49 ACOUSTIC ENERGY LOSSES ON REFLECTION FROM

O C E A N O L O G Y NO 11 (1979)
49
MAŁGORZATA BRZOZOWSKA
BARBARA JANUSZEW SKA
Polish A cadem y of Sciences
Institute of Oceanology — Sopot
ACOUSTIC ENERGY LOSSES ON REFLECTION FROM SELECTED
FLOORS OF THE SOUTH BALTIC
C o n t e n t s : 1. Introduction, 2. Theoretical assum ptions and method of m easure­
m ents, 3. Experim ental techniques, 4. R esults of m easurem ents and their discussion,
5. Conclusions; Streszczenie; R eferences.
1. INTRODUCTION
/
In th e p ropagation of sound in a shallow sea, such as th e B altic,
a p a ra m o u n t role is played by th e acoustic p ro p erties of th e sea floor.
A couple of p a ra m ete rs, such as p ressu re reflec tiv ity , reflection losses,
sc atterin g factor, and sc atterin g pow er a re u su a lly em ployed in th e q u a n ti­
tativ e description of th e reflectiv e and diffusive p ro p erties of th e sea floor.
T he m ag n itu d e of reflec tiv itie s fo r n orm al incidence of th e acoustic w ave
on th e sea floor d eterm in es th e m ax im u m losses of en erg y to be a n tic ip a ­
ted in th e reflectio n of sound.
T he purpose of th is stu d y w as to find th e rela tio n sh ip b etw een th e
acoustic re fle c tiv ity on th e sea bed and th e freq u en cy of incid en t acoustic
w aves, angle of incidence, and ty p e of sedim ents. T he m easu rem en ts w ere
conducted a t frequencies from 2 to 10 kH z (at te rtia ry in te rv als, i.e. for
frequencies of 2, 2.5, 3.15, 4, 5, 6.3, 8, and 10 kHz) fo r th re e ty p es of sea
bed: fine sand of 0.145-mm m edian, v e ry fin e sand of 0.120-mm m edian,
and soil consisting m ain ly of m acrofauna (shells of m olluscs and snails).
2. THEORETICAL ASSUM PTIONS AND METHOD OF MEASUREMENTS
The e x p e rim e n ts w e re conducted by th e m ethod of stan d in g w aves,
w hich yields th e m ost a ccu rate resu lts [9, 10], T he m ethod consists in th e
m easu rem en t of a re s u lta n t acoustic p ressu re a t th e m inim a and m ax im a
of a stan d in g w ave th a t re su lts from th e in te rfe re n c e of tw o acoustic
signals, th a t d ire c tly from a tra n s m itte r and th e b ed -reflected one (Fig. 1).
Fig. 1. Configuration of transm itter (N) and receiver (0) for the incidence angle of an
acoustic Wave on the bottom ¡9 ^ 0°
n
r:
hi
hs
r
— d is ta n c e
— d is ta n c e
— d is ta n c e
— d is ta n c e
— d is ta n c e
tr a v e lle d b y in c id e n t a c o u s tic s ig n a l
tr a v e lle d b y r e f le c te d a c o u s tic s ig n a l
f ro m b e d to r e c e iv e r
f ro m b e d to t r a n s m it te r
b e tw e e n r e c e iv e r a n d tr a n s m i t t e r (in h o r iz o n ta l p la n e )
Rye. 1. U sytuow anie nadajnika (N) i odbiornika (0), gdy kąt padania fali akustycz­
nej na dno &
0°
ri
r;
hi
hi
r
—
—
—
—
—
d ro g a p r z e b y ta p rz e z s y g n a ł a k u s ty c z n y b e z p o ś re d n i
d ro g a p r z e b y ta p rz e z s y g n a ł a k s u ty c z n y o d b ity od d n a
o d le g ło ść o d d n a do o d b io r n ik a
o d le g ło ść o d d n a d o n a d a j n ik a
o d le g ło ść p o m ię d z y n a d a jn ik ie m i o d b io r n ik ie m (w p ła s z c z y ź n ie p o z io m e j)
F or a point source of sound, th e p ressu re of th e acoustic w ave tra v e llin g
d ire c tly from a tra n s m itte r to 'a receiv er has th e form :
P i = P o /ri exp (— ikrj)
(l )
in w hich:
Po — acoustic p ressu re a t a u n it d istance from th e tra n sm itte r,
ri — distance th e acoustic signal tra v e ls d ire c tly from tra n s m itte r to
receiv er (Fig. 1),
k = 2n/\ — acoustic w ave nu m b er,
w hile th e p ressu re of th e b ed -re fle cte d w ave is given by
P s = Po/r 2 • exp (— ik r2)
(2)
in w hich:
r 2 — distance tra v e lle d b y th e acoustic signal from tra n s m itte r to
receiver, a fte r reflectio n from th e sea bed.
T he p ressu re re fle c tiv ity is given b y th e follow ing form ula:
V = V0 • exp (icp)
(3)
in w hich:
V 0 — absolute v a lu e of reflectiv ity ,
cp — phase of reflectiv ity .
T he re s u lta n t acoustic p re ssu re of th e point of reception equals
P = Ps + Pi
(4)
A fte r tak in g in to account Eqs. (1, 2) and (3) one o b tains th e follow ing re ­
latio n sh ip for absolute p ressu re:
,=
|p h
f
(^
+
1
-2
V 2o
,
2 p 20 V o
r 2|
cos [k (r2 — ri) — tp]
n1 Xr22
*
)
( 4)
T his p ressu re w ill be m ax im u m if cos [k (r2 — ri) — <p] = + 1 and m in i­
m um if cos [k (r2 — ri) — cp] = — 1. U pon s u b stitu tin g consecutive cosine
values of + 1 and — 1 in Eq. (4) and solving th e re su ltin g system of tw o
eq uations (w ith resp ect to VQ) one obtains
P m a x _______ P m in
TT
rim in
v °=
rim a x
( 5)
--------------- -------
Pmax
|
Pmin
r2min
r2max
in w hich: r ij2 m ax and ri ,2 m in = d istances ri and r2, determ in ed above,
a n d counted to th e receiv er in th e phases of stan d in g -w av e m ax im u m
an d m inim um , respectively.
g
mu I
I
e
cwo
*<>
Fig. 2. Schem atic diagram of transm itting, receiving, and recording configuration
G
T
N
O
W, F
M
—
—
—
—
—
—
p o w e r g e n e r a to r , ty p e PO-21 (K A B ID -Z O P A N )
tr a n s f o r m e r
t r a n s m i t t e r : c y lin d r ic a l p ie z o c e ra m ic tr a n s d u c e r
r e c e iv e r : 8101 B r u e l- K ja e r h y d r o p h o n e
a m p lifie r a n d s e t o f f ilte r s : 2120 B r u e l- K ja e r a n a ly z e r
m a g n e tic v o lta g e r e c o r d e r , B r u e l- K ja e r 7003
Rye. 2. Schem at układu nadaw czo-odbiorczo-rejestrującego
G
T
N
O
W, F
M
—
—
—
—
—
—
g e n e r a to r m o c y ty p P O -2l (K A B ID -Z O P A N )
t r a n s f o r m a to r
n a d a jn ik : c y lin d r y c z n y p r z e tw o r n ik p ie z o c e ra m ic z n y
o d b io r n ik : h y d r o f o n 8101 (B rU e l-K ja e r)
w z m a c n ia c z z z e s ta w e m f iltr ó w : a n a liz a to r 2120 (B rU e l-K ja e r)
r e j e s t r a t o r m a g n e ty c z n y n a p ię c ia 7003 (B riie l-K ja e r)
W here th e distance tra v e lle d b y an acoustic signal from th e tra n s ­
m itte r to receiver, is m uch g re a te r th a n th e acoustic w a v e -le n g th (and
co n seq u en tly — th a n th e d istance b e tw e en a d ja c e n t stan d in g -w av e m in i­
m um and m axim um ) one can assum e ri min ~ ri max a n d r 2 min ~ r 2 max.
A ccordingly, th e u ltim a te fo rm u łą for th e absolute v a lu e of re fle c tiv ity
reads:
-y
_ _ 1*2
Fl
Pm ax
P m in
P m a x “ I- P m in
H ence, th is absolute v a lu e m ay be found for know n ri and ri (from th e
geom etrical con fig u ratio n of ex p e rim e n tal environm ent) an d for acoustic
p ressu re m easu red a t th e nodes and antinodes of stan d in g w aves.
3. EXPERIMENTAL TECHNIQUES
The block diagram of th e m easu rin g system is show n in Fig. 2. The
tra n sm ittin g com ponents consisted of a PO 21-type p o w er gen erato r,
and a piezoceram ic tra n sd u c e r w ith a conical shield w ith 45° angle of
a p e rtu re . T he shield w as n ecessary to elim in ate in te rfe re n c e d u e to scat­
te rin g of sound off th e u n d u late d sea surface. T he recep tio n com ponents
included a hydro p h o n e (type 8100), am p lifier w ith a set of filte rs (type
2120), and m easu rin g tap e reco rd er (type 7003), all in stru m e n ts m an u ­
fac tu re d by B riiel K ja e r. A special s tru c tu re (Fig. 3) to m ove th e h y d ro ­
phone along th e v e rtica l axis w as designed to d e te rm in e th e m ax im a and
m inim a of stan d in g w aves. T he positions of th e hydro p h o n e w ere found
from indications of a co unter, w hich recorded th e n u m b er of rev olutions
of a th read ed rod, along w hich th e m otion occurred. T he accuracy of re ­
ceiver se ttin g a t th e e x tre m u m of th e stan d in g w ave w as 3.5-m m (pitch
of rod th read ), w hich w as su fficien t fo r th e w av elen g th e n co u n tered (75...
15 cm). T he su p p o rtin g s tru c tu re of th e hydro p h o n e w as set firm ly on th e
bed, w hile th e tra n s m itte r w as low ered on a jib to a d ep th of 1.5... 2 m
below th e w a te r surface. A fte r th e au to m atic sw itching-on of th e h y d ro ­
phone (this being controlled by an electric m otor), th e deflection of a
po in ter, corresponding to changes in acoustic p ressu re, w as observed on
the m easuring am p lifier scale. T he hydro p h o n e w as stopped fo r m axim um
and m inim um p o in ter deflections, and a 100-second m agnetic recording
taken. F o r each fre q u e n c y th e m easu rem en ts w e re rep e a te d fo r several
consecutive m inim a and m axim a. T he v a lu e obtain ed from th e 100-tsecond
averaging w as accepted as th e p ressu re fo r stan d in g -w av e m inim um and/
or m axim um . T his p ro ced u re p e rm itte d random in te rfe re n c e and flu c tu ­
ations of th e acoustic signal to be avoided a t th e p oint of reception. T he
averaging w as accom plished a u to m atically w ith an in te g ra l system b u ilt
in a B riiel K ja e r 2120 analyzer.
T he hydroacoustic m easu rem en ts w ere accom panied by sam pling of
bed surface sedim ents, th e la tte r hav in g been su b ject to g rain size analysis.
T he an aly sis w as perfo rm ed on a set of 13 sieves bounded by 5 an d 0.063
m eshes.
Fig. 3. Schem atic diagram of hydrophone structure
1 — h y d r o p h o n e c la m p
2 — e le c tr ic m o to r in w a te r p r o o f c a s in g
Rye. 3. Schem at konstrukcji nośnej hydrofonu
1 — u c h w y t d o h y d r o fo n u
2 — s iln ik e le k tr y c z n y w o b u d o w ie w o d o s z c z e ln e j
4. RESULTS OF MEASUREMENTS AND THEIR DISCUSSION
\
H ydroacoustic m ea su rem e n ts w ere conducted for th re e types of sea
bed. The firs t ty p e consisted of m acro fau n a an d v e ry sm all a d m ix tu re s
of fine, as w ell as m edium sand. T he m olluscs M ytilus edulis a n d Cardium
Lcimarcki a n d tin y snails 2... 30 m m in size dom inated in th e species com­
position. The second ty p e of soil w as fine sand w ith a m ax im u m sh are
of g rain s of ab o u t 0.16... 0.10 m m , th e m edian being 0.145 m m . The
T ra sk so rtin g fa c to r [7, 8] fo r th ese sedim ents w as fa irly high: 1.75. The
th ir d ty p e of bed w as c h a ra c te riz e d by th e occurrence of tw o d istin ct
lay ers. T he u p p e r lay er, ab o u t 0.2*m thick, of v e ry high w a te r content,
consisted of fine a n d m ed iu m sands. The m edian of th is la y e r w as 0.195 m m
w hile th e T ra sk so rtin g fac to r rea c h e d 1.70. T he o th er la y e r included
h ighly consolidated fine a n d v e ry fine sands. T h eir m ed ian w as 0.120 m m ,
a n d th e so rtin g fac to r w as 1.37. T he in te g ra l curves obtained fro m the
gi'ain size an aly sis of th e soil sam ples a re show n in Fig. 4.
Fig. 4. Integral curves of grain size
-----------------
m e a s u r in g p o in t 2: fin e s a n d
m e a s u rin g p o in t 3: la y e r I (U p p e r), f in e s a n d
--------- -------m e a s u rin g p o in t 3: la y e r II (lo w e r), f in e a n d v e r y f in e s a n d
Rye. 4. W ykresy uziarnienia gruntu (krzywe kum ulacyjne)
-----------------------------------------------
d la p u n k t u p o m ia ro w e g o 2: p ia s e k d r o b n o z ia r n is ty
¿ la p u n k t u p o m ia ro w e g o n r 3: w a r s tw a I (g ó rn a ), p ia s e k d r o b n o z ia rn is ty
d la p u n k tu p o m ia ro w e g o n r 3: w a r s tw a II (d o ln a ), p ia s e k d r o b n o - i b a rd z o
d r o b n o z ia rn is ty
T he ab so lu te values of th e reflec tiv itie s for n orm al incidence of
acoustic w aves a t th e sea bed, av erag ed over freq u en cies of 2 to 10 kHz,
fo r th e th re e ty p es of soil, w ere as follows:
i
m acro fau n a
V0 = 0.59
fine sand
(Md = 0.145 m m )
VQ= 0.55
v e ry fin e sand (Md = 0.120 m m )
V 0 = 0.61.
The re su lts o b tained ag ree w ith th e fin d in g s for o th er w a te r bodies
u n d e r sim ilar conditions and e x p erim en tal technology [2, 5, 10].
F o r instance, o u r d a ta fall into th e in te rv al of v a ria b ility of reflectiv ities
given by L ib e rm an n [5], w ho obtained 0.50... 0.60 for a stony bottom ,
0.40... 0.86 for sand, and 0.40... 0.60 for soil consisting of sand and clay.
T he d iam eters of th e p a rticles of th e stony floor studied by L ib erm an n
coincide w ell w ith th e size of ou r m acrofauna, so th a t th e reflectiv ities
in both cases w ere v e ry sim ilar. M oreover, it should be concluded th a t
th is coincidence w as fo rtu ito u s, as th e acoustic p ro p erties of th e sea floor
a re d e te rm in e d not only by th e size of bottom sedirpents. O u r figures are
lo w er th a n th e data given by G ru b n ik [2] for th e C aspian Sea, w ith m ean
ab so lu te v alu es of reflec tiv itie s of 0.76 and 0.68 for sand and silty sand,
respectively. M ackenzie [6] also detected reflection losses of 6 ... 8 dB for
v e ry fine sand, w hich correspond to an absolute m ag n itu d e of reflec tiv ity
of about 0.5... 0.4.
The dispersion of d ata in d icated by th e above re su lts o b tained by
vario u s a u th o rs can be due to b ottom irre g u la ritie s, d iversified in te rn a l
stru c tu re , a n d r a th e r in ac c u ra te n o m en c latu re of bed sedim ents (since
d iffe ie n t c rite ria of th e classification of sedim ents a re in use, it is necessary
to specify th e d istrib u tio n of resp ectiv e sed im en t fractions, to g e th e r w ith
th e ty p e of soil, e.g. sand or clay).
T he absolute values of re fle c tiv ity found in th is stu d y are show n in
Fig. 5 as a fu n ctio n of th e fre q u e n c y of th e acoustic signal. No c le a r dé­
pendance of th e re fle c tiv ity on th e len g th of th e acoustic w ave w as found
in th e stu d ie d in te rv a l of freq u en cies from 2 to 10 kHz, a t te r tia ry spacing.
S im ilar re s u lts w ere obtained by G ru b n ik [2], see Fig. 6. H ow ever, a fte r
th e re s u lts of ou r ex p e rim e n ts w ere obtained, a stu d y by G oncharenko et
al. [4] ap p eared , in w hich th e above rela tio n sh ip w as exposed for tw c lay e r bottom . T h eir findings conform rea so n a b ly w ith B re k h o v sk ik h ’s
co m putations fo r a m u ltila y e r bed. T he function of absolute reflec tiv ity
v e rsu s fre q u e n c y is illu s tra te d in Fig. 7. T he function VQ(f) is oscillatory,
its period changing w ith th e incidence angle of acoustic w aves a t th e sea
floor (w ith th e exception of th e case C2 C0, in w hich C2 and CQ are
speeds of sound in th e u p p e r floor la y e r a n d in w a te r, respectively). The
freq u en cy relatio n sh ip s obtained for vario u s angles of incidence can be
used in com putations of the ch a ra c te ristic s of respective bottom layers.
T he lack of a freq u en cy fu n ctio n for th e absolute reflec tiv ity , in our
Fig. 5. A bsolute valu e of reflecitivity, V0, versus frequency of acoustic w ave f(# = 0°)
o d a ta f o r m a c r o fa u n a
x d a ta f o r f in e s a n d
A d a ta f o r v e r y f in e s a n d
Rye. 5. Zależność m odułu w spółczynnika odbicia (V0) od częstotliw ości fa li akusty­
cznej f (# — 0°)
o d a n e d la m a k r o f a u n y
x d a n e d la p ia s k u d r o b n o z ia rn is te g o
A d a n e d la p ia s k u b a rd z o d ro b n o z ia rn is te g o
0 ___ 1___ I___ 1____I___ 1___ I___ 1___ I--------1--------L
2
U
6
8
10
/[k H z ]
Fig. 6. Experim ental absolute valu es of sea-floor reflectivity, V0, versus frequency.
D ifferent notations correspond to various m easuring points. According to Grubnik
[2]
Rye. 6. Eksperym entalne w artości m odułu w spółczynnika odbicia V0 od piaszczyste­
go dna w funkcji częstości. Poszczególne oznaczenia odpowiadają różnym punktom
pomiarowym. Wg pracy N.A. Grubnik [2]
Vo
f[H z]
Fig. 7. A bsolute value of reflectivity versus frequency for a tw o-layer m odel of the
sea floor (the case Ca
C0, in w hich C 2 , Cc — speeds of sound in the upper layer
and ir. w ater, respectively). A ccording to Goncharenko, Zakharov, Ivanov, K irshov
[3]
O
--------
e x p e r im e n ta l d a ta
t h e o r e tic a l c u r v e
Rye. 7. Zależność częstotliw ościow a m odułu w spółczynnika odbicia dla dw uw arstw o­
w ego modelu dna (przypadek gdy C2 <^C0; gdzie Ca, C0 — prędkości dźw ięku od­
powiednio w powierzchniowej w arstw ie dna i w wodzie). Wg pracy B. I. G onczarienko, L.N. Zacharów, W.E. Iwanow, W.A. Kirszow [3]
O
--------
d a n e e k s p e r y m e n ta ln e
k rz y w a te o r e ty c z n a
Fig. 8. Experim ental absolute valu es of reflectivity for very fine sand, versus
frequency of acoustic w ave, at tw o angles of incidence
A
d a ta o b ta in e d f o r Si = 0°
d a ta o b ta in e d f o r
= 25°
Rye. 8. Eksperym entalne w artości m odułu w spółczynnika odbicia od piasku bardzo
drobnoziarnistego w funkcji częstotliw ości fali akustycznej dla dwóch kątów pa­
dania
A
d a n e o tr z y m a n e p r z y fti = 0°
j^
d a n e o tr z y m a n e p r z y fit = 25°
resu lts, m ig h t h av e been due to too w ide in crem en ts in th e freq u en cy
scale.
T he v a ria tio n of re fle c tiv ity fo r d iffe re n t angles of incidence of
acoustic w aves a t th e bottom is illu stra te d in Figs. 8 and 9 by th e exam ­
ples of tw o angles selected for b oth m acro fau n a an d v e ry fin e sand, i.e.
Oj = 0°, 02 — 15°, and i>i = 0°,
= 25°, respectively. T he absolute r e ­
flectivities, av erag ed over th e w hole ran g e of freq u en cies fo r th e fo u r
angles are:
m ac ro fa u n a
v e ry fine sand
=
d2=
0i =
=
0°
15°
0°
25°
V0 =
V0 =
V0 =
V0 =
0.59
0.80
0.61
0.57
A ccording to th e g e n e ra l th e o re tic a l rela tio n sh ip [2, 9], th e data obtained
in th is s tu d y p o in t to th e g ro w th of re fle c tiv ity w ith th e increasing an g le
of incidence of th e acoustic w ave. F ro m Figs. 8 a n d 9 it ap p ears th a t th e
above rela tio n sh ip is satisfied in th e w hole ran g e of freq u en cies fo r th e
m acro fau n a soil and in th e 2... 6.3 kH z frequencies for v e ry fin e sand.
Fig. 9. Experim ental absolute values of reflectivity from a sea floor consisting of
m acrofauna (molluscs, snails), versus frequency of acoustic w ave, for tw o angles
of incidence
O
•
d a ta o b ta in e d f o r fh = 0°
d a ta o b ta in e d f o r dt = 15°
Rye. 9. Eksperym entalne w artości m odułu w spółczynnika odbicia od powierzchni
dna utworzonej z m akrofauny (małże, ślimaki) w funkcji częstości fa li akustycznej
dla dwóch kątów padania
O
d a n e o tr z y m a n e p r z y di = 0°
•
d a n e o tr z y m a n e p r z y 9« = 15°
T he re su lts fo r freq u en cies of 8 and 10 kH z for sand differ from th e
rem a in in g findings. To fin d w h e th e r th is deviation is due to a m ore com ­
p lex rela tio n sh ip b etw een re fle c tiv ity and th e angle of incidence, coupled
w ith selective absorption in a tw o -la y e r bed, or is caused by e rro rs in
m easu rem en ts, is im possible because of th e lim ited n u m b er of e x p e ri­
m en ta l data.
T he m ea su rem e n t of re fle c tiv ity by th e m ethod of stan d in g w aves
is one of th e m ost a ccu rate techniques. T he e rro rs caused by m u ltip le
reflections from a free su rface and th e sea floor have been estim ated
elsew h ere [3]. In ou r e x p e rim e n ts th is ty p e of e rro r does not exceed 5 p e r
cent. T he d e te rm in a tio n of th e e x tre m a of stan d in g w aves is also easy,
as show n in Fig. 10 a, b, c, d; th e flu c tu atio n s of th e signal received do
not exceed 0.1V, w h ile th e differen ce b etw een stan d in g -w av e m ax im a
and m inim a reaches 0.4... 0.5 V. Hence, th e accuracy of d e te rm in a tio n of
acoustic p ressu re a t th e m axim a arid m inim a of th e field of in te rfe re n c e
is basically eq u iv alen t to th e accuracy of voltage rea d -o u t on a v o ltm e te r
scale
5. CONCLUSIONS
1. T he fig u res obtained fo r th e sea-floor reflec tiv ity in th e south
B altic a re com parable w ith d a ta for o th er w a te r bodies and a re contained
w ith in th e lim its given by o th er a u th o rs for sim ilar ty p es of sea floor.
2. In th e case of n o rm al incidence of acoustic w aves th e av erage
absolute v alu es of re fle c tiv ity a re 0.55 for fine sand, 0.61 for v e ry fine
sand, a n d 0.59 fo r m acrofauna.
3. No d istin ct rela tio n sh ip w as found b etw een th e absolute v a lu e of
re fle c tiv ity and th e fre q u e n c y of acoustic w aves in frequencies of 2...
10 kHz.
4. It w as found th a t re fle c tiv ity increases w ith th e increasing inci­
dence an g le of acoustic w aves on a fla t u n stra tifie d bed, w hich confirm s
th e ex istin g th e o ry [2,9], In th e case of a m u ltila y e r bed th is d ep en ­
dence is m uch m ore com plex an d is controlled b y ch a ra c te ristic s of indi­
v id u al lay e rs a n d acoustic w a v e -le n g th [1,4],
5. In o rd e r to an aly se th e e x p e rim e n tal reflectiv ities fo r vario u s sea
floors c o rre c tly not only is a know ledge of th e u p p e r la y e r of th e floor
im p o rta n t, b u t also th e in te rn a l s tru c tu re of th e bed, especially if acoustic
signals of high in te n sity are em ployed.
fe
Fig. 10. Exam ples of recorded interference fields, obtained w hen the hydrophone
m oved along the z-axis (vertical axis oriented upwards from the bottom). The
drawings correspond to various frequencies of acoustic w aves
Rye. 10. Przykłady rejestracji pola interferencyjnego otrzymane podczas ruchu h y ­
droforni w zdłuż osi z (tj. osi pionow ej, skierow anej ku swobodnej powierzchni m o­
rza). Poszczególne rysunki odpowiadają różnym częstotliw ościom fa li akustycznej
5 — O c e a n o lo g ia N r 11
62
O C E A N O L O G IA N r 11 (1979)
MAŁGORZATA BRZOZOWSKA
BARBARA JANUSZEW SKA
Polska A kadem ia Nauk
Zakład Oceanologii — Sopot
STRA TY EN ER G II A K U ST Y C Z N E J PR ZY ODBICIU OD W YBRANYCH
RO D ZA JÓ W DNA PO ŁU D N IO W EG O BA ŁTY K U
STRESZCZENIE
Przedstawiono w yniki pom iarów w spółczynnika odbicia fal akustycznych od
dna Bałtyku dla trzech rodzajów m ateriału osadowego i trzech kątów padania w
zakresie częstości 2—10 kHz (w odstępach tercjowych). Dla piasku drobnoziarnistego
o średniej średnicy ziaren 0,145 m m w spółczynnik odbicia od dna, uśredniony w ca­
łym zakresie częstości, w ynosi 0,55, dla piasku o średnicy 0,12 mm—0,61 mm i dla m a­
teriału złożonego w przeważającej części z m akrofauny (M ytilus edulis, Cardium
lamarcki, Macoma balthica) — 0,59.
Przytoczone w artości określają m aksym alne straty energii akustycznej otrzymane
przy prostopadłym padaniu fali na dno. W zakresie częstości 2— 10 kHz nie zaobser­
w ow ano w yraźnej zależności częstotliw ościow ej. U zyskane w artości w spółczynnika
odbicia zbliżone są do rezultatów otrzym anych przez Grubnika [2] dla Morza K aspij­
skiego.
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