BSTJ 60: 4. April 1981: Magnetic Localization of Buried Cable by the SCARAB Submersible. (Francis, S.H.; Lunde, H.R. ; Reinold, G.A.; Talpey, T.E.)

22.4 MB·English

Magnetic Localization of Buried Cable by the
SCARAB Submersible

By 5. H. FRANCIS, H.R. LUNDE, T.E, TALPEY, and G. A. REINOLD
(@toruscrt recived June 18,1080)

A magnetometer arrey has been installed aboard the scaman
‘submersible to enable it to detect and track buried undersea telephone
‘able. Four three-axis magnetometers sense the magnetic field of a
25 Ha eupront applied to Dhe euble. The magnetometer signals ure
Filtered, amplified, digitised, and multipeed onto Scanan’s uni

‘cal cabie for transmission tothe contr ship. A ahipborne minicom

puter processes these signals in real time to determine the cable
‘oeation, which is continuoualy displayed to scanau’s operator on a
_graphies terminal. This paper ceseribes the development and capa:

Dilitien ofthe system. Amine the topics dicussed are a history ofthe
‘cablelocating problem, an analysis of the factors governing the
‘achiecable 25-1 signal level, a description of the magnetic ole
‘spectrum and its sources. and a discussion of the signat processing
‘techniques. We also examine the dependence of system performance
‘om signal anal noise levels

1. mrRODUCTION

scauau (Submorsibla Craft Assisting Hepair and Burial) is a new
‘ls of unmanned cable-contrlled submersible craft whose mission f=
{u faite che maintenance of undereeslephone cable ystems. The
first seauas, now under development, is owned and will be operated
Thy » consortivm incling AT&T Long Lincs und British, Canadian,
and French tlecommunicalions companies A second acanab belongs
{a Transpacific Communications, inc, # walnidinry of ATST Long
Line. The scazas vehicle, shawn in Fig. 1, is typical of other rently
designed unmanned submersiblea. "The base structural element ia

ec Fre Bre Tl

cag MU ae ate

tes Tee
ig seaman aor gs

frame of welded aluminum rod, Flotation tanks axe mounted oa the
Feaaye to provide buoyancy, und seven thrusters are used for propul-
sion Two of the thrusters are oriented vertically wo allow the vehicle
{ dive and ascend. Two manipulators are provided for cutting and
‘ripping cable. A water pump in sed in conjunction with dredger and
Jetier attachments for unburying and burying cable. Additional equip

‘ment includes TV aystems, aoutic devices, and a sensor eye that
includes the eable detection apparatus,

‘Operationally, scanas is designed to perform two specific functions:
‘us anat dung the repair of faulted eable, and to perforin surveys of
‘operational eables. During a repair operation, scaxan magnetically
Toextos tho cable track and, subsequently the point ofthe feult. then
lunbaries and cuts the cable ax requlrad, and attaches grippers 9 the
cable ends can bo lifted tothe cable ship. Once both ends are aboard
the ship, the cable ia spliced, tested, and then lowered tothe sea bed.
‘The scaxau then cuts away the lowering lines and rebures the exposed
length of cable

‘usially, the survey operation isto fellow the cable and measure
the burial depth with the use of the magnetometer detection sytem,

506 THE BELL SYSTEM TECHNICAL JOURNAL, APRIL 1991
1a point of shallow burial or eable exposure were det etn the vehicle
‘would vebury using the jetta.

"The purpose of the prosnl papor ix to dosevibe in some detail
scauau's cable Joeating subsysuem, feign and constructed by Bell
Laboracones. More previstly, ScaRas has ovo distinct eable-locwting
systems or reason essed Helo. The ae system described hersin
{& conkiderod the privy system; the de stam {not deseibed) is
{ncended to serve asa backup under certain operation eimmstances

I. HISTORY OF CABLE-LOCATING PROBLEM

‘Pelophone cable systems use aingls coarial onblos whose outer
tiametors are between 2 and 5 em, with repeaters interied every 620
40 nautical eile, depenving onthe ayscem. (The newer sytem have
the larger valle and clooer repeater spacing) Sine the Brat ans
Tune tweptione cable was Ind in 1956, Une major ehreat vo underscu
cables at been the trawling and dredving operations of fsharmon on
the Ailantic continental shelves The, about 95 percent of worsen
telephone cable faitree have bee al ributed to Gshing activities, The
brincipal response to his thrcut, we from more heavily armoring
Ue self asetiona of oahles, wa the davelopmenr in 194 of tv st of
{series of plows (Sea Plows T 1a TV) for Burial of ocean cable Rant
‘ensions of the sea plow burt the cable toa depth of 70 om, leaving
‘the repeater enpoead: che latet plow” Buren tne repeacera as well

‘Burial veduces the lkelihwad of abla faults but by 10 sane
‘ovestalle them. Even «buried cableqyetom is vulnunle near unburied
‘repeaters, or whore cable burial was impeded by adverse bottom
Conditions ur where hollon erosion occurs after plowing, oF where
ships anchors may dra

‘This vlcrabiity, though mininived, neverheless pesca a problem:
if Iuried cable iz damaged, how can it be found nnd its fault
Tocalized? An approxinnlion tothe feullocation ean by ob from
shore by combining knosledge ofthe cable's route with am estimate of
the olfshore distance tothe tral, the Inter obeained by any of several
‘hectroniefaule-locating techniques applied atthe cable's shore Ler
trina Phis procedure ean ienlify the fault repeater soction. thereby
ibuling the fault to within» few miles, but the problems renner
the eable-tepair ehip wo pinpoiot the faut lation und recover the
"able, hie problem ie corieraly exacerbated by bural ofthe cable
‘Tact recognised whe tte the fer sex plow became uperational. TO
provide repair hips with the eapabiity of Rng buried cables, Hall
Eaborwvorre developed three varioties of electromagnetic probe: an
cloeteeda pats ta be strernel Irkind the repair hip with a horizontal
Simcing af about 60 me 76m long magnatie inéueion ool ts be

THE SCARAB SUBMERSIBLE 537
towed ac low speeds ina wericloriencaton near che 2e8 oor; and a
salle inelacton ceil for hand-held ua by diver.

"To use the probes, 2 lov-frequency signal 29 co 40 Ha) is wppliod
Dbecwoon the eabl'scontar conductor and ground at the shore euble
‘terminal noaror the fault. This signal propagates along the cable, and
ita slactzie iol enn tne deluctadl al sen Dy the slecteeda aystom
sircumed behind the repair ship allowing the hip to trac he eable
at relatively high speeds by zigzagging across it Unless signal ater
tation is exces che mnst seaward location at which the signal i
Allectel pve crude ful-lcatin eainate, which can be rte
by towing the induction eal at slow speeds ao that it bangs neszly
vercically, ndicacng by a wll when itis above the cable. It was
Intended ac this sago ofthe repair operation that n diver, using the
hand-held probe vo pinpoint the faultlocaton, attach Ling lines ad,
itnecesay. cor the cable. However, because of che depth Limitations
and inherent danger of diving operations, the urual procedure isto
Pick up the cable by grappling. Although erapoling is quite straight
forward for unburied cable, grappling bas obvious disadvantages when
spplied to buried cable. The grapnel may not penstrate the bottom
consistently co the eablo's burial depth, thereby running the risk of
missing the cable completly; or the cuble may nol have adexmate
tensile siength to withstand being pulled obliquely (often nearly
‘horizsptally} out of che bottom thraugh more than a meter of ad or
‘and, ‘Tha problema and similar ones motivated seanan's develop-
I, CABLE LOCATING BY SCARAB: OPERATIONAL REQUIREMENTS AND

SYSTEM CONCEPTS.
3.1 SCARAG’S catfocating requirements

‘During a pica repair operation, seats i called upon to ind a
‘buried cable and track it for possibly a mile or more unl the exble
fault is Toeated. Thereupon it must postion itself onthe bottom, eloee
to the euble, eis dredger wel rinipulaors eon he used to unary
Ue abe, ea Hi meen aad allach & Kiting Vine. Cable eer
follows cable repair. scaRAB may also be required to esimate the
cables ural fepth, both during repair operations and during survey
‘of operational cable, ln determine wheller Une cate shouldbe buried
‘more deeply.

Taview ofthe complexity of these tas and the difficulty ofremotely
smuneuvering a evimiming vehile within a fow meters of the boltom
‘the eableJoeating system aboard soaRa must be eansiderably mare
sophisticated than the aip-comed spacer described shave. principal
limitation ofthe ship-Lowed aylem, From the viewpoint of ils possible
application abort” ncanan (eg. By mouncing 4 single mduetion coi

500 THE BELL SYSTEM TECHNICAL JOURNAL, APRIL 1081
fon the Vehicle, ia its reliance on a single sensor, which measures only
fone component ofthe electric or magnetic Geld vector, and at only one
Point in space. Cloasy, one instantaneous measurement ofthis kind i
{nauficint to localize the eable, which therefore ean only be found by
maneuvering the probe toa variety of positions near the cable (eg,
‘igzagging), noting the field component as a fonction of time (and
therefore of space), and using this time series to estimate the cable's
location.

“This procedure serves well where th information required concern
ing the cable's lation is minimal and qualitative, such as during a
frappling operatin, where one uanally neods only ta knose which of
eo eppesite courses wil eros the eable. However, fr quantitative
‘able localization (eo determine cable burial depth, thi procedure
‘would of course require real-time computer procestings but further-
‘more the processing algortht would require knowledge ofthe vehicle's
track over the oltom ga th past history could he uted to determine
‘resent cable location. Hence the computer would need accurate daca
concerning vehicle speed and ocean current; of these, the former is
Aificlt to measure accurately at slow epeeda and the lator vreually
Impossible co measure from a swimming vehicle. (Tho problem is
snulogous to ship navisation using running favs, u procedure whereby
past and provent lines of position are conjoined to determine the ship's
Position. The past tine af position ix update sing dead:-reckoned,
hip's track, which is eubject lo eerort In the wtimated course and
‘peed of tho ship und the ve. aad drift uf the eurruat. Tho procedur
is viable only because the ship’ speed is usually large compared with
the errors in il assumed speed over the bottom, and because the
quirements for postinal avuraey are not stringent. Neither of these
helpfl condicions pertains to acta’ eablelocalzscon problem)

‘Mo that satimaten of tack over the bottom do not have tobe relied
‘upon, » precision cable-loeating system must have enough sensors so
‘that their outputs, taken ata single instant of timo, ore sufficient to
determine cable location without ring any past dats, Por thie prpoe,
1 miaimura of five seneors is required, since four parameters ae
rede ln spel a giver Tie in sic (Ube enble lotion) and one
parameter is needed to specify the cable curent. In fac, for reasona
Aiseussed below. it proves convenient to use 12 sensors, packaged in
{our throe-axie modules

2 Cholee of sensor

‘An attractive possibility for a eable-Locating sensor is a bottom
penetrating son. fact parametric sonar haa succeeded in detect
Ing telephone enble buried saver 90 cin af san in laboratory tank,
cxperiment* Although the feasibility of « sonar cable detector has

THE SCARAB SUBMERSIBLE 599
been thereby strongly suggested, the time and funding required to
‘develop sn operational sonar aystem for scaxau were judged to excood
the available eoounces. Accordingly, attention was focussed on passive
slactromagnetic sensors

"The next choice to be made was between cleetric and magnetic
sensors, Hacause at least five aencors would bave to be mounted on a
small vehicle crowded with other equipment, compactooss was an.
Averrding concern. Krom this viewpoint, electric sensors 0. clectode
pals) are a dissdvantage because their io, dotermine hy thei
thse Tongth, b normally consierably Inger than & comparat
rensitive magnetic deterio. Furthermore, Use technology of commer.
ally available magnet sensors ie well advanced, having benefited
from long-standing geophysical and military interest in compact and
‘liable mayneticdetectore (a interest with no counterpart in elect.
field detection). For these reasons, and becuuse the geometry of the
cable's magnetic field offers some processing advantages over ils
cloctric-icld goometry, magnctie cable detection was chosen.

“Mugnotic detectors fll into three extagorive, wording Wo Ube may

by menor: total Belt mangrelometere (racing Ure

field magnitude), component magnetometers fmeseuring individual
veecor feld components), and induction cola tmessuring the time
‘erivacve of individual fila componente

OF these, induction cols can be readily dismissed from farther
consideration, Their aol advancage over magnetometersis their lower
sclfnoize level, which ie fislevant in thin applicacion because other
noise sources dominute olf noize, Their principal disadvantages are
the size and the weight of theic cores (iapurtant considerations fr &
sanall equipment laden vehicle Una rae be poilively buoyant

"Tho choice becween tetal-feld and component magnerometars is
‘ot as strightforvard. To clarity the functional difference between
‘thom, consider the oaso of «total eld magnetomecer inthe presence
of the earth's Held and a eable field Mh, where |, | || (asi
always che case in practice. fb is « unit vector parallel to, chen
tho magnotometor ourputis|B, + By| = |B,|-+ b-B, which i amply

he component of the total field parallel co the earths field (ie, the
perpendicular componerits of the cable field dy mat contribute). A
{otal-feld magnetometer ia therefare completely equivalent to con
ponent musnetometcr oriented along the earth's fcld,

Suppose now that B. is time-varying 20 that its contribution to the
magnetometer ouput can bo sopurave rom that of tho earth's static
field by ftering, leaving w signal of B-B, from the total-Seld device
‘and 6-B, from the eomponant device (here &is a unit vector parallel
to the magnetometer ass). The sole ditinction between these output
{is thatb is Gixed while is changeable Ge. a component magnetometer
‘an be oriented to mensuro any dered component)
‘ThisGlaibility ofthe component magnetometer is both an advantage
and » disadvantage, The advantage ia thal Uhre component magne
tometer with mutually orchogonal arcs can be vombinel in ningle
package co create a vector sensor (i a sonsor which dereets all three
‘omponents ofthe veclor fe), Burwuse the electromagnetic fel is
1 yettor field, there ure crucial siaal-promming atlvanages to be
{gained by employing a vector senso, os wil be seen. The dissdvantage
of the component mnguetomelnr is that ite abject to a potenally
severe source of nese if mound on a moving vehicle. As the vabiclo
‘mumneuvers, ptehes, and rolls it changes the orientation ofthe mag:
‘netometers uals selaive I Uhe enrth's eld, 20 that in the earth
Coordinate system at is time-varying. The magnstometse ourpat is
therefore [B,(¢) + B,]-A(t, and the concribution from the sari’ field
fan no longor be rsnove hy fering unless a(e) baa negligfle
Tequency componente in the frequency band of the signal BA.
‘Because the each’ field normally excood the exible ild by nearly
four ardaa of magnitude, even slight while motions ran produce
intolerable levels of motional noise i thewe motions are inthe signal
frequency band.

"The choice betwoon cotal-feld and component maynetameters
thanafora hinges on whether a usable signal froqucy exit, at which
‘motional noise is acouptably ual. Av will be shown helo, scasas,
has auficiene inertia and stability, 0 when is submerged, mtional
noize is negligible at froquoncics ubove w few Hertz This fact sas not
ows wim lle eloioe of ensox ed ro be made, becatna scat hae
not yet been ball: eccordindy, an experiment was conducted using
the Navy's cuny 1, a eable-conerolld underwater recovery vehicle
ile viewlar in design to. scaman, A component. magnetometer
outed aboar GURY tt revealed not only that motional noize was
Aeeoptably Tow ac all frequenciea of poasible interet, but wba thal
ringelic nove from all aources (notably the thrusters) wus low eragh
To encourage the development of the eablelocating system described
belo.

2.3 Cholee behween ae and designate

‘A further preliminary question concerned whether the syetwm chould
operate at ac or de. An ae ayatom would detoct the ae magnelic Geld
bs low-frequency eurent on be cable, Ade wpauem wou detect The
superporition of de Gelde from (wo xoiren: de curren on the cable,
tnd permanent magnetialiva of the cable's etel components (central
Strong eli or external senior wie)

"The nvervling difference between ac and de cable detection derives
from the existence of a lange de component of the earth's Bed. To
‘leteot the small de cable Bld in che presence ofthe varth’ fe, ome
fun use the fat that the earths field ix mata homogeneous,

THE SCARAB SUBMERSIOLE 541
seherens the cable fel ea Joenlized anomaly. Therefor, to etontthe
fable one must delet diferencee betwuut field valuew at different
Toeations in space; if hove valuee differ, the exible in weary. This
‘pmcrde can he inyplementel with one sensor by moving itn space
find watching for changes m signal. Ori can be implemenced sith
‘multiple sensors by looking atthe differences among their signals. The
fit of these techniques must employ atotal-fsld ene, snes a vector
sensor would be aubject to changea in riencaton as it was moved (the
‘Acequivalent of motions noire). The second ofthese techniques could
empl either total-Held or component sensors, though usually com-
ponent sensor are used, with reo being packaged inthe same housing
to maintain their parllelem, Such a configuration is termod a gradi
fmeter, ance itt output it proportional t the Seld gradient i the
‘anaor separation is small

‘ne contrast hetwec ane de wxble detection, Unerefore, ix in he
tyme of wero Uhat canbe use we detection can wae a vector magne
twmeter, while de detection requires wu tlal fed magnetometer o
anuiomoier. Pal anotlir way, the ae sensor measures 9 vector quan
Hy while de de ewor mentces ether a scalar ora tensoe quand
Bocuuee the maxawie Feld i w vector, one eight expect the vector
sensor to Tend itself more readily to data processing for cable localiza
tion, chereby favoring ac over de. In fact, this expectation is ome ov
tho ac cablelocating nigerithm doscribed holo is extremely simple,
‘wherens x contrat de algorthen hes nol ye hen devine

[Nonetheless ac cablelocaling system possesses erlainaavancages
caver ae. Since undersea telephone cables normally cary a de eureent
(Gch sen return to power the repeaters a de detection aystem does
‘hn euiness pecial sal lr e pace Ue cabe,and the sytem
‘Worle an well for an operational eable a for a broken cable. Further-
‘more, unlike an ae signal, the de current does not diminish expenen-
‘all with cistance from chore, go both ends of a brokon cabo can bo
powered with wompuruble euzrene levels In fuel, when he able ix
Arnona Ides nil al Ube power all fr de detection, which,
an take advantage of the relatively lage fields of the seel amnar
‘wien eonmtar, an ac signal cannat propagate across the osean, and
therefore, at bet izean reach only one side freak na trannoveani
‘able, Under seviain circumstances, therefore, the unique capabilities
(of a de system may be necessary

im, de sensora provide magnetic data of a wype that is not

or table Tosliation, wills Uw resale si xn ae le
rarially preferable. Hoseever, onernional eicmimadariaet exit Tar
‘which an ae system vould be ineffective und « backup eystem i
Aesirable. For these reasons, a de system is ror develepmnt for
cana to supplement the ac aystem

‘42 THE BELL SYSTEM TECHNICAL JOURNAL, APRIL 1081
{The remainder of thie paper desaibee the implementation and
‘operational performancs of the we syste

3.4 Cobtefocaiztion ageritnm and array design

Tn thw preceding secions we propased that the epsimum eable-
Icing sytem should employ w veevor senor (8, senso detecting
Al these Feld component}, ensue some sense a vector sensor i
Teller suiled lo characterizing vertor fed. A corollary was that tho
cplimiin syste wes an a ayatem, wo avd the earth's de fil. A
further proposition was thatthe optimum system should have enough
seats to determine cable Iocarion ietantaneoualy, without having 1
ely on past history

‘To particulaize these general considerations, this wotion eseries
the method implemented aboard seanas for instantaneous cable lo
‘alization by a number of ac vector senaors, The mothod relies on the
fact that the magnetic fold lines caused by the cube’ curent nee
cireles (assuming the cable lies in a staieht Linc). This geometry
permits cable localization ky a simple tranquacion procedure mast
fauly visulizul in we dimensions (ig 2}. Veetor agora are used to
Aletermine the drei ofthe fed at tien lneation®. At both locations
‘line perpendicular to the field is constracted. Bocauge those ew Lin
Tie long ral af concentric ieles, their point of intersection isthe
common center: the cable Ination. "The generalization of this proce:
<datevo chvee dimensions strvightformand nwo locaons, cancel
planes perpendicular (othe field direction, and decermine the line at
‘which these planes intenwel, This line of incereetion is the cable's
Tova.

Tr principle, this procedure allows eablelaalization with only ewe

sci 2, ston oe penn samp ml eon
vector magnctometers. Marcaver, even this hac syatem overdcter.
Tres the problem line in aes can be dsaeribed sit four para
lors, whefeas two vector aenaora provide six aalar meaeurenents
"These six meauirementa can be manipulatod to provide information
fon felt direction (four angle) and size (cwo magnitudes). Cable
Iealizaion requites only the four angles; the bro muznimes cn
provide theo cableeurrent estimates, which would of course coincide
Jithe absence af noise and other anarees of ld orton.

However, chia simple ayatem of fro vector wentors rune embel-
linhad, for i fall when tho two seniors ond Uhe cable al ie in the
same plane (or, ia the presence of noe, neatly in che eame plans).
‘This degenerate ease ean be ignored, for it eccure quite commonly
for any cwo-aonser array. For example, to optimize cable tracking it
‘sould he logical to place the evo sensors symmetrically to port wel
board, eo thal the sensors straddle the eablo when seasas docs
For such a placement, however, canas ould be unable to do cable
Toealiztion while heading perpendicular to the eable, uch as during
Inia able aequisicion

‘The solution isto provide mone than {wo vaso aol lo Implement
procedure for decdina which secarsshalleused and which ignored.
‘To ensure that no cable location shall be coplanar with all pais of
senaors, a nonplanar stray of at leat four angers is required.

‘The optimum position of these fonraensors is governed bythe desire
shat tho planes whose inlereclion determines cable lation abould
Interevel rene nt right-angles since then the eable location errra
‘unwed by noiserinel etuations of these planes will be minimized.
For rightcangle intersection, the baaeline length between two arnzors
ust be evice the ditanoe from the biseline 1p Ihe eae. However,
‘soaRAB's ruta boetineleng(h connirined toe las than ro
‘meters (snee booms extenting beyond the vehicle's perimeter are
Lulesirable), while scaRan'a height ahove the sea ocr will generally
‘be al Test one meter Consequently, right-angle triangulation i i-
omible to achieve, and the effects of noe are minimized simply by
spacing the sensors as far apart a possible, maximizing tho distaneo
‘etieen ele enor and the plane defined hy the other three. Infact
‘ven this modest goal ia impractical, because mast potential sensor
‘tes inthe center and after parte of tho vehicle wre continated hy
‘thruster noise and frame-curtent distortion of the magnetie Bald abe
‘elo. Consequently, the hes vailale array configarstion comprinse
‘our coplanar sensors in & neney vertical plane in the forepart of cho
‘ehicle (Fig. 1), This array is not well uite ro localizing a cable which
Ties arhwareshipe and underteor (i.e, a eable in the plane of the
sensors), but for al ober cable Hocavirs i erven wal

‘Such an array of four sensors provides six possible sensor pairs and
therefore sx exbli-location eatin, Ue oul pls from all pais are

‘Sia THE BELL SYQTEM TECHNICAL JOURNAL, APRIL 1081