Design and Experimental Research on Shallow WaterVolume Reverberation Signal Acquisition System

Yongwei LID * , Qi LI, Mengying CHEN

Abstract - Volume reverberation is the main interferenceof active sonar in shallow coastal waters. This greatly limitsthe sonar's working range and the performance ofparameterestimation. A lot of research on the properties of deepscattering layer, sea surface reverberation, sea bottomreverberation has been done. However, the research onvolume reverberation at 10 to 40 kHz in shallow coastalwaters is only a little. It is difficult to collect shallow watervolume reverberation signal by conventional underwateracoustic signal acquisition equipments with an electricalswitch, due to circuit obstruction. Therefore, based on thecharacteristic of volume reverberation, a signal acquisitionsystem was designed. The control core was made up ofFieldProgrammable Gate Array (FPGA). The signal acquisitionsystem was tested in shallow coastal waters. The law ofvolume backscattering intensity with temperature, salinity, theconcentration ofsuspended sediment particles, and frequencyis summarized. The results demonstrate that the volumebackscattering intensity may be changed by 30 dB, due to theconcentration change ofsuspended sediment particles. 1

Index Terms -Volume Reverberation, Signal AcquisitionSystem, FPGA, Volume Backscattering Intensity

I. INTRODUCTION

When acoustic energy is transmitted in the ocean, part ofthe transmitted energy is scattered by small inhomogeneitiesin the ocean volume. These small inhomogeneities may be fish,suspended sediment particles, small air bubbles, or oceantemperature fluctuations. Energy from all of these possiblesources combines to form the scattered sound field, which ischaracterized as volume reverberation. The characterization ofvolume reverberation is important in a wide variety ofscientific, commercial, and military applications. Examplesinclude: seafloor imaging, swath bathymetry, air seainteraction and mixing studies, bio-acoustic surveys, fisherystock assessment, underwater communication, naval mine­hunting, and covert, environmental reconnaissance in supportof special operations. The properties of deep scattering layer,sea surface reverberation, sea bottom reverberation have beenstudied in recent years [1]-[6]. But the research on low

This work is funded by the China National Foundation Research under grantNo.5132101.Yongwei LIU is with the College of Underwater Acoustic Engineering,Harbin Engineering University, Harbin, China. (e-mail:liuyongwei3000@hrbeu.edu.cn).Qi LI is with the College of Underwater Acoustic Engineering, HarbinEngineering University, Harbin, China. (e-mail: leechi@hrbeu.edu.cn)Mengying CHEN is with the College of Underwater Acoustic Engineering,Harbin Engineering University, Harbin, China. (e-mail:chen03051302@126.com)

978-1-4244-3870-9/09/$25.00 ©2009 IEEE

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frequency volume reverberation in shallow coastal waters isonly a little [7]-[10]. Based on practical condition of shallowcoastal waters, it is difficult to collect the volumereverberation signal by conventional underwater acousticsignal acquisition equipments with an electrical switch, due tocircuit obstruction. A signal acquisition system, composed ofpower board, analog receiving board, and control board wasdesigned [11]-[12]. The volume reverberation signal wascollected by the analog receiving board, then amplified,filtered, and stored in hard disk of host computer through PCIboard. The core of the control board was made up of FPGA[13]-[16]. The signal acquisition system's working condition,sampled frequency, initial gain code, and filter bandwidthcould be set up remotely by host computer through the controlboard and PCI board.

II. THE SIGNAL ACQUISITION SYSTEM

A. Analog Receiving Board

The volume reverberation signal follows the transmittedsignal closely, but the transmitted signal usually makesconventional underwater acoustic signal acquisitionequipments obstructed, so the designed signal acquisitionsystem is required to switch off, when the signal is beingtransmitted. Meanwhile, the designed signal acquisitionsystem is required to switch on quickly, and collect thevolume reverberation signal after the transmitted signal.Because the attenuation of volume reverberation signal isproportional to the square of time, there also needslogarithmic time gain control in the system. At last, thevolume reverberation signal is susceptibly influenced by thenoise; it is necessary to be amplified and filtered beforestorage. The block diagram of the analog receiving board isshown in Fig.l.

Digital Optical Coupler

Fig.I The block diagram of the analog receiving board design

There are 12 channels on the analog receiving board. Theanalog switch (MAX4678) is the switch control of eachchannel and can be controlled remotely by host computer. Its

main function is just as an anti-saturation circuit and used tosolve the problems of circuit obstruction, when volumereverberation signal is being collected. MAXIM MAX4478 isadopted to be the emitter follower and first amplificationcircuit, due to its high impedance. Therefore, the impedancematching effect is good when the analog receiving boardconnects acoustic transducers. The Logarithmic Time Gaincontrol is carried out by a series of digital potentiometersAD8400, of which the resistance can be controlled remotelyby host computer through sending data into the DAC registerinside AD8400. Namely, the logarithmic time gain code canbe set up remotely, and gives the output expressed in decibels,so as to simplify the processing algorithms. The band-passfilter is automatically matched to the transmitted signal, inorder to achieve the optimal signal-to-noise ratio (SNR). TheBurr-Brown OPA2345 is as second amplification circuit. Inorder to reduce the interference between the analog signal andthe signal converted by ADC, the linear optical coupler (OC)IL 388 is adopted to isolate the interference from the analogreceiving board. The digital OC ACSL-6400 is adopted toisolate the interference from the control board.

B. Control Board

The control board is made up of FPGA, DSP, ADC, DAC,fiber receiver, fiber transmitter, power module, and SDRAM.The architecture of the control board is shown in Fig.2. Theanalog power supply TI LDO REG104 mainly provides thepower for the analog receiving board; the digital power supplyTI PTH08T260W mainly provides the power for the controlboard. The redundancy design is adopted to guarantee thereliability of the system. Namely, there exist reserved powersupplies. When each of power supplies fails, the reservedpower supplies can be used immediately.

Fig.2 The architecture of control board.

The architecture outlined above was implanted on theXilinx Virtex-4 XC4VLX25 FPGA device. Before the signalacquisition system works, whether the system is normal canbe checked by self-test. The self-test command is sent fromhost computer and is received by FPGA through PCI board.Then, FPGA sends an interrupt to the digital signal processor(DSP TMS320V5509A). The self-test data output from DSPserial port 1. After DAC, the self-test signal generates and

sends to each channel through linear OC. Here, the self-testsignal is sine wave, and its frequency is determined by thefrequency ofDA clock.

The modules in FPGA can be mainly separated into clockconfigure module, ADC control module, DAC control module,gain code configuration module, data transmission andpackage module, the fiber receiver and transmitter module, etc.FPGA provides the clock and control signal for ADC andDAC chips of twelve channels. The data from ADC have beenpacked in FPGA, and sent to store in the hard disk of hostcomputer. Meanwhile, FPGA receives various commandcodes from host computer through PCI board, and controls theanalog receiving board's working condition.

C. PCl Board

The connection between the signal acquisition system andhost computer is through fiber and PCI board. The uplinkfiber is to send the collected data package through fibertransmitter HOTLink CY7B923 [17]-[18]; the downlink fiberis to receive command codes from host computer throughfiber receiver HOTLink CY7B933. The PCI board mainlyrealizes the function of interface exchange, namely, theexchange between the fiber interface and PCI interface. It canalso carry out the data transmission between the signalacquisition system and host computer. The data from theuplink fiber are firstly decoded by the chip CY7B933, andsent to FPGA with 8 bits width, then sent to PCIDP with 32bits width through combination. In PCIDP, a double portshared 16 KB RAM is separated into two 8 KB ping-pongRAMs. If the ping-pong RAMs are full, an interrupt is sent tohost computer. And host computer sets up the DMA controllerin PCI-DP as a response of the interrupt. Then the data inping-pong RAMs are sent to DMA buffer in the memory ofhost computer. A convenient WINDOWS 2000-basedoperator interface allows full control of the signal acquisitionsystem, as well as providing multi-window color displays forthe collected volume reverberation signal. The block diagramof PCI board is shown in Fig.3.

Fig.3 The block diagram of PCI board

The designed signal acquisition system is with widedynamic range, 100 dB. The fastest switch speed is 25 us .The system tested in the sea was described in detail next.

III. THE PRINCIPLE OF VOLUME REVERBERATION

The parameter used here to describe the ability of thevolume to produce acoustic reverberation is its backscatteringintensity Ss,v. This is defined as the ratio in decibels of the

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40

40

40

35

35

35

20 25 30Frequency (kHz)

20 25 30Frequency (kHz)

20 25 30Frequency (kHz)

15

15

15

Fig. 5 The volume backscattering intensity of different experiment sites.The flags denote the concentration of suspended sediment particles,temperature, salinity of each experimental site, respectively.

-80 •••'11 mgllL 22SC 319llo 5 cycles~ II mgllL 22SC 31 9llo 10 cycles 1

=- 1 1 : 1 1~~ ----r----r----r----T----T----i' 1 1 1 1 1J 1 1 1 1 1.:l 1 1 1 1 1,~ ----r----r----r- --T----T-

t .~. : ~: 1 : .~

J -95 -'~ ~, ~ -!\--~ --- ~/ -',-~;--f :"j ., : 1 I) ~ .. ~ 6 1

~ -100 - - - - ~.J -V~~I ~.p: -··i"~ - - -l.!·~.:~ - ~ - - --~ 1 i.·· 1 -I U 1

1 1 1 I. 11 1 1 1 1

Fig.6 The comparison of volume backscattering intensity betweendifferent pulse length at one of the four experiment sites.

-80 .4). 31 mgIL 24.6'C 269llo 5 cycles~ 31 mgIL 24.6'C 269llo 10 cycles

Fig.7 The comparison of volume backscattering intensity betweendifferent pulse length at another experimental site. The flags denote theconcentration of suspended sediment particles, temperature, salinity,different pulse length, respectively.

work after the single frequency pulse transmitting 1 ms, andstopped before the coming of sea surface reverberation orbottom reverberation, through remote control of hostcomputer. These actions ensured that there was no circuitobstruction in the signal acquisition system, and the volumereverberation signal was collected correctly. The selectedfrequency was 10 to 40 kHz, in steps of 1 kHz. Meanwhile,temperature, salinity, and concentration of suspendedsediment particles were recorded by other instruments at theexperiment sites.

The volume backscattering intensity changing withtemperature, salinity, concentration of suspended sedimentparticles, and frequency is shown in Fig. 5. The comparison ofvolume backscattering intensity changing with different pulselengths is shown in Fig. 6, and Fig. 7, respectively.

backscattered sound intensity, lseat' from a unit volume of the

medium to the incident intensity, line' if the former is

measured in the far-field of the scattering volume and referredback to a unit distance from its acoustic center. Then, thevolume backscattering intensity will be expressed as [19]

1S = 10log zssu: (1)

s,v line

IV. THE SYSTEM TESTED IN THE SEA

In order to test the designed signal acquisition system inshallow coastal waters, an experiment was conducted in thesea area near Yangtze River estuary, China. At the experimentsites, the transmitting transducer and hydrophone were put at asuitable depth, according to sea condition. The signalgenerator (Agilent 33120A) transmitted single frequencypulse. At the same time, the signal generator transmitted asynchronous TTL signal to external trigger port of the PCIboard in the signal acquisition system. Then, the pulse signalwas amplified by the power amplifier (NI L6), and sent to thetransmitting transducer (PCB 6829-24). After 1 ms, the signalacquisition system worked and recorded the volumereverberation signal, which was collected by the hydrophone(PCB 6829-30). The reason why volume reverberation signalcan't be collected by conventional underwater acoustic signalacquisition equipments with a transmit/receive (T/R) switch isas follows. First, the surface or bottom reverberation comesafter a few milliseconds, usually 13 ms, e.g., the depth of thesea is 20 m, and the sound speed is 1500 mise And the soundbackscattering intensity of the surface or bottom reverberationmay be 40 to 60 dB higher than that of volume reverberation.So if the volume reverberation signal is collected byconventional underwater acoustic signal acquisitionequipments with a T/R switch, it is also be obstructed after thesingle frequency pulse transmits several milliseconds. Thiswill cause no volume reverberation signal collection. Second,the low frequency volume reverberation signal is susceptiblyinfluenced by the T/R switch's crosstalk noise (typically, 1mV electrical noise, e.g., Agilent 3499B), and is submerged inthe noise. So there were two acoustic transducers, the one wasa transmitting transducer for transmitting the single frequencypulse; the other was a hydrophone for receiving the volumereverberation signal. The signal acquisition system began to

I,,,,.. ~..l __!' /' "<, //1 »<>: __ <,

,1/ 1/1 "'\1/'1' I .. J. <, '\/ \

/1 / I /.y..~ -, \ \'1/ /{ '0 L\ I

\ J J J'- V>.........// / /

\ <, - /'A/" ....... _-- /" /'

........ -----Fig.4 The block diagram of volume backscattering intensity

The block diagram of volume backscattering intensity isshown in Fig.4.

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From Fig. 5 to Fig. 7, the volume backscattering intensity inshallow coastal waters is -60 to -100 dB re m". The volumebackscattering intensity does not change with the pulse lengthof the transmitted signal. There is a little effect of temperatureand salinity on volume backscattering intensity. But theconcentration of suspended sediment particles may have agreat effect on volume backscattering intensity. Therefore, ifthe concentration of suspended sediment particles changesfrom 31 mg/L to 47 mg/L, the volume backscattering intensitymay be changed by 30 dB.

v. CONCLUSIONS

In the paper, a signal acquisition system was designed,based on FPGA. The system solved the problem that volumereverberation in shallow coastal waters could not be collectedby conventional underwater acoustic signal acquisitionequipments, due to circuit obstruction. The system was testedin the sea and worked quite well. This provides a way toinvestigate the volume reverberation in shallow coastal waters.It is concluded that the results in the paper is also appropriatefor estimating volume reverberation in nature coastal watersof suspended sediment particles, such as the Yellow Sea, theEast China Sea offshore of the Yangtze, Yellow rivers,Persian Gulf, the South Atlantic offshore of the Amazon river,Bristol Channel, and so on.

ACKNOWLEDGMENT

The author would like to thank Professor Ping CAl,Shoubin KE, Chao ZHONG, Fengyang CHI, Van XIAO fortheir assistance with data processing.

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