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DEEP-OCEAN ASSESSMENT AND REPORTING OF TSUNAMIS

Chapter 1

INTRODUCTION

This paper describes the system components that make up the second-generation

Deep-Ocean Assessment and Reporting of Tsunamis system, known as DART II1.

Tsunami data from the DART system can be combined with seismic data

ingested into a forecast mode to generate accurate tsunami forecasts for coasta areas!.

The moti"ation for de"eoping a transportabe, rea-time, deep ocean tsunami

measurement system was to forecast the impact of tsunamis on coasta areas in time to

sa"e i"es and protect property. O"er the past !# years, $%&' has identified the

re(uirements of the tsunami measurement system through e"oution in both technoogy

and knowedge of deep ocean tsunami dynamics. The re(uirement for transportabiity was

a conser"ati"e approach to a phenomenon that had itte data to guide strategies for

choosing depoyment sites. The re(uirement for rea time was to pro"ide data in time to

create a forecast. The first-generation DART design featured an automatic detection and

reporting agorithm triggered by a threshod wa"e-height "aue. The DART II design

incorporates two-way communications that enabes tsunami data transmission on demand,

independent of the automatic agorithm) this capabiity ensures the measurement andreporting of tsunamis with ampitude beow the auto-reporting threshod. *or more

accurate forecast modeing and subse(uent, more reiabe decision-making, this capabiity

is "ery important because +a a "ery arge, destructi"e tsunami may, in fact, ha"e a "ery

sma ampitude at any particuar DART station position, and +b sma, deep-ocean

tsunami ampitudes can reach destructi"e "aues, due to arge, ocaied, shaow-water

ampification factors. This atter concern was dramaticay affirmed and demonstrated

after measurement of a !cm wa"e of a tsunami generated in Aaska was ampified to

become a #cm tsunami on the north shore of Oahu, /awaii.

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Chapter 2

OVERVIEW AND BACKGROUND

2.1. Reaction to the Indian Ocean Tsunami of December 26, 2004.

The 0.# earth(uake of December !, !## and the resuting tsunami kied more

than 2##,### peope 3 more casuaties than any other tsunami e"er recorded. It was the

argest trans-oceanic tsunami in o"er # years. A second earth(uake ony three months

ater of magnitude 4.5 has instied more fear in the sur"i"ors of the first disaster, and has

e"eryone wondering when the ne6t tsunami wi occur, and how their coasta community

can be made tsunami-resiient.

As a resut of this tragedy, there is strong interest in instaing a goba tsunami

warning system as soon as possibe. Additionay, the 7ush Administration has pro"ided

funds to e6pand the current tsunami warning system by depoying 25 additiona DART

systems in the $acific, the Atantic, and the 8aribbean. Thus in(uires ha"e come to

9OAA:s $%&' about the technoogy and methodoogy that has been de"eoped for

tsunami warning and haard mitigation) especiay the DART technoogy.

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2.2. DART System !o"ution

DART systems, de"eoped by 9OAA:s $acific %arine &n"ironmenta

'aboratory +$%&', ha"e pro"en to be robust and reiabe. A si6-buoy array has been

transitioned to operationa status and is monitoring and reporting water coumn heights inthe $acific Ocean at ree"ant ocations for tsunami propagation. The cumuati"e data

return ratio for the array from 1005 3 !##2 e6ceeded 01;. The data return ratio is

computed by di"iding the tota data recei"ed on shore by the tota e6pected data. The

DART systems ha"e reacted to and reported si6 seismic-induced wa"e e"ents that

contributed to operationa decisions, a"oiding fase aarms and the resutant costs

associated with them5. One incident, a magnitude 5.< earth(uake in the Aeutian Isands

that occurred on 9o"ember 15, !##2, triggered a tsunami =watch: in /awaii and Aaska.

Data from three tsunameters showed the wa"e was not significant, and no warning was

issued, thus sa"ing /awaii >?4% in e"acuation costs. /owe"er, the 9o"ember 15, !##2

! cm tsunami obser"ed at a DART buoy +ess than the 2 cm sef-trip threshod resuted in

a surprisingy arge # cm wa"e recorded on the north shore of Oahu, /awaii, as depicted

in *igure !. A sma tsunami that coud potentiay be dangerous ceary demonstrated the

need for bi-directiona communications with the DART system.

$ost-e"ent data from DART systems ha"e been combined with forecast modes

and compared with coasta tide gage records to show a high correation in ampitude and

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arri"a times, "aidating the system parameters. Additionay, the timeiness of DART

reporting has pro"en to be highy "auabe to the warning centers.

The success of DART can be attributed to steady progress in the de"eopment of

four main components@ deep-ocean pressure measurements, acoustic data transmission

from the tsunameter to the buoy, buoy and mooring technoogy, and imbedded software.

The DART II systems, which incorporate the atest technoogies and ad"ances, ha"e

onger maintenance inter"as, and feature two-way communication.

2.2.1. Deep-Ocean Pressure easure!ents

During the past !< years $%&' has been designing and depoying bottom-sensor

patforms with $aroscientific pressure sensors for the purpose of tsunami and cimate

research The eary efforts used interna recording systems that were depoyed and

reco"ered at one -year inter"as, and were in"auabe in refining and "aidating the

measurement process in deep water.

Tsunamis ha"e wa"eengths of hundreds of kiometers, and are considered

shaow water wa"es. They =fee: the bottom in deep water, and increase in height ony as

they shoa in areas of decreasing depth. ind wa"es are caed =deep water wa"es: where

they ha"e negigibe impact beow haf of their wa"eength. This we-studied e6ponentia

decay of orbita motion with depth was deri"ed and presented by Beorge Ctokes in 145.

This phenomenon makes the deep ocean an idea ow-pass fiter, and aows tsunamis,

tides, and other ong-period e"ents to be detected by simpy measuring the pressure at a

fi6ed point on the seafoor.

The earth(uake wa"es shown in *igure ! are an interesting feature of tsunami

monitoring with bottom pressure sensors. &arth(uake wa"es tra"e significanty faster

than tsunami wa"es, and fre(uenty trip the tsunameter into =&"ent %ode: before the

tsunami arri"es. The "ertica shifting of the seafoor from the earth(uake acts to ift or

compress the water coumn abo"e, showing an increase in pressure as the seafoor rises,

or decrease in pressure as the seafoor fas.

The reati"e (uiet en"ironment on the seafoor in deep water can be used to

"aidate the performance of pressure measuring systems. Obser"ed "s. predicted tida

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"ariations can be used to "aidate the gross measurement, and the finest resoution can be

seen by the sampe - to - sampe "ariation at periods of high and ow tide. The ow noise

and high resoution in the tsunameters ha"e precuded fase aarms from occurring on the

depoyed DART systems.

*or our purposes in this paper, when we refer to measured =sea e"e height:, it

is inferred from the seafoor pressure measurements by assuming a constant 1 psi 5#

mm of water height. The errors resuting from negecting "ariations in acceeration and

density terms are not significant for tsunami detection, where ony the reati"ey sma

changes are used in the anaysis of wa"e characteristics.

2.2.2. Ac"ust#c Data Trans!#ss#"n

$%&' began testing Datasonics acoustic modems in 100 for teemetering data

from the Ceafoor to a surface buoy. $%&' engineers worked cosey with industry on the

de"eopment and refinement of the technoogy that has ed to the systems now a"aiabe

from 7enthos, Inc. These systems are reiabe and robust, and are commerciay a"aiabe

in a usabe form. A significant effort has gone into de"eoping a transmission protoco

that is used on the DART II systems. The scheme was taiored to the specific acoustic

teemetry re(uirements of the task and is described in section 2.

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seafoor and the desktop. An important goa in the de"eopment of a the software toos

and tasks has been to keep the tota power re(uirements ow.

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Chapter $

DART II **TE COPONENT* AND

CARACTERI*TIC*

#.1. O!er!ie$

A DART II system, shown inside the dashed ines of *igure 2, consists of two

physica components@ a tsunameter on the ocean foor and a surface buoy with sateite

teecommunications capabiity. The DART II systems ha"e bi-directiona communication

inks and are thus abe to send and recei"e data from the Tsunami arning 8enter and

others "ia the Internet. The web site for the DART data is supported by the 9ationa Data

7uoy 8enter and can be seen at@[email protected]"Edart.shtm.

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#.2. DART II %haracteristics

DART II performance characteristics are summaried in Tabe 1. These

performance characteristics heped to dri"e the research and de"eopment of the DART II

system. Cpecific engineering detais about the tsunameter and the buoy foow.

$.$. Tsuna!eter

The bock diagram in *igure shows how the components of a tsunameter

function together. The computer reads pressure readings, runs a tsunami detection

agorithm, and sends and recei"es commands and data to and from the buoy "ia an

acoustic modem.

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$.$.1. Pressure *ens"r

The DART II pressure sensor is a #-1#,### psi mode 1#F Digi(uartG unit

manufactured by $aroscientific, Inc. The transducers use a "ery thin (uart crysta beam,

eectricay induced to "ibrate at its owest resonant mode. The osciator is attached to a

7ourdon tube that is open on one end to the ocean en"ironment. The pressure sensor

outputs two fre(uency-moduated s(uare wa"es, proportiona to the ambient pressure and

temperature. The temperature data is used to compensate for the therma effects on the

pressure-sensing eement.

$.$.2. Rec#pr"ca' C"unter

The high resoution precision reciproca counting circuit continuousy measures

the pressure and temperature signas simutaneousy, integrating them o"er the entire

samping window, nominay set to 1< seconds. There is no dead period between the

samping windows. The circuit has a sub-miimeter pressure and sub-miidegree

temperature east-count resoution. The reference fre(uency for the reciproca counter is

deri"ed from a ow power, "ery stabe, !.#051

osciator. A rea time caendar-cock in the computer aso uses this reference for a time

base. At the end of each samping window, the computer reads the pressure and

temperature data and stores the data in a fash memory card. A 1

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$.$.). Ac"ust#c "(e! an( Trans(ucer

A 7enthos AT%-44# Teesonar acoustic modem with an AT-!1'* directiona

transducer has a #H conica beam which is used to transmit data between the tsunameter

and the surface buoy. %odems transmit digita data "ia %*CF moduated sound signas

with options for redundancy and con"outiona coding. Transducers are baffed to

minimie ambient noise from entering the recei"er.

$.$./. T#'t *ens"r

&ach tsunameter has a Beometrics 0##-< tit sensor mounted in the base of one

of the housings. This is used to determine the orientation of the acoustic transducer when

the system has setted on the seafoor. If the tit is greater than 1# degrees the tsunameter

can be reco"ered and redepoyed. The watch circe of the surface buoy coud carry it out

of the acoustic proection cone from the tsunameter if the ange from the "ertica is too

great.

$.$.0. Batter#es

The tsunameter computer and pressure measurement system uses an Akaine D-

8e battery pack with a capacity of 1

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*tan(ar( "(e reports once e"ery si6 hours. Information reported incudes the a"erage

water coumn height, battery "otages, status indicator, and a time stamp. These

continuous measurements pro"ide assurance that the system is working correcty.

E&ent "(e reports e"ents such as earth(uakes and Eor tsunamis when a detection

threshod is e6ceeded. The Tsunami Detection Agorithm triggers when measured and

predicted "aues differ by more than the threshod "aue. a"eform data are transmitted

immediatey +ess than a three-minute deay. Tsunami wa"eform data continue to be

transmitted e"ery hour unti the Tsunami Detection Agorithm is in a non-triggered status.

At this point the system returns to the Ctandard %ode.

$.). *ur+ace Bu"%The DART II surface buoy, shown in *igure

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$.).2. C"!puter

The computer is the same type used in the tsunameter as described in 2.2.2. It

processes messages from both the sateite and the tsunameter.

$.).$. Ir#(#u! Transce#&er

A %otoroa 0

famiiar dia-up modem connections. A typica Ctandard %ode report takes appro6imatey

2# seconds, incuding the time it takes to compete the connection, transmit the data, and

disconnect.

$.).). GP*

A 'eadtek mode 0

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$./.1. W"r4stat#"n - t" - Bu"%

A DART II inno"ation is the abiity to send messages from a workstation on and

to the buoy and the tsunameter. This bi-directiona communication enabes commands to

be sent to the DART II system.

$./.1.1. Te'e!etr%

The warning center issues commands that are (ueued in a ser"er unti the DART

II buoy is in 'isten %ode.

$./.1.2. C"ntent

Once the connection is estabished, the foowing commands can be sent@ MTurn

on Depoyment %ode for 2# minutes in the tsunameter

MDownoad one hour of high fre(uency data +1

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$./.2.2. C"ntent

*tan(ar( "(e

9ormay, the tsunameter is in its ow-power Ctandard %ode and transmissions are

made ony once e"ery si6 hours. Ctandard %ode messages contain the foowing data@

M%essage ID, a se(uentia number

M%essage status, 8 corrupted, I intact

MDate month day year

MTime hour minute second

M%ain battery "otage,or error code

MAcoustic modem DC$ battery "otage

MAcoustic modem battery "otage

M*our "aues for water coumn height in miimeters corresponding to 1

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"aues. &nsuing messages are simiar, but incude 1< one-minute a"erage height "aues,

where the one-minute "aues consist of the a"erage of four 1

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$./.2.$. 5"r!at

The format of the messages is a space-deimited te6t string of "aues, foowed

by an asterisk, foowed by a checksum. The de"iation "aues are coded as four

he6adecima digits.

$./.$. Bu"% t" *ate''#te

$./.$.1. Te'e!etr%

&ach DART II buoy sends its data to the Iridium Cateite 9etwork using an

Iridium transcei"er. The radio fre(uency transmission in the 1

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sends B$C coordinates instead of water coumn height data. The reported position is

checked to ensure that the buoy has not parted from its anchor.

Dep'"%!ent "(e

The tsunameter wi enter into Depoyment %ode prior to depoyment. This

mode enabes the user to "erify that the system is working on the seafoor before ea"ing

the site. Depoyment %ode wi transmit data to the buoy e"ery other minute for four

hours. Once the buoy has recei"ed a few messages, it wi transmit these messages thru

the Iridium system. The data wi show the tit of the tsunameter, a (uaity parameter of

the acoustic modem channe, and four 1

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9o messages are stored in the sateite network) rather messages are simpy

reayed from the buoy to ser"ers or workstations. The format of the messages is te6t using

T8$EI$.

$.0. *#te Character#st#cs

To reiaby send and recei"e the acoustic packets to and from the tsunameter,

which might be submerged between 1### and ,### meters beow the buoy, the

tsunameter must be ocated on a reati"ey fat portion of the ocean foor, and the buoy

must be moored such that it stays within a #-degree cone) a cone whose "erte6 is at the

tsunameter, and whose base encompasses the buoy. Outside of this cone, the signa - to -

noise ratio deteriorates rapidy, and data integrity wi be compromised. The mooringneeds to be strong enough to withstand harsh ocean conditions of wind, wa"es, currents,

fish bites, and "andaism.

Chapter )

CONC6U*ION

The DART system has e"o"ed to become a reiabe and robust system. In the

years since its initia funding in 100, many essons ha"e been earned, and progress has

been steady in four areas@ tsunameters, moorings, teecommunications, and software

agorithms. These scientific and engineering ad"ances at 9OAA:s $acific %arine

&n"ironmenta 'aboratory +$%&' ha"e ed to a highy-reiabe system that ac(uires and

dei"ers direct tsunami measurements from deep ocean ocations between the tsunami

generating e"ent and distant at-risk communities, and transmits these data in near rea

time to tsunami warning centers and the Internet.

The si6 DART systems that are depoyed in the $acific Ocean ha"e faithfuy

monitored tsunamis since their depoyment, beginning with the first buoy in 1005. &ach

year has seen engineering ad"ances in both the reiabiity and robustness of the

depoyments, and in the maintenance inter"as of both the tsunameters and the buoys1

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RE5ERENCE*

1S 8hristian %einig, Ccott &. Ctain, Ae6 I. 9akamura J Rea- Time Deep Ocean

Tsunami %easuring, monitoring and Reporting CystemK $ubished in

www.ndbc.noaa.go".

!S D.F. Roy ICRO scientist, JCateite 7ased Tsunami and earth(uake &ary arning

CystemK, in &ectronics *or ou August U Ceptember !##

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Sem%#$ Re&"t 2005

CONTENT*

8hapter 1. I9TRODQ8TIO9 1

8hapter !. O&RI& A9D 7A8FBROQ9D !

!.1 Reaction to the Indian Ocean Tsunami of December !, !##. !

!.! DART Cystem &"oution 2

8hapter 2. DART II CCT&% 8O%$O9&9TC A9D 8/ARA8T&RICTI8C 5

2.1 O"er"iew 4

2.! DART II 8haracteristics 4

2.2 Tsunameter 4

2. Curface 7uoy 11

2.< Data 8ommunications 1!

2. Cite 8haracteristics 15

8hapter . 8O98'QCIO9 14

8hapter

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Sem%#$ Re&"t 2005

AB*TRACT

As part of the Q.C. 9ationa Tsunami /aard %itigation $rogram, the Deep Ocean

As-sessment and Reporting of Tsunamis $roect is an ongoing effort by the $acific

%arine &n-"ironmenta 'aboratory +$%&' to de"eop and impement a capabiity for

the eary detection andnear rea-time reporting of tsunamis in the open ocean. A DART

system consists of a seafoor system,capabe of detecting tsunamis as sma as 1 cm, and a

surface buoy for near rea-time communica-tions. An acoustic ink is used to transmit the

data from the seafoor system to the surface buoy. Thedata are then reayed "ia a 9OAA

Beostationary Operationa &n"ironmenta Cateite +BO&C sate-ite ink to ground

stations, which demoduate the signas for immediate dissemination to Tsunamiarning

8enters and $%&'. A DART (uaity contro page is made a"aiabe to the pubic in

nearrea time "ia the ord ide.

eb [email protected]"Edart(cEa"eatcher.

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