Ayres Neto Et Al 2013

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Geotechnical Influence on the Acoustic

Properties of Marine Sediments of theSantos Basin, BrazilArthur Ayres Neto

a, Joana de Noronha Teixeira Mendes

a, Juliana

Maria Gonalves de Souza

a

, Miguel Redusino Jr.

a

& Rodrigo LeandroBastos Pontes

a

aGeology Department, Fluminense Federal University, Brazil

To cite this article:Arthur Ayres Neto , Joana de Noronha Teixeira Mendes , Juliana Maria Gonalves

de Souza , Miguel Redusino Jr. & Rodrigo Leandro Bastos Pontes (2013): Geotechnical Influence onthe Acoustic Properties of Marine Sediments of the Santos Basin, Brazil, Marine Georesources &

Geotechnology, 31:2, 125-136

To link to this article: http://dx.doi.org/10.1080/1064119X.2012.669815

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Geotechnical Influence on the Acoustic Properties ofMarine Sediments of the Santos Basin, Brazil

ARTHUR AYRES NETO, JOANA DE NORONHATEIXEIRA MENDES, JULIANA MARIA GONCALVESDE SOUZA, MIGUEL REDUSINO JR., ANDRODRIGO LEANDRO BASTOS PONTES

Geology Department, Fluminense Federal University, Brazil

There are several problems associated with core location and sediment deformationduring the coring procedure. Geophysical investigation of the seafloor using acousticsystems, such as high-resolution seismic and single-beam sediment classification,

provides a very cost-effective evaluation on the distribution of sedimentary environ-ments. The present study investigates the relationship between acoustic impedanceand geotechnical properties of shallow marine sediments from the Santos Basin,Brazil. A total of eight cores from different geological settings were collected andmeasurements of P-wave velocity (Vp), gamma density, mean grain size, void ratio,water content and shear strength were conducted. The results show that, despite dif-

ferent trends according to the sedimentary environment, the acoustic impedance cor-relates very well with all geotechnical parameters analyzed.

Keywords geotechnics, marine geophysics, sediments

Introduction

The main challenge of the major deep water oil and gas projects is to transport theproduction to on-shore processing facilities. The option often lies on submarine pipe-lines connecting the production area to these facilities. However, the transportationof large quantities of sometimes very hazardous products over great distancesthrough a pressurized pipeline system, often with zero-leak tolerance, is not a trivial

thing. Submarine production facilities are very complex, carefully engineered struc-tures placed into an enormously variable, ever-changing, and usually hostile environ-ment, the seafloor. Variations in the structure-seafloor system may be enormous andmaterial and environmental changes over time are of chief concern (Muhlbauer2004). The selection of pipeline routes and the location of the associated tie-in struc-tures require detailed knowledge of the seabed physical properties and shallowgeology demanding that the areas along the routes, and regionally around the routes,be accurately surveyed.

Received 14 July 2011; accepted 15 February 2012.The authors want to thank REPSOL SINOPEC Brasil for the funding of the project.

Address correspondence to Arthur Ayres Neto, Geology Department, FluminenseFederal University, Av. Gen Milton Tavares de Souza S=N, CEP: 24210-346, Niteroi, RJ,Brazil. E-mail: [email protected]

Marine Georesources & Geotechnology, 31:125136, 2013Copyright # Taylor & Francis Group, LLCISSN: 1064-119X print=1521-0618 onlineDOI: 10.1080/1064119X.2012.669815

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The use of acoustical geophysical tools is the most time- and cost-effectiveway to characterize the seafloor by allowing large areas to be surveyed in rela-tively short time. However, the geophysical information is indirect information.In order to precisely understand the geotechnical and geological characteristics

of the seafloor it is necessary to sample the seafloor, both superficially andsubsuperficially.

The acquisition of geological and geotechnical samples has several constraints,especially in deep water. The first is to determine the exact location of the sample.Subsea acoustic positioning systems are used to determine the position of the sam-pler device at the moment it touches the seafloor. However, the precision of thesesystems depends on the frequency used, the horizontal distance of the equipment,and the level of environmental noise. Another critical issue is the deformation ofthe samples. Inevitably, every sampling technique affects the sediment column in aparticular manner, potentially introducing dimensional and structural changes.Moreover, the representativeness of the geological and geotechnical information interms of areal distribution is not yet well-established and is still a large disagreementamong engineers.

Since acoustic geophysical data is the basis for most of the information aboutthe seafloor used in geohazards and pipeline route studies, it would be convenientif the geotechnical information could be extracted from the acoustic data. Theadvantages would be the continuous coverage of the geotechnical information, incomparison with discrete characteristics of the samples, with minimum samplingand laboratory testing. The objective of this paper is to investigate the correlationbetween acoustic properties, namely the acoustic impedance, and geological=geotechnical parameters from near surface sediments and its applicability to marine

geotechnical investigation.

Previous Studies

Studies on the acoustic behavior of shallow marine sediments have beenconducted since the 1980s (Hamilton 1980; Hamilton and Bachman 1982; Tatham1982; Domenico 1984; Castagna et al. 1985; Han et al. 1986) demonstratingsimilar, but different, relationships between compressional (P-) and shear (S-) wavevelocities and attenuation and porosity, water content, sand=silt=clay content,wet bulk density, shear strength, overburden pressure, and other petrophysicalproperties.

Investigations focused on the correlation between geoacoustic parameters andgeotechnical properties have shown that, despite low correlation coefficients, it ispossible to note a tendency on correlations between P-wave velocity (Vp), acousticimpedance, attenuation coefficient and properties such as density, water content,porosity, shear strength and Atterberg limits (Liquid limit, plastic limit and plasticityindex). Buchan et al. (1972) showed good correlations between the attenuation coef-ficient and several geotechnical properties while Vp showed, in general, relativeweaker correlation with the same properties. In this study it was observed thatacoustic impedance showed a wider range of correlation values, correlating verystrongly (r> 0.9) with properties such as wet bulk density and porosity and weakly

(r< 0.5) with mean grain size and clay content. The authors concluded that by farthe most important factor in the variability of the acoustic data is that of particlesize. Thus, for a deep-sea sediment, the greater the percentage of large size particles,

126 A. Ayres Neto et al.


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or the larger the mean particle size, the greater the attenuation coefficient, soundvelocity, and acoustic impedance. They also noted reasonable correlations betweenthe Atterberg limits and some of the acoustic properties. Ayres and Theilen (1999)demonstrated that, despite the relatively low correlation coefficients between Vp

and porosity, water content and wet bulk density, it was possible to determineregression equations. As expected, Vp increases with increasing wet bulk densityand decreases with decreasing porosity and water content. Fener et al. (2005) showedthat Vp correlated very well (correlations coefficients up to 0.89) with plastic andliquid limits of cohesive soils. Bartetzko and Kopf (2007) in a compilation of datafrom 46 Ocean Drilling Program (ODP) legs, demonstrated that shear strengthand porosity show an increase down hole in about half of the drill sites. This increas-ing with depth trend was also noted by Ayres Neto (1998) and Hamilton and Bach-man (1982) for the velocity of P-waves in shallow marine sediments.

On the other hand, the main problem with geotechnical (and geological)analysis of the seafloor is the deformation of the sediments due to differentfactors. Skinner and McCave (2003) analyzed the effects of gravity- and piston-coring techniques on soft sediment cores using principles of soil mechanics. Theyshowed that the extent of piston over-sampling and the interval of sediment affectedappear to be directly related to the weight of the coring apparatus and the lengthand elasticity of the coring cable, and that stratigraphic thinning increases withdeeper penetration, with essentially intact sediments being recovered only down to3 m penetration. Lowemark et al. (2006) estimate differences in core shortening inthree gravity cores taken using different coring devices. The comparison of X-rayradiographs and stratigraphic records of three cores showed that core shortening(sediment thinning) in gravity cores can be recognized in X-ray radiographs even

if it is not visible in fresh sediment. Jackson and Richardson (2007) state thatthe determination of properties such as density, porosity, and water content is sub-

jected to errors associated with sediment disturbance during collection, transport,and storage in different degrees depending on the nature of the sediment (muddyor sandy). Laboratory testing also does not yield appropriate properties of soildue to different loading and drainage conditions as compared to the actual in-situsoil condition.

In order to minimize measurement errors due to sampling procedures somein-situ measurement techniques were developed. Arulrajah et al. (2006) used in-situdissipation tests to provide a means of evaluating the in-situ coefficient of horizontalconsolidation and horizontal hydraulic conductivity of soft clays from the ChangiEast Reclamation site in the Republic of Singapore. They noted that in-situ testsusing the cone penetration test (CPT) are recommended as the most suitablemethod for the determination of the coefficient of horizontal consolidation in soilimprovement schemes involving vertical drains. Seifert et al. (2008) presented in-situstrength and pore-pressure measurements from 57 dynamic CPTs in sediments ofMecklenburg, Eckernforde, and Gelting bays, western Baltic Sea. The authorsconcluded that dynamic CPT measurements showing higher penetration velocitythan for the standard CPT may overestimate the soil stiffness. However, given theoverall good agreement between the soil classification data and ground-truthingevidence gathered from the gravity cores, measurements using a dynamic CPT device

represents a versatile approach in muddy sediments. This would be particularly truein gas-rich muds on which, for stability reasons, standard CPT rigs cannot bedeployed.

Acoustic Properties of Marine Sediments 127


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Correlation Between Acoustic and Geological Data

The acoustic impedance is the main geoacoustic parameter which allows the corre-lation with geological and geotechnical properties through acoustic geophysicalmethods (Brand 2003). The reflection coefficient is the controlling property of acous-tic energy transmission across a boundary. For a boundary separating two media ofimpedance Z1 and Z2 (such as seen on the water=seafloor boundary), the amount ofenergy reflected at the interface for a normally incident acoustic wave, which is thecase of high-resolution seismic and sediment classification systems based onsingle-beam echo-sounders, travelling in Z1 is given by:

RZ2Z1

Z2Z1 1

As seen in Equation 1, the amount of energy reflected by the seafloor depends on the

contrast of acoustic impedance between the two media. This energy is represented bythe peak amplitude of the reflected signal. If the acoustic impedance contrast at thewater=sediment interface is too low, energy transmission into and out of the sedi-ment is facilitated. Low frequency signals also have greater ability to penetrate theseafloor. In both cases it is recognized that sediment volume heterogeneity in theform of layering, bioturbations, gas bubbles, and the sediments inherent granularityplays a major role in controlling the acoustic response of the material (Jackson andRichardson 2007). On the other hand, the responses from high-frequency acoustic

Figure 1. Study area with core locations.



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signals which show very small or no penetration at all, are primarily related toseafloor roughness.

Study Area

The study area is located at central portion of the Santos Basin, southeastern Brazil,between the 70 m and 2200 m isobaths (Figure 1). The Santos Basin is one of themost important off-shore sedimentary basins in Brazil and has an area of approxi-mately 200,000 km2. The continental shelf at the study area has a maximum widthof 250 km and the shelf break is located approximately at the 200 m isobath. Thesedimentary coverage at the inner continental shelf is composed of muds and siltsof continental origin with small amounts of sands and carbonatic debris. The outershelf sedimentary cover is characterized by a mixture of coarse to very coarse carbo-natic sediment, mainly composed of shell and corals fragments, with muddy matrix.At the continental slope the seafloor is characterized by essentially muddy sediments

with minor amounts of coarse material.

Methodology

Sediment samples were collected using a large (50 cm x 50 cm) box-corer. From thematerial collected three sub-samples were taken with a 100 mm PVC liner for geo-logical, geotechnical, and physical properties analysis. The Vp and gamma density(q) were determined using a Multisensor Core Logger (MSCL) from Geotek whichautomatically scans each core section as it passes through the sensors and expressesmeasurements in meters per second (m=s), grams per cubic centimeter (g=cm3),respectively. Porosity was calculated directly MSCL by assuming fully saturated

conditions and sediment matrix and pore water density to be 2.6 g=cm3 and1.030 g=cm3, respectively, according to Evans (1965) and is expressed by thedimensionless void ratio. Grain size analyses were conducted using a laser particleanalyzer Mastersize 2000 and the results are expressed mean grain size (/) accordingto Pettijohn (1975). The acoustic impedance (Z) was calculated by multiplyingthe sound velocity by the density of the medium and is expressed in SI units(kg=s m2). Water content (W) was determined using conventional laboratory proce-dures and is expressed in weight percentage (W%). Shear strength was determinedusing a hand held Torvane and is expressed kPa. Measurements were made everytwo centimeters on the eight cores collected.

Table 1. Coordinates and depth of the core

Core number Latitude (S) Longitude (W) Depth (m) Sample length (cm)

S1 2506,835 04317,803 2104 45S2 2428,653 04416,386 415 50S3 2401,109 04511,674 70 30S4 2458,029 04443,925 350 50S5 2513,527 04459,627 170 25S6 2521,992 04415,144 1930 45

S7 2411,567 04336,076 200 50S8 2326,121 04308,194 115 45



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The coordinates and depth of the cores are presented in Table 1 and Figure 1.

Results

According to the results of the average grain size for each sample, we were able toidentify three groups of sediments (Table 1). The first one (G1), which includes coresS1 and S6 (the deeper ones, collected at 2104 m and 1930 m water depth, respectively)are predominantly characterized by muddy sediments (clay silt >85%) and showmean PHI values above 6. The second group (G2) comprises cores S2, S4 and S7which are located at the shelf-break and upper continental slope (water depths of415 m, 350 m, and 200 m, respectively). Despite the large silt content, the sedimentsof this group show higher contents of the sand and gravel fraction (up to 41%) result-ing in mean PHI values between 4 and 6. The third group (G3) contains the coarsersediments, with gravel plus sand content higher than 75%, and mean PHI values

below 4. The cores pertaining to this group (S3, S5, and S8) are located at the con-tinental shelf. The coarse fraction is basically composed of shell and coral fragmentswith some quartzose sand associated.

Table 2 shows that the other physical parameters behave approximately thesame way and follow the same distribution within the groups. Sediments from group1 showed the lowest average value of acoustic impedance and the highest for voidratio, water content and shear strength. On the other hand, sediments from group3 showed the highest average value for acoustic impedance and the lowest valuesfor void ratio, water content and shear strength. Sediments from group 2 showedvalues between the two extremes for all investigated physical parameters.

To understand the behavior of the acoustic impedance in the sediments of the

study area, two plots (Vp versus impedance and density versus impedance) areshown in Figures 2 and 3. It can be seen that the acoustic impedance is much more

Table 2. Physical properties according to sedimentological groups (Vp measure-ments were conducted with 250 kHz transducers)

Vp(m=s)

Density(g=cm3)

Acousticimpedance(kg=s m2)

Voidratio

Watercontent(W%)

Shearstrength

(kPa)

Meangrainsize(u)

G1 Minimum 1440.35 1.157 1.719 1.787 46.967 1.177 5.416Maximum 1486.03 1.645 2.409 12.193 168.620 26.478 6.994Average 1460.05 1.489 2.174 3.197 85.248 11.231 6.513St. Deviation 9.23 0.12 0.18 2.07 37.47 7.99 0.38

G2 Minimum 1481.54 1.498 2.231 1.200 37.048 0.196 2.728Maximum 1559.18 1.810 2.815 2.654 92.942 25.890 6.540Average 1508.74 1.658 2.502 1.766 58.211 9.390 4.801St. Deviation 25.68 0.07 0.14 0.32 14.61 7.34 0.95

G3 Minimum 1447.86 1.365 2.095 1.015 9.338 2.452 0.799Maximum 1603.00 1.882 3.017 4.084 81.641 19.613 4.398

Average 1529.10 1.684 2.577 1.724 53.398 8.910 2.947St. Deviation 36.90 0.12 0.23 0.61 24.58 5.11 0.96



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sensitive to variations in the sediment density (r 0.98) than to variation in the Vp(r 0.79). This is because P-wave velocity shows a variation of 11% between themaximum and the minimum values. Density, on the other hand, shows variationof approximately 63%. It is also noted that sediments from the deep sea (G1) havethe lowest Vp and density values, while the shallower sediments (G3) show thehighest Vp and density values.

Figure 4 shows the plot between acoustic impedance and mean grain size. There

is a clear trend of increasing acoustic impedance with decreasing PHI, i.e., thecoarser the sediment the higher the acoustic impedance. The calculated correlationcoefficient is 0.60. Despite some overlapping of the data, sediments from group1 showed the lowest range of acoustic impedance, sediments from group 3 thehighest range of acoustic impedance and sediments from group 2 essentially plotsbetween the two other groups.

Figure 2. Plot between acoustic impedance and P-wave velocity.

Figure 3. Plot between acoustic impedance and density.



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Figure 5 shows the plot between acoustic impedance and void ratio. Since voidratio was calculated based on the density values, a very good correlation betweenacoustic impedance and void ratio was to be expected (r 0.80). However, thequasi-linear positive correlation observed between acoustic impedance and densityturns into an obvious negative logarithm correlation.

The correlation between acoustic impedance and water content is shown inFigure 6. There is a negative trend of increasing impedance with decreasing watercontent. Also, there is a large overlap of data from the three groups especially forwater content between 40 and 100%.

Figure 7 shows the plot between shear strength and acoustic impedance. It isworthwhile to note that the sediments from the three groups showed the same rangeof shear strength values but are separated according to the acoustic impedance.

Figure 4. Plot between acoustic impedance and mean grain size.

Figure 5. Plot between acoustic impedance and void ratio.



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Moreover, the samples from the three groups showed an increasing trend of shearstrength with acoustic impedance. The overall correlation coefficient between thetwo variables is weak (r 0.30). However, when considered separately the correla-tion coefficients are significant for groups 1 and 2 (G1 )r 0.72; G2 )r 0.57)and remaining weak for group 3 (G3 ) r 0.38).

Discussion

Undisturbed samples were collected from eight different sites, from the inner

continental shelf to the lower slope at the Santos Basin and their geological andgeotechnical properties were plotted against acoustic impedance. With the exceptionof the relationship between shear strength and acoustic impedance all other

Figure 6. Plot between water content and acoustic impedance.

Figure 7. Plot between shear strength and acoustic impedance.



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relationships investigated showed a strong correlation with correlation coefficientvalues above 0.6 (Anderson and Finn 1997).

Richardson and Briggs (1993) postulated that Vp and density dependency ontemperature, pore water salinity, and water depth, could complicate the apparent

straightforward prediction of values of sediment properties from remotely sensedsediment impedance. However, the results demonstrated that the acoustic impedanceis a good parameter for the indirect investigation of geotechnical properties of theseafloor such as grain size, density, void ratio, and water content. As described byJackson and Richardson (2007) the fine sediments collected at the deeper sites, com-posed mostly of clay or silt-sized particles, showed average Vp 4% less than oceanwater, while the sediments composed mostly of sand-sized particles collected atthe shallower sites showed average Vp approximately 2% greater than sea water.Fu et al. (2004) reported Vp values usually above 1600 m=s for coral sands off thecoast of Hawaii. However, despite the relatively high content of coarse carbonaticparticles in sediments from group 3, they presented Vp essentially below 1570 m=s,with only one sample showing Vp values above 1600 m=s. The amount of coarseparticles was also shown to affect the bulk density of the sediment.

These facts resulted in sediments from the lower slope showing acoustic impe-dance values up to 43% lower than the shallower sediments. Such differences wouldgive rise to considerable differences in the amount of acoustic energy, generatedeither by high-resolution seismic or by sediment classification systems, and reflectedback from the seafloor. The reflected energy is expressed by the peak amplitude andthe echo-length. The latter is a function of the signal frequency and impedancecontrast between the sediment and the overlying water, which are the main factorscontrolling energy penetration. In general, signal penetration is expected to decrease

with an increasing sounder frequency and impedance contrast (Van Walree et al.2005).

The relationship between shear strength and acoustic impedance needs morecareful analysis. Shear strength is a soil mechanics parameter used to describe themagnitude of the shear stress that a soil can sustain and is a result of friction andinterlocking of particles, and possibly cementation or bonding at particle contacts(Buchan et al. 1972). In fine soft sediments the shear strength is a result of electro-chemical bounding between particles. As the compaction takes place, pore water issqueezed out and friction between grains gets more important as the main factorcontrolling shear strength. Naturally occurring marine sediments may present vari-able quantities of coarse material disturbing the structural lattice and providing itsown mechanical structure by grain-to-grain contact. In this case, the contact areabetween grains is much smaller, reducing the strength of the sediment. This factwould explain the lowest and more random shear strength values observed insediments from group 3. Brand et al. (2003) studying deep water sediments fromthe southern Green Canyon area of the Gulf of Mexico, observed a relationshipbetween shear strength and acoustic impedance establishing an exponential trendline equation. This result is very similar, including the impedance values, to theone observed in the present study for the sediment from groups 1 and 2.

Conclusions

The main difficulty of assessing geotechnical properties of the seafloor is related todeformation of the sediments due to the different sample procedures. This problem



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has been partially resolved with the development of in-situ measurement techniquesfor the evaluation of the geotechnical condition of the seafloor.

The present study suggests that the study of the acoustic properties of seafloorsediments, and the manner in which they vary, can lead to an assessment of the

regional variation of some geotechnical properties of the sea-floor. The results indi-cate that this is especially true concerning the correlation of the acoustic impedance,rather then the Vp values, with some geotechnical properties such as mean grain size,void ratio and water content. The most common application of this result is theseismic amplitude map of a reflector of interest. As acoustic impedance controlsthe reflectivity of the sea-floor, it should be possible to measure this property fromechosounding and high-resolution seismic responses and assess in some way themechanical strength of the sea-floor.

In the present research, the effects of some soil properties such as particle shape,mineralogy of the soil particles, and chemistry of the pore water on density andP-wave propagation velocity were not taken into consideration. Moreover, somedata dispersion may result from the techniques used to determine sediments physicalproperties, especially concerning shear strength.

Further investigations should focus on the relationship between acousticimpedance and peak amplitude and echo-length of the traces, from both high-resolution seismic and single-beam sediment classification systems.

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Hamilton, E. L. 1980. Geoacoustic modeling of the sea floor. Journal of the Acoustical SocietyAmerica68: 13131340.

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Ayres Neto Et Al 2013