An Ocean Bottom Magnetometer for Marine Geomagnetic Field

Survey: Developed dIdD and Fluxgate Sensor

Xiaomei Wang

, Yuntian Teng, Jiemei Ma, Chen Wang, Qiong Wu and Zhe Wang

Institute of Geophysics, China Earthquake Administration, No.5 Minzudaxue Nanlu, Haidian District,Beijing 100081,

China

E-mail: wxm@cea-igp.ac.cn

Keywords: Ocean bottom magnetometer(OBM), dIdD, Overhauser effect, fluxgate sensor, marine geomagnetic field

Abstract: A new type of ocean bottom magnetometer (OBM) for investigations of the basic magnetic field is

developed.The data of marine geomagnetic field could not only improve the complete basic model of the

geomagnetic field with high spatial and temporal resolution, but also satisfy the global geophysical field

research needs. The developed OBM is according to the philosophy of design that it should be high

sensitivity, low power consumption, light in weight, robust enough against mechanical shocks, reliability for

long-term observations. So the developed prototype includes a compact spherical coil system and

Overhauser effect sensor for measuring the total strength of the geomagnetic field and the vectors of

strength, Delta inclination - Delta declination (this kind instrument called dIdD), meanwhile we also

improve the performances of tri-axial fluxgate sensor for geomagnetic vector filed measurement (H,Z and D

components). The advantages of this method are be calibrated by each other sensor and get the whole

elements of basic geomagnetic field. Then we had carried out comparative measuring test in the Jinghai

geomagnetic observatory. As a result, the developed OBM shows better performance in the accuracy of

magnetic field and angle less than 0.2nT, 0.004° respectively.

1 INTRODUCTION

The marine magnetic field survey is very useful for

reflecting the crust-mantle structure , and inferring

global tectonic activity characteristics. The

geomagnetic anomalies reveal much valuable

information about the seafloor spreading and

distribution of hydrothermal vents. With the

progress of technology, in recent years the ocean

magnetometer has been improved rapidly with

accuracy, sensitivity and resolution (Auster et al.,

2007;Antoni et al., 2012;Wang, 2015). At present,

the method of marine magnetic field survey mainly

uses the ship to carry the measurement instrument to

carry on the survey, which include three methods:

the first one is the geomagnetic measuring

instruments installed in the non-magnetic, the

second one is the geomagnetic measuring

instruments in towed operation, the last one is free

fall to the seabed for measurement (Guan,

2010;Kasaya et al., 2013).

The major ocean magnetic technology from total

field strength measurement is first to total field

gradient measurements, then to the vector(almost

three components) measurement and full tensor

multi-parameter measurement. Recently, the major

instrument for ocean magnetic survey are still

optically pumped magnetometer, The Overhauser

magnetometer and proton procession magnetometer,

which used in measuring the scalar parameter of

total strength or the gradient value. Compared with

the scalar magnetic survey method, vector magnetic

survey is able to obtain the size and orientation

information of the magnetic field simultaneously. So

the measurement of ocean bottom geomagnetic

vector has received more attention compared to

geomagnetic scalar measurement by the ocean

geophysicists, because of the geomagnetic vector

measurement could effectively reduce multiple

solutions in inversion using geomagnetic scalar data,

help to qualitative and quantitative interpretation of

magnetic field, to improve the detection resolution

and positioning accuracy of underground orebody.

Wang, X., Teng, Y., Ma, J., Wang, C., Wu, Q. and Wang, Z.

An Ocean Bottom Magnetometer for Marine Geomagnetic Field Survey - Developed dIdD and Fluxgate Sensor.

In Proceedings of the International Workshop on Environment and Geoscience (IWEG 2018), pages 511-515

ISBN: 978-989-758-342-1

511

So the new developed OBM with a compact

spherical coil system and Overhauser effect sensor

for measuring the total strength of the magnetic field

and the vectors of strength, Delta inclination - Delta

declination(this kind instrument called dIdD),

meanwhile we also improve the tri-axial

fluxgate performances of the traditional instrument

for geomagnetic vector filed measurement. This

paper is mainly focus on the key technology of the

measurement sensor, and also the prototype

development.

2 MAGNETOMETER

2.1 didD Sensor

The developed dIdD consists of an Overhauser

effect sensor centered inside two orthogonal

spherical coil systems (see Figure1). Coils are

eventually aligned to be approximately

perpendicular to the local geomagnetic field

direction in the horizontal and geomagnetic meridian

planes, respectively. One series of measurement

consists of five consecutive measurements. There is

no current measurement in the coils at first and later

currents are applied to produce a deflection field.

During these steps we have current in one coil in

one direction then the same current to opposite

direction. Later these steps are repeated with the

other coil. At the end of the series we have five

readings and using these five readings the absolute

value of the vector and two angles can be calculated

(Schott et al., 2001;Heilig, 2006;Hegymegi et al.,

2004;Eugen, 2015).

To have enough volume of homogeneous

magnetic field in limited space of glass sphere and

the size of the Overhauser sensor which has to be

placed into the I-coil we use spherical coil systems

based on the Braunbek coil by increasing the coil to

34 sets coils and non-homogeneous field better than

5‰ in the volume of 70 mm in diameter and 140

mm in long, which could ensure the

OVERHAUSER sensor normal operation developed

by China University of Geosciences (CUG).

The orthogonal spherical coil systems is built for

a conventional PPM material and the diameter of D-

coil and I-coil was 35cm and 30cm respectively.

High level of orthogonality of the two bias coils can

be achieved experimentally by the method of Heilig

B (Heilig, 2012) introduced and tested in Jinghai

geomagnetic observatory. To repeat adjusting angle

and testing, we have got good orthogonality of the

two bias coils about 90.04°and the coils constant of

1057nT/mA, 1265nT/mA respectively.

Overhauser sensor

D-coil

I- coil

Figure 1: The developed dIdD sensor (left is structure

schematic, right is physical photo)

2.2 Fluxgate Sensor

The developed fluxgate sensor is based on second

harmonic Fourier component with the requirements

of high resolution, low noise and long-term stability

characteristics for geomagnetic relative observation

instrument, suppressing the influence of outside

temperature on the sensor output and obtaining low

temperature coefficient (Mitra, 2011;Owais and Atiq,

2011). So the core as magnetic sensitive element is

formed by alloy frame of high plasticity, endurance

strength wrapped by high permeability permalloy

material strip and at least two coils, a drive-coil and

a pick-up coil. In order to better suppress the

fundamental wave signal, the magnetic core adopts

the ring frame and is driven periodically into

saturation by a square current that flows through the

drive-coil. It can be shown that the amplitude of the

second harmonic frequency component will be

measured by external magnetic field. Due to

alternating amplification and deep feed-back loop,

the stability and linearity of the magnetometer are

promoted. Figure 2 shows the developed fluxgate

IWEG 2018 - International Workshop on Environment and Geoscience

512

sensor, and which performances of sensitivity

40uV/nT, noise 0.2nT(p-p).

Fluxgate sensor

H component

Z component

D component

Aluminum frame

Base

Figure 2: The developed fluxgate sensor (left is structure

schematic, right is physical photo)

3 THE MECHANICAL DESIGN

OF THE OBM

In our design, the developed OBM fall free to the

seabed for measurement without communication

cable, so the prototype could not communicate with

the main control device on the boat or ground

observatory when it measuring in the ocean bottom.

Because of the longer deployment duration, the

timing requirements is necessitate a more accurate

internal clock and timer is required to product signal

to the main control unit to fuse wire releasing the

OBM structure from the counter weight so that it

rises due to its ascending buoyancy. Using the

release acoustic sends the signal to communicate

with the main control device for the OBM recovered.

The counter weight made by the Non-magnetic

white cement material provide enough sinking

buoyancy to let the OBM down to the seabed.

Figure 3 shows the developed OBM structure,

which includes six thick glass spheres which

contained the developed magnetic sensors, release

acoustic electronics, main processing unit and the

batteries for the electronic system , thus providing

both ascending buoyancy and the pressures housing.

To cushion the glass, the spheres then go inside

yellow plastic "hardhats". The overall support

platform is made of non-magnetic material PVC

board, considering the corrosion resistance, hardness

and nonmagnetic.

Figure 3: The construction of developed OBM (left is

mechanical assembly drawing , right is physical photo)

4 MEASURING TEST AT

JINGHAI GEOMAGNETIC

OBSERVATORY

The developed prototype has been set up in

geomagnetic Laboratory of Jinghai observatory.

Because the bias current of dIdD is disturb to the

fluxgate sensor output, so the distance between the

two sensors was far enough in this comparative

measuring test. During the trial operation (from 28th

September to 31th October, 2017), we collected the

raw observation data of prototype with 1Hz

An Ocean Bottom Magnetometer for Marine Geomagnetic Field Survey - Developed dIdD and Fluxgate Sensor

513

sampling. Figure 4 shows the geomagnetic diurnal

variation curve and noise of two developed sensors

on 8th October 2017, and we calculate the noise to

evaluate performance of developed prototype using

the time series from16:00 to 20:00(UTD) of 8th

October because artificial noise and natural

geomagnetic variance are small. The corresponding

results are shown in Table 1. We could see from the

results that the two developed magnetic sensors

could record the geomagnetic diurnal variation

precisely, the accuracy of magnetic field and angle

less than 0.2nT, 0.004° respectively.

Table 1: The results of noise between developed dIdD and

fluxgate sensor.

Instrument

type

dIdD Fluxgate sensor

Magnetic

component

(nT)

(°)

(nT)

Noise (P-P

value)

0.19 0.0029 0.0033 0.09 0.16 0.09

Figure 4: The geomagnetic diurnal variation curve of two

developed sensor on Jinghai observatory(upper is dIdD,

down is fluxgate sensor ).

5 CONCLUSIONS AND FUTURE

WORK

In this paper we developed two prototype magnetic

sensors for marine geomagnetic field survey, and

carried out parallel observation to evaluate the

performance of each developed sensor. Then present

the design and construction of developed OBM.

This sensor combination observation method could

be mutual calibrated by each other and get the whole

elements of basic geomagnetic field. But during

parallel observation, we didn’t consider the distance

and volume of developed OBM, so the next work

we would focus on the final prototype to eliminate

the bias current interference to fluxgate sensor

output.

Follow-up work would also focus on the

optimization of the dIdD sensor circuit to improve

the bias magnetic stability and reduce the noise

which can improve the effective resolution, the

fluxgate sensor mechanical structure by using

suspension device to improve stability. Because of

the developed OBM is free fall to the seabed for

measurement, we could know the initial attitude

information by using attitude device to measure the

measurement coordinate system, then analyze the

mathematical logic relationship between the

geographic coordinate and measuring coordinates by

using coordinate rotation algorithm, so we need take

a series of experiments and tests which the

developed OBM tiling arbitrary angle, it still could

get high quality geomagnetic observation data.

ACKNOWLEDGMENTS

The authors would like to thank cooperators from

China University of Geosciences(CUG) and Zhuhai

Ted Enterprise Co, Ltd for their contributions to this

work and staffs from Jinghai and Zhaoqing

Geomagnetic Observatories for supplying test sites.

This work is supported by the National Key

Scientific Instrument and Equipment Development

Project of China (Grant No. 2014YQ100817-2) and

the National Natural Science Foundation of China

(Grant No. 41404141).

-6.80

-6.75

-6.70

-6.65

-6.60

57.45

57.48

57.51

57.54

54000

54010

54020

54030

54040

54050

dI Comonent

24:0020:00

16:00

12:0008:00

04:00

2017-10-08

00:00

Angle/。

dD Component

Magnetic/nT

F Component

-105

-90

-75

-60

-45

-30

-15

135

140

145

150

155

160

D Component

H Component

2017-10-08

Magnetic/nT

Z Component

24:0020:00

16:00

12:0008:00

04:00

00:00

IWEG 2018 - International Workshop on Environment and Geoscience

514

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