 uv/(noise level based on 1Hz frequency), the frequency range is 0.004Hz-210Hz, and the compression strength is 3 MP.

In the technical scheme, the preparation method of the all-solid-state Ag/AgCl electrode comprises the following steps: AgCl particles are prepared by a solid-phase ball milling method, and the obtained particles are spherical and uniform in size, and the particle size is about 5 um; uniformly mixing silver powder into AgCl particles according to a certain proportion, adding 8% of PVA and an additive to granulate to generate mixed particles, and pressing by using a grinding tool to form an electrode blank; sintering the electrode blank body to form a blank, setting the sintering temperature to be near the melting point of AgCl, and carrying out heat preservation for a period of time; and carrying out surface treatment and activation on the blank to form an electrode.

In the above technical scheme, the potential distribution expression in seawater in the three medium spaces around the fault point is as follows:

the potential distribution expression in the seawater in the double-layer medium space around the fault point is as follows:

the sea depth of a fault point of the submarine cable is h, the horizontal distance between a certain point A near the fault point and the fault point is r, the vertical distance from the sea level is z, I is a constant current value of the submarine cable, and the conductivity of the sea bed is gamma 2, gamma 1, gamma 3, wherein gamma 2 is 0;

n is an integer.

After the position of the fault point is roughly measured at the shore end, the submarine cable maintenance ship is driven to the fault sea area to be accurately positioned. The dragging electric field sensor detects electric field signals, and when the amplitude of the received electric field signals is found to be changed from weak to strong and then from strong to weak, the lower part of the strongest position of the signals is the submarine cable fault point. The process of offshore measurement takes a ship as a carrier, and is easy to operate. The electrode sensors are connected together by using the cables and are arranged behind a ship, the electrode sensors are easy to realize, the distance between the sensors is not influenced by the size of the ship, and the electrode sensors are not easily interfered by the electric field of the ship. The electrode adopted by the invention can meet the requirements of engineering application, and has the characteristics of high measurement sensitivity, low noise and capability of efficiently converting electric field signals in a liquid phase environment into signals which can be identified by a solid electronic measurement system. When the sensor shell adopts the high-resistance spherical body, the sensor shell has an amplification effect on an underwater electric field, and is favorable for positioning a fault point of the submarine optical cable

Drawings

FIG. 1 is a schematic structural view of the present invention;

FIG. 2 is a schematic diagram of a three-layer medium space model

FIG. 3 is a diagram of a two-layer simple space potential distribution

FIG. 4 is a diagram of three-layer medium space potential distribution

FIG. 5 is a diagram of potential distribution simulation in two models, where z is 0 and z is 90m

FIG. 6 is a schematic diagram of horizontal probing

FIG. 7 is a potential difference simulation plot for horizontal sensing when the sensor is in the plane of z 19m

FIG. 8 is a simulation of the variation of Uba at different z

FIG. 9 is a schematic view of a vertical probing method

FIG. 10 is a simulation diagram of a vertical method when two sensors are respectively located in a plane where z is 19m and z is 24m

FIG. 11 is a schematic drawing of a towed-detection sensor position

FIG. 12 is a schematic drawing of a towed detection method

FIG. 13 is a simulation diagram of the towing method when the two sensor coordinate points are (r, θ, 24), (r +20, θ, 19)

FIG. 14 is a simulation diagram of the towing method when the two sensor coordinate points are (r, θ, 29), (r +5, θ, 19)

FIG. 15 is a comprehensive schematic diagram of three detection methods

FIG. 16 is a simulation of changes in Uca at different z

FIG. 17 is a flow chart of a process for preparing an Ag/AgCl solid electrode body

FIG. 18 is a schematic block diagram of the AD624

FIG. 19 is a current diagram of AD624 setting arbitrary gain value

FIG. 20 is a schematic diagram of a signal filtering circuit

FIG. 21 is a schematic diagram of distortion potential calibration

FIG. 22 is a graph showing the variation of calibration factor with resistivity ratio

FIG. 23 is a schematic diagram of a sensor configuration

FIG. 24 is a schematic view of an electric field signal measured by the pull-probe method

Detailed Description

The invention will be further described in detail with reference to the following drawings and specific examples, which are not intended to limit the invention, but are for clear understanding.

As shown in fig. 1, the present invention provides a deep sea optical cable fault detection device, which is characterized in that the deep sea optical cable fault detection device comprises a first electrode sensor and a second electrode sensor which are connected in series with each other, the first electrode sensor and the second electrode sensor are arranged in water and fixedly arranged on the same cable, the first electrode sensor and the second electrode sensor are electrically connected with a receiving device on the sea surface through the cable, the receiving device moves on the sea surface of an area where a fault point of a to-be-positioned cable is located and drags the first electrode sensor and the second electrode sensor to move with the receiving device underwater through the cable; the shore signal output equipment provides constant current for the optical cable positioned at the sea bottom; the first electrode sensor and the second electrode sensor feed back the positions of the first electrode sensor and the second electrode sensor and the potential difference between the first electrode sensor and the second electrode sensor to the receiving device in real time, and the receiving device judges the fault point of the submarine cable according to the positions and the potential difference of the first electrode sensor and the second electrode sensor.

The basic principle of the submarine cable fault point positioning technology based on the electric field method is as follows: and D, electrifying direct current in the submarine cable conductor, and connecting the current leakage with the seawater and the ground at the fault point to form a loop. According to the electromagnetic field theory, constant current generates a constant electric field around a fault point, the electric field is distributed in a certain rule in seawater near the fault point and is closely related to the position of the fault point of the submarine cable, and then the positioning of the fault point of the submarine cable can be achieved through the measurement of the electric field.

The invention constructs a mathematical model of the potential distribution near the submarine cable fault point.

The following assumptions are in the model:

(1) the current signal injected from the shore end flows into seawater through a fault point, and the current is used as a constant current which takes the fault point as a starting point and is dispersed to the periphery, and the magnitude of the constant current is set to be I, so that the constant current is a source for generating an electric field and is also a source for electric potential.

(2) The conductivity of seawater is unchanged, and seawater is considered to be a medium with the conductivity equal everywhere. The seabed does not contain a medium such as sludge and the like with different conductivity from the seabed, namely, the conductivity of each medium is uniform.

(3) The point of failure of the submarine cable is complete exposure to sea water, connection to ground, and does not take into account the influence of the submarine cable itself on the surrounding electric field.

(4) Influence of the geomagnetic field, the sea wave and the like on an electric field generated by a fault point is ignored.

(5) The sea surface and the sea bottom surface are both plane and parallel to each other.

(6) The infinite distance is a zero potential point.

According to the assumptions, a potential distribution mathematical model taking the current at the fault point as a field source is established:

air above the sea bed and sea level can affect the electric field emitted from the fault point of the submarine cable in the sea water. And establishing a three-layer medium space model for the three-layer medium space of the air, the seawater and the seabed. In consideration of the symmetry of the potential, in any plane parallel to the sea level, the potential of all points on the circumference is equal to that of any circle taking the projection of the source point on the plane as the center of the circle. Thus, a cylindrical coordinate system is established, and the model schematic diagram is shown in FIG. 2.

In fig. 2, the sea depth is h, and the model assumes that the fault point is connected to the seabed ground, so that the fault point is located h below the sea level, i.e., the constant current source is located h below the sea level. The electric conductivities of air, seawater and seabed are gamma 2, gamma 1 and gamma 3 respectively, and the electric potential zero point is at infinity. And taking the projection O of the source point on the sea level as the origin of coordinates, taking a straight line connecting the O and the source point as a z-axis, and taking the vertical sea level downwards as the positive direction of the z-axis. The potential at any point a (r, θ, z) in the seawater is related to r, z. The invention adopts a mirror image method to solve.

The point charge is in the medium space 1, it reflects back and forth between two interfaces, transmits infinite times, each time of reflection, the transmission generates a mirror charge, the infinite times of reflection and transmission are equivalent to the superposition of infinite mirror charges, the potential phi is an infinite number of sums, the reflection coefficients of two interfaces are respectively:

thus, the three-layer medium space potential distribution expression:

according to the analogy relation, I is respectively used for replacing q, gamma 1, gamma 2 and gamma 3 for replacing epsilon 1, epsilon 2 and epsilon 3 in the above formula, and the potential distribution expression of the stable constant current field in the three-layer medium space can be obtained:

in the formula

The medium 2 is air, and the conductivity γ 2 thereof is 0. The model assumes that the fault point is connected to the seafloor, so there is d ═ h. Assuming that the sea depth of a certain submarine cable fault point is h, the horizontal distance between a certain point A near the fault point and the fault point is r, and the vertical distance between the certain point A near the fault point and the sea level is z, calculating to obtain the potential distribution of the point, namely the potential distribution expression around the fault point is as follows:

this is the potential distribution model in the seawater of the three-layer medium space.

Let γ 3 be γ 1 and k13 be 0, change the model into a two-layer medium space model, and substitute the above formula to obtain

The deep sea area can be equivalent to an air-seawater double-layer medium space model, and the shallow sea area can be equivalent to an air-seawater-seabed three-layer medium space model. The potential distribution models of the two medium spaces are analyzed and obtained, and the two distribution models are simulated to discuss the specific distribution rule of the potential.

According to the practical situation, relevant parameters are taken, and the electric conductivities of the three media of the air, the seawater and the seabed are respectively equal to gamma 2 and 0S/m, gamma 1 and 3S/m, and gamma 3 is equal to 0.003S/m (the electric conductivity of the seawater is 3-5S/m, and the electric conductivity of the wet soil is 0.003-0.03S/m). The current I injected into the submarine cable at the shore end is 10mA, and the sea depth h in the sea area at the fault point of the submarine cable is not assumed to be 100 m.

When the actual submarine cable fault point is positioned, the general shore end can position the submarine cable fault point area to a certain range, so that the simulation horizontal range r can be within +/-200 m.

Fig. 3 and 4 are respectively a simulation diagram of the spatial potential distribution of the two-layer medium and a simulation diagram of the spatial potential distribution of the three-layer medium. Comparing the two figures yields: the distribution rule of the potential in the seawater in the double-layer medium space and the three-layer medium space is the same, namely the potential is the same at each point which is on the same sea depth plane and has the same horizontal distance with a fault point, and the symmetry of the potential is verified; the closer to the fault point, the larger the potential, the faster the change. But the magnitude of the potential at the same point in the two models is different. Fig. 5 is a diagram of potential simulation in the plane where z is 0 and z is 90m in two models.

In the double-layer medium space model or the three-layer medium space model, the potential of each point on the sea level, namely the z-0 plane is far smaller than the potential of each corresponding point on the z-90 m plane, and the seawater has an attenuation effect on the electric field, namely the farther the propagation distance is, the greater the attenuation is, so when the electric field method is used for positioning the fault point of the submarine cable, the sensor is required to be placed on the plane close to the sea depth of the sea area as much as possible, but cannot be placed on the sea surface.

As can be seen from fig. 5, under the same external conditions, the potential at the point in the three-layer medium space model is higher than the potential at the point in the two-layer medium space at the same distance from the fault point. The reason is that seabed media are considered in the three-layer medium space, and all layers of media can influence an electric field.

And positioning a submarine cable fault point by an electric field method, and driving the submarine cable maintenance ship to a fault sea area to perform accurate positioning after the fault point is roughly measured at the shore end. The dragging electric field sensor detects electric field signals, and when the amplitude of the received electric field signals is found to be changed from weak to strong and then from strong to weak, the lower part of the strongest position of the signals is the submarine cable fault point.

In the actual operation of locating a submarine cable fault point by using an electric field method, it is difficult to find a reference point with constant potential, but if the distribution of the potential is known, the potential difference distribution of two points at a fixed distance can be solved. Therefore, two electric field sensors can be used for measuring potential difference, and fault point positioning can be carried out through potential difference distribution. According to the theoretical simulation, three methods can be adopted for detection according to different postures of the two sensors, namely, the two sensors are placed on the same horizontal plane, the same vertical plane and different vertical planes which are not different from the horizontal plane, and the submarine cable fault points are positioned according to the distribution rule of potential differences detected by the electric field sensors under the horizontal posture, the vertical posture and the towing posture, and the methods can be respectively called as a horizontal detection method, a vertical detection method and a towing detection method.

The closer to the fault point in the horizontal direction, the faster the change in potential difference. The horizontal detection method is to place two sensors on the same horizontal plane and move the sensors in the horizontal direction, and the horizontal distance between the sensors is kept unchanged in the process. The method measures the potential difference of two points on the same horizontal plane, and locates the fault point according to the measured potential difference distribution.

According to the seawater potential distribution theory in the three-layer medium space model, the distribution condition of potential difference measured by the sensor around a fault point when a horizontal detection method is applied is solved. Assuming that the sea depth h is 30m, the horizontal distance l between the two sensors a and B is 20 m. For the convenience of analysis, assuming that the projected tracks of the two sensors and the submarine cable fault point are on the same straight line in the detection process, the coordinate A is (r +20, theta, z), and the coordinate B is (r, theta, z), so that the potential difference U between the two sensors can be obtained_baComprises the following steps:

and substituting the three-layer medium space model and the double-layer medium space model into the formula to obtain:

during detection, the sensor needs to be arranged near the sea depth of a fault point sea area, and the depth z of the sensor is 19 m. The simulation parameters are as follows: gamma ray₁＝3S/m，γ₃0.003S/m, 0.01A, 30m, 20m, 19 m. According to the above formula, the potential difference U between the electric field sensors B, A_baIs shown in fig. 6. Fig. 7 is a diagram of potential difference distribution obtained when a horizontal detection method is used in a three-layer medium space model, that is, a relationship between potential difference distribution between sensors and a position of a submarine cable fault point when the sensors are horizontally arranged. When the sensor is dragged horizontally, the submarine cable fault point can be found according to the measured potential difference and the change condition thereof. When the absolute value of the detected signal is changed from small to large and then changed to 0 and then changed to large, when the signal is 0, the fault point of the submarine cable is positioned in the middle of the projections of the two sensors on the seabed. As can be seen from the figure, when r is-10, that is, the horizontal coordinate of the sensor B is-10, the horizontal coordinate of the sensor a is 10, the fault point is on the perpendicular bisector of a and B, the distances from the sensors a and B to the fault point are the same, and the potential difference is 0. The electrode potential difference distribution is in odd symmetry with the fault point as the center. It can be seen from the figure that when the horizontal detection method is used for detection, the change rule of the potential difference signal is obvious, the absolute value is changed from small to large and then becomes large after being changed from small to 0, the detection personnel can accurately position the fault point according to the rule, and the detection precision is high. According to the above formula, comparing the change of the potential difference Uba between B and a at different sea depths z, assuming that the range of the sea depth z is 16m-21m and the distance between the sea depths is 1m, the simulation diagram is as shown in fig. 8, and the change rule of the potential difference signals detected by the horizontal detection method is the same at different sea depth levels, and when r is the same, the larger z is, i.e. the closer the sensor is to the fault point, the larger the potential difference is.

The vertical detection method is to place two sensors on the same vertical line and keep the vertical spacing of the sensors unchanged during the horizontal movement. The closer to the fault point in the vertical direction, the faster the potential difference changes. The method measures the potential difference between two points on the same vertical line, and locates the fault point according to the measured potential difference distribution.

According to the three-layer model theory, the distribution condition of the potential difference detected by the electric field sensor around the fault point is solved when the vertical detection method is applied. Let the sea depth h be 30m, the vertical distance l' be 5m between the two sensors, the depth z be 19m, and the depth z be 24 m. For the convenience of analysis, assuming that the projected tracks of the two sensors in the detection process are on the same straight line with the submarine cable fault point, and using the same coordinate system as the horizontal method, the coordinates of the two points C and B are (r, θ, 24), (r, θ, 19), respectively, so that the potential difference Ucb of the two sensors is:

and substituting the three-layer medium space model and the double-layer medium space model into the formula to obtain:

the simulation parameters are as follows: gamma ray₁＝3S/m，γ₃0.003S/m, 0.01A, 30m, 5m, 19m and 24m respectively. According to the above formula, the potential difference U between the two sensors_cbThe simulation diagram 10 is as follows.

When a vertical detection method is used in the three-layer medium space model, the obtained potential difference distribution is a relational graph of the magnitude of the potential difference between the sensors and the position of a submarine cable fault point when the sensors are vertically placed. When the sensor is dragged horizontally, the submarine cable fault point can be found according to the measured potential difference and the change condition thereof. When the detected signal changes from small to large and then changes from small to small, the part right below the position where the signal is maximum is the fault point of the submarine cable, at the moment, the fault point is positioned right below the sensors B and C, the distance between the sensors and the fault point is the shortest, the electric field changes most severely, and the potential difference is the largest. The electrode potential difference is even-symmetric with the connecting line of points projected on the seabed through the fault point as a symmetric axis.

As can be seen from the figure, the potential difference changes significantly only closer to the point of failure, while at a distance slightly further the potential difference is small and changes very slowly. Therefore, when the vertical detection method is adopted, the fault point is easily missed, and the misjudgment is caused.

According to the above formula, the change of the potential difference Ucb between C and B is compared at different depths z, and the range of the depth z is assumed to be 16m-21m, and the depth distance is 1 m. The change rule of the potential difference signals detected by the vertical detection method is the same on different sea depth planes, and when r is the same, z is larger, namely the closer the sensor is to a fault point, the potential difference is larger.

When using the towed detection method, the two sensors are placed on different horizontal planes and different vertical planes. The method is also used for measuring the potential difference between two sensors and positioning the fault point of the submarine cable through the distribution of the potential difference.

When the towing method is used, the two sensors are not on the same horizontal plane or the same vertical line, the two sensors are towed behind the detection ship through cables, and the position relationship is schematically shown in fig. 11. The underwater depth of the sensor A is z, the underwater depth of the sensor C is z + l', and the horizontal distance between the two sensors is l. When the ship moves at a constant speed and the sea is stormy waves and swells less, z, l and l' can be basically kept unchanged.

For the convenience of analysis, the projected tracks of the two sensors in the detection process and the fault point of the submarine cable are assumed to be on the same straight line, the coordinate system which is the same as that of the horizontal method is still used, and the posture of the sensors is assumed to be unchanged in the dragging process after the sensors are placed in seawater. If l is 20m, l' is 5m, and the sea depth z of the sensor a is 19m, the coordinates of the two points C and a are (r, θ, 24), (r +20, θ, 19), so that the potential difference between the two sensors is:

substituting the model into the formula to obtain:

the simulation parameters are as follows: gamma ray₁＝3S/m，γ₃0.003S/m, 0.01A, 30m, 20m, 5m and 19 m. The potential difference U between the two sensors_caIs shown in fig. 13.

Fig. 13 and fig. 14 are graphs showing the potential difference distribution obtained when the drag detection method is used in the three-layer medium space model, that is, the relationship between the magnitude of the potential difference between the sensors and the position of the submarine cable fault point when the two sensors are not placed on the same horizontal plane or the same vertical line. When the sensor is moved horizontally, the submarine cable fault point can be found according to the measured potential difference and the change condition thereof. From the two figures it can be derived that when the absolute value of the detected potential difference signal changes from small to large, then small and then large to a maximum, the fault point of the submarine cable is just below this point. The maximum and minimum values of the signal are not equal because the two sensors are not in the same plane, nor in the same vertical plane. When the potential difference between the two sensors is maximum, the fault point is just below the sensor C, namely the sensor with larger distance from the sea surface.

Comparing the towing simulation plots of the three methods shows that when the horizontal spacing of the sensors a and C is larger than the vertical spacing, the simulation plot of the towing detection method is closer to the horizontal detection method, and conversely, the simulation plot of the towing detection method is closer to the vertical detection method. From the theory of the three detection methods, it can be known that the two sensors a, C in the towing attitude form a horizontal attitude between the two projections on the same horizontal plane, and the two projections on the same vertical line form a vertical attitude, as shown in fig. 15.

U_ca＝U_cb+U_ba

Where Uca is the measured potential difference in the trailing attitude, Uba is the potential difference between horizontal projections and Ucb is the potential difference between vertical projections.

Uba plays a major role when the sensors A, C are spaced horizontally closer than vertically, i.e., the simulation of the towed probe approach is closer to the horizontal probe approach, and vice versa, is closer to the vertical probe approach, when Uca is closer to Uba.

As can be seen from fig. 13 and 14, the positive and negative values of the potential difference can determine which sensor has a smaller distance between the fault point and the sensor, and when the potential difference is a negative value, the fault point is closer to the sensor having a smaller depth of water entry, whereas when the potential difference is a negative value, the fault point is closer to the sensor having a larger depth of water entry.

When the dragging detection method is used, the amplitude of the potential difference is large, and the change is obvious in the moving process of the sensor. Therefore, in practical application, detection personnel can accurately position fault points according to the change rule of the potential difference.

Comparing the change of the potential difference Uca between the electric field sensors C and a at different depths z, assuming that the change range of the depth z of the sensor a is 16m to 21m and the depth distance is 1m, the simulation diagram is shown in fig. 16: the change rule of the potential difference signals detected by the towing detection method is the same on different sea depth levels, and when r is the same, z is larger, namely the closer the sensor is to a fault point, the potential difference is larger.

The three detection methods are all used for positioning the submarine cable fault point by analyzing the distribution rule of the potential difference between the two sensors. According to the simulation graph, the change condition of the potential difference depends on the distance between the two sensors and the fault point, and under the condition that the distribution rule of the potential difference is known, the position of the fault point is obtained through the change condition of the detected potential difference signal.

The three detection methods are derived based on the potential distribution rule of the point power supply in the three-layer medium space model in the seawater, and the internal relation among the three detection methods can be obtained from the theories of the three detection methods.

The core of the three detection methods is according to: the distribution rule of the potential difference of the seawater near the fault point is closely related to the spatial position of the fault point. The received potential difference signal depends on the attitude between the sensors and the distance between the sensor group and the fault point. Thus we can locate the point of failure by the detected change in potential difference.

(1) When the horizontal detection method is used, when a fault point is positioned on a vertical bisector of the two sensors, the received signal is almost zero, so that the position of the fault point can be judged, and the received signal distribution is in odd symmetry by taking the position as a central point. The absolute value of the received signal changes from small to large, then changes from small to large and then changes from small to small, and an operator is sensitive to the change process, so that the position of a fault point of the marine cable can be timely positioned, the detection precision is high, and the fault point is easy to miss.

The vertical detection method receives signals, the signals reach the maximum value at a fault point, and the positions where r is equal to 0 are taken as symmetry axes, and the received signals are in even symmetry. The received signal changes from small to large and then becomes small, and the signal changes slowly. However, if the signal is changed from small to large and then small due to small fluctuation of the received signal in a small range, the fault point is misjudged by an operator, so that the operator cannot easily and accurately judge the fault point when using the method, the detection precision is low, and the fault point is not easy to lose.

When the dragging detection method is used, the absolute value of the change rule of the received signal changes from small to large, then from small to large, and when the absolute value reaches the maximum value, the fault point is right below the absolute value. The dragging detection method is equivalent to the superposition of a horizontal detection method and a vertical detection method, and under the same external condition, the amplitude of a useful signal received by the dragging detection method is maximum.

(2) The three detection methods have different requirements on the position of the sensor. The basic carrier of the marine electrode is a ship, so that the distance between the sensors cannot be too large. When the horizontal detection method is used, the sensors are on the same plane, if a ship is used as a carrier, the depth and the distance of the two sensors are small, the useful signals which can be received are weak, the two sensors are easily interfered by an electric field of the ship, the useful signals cannot be extracted, and the positioning of a fault point is not facilitated; if a ship is not used as a carrier, the sensors are directly arranged behind the ship, one sensor cannot be ensured to be on the same plane, and the other sensor is required to be arranged in a deeper position underwater, so that the engineering is difficult to realize. These problems also exist with the vertical probing method. The sensors of the towing detection method are connected together by cables and are arranged behind the ship, so that the towing detection method is easy to realize, the distance between the sensors is not influenced by the size of the ship, and the sensors are not easily interfered by the electric field of the ship.

In summary, of the three detection methods, the towing method is most suitable for practical application, so that the design of the sensing system as the towing method is scientific and practical.

In the technical scheme, the first electrode sensor and the second electrode sensor are the same in structure, both adopt a single-shaft structure and comprise an electrode 3, a motor base 1, a cavity 6, a signal conditioning circuit 7, a shell 4 and a multi-core watertight connector 5; one end of the electrode 3 is exposed outside the cavity 6, and the other end of the electrode 3 is arranged in the cavity 6; the multi-core watertight connector 5 is arranged on the cavity shell, the signal conditioning circuit is arranged in the cavity, and one end of the electrode, which is positioned in the cavity, is electrically connected with the cable through the signal conditioning circuit and the multi-core watertight connector in sequence; the electrodes and the cavity are fixedly arranged in the shell, and the shell is a honeycomb spherical porous cover. The electrode holder 1 is arranged outside the cavity, one end of the electrode penetrates through the electrode holder 1 and is arranged inside the cavity, and the sealant 2 is arranged at the joint of the electrode 3 and the outer surface of the electrode holder 1. The shell of the electric field sensor is made of ABS (acrylonitrile butadiene styrene), the material is high in toughness and not easy to corrode seawater and does not react with seawater, the shell is made into a honeycomb spherical porous cover, the main function is to slow down the mutual movement between the Ag/AgCl solid electrode and the seawater, and the protective electrode is not easy to damage in the detection process. Because the cover body is a double-layer honeycomb porous cover, the speed of seawater outside the cover when entering the cover is reduced, the flow speed of the seawater inside the cover is much slower than that outside the cover, and the cover is filled with the seawater, so that the detection of potential signals is not influenced. And sealant is coated above the electrode seat, so that the rear signal conditioning circuit is ensured not to have water inflow. The cavity and the shell are also made of ABS materials. And the serial number 7 is a signal conditioning circuit which comprises a preamplifier and a filter circuit, wherein a power supply supplies power to the amplifier through a multi-core watertight cable, and conditioned electric field signals are transmitted to receiving equipment on a ship along a cable.

The electrode is the main sensing element of the sensor, and needs to have high sensitivity, high stability, small range and good voltage resistance. Theoretical calculation shows that useful electric field signals are weak, so that the electrode sensitivity is required to be high and the stability is required to be good; electric field signals are transmitted in seawater, and the electrodes are required to have the capacity of efficiently converting the electric field signals in a liquid phase environment into signals which can be identified by a solid electronic measurement system; the working environment of the sensor is hundreds of meters deep in the sea, and the electrode is required to have certain pressure resistance. The signal conditioning module preprocesses the detected electric field signal at the first time. The useful electric field signal is small; seawater attenuates electromagnetic signals; the detected signals need to be transmitted by cables of hundreds of meters, the amplitude of electric field signals can be as small as microvolt, and the signals are easily submerged by background noise, so a signal preprocessing circuit needs to be added in the sensor for filtering the background noise, amplifying useful electric field signals, accurately extracting target signals and accurately positioning fault points. The casing material must have excellent pressure resistance and certain shape gain. The working environment of the electric field sensor is a sea area with the depth of hundreds of meters, the pressure of water at the depth reaches dozens of atmospheric pressures, and in order to ensure the stability and accuracy of the system measurement work, the shell material must have good pressure resistance. The useful electric field signal is weaker, in order to accurately position a submarine cable fault point, various methods are adopted to increase the useful signal, high-resistance materials are adopted, and the shell is designed to be spherical to generate certain shape gain.

In the prior art, the measuring electrodes can be divided into inert electrodes and chemical electrodes according to the materials. They are not suitable for subsea electric field measurements. The inert electrode mainly comprises silver, gold, titanium, carbon and the like, and is not suitable for being used as an induction module of the electric field sensor due to the polarization phenomenon in seawater and poor electrode potential stability. The chemical electrodes mainly comprise silver chloride electrodes, copper/copper sulfate electrodes, lead/lead chloride electrodes and the like, and the electrodes cannot meet the engineering application of the electric field sensor. Firstly, such electrodes usually use some solution as their internal reference solution, which easily contaminates the system to be tested. Secondly, the shell of the electrode is made of glass and is easy to break, the application environment of the electrode is deep sea, the pressure reaches above MP level, the electrode is subjected to great pressure when measuring electric field signals, the glass shell is required to be replaced by a pressure-resistant material, and a proper pressure-resistant device is required to be added to the internal reference liquid, so that the structure of the electrode is extremely complex.

The measuring electrodes can be divided into land measuring electrodes and underwater measuring electrodes according to the purpose. The conventional electrodes such as Cu/CuSO4 electrode and Pb/PbCl2 for land electrical prospecting and measurement are not suitable for measurement of marine environment. Because the amplitude of the electric field signal on the land is large, the influence caused by poor extreme stability of the electrodes does not cause the useful signal to generate serious distortion, so the requirement on the performance of the electrodes is not high, but when the submarine cable fault point is positioned by using an electric field method, the seawater electric field signal around the fault point is weak, and at the moment, the amplitude of the extreme difference of the Cu and Pb electrodes is at least the same order of magnitude as the useful signal, even larger than the amplitude of the useful signal, so that the detected data cannot effectively reflect the spatial information of the fault point.

In order to solve these problems, the development of all-solid-state electrodes without liquid contact potential has been carried out very early abroad, and in recent years, with the progress of research, all-solid-state electrodes have been developed to some extent.

The electrode sensor needs to meet the requirements of practical engineering application and solves a plurality of problems. The key point is to develop an all-solid-state electrode which has the advantages of small range, good stability, high measurement precision and seabed high pressure resistance. Only if the test electrode meets the engineering application, the electrode sensor may meet the engineering application requirements.

From the above analysis, it can be seen that: the all-solid-state electrode can be used for measuring underwater electric field signals, but the electrode is required to have the characteristics of good reversibility, difficult polarization and the like, namely the current exchange density of the electrode material is required to be high. The following indexes are compared through comprehensive analysis, and an all-solid-state Ag/AgCl electrode is selected as a measuring electrode of the sensor.

1. The current exchange density is high, the depolarization capacity of the Ag/AgCl electrode is strong, and the stability is good.

The Ag/AgCl electrode has two phase interfaces in seawater, namely Ag | AgCl | Cl, and the anode process and the cathode process of the Ag/AgCl solid electrode are respectively shown as follows:

Ag→Ag⁺+e

Ag⁺+e→Ag

since AgCl is a poorly soluble salt, the electrode also possesses the following balance:

when an electric field signal is transmitted, a weak current passes through the electrode interface, and Ag + at the cathode interface is precipitated on the cathode, so that the concentration of Ag + in the solution near the cathode is reduced, and if the Ag + in the bulk solution is not enough to supplement the Ag + in the bulk solution, the concentration of Ag + in the bulk solution is higher than that in the liquid layer near the cathode, and the balance is broken. If the electrode reaction rate cannot keep up with the electron movement rate, resulting in the accumulation of charges on the interface, the electrode polarization will become more and more severe, eventually leading to the flooding of the useful signal. On the contrary, if the speed is large enough, the polarization and depolarization tend to be balanced, the polarization overpotential is small, and the detected signal can better reflect the space information of the fault point, thereby being beneficial to the whole fault point positioning process.

The reaction speed of the electrode conforms to the Bultler-Volmer equation, and the expression is as follows:

where j is the net reaction velocity of the electrode; j0 is the current exchange density associated with the electrode material; alpha is a transfer parameter, and F is a Faraday constant; r is an ideal gas constant; t is the ambient temperature; Δ U is the polarization value of the electrode when current is passed.

From the above formula: when T is constant, at the same net reaction speed, the larger j0, the smaller the Delta U, i.e. the tendency of deviating from the equilibrium state is weaker, and the electrode is easy to stabilize. When weak current flows through the electrode, the net reaction speed is high, new balance can be established quickly, and the depolarization effect is strong.

Table 1 below shows the current exchange densities of some commonly used metal electrodes, from which data it can be found that the current exchange density j0 of Ag is the largest and much higher than other metals, more than four orders of magnitude higher than Cu, Zn, etc. When the electrode is polarized, the net reaction speed of the Ag electrode is higher, the balance can be quickly reestablished, and the depolarization capability is stronger, so that the Ag/AgCl electrode has better stability.

TABLE 1 Current exchange Density of some metals

Metal electrode	Current exchange density j0
		(a) Sulfate solution
Cu/Cu +2	4×10-5，3×10-2
		Fe/ Fe +2	10-6，2×10-5
Zn/Zn +2	3×10-4
		Ni/Ni +2	2×10-5
Ti/Ti +	2×10-3
		(b) Perchlorate solution
Ag/Ag+	1.0
		Pb/Pb+2	8×10-4
Zn/Zn +2	3×10-8
		Ti/Ti +	10-3
Bi/Bi +3	1×10-5
		(c) Chloride solution
Zn/Zn +2	3×10-4，7×10-4
		Sb/Sb +3	2×10-5
Sn/Sn +2	3×10-3
		Bi/Bi +3	3×10-2

2. The potential of the Ag/AgCl electrode in seawater is basically unchanged, and the extremely poor stability is good.

According to the electrochemical theory, after the metal Ag is contacted with seawater, a polarization phenomenon appears near the interface of two-phase media of the metal Ag to form an electrochemical overpotential, which can be described by a Nernst equation, wherein the expression is as follows:

phi in the formula represents the electrode potential when the redox reaction on the interface of the two-phase medium reaches the equilibrium; phi 0 represents the electrode potential of the reference electrode of the silver electrode and the hydrogen electrode in the standard state, and can be obtained by looking up a table; r is an ideal gas constant, T is the absolute temperature of the environment, F is a Faraday constant,

is the Cl-concentration in the liquid layer near the cathode.

According to the above formula, in seawater, such electrode potential is related to seawater temperature, Cl-concentration. On the seabed, the temperature of the seawater and the salinity of the seawater in a certain area are nearly constant within a period of several months, so that the potential of the Ag/AgCl electrode in the seawater is nearly constant according to the formula, and the range between the electrode pairs is relatively stable.

3. AgCl is insoluble salt. Therefore, the speed of decomposing the ions into the ions in the seawater is low, which is beneficial to reducing the change speed of the substance concentration along with the time in the electrode oxidation process, and the ions participating in the reaction in the seawater are Cl-which belong to the same substance as Cl-in the electrode, and is also beneficial to reducing the change speed of the substance concentration along with the time in the electrode reduction process.

4. The silver ions have the function of a biological insecticide and can effectively reduce the accumulation of marine biological dirt, so that the all-solid-state Ag/AgCl electrode has the capability of protecting the electrode.

5. The electrode is positioned at the seabed during detection, so that the problem that AgCl is sensitive to light can be effectively solved. Because the electrode is arranged at the depth of hundreds of meters below water in the detection process, light cannot reach the electrode, and the problem that AgCl is sensitive to light can be effectively solved.

In conclusion, the all-solid-state Ag/AgCl electrode has high stability and small range in the marine environment, and is most suitable for being used as a measuring electrode of an underwater electric field sensor, so the all-solid-state Ag/AgCl electrode is selected as the measuring electrode of the electric field sensor in the design.

The following analysis was performed on the performance parameters of the all-solid-state Ag/AgCl electrode:

the electrode is used as an inductive element of the sensor, and the performance of the electrode directly determines the performance of the sensor. The above analysis can yield: the all-solid-state Ag/AgCl electrode is most suitable to be used as a measuring electrode of an electric field sensor. In order to better represent the performance of the electrode, the main parameters of the all-solid-state Ag/AgCl electrode performance are provided by combining the characteristics of the electrode and the detection application. Analyzing and researching the external conditions influencing the parameters, the variation trend of each parameter along with the difference of the external environment, the influence of the variation of each parameter on the electrode performance and the like.

1. Pole difference potential

Definition of the range potential: under the condition of no external electric field, the inherent potential difference exists between the all-solid-state Ag/AgCl electrode pair, when the all-solid-state Ag/AgCl electrode is contacted with seawater, the oxidation-reduction reaction occurs on the two-phase medium interface of the electrode, and then the dynamic balance is achieved, the potential difference phi exists between the electrode and the seawater, which is generally called electrochemical overpotential. When two all-solid-state Ag/AgCl electrodes are used for detection at the same time, each electrode has electrochemical overpotential, and if the two electrodes are completely consistent and the external environmental conditions such as seawater temperature, salinity and the like of the two electrodes are also completely the same, the potentials of the two electrodes are equal, and no range difference exists. However, due to the limitation of the preparation process, the two electrodes are difficult to be completely identical, the environment of the seawater is variable and complex, and the environment between the measuring electrodes is difficult to be completely consistent, so that the electrode pair inevitably has extreme differences. Thus, the range is one of the inherent characteristics existing between the electrode pairs.

The existence of the range potential can reduce the resolution of the electric field sensor and weaken the capacity of the electric field sensor for identifying weak electric field signals. It is therefore necessary to test the range potential, which can theoretically be eliminated in a certain way as long as the range potential is unchanged.

2. Extremely poor stability

Extremely poor stability definition: the electrode pair has the performance that the extremely poor potential of the electrode pair does not change after long-term use. Its two dependent variables are time and environment. When the external environment temperature is not changed, the change of the pole difference potential along with the time is generally called time drift; the variation of the pole difference potential with the temperature of the seawater is generally called temperature drift. The application environment of the electric field sensor is hundreds of meters deep sea, and in the same sea area, the external environment conditions are basically the same, so the range stability is generally measured by using the range drift index in a certain time period.

The range potential drift can be superposed on a useful signal detected by the electric field sensor, so that the signal generates a disturbance phenomenon. Therefore, the electrode pair with good range stability is selected for testing the range stability of the electrode pair, the interference of range potential drift on useful signals is weakened, and the detection and the positioning of fault points are facilitated.

3. Self-noise

The all-solid-state Ag/AgCl electrode self-noise mainly refers to potential fluctuation caused by chemical reaction on a two-phase medium interface after the electrode contacts seawater, namely electrochemical noise. The method is one of the most important parameters of the all-solid-state Ag/AgCl electrode, is a source for influencing electric field signal measurement, and can radically reflect the performance of the all-solid-state Ag/AgCl electrode. The change of the temperature of the seawater, the change of the concentration of the reaction substance in the solution, the change of the reaction activity of the local cathode and anode of the electrode and the like all cause the change of the potential of the electrode, and cause the electrochemical noise.

The self-noise of the all-solid-state Ag/AgCl electrode mainly comprises thermal noise, random electrochemical reaction noise, noise generated by the change of seawater temperature and salinity between electrode pairs and noise generated by electrode vibration. The electrode self-noise is generally measured by using an index of 1Hz bandwidth root mean square. Since the self-noise of the all-solid-state Ag/AgCl electrode is imaged by various factors, the calculation is difficult to adopt a theoretical formula. Therefore, a more feasible way is to obtain the self-noise parameters of the all-solid-state Ag/AgCl electrode through laboratory measurement.

4. Frequency response range

Under the influence of interface reaction, the all-solid-state Ag/AgCl electrode corresponds to an optimal frequency response range when the ocean electric field signal detection is carried out. Beyond this frequency range, the electrode cannot function properly.

The amplitude-frequency characteristics of the all-solid-state Ag/AgCl electrode are researched by the predecessor, and the conclusion is reached: when the amplitude of the source signal is unchanged, the amplitude of the electrode detection signal is gradually reduced and the attenuation is continuously increased along with the increase of the frequency of the source signal. When the frequency of the source signal is less than 30Hz, the attenuation speed of the signal in seawater is high along with the increase of the frequency and almost linearly attenuates, when the frequency range of the source signal is 30Hz to 210Hz, the attenuation speed of the signal is slow along with the increase of the frequency, when the all-solid-state Ag/AgCl electrode is used as a sensor sensing unit, the all-solid-state Ag/AgCl electrode corresponds to a frequency range, and when the frequency range is exceeded, the signal detected by the electrode is distorted or the signal cannot be detected, namely the electrode fails.

When the electric field method is applied to positioning the fault point of the submarine optical cable, the source signal is direct current, and according to the conclusion, the all-solid-state Ag/AgCl electrode has the most sensitive signal reaction, can accurately measure useful signals and is beneficial to subsequent positioning.

5. Pressure resistance

The pressure resistance is the capacity of measuring the seawater pressure resistance of the all-solid Ag/AgCl electrode. The working environment of the electric field sensor is in a sea depth of hundreds of meters, the pressure of seawater reaches MP level, and an all-solid-state Ag/AgCl electrode is required to have strong pressure resistance. Therefore, in order to ensure that the all-solid-state Ag/AgCl electrode can work normally under the sea, the voltage resistance of the all-solid-state Ag/AgCl electrode needs to be tested.

The electrode is the key for determining whether the electric field sensor has excellent performance and stability. In order to improve the sensitivity and stability of the all-solid-state Ag/AgCl electrode, improvement of the component composition, properties and process preparation of the electrode is required. And the AgCl powder prepared by different processes has great influence on the electrochemical noise of the electrode, the analysis shows that the electrochemical noise is the root of influencing the measurement of electric field signals, and in order to prevent the weak electric field signals from being covered by the noise, multiple ways are needed to reduce the noise intensity as much as possible. Therefore, it is necessary to summarize the performance of electrodes prepared by various processes, and to select the best preparation process from them.

The preparation of the Ag/AgCl electrode comprises two parts: firstly, preparing electrode precursor powder, and secondly, forming an electrode main body. The preparation method of the superfine AgCl powder comprises a grinding method, a solid-phase ball milling method, a liquid-phase precipitation method and a micro-reactor method. The electrode main body is formed by a powder tabletting method, the preparation process is simple, the reproducibility is good, the stability and the bearing capacity of the electrode are greatly improved, and the method is beneficial to practical application.

The grinding method is to mix AgNO with a molar ratio of 1:1₃Mixing with NaCl raw material powder, fully grinding in an agate mortar to enable the NaCl raw material powder to completely react, repeatedly washing with deionized water, and drying to obtain precursor powder.

The solid phase ball milling method is to mix AgNO with the mol ratio of 1: 1.1₃And placing the NaCl raw material powder in an agate tank of a ball mill, adding a grinding aid, namely absolute ethyl alcohol, and carrying out ball milling for 2 hours, and repeatedly washing a product with deionized water after finishing ball milling to obtain a precipitate.

The liquid phase precipitation method is to precipitate AgNO₃The solution and NaCl solution are mixed according to the stoichiometric ratio and react to obtain AgCl white precipitate.

When AgCl is prepared by the microreactor method, NaCl solution, sodium dodecyl sulfate and isooctane are subjected to ultrasonic oscillation and mixing to obtain microemulsion A1, and AgNO is added₃And carrying out ultrasonic oscillation mixing on the solution, SDS and isooctane to obtain a microemulsion A2, then carrying out ultrasonic oscillation on A1 and A2 to prepare AgCl microemulsion, and finally separating solid-phase particles of AgCl by using a centrifugal machine.

The AgCl powder prepared by the four methods has different shapes, particle sizes and distribution of the AgCl particles. The particles prepared by the grinding method have poor shape and poor particle size distribution, and the particle size is smaller by about 0.5um through experimental measurement. AgCl particles prepared by a solid phase ball milling method are spherical and uniform in size, and the particle size is smaller by about 0.5um through experimental measurement. AgCl particles prepared by a liquid phase precipitation method are irregular in particle shape and large in particle size, and are about 1um through experimental measurement. The AgCl particles prepared by the microreactor method have the largest particle size, and are about 2um as measured experimentally. The AgCl powder is required to be small in particle size and uniform in shape, so that the AgCl powder is favorably in full contact with the Ag powder, more active centers are generated on the surface of an electrode, the stability of the potential of the electrode can be improved, and the electrochemical noise can be reduced. From the comparison, the AgCl particles prepared by the ball milling method have the best performance among the four preparation methods.

The stability of the electric field sensor is characterized by the stability of the electrode body, so it is necessary to explore the performances of the electrode bodies prepared by different processes and screen out the optimal preparation process. The Ag/AgCl solid electrode main body is formed by adopting a powder tabletting method, electrode raw materials are uniformly mixed according to a certain proportion, then a binder is added, and the raw materials are subjected to granulation, pressing, sintering and other steps to prepare a raw blank, and then surface treatment and activation are carried out to form the electrode. The flow chart of the preparation process of the Ag/AgCl solid electrode main body is shown in figure 17.

As can be seen from the flow chart, the main differences of different preparation processes are in the parameters of the concentration and the addition amount of the binding agent, the granulation time, the sintering stability and the like. The experimental study shows that: the PVA binder with higher concentration and a small amount of additive are selected, so that the water content in the electrode green body can be effectively reduced, the air holes generated by the evaporation of water in the sintering process are reduced, the density of the electrode is increased, and the pressure resistance of the electrode is improved. In the sintering process link, proper temperature should be selected, and too low temperature can cause more air holes and defects inside the electrode; the AgCl is melted when the temperature is too high, and the electrode is covered by the AgCl, so that the signal transmission is not facilitated. Through repeated experiments, the optimal sintering temperature for electrode forming is near the melting point of AgCl, and heat preservation is carried out for a period of time at the temperature point during the preparation process.

The complete set of preparation process at the present stage is basically determined by combining the discussion of the preparation process of the Ag/AgCl electrode. AgCl particles are prepared by a solid-phase ball milling method, and the obtained particles are spherical, uniform in size and small in particle size of about 5 um; in the electrode main body forming process, PVA with a content of more than 8% and a small amount of additive are selected, so that air holes can be effectively reduced, and the pressure resistance of the electrode is enhanced; the sintering temperature is set to be near the melting point of AgCl, and heat preservation is carried out for a period of time, so that internal pores and surface defects can be reduced, various factors influencing the working performance of the electrode are fundamentally reduced, the overall performance of the Ag/AgCl electrode is improved, the stability is improved, the self-noise is reduced, and the like. The Ag/AgCl electrode produced by the preparation process has excellent performance, is suitable for detecting submarine weak electric field signals, can effectively extract the spatial information of fault points through the detected signals, and is beneficial to positioning the fault points by a submarine optical cable electric field method.

According to the analysis, in order to effectively detect the weak electric field signal emitted from the fault point, the technical parameters of the all-solid-state Ag/AgCl electrode selected in the design are as follows:

(ii) a range potential between electrodes: within +/-1.0 mv

② 12 hours extremely poor stability: is better than 0.05mv

③ self-noise of electrode: is less than

(based on the noise level at 1Hz frequency)

Frequency range: 0.004Hz-210Hz

Compressive strength: 3MP

The key point to be solved by the invention is the transmission problem of the submarine weak electric field signal. Electric field signals emitted from fault points are weak; seawater has an attenuation effect on electric field signals; the working environment of the electric field sensor is in a depth of hundreds of meters, detected signals can reach a receiving device on a ship only through a cable of hundreds of meters, and due to the influence of many factors, useful signals are greatly influenced, so that the spatial information of fault points cannot be effectively extracted.

For the problem, the solution is to immediately preprocess, i.e. filter and amplify, the signal after the electrode detects the signal, so as to reduce the influence of noise as much as possible and amplify the weak signal, thereby enhancing the anti-interference capability of the signal in the transmission process.

In the present embodiment, AD624 manufactured by ADI corporation is used as the electric field signal amplifying element. The AD624 is a confidential instrument amplifier which is developed by ADI company in America and is suitable for a high-speed data acquisition system, has high-precision adjustable gain and can reach 10000 times to the maximum; the common mode rejection ratio is larger than 80dB (when the gain is 500 times, the common mode rejection ratio reaches 130dB), and the common mode part contained in the signal can be effectively rejected; the unit gain bandwidth is 25 MHz; the non-linearity is less than 0.001%; equivalent input noise less than

Noise when the source signal frequency is below 10HzIs less than

The temperature drift of the input offset voltage is less than 25 uv/. C. The high-speed data acquisition system has the characteristics of higher gain, low noise, excellent linearity, high reliability, small volume, suitability for a high-speed data acquisition system and the like, so that the low-noise amplification requirement of submarine cable detection signals can be completely met.

The AD624 is a novel precision instrumentation amplifier modified by a typical triple op-amp, the functional block diagram of which is shown in fig. 18.

The AD624 has high common-mode and differential-mode input impedance, large common-mode rejection, and consistent amplification for different signals as long as the gain setting is unchanged. Errors generated by drift of the first stage and common mode gain imbalance can be offset, the second stage suppresses common mode signals, and double-ended output is changed into single-ended output so as to meet the requirement of a grounding load.

The gain values of the external wiring setting circuit of the instrumentation amplifier pin can be changed to four cases of 1, 100, 200, 500 and 1000, respectively, as shown in table 2, and it can also be connected to an external resistance setting gain value ranging from 1 to 10000, as shown in fig. 19.

TABLE 2 AD624 fixed gain settings

Through a resistance R_GTo set a gain value, the expression of which is:

where A is the amplifier gain.

In weak signal amplification, the influence of interference and noise is not negligible. Effective suppression measures are therefore to be taken. In order to reduce the impact of the interference on the circuit, it should be far from the source of the interference, where possible. However, the electric field sensor works in seawater, and the main interference source is a complex seawater electromagnetic environment, which cannot be far away from the interference source and can only be inhibited on other factors. A filtering link is added at a power supply access circuit, and a tantalum capacitor of 10-30 uF and a monolithic capacitor of 0.01-0.1 uF are connected in parallel at the power supply access position.

The signals detected by the electric field sensor comprise electric field signals emitted from fault points and marine environment noise and high-frequency interference signals. IN order to effectively extract useful signals, filter background noise and more quickly and accurately extract space position information of fault points, a field source is direct current, therefore, a passive low-pass filter is adopted to carry out simple filtering anti-aliasing processing on the signals, a filter circuit diagram is designed as shown IN fig. 20, IN1 and IN2 are respectively differential input ends of detection signals, and AIN1 and AIN2 are output ends of the signals after low-pass filtering.

The sensor housing is typically made of high strength engineering plastic. When the electric field sensor is used for measuring in seawater, the sea is at a depth of more than hundreds of meters, the pressure of the seawater is more than dozens of atmospheric pressures, and in order to ensure that the electric field sensor can normally work in the sea bottom, is not corroded by the seawater and does not react with the seawater, high-strength engineering plastics are adopted.

The sensor is designed to be spherical and made of high-resistance materials. Through theoretical analysis, if the sensor is made of a spherical shape and high-resistance materials, the sensor has certain shape gain and can amplify electric field signals, and therefore the shell of the electric field sensor is made of the spherical structure and the high-resistance materials.

When the electric field sensor works in seawater, the outer shell is made of engineering plastics made of high-resistance materials, the electric field sensor can repel external current, an electric field near the electric field sensor is distorted, and the measured value of the electric field sensor is inconsistent with the true value. To quantitatively investigate the effect of spheroids on the electric field, the following assumptions were made: assuming the spheroid resistivity is ρ₁Radius r₁Sea water is an infinite homogeneous medium and the resistivity is rho₂The external electric field has a current density of j₀A uniform current field. The existence of the spheroid can generate distortion phenomenon to an underwater electric field, and the distortion degree is gradually reduced along with the increase of the distance. The distortion is greatest at the interface of the spheroid and the seawater, and is essentially negligible when the distance from the spheroid exceeds four radii. When the resistivity of the spheroid material is higher than that of seawater, the electric field distortion is positive, and a certain gain is provided for an external electric field; on the contrary, when the resistivity of the spheroid material is lower than that of seawater, the electric field distortion is negative, and the external electric field has a certain counteracting effect. Therefore, the shape of the shell of the electric field sensor is selected to be spherical, and weak electric field signals can be increased when the material is selected to be high-resistance, so that detection is facilitated.

Due to the difference between the resistivity of the spheroid material and the resistivity of the seawater, the existence of the spheroid can cause the distortion of the underwater electric field. In order to accurately locate the submarine cable fault point, the actually measured data needs to be corrected, and the calibration coefficient of the spheroid is u. Assuming a spheroid diameter of 2m, the radius r1 is 1 m. The measuring electrode is located at point a, along the x-direction, where rA-r 1 and θ a-0 are located at the spheroid-seawater interface. In the absence of spheroids, the potential at point A is U₀(r_A,θ_A) In the case of spheroids, the potential at point A is U₁(r_A,θ_A)。

Wherein the expression of u is:

as shown in fig. 21, the calibration coefficient u becomes larger as the resistivity ratio k increases. When k is less than 6, the calibration coefficient u is increased sharply along with the increase of the resistivity ratio k; when the resistivity ratio k is larger than 45, the calibration coefficient u has a smaller increasing trend and is continuously close to 1.5. The spheroid is generally made of non-metal material, and the resistivity is large, and according to the result in fig. 22, the calibration coefficient u is 1.5. Therefore, when the sensor shell adopts a high-resistance spherical body, the underwater electric field amplification effect is 1.5 times, and the submarine optical cable fault point is favorably positioned.

Details not described in this specification are within the skill of the art that are well known to those skilled in the art.

Claims (8)

1. A deep sea optical cable fault detection device is characterized by comprising a first electrode sensor and a second electrode sensor which are connected in series, wherein the first electrode sensor and the second electrode sensor are arranged in water and fixedly arranged on the same cable; the shore signal output equipment provides constant current for the optical cable positioned at the sea bottom; the first electrode sensor and the second electrode sensor feed back the positions of the first electrode sensor and the second electrode sensor and the potential difference between the first electrode sensor and the second electrode sensor to the receiving device in real time, and the receiving device judges the fault point of the submarine cable according to the positions and the potential difference of the first electrode sensor and the second electrode sensor.

2. The deep sea optical cable fault detection device of claim 1, wherein the second electrode sensor is located below the first electrode sensor, the second electrode sensor being located at a greater horizontal distance from the receiving device than the first electrode sensor; when the receiving device measures that the potential difference between the first electrode sensor and the second electrode sensor is the maximum value in the process of reciprocating movement on the sea surface of the area where the fault point of the submarine cable to be positioned is located, the receiving device judges that the fault point of the submarine cable is located right below the position where the second electrode sensor is located.

3. The deep sea optical cable fault detection device of claim 1, wherein the first electrode sensor and the second electrode sensor are identical in structure and each comprises an electrode, a motor base, a cavity, a signal conditioning circuit, a housing and a multi-core watertight connector; one end of the electrode is exposed outside the cavity, and the other end of the electrode is arranged in the cavity; the electrode seat is arranged on the outer wall of the cavity, one end of the electrode penetrates through the electrode seat and is arranged in the cavity, and sealant is arranged at the joint of the electrode and the outer surface of the electrode seat; the multi-core watertight connector is arranged on the cavity shell, the signal conditioning circuit is arranged in the cavity, and one end of the electrode, which is positioned in the cavity, is electrically connected with the cable through the signal conditioning circuit and the multi-core watertight connector in sequence; the electrodes and the cavity are fixedly arranged in the shell, and the shell is a honeycomb spherical porous cover.

4. The deep sea optical cable fault detection device of claim 3, wherein the housing and the cavity are made of ABS material.

5. The deep sea optical cable fault detection device of claim 3, wherein the receiving device is provided with a power supply, the power supply supplies power to the first electrode sensor and the second electrode sensor through cables, and grounding points of the first electrode sensor and the second electrode sensor are both power grounding ends.

6. The deep sea optical cable fault detection device of claim 1, wherein the second electrode sensor weighs more than the first electrode sensor.

7. The deep sea optical cable fault detection device of claim 3, wherein the signal conditioning circuit comprises an instrumentation amplifier and a filter connected in series, wherein a power supply access end of the instrumentation amplifier is connected in series with the filter circuit; the filter adopts a passive low-pass filter.

8. The deep sea optical cable fault detection device of claim 3, wherein the electrodes are all solid Ag/AgCl electrodes, and the technical parameters are as follows: the electrode range potential is within plus or minus 1.0mv, the 12-hour range stability is better than 0.05mv, the electrode self-noise is less than 1 uv/(noise level based on 1Hz frequency), the frequency range is 0.004Hz-210Hz, and the compression strength is 3 MP.

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