4 Prepared underNR 083-157CONTRACT Nonr 1841(74)OFFICE OF NAVAL RESEARCH

BEST AVAILABLE COPY

ISPEED OF SOUND IN UNCONSOLIDATED

SEDIMENTS OF BOSTON HARBOR, MASSACHUSETTS

BEST AVAILABLE COPY

by

loyd Frederick Lewis

Massachusetts Institute of Technology

Cambridge, Massachusetts 02139 September 1966

TI

SPEED OF SOUND IN UNCONSOLIDATED SEDIMENTS

OF BOSTON HARBOR, MASSAC-IUS7?TS

by 6

LLOYD FREDERICK LEdIS

B.Sc., University of California (Berkeley) ..

(1965)

SUBM4ITTED IN PAATIAL FULFILLIENT

OF THE AE0UIzEMENTS n•h ThE

LERhES OF MASTER OF

SC I ENC E

MASSACHUSETT'S INSTITUTE CF TECHNOLO2Y

September, 1966

Signature of Author./ ... ........................Department of Geolory and Geophysics, September 20, 1166

Certified by...............- "Tes,i7Aedvisor

Accepted by ................. .................CIr man, Departnent Committee

on G2raduate studerets

1,, ;6

SPEED OF SOUND IN UNCONSOLIBATED

SEDI.iEN`S OF BOSTCN -ArU!Oh, iASS.

by

Lloyd Frederick Lewis

Submitted to the Department of Jeology and Geophysics onSeptember 16, 1966 in partial fulfillment of the require-ments for the degree of m~aster of Science.

ABST.HACr

In situ measurements of the speed of sound in surficalmarine sediments of roston Harbor have been made at approx-Imately 100 stations. A simple spark discharge of chargedcapacitors created the sound pulse wnicn was received by aconventional nydrophone-amplifier-oscilloscope system.Photo--raDhs were taken of the trli.•er pulse as displayed ontne oscilloscope screen. Detailed time records were ob-tained using a delay time base. first arrivals transmitte'by the hydrophone appeared in the frequency ranxe of 10 to30 kilocycles/seconc' while the sound source likely emttteAa broad spectrum of frequencies.

Sediment samples at all stations have bepn obtainedeltier by zravity coring (aided by nammar blows) or bucke+rrabs. Laboratory analyses of zraln size distritution an,'water content nave oeen made. Porosity was calculatedassumin•- complete water saturation. "Lhe author attemptedto correlate th•ese various pnysical properties with In situsoune speed mpasurements and has compared his work tostudies of similar sediments by other investigators. Thepresence of methane and nydrogen disulfide ases in thesediment limited tne de.-ree of simple correlation betweensounrd transmission and other physical properties.: 1

Ihesis Supervisor: .-r. -:arold 6. Mdcdrton1itle: Professor of .lectrical -nxineerinv and

Institute Professor

4L

CONTENTS

Pa ~eABSTRACT

I!CONIENTS ILIST OF FIGURES AND TABLES

LIST OF SYMBOLS IVACKNOWLEDG EMENTS v

I. INTdODUCTIONA. Object of ResearchB. Previous Investizations 2

TIT. SCOPE OF PROJECTIII. FIELD PHOCEDUIES

10A. Site Location

10B. Sound Speed Measurements 10

a. Equipmentb. Technique

C. Sediment Samp5inz 16r. Equlptentb. Technique

IV. LABORATORY PnOCEDrT)ES 2'

A. Water ContentB. Sieve Analysis

26C. iydrometer Analysis 22D. Summary of Grain Size Analysis 3i

V. RESULTS AND DISCUSSION 43

A. Sound speed versus Sediment Properties 13B. Sound Speed Profiles 10C. Comparison to other work. QD. Error Analysis

VI. CONCLUSIONS AND UECOMMENDATIONS C?

BIBLIOý;RAPHY

LISI OF FIGMUES AND TABLES

Figure 1 Sound Speed and Sample Stat ions 8

Figure 2 eesearch Vessel h.h. Shrock 12

Figure 3 Field Equipment 11

Figure 4 Oscillograph:Sta.283,4ater Path 17

Figure 5 Oscilloaraph:Sta.283,Deep Sediment Path 18

Fliure 6 Oscillograph:Sta.283.Shallow Sediment Path 19

'fable 1 Sound Speed Data and Results 20

Form A Sediment Sample Analysis Summary 27Form B Hydrometer Analysis Summary 30Form C Nomographic Chart for Hydrometer Calrulations 31

Fizure 7 lydrometer Calibration Curve 32

Figure 8 ;rain Size Distrlbut..on 34

Fl~ure 9 Sediment Nomenclature Scheme 36fable II Sediment Sample Data and Analysis 37

Fi4ure 10 Sound Speed Ratio vs. Porosity (7as absent) hht

Fi-ure 11 Sound Speed iatio vs. Porosity (gas present) 46

Fi.ure 12 Sound Speed natio vs. dater Content 48

Figure 13 Sound Speed Profile Locations 49

r'iures 14-l7Sound Speed Profiles 50

Table III Sound Speed Comparison to Other .Work 51

-V-

ACKNOM LEDGENl S

The author wishes to express his gratitude to Dr.riarold S. Edgerton for the advice and criticism which

guided this thesis topic from bezinning to end. Apprec-lation is offered to V. eLcrioberts for his suegestions

on electronics and to Miss J. 1,iooney for her help in

administrative affairs. ,.uch of this work was accom-

plished with the Instruments and facilities of the

Stroboscopic Laboratory at M.I.T. and the patience of the

personnel there is herein acknowled:et.

Field work was accomplished in close co-operaiton

with the Boston -.arhor Group under the direction or Dr.

Ely Mencner and supervision of A. Copelane and E. Payson

Jr. The use of the Sedimentary Petrology Laboratory as

well as the snarin; of the use of tne m/V R.R. Shrock is

greatly appreciated.

Financial support was derived from U.S.Navy contract

NONh 1841(71) administered by Dr. Frank Press and Nat-

ional Science Foundation ;rant GP 2628 administered by

Dr. Ely Mencher. Calibrated hydrophones were loan-ed bý

the U.S.Navy Underwater Sound deference Laboratory,

Orlando, Florida.

Finally, the author thanks his wife for her help in

manuscript preparation and her patience throuznoi* tho

prolect.

I. Int rorluc t Ionl

A. Ciblect of stesearci

-ýnis researchi was un'ierl~aken in an attempt by Ineaut~ior to rela-.e r.-e speed of propozarion o' ap-oust'ic enpr2-y

:-irou q~ na'urally ot~currl~n marine sedimonts to o'her p'iysi-

cal proper.-Aes of tie sa~dirent. Labora-ory meas'iremen,-s

of sound speed on core samples niave yielded' ro-sulls In close

a;.reement -.o irI situ sound spee& ms!asuremen's only in -.iosia

Instances where :ie sedicaent_ was main-alned In 1*5 or!71nal

..as-:'ree S:aze and wrnen due consideration was Iven 1,o

chanes In pressure and tempera,ýure of nre sam~ple (Hamil-

ton 22, oyles 4 ). In Joston Harbor tne presence of an unknown

amoun:. o! -~y-.rozen disulfide and/or me-.-ane was obvious from

tn-e odor of sarples collected. The :-empera:ure of 7ne wa-er

and sedimen7 varies a -reat deal In very shallow re-ions over

a tidal period an,' daily witn weatier condli'ions. Con-

sidlerin-z the po-en'.1al Inconststency In relat-In laboralory

ýoi sir~u condiitions, In'e aut'nor decided to make soundl

speed measurements In sitýu anrH ob:atn samples oP sPdimen-.

'or labora'.ory analysis of ph~ysical properties whici would

be unaf"ected, by transporttnz t-ie sample to !-ie laboratory.

-dgertonl3 nas srnown tciat penetra-ion ol 12 kilocycl'p/

spcona soune Is possible In 6os,:on harbor sediments only In

tnose areas~whioh are no-: covered by a black, firie-rained

odoriferous mud. The latter acts as an almos' perfect

re~lector of souna enxer-y even when only Inches t-iick. ihe

author inves:izated 7nis layer as well as tie unnerlytnx

compact clay and sand layers In an aremp- to assiwn *,ypi-

call sound speed values :or use in accura--ely converting rec-

ords ol travel time(from continuous seismic pro' iles) -o

zeolocical cross-sec-ions.

?rom seismic Invesit,-ations o deeD-lyinz sedtmen~s, a

re'rac ion -lec-.nique yields an avera e so-in,' speed to use in

computin.w dep-h (ý'wtnz 1 , tiourz 2, Shor 42 This-1-

technique does not eiscriminate between layers of low acous-

tic contrast and effectively masks the distinction of thick-

ness of these layers.

In the present study a horizontal variability In sounH

speed amounting to 404 or more is noted in the surfical

sediments over the 30 square wile study area of Boston =arbor.

Vertl.cal variability in sound speed amounted to 30% In the

first few feet at some locations. Assignment of sound speeds

averaged over the Harbor would certainly produce significant

errors in calculated layer depths locally.

A further application of sound speed measurements Isin the field of soil mechanics. Once the speod of the com-pressional wave, tne density and the compressivility ofa sediment are determined, It Is possible to calculate the

other elastic properties Including: Poison's Entio, Shear

Modulus, speed of shear wave, Young's Modulus, and Lame's

constant (Jaezer 27). Assumptions and techniques for

carrying out these calculations have been given by iamil-

ton 18 and will not be repeated here.

B. Previous Investigations

Hamilton 22 revorted In situ sound speed mmasurements

in 1956 off San Diero. Operating In 90 feet of water,SCUBEA divers inserted acoustic probes Into the sediment andrecordIn~z was done with oscilloscopes on a surface ship.Samples were collected and kept 'air-free' until laboratoryanalyses of density, porosity and grain size were completod.

Hamilton noted that sound speed in sediments of high porosity

was less than that in sea water and explained this by Dart-

Icle movement in a sound field causing frictional losses eue

to viscous dra... In situ sound speed measurements were20

conducted azain in 1963 (Hamilton ) in 1000 feet of water

using the bathyscaphe Trieste. Laboratory analys-s of scdl-

ment properties were conducted as in the previous study.

The zeneral findings of tnese measurements are listed in

Table III, Section V of this paper.

-2-

"* 3ound speed measurements were ma.e in situ in a fresh28 -

water lake by Jones in 19ý8. Iwo hydroprones were burled

in the lake bottom to known deptis and a known setaration.

The time delay in sensing a spark discharze in tne w&fr (a,

a known depth) indicated by an oscilloscope rerord of the

hy'rophone receptions provided a means of determInln: sound

speed. Divers noted a wreat amount of oreanic debris e'ocav-

ing and generatina free zas in tna sediment. Ustnw tnis 1wc

nydrophone technique, Jones was able to de*ermine tha* h-

sound speed throuzn the zas cnar-el bottom was about one *en-

th tne sound speed in the lake water.

Sykes48 used acoustic probes (modifIes from 'oo& and

Weston' )of small radiating area to pulse 3Y0 kilocycle/

second sound throuzh various strata in deep sea cores ob-

tained by tne 4ood's mole Oceanographic Institution In 19 9.

Assuming the ratio of sound speed In sediment to sourd speedin water remained constant for in situ and laboratory cond-

itions, Sykes was able to caloulate on the basis of salinity

and temperature measurements (Albers1 ) the speed of sound

in sea water in situ and thus the speed of sound in sediments

in situ. The results thus obtained are listed in lablp Il1,

Section V of this paper. The basic difficulty with Sykes'

system is in the probe size and inherent frequency lImita-

tions. In order to maintain tne radiatin- area small win

respect to core diameter and to emit souind wiose '•a-elenz'i

was smaller than any particle size, Syke resorted 1o ul~ra-

sonic frequencies. Transmission was possible in 1Lzhly

porous fine clays but signal attenuation and sca-erinr rro-

hibi t ed reception tnrou-h silts and sands. [no-e: .- Irurps

8 and 9 of this paper explain the size termns men-toned].

3yies also determined water content, rain size, porosity

and density assumin7 the cores had not dried apprepiatly

over tne year period between collection and analysis.

ihe use of lower requencips in analyzinz small samplas

in the laboratory for sound speed is possible usin: A

tec-inique developred hy Coulis an .Phumway in 13-6.

-3-

ine sedi;ent sample is placed in a compliant-wallpd cylin,-

er and set into resonance by one acoustic probe. the

frequency at wnicn t~is resonznce occurs Is measured by

another probe and indicated accurately by a counter-ampli-

fler voltmeter system. Cver a frequency ranve of 24 to 35kilocycles/second, tne speed of sound was determined from

frequency !Leasurements and resonance mode assumptions. At

the same time a sediment sound attenuation factor was deter-

mined from the '.' of the frequency resonance. An inricarion

of Snunway's resilts Is =Iven in Table III, Section V of *h5s

paper. Ihe major criticism of thls technique is in tnat it

does not provide for repeated measurements on tni same

sample. Invariably zas forms on decreasing pressure and in-

crrarin; temperature as a result of setting the sample into

resonance. mitn the gas present, the attenuation is much

too nihn to repeat the measurement.

Nolle37 worked with artifically compacted, sorted sangsin an attempt to characterize tneir sound transmission

properties. Sound speed was not measured in these experi-

ments but wnen otner factors were analyzed it became appar-

ent that gas was cominw out of solution and depositing on

the sanr grains, creating- hign attenuation and scattering

coefficients at tie operating frequencies of 400 to 1000

kilocycles/second. A solution to this difficulty was the

continuous boiling of the sample durinz experimentation to

mnlnaln gas-free conditions. From an assumption of no

ri.zidty (u = 0 for nriinly porous systems) the speed of a

compressional wave Is aiven by (Jaezer 27):

V ./d = /c(1)

A^ere V = sound speed, k = imcompressibility, d = density

and, C = compressibility. If the system has a slight amountof gae entrainment it becomes highly compressible without a

comparative density decrease and the net sound speed is

reruc ed.

berson3 and i•randt 7 nave snown by ratner indepenlonr

analytical means that a drastic redurtion in sounr spo-d

occurs for only a small percentage of free ias by volume

tn a solid-liquid-zas system of components. 7he sound spead

for a 0.24 fraction of gas in the void volume of a solt'4-

liquid system is only .04 of tne sound spead In the later.

Pnyslcal reasonin7 points out tnat if was Is present as free

bubbles, these bubbles will expand and contract absorbing

sound energy and lenwthening tne time of propoaation. In

addition, tie bubbles scatter and otherwise attenuate the

si.nal.

Assuminz the possiblilty of an iPeal mixture of one

solid (s) and one liquid (1) component, Cfficer3 8 has der-

ived an equation expressing tne sounH speed (V) in torms of

porosity (n), density (d) and compressibility (c):

21

In d1 + (1 - n)d,) [ n C1 + (1 -n)Cs] (2)

eor n = unity, tnat is all liquid, the sound speed retluces

to that of the liquid (see one-component relation, equation 1)

Va = 1 =dIC1 1 (3)

For n = 0, that Is all solid zrains, the sound steed reeucesto that of the solid (see one-component relation, equation 1)

1s~ = • a (Li)d sC s

As the porosity decreases sliahtly from unity, ronsiderinx

densities and compressibilitles relatively uncnan:¶ng, the

denominator in (2) remains such *hat thne sound speed de-

creases since the 'n' terms predominate and liquid compress-

Ibility is much greater tnan that of sollAs whilP liquPldensity is less tnan -hat of solid. Further decrease of

porosity causes tne '(l-n)' terms to becomp dominant an'

since Vs Is always greater tnan VI, tnere occurs a minimum

-5-

Where the sound speed of the mixture Is less than that in

tne liquid alone. This concept is furtner discussed In

Section V of this paper In relation to the experiments of

I'Afe and Drake3 6 .

-6-

.;I SCOPE OF PROJECT

This research was undertaken in co-oporation with tthe

Bostm Harbor ýroup here at M.I.I. under the direction of'

Dr. Ely Mencher. The objective of this croup was to sample

the surfical sediments over mort of Boston -arbor and using

conventional laboratory techniques to work out the recent

-eolozical history of this area. The author ortrinally in-

tended to occupy a small number of stations with the harbor

.roup and to develop a sound speed measurement technique.

It soon became apparent that numerous stations would have to

be occupied in order to find sites where similar sediments

could be compared and to note significant trends In the re-

sults of the sediment analyses. The author therefore chose

to work with the Harbor 'Group through the summer of 1966 to

collect data at each of 100 stations as shown in Figure 1.

The stations are on an arbitrary Frid nptwork and apDaren'

gaps in the crld indicate sites where shallow warer and/or

a rocky bottom prohibited sound speed measurements.

!-he surficial geolocy of the Boston 9arbor has been re-40

viewed briefly by PhippsO. One or more glacial till layers

occL.ring as drumlins or drifts are evidence of the las*

Pleistocene glaciation. The zlacial till is an unsortpd

mixture of sands and zravels with fine clay-size rock flo, r,and some clay minerals. It is postulated that at the waning

of the ice, the land rose and was eroded slizhtly an- "nen

san" to leave depressions In wnich fresh and salt water ppats

and black silty fossiliferous sediments were deposited. A

hiun rate of discnarze of or-.anic wa3tes by man n .- elpe.

to create the surfical, black, odoriferous, soft :-ud layer

that covers most of the undredged area of tie -iar'or.

Probably the best sorted and most homozpneous devosi:

is tne very stiff Boston Elue Clay (Lambe3 1 ) tia- occurs as

rhick as 100 feet under a layer of black mu, or a layar o

saneý and ravel over most of the Harbor. Where *ie cover',,

nas been drpd-ed, the clay apts as an acoustic Absorber bur

where the black, aspou- mud Is as tqin as a few Inc.is, '-e

-7--

luw WOO.

+Vo AHm""

4,, Ipa

Moslem Pfp UKJ1ACJSET

( -8-

bottom Is a nearly perfect reflector of sounr! eneray.These two 11tholozips--t-ie tlae-Ic mud anr tlie *?os+on BlueClay--In a'11ition to an or'casional mandy to-orr In 4r^A701

areas waere t.ne materials most often enrounteree' in sur-facp samplinz andý soun-4 spee& measurements in ti'ts re-ton.

III L 0•• P n CC _'D 'n -S

A. Site Loca'ion

,lost of the samples an,' all of the sounO soeA mreagure-

mtnts w-re taken rrom the X.I.r. n-searcn Vessel zi.r..Shrork

(Fizure 2). Aitt reference ýo ai artirrary 'rld notwork

plolte" on 'no United States Coas- anA jo'4e-Ic Survey

Ccart 2h6, tie vessel was anchored at a proposee sa'tion ane t

a position was established usinz sex-ant fixes on three

visible landmarks and resection plottin uslng a three-arm

protractor. The estimated accuracy of location by tnlistecnnique is 2, yards and is fixed by thne one minute reaAinz

precision of the sextant (.;.auzes and Sons Ltd.l•12997) an1

scale of tee chart. Several stations occurred adjacent

cnannel bouys wnicn facilitated location.

B. Sound Speed ,,ieasurements

Equlpmen7 used on the vessel is snown In ""Icure 3. 1 noe

sonic probe and samplIn- Instrlmen t s were s13qDenre;. A from the

snip's A-frame as snown in Figure 2. -.avin7 anrhorpJ an•

obtainp- a position, a zrab sample using tne Van Veen

('g',Fiý.ure 3)or a core usin- the square corer 'a',: I-,'re

3) was obtained to determine the coarseness or toe bottom

an- to obtain a sediment sample. If a sample was 1-aken, the

sonic probe was lowered aft and sound speeA measurements were

made.

ihe sonic probe (f, Figure 3) was cons t ructed of 21"2diameter cast iron pipe with 1" probes of C.I.P.. Threa'ed

into 'T' couplin:s spaced approximately two feet apart on

tie 2 1/2" c.i.p. cross member. 1he supportinr members

were weiznted witn approximately 120 pounds of leae'doutnnuts'

provldkg a total weight of 190 pounds an, a bearln7 pressure

of approxinately i10 pounds/inch a a, tne ene of each Drobe

(in air). qhis weignt anA confizuratIon was foun, -o he

sufflcientWly statle ro maintain the protps In a vertical

position In -Te bottom except when Ine *If•al rurren- was a-

-10-

FIGURE 2 RESEARCH VESSEL

FIURE 3 FIELD EQUIPMENT

-12-4

b. e

9,4,

EQ~UIPMENT

G .S q u ar e c o re r qS f~ V a n V e n a m l e

b. oscilloscope K spark cbns le r

c. Camera mounlt i. spark cable

d . 1 2 11 s c a l e s p.r s o u r c e~

e.amplifier pr~suc

FIGUR~E 3

-13-

a maximum and/or the surface wind caused the vessel to

swing rapidly and tijhten the cable pulling the probes

out of the sediment. A heavier probe arrangement ane

better anchorinr tecnnique would solve these problems.

Fixed to the end of one probe was a two-coneiictor,

snielded, No. 14 copper wire cable ('h', Figure 3). Approx-

Imately 100 feet of this cable led back to the ship ane was

connected to tne spark source ('J' Figure 3). The latter

Is a high voltaze capacative discharge device designed by

V. Mckoberts, Stroboscopic Laboratory, Ni.I.T. It was

operated at an electrical energy output of about 80 watt-

seconds (3200 volts across 4 microfarads) which, when

trizgered once per second, provided 80 watts of acoustic

power at the short circuit discharge In sea water across the

two #14 wire leads ('h', Figure 3)At the end of tne other probe ('i', Figure 3 and LC34 a

hydrophone (Atlantic desearch Corporation, Serial #152) was

fitted Into a groove cut Into the 1" c.I.p. The hydrophone

Is a plezeoelectric device (Hueter 26) constructed of coaxial-

ly mounted lead zirconate-lead titanate cylinders in a neo-

prene rubber sheath with an overall length of 4.3" and dia-

meter of 0.75". When caused to contract and expane by the

acoutic pressure wave from the shock associated with the

spark discharge, tne cylinders set up a potential difference

across face-mounted electrodes. The voltaze was transmitted

back up to tne surface by a two-conductor, low-imtedance

cable and to the vertical Input of an oscilloscope. Accord-

Into to Its speciflcations (UNSUSRL 0 )the hydrophone has an

omnidirectiotAl sensitivity In the X-Y plane If held such

tnat Its long axis is in the Z direction. Since its free

field voltaze sensitivity (over the frequency range 10-100

kllocycles/secund) Is-106 decibels relative to 1 volt/micro-

bar and the voltaze received st the oscilloscope was approxi-

mately 0.8 volts (a maximum), the acoustic wave transmitted

over two feet of sea water had a pressure effect a: tne hyd-,

rophone of about 1.7c pounds/Inch 3 (approximately 0.12 bars).

-14-

When sotnd was transmitted throug-h particularly 'lossy'

sediment, the signal from the hydrophone was sent throuzn a

lOX or lOOX voltage amplifier (.iewlett Packard Model 466A).The amplifier('e', Figure 3) could be used only In those

instances where the received voltc_.e was 50 millivolts or

less since signal clipping occured'for higher voltaaes.

The received signal was further amplified and displayedby the oscill icope(Tektronix Model 564, #003378; Dual Trace

Amplifier #006623; 3A3 Delayed Time Base #00229- as shown

'b', Figure 3). The received signal, togetner with the

trigger signal from the spark siurce were displayed in the

0.1 millisecond 'normal' time mode and then the received

signal only was displayed in the 10 microsecond 'delayed'time

mode. In both cases a photographic record was obtained on

35 mm, film using the camera mount(author's desIrn; 'c', Fi'ure

3)and a single-lens reflex camera with close locus rings

(Nikkorex Model F,#399935; Nikkor hodel H 50 mm fl.2 lens: not

shown in Figure 3).

The technique used in making the sound speed meas•irementwill be reviewed briefly witn reference to :ne data recorded

at Station 283 and shown in Figures 4 tnroucn 6. The pro.-

was lowered slowly through the water column with the snip's

hydraulic winch. lIhe spark was discharged once per second anr4

a record was made of the sound transmission In sea water (rlg-ure 4), having noted the voltage, time and time delay settin~s

on the oscilloscope and the original spark-hydropnone separa-

tion at the probes. The probe was lowered until the winch

cable slacked and a measurement was made in the sediment

(Figure 5) noting voltage and time. After beinw raied azain

to the surface, note was made of the penetration from the

sediment marks on the probes, tie probe spacinx was checked

and the probe was lowered aaain to obtnin a measurement nearer

the depth from which the sample was taken (Figure 6). Con-

parlson of strata was also possible since the protas wereopen-ended pipes and collected cores from tnetr point of

deepest penetration. Finally the probes were raised, hosed,

-15-

the spacina was checked aoaln and the equipment was setured

for the move to the next station.In the example shown in Figures 4 tnrough 6, the deeper

measurement (48") showed the speed of sound transmission

to be 9. wreater than that in water, while the shallower

measurement (20") snowed the speed to be actually 3c less than

that in water. A moderate amount of hydrogen disulfide gas

was noted in the core sample from the surface layer but none

was noted at depth.

Table I with explanation summarizes the data and re-

sulting sound speeds calculated for the various stations

occupied. An estimate of the maximum signal voltage in both

sediment and water was recoraed bl:t this is only an estimate

since the power output of tne spark source varied by as much

as 10c between discnarges.

C. Sediment Sampling

ihe sediment sample was obtained with either the Van

Veen zrab sampler ('I', Figure 3) or square corer ('a', Fig-

ure 3). As the Van Veen struck the bottom the trip bar releas-

ed and the jaws closed to a depth of about six inches. The

instrument was simple to operate and gave a quick indication

of the coarseness of the sediment burface. The square corer,

deslined by h. Payson, Department of Geology and ceophysics,

;..I.T., was used where samples of both the surface and Immed-

lately underlying sediment were debired. This device was

lowered over the stern, held vertically at the sediment sur-

face and pounded into the bottom with a 30 pound lead 'dough-

nut' drop weight.

Samples from either instrument were examined and placed

In i-lass jars, capped, and labeled. Lote was made on a core

lo0 of the estimated gas content(strengtn of odor), tne

coarseness of grain, method of sampling, location of station

and other pertinent Information. The sample was then taken

to trie laboratory for further analysis.

-16-

0 .1 f t l g c n d

0 tirme delay

LiL10 micrasecoade

LIA A4, M 19 0.375 mlllise4CWW111 delay

FIGURE 4Station 2 83: Water Path Oscillogrophs

initial arrival time =0.4?-3 milliseconds

Probe spacing =2.00 fe~et

Sound speed =4.,73 feet/second

Max imum -;gnal voltage =044 volts

"7(3

0.1 miltieconhas

0 37tif lieC~ delay

Maximu signa vol~oe.00.0 volt

OJ mingkonde

0 time delay

0.O5voltsJ

FIGURE 6

Station 283: Sediment Path (20'"deep) Oscillogrophs

Initial arrival time 0.434 milliseconds '

Probe spacing =2.00 feet

Sound speed x4,610 feet/second

Maximum signal voltage -CQ20 volts

-19-

SABLE I: SOUND SPEED DATA AND RESULTS

Symbol Explanat ion

No. Station number as snown on Figure 1.'b' indicates stations are at same location.Station 26: changed to Station 202.Station 140: changed to Station 205.

Location Approximate co-ordinates as shown on Figure 1.

Date Date of sound speed measurement.Not necessarily same date as sample collected.

Depth Penetration in inches of sound speed probes.'a' indicates no change in sound speed overdepth.

Vs Sound speed in feet/second through the sedi-ment at the Station and Depth shown.May be more than one sediment sound speed ata given station.

V1 Sound speed in feet/second through the seawater at the Station.

R The ratio: Vs/Vlat a Depth at a Station.

a The approximate ratio of signal amplitude insediment to that in water at a Depth and Station.

5as Content Subjective decision on intensity of odor ofhydrogen disulfide. A few stations had a weakmetnane odor.

Comment Estimate of the coarseness and or consistency

of the sediment adhering to the probes.

-20-

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-21-

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-23

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-24

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iV LAbOz1ATOmY PROCKDUk'S

All samples collected in Boston 4arbor were analyzed for

water content, Rrain size distribution, total iron and carbon

contents and clay minerology. Of these, water content and

grain size analyses only are of relevance to the sound speed

measurements. Sediment-porosity was calculated from the masses

and assumed densities of water and solids. No analysis

technique was developed for determining the amount or kind

of gases entrained in the sediment.

A. Water Content

Form 'A', Part 'A' outlines the data collected in deter-

mining water content for sample #283. A representative sample

of the jar contents was selected, weighed, dried at 10 oC. for

24 hours and weighed again. The water content is determined

as the ratio of weight of water to weight of solids (Lambe 3 l)

Several samples collected prior to Summer, 1966, nad to be

discarded since they were Improperly stored and had obviously

undergone considerable drying before tney were to be analyzed

for water content..This is the reason for the breaks in number

sequence as noted in Figure 1 and Tables I ant II.

B. Sieve Analysis

Form 'A', Part 'B' outlines tne data collected in

sieve analysib of Sample #283. A representative sample of

the jar contents was selected and welgned. After wel;nlnw,

the sample was mixed witn distilled water In an electric

mixer. Ihis sample was then wet sieved througn sieves

selected for the size ranges: greater than 0.500 mm: 0.250

to 0.00 mm: 0.125 to 0.250 mm; 0.063 to 0.125 mm. The

fraction collected on eacn sieve was weIgned and the result

entered in tne table of Form 'A'. The fraction that passed

through the 0.063 mm sieve was placed in a one liter grad-

uated cylinder for a hydrometer analysis (discussion followlnc).

Once the hydrometer analysis was completed, a few milliliters

-26-

FORM A

SAMPLE ANALYSIS SUMMARY

Samplofe # 8s Location '..-"--, ...-

Date -- 4-o- - L4 Core Depth 0' t" -o ,Analysis y

A. Water Content

l. Weight of crucible /6. rb. Weight Of crucible + wet samrIe ro. 0.C. Weaht of crucible + dry sample C1.6. I

d. Wate content •b- € ,2--' -7:7 %

E Seie Analysis

a Weight of dish .1 g.f Weght of dish + wet bompile r, gL,a Woim of wet oemple (f-0) 06, t

k Weight of dish CA,/ g.i. Weight of dish + dry hydrometer coluftan deoult 8Ag

J. Waght of fraction lost tins 0.063 wvllimo.'es diamew (I -h)

Solve R•ne Dish WighMD~isht+aweWei* SaqielWe ight Weight % FinermIm 1 9 9 (of toal wetgtlt)

> 0.5mx S$____J 0, 1 1.6 1**._I_

0250 to 0500 PicA 1•. a o ,- .9

0125 o 02501 1a..a.. .vw. A-0063 to 0125 1- /$. '? _/,_ x

< 003 (from j above) )by .,,eorTota fZ ' 0___

(Ws)

C. Check on Dry Weight (Ws)It. Weigh! of water= (d) X (g) z 1. $3)X( 1 x .%/.v I

I Dry weight = (9)- (b) = (•f)-(,s).

-27-

of 6N ;CL was added causin: tne suspension to flocculale

and settle rapidly. The cylinder was decanted and tne

deposit drird and wel-ned. 1he latter amount, added to the

sieve weignings gave the total dry weight of sediment

analyzed (As ).

At this point the 'porosity' was calculated for tie

unconsolidated sediment. .eorosity is defined as the volume

ratio of voids to total sample. A density in gm/cm' of

2./i for the sediment solids based on data from Lambe 3 1

was assumed: 2oston blue Clay = 2.79: quartz - 2.(5;

, ar = 2.70. The density for sea water was taken as

1.03 .Sverdrup 6). From these assumptions the porosity (n)

void volume -mass of sea watern = bulk volume density of 3ea water (5)

mass of sea water + solid massdensity of sea water solid denslty;

and "or samDle #283, refering to From 'A':

,97 -n =1.03 - [1001$ g -' 4 S + * s

1.03 75

= 440.4 - 18.21.03 £100]

707. - 18.2 18.21.03 2.75

n =7ý

.his number should not be compared to the wa-er contenr

since porosity is an estimated volume ratio while water

content is determined as a welwht ratio.

-28-

C. Hydrometer Analysis

korm 'B' outlines the data collected in tne hydrometer

analysis of sample #283. That portion of the sample which

was wet sieved through the 0.063 mm opening sieve was

placed In a one liter Frlv,,ated cylinder with 100 milliliters

of sodium oxalate dispersing aaent (approximately one part

per thousand parts by weight) and distilled water to mike

one liter of suspension. The hydrometer (Fisher Scientific

Instruments #864209) was read at the time intervals shown

or until tne least rsading approachee 1.0000 + 0.0005.

lemperature In oC. was read sufficiently often to monitor

the temperature to P 0.5 0 C. The hydrometer reading (ah)

was corrected for miniscus rise (constant for a given hydro-

meter) anI to this was added a correction for temperature (Im').

1he percentage ('h') of sample #283 finer than a given grain

diameter for an equivalent sphere was found from the relation:

N L-d ds •h + mL d -d]S ](loo) (6)

2.75 h ]m (100)2-75 - 17o3 16.2

N = 8.79 [A h + m] In%

lo determine the diameter 'l' of tne equivalent spherical

particle for wnicn 1:1' is the percentaze finer, tie nomo-

,raphic chart, F•orm 'C' was used. A calibration was run for

the hydrometer (Fizure 7) as explained in Form 'C' and the

resulting hydrometer readinzs were plotted on the scale

"Zlelixt in C.," on Yorm 'C'. Using tne assumed density for

solids and tne temperature as measured,a-Doint on 11e scale

"•- x 103" was determined (see "Key", Form 'C'). Uslnz *P

-29-

S.

FORM 8MASSAC04USETTS INSTITUTE cW TECHNOLOGY

I&L WtCN&AlICS LA4SOATOAI

HYDROMETER ANALYSIS

SOL SAWLE W SL SAMOPLF WEIGHT TEST NO0.'J...l. Aglmoc *loj' (Z SA CU*4C S DATE 4-, s"

LOCATIN A-0710*12 A-f 1. c"c"st" AP A

LOCAIO L.~ 'S~* I~ 4z~l, caWY $OIL IN .. A&.. TESTED BY iq

II -1'Is If-to) aO 10 -~-~ % FINERNO" 200' 1 PtoS.ko o

-a W t 'WLSSOL

- Tw et It -

4- ~A2 f, Z LL ~

- ~ -_/9-z -- c-- 0 -

-02 / .t 2,!. - - -

-EAK "'o~.. S..L....Z . L2. __

- /A 22I4d~L. ___ 30_

~ 14

L IL

0.3 *~ -w .4100V37A

4q

-I I '-

FIGURE 7

CALIBRATION CURVE

,HYDROMETER ,AitO--

'5

14 -

z13-

0IL-J

-0

w IIU

S0

U)a

zII=YDROMETER READING

-32--

hydrometer reading corrected for miniscus rise (but not for

temperature)and tne measured time, a point on the "Velocity"

scale was determined. rinally using the "Velocity" point

and tne "t x 100 point, the diameter '0' in millimeters

was found.

D. Summary of Grain Size Distribution

laving completed the sieve and hydrometer analyses,

a Grain Size Distribution (cumulative curve) was plotted

as in Figure 8 for sample 4283. This plot was made from

the columns "% Finer" and "Sieve Ranze" (minimum sizesieve used) on Form 'A' and columns 'N' and 'D' on Form 'B'.The final form zives the diameter of particles for whichall lesser diameters !orm a :Iven percentare finer by weightof tne total wiewnt. irom thts cumulative distributioncurve the sand, silt and clay percentage (,.I.T. classi-fication) were read and a •rapnic ýiean Size was calculated.Since the diameter scale ir lozaritnmic, conversion ismade to phi units (Folk1 5 ) in calculating the G...i.S.:

Dphl = -log 2 Dmm (7)

where for example; 0 pni = 1 mm, 1 phi = 1/2 mm, 2 phi =

1/4 mm. rrom Folk15 the i.i..S. was calculated as:

D844 + D0- + D169 (8)3 in phi units

where D84% represents the diameter for the 84 tA percentile

on tne cumulative curve aný from a scale converting mm to

pnI units, the zrapnic mean size for sample 283 (refer to

:izure 8) is:

= 3.6 + 6.1-+ 8.9 = 6.1 pli= 0.015 mm.3

-33-

4N I

meL

*-3-

A sediuent name was assigned the sample according to

the scheme given by Folk ' and shown In Flizure 9. -rom

tre grain size distribution curve the percent sand Is com-

pared to the ratio of percent silt to percent clay. For

sample .1283:

A Sands 20%

Silt:Clay = 4-.3:1

and from Figure 9 the sediment name is *sandy slit". Since

the core log did not Indicate any pebtles or shells In the

sample, tnis name is applicable.

Table II with explanation summarizes all the data forthe field and laboratory seldment analyses.

-35-

I • I e e e | | |

9C S M zS Z

sC sMLA CuL ZE

CaMU MUMZD

Z kSILT ZwSILTY

FIGURE 9. Sedment Nomfonctoture ol

-36-

TABLE II: SEDIAEIII *4A.,.PLE DATA AND ANALYSES

Symbol Explanation

No. Station number as shown on Figure 1.See Figure 1 and l1able 1 for co-ordinatelocation.

Date Date sample was collected.Not necessarily the same date as sound speedtaken.

Depth Deptn In incnes Into bottom from which sampletaken.

Inst. Sampler used as illustrated In Figure 3.

VV = Van VeenSC = Square C3rer

C = Corer(cylindrical tube used on squarecorer)

SandSilt Percentages as determined from Figure 8.Clay

Name As determined from Sand, Silt, Clay %and Figure 9.

G.•. .5. araphic mean size in mm x 10"3 (explained intext)

vis ,Aiass of dried solids In grams.

14,.ass of liquids In grams.

B hater content In % (explained in text).

n 'porosity' In % ( explained In text).

-37-

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"'-42

V HESULTS AND DISCUSSION

Specific sound speed and sediment properties for each

station are listed in Tables I and II of the preceeding

sections. In Table I are found the sound speed ratto(H) of

transmission in sediment to transmission In sea water; -ne

signal attenuation ratio (a) and pertinent field da-a as -o

location, description, date measured and depth of penetration.Table II lists the sediment name, graphic mean size, water

content and porosity as well as field and laboratory data

concerning collection and sample analysis. The followin-

is a discussion of these results witn comparisons made to i-e

work of other investigators.

A. Sound Speed versus Sediment Properties

Figure 10 is a plot of the sound speed ratio 'i'

versus porosity 'n' for stations and samples Investiaated Inthis study. The solid line is a 'best fit' curve for the

plotted points. Only those stations (55 In number) atwhich the odor in the sediments was estimated as weak or

absent are plotted in Fizure 10. Approximately 65 4 or

the points lie within or on the two curves labeled: "b=L1

and "b=5", which are exponents in the followina veneral equa-

tion(9) and defining relations (10, 11) after the statistical

analysis of Nafe and Drake3 6 :

Va = n Vza [ 1 + d1 (1-n)] + Vsa[ ds ( 1 -n)bd ] (9)d

wnere V zcomes from:

1 _ n + (1-n] [1 +(4/3)(us/ks)) (10)dI d1 dsVs

and d is:

d = d1 n + ds(l- n)

-L13-

1 O.

I-

.2 .S

I! I

1 2

1/ 01.¥Clid IO •

V1 - speed of sound In liquid - 1.'2 km/sec

VS = speed of sound In solid = 6.00 km/sec

ds = denstty of solids = 2.6s gm/cm3

d = density of sea water - 1.03 5m/cm3

U =/ks a structure factor = 0.60

The above factors, used in equations (9,10,11) result In:,. V Ln+ (l.03n) (l-n) ]+[ 95.; )(1-n) b

= z n (2.6 - 1.62n) 2.6S - 1.6n (- l.62n (12)

V 1 (13z (2.65 - 1.62n)(0.U05,r '+ 0.019)(

Lettln: n = l(liquid only), the bulk sound speed reduces to

the liquid sound speed:

Vza = 2.29= VIa - Va

and leiting n = 0(solids only), the bulk sound speed reduces

to the solid sound speed:

Vz•= 2.00

V2 = 36.00 = Va a

At intermediate porosities, the sound speed is as shown with

a ratio h'' less than unity ove* tne porosity range: 65 % to

100%. This effect has been explained by Off1cer 3 8 and Is

discussed In tie Introduction to this paper.

Figure 11 Is plotted in complete analogy to FI:ure 10

except trhat all tne points represent stations where tne zas

odor was particularly punzent('moderate' to 'stron-' In Table

I). Tie solid line 'best fit' curve falls considerably belowra-rer than Intermediate to the Nafe, Drake3 6 rela:ions. :he

author postulates that since the sound speeds at :nese

stations are low with respect to similar stations where no

odor Is present, tne gas odor represents zases at least par-

tially in a free bubble state. These bubtles are l~kely_45-

"�"I%S

At

%h

9 U

• ~uv I.s o|o vi••°7 . .. - 6

OIkY¥1 Olid$ QNn•C. -*l

-46-

entrained in the soft organic ooze and are being generated

by organic decay in an anerobic environment. the bubbles

act as sound absorbers and effectively attenuate and other-

rise slow the speed of propogation. The effect 13 pronounce 4

over a wide range of porosities in comparison to the non-

gaseous sediments: n from 486 to 100%. For much lower

porosities(35.i or less) compaction effects of crain to zrain

contact outweigh the gas presence and 'n' is 7reater tnan

unity. At In' equal to unity, 'Ii' probably rises to unity

since from density considerations, even in a gas saturated

liquid, tne gas would not appear as free bubbles. Since t•e

gas would be in solution, it would have lilttle sound trans-

nibsion Imnibiting effect.

An attempt was made to relate mean grain size -o ra'to

of sound speeds. The resulting plot Is a scat-er dlazram with

no apparent relationship between the two factors. A.an,gaseous sediments plotted well below the 'I' equal to uni'Y

ordinate and clustered in tne finer grained re .on. "he

lack of correlation is explained by tne unsorted nature o"the sediments, cnaracteristic of -lactal tills and glacialdrift. For these deposits, mean grain size ras little realsignif icance.

ii,.ure 12 is a log-linear plot of 'Ii' versus water

content. Althou~h the scatter Is severe, "or Those samplep

wnich are nonzaseous, a relati.on similar to that ror 'A'

versus 'n' 43 distinguished(soltd line in Piure 12 is best

fit for nonzaseous sediments only). At low water conlent,

tie sound speed approaches tiat of tie sollds sn- a, ;iq

water contents near ]7O0 an' is less than uniy correspon•inz

to the case for porosi~y greater -Ian 6r%.

E. Sound Speed Profiles

The ieavy do-ted lines in kI ure 13 represen, -ne

locations of tne sound speed profiles as plotted in •I ures

14-17. The ordinate is The soun-! speed ra-lo Or' an,' *:

-47-

itss

OUVW ~ aad IWO

+ ++

owl am"" J - ? _ .. . " "_f\g

'I .. '.." /-

ALI

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~ L* I W IJ ' ,use s n.&-

3ftas•. i 'PON

• I • •, •,vd I

FIGURE 13 SOUND SPE•ED PROFILE LOCATIONS-49-

- - -i • • I i I l I I l1 I .l I l l I I [

I

L2

0

o.T zooo YARDS 480,

FIGURE 14. PROFI LE A - A,"•

LI

0

o

LO

-k .P . .. ....

N

YARDS 40

zI . PROFILE A3.9.

0- 4n I-of

C40 W

Q YARDS

FUM. P5 PROMiE a 9,

2

03

a a-

YARDS

F I AM 16. PROPUL C- C

12

200

JYAMMUN rB *FL -

a -scIss so. 1, Ir'an r e lI, varls .rom m'e or' wes'r

sa, ior, on , e pro' tle. ;-oinsr represeno :as ren r ainr,

crosses ar -;aseous s*aior. an-' boxes are s'a*Wons !n

drelizet aress. ,hese pro, lies are rematrkabl'y smoo-n an1! In-

,41cate -,,e ratýer abrup, increase In sobnl speed In passln;

'rot :-e gaseous black mud of tr.e shallow bays to tie as :ree

silts an:, sands o: the dred-ed cit.annels. :Is concept corre-

la-es wlt. -. , :indin s of .14 erton" and ut;les that tie

souni penetration characteristics of shallow, undredzed

bays in Loston narbor are much in' erior to those o" dredzed

channels.

L. (omparison to ctner hork

Jven t iou;h a plot of mean .rain size versus 'I ' forall stations showed no apparent correlation, iV one .roups

?te sound speed results in terms of sediment type, one finds

sound speeds limited to ratier specific numbers witn ralier

small s'anrard deviations. !able III expresses tie sed-Inen' soind speed as determined from average 'n' values and

an avera-e sea water sound speed of 4880 ft/sec. Also

lisred are ie mean and standard deviation in 'hI' and tie

number o: samples represen•An.z _tne sedimenr type, with

paren'teses indicating sediments specific to -.nis study.

;onslcerln. tie rather nig.t standard deviation ,iven for tie

mean 'a' values listed, !able III snows a general azreement

for mean sound speeds of broad sediment types amonQ the

various worxers. All comparisons are made for sediments

free o: ;as.

" "'inal note Is tne "ac- tha- botrh Yul-s56 and Phipps

asnued In -telr Boston 'arbor seismic work tia- -,e Eos-on

Blue Clay -.ad a sound propozation speed equivalon- to -na4

o' sea wa*pr. FTis assumption was actually no- rar in error

as shown by 'able 111. Depths to norizons wi~nin -nis clay

as determined from *ieir travel time curves were probably

in error by less tnan 2s under •nis assumption.

-52-

31: 0 c)

00

cr.)

\40)

00 C-l IDC1

010214

4

:-44

ed02

a4,

os z

mI N)C

0\ 02 (U '-

co -4 4e

Lt. .:rror Analysis and ,.easureren: Consistency

.he precision of any soind speed measurement in tis

study is limited by sparA cable-hydroprcae separation and

tnus by :re relative spacing of tne probes. The author

assumed after repeated use that the probe spacing -ermained

fixed to within 0.15 inches in 24.00 Inches. Assumin., amean sound speed of 4880 feet/second, tnis spacing Indica-es

.tnat time measurements were accurate to 'our microseconds

in 410 microseconds or approximately 14 wnich represents

approximately 50 feet/second in )000 feet/second. Cn tie

oscilloscope 10 microsecond 4elayed *ime base scale, :Ime

could be read easily to two microseconds.

A test of precision at a riven station is represented

in tne 'It value at each of four stations occupied on two

different dates:

Starion Date Depth(inches)

28 7/04/66 7 1.248/22/66 20 1.20

38 ?/04/66 25 0.958/22/66 31 0.92

8? 8/06/66 2? 1.008/12/66 48 1.03

245 7/12/66 10 0.947/16/66 26 0.94

It is noted tha an 'A' value could be repeated to wltnIn

39 of its original value considerinq all tne possible errors

in relocatin. on station and sinkin- tne probes to tie same

horizon.

ihe sea water sound speed was averaged from 104 measure-

ments anc found to be 4880 feet/second with a standard devia-tion of 110 feet/secon,!. .nis discrepancy is explicable witi

respect t3 the area studied. Boston '.'r)or has several

shallow bays that warn considerably compared to deeper

snip's channels. The amount of 3ewa;o and otier debris in

the water b.'tn altPr its temperature antr Its dispersive

character with respect to sound transmission. The entire

harbor also warmed somewhat over the summer during which

this study was conducted. Various amounts of sewage an"

'fresh' water effluent also alter tie :7allnity of tie water

locally. Corisidering the increments of -.. 7 Poot!- 'cd pero-.

increase in temperature and 4.3 feet/second per one tiousaneth

part increase in salinity, it Is not surprisinp that the water

sound speed was variable witnln the limits of 4720 to ,OcO

feet/second over the summer in the Earbor.

As a test of consistency in laboratory procedures

and results, sediment samples from three stations were chosen

on whinn to carry cut complete analyses by two different

labo-atory personnel. Samples 193, 194 and l9• as shown

in lable II have duplicate readlr" for all parameters deter-

mined. Considering the unsorted nature of most samples

collected, the comparisons of graphic mean sizes ani per-centages of sand, silt and clay are within reason. In

the three comparisons, porosity varied by as much as 104

and water content by as much as 100%. The latter is due

mainly to the difficulty in determininz water con-ent on

a sample that is poorly sorted and not fully dtsa-xrevater'.

Estimates of accuracy considering the laboratory tecn-

niques used are as follows:

Sand, Silt, Clay j.1..S. Aater Content Porosity

± 5% +10% + 25+

This variation in percentage of size component does notaffect the c.noice of sediment name. .ean size is not an

appropriate characterization of unsorted materials. Piater

content was not a critical factor in this study an( tie

technique used for its determination was not repeatable

in tne same sarple. Porost~y was calcula~ed Irom accura~elv

de ermined solid~ anc llquliý weIzn~s since comple~e 11sap-xre-

ga~ion inst~red cocple-e dryin o-" solid cor'ponen's.

-56-

VI CG1NCLUSIUJIS Afri _ýC L2N DAIL IONS

7he object of this investi. ation was to relate the

speed o.' sound transmission in marine sediment to oi-ier

pn;sical properties of "ne sediments. Ihis scoal was accom-

plished usin the equipment an,! tec~iniques herein desc~ribed.

Considerin. the unsort.ed and altered conditiIon of tie

sediments exam~ined In Loston -iarbor, tie correla*1on tetw~en

soiund speed and~ sedInmen- properties Is ralner remarkable.

iata ob~ained In %nis s'uly compare favorabtly wirn analo.-ous

work of" o;-Ipr investt;a-ions and results assocla-ed witi

particularly aseous sediments have been explainer. The

reneral character of varta-lIon of sound speed in -ie slirfical

ser~lmýn:_ layers over rhe iarbor 'ýas been described.

I is -e au' ior's opinion t ia* -~ie desig~n of -,^.e

seliiien% sound protb- -ouli be Improved! wit-2 respec- lo s~ab-

iltly and belzer moni-.orin; of dep-ni of penetra~ion. Com-

parison on I- e basis o: p~iyst:;al proper'.les would probably

be miic-. Ilprovel if care were t~aken -o select samples 'rom

exactly tie deplti a-. wrici tie sound speed Is measured.

I! a ii-h ener-y, controlled-outpilt sound source were

used, transrnissiion t..ro, ni aseous .3edimpnts would befacilitated. If, In alditiori, a quantitive es~imate oftie free .as could be mrale, tits coul,ý be correlaeýP i-o tie

SOUnd st.-nal ampli~tule attenuation.

E IF 0 IC: hAF .-iY

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-59-

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