Low Power AMT Acquisition Network Based on

ZigBee and GPS

Xiaolu Xi, Haicheng Yang

Nanjing University of Science and

Technology

Nanjing, P. R. China

Xxl00001@gmail.com

Xuefeng Zhao，Hongchun Yao,

Jieting Qiu, Hai Dong, Fabao

Yan, Shenglong Tan, Ruijie Shen,

Hong Wu

Champion Geophysical Technology

Changsha, P. R. China

yhc511025@163.com

Xing He, Rujun Chen School of Geoscience & Info-physics

Central South University

Changsha, P. R. China

chrujun12358@gmail.com

Abstract—Audio frequency magnetotullric (AMT) is widely

used in the exploration of mineral and underground water.

Three-dimension (3D) AMT exploration makes the imaging of

underground geological body or structure with the best precision

and resolution. And it needs a lot of AMT acquisition units to

carry out exploration. Current commercial AMT unit was

suffered from high power consumption and low work efficiency

for 3D AMT exploration. We design and realize a low power

AMT acquisition network based on ZigBee, GPS and ARM based

embedded control system. Each AMT acquisition unit is

composed of power supply module, GPS module, ARM module,

and data acquisition module. The power supply module is

controlled by the GPIO of the ZigBee Pro module. Power supply

for GPS module, ARM module, data acquisition module, and

induction coils can be switched on or off remotely by ZigBee

network. The GPS module offers clocks and timing signals for

the data acquisition module. The ARM based embedded control

module is composed of AT91RM9200, 1GB NAND flash, 64 MB

NOR flash, WI-FI, Ethernet and 64 MB SDRAM. It controls the

data acquisition, calibration, and self-testing in AMT exploration.

The data acquisition module is composed of 4 channels for the

signal conditioning of weak AMT signals, 4 channels 24-bit ADC,

and 24-bit fixed DSP. The low power differential amplifiers and

low power audio amplifiers are used for the amplifying and

filtering of the input signals. All AMT acquisition units are

configured as ZigBee routers to build a wireless sensor network

in mountain area successfully. A notebook with ZigBee router is

used to control the AMT acquisition network. Software is

developed in notebook to monitor network, send control

command, retrieve the status of each AMT acquisition unit, and

control the data acquisition process. One hundred AMT

acquisition units are made, and tested in Tibet. The tests were

successful in most area, but it was difficult to build a ZigBee

network in some area where the tough topology was faced.

Keywords—data acquisiton; AMT; ZigBee, sensor network,

GPS

I. INTRODUCTION

Audio frequency magnetotelluric (AMT) is a useful tool in natural resource and underground water exploration[1][2]. It’s also used in engineering geology and environment geology to image to distribution of underground geological body and

pollution[3]. Traditionally, AMT is mainly used for two-dimensional exploration (2D) that considers the variation of resistivity distribution in two directions[1][4]. However, most geological body is three-dimensional (3D). Its resistivity distribution is varied in three directions. A big error may be occurred if 2D exploration is used to the exploration of 3D geological body or 3D ore body. Therefore, 3D AMT exploration is the best choice for the most conditions. Current commercial AMT acquisition station suffered from high power consumption and low work efficiency in 3D AMT acquisition because one AMT acquisition station needs one operator, and one heavy battery is needed for each AMT acquisition station. The total weight and power consumption of a commercial AMT acquisition is about 10 w and 20 kg respectively. Therefore, we developed a low-power AMT acquisition network based on ZigBee and GPS for 3D AMT exploration. The power consumption and total weight of our AMT acquisition unit is less than 3 w and 4 kg respectively. And one operator can control more than 40 AMT acquisition units by ZigBee network in the field. The paper discusses the design, realization and the test of low-power AMT acquisition network proposed by us.

II. THE PRINCIPLE OF AMT EXPLORATION

The AMT method uses the naturally occurred electromagnetic field, which usually produced by lighting and solar wind, to explore the underground geological structure[5]. Because the source that produces the audio-frequency electromagnetic field is usually near the equator. The naturally occurred audio-frequency electromagnetic field can be considered as plane electromagnetic wave. The direction of its propagation is vertical to earth's surface. The underground

rock’s resistivity ranges usually from 10 .m to 1000 .m, so the electromagnetic field will be attenuated. The skin depth defines the degree of attenuation as following

(1)

978-1-4673-6386-0/14/$31.00 ©2014 IEEE

Where is earth's resistivity in .m, f is frequency in Hz,

is skin depth in meter. We can find that the skin depth is

proportional to and . . The ratio between electric field

strength and magnetic field strength is called wave impedance

Z as following

(2)

Where Ex and Hy are electric field and magnetic field

strength respectively. They are orthogonal and parallel to

earth's surface. If the earth is uniform with constant resistivity,

there exist simple relationship between wave impedance and

earth's resistivity as following

(3)

Where Ex is electric field strength in mv/km; Hy is field

strength in gamma, f is frequency in Hz. Formula (1) and formula (3) are the foundation of AMT

exploration. Formula (1) shows the relationship among frequency, skin depth and resistivity. When the resistivity is fixed, the skip depth increase if the frequency of electromagnetic wave decreases. We can change the depth of exploration by changing frequency. Formula (3) shows the relationship between earth's resistivity and wave impedance. The resistivity at different frequency is related to earth's resistivity at different depth. However, we need to measure orthogonal electric field Ex and Ey, and measure orthogonal magnetic Hx and Hy to determine underground structure if the underground earth is not uniform.

III. DESIGN AND REALIZATION OF AMT DATA ACQUISITION

UNIT

A. General Design

Fig. 1. Diagram of AMT data acquisition unit and sensors

Fig. 1. shows the diagram of AMT data acquisition unit designed by us and corresponding sensors. The electrodes located at north and south direction measure electric field Ex; and the electrodes located at east and west direction measure electric field Ey. The orthogonal induction coils Hx and Hy measure magnetic field. ZigBee is used to build a wireless

sensor network in the field. GPS ensures all AMT data acquisition unit (DAU) to acquire data at same time.

ARM Module

Power Supply Module

RS-232

GPS Sync Module

RS-232

Data Acquisition Module

GPS Antenna

ZigBee Antenna

WI-FI Antenna

SSC SPI UART PPS ADC Clock

Ch1 Ch2 Ch3 Ch4

Fig. 2. Diagram of AMT DAU

Fig. 2. shows the modules designed for AMT DAU. The DAU’s power supply can be controlled remotely by the GPIO of ZigBee OEM board. The serial port of ZigBee OEM board is connected to ARM module by RS-232 interface. The ARM module is the center of AMT DAU. It communicates with control center, which is a portable computer with ZigBee adapter as router, by ZigBee network and WI-FI. WI-FI is used for fast data communication between control center and AMT DAU for testing, data transfer, and software update. Because all ZigBee OEM boards of each AMT DAU are configured as router, this ensures robust network maintained in mountain area. The GPS sync module provides PPS and A/D converter clock to data acquisition module to ensure all AMT DATUs acquiring data at same time. It also output NMEA-0183 format data to ARM module. The control center can get these information by ZigBee network. Therefore, the location and exact time of each AMT DAU at field can be got, and be shown in the screen of the control center. The data acquisition module is low power consumption, and it conditions input analog signals from electric electrodes and induction coils then realize A/D conversion. Finally, a series of digital signals processing are performed to the output of ADC then the processing result is output into ARM module. The ARM module communicates with the data acquisition module by interfaces SSC, SPI, and UART. UART is used to upload DSP program and send control command to DSP. The SSC ensures fast data transfer between ARM module and data acquisition module. The SPI is used to configure the data acquisition module, including start/stop ADC, controlling the gain of amplifiers, start/stop calibration, generating calibration signal, and so on.

B. Design of Power Supply Module

Because it needs several hours to deploy AMT DAUs in the field, it’s important to let AMT DAU at low power mode when data acquisition is not start. As shown in Fig. 3. , just ZigBee

Pro OEM board is on when DAU’s power is on. The GPIO of ZigBee Pro OEM board let the power supply of other modules and induction coils off. Therefore, a wireless sensor network is maintained at low power mode. The control center can control the power supply of DAU by ZigBee network.

ZigBee ProOEM

11.1V/9 Ah Li-ion

Battery & Power Switch

9~18V to +5.5V DC-DC

& +3.3V DC-DC

9~18V to+/-15V DC-DC

9~18V to+3.3V DC-DC

9~18V to+3.3V DC-DC

9~18V to+5V DC-DC

9~18V to+3.3V DC-DC

Analog Power Supply

Induction Coil Power Supply

GPS System Power Supply

ARM System Power Supply

USB WI-FI Power Supply

Analog Power Control

Induction Coil Power Control

GPS System Power Control

ARM System Power Control

USB WI-FI Power Control

Fig. 3. Diagram of Power Supply Module

C. Design of GPS sync module

We adopt timing GPS OEM board in the GPS sync module.

The timing GPS OEM board output PPS, NMEA-0183 format

data, and 8MHz clock. The 8MHz clock is synced to GPS and

converted into 12.288MHz ADC clock by frequency synthesis.

D. Design of ARM module

The AT91RM900 is adopted in this module. It’s a

180MHz ARM9 SOC, and it supports interfaces as 5 UARTs,

1 USB, 4 SPIs, 3 SSCs, I2C, and so on. The ARM module

also includes 1GB NAND flash, 64MB NOR flash, WI-FI,

Ethernet and real-time clock. Programs and Linux are stored

in NOR flash. Data are stored in NAND flash.

E. Design of data acquisition module

Fig. 4. shows the diagram of data acquisition module. We

adopt different method for the amplification and filtering of

electric signal and magnetic signal. Because the signal

conditioning circuit of induction coil amplifies magnetic

signal to a level of 200mV/nT at 100Hz, we just use

programmable main amplifier and anti-aliasing filter to

magnetic signal. The signal from electrodes is weak; so the

two-stage amplification is adopted. The protection circuit is

used to prevent ESD and lighting strike in the field. The radio

frequency interference (RFI) filter prevents amplifier

saturation. A four-channel audio ADC AK5388 converts

analog signal into digital signal. A low-power CPLD

LCMX02 receives the output from ADC then sends it into a

24-bit fixed point DSP56309 after converting data format into

what needed by DSP. The operational amplifiers used in

amplification and filtering are carefully selected for low-

power consumption. The amplifiers OPA2836 and THS4521

become our choice for their low-power consumption

characteristic. The DSP does multi-decimation and multi-

notching for input signal then outputs digital signal data at rate

as 24000Hz, 2400Hz, and 300Hz simultaneously. Protection

Circuit & RFI filter

Protection Circuit & RFI

filter

Protection Circuit & RFI

filter

Protection Circuit & RFI

filer

Pre-Amplifier(9)

Pre-Amplifier(9)

Low pass filter

Main Amplifier(1,3,5,9)

Main Amplifier(1,3,5,9)

Main Amplifier(1,3,5,9)

Anti-alias filter

Low pass filter

Main Amplifier(1,3,5,9)

Anti-alias filter

Anti-alias filter

Anti-alias filter

4-ch ADC, low-

power CPLD,&

DSP

Ex

Ey

Hx

Hy

Fig. 4. Diagram of data acquisition module

F. Realization of AMT data acquisition unit

Fig. 5. Realization AMT DAU

Fig. 5. shows our realization of AMT DAU. Each module

is implemented as a PCB board with same size. Data

acquisition board (DAB), ARM board, GPS board, and power

supply board are stacked from top to bottom respectively. All

boards and battery are fixed on fastening structure to resist

vibration in the field.

IV. SOFTWARE DESIGN OF DATA ACQUISITION SOFTWARE

BASED ON ZIGBEE NETWORK & ETHERNET

Fig. 6. is the diagram of data acquisition software. It includes control center software run on portable personal computer (PC) and data acquisition unit (DAU) software run on DAU. The control center software is composed of user interface (UI) module, DAU management module, DAU agent

module and communication module. The UI module is responsible for the human-computer interaction between user and control center. It includes interfaces for DAU management, parameter setting, status information, real-time monitor, and data transfer. The DAU management module is responsible for the management of AMT acquisition unit in the field. The DAU agent module offers the interface to access the server of DAU for top layer. This module reduces the complexity of top layer by masking the detail of realization in the bottom layer. This reduces the coupling among modules, and increases the portability, extension ability and expansibility of modules. The communication module realizes the wired and wireless communication among DAUs, offers reliable data transfer interface to top layer, and offers the interface of the power supply control of the DAU.

User Interface Module

Data Acquisition Agent

Module

Communication

Module

Data Acquisition Unit

Management Module

Server Module

Communication

Module

Execution Module

Feebback

Request

ZigBee/Ethernet

Control Center Software

Data Acquisition Unit

Software

Fig. 6. Diagram of data acquisition software

The DAU software runs on embedded Linux operation of the DAU. It mainly includes execution module, server module and communication module. The execution module is responsible for data acquisition, DAU calibration, induction coil calibration, contact resistance measurement, GPS sync, and status LED control, and so on. The server module is responsible to offer service interface to control center. If a request from control center is received by the server module, the server module lets the execution module do responding action, and return execution result to the control center. The communication module of DAU does the same job as the communication module of control center does, it offers reliable data transfer interface to the server module in the top layer.

V. INDOOR TEST AND EXPLORATION IN TIBET

We tested power consumption and input noise indoor, and

compared the result with that of a commercial product MTU-

A. The result (Table 1 & Table 2) shows our design is

successful. The input noise is different between magnetic

channel and electric channel because of different amplification

and filtering circuit for electric channel and magnetic channel.

The noise of magnetic channel is less than that of electric

channel because magnetic channel uses fewer amplifiers in

each channel than electric channel does. The noise of

magnetic channel for AMT DAU is also less than that of

MTU-A. But the noise of electric channel of AMT DAU is

bigger than that of MTU-A when the gain of electric channel

of AMT DAU is equal to 27 or more. This may be caused by

different design idea adopted between AMT DAU and MTU-

A.

The power consumption of AMT DAU is much less than

that of MTU-A. In data acquisition mode, the power

consumption of AMT DAU is only 1/3.6 times power

consumption of MTU-A. In idle mode, the power

consumption of AMT DAU is only 1/4.8 times power

consumption of MTU-A.

We build 100 AMT DAUs and carry out an exploration in

Tibet (Fig. 7.). The tests were successful in most area, but it

was difficult to build a ZigBee network in some area where

the rough terrain was faced.

TABLE I. POWER CONSUMPTION OF AMT DAU AND MTU-A

Instrument Test condition & Result

Condition Power (w)

AMT DAU

ZigBee On 0.26

Startup 2.28

Idle 2.16

Acquisition 2.88

MTU-A

Startup 12.72

Idle 10.44

Acquisition 10.44

TABLE II. INPUT NOISE OF AMT DAU AND MTU-A (SAMPLE RATE: 24000 HZ)

Channel

No.

Gain & Result in uV

AMT DAU

Gain

MTU-A

Gain

AMT DAU

Noise

MTU-

A

Noise

1 9 4 2.77 5.92

2 9 4 2.50 6.28

3 1 1 3.68 24.35

4 1 1 3.54 24.85

1 27 16 2.92 1.73

2 27 16 2.77 2.53

3 3 4 1.61 8.48

4 3 4 1.57 9.68

1 81 64 3.07 1.33

2 81 64 3.09 2.30

3 9 16 1.16 6.86

4 9 16 1.05 8.57

Fig. 7. Field test in Tibet

VI. CONCLUSION

The ZigBee network can play an important role in geophysical instrument. It’s necessary to reduce the power consumption of geophysical instrument because it can decrease the total weight of data acquisition units. Our research shows that there exist a big potential to reduce the power consumption of traditional geophysical instrument usually operated by one person. The wireless sensor network may bring a revolution to geophysical exploration because it can image our underground earth with high precision and high resolution.

ACKNOWLEDGMENT

We thank Hongshuang Li, GuiHui Tao, Pei Zeng, Qiang Ren, Shaohua Wang, Fuguo Chang, Xiao Liang, Lei Liu, Fengling Wei for their contribution in the indoor and field test of the developed 3D AMT instrument.

This work was supported by geological prospecting fund (12120113095200) of Chinese Geological Survey.

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