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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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-22-
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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
0 ~ ~ ~ * nou b0ofojw o ;op,ow ',. popo" MA SASE r owfts, . s (4 a
~ 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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