Acoustic anisotropy in marine sediments and sedimentary rocks

Bachman, Richard T.

doi:10.1029/jb084ib13p07661

Cited by 44 publications

(32 citation statements)

References 25 publications

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“…Potential causes for acoustic anisotropy in aggregates are preferred orientation of anisotropic mineral grains, preferred orientation of pores and cracks, and bedding (Bachman, 1979;Christensen, 1977,1979;Milhollandetal., 1980;Carlson, 1981;Kim et al, 1983Kim et al, , 1985Carlson et al, 1984;Schaftenaar and Carlson, Velocity anisotropy (%) 1984). In the following discussion, we will try to determine which of these mechanisms might be the principal cause for acoustic anisotropy measured in calcareous oozes from Ontong Java Plateau during Leg 130.…”

Section: Possible Causes For Acoustic Anisotropy In Calcareous Oozes mentioning

confidence: 99%

“…A small part of these efforts has been dedicated to the understanding of acoustic anisotropy in marine sediments. It has been observed that horizontal P-wave velocities (measured parallel to bedding) are usually higher than vertical velocities (measured perpendicular to bedding) (Bachman, 1979;Christensen, 1977,1979;Carlson, 1981;Kim et al, 1983;Carlson et al, 1984;Schaftenaar and Carlson, 1984). Velocity anisotropy is important for the interpretation of seismic refraction and reflection data as it affects seismic-wave propagation in the sediment column (Carlson and Christensen, 1979;Bachman, 1979;Milholland et al, 1980).…”

Section: Introductionmentioning

confidence: 99%

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Velocity Anisotropy in Calcareous Sediments from Leg 130, Ontong Java Plateau

Bassinot¹,

Marsters²,

Mayer³

et al. 1993

Proceedings of the Ocean Drilling Program

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During Leg 130, an intensive program of P-wave velocity measurements was conducted on calcareous sediments of varying induration (ranging from ooze to limestone) that cover the Ontong Java Plateau (west equatorial Pacific). The high quantity and quality of shipboard acoustical property measurements seemed to be particularly appropriate for gaining a better understanding of acoustic anisotropy in calcareous sediments. Major results can be summarized as follows:1. In calcareous oozes, acoustic anisotropy exists even at shallow depths of burial. Values are low, ranging from -0.5% to 3.5% and averaging 1.2%. Anisotropy profiles are affected by coring disturbances, but they still roughly correlate between holes cored at a same site. There is no general increase of anisotropy with depth of burial in ooze sections. These observations imply that most of the anisotropy measured in oozes is controlled by original changes in sediment composition rather than by reorientation of anisotropic minerals or flattening of pores during burial. Given the data available, bedding lamination seems to be the most satisfactory explanation for acoustic anisotropy in calcareous oozes of Ontong Java Plateau. However, bioturbation affects the entire ooze sections cored and much of the original depositional features may have been removed. Further analysis such as scanning-electron-microscope (SEM) observations of epoxy impregnated ooze samples are necessary to test the "bedding lamination" hypothesis.2. Ooze to chalk transition is marked by a tremendous change in the anisotropy profiles. In chalk sections the anisotropy values are higher (reaching 20%) than in ooze sections. Most of the anisotropies are negative (horizontal P-wave velocities lower than vertical velocities), whereas positive values are expected. Weakly cemented chalk samples are affected by cracking when compressed between the Hamilton Frame transducers for P-wave measurement. Cracks occur along bedding planes during horizontal P-wave velocity measurements, thus leading to anomalously low horizontal P-wave velocities and negative anisotropies.

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Section: Possible Causes For Acoustic Anisotropy In Calcareous Oozes mentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Velocity Anisotropy in Calcareous Sediments from Leg 130, Ontong Java Plateau

Bassinot¹,

Marsters²,

Mayer³

et al. 1993

Proceedings of the Ocean Drilling Program

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“…In clay-rich marine sediments, positive transverse anisotropy will vary from 0% near the surface (complete isotropy) to >12% at depths of several hundred meters. In the interpretation of seismic reflection and refraction data, velocity anisotropy has to be taken in consideration because it will determine the mode of wave propagation in the sediment (Bassinot et al, 1993;Carlson and Christensen, 1977;Bachman, 1979;Milholland et al, 1980). Compactional anisotropy, parallel alignment of pores and particles parallel to bedding because of gravitational compaction under increasing overburden, is commonly assumed to be the most important single source for the downhole increase in acoustic anisotropy in fine-grained, clay-rich sediments (Hamilton, 1970;Kim et al, 1983Kim et al, , 1985O'Brien, 1990).…”

Section: Acoustic Anisotropymentioning

confidence: 99%

Directional properties of P-wave velocities and acoustic anisotropy in different structural domains of the northern Barbados Ridge accretionary complex

Brückmann¹,

Moran²,

Housen³

1997

Proceedings of the Ocean Drilling Program, 156 Scientific Results

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Ocean Drilling Program (ODP) Leg 156 revisited the northern Barbados Ridge, where the previous Deep Sea Drilling Program Leg 78A and ODP Leg 110 studied the frontal part of this accretionary prism. Drilling and logging-while-drilling at Sites 947, 948, and 949 successfully identified major thrust faults and the décollement, which was the target of several downhole experiments. Two of the eight holes drilled were equipped with borehole observatories that will monitor temperature, pressure, and fluid flow over the next years. Coring at Hole 948C recovered 180 m of sediment, centered around the décollement, which was positively identified based on structural information. The aim of this study is the evaluation of the possible correlation of preferred orientation of acoustic properties and the direction of maximum compressive strain in the frontal part of the accretionary prism. For this purpose, shipboard P-wave velocities from Holes 948C and 949B were reoriented. This information was then used to compare the directional properties of accreted and subducted sediments. In Hole 948C, lowest transverse velocities (T min ) were observed to be consistently oriented perpendicular to the maximum horizontal compressive stress, believed to be parallel to the convergence vector. In the underthrust domain of Hole 948C, several preferred orientations for T min were detected, but no correlation with the geotectonic reference frame could be identified. Acoustic anisotropy does not show a comparable pattern in Hole 948C. It is concluded that the observed directional dependence of P-wave velocity in the accreted sediment domain in Hole 948B is the result of moderate to steeply inclined bedding, although this conclusion can not adequately be tested due to the lack of corrected structural data.

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“…See Laughton (1957); Carlson and Christensen (1977);and Bachman (1979) for discussions of anisotropy.…”

Section: Sound Velocitymentioning

confidence: 99%

“…5) is important for estimating vertical velocities (for seismic reflection profiles) from (1) the horizontal velocities determined by refraction techniques, and from (2) oblique velocities determined by sonobuoy techniques. Acoustic anisotropy in sedimentary rock may be created by some combination of the following variables, as summarized by Press (1966), Carlson and Christensen (1977), and Bachman (1979): (1) alternating layers with high-or low-velocity materials; (2) tabular minerals aligned with bedding, which create fewer gaps (containing pore water) in a direction parallel to bedding; (3) acoustically anisotropic minerals whose high-velocity axis may be aligned with the bedding plane; and the (4) foliation parallel to bedding.…”

Section: Scatter Diagramsmentioning

confidence: 99%

Electrical Resistivity, Sound Velocity, Thermal Conductivity, Density-Porosity, and Temperature, Obtained by Laboratory Techniques and Well Logs: Site 462 in the Nauru Basin of the Pacific Ocean

Boyce¹

1981

Initial Reports of the Deep Sea Drilling Project

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At Deep Sea Drilling Project (DSDP) Site 462, from mudline to 447 meters below the sea floor, Cenozoic nannofossil oozes and chalks have acoustic anisotropies such that horizontal sonic velocities are 0 to 2.5% faster than those in the vertical direction. In laminated chalk, anisotropy of 5% is typical, and in limestones, radiolarian oozes, porcellanites, and cherts, the anisotropies range from 4 to 13%. Middle Maestrichtian volcaniclastics from 447 meters to 560 meters below the sea floor have an acoustic anisotropy of 2 to 32%; 4 to 13% is typical. Basalt flows and sills occur between 560 meters and 1068 meters, and have no apparent anisotropy, but minor interbedded volcaniclastics have anisotropies from 0 to 20% (5% is typical). These volcaniclastics frequently have very small anisotropies, however, compared with the volcaniclastic sequence above the basalt section.Data in cross-plots of the laboratory-measured compressional sound velocity versus wet-bulk density, wet-water content, and porosity of sediments and sedimentary rock typically lie between the equations derived by Wyllie et al. (1956) and Wood (1941); the Wyllie et al. (1956) equation has a fair fit with similar basalt velocity cross-plots. Crossplots of thermal conductivity and sound velocity indicate only a fair correlation. Cross-plots of thermal conductivity versus porosity, wet-water content, and wet-bulk density correlate well with equations derived by Maxwell (1904), Ratcliff (1960), Parasnis (1960), and Bullard and Day (1961). Electrical formation factor versus porosity for sediments and sedimentary rock agrees with the Archie (1942) equation, with m values of 2.6; for basalt, an m of about 2.1 is typical. Basalt pore-water resistivities do not appear to be greatly different from sea water. Formation factors are greater than those derived from equations in Maxwell (1904), Winsauer et al. (1952), Boyce (1968), and Kermabon et al. (1969). An "apparent interstitial water resistivity" (i?J curve was derived from the density and induction logs. This R wa curve indicated an anomaly, at 393.5 to 396.5 meters, which could be interpreted as (1) 76% hydrocarbons, (2) relatively fresh pore water (1.8‰ salinity), or (3) low-grain-density (2.2 g/cm 3 ) semi-lithified porcellanite-chert. Porcellanite-chert is the most plausible interpretation.In situ temperatures measured by the Uyeda temperature probe were about 2 to 5°C (50%) higher than the equilibrium temperature (Lachenbruch and Brewer, 1959) extrapolated from two Gearhart-Owen continuous temperature logs; this discrepancy probably arises because the hole was washed out in this depth interval, so these extrapolated temperatures are probably not reliable. If one ignores all precautions as to temperature artifacts, then the equilibrium temperatures of the Gearhart-Owen temperature logs suggest that hydrothermal circulation is occurring in at least the upper 40 meters of the basalt section and heat is transferred by convection and not conduction. Hydrothermal circulation is probably not indica...

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Acoustic anisotropy in marine sediments and sedimentary rocks

Cited by 44 publications

References 25 publications

Velocity Anisotropy in Calcareous Sediments from Leg 130, Ontong Java Plateau

Velocity Anisotropy in Calcareous Sediments from Leg 130, Ontong Java Plateau

Directional properties of P-wave velocities and acoustic anisotropy in different structural domains of the northern Barbados Ridge accretionary complex

Electrical Resistivity, Sound Velocity, Thermal Conductivity, Density-Porosity, and Temperature, Obtained by Laboratory Techniques and Well Logs: Site 462 in the Nauru Basin of the Pacific Ocean

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