The Ninth International Conference on Electronic Measurement & Instruments ICEMI’2009

Research on Modeling and Experimentation on Detecting Buried Depth of Optical Fiber Submarine Cables Based on Active Detection Technique

Xuejun Zhou Yingjian Wang Yuanyuan Zhou Electronic Engineering College, Naval University of Engineering

Wuhan, China Email: zyy_hjgc@yahoo.com.cn

Abstract –In recent years, detecting buried depth of optical fiber submarine cables is a difficult problem for the building and the maintenance of optical fiber submarine cable communication system. Based on magnetism detection method, this paper establishes the basic model of buried depth detection and designs the biorthogonal-coil depth detection method. The characteristic of the biorthogonal-coil depth detection method is that it is guaranteed that the detection signal obtains the maximum value in spite of the orientation of coils as long as the coil array is above the optical fiber submarine cable and root-mean-square value of the two coils from each group is calculated. The correctness and feasibility of the basic model and the biorthogonal coils model are validated by the experiments. Keywords –optical fiber submarine cable, buried depth detection, magnetism detection method, biorthogonal-coil depth detection method.

I. INTRODUCTION

With the rapid development of optical fiber submarine cable communication, all of the builders of optical fiber submarine cables think that it is inevitable that optical fiber submarine cables are deeper buried [1]. The buried depth is the important factor affecting the health, the vitality and the maintenance of optical fiber submarine cables [2]. Therefore, study on detection technique of the buried depth of optical fiber submarine cables is very urgent, and significant to develop the related equipments.

At present, TSS-340 and TSS-350 are two kinds of internationally advanced equipments which can carry on detecting buried depth of optical fiber submarine cables. The former is based on passive detection technique, and only can detect the buried depth less than 70cm. The latter is based on the active magnetism detection method, but it is exigent to the carrier. Usually the underwater robot (ROV) is used as the carrier. Not only the cost is very expensive, but also the operation is complex. So, it is unsuitable for detecting buried depth of optical fiber submarine cables for common purposes.

The remainder of this paper is organized as follows. In section II, we review the basic principle of buried depth detection. In section III, we establish the basic model of buried depth detection and design the biorthogonal-coil depth detection method. The correctness and feasibility of the basic model and the biorthogonal coils model are

validated by the experiments in section IV. Finally, a conclusion is contained in section V.

. THE BASIC PRINCIPLE OF BURIED DEPTH

DETECTION

The basic principle of buried depth detection based on the magnetic induction is as follows:

Usually, there is metallic conductor in the optical fiber submarine cable’s core. When the alternating current passes in the conductor, there will be an alternating electromagnetic field around the optical fiber submarine cable which will disseminate to the periphery. The tailor-made probe resonated with the sending frequency can detect the voltage signal with the same frequency nearby [3]. In practice, the radius of the area from the researched point to field source considered is smaller than 1/6 of electromagnetic wavelength [4]. Therefore, the optical fiber submarine cable may be considered as an infinitely long wire. Usually the cable is buried under the ground and is parallel with the surface. When the certain electric current passes in the cable, magnetic field line is a concentric cylinder taking the cable as the core. As shown in Figure 1, x is the lateral distance between probe head and cable. B is the magnetic induction intensity of the point with a certain distance from the optical fiber submarine cable [5], and its expression is

2IBr

��

� �� �

(1)

where u is a magnetic conductivity and I is the electric current in the optical fiber submarine cable. r is a radius vector, and � is a transverse unit vector which has right hand screw relations with I. Thus, the magnetic field around the infinitely long straight wire is a vertical vector with I and r. The proportion of B to r is inverse. Usually this characteristic is used to detect the buried depth of the cable on land. According to the above theory, the buried depth of the optical fiber submarine cable also can be detected.

According to the present situation of optical fiber submarine cables buried, the optical fiber submarine cable is considered that it satisfies the condition of the long straight wire in the electromagnetic theory analysis. Certain electric current passes in the optical fiber submarine cable, and there will be magnetic field around the electric current. The relations between magnetic field intensity and the space position are studied, and the

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The Ninth International Conference on Electronic Measurement & Instruments ICEMI’2009

optical fiber submarine cable’s position is directly determined with an appropriate measuring technique.

Without regard to the attenuation of electromagnetic wave caused by the sea water, we establish coordinates which take a lateral section of a submarine cable as the origin.

In Figure 1, x is plane distance between the probe head and the optical fiber submarine cable, and y is vertical distance between the probe head and the optical fiber submarine cable.

With the related theory of electromagnetic field, the induced electromotive force in probe head is

c o s2

S d Ir d t

�� ��

� � (2)

where � is the included angle between the field intensity

vector and the unit of area vector dS . From (2), we know that the induced electromotive

force � is direct ratio to the section area S of the close coil, to changing frequency of the electric current in the long straight wire, and to the cosine of included angle between the field intensity vector and the unit of area vector's respectively.

1r

� � (3)

The induced electromotive force of the close coil is inverse ratio to the distance between the close coil with the long straight wire, which is also a physical quantity related with the spacial physical position. cos� is also the physical parameter directly related with the direction of the close coil referring to the long straight wire. Detection of the optical fiber submarine cable’s position is to indirectly determine r through measuring induced electromotive force of the close coil.

Figure.1 Diagram of detection coordinates

. THE ELECTROMAGNETIC FIELD MODEL OF OPTICAL FIBER SUBMARINE CABLES

A. Basic coils

The detection method with two coils is presented in [6]. We denote I as the alternating current of the optical fiber submarine cable. In Figure 2, A and B are two identical coils and the distance between the coils is l, and

h is the buried depth of the optical fiber submarine cable. The voltages of coil A and B can be expressed as

cosAIu Kh

�� (4)

cosBIu K

h l��

� (5)

From (4) and (5), the ratio A

B

uu

is

A

B

u h lu h

�� (6)

Assuming A

B

u xu

� , we have

1lh

x�

� (7)

B

A

l

hSea bed

Coil

Coil

CableMagnetic field

Figure.2 Diagram of detection method of double coils If uA, uB and l are known, we can calculate h, namely,

the optical fiber submarine cable’s buried depth. (7) shows that the depth is only related with the voltage ratio of the two coils and the distance between the two coils. But it has nothing to do with its absolute magnitude. Namely, it has nothing to do with probe head's parameters, the rotary orientation, and the intensity of electric current of the optical fiber submarine cable. In practice, however, we take above parameters into account in order to enhance the measuring range and increase the detection precision.

B. The detection model of two group vertical coils

The included angle between coil normal and magnetic field direction don’t be taken into account above. The bigger the included angle is, the less the induction signal is, which will result in the increase of detection error and decrease of the probing range. The induced voltage is zero on the extreme condition �=900.

We have designed four coils depth detection model. As shown in Figure 3, two coils A and B are two group mutually vertical coils. Four coils’ characteristics are completely consistent. Therefore, the voltage value of two coils from group A respectively is

1 cosAIu K

h l��

� (8)

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The Ninth International Conference on Electronic Measurement & Instruments ICEMI’2009

2 cos( 90 )AIu K

h l�� �

� (9)

The root mean square value of the 1Au and 2Au is

2 21 2A A A

Iu u u Kh l

� � ��

(10)

Likewise 2 2

1 2B B BIu u u K

h l� � �

� (11)

The value of uA and uB are the maximum value of single coil, when � is 900. As long as the coil array is above the optical fiber submarine cable and root-mean-square value of the two coils from each group is calculated, it is guaranteed that the detection signal obtains the maximum value in spite of the orientation of coils. The conclusion is very significant to practical operation. Next we will prove the above conclusions by experiments.

Figure.3 Diagram of detection method of biorthogonal coils

. EXPERIMENTS AND DATA ANALYSIS

The experiments aim to validate the correctness of the two mathematical models. Firstly, we validate the correctness of the basic model and the feasibility of depth detection method. Secondly, we validate the correctness of the biorthogonal coils model and the feasibility of the depth detection method.

A. Experiment design and data analysis of the basic model

The experiment design is shown in Figure 4.

Figure 4 Diagram of basic experimental principle

(1) A long wire passed the 25Hz alternating current is used to simulate the optical fiber submarine cable.

(2) The 10000-circle coils with ferrite core is used to simulate detection coils. The induced voltages of the coils’ both ends can be directly measured with a multimeter.

In Figure 4, the sender generates a 25Hz alternating signal to pour into the wire. We lay the coils horizontally above the wire, make the coils be vertical ( 0� � ) to the wire, measure the coils' induced voltage on different heights, and record the results in Table 1.

Table 1 The voltage values of different heights and the calculated

depth value V=300V f=25Hz I=314mA

NO. Heightm

VoltagemV

calculated depth m

Average relative

error

1 0.43 68 h12=0.43 h13=0.44 h14=0.43

2.3%

2 0.86 34 h23=0.89 h24=0.86 3.5%

3 1.29 23 h34=1.21 -6.2% 4 1.72 17

Note: hij is denoted as the calculated depth value with the ith and jth data

Table 1 shows that the buried depth of optical fiber submarine cables can be completely determined by the relative value of the induced voltages of the coils on two different heights

B. The experiment and data analysis of biorthogonal coils model

The experiment design and experiment environment are same as above. In this experiment, the rod is on a platform with a fixed height. The induced voltages of the rod’s both sides are measured on different orientation relative to the cable. The results are recorded in Table 2 and Table 3.

Table 2 The induced voltage values of the rode on different heights

(height: 0.86m electric current: 1000 mA)

Quadrant �( ) 0 15 30 45 60 75u0 (mV) 304 298 266 210 154 67

Quadrant �( ) 90 105 120 135 150 165u90 (mV) 5.1 65 133 204 260 287

Root Mean Square

2 2

0 90u u u � � 304 305 297 293 302 295

Average E[u] mv 299 Error(compar-ed with

E[u]) �u (mv) 5 6 -2 -6 3 -4

Mean Squared

Error � 5

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The Ninth International Conference on Electronic Measurement & Instruments ICEMI’2009

Table 3 The induced voltage values of the rode on different heights (height: 0.43m electric current: 1000 mA)

Quadrant �( ) 0 15 30 45 60 75u0 (mV) 705 664 593 491 310 135

Quadrant �( ) 90 105 120 135 150 165u90 (mV) 150 200 396 524 630 681

Root Mean Square

2 2

0 90u u u � � 705 690 709 713 697 687

Average E[u] mv 700 Error(compare-ed with

E[u]) �u (mv) 5 -10 9 13 -3 -13

Mean Squared

Error � 10.5

The table 2 and table 3 shows that the root mean

square value of the coils with intersection of 900 is approximately equal, and is the maximum value when � is 900. That has validated (10) and (11).

. CONCLUSION

The buried depth detection of optical fiber submarine cables has been a difficult problem in the construction and maintenance of the optical fiber submarine cables. This paper has studied biorthogonal coils detection depth method based on active detection method, and confirmed the rationality and feasibility of the method by experiments. The study is very significant to develop the related equipments. But this paper does not study more about the detection parameters, such as measuring range, measuring accuracy and so on which are the important factors in practice, we will take on a more in-depth research.

REFERENCES

[1] P. Du, S.Z. Fu. “Brief analysis on several kinds of detection technologies of optical fiber submarine cable,” Guangxi Journal of Light Industry, vol.12, pp. 67-68, 2007.

[2] J.F. Chen. “Thinking about the situation of the international optical fiber submarine cables and some problems of East China Sea,” Submarine Cable Technology, vol.1, pp.6-8. 2009.

[3] D.X. Bi. “Electromagnetic field theory,” Publishing House of Electronics Industry, Beijing, pp.126-129,1985.

[4] Y.Kang. “College physics (Part ),” Science Press, Beijing, pp. 56-62, 2006.

[5] C.R.Chang, X.J.Liu. “Electromagnetism,” Harbin Institute of Technology Press, Harbin, pp. 98-102, 2000.

[6] X.J. Zhou, H.X. Wang. “Study on the detection technique for the buried depth of submarine cables,” Electric Wire & Cable, vol. 3, pp. 67-69, 2005.

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