Recent measurement and analysis techniques developed at National Institute of Oceanography

RECENT MEASUREMENT

_{AND ANALYSIS TECHNIQUES}

DEVELOPED AT N.I.O.

by

M. J. Tucker

Conference ori Ocean Wave Spectra

Eaa ton,

_Maryland

May 1-4,

₁₉₆₁

RCT

SUE7 AND ANALYSIS TECiNIQUEs

DEVLQP

LT N.I.O.

M.J. Tucker

Ntional Ins bitute of Ocuoaphy

Worniley, Surrey, 1gland.

Introduction

N.I.O. and its

predecessor, the Oceanographic Group of the Admiralty Research Laboratory, have devoted considerable ef f ort over a period of sixteen years to wave

research.

In

_{the field of technique, our efforts to record and aza1yse waves for} research purposes have on the whole been successful: though "he who never reade s mistake never made anything", end we have made our share of

mistakes.

Our efforts to provide wave recorders for civil engineers have

on

the whole been less successful,

though this is a difficult problem and the number of really successful wave reocrdor dvaloped for this application anywhere

in

_{the world,}

_{can be}

_{ooited on the fingers of} one hand, leaving perhaps three or four spare fingere. very civil engineering

installation

presents different problems, and one may devote a lot of effort to

over-ociing

these in

one case, only to find a new

_{and equally difficult set facing one in}

the

next.

A further difficulty is that the staff on the site, particularly when this is overseas and one cannot spe*k to there personally, are often synathetio to the equiçnent, and.

get

annoyed and. lose interest at the first diffIculty. Probabi,y

most installations use bottcxn-pressuro

meter

type wave recorders, and in inoderete winds producing short waves, those give apparently false

recordings

owing to the attenuation of the shorter waves with depth. Correction using the apparent period and. attenuation factors

frczn

tables is little

help:

the apparent period of the waves on the surface may be four seconds, end cii the recording six seconds.

Use f the

factor corresponding to either of these periods will give the rong answer. Th,e

reaction of the

engineer on the site, 'suo doesn't believe in instruments anjway, is to say "It's no good", arid loso interest.

&ver the years we have

built up a

number of ideas on how to set about measuring and analysing waves, and theso will be set out below. They owe a lot, of course, to other people's ideas and experience.

inoiples governing the development of 'save iae3suring instrumente

In the desi of instruments, two factors of fiidmental importance sometimes

get

overlooked.

These

aro that the

user must have confidence in the accuracy of the measurements, and that the easier the analysis of the results, the more likely they are to got used. In the oase of wave-recording, both thase factors point to the measurement of absolute height (or pressure) because recording of a tidal cycle will

thon give a convincing check on

the

_{calibration of the equipment, and there is no} rieoesaity for

any correction of the results

for the variation of the response of the

instrument itself with frequency, though hydro&ynomio response factors will usually still need correction. Such an instrument also has the advantage that if it is

sificiontly accurate, the long 'savez

(of several minutes' period) mey be filtered fran its output and recorded separately. It is not, of course, practicable to record absoluto height on the deep sea whore no fixed pintfonn is available as a reference.

Fixed aerisitivity is deireble, since thon

_{the recorda are not}

wasted if

coone

forgets to noto the setting

_of

the itivLty

control, end the r300rcls do not go of?

ecale by mistake.

_{This may necezsitcte high resolution recording to}

obtain adequate

precision over the

possible range of

_{vo-heigJt to be mussurod, but this}

_{range is}

not usually v'ry grec t.

In a shore-based installation, the

_{biggest single item of}

_{epanditure is often} the oost of the cable,

and if it ha3

_to

_{he specially manufactured, the}

time reqir'et

for this can be the factor

determining the earliest pozsible d.ate of installation.

It is therefore an advantage if the

_{thatrznent requires}

_{cnly a simple co-axial or}

2-core oable,

_{Such cables aro relatively cheap and}

_{can often be}

_{obtained "off the}

ahe].f",

Those last

two considera tions indicate a measuring

head

ihioh gives a frequency

depe4ent on the

wave-height, that is, an

f.m. system.

_{Prequonoy can be rnoasured}

and recorded with

_extremely

_{high precision; two ocres suffic'e to}

oar' both the power supply to the

_measuring

_head

_and

_{the output signal back}

_{to the shore;}

_{in addition}

the characteristics of the cable do not

affect the oalibration,

_{and the output can} be tolometored

by radio if necessary.

For roi.rtine wave-recording,

self-contained wsvo recorders, that is, inatrnts

laid on the

sea-bed for several weeks, _{or possibly months, at a timo, end recording} ithin themselves, hava been f o.nid to be _{unsatisfactory.}

_{ly too often it te found}

on recovery that sc*xeththg has gone wrong with

the device and the records ars lost.

Such a loes may spoil a year's set

of records. For SOEDO _{research purposes where The}

wave recorder is only in op-atìon for

3 short time

_{before it is recovered ath the} recorda exsmined, this is not

30

important, but is still a siìifioant factor.

For fixad instalistions,

_instrtzuants

which measure

the

_{variations in pressure} as the waves peas overhead are still th3 rost practical

_{in most oiro'ixistanocs, in}

spite of the severe disadvantage _{f their insblity to measure short wsvea}

..etiafao-tori].y. This is partly

_{raJC&U.$O of the great oxperae of insta).ling structures}

capable of supporting on ìnctr.aent

p 8ciflg

_{bhrDugh the water curfooe, end}

_partly

duo to the leak of

_{roci),y sctisfotory s'rfaoci-}ìeight recarder}

_{capable of handling}

the range of lev1a required, which rca:c be sa.'enty feet in an exposed situation

(waves plus tides). _{Foi' domo otvil. cngine:ring tpplications, the}

insensitivity to short waves may

be an advantage, aince it i

_{sciotimaa only}

the longer waves which

are importent.

For near-shore reoord.ing, then,

it eraa that on sbolute pressure gauge, mo'.tted a alose to the surface as practicable, with an f.m.

_{output, and oonncoted by cable to}

a recorder

ashorc,i3 likely

to be the roost satisfactoxy instrtxnont for

_{general use.}

As far as the author ±s aware, Snodgress, Munk end

_Tucker

₍₁₉₅₈₎

_{2t the Scripps}

Tnstitution of Oceanography were the first to develop

_{such e system.}

For recording 'nves on deep water, teeny people have

_{tried using the still water}

below the action of the waves

as a

reference, Such

_{3StCflLS typically have either}

_a

surface-height measuring device held steady by

a drogue

in deep vater, or arrange for

a buoy on the surface to raise and lacier a pressure matar or integrating

current-meter suspended in the deep water.

In order ta obtain reasonable

_{responso to the}

longer waves, such a system

requrea a

sUkpL3nBiÒn at least 100

s.

Long.

'Ñ have

-3-fod such systems to be

_atifctory.

The equiçznent is usually clumsy and

difficult to handle in stormy 1eb

4i.

ìi

xm&y bQ

i.hject to sppx'eoiable

stretch;

difforeritial currents, wind dr3g, or drag through the cable cornisoting the

instri.znent to the ship cn cause the viro to tilt from thu vrtiol, introducing

drift into the record and unknown factors into the

responce.

The most aatisfaotorj doap-su v.evo recorders so

far

developed are those which

measure the motions of a buoy.

In their simplest form these contain a vertical

gooelsroter measuring the oomponent of acceleration perpendicular to the vater

surface, or in the direction of the resultant acceleration, vthich is, in principle,

the sama thing.

Better accuracy con be obtained if the aoco1eroneter is motmted on

roocpe so that it measures the oomporiunt of acceleration in the triia vertical.

The silo can either be integrated truce before recording to give wave elevation,

or if the poier spectrum is being obtained, the acceleration spectrun can be

deter-mined and ccnverted to the elevation spectrum by multiplying by 1/cr'.

By also

measuring pitch and roll, a useful amount of information can be obtained about the

directional speotrus of the waves (see Cartvuright'z paper presented to this oonf er-.

ence).

Such

systems are eminently praotiooble, and can be considered 85 fully

establishßd.

To obtain

more

ifozation about the directional speothri is d.ifficult, and

N.I.O. hes not yet succeeded in producing any

equipment which will

do this.

The

problem is, of course, receiving a

'oat deal of attention by workers in several

countries.

The f,m. pressure recorder

This is a pro asure-reatar typo wave recorder crinected to the shore by a cable.

A diagaei of the measuring head is ahocn in figure 1

The pressure acts on a

disphragn carrying one plato of a parallel ploto capacitor.

The

parntiOE, of the

capacitor plates at zero pressure is 0.015 inch and is reduced to 0.0015

inch at a

pressure corresponding to 200 lt. of water.

This capacitor is the timing ospacitor

of an L.0. oscillator, ïhoso frequency is thus a measure of pressure.

The measuring-head circuit is shown in figure 2.

The paver supplies are

fed

down a twin-core or co-axial cable.

The oscillator is not deooi.led, so that the

supply current fluctuates and approximately 100 mV of sial is devaloped.,aoross the

680 load resistance at the shore end (if the cable is short enough to produce little

attonuatixt).

The oscillator has bean desi-ìod to give maximum stability against changes in

terxorature and supply voltage.

Tho overall temperature cocf'iciont of the

measur-ing hcad at o'bnospheria

pressure is approximately I pt.

in iO

of full scolo

(corresponding to approximately 0.25 inch of rater) per °C.

It ha

not boon possibl3

to measure it at other pressures, since we have not so far boen able to maintain

sufficiently constant pressures.

Supply voltage changes produce a negligible effect.

The calibration o'o of frequency against pressure is not linear, but over a normsl

working range it may be regarded as linear for most practical purposes.

The froquency-iueaauring arrangement used for routine wave rocording is shown in

In this inatnxnent en air-filled rubber

ag is fixed to the sea-bed, or suspondßd

a fixed distance above it by a float.

The sir iii the bag takes up the pressure of

the water around it, which v-aries as the waves pass overhead.

The bag is oonneotod

by s hosepipe to a pressure recorder ashore.

The principle is simple and attractive, not least because no power aplies ero

n006ssary.

However, a ntxnber of unexpected snags arise in praotioo, mainly in

octineotion with the design of the sec unit and with the arrangements for f illin,g it

with the correct amount of oir.

(e might cite the operators in the Persian G&2)Ì,

who, when the temperature is 120°F in the shade, find that the effort required to

ptp

up

the sea unit using a hand pup is more than they can manage.

N.I.O. la

a lowly overcoming the problema, but can't o la im tha t the ins truent is I 0()

satisfactory yet.

Por unattended intermittent c)er3tion In remoto places, a synchronous-motor

chart drive is usod, driven from a transistor oscillator, arxd

witohed by a

spring-drive proanng olook.

Wave-moe suing bucys

A desoripticxi of the prinoiplo and practice of wave measurement using the N,I.O.

bucys is given by Longuet-Higgins, Cartwright and Smith, and an example of their

use

in a practical problem

is given by Cartwright, in papers presented to this oorEerenee,

ily a very

brief

description will therefore be given her..

The self-contained buoy is ca st in alimiinitzn, is circular,

about aix feet in

diarmetez, and one foot deep.

It contains batteries, a vertical

accelerometer,

pitch and

roll gyros, electronics,

a multi-channel photographic

recorder and e

prcgrarxming clock.

It is lined up in the wind direoton by means of a

_{nall drcg}

attached to it by a rope bridle. It

has

been used

satisfactorily in winds up to

Force

7.

Tboui this buoy is a fully praotioal device and

has been used

s great deal, it

is rather large and heavy to handle, and in cQnmon with all se f-contained devices,

since the use of transistors presents some problems (for emple, the iiedanoea in

the wave/tide filter are too high for

'ensistor circuits).

o advantage of this inatrncnt is its flexibility.

An R.C. oscillator using

thermistors in the frequency-determining nctwork h3 boon connected in parallel with

the pressure oscillator.

Thus, e frequency dependent on temperature also appears

across the 68c load resistor and is arranged to be a l

frequency so that it may be

filtered out and measured.

Long waves may bu acasured by connecting a suitable

filter and output stago to the output of the diodo-punp frequency meter:

such a

system is 8t present in use and appears to be satisfactory.

For research purposes,

the output ay be recorded digitally using a counter-typo froqucncy metor feeding a

tape-punch.

o minor disadvantage of the present design le that it is rather too heavy for

ease of' handling during calibration.

-5-.

soffers fri the disadvantage,

alreay

d.icuosed, that occasionally

.t i

foimd OEZ recovery that snething has not ceratcd correctly,

and no

records have been obtaird A naller and lig)tcr buoy has therefore been macle which is connected by a cable to the ship where the outputs are recorded. In this buoy, a single gyroscope with a vertical axis carries light vertical accelerometer, and has pitch end. roll pick-s

its gimbal bearings.

This

and its

associated

elootrics are contained in a watertight

can fixed in

the middle of

a circular

wooden

disc four

feet in

diameter

ami four inches

thick.

The light multicore

cable which conneots it to the ship la kept slack, but the tenaion is sutficient to

control the aliment

of the buoy.

These buoys allow the zero order, first and second angular harmonics of the wave ape otrun to be determined.

Principles of recording and

analysis

The

analysis of

wave records falls into three

oateories

-(i) Por moat routine recordings, only a height and period are required frun each record, and these ars then subject to statistical analysis.

Por researoh purposes and for the development of prediction formulae, the enerr-.apeotrxn of a non-directional

reoording may

be required.

Por other research purposes such

83 the measurement of directional spectra

and the sttxy of ship motion, cross power spectra between records may be

required.

For the

simple

analysis, it is possible to devise systeme which record mean wave

height and

period directly: for

emple,

that described by ßnodgrass

end

Putz

₍₁₉₅₈₎

for

ettaahing to the

output of an electrical wave mater or that developed by

Wcmeletelder (de acribod in

Ferguson, Weinalafelder and Santema,

1957)

for a

mechanical system.

Such inatrunents are usually

arranged to talca en oocesional

ordinary wave record.

We have

not, however,

so far felt the need for such systxna.

We believe that full

wave

reoorda should in

any case be

taken

at

freqtnt intervals,

pertly because they usually show any fault which has

develqod in

the system, and

partly because

we often subBequently select a series cf these records and oiipute their frequanoy spectra for reeearoh purposes. Having got the

r000rds, a manual

analysis

baeed on

sound

statistical principles has been devised which is quiok end simple (see bolow).

Por obtaining the power spactrun of a non-directional

recording,

the

photo-electric analogus analyser

(Tucker 1956) is still mostly used.. This has the advantage that elmoet any fcam of

record can be converted into a form suitable for

analysis on it by one of a

nuiber of

simple, if

acnetimes rather laborious, teohniqucs

and it gives a more detailed analysis

than the usual digital procedure. It has two major limitations. Firstly, the records must not contain more than about 200 waves

of the highest frequency of interest, which limita the

statistical

aocuracy. Seoondly, it in limited in accuracy of calibration to about ±1C end in relative

accuracy within

e spectrun to about ±5%.

These

record lengths and accuracies soffice

for many ptooees, but

when better accuracy is required, digital

methods are

used.

Por this, the records

can either be recorded directly on 5-hole

tape, or recordad in

grsphio form and digitized on the

equipment described below.

For obtaining direoticial

_{spectra and for some specisl}

_{purposes, sh 85 the}

study af ship motion, the

_{cross power spectra between pairs of}

reoords are required.

When thia problem first

serious),y arose, N.T.O. had no analogue equipment capable

_of

performing such analysis.

_{The photo-electrio corralator (Tuoer}

1952) would go

part of trie way, but its accursoy is poor, it will not handle

_{output oorrelo'ams}

_{long reoorda,}

and its

require Fourier analysis

_{to give the cross-epetra.}

_Development

cf a speoíal analogue

_{nichine would be expensive,}

_and

the resulting

_{irìatrxnent wo&4}

not be very flexible.

_{Suitable digital cctnputer proarns wero, however, already}

available, and ere

_{comparatively easy to raodffy}

_{as required.}

Thus, everything

pointed to the use of digital

_{techniques for this purpose, and these hava, in}

fact, been adopted.

Having started

_{on the use}

of high-speed digital computers1 N.I.0. has been

steadily extending its

range of proams

_{and of ancillary equirent.}

Many of these

techniques

are not, of course, N.I.0.a

original conception, but we hope that before

lg we shall

_{have an t*iusual}

_{range of facilities}

available. Sinpia mea8lxrement cf _{wave reoords}

Though this system is not yet fully standardised

oven at N.I.O., it is thought

to be worth presenting

here in the

hope

that it may be disoussod

at the meeting.

The

instructions are as follows, and are illustrated

in figure ii.,

Measure off a 10-minute

_{length of the record,}

and oonsder on3.y

_{waves in this}

mt

a rva i.

Draw in a mean water-level

_{line by eye.}

Coa.nit

the nber of crests N0.

A crest is defined sa e point where the water level

_{la mentarily oo13tant,}

falling to either aide.

_{Sotse crests may be belcr4v}

_{mean water level.}

Coimt the riaber of tunca

_{N5 that the reo1}

_croases

the zero line nioving in en

upward direction.

Measure the height A of the

highest crest and the

_{height B of the seoond highest}

crest, measuring frun the

zero line.

Measure the depth C of the

_{lost trough and the depth}

D of the second lowest

trough, measuring from the

_{zero line and}

_{taking both qintitiea as poeitve.}

Record H,

A+C

B + D

T0

_{period of cro 3t5}

600

T5 -r- _{period of zero crossings}

rge

The theoretical basis for this system of measurement is

_{g4van by Ca1-twL'ight and}

Longuet-Higgina

(1956)

and by

_Cartwright

_(1958),

(See also Putz, 19

) and is

The statistical distribution of wave-heighta is governed by the

r.m.a. wave

height Hr.m.s

and by a speotra1-idth parameter e

From the measurements, the best estimate of

e

is

i

-Hr.m.s

is the r.m.s. deviation of the surface from

_{mean water level, and is estimated}

as follows.

where

O

1og6

These are the best estimates to a good dcgree of approximation.

The statistical errors in these estimates are less than might be oeoted, and

are not mth worse than that of the mean of the highest

-ç waves in the reoords.

The

fulae fr them are cc2nplicatod, but in

a typical case where N0

100,

e

0.8, the

proportional standard error in the estimate of Hr.m.s. frc2n H1 is approximately 13%

and frcin E2 about 10%.

In practice, for many civil engineering purposes, the relevant wave-height ia

and the relevant period T.

Digital recording. and d±itisaticn of analojue records

N.IP0. novi has a digital recorder which will record, an input voltage as

s

patten-of holes on 5-hole teleprinter tape.

The equipment can record one, two or four

channels at a maximin rate of five readings per second with

_{a resolution of 0.1% of}

full scale.

Each channel has a reoording potenticiiìeter fitted with

a ooddßiso on the pen

drive shaft.

The patton on the disc corresponding to the angular position of the

shaft is read photo-electrically, stored in relays and then pimehed onto the tape

using three successiva rows of holes.

The code used is Watts Rfleoted Deoiral with

a parity digit added.

This code is a form of cyolic progressive binary-coded

decimal and is used for instrtutental reesoris.

_{After every four readings, an}

identi-ficeton "o'zrna" is pmched.

The instrtuent wa manufactured by liilgor axi Watts

Limited.

This digitizer can be used either to record the output of a wave meter directly,

or it can be fItted to th3 output of a photo-electric curve-follower.

This curve

fol1.ier is an improved version of that described by Tucker and Collins (1947),

az

will follow line records and convert them Into a varying electric voltage.

_{It must}

have records in the form of lìrvs on plain paper or on transparent fiLn.

Records in

other forms can often be traced onto ohart suitable for feeding into the machine.

-7-I

-h1 (2o)

(i + 0.289 6'

- 0.2l.7

a2)

--i-E2 (20) 2 (1 - 0,211 &

- 0.103

2) FrOEn H1

_{Hrm s.}

Frcxn FI2 :

FIrme

Sctne reoords, notably those from the self-contained N.I.0. buoy, are more easily digitisad on semi-manual digitisers (medo by Southern Instrunents or

Benson-Lehner)

to

which we

have access at

the

Royal Aircraft Establishment. These records, cn 70 nia, film, have several tracas which may cross end

_{which subsequently have}

_{to be}

cross-correlated, so

that matching cf time-scales is important. _{In the R.A.L inatrtznents} a cross-wire is moved onto the record, and depressing a pedal then oausss the reading to be punched onto a computar card. The readings from all the traces et a partioit1.r

time are punched in ti.u:'n onto the

seme card:

the

record is

moved forward, the card

replaced by a new one, and the process repeated.

A system for recording digitally the output of an F.M. measuring head. is in process of being developed. In prinoiple it is similar to that used by Munk and Snodgrasa et Scripps Institution of Oceanography. The number of cyclos in the siial is counted for, say, I second on an electronic counter, end the state of the

decades is then punched onto 5-hole tape.

Such a

system is capable of very high resolution, and using the F.M. pressure recorder described above _{arid digital}

filter-ing techniques,

it

_{should allci long waves to be filtered out of the ordinary wave}

re cord.

r000ssingof digital records

The digitiser recording on 5-hole tapa is

unfortunately not too

reliable, and mecea on

the

average one error in about 200 readings. The code is alao not one cf the usual digital computer oodos. The

first stage in hsnd.Jing a

tepe f r this instrument is therefore to feed it throu,gh a computar p rrmed to change the code and look for errors. It first makes sure that the pattern of holes forras a

legitimate codo, than decodes them and thon takes first differences. if these exoeed a certain value (fed in with the prograrrgno), it regards the reading

as en

error. It also makes sure that the correct

number of roia of holes occur

between

"OLias&'.

_{The record is thon punched out in standard coda with zeros where the}

errore occurred. The serial number et the errors is also

given, so that the

operator can go through the tape and punch in corrected or interpolated readings by

hand.

The prograninos for

correlation and

Fourier

tranaformation ere besad on the well-laiown principles first sot out by Tukey, end will not be described here.

Ac kncv].o dgements

The techniques described above hava been developed by quite a numerous team at

NI.O.

_{These inoltide (in alphabetical order) L.A. Baxter, R. Bowers, Miss D. Catton,} D. E. Cartwright, L.A. Thaper, U. Harris, M. S. Longue t-Higgina, F.

L Pierce and

Bof

cronaca

Cartvright, D.E. and Longuot-Higgins,

u.S.

₁₉₅₆

"The statistical distribution

of the maxima of a rDnd1l function".

Proc. Roy. Soc. A,

Vol. 237,

p.212.

Cart-drigììt, D.E.

1958

"On estimating

the moan oner of sea

evea from the

hig1ost waves in a record".

Proc. Roy. Soc. A, Vol. 2L7, pp. 22-48.

Snodgraos, F., Munk, TI. and Tucker, M.J.

1958

"Offahoro recording of

Li-fraquency

ocean

Trans. Am.

Goophys.

Union, Vol.

_39,

pp, iii-1a.

Putz,

R.R. ₁₉₅₎ "Statistical

An3lysis of

3VO

Records".

Proc.

Lth Conf, on

Coastal

ßngg., pp.

13-2L,-.

Snodgrass, F.E. and Putz, R.R.

1958

"A wave height end frequency meter".

Proc.

6th Conf, on

Coastal

ìgg., pp.

209-22L

Ferguson, H.A., Wumoisfeldor, P.J. and Santeros, P. 1957 19th

Internsti.al

Navigation Coness, London, pp. 207-231.

Tker, M.J.

1952 "A photoelectric correlation motor".

J. Soi. Instrun., Vol. 29, pp. 326-330.

Tuckor, M.J. 1956 "The

N.I.O.

Víavo

Analyser".

Proc. Ist Conf. on

Coastal

Bigg. Instrtiucnts, pp. 129-133.

Tixkor,

M.J. and

Collina, G. 1917 "A photo-electric cure-fo1lower".

(i) Cross-scotion of F.M. prossura

r000rdcr measuring hoad.

F.M. prosso recorder

mosuring-head

circuit.

The oscillator froqinny ja

approximately 500 ko/o

st enosphoric

pressura, end 150 ko/a at a proasurc

oorre spending to a depth of 200 ft.

The froqncy measuring circuit and. wave/tide filter for routine

wave recording.

(4.)

To illustrato the simple measurement of

_{a wave reoord}

_{(only five minutea of tha}

r000x'd is shown haro).

_{The points marked v;ith a dash aro wave croata, and}

with

3

circle aro zero crosainga in an

ward.s direction.

Interior is

evacuated

Terminal

box

filled

with

compound

Oscillator coil

/

Support for electronics

ll/4 inches

Plate of capacitor

Mond waphrogm

Gunmetal case

irjmierjiu

t

C

o

C

o

o'

o a

O2,F

5pF

47Ort

122r

0C44

L1

_/

(J

L,'

-e

-6V

68 r.

.

I From

meoauring

h cad

'1H

B A RE

Ampi itier

r

8,.jF I

iOMf

L

Crystal

Oscillator

Mixer

Low pass

Filter $

Amplifier

Diodepumpi

o

fi

Approx Ql V D.C.

per foot

SWITCH POSITIONS

Recorder

A

_{Waves a tides}

B

Waves only

Stated

and _{OtIt put}

C

Tidts only

dessicated

box _Stage

+ 300 V

1 .t.4.L1

i'i°&A.

A.L.L..&

.

l.A

tL. .1

.

L.&.IA1..

H!!

I

!1!Ttt

!

j''; !!!!'''

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