Acoustic and other physical properties of deep-sea sediments in the Tyrrhenian Abyssal Plain

MU k

Geology - Elsevier Publishing Company, Amsterdam

ACOUSTIC MENTS

AND

OTHER

IN THE

A. KERMABONl,

PHYSICAL

TYRRHENIAN C. GEHIN,

- Printed in The Netherlands

PROPERTIES

ABYSSAL

P. BLAVIER

AND

OF DEEP-SEA

SEDI-

PLAIN

B. TONARELLI

Suclant A.S. W. Research Centre, La Spezia (Italy) (Received May 6, 1968) (Resubmitted October 19, 1968)

SUMMARY

The Tyrrhenian

Abyssal

Plain, lying within an enclosed

basin 70 miles west

of Naples (Fig. l), was chosen as a model zone for sub-bottom sound propagation experiments by the Saclant A.S.W. Research Centre. High-density coring was performed in this zone along a IO-mile line, located between positions 40 “20’N 12 “45/E, and 40 “10’N 12 “50’E (Fig.2). Twenty-one cores taken along the profile were afterwards analyzed for acoustic and mass physical properties. The results of these analyses are given in this report. The correlation between echo soundings and some of the physical properties of the cores was studied. It was found

that the horizontal

sub-bottom

layering

observed

on P.G.R.

(precision graphic recorder) records corresponds roughly to the interbedding found in cores. However, correlation studies on cores taken close together prove the presence of microtectonics which could not be observed on P.G.R. records. The micromorphology of the area studied proved the zone could be separated into two areas, north and south. The northern area presents a rather flat microbathymetry in comparison with the rather complicated bottom configuration of the southern

area. The bottom

morphology

seems to be well related to the tectonics

of sub-bottom layers. Sound velocity and porosity measurements on cores seem to be poorly correlated. The scatter is large, especially for sand-size sediments. Electrical resistivity measurements relation found between this parameter different report.

were also performed on cores. The corand the porosity will be described in a

METHOD OF SAMPLING

The coring

operations

1 Present address: Compagnie

were performed

Maritime d’Expertises,

with a large-diameter

“Sphincter”

Marseilles (France). Marine Geol., 7 (1969) 129-145

A. KERMABON

et al.

Fig.1. Topographic chart of the Tyrrhenian Abyssal Plain with SOO-fathom contour intervals with geographic position of the model zone. (From maps of Woods Hole Oceanographic Institution, Lamont Geological Observatory, and Saclant A.S.W. Research Centre).

corer,

with split piston,

core lengths

obtained

developed for this purpose by KERMABON et al. (1966). The were from 3 to 10 m. The cores were secured in a P.V.C.

liner made watertight at both ends by means of rubber caps sealed with paraffin. The cores were analyzed after the proper temperature stabilization was reached in the laboratory. In order to keep the possible

disturbances on the cores as constant as possible, the coring characteristics, i.e., the total weight of the corer and the drop, were kept identical for all coring operations. Loran-C was used as a navigational system. Twenty-one long cores were sampled along the IO-mile profile. THE ACOUSTIC AND PHYSICAL CHARACTERISTICS MEASURED ON CORES

The method of analyzing the cores at the NATO Saclant A.S.W. is fully described in KERMABON and BLAVIER (1967; see also (I) Longitudinal sound velocity is measured perpendicular to the of the core on an 85 mm base, through holes drilled through the P.V.C. sound velocity is calculated from the time separating the emission and the of predetermined levels of a 70 kHz pulse sent and received through the by two piezo-electric crystals. Centre

Research Fig.3-8). main axis liner. The reception sediments

Marine Geol., 7 (1969) 129-145

131

DEEP-SEASEDIMENTSIN THE TYRRHENIANABYSSALPLAIN

$_ ;

‘!IgIIlJ__

I

I’!

c

‘~~‘~:~lr-,~,~~~,,,:!,,~.~,~~~~~’ IO

Fig.2. Microtopography are purely arbitrary).

of the Naples zone, showing coring stations (the core numbers

The accuracy of sound-velocity

measurements

can reach lx0 in clay structures,

but be as low as 4x0 in sandy layers. As sound-velocity measurements are made at laboratory pressure and temperature (22-27 “C), a correction factor has to be added to this result to obtain the in situ sound velocity of the sediment at bottom pressure and temperature. This correction

is calculated

from the Wilson

formula

(WILSON, 1960): it is

the difference between the speed of sound in sea water at bottom temperature, pressure and salinity, and the speed of sound in sea water at bottom salinity but at laboratory pressure and temperature. The corrections relative to temperature and pressure for sound velocity in the sediments are the same as those for bottom sea water, as shown by HAMILTON(1963) in laboratory versus in situ tests (from the bathyscaph “Trieste”). (2) The porosity is the ratio of the volume of water to the total volume of wet sediment. The volume of water evaporated is measured by weighing the sample Marine Geol., 7

(1969) 129-145

132

A. KERMABONet al.

before

and after dessication.

The total

by means of a helium pycnometer.

wet volume

of the sediment

is obtained

The results are given with an estimated

accuracy

of 1%. (3) The water content is the weight of water expressed as a percentage of the weight of dry material. The accuracy of this measurement depends on the drying process.

It was found

that keeping

the samples

in the oven for 48 h at 105”C, the

water evaporated irrespective of the place where oven. The water content is given with an accuracy (4) The wet or mass unit weight is the weight mass saturated to 100% with interstitial water. It

the sample was treated in the of 2x,. per unit volume of the sediment is assumed that the saturation

level of the sample is very close to 100% when analyzed. The wet unit weight is given with a maximum error of 1%. (5) The void ratio is the ratio between the volume of the voids and that of the solid particles. This measurement involves the use of a helium pycnometer, the accuracy of the measurement is estimated to be made with a relative error of 7%. (6) The density of solid particles is the ratio between the weight and volume of a dried sample. This value is given with a maximum error of 6%. (7) The electrical formation factor F.If R, is the bulk electrical resistivity of a sediment saturated with its interstitial water, and R, the electrical resistivity of the interstitial water, then ARCHIE (1942) has defined the formation factor F by the following relationship:

R, = FR, It has been shown experimentally the sediment.

that

F can be related to the porosity

of

The measurement of electrical resistivity is, or can be, a fast and accurate way of establishing the value of the porosity of unconsolidated sediments. The correlation of F with the physical characteristics of the sediments will not be treated

in this report.

SEDIMENTS As already shown by RYAN et al. (1965), the cores of the Tyrrhenian Abyssal Plain present layers of two textural types: (1) Brown, gray and tan clay layers from a few millimetres to 3 m thick. The compactness of these layers varies considerably from the top to the bottom of the core. (2) Gray sand and silt layers from 1 cm to 1 m thick. The coarse layers show a sharp contrast with the clay layers between which they lay. The strong grading of mineral size in the coarse layers is indicative of a Marine

Geol., 7 (1969) 129-145

DEEP-SEA

SEDIMENTS

IN THE TYRRHENIAN

ABYSSAL

133

PLAIN

Fig.3. Physical parameters versus distance; the corresponding underneath the figure.

El

clay

q

~

q

SiR

q

section of the core is shown

ctrg&M

Fig.4. Physical parameters versus distance: the corresponding underneath the figure.

section of the core is shown

Marine Geol., 7 (1969) 129-145

134

A. KERMABON et al

El f-Y

q -d

I=

qDpy*L

Fig.5 Physical parameters versus distance; the corresponding underneath the figure.

q Fig.6.

mderneath

Phvical

Clay

m

Sand

q

silt

q

O,parJc

section of the core is shown

d&,,,

Parameters versus distance; the corresponding

section of the core is shown

the figure. Marine Geol., 7 (1969) 129-145

135

;S WET DENSITY

,

g/cm3

90 8o Wl?oSlM 7o % 60

wet volume

50

m/s 1450:

0

* 100 200 --cm-D

360

400

500

6OU

700

600

900

1000

Fig.-/. Physical parameters versus distance: the corresponding underneath the figure.

section of the core is shown

Fig.8. Physical parameters versus distance; the corresponding underneath the figure.

section of the core is shown

Marine Geol., 7 (1969) 129-145

136

A. KERMABON

fast sedimentation

process, most probably

ash deposition. Fig.34 show for each individual

caused by turbidity core the physical

currents

properties

et al.

or volcanic listed above

versus

the depth in the core. In certain layers some measurements are missing because in some of the coarse-grain layers, sound-velocity measurements were difficult to perform. From inspection

of these cores it is evident

that a strong change of porosity

occurs within the top 100 cm of all cores. This is due to the common presence of a series of sand layers within this depth range. Layers deeper than 100 cm are shown by radical changes of porosity or sound velocity at the interface between turbidites-clay. The gradual change of grain size in the turbidites is clearly reflected by the corresponding change of porosity within these layers. It can be seen that all the clay-size layers have a velocity of sound lower than that of the bottom water. In the coarse layers, usually represented by sand layers, the velocity of sound can be as much as 10% higher than sound speeds in the clays. In all cores (especially in the clay layers), there is a general decrease of porosity with depth (Fig.3-8). The clay sections, below the coarse turbidites, often have a high porosity in comparison with the environment. This phenomenon has also been noticed and confirmed by deep-sea electrical probing. No explanation can be given for this fact which is not matched by an equivalent change of sound speed. BOTTOM TOPOGRAPHY

AND ECHO SOUNDINGS

The bottom topography of the zone has been carefully studied from P.G.R. records. A tentative bathymetric chart is shown in Fig.2. The microtopography seems to be well related to sub-bottom microstratigraphy, a flat layer on the echo-sounder

Fig.9.

corresponding,

Echograms

often, to a flat bottom.

as per sections

of Fig.2

(sections

AA’ and BB’). Marine Geol., 7 (1969) 129-145

DEEP-SEA

SEDIMENTS

IN THE TYRRHENIAN

Fig.10. Echograms

ABYSSAL

PLAIN

137

as per section of Fig.2 (sections CC’, DD’ and EE’).

During crossings of the abyssal plain, the P.G.R. was generally adjusted to record with a short pulse length (0.4 * 10m3 set) on a 40 fathoms scale, which permits a resolution of 60 cm. As seen in Fig.2 the zone studied can be divided into two areas: (I) the area north of core no. 40, which is relatively flat; and (2) the area south of core no. 40, where the microtopography seems to indicate that the zone has a more complicated geology.

Fig.11. Echograms

as per sections of Fig.2 (sections FF’, G’G, II’, HH’, LL’, M’N and

N’N). Marine

Ceol., 7 (1969) 129-145

138

A. KERMABON et al.

This is illustrated

by the echo-sounding

sections

AA’ and BB’ of Fig.9, and the sections

sections

presented

in Fig.9-I

I. The

CC’, DD’, and EE’ of Fig.10 are

drawn parallel to the main axis of the zone (see Fig.2) thus interpolating

the actual

recordings of bottom echoes made with the P.G.R. These sections also show the continuity of the sub-bottom echoes in the northern zone while the southern zone appears more acoustically complicated. The same conclusions can be reached by FF’,

inspection

of sections

the various

coring stations.

GG’, /I’, HH’,

M’N, N’N (Fig.1 I), taken

through

THE CORRELATION BETWEEN CORES AND ECHOGRAMS

The P.G.R. was used with an E.D.O. transducer in these studies. The sound pulse is emitted at a frequency of 12 kHz, the duration of which is variable. The records, as previously mentioned, have been obtained with a pulse length of 0.4 . 10m3 sec. The transducer’s radiation pattern consists of a main lobe having a half angle of 15”. The insonified area of the bottom is a circle with a diameter of a little more than 1 mile when the depth is 3,600 m. Furthermore, the P.G.R. recorder is adjusted for a sound speed of 800 fathoms/set or 1,463 m/set, while the average speed of sound propagation in the

Core Depth

18 m 3597

Fig.12. Relationship between echogram and core. (The wavy appearance is due to the sea surface wave motion.) Marine

of the reflectors

Geol., 7 (1969) 129-145

DEEP-SEA

cc - Ire

42

De pth

m

SEDIMENTS

IN THE TYRRHENIAN

ABYSSAL

PLAIN

139

3597

Fig.1 3. Relationship

between echogram and core.

sediments remains close to 1,564 mjsec. This may introduce an error of 7 % in the depths recorded by the P.G.R. It seems that within the range of error expected, the main reflectors present in the records appear to correspond to the porosity and sound-velocity changes noticed in the cores. A few echograms with the corresponding cores are shown in Fig.12-16. Fig. I7 shows the porosity Excellent

correlation

logs of all the cores taken in the northern can be noticed

for some cores taken

area.

close together,

such as cores 48 and 53, or even 55 and 56. The same applies to cores 43 and 49, and cores 39 and 40. However, although the zone where these cores were taken seems acoustically uniform,

Fig. 17 shows that strong variations

exist in the position

of the reflectors

between some cores; variations that are not detectable on the P.G.R. records. The sampling process could be responsible for this anomaly. However, this is not probable, because all the cores were taken with the same corer, and with the same coring characteristics. If the cores are disturbed, there is no reason why they should be disturbed differently from one another. A more valid conclusion is that the microstratigraphy of the bottom layers does not appear in the echograms because of the integrating process due to the large insonified area. This conclusion is confirmed by visual inspection of the cores. The layers appearing in the cores are roughly continuous in their sequence, but Marine Geol., 7 (1969) 129-145

Fig.14. Relationship

Core

53

Depth

m 3597

between echogram and core.

40

4

Fig.15. Relationship

F Lo 7c

between echogram and core. Marine Geol., 7 (1969) 129-145

DEEP-SEA

SEDIMENTS

Core

54

Depth

m 3597

IN THE TYRRHENIAN

ABYSSAL

PLAIN

141

Fig. 16. Relationship between echogram and core.

the actual positions

of the layers vary from one core to another.

Some of the layers

disappear in cores that are taken close together. The P.G.R. records, taken with a hull-mounted transducer, should be considered as a rough method of studying the bottom reflectors of any area which is as geologically complicated To get a clearer picture

as the one studied. of any surveyed area, high-density

coring should be

performed. This method is time consuming. In order to overcome this problem the authors have developed a deep-sea resistivity probe (report unpublished) which records vertical porosity profiles of the sea floor. The density of information obtained in a limited area makes this a good method

where porosity

CORRELATION

BETWEEN

variations

PHYSICAL

are complicated.

PROPERTIES

MEASURED

ON CORES

Wet density versus porosity (Fig. 18) The well-known linear relationship has been checked for the set of data available. From the results of ten cores the following regression equation was found: d = 2.60 -

0.0158 n Marine Geoi., 7 (1969) 129-145

core

POROSITY

core

Fig. 17. Porosity logs of a few cores taken in the northern part of the model zone.

60

Porosity

Fig.18. Relationship

70

% wet

volume

80

between wet density and porosity. Data from ten cores. Marine Geol., 7 (1969) 129-145

DEEiP-SEASEDIMENTSIN THE TYRRHENIANABYSSALPLAIN with d = mass unit weight of sediments The standard The standard

and n = porosity

error made on the determinations

error of estimate

143 of the sediments,

of the slope is 1.06 . 10e4.

S,. n is 2.78 * 10T2; the correlation

coefficient

is

r = -0.97. Sound velocity versus porosity (Fig. 19) Some scatter

correlation

was found

is large, especially

between

for sand-size

sound

velocity

and

porosity.

The

sediments.

As already noticed by RYAN et al. (1965), the velocities in this area are higher than expected.

measured

in clays

The experimental curves defined by NAFE and DRAKE (1957) have been compared to our set of data. No clear correlation exists between the experimental points and the curves n = 4 and n = 5 of the equation established by these authors:

4,

V2 = 41 Vi + p2” $2” V2’ P = porosity, 42 = 1 - 41;

Pl

= density

P2

=

P

= density

V, V

density

of interstitial

water;

of solid particles.

pZ = 2.645 g/cm3;

of sediments. p = dIpI + 42p2; = 6000 m/set; the estimated velocity of sound in the solid matrix alone; = sound velocity of sediments;

VW = sound velocity

of bottom

water.

CONCLUSION The study of a 10 mile long profile in the Tyrrhenian Sea proved that the main sub-bottom layers noticed in the P.G.R. records existed in cores taken in the area. As already composed

found

of volcanic

The cores taken on the P.G.R.

records.

by RYAN et al. (I 965), the reflectors ash and terrigenous demonstrated

sands.

the presence

The microstratigraphy

are often turbidites,

of microstratigraphy

is related

not seen

to the microtopography

of the area. The necessary high-density information could be obtained only by taking a number of cores. This operation, as well as the subsequent analyses, are expensive, laborious, and time consuming. It is therefore suggested that fast methods of obtaining data of the bottom be developed. Electrical, or nuclear, probing might be an answer. This study illustrates the necessity of taking a sufficient number of sediment samples (“high-density sampling”) in an area in which there is apt to be marked lateral and vertical variations in mass physical properties. Mass physical properties measured in one or two cores in environments of the type studied for this report could not be safely extrapolated over the whole environment. Marine

Geol., 7 (1969) 129-I 45

144

Fig.19.

Relationship

The number

between

of samples

sound

velocity

required

and porosity.

Results

to define adequately

from

21 cores.

sediment

properties

in any area is a function of data required, expected variations, time available, and expense; unless already known, a reconnaissance survey may be advisable, in some cases, to define these parameters. The frequently-held concept that a few cores will define sediment

properties

over large areas is tenable

only in some of

the larger environments (such as abyssal-hill, “red” clay environment) where variations are apt to be small; even in these environments safe extrapolation of data is subject to statistical proof. REFERENCES ARCHIE, G. E., 1942. The electrical

resistivity log as an aid in determining some reservoir characteristics. Trans. Am. Inst. Mining Me/. Eng., 142: 54. HAMILTON, E. L., 1963. Sediment sound velocity measurements made in situ from bathyscaph TRIESTE. J. Geophys. Res., 68(21): 5991-5998. HERSEY, J. B., 1965. Sediment ponding in the deep sea. Geol. SW. A!77., Bull., 76: 1251-1260. KERM~BON, A., BLAVIER, P., CORTIS, U. and DELAUZE. H., 1966. The “Sphincter” corer: A wide diameter corer with watertight core catcher, Marine Go/., 4: 149-162. Marine

Genl., 7

(I 969) I29 I45

DEEP-SEA

SEDIMENTS

IN THE TYRRHENIAN

ABYSSAL

PLAIN

145

KERMABDN, A. and BLAVIFR, I’., 1967. Principles and Methods of Core Analysis at the SACLAN7

A.,Y. W. Research Centre. Tech. Kept., 71. NATO Saclant A.S.W. Res. Centre, La Spezia, in press. KFRMABON. A., GEHIN, C. and BLAVIER, P., in prep. Nunrerical Kesrllts of Coue Analysisfiw the Tvrrhenian Ahyssal Plain. NATO Saclant A.S.W. Res. Centre, La Spezia. NAFE, J. E. and DRAKE, C. L., 1957. Variation with depth in shallow and deep water marine sediments of porosity, density and velocities of compressional and shear waves. Geoph_vsic.s,22: 523-552. RYAN, W. B. F., WORKUM, F. and HERSEY, J. B., 1965. Sediments on the Tyrrhenian Abyssal Plain. Geol. Sot. Am., Bull.. 76: 1261-1282. WILSON, W.. 1960. Speed of sound in sea water as a function of temperature, pressure and salinity. J. Acoust. Ser. Am., 32: 644.

Marine Geol., 7 (1969) 129-145