Search

This study is the first attempt at describing and interpreting incised-channels on the continental shelf of the South Sea, Korea. The distribution, pattern, shape, and infilling of submerged channels incised across the continental shelf of the South Sea have been investigated based on approximately 1940 line-km of chirp and sparker data. The incised-channel system of the paleo-Seomjin River on the continental shelf is approximately 109 km long, 800–5000 m wide, and is more than 890 km2 in area. Meandering and straight types of channel are dominant on the inner shelf, with a change to straight and braided planforms on the mid to outer shelf. Sparker seismic data suggest the paleo-channels formed under fluvial systems when the shelf was exposed during the Last Glacial Maximum (LGM). Channel fills are divided into five types, based on erosional surfaces and internal seismic reflectors: (I) transparent to semi-transparent incised channel fill; (II) parallel to sub-parallel incised channel fill; (III) complex incised channel fill; (IV) divergent incised channel fill; and (V) chaotic incised channel fill. The acoustically transparent facies of the channel fill type (Type I) suggest the presence of basin muddy deposits in a low energy environment, which were dominant in the inner shelf. These muddy sediments were likely during a period of higher sea level during the Holocene transgression. Types II, III, and IV are characterized by onlap, downlap, and wave or tidal ravinement surfaces, respectively, indicating that they were deposited during a transgressive stage, which type V represent basal incision surface. The evolution of paleo Seomjin River has been strongly controlled by sea level change and sediment transport processes since the LGM.

Laboratory measurements of sound velocity in unconsolidated marine sediment were performed to establish specific correction curves between temperature and sound velocity. Cores from the Hupo Basin ...

A total of 157 piston and box core samples were collected and analyzed to characterize physical properties and geoacoustic provinces of surficial sediments in the southwestern part of the Ulleung Basin in the East Sea. The results and distribution patterns of surficial sediment texture, physical and acoustic properties reflect well the sedimentary environment like distribution and composition of sediment in the study area. The inner shelf and basin slope in this study are dominated by fine-grained sediments and show high porosity and water content, with low bulk density and velocity as well. The trough of the Korea Strait and outer shelf, which is composed of coarse-grained sediments, shows low porosity and water content and high bulk density and velocity. Based on in-situ velocity and sediment properties, the study area is divided into five provinces: (1) Province IA is hemi-pelagic mud partially mixed with intermittent sandy sediments which are derived from the outer shelf due to slide/slump or mass flows (in-situ velocity: 1445 m/s, mean grain size: 8.4Φ, porosity: 84%, and bulk density: 1.24 g/cm3), (2) Province IB is covered with muddy sediments that were deposited during the Holocene (in-situ velocity: 1445 m/s, mean grain size: 7.9Φ, porosity: 78%, and bulk density: 1.35 g/cm3), (3) Province II is marked by mixed recent and relict sediments (in-situ velocity: 1477 m/s, mean grain size: 6.1Φ, porosity: 68%, and bulk density: 1.53 g/cm3), (4) Province III is dominated by coarse-grained relict sediments formed during the Pleistocene (in-situ velocity: 1564 m/s, mean grain size: 3.8Φ, porosity: 54%, and bulk density: 1.77 g/cm3), and (5) Province IV consists of coarser sediments than Province III (in-situ velocity: 1671 m/s, mean grain size: 3.0Φ, porosity: 46%, and bulk density: 1.91 g/cm3). The compressional wave velocity, mean grain size, and bulk density increase from Province IA to Province IV, whereas the porosity and water content decrease from Province IA to Province IV. Variability of the physical and acoustic properties tends to follow the general trend of the mean grain size. The classification of each province using the in-situ velocity corrected with the temperature and pressure provides a better reflection of the sediment properties and sedimentary environment.

기존의 자동음파전달속도 측정시스템을 이용하여 해양퇴적물의 음향특성을 측정하는 경우 최초도달신호시간의 결정에 발생하는 오류와 측정 절차의 번거로움 등의 몇 가지 문제점들이 존재한다. 이러한 문제점을 개선하기 위하여 PXI모듈을 이용한 LabVIEW기반의 측정 시스템을 개발하였다. 새로운 측정시스템을 검증하기 위해 동일한 시료와 실험환경에서 기존 시스템과 병행하여 측정하였다. 그 결과 1~2%의 음파전달속도 측정오차를 보였으며 음파감쇠 역시 기존 시스템과 유사한 결과 값을 도출하였다. 새로이 개발된 PXI(PCI eXtentions for Instrumentation)를 이용한 LabVIEW기반 음향특성 측정시스템은 퇴적물의 음향특성을 보다 신속하고 효율적으로 측정할 수 있고 측정 자료뿐만 아니라 분석된 신호의 원시자료에 대해서도 지속적인 데이터베이스를 구축할 수 있다.Abstract − A previous velocity measurement system for marine sediment had several problems such as the errorsoccurred when picking first arrival time and the inconvenient measurement procedure. In order to resolve theseproblems, we developed a new acoustic properties measurement system by using PXI (PCI eXtentions for Instru-mentation) module based on LabVIEW. To verify the new system, we measured the velocity and attenuation ofsediment using the new system in a parallel with the previous system under the same experimental environment.The result of measurement showed 1~2% margin of error for the velocity as well as similar attenuation values. Weconcluded that the new system can efficiently measure the acoustic properties of marine sediment. It also has anadvantage to construct the database of acoustic data and raw signal.Keywords: LabVIEW, PXI, Acoustic properties measurement system(음향특성 측정시스템), Marine sediment(해양퇴적물), Velocity(음파전달속도), Attenuation(감쇠)

Core sediment sampling has been conducted to investigate the physical and acoustic properties of inner shelf sediments in the South Sea, Korea. Thirty-two piston core samples were analyzed for sediment texture (grain size as well as, fractions of sand, silt, and clay), physical properties (porosity, water content, bulk density, grain density, and shear strength), and acoustic properties (compressional wave velocity and attenuation). The sediments in the study area consist mostly of homogenous mud. The study area can be divided into two different areas (south of Namhae Island and southeast of Naro Island) by texture, and physical and acoustic properties. The mean grain size generally decreases seaward from the mouth of Yeosu Sound and eastward in the southeast area of Naro Island. The compressional wave velocity decreases southeastward from 1500 to 1480 m/s and then increases to 1520 m/s due to relict sediments deposited in mid-shelf of the South Sea. Porosity, water content, and bulk density show an inverse pattern to velocity. The geoacoustic properties of the study area are generally similar to those of continental terrace sediment studied by Hamilton in the North Pacific. The area south of Namhae Island area matches Hamilton's model better than the area southeast of Naro Island. This result is believed to be related to the higher silt and sand contents as well as lower clay content than southeast of Naro Island.

High-resolution (Chirp and Sparker system) seismic profiles were analyzed to investigate the sequence stratigraphy of late Quaternary deposits in the southeastern Yellow Sea. Approximately 1560 line-km data of chirp and sparker profiles were acquired, together with 11 piston cores. Two previous long drill cores (YSDP-102 and 103) were used for comparison with seismic data. High-resolution seismic profiles image the Holocene mud deposits and the complex sedimentary structure in this area. Sequence analysis of high-resolution seismic profiles reveals that the shelf deposits form a succession of high-frequency (five-order) sequences consisting of one depositional sequence developed during the late Quaternary. The depositional sequence includes five sedimentary units, each with different seismic facies and geometry: (1) incised channel fill and lowstand deltaic wedge (unit SY1), (2) sand ridges (unit SY2), (3) estuarine/deltaic mud (unit SY3), (4) redeposited mud (unit SY4), and (5) recent distal mud (unit SY5). Based on the interpretation of high-resolution seismic records and correlation with the YSDP-102, 103 long cores and piston cores, late Quaternary deposits in the southeastern Yellow Sea consists of a set of the lowstand (unit SY1), early transgressive (unit SY2), middle transgressive (unit SY3), late transgressive (unit SY4), and highstand systems tract (unit SY5) formed since the last-glacial period. The rather unusual stratigraphic architecture including three systems tracts is largely controlled by the postglacial sea-level changes and regionally circulation pattern associated with sediment erosion and redeposition. The results of this study present firstly clear seismic evidence that the southeastern Yellow Sea mud belt (SEYSM) can be divided into three stratigraphic units (units SY3, SY4, and SY5) bounded by distinct bounding surfaces.

High-resolution seismic profiles coupled with sediment sampling were analyzed to investigate the acoustic characters and distribution patterns of the late Holocene sediments in Gamak Bay of the South Sea, Korea. The mean grain size of surficial sediment lies around . Sediments in the bay consist of silt and clay with progressive decrease toward the inner bay. The seismic sedimentary sequence overlying the acoustic basement can be divided into two sedimentary units (GB I and II) by a prominent mid-reflector (Maximum Flooding Surface; MFS). The acoustic basement occurs at the depth between 20 m and 40 m below the sea-level and deepens gradually southward. The GB I, mostly occupying the channel-fill, is characterized by reflection-free seismic facies. It can be formed as late Transgressive System Tract (TST), interpreted tidal environment deposits. MFS appears at the depth of about 15~28 m below the sea-level and is well defined by even and continuous reflectors on the seismic profile. The GB II overlying MFS is composed of acoustically transparent to semitransparent and parallel internal reflectors. GB II is interpreted as the Highstand System Tract (HST) probably deposited during the last 6,000 yrs when the sea level was close to the present level. Especially, it is though that the GB II was subdivided into two layers (GB II-a and II-b) by a HST-reflector and this was classified by wind, sea water flux, and tidal current.

A long core of 23.6 m was acquired at the DH-1 site (37°36.651′N and 129°19.709′E) in the Korean continental margin of the western East Sea. The core site is located near the Donghae City and the water depth is 357.8 m deep. A geoacoustic model was reconstructed for 7 geoacoustic units of the core sediments, based on the measurements of 125 P-wave velocities and 121 attenuations. The P-wave speed was compensated to in situ depth below the sea floor using the Hamilton method. The geoacoustic model DH-1 probably contributes for reconstruction of geoacoustic models reflecting vertical and lateral variability of acoustic properties in the Korean continental margin of the western East Sea.

Transverse Acoustic Variability Experiment (TAVEX) is a joint ROK-US acoustic experiment for testing transverse acoustic variability in the northern limit of the East China Sea. TAVEX was performed in a water depth of 70–90 m around 32°30 to 40N, 125°20 to 30E, about 150 km southwest of Cheju Island, in August 2008. Purpose of this paper is to provide geoacoustic models of the TAVEX area, integrated and detailed as available. Two geoacoustic models of three layers were reconstructed, located at 32°40N/125°30E and 32°30N/125°30E of the northern East China Sea, respectively. The models were based on the data of the high-resolution subbottom and air-gun seismic profiles with cores. Geoacoustic data of the cores were extrapolated down to a depth of 100 m of the geoacoustic models. For actual underwater experiments, the P-wave speed of the models was compensated to in situ depth below the sea floor using the Hamilton method.

High-resolution (Chirp and Sparker system) seismic profiles were analyzed to investigate the sedimentary environment and sequence stratigraphy of the late Quaternary deposits in the East China Sea. Echo types in the study are divided into three major echo types which are classified as flat (transgressive sediment sheets, relict sands, and Holocene muds), mounded (tidal ridges), and irregular echoes (channels). Sequence analysis of high-resolution seismic profiles reveals that the shelf deposits form a succession of high-frequency (fifth-order) sequences consisting of three depositional sequences (DI, DII, and DIII) developed during the late Quaternary. The depositional sequence includes five sedimentary units, each with different seismic facies and geometry: (1) regressive estuarine/deltaic wedge (unit J1), (2) regressive deltaic/offshore deposits (unit J2), (3) low-stand deltaic wedge with incised-channel fill (unit J3), (4) transgressive estuarine/deltaic complex (unit J4-a) and sand ridges/sheet (unit ...

In order to characterize geoacoustic provinces of the South Sea shelf of Korea, sediment texture, physical, and geoacoustic properties (p-wave velocity) were measured and calculated based on approximately 350 core samples collected from the study area. The empirical relationship between mean grain size and velocity suggested by Hamilton was used for calculation of p-wave velocity. The inner shelf sediments of the South Sea are characterized by Holocene muddy sediments derived from nearby rivers (i.e., mostly supplied from Nakdong and Seomjin rivers near the Korean shore), whereas the outer shelf areas are dominated by Quaternary sediments (e.g., relict sediments, due to a rising of sea level during the Quaternary) depending on seafloor bathymetry. In this study, the seafloor of the South Sea shelf is differentiated into 4 provinces based on sediment properties (esp. mean grain size and velocity). Province I consisted of muddy sediments affected directly by the Nakdong and Seomjin Rivers discharge. Province II was covered by the sediments transported from two rivers. Province III is marked by mixed Holocene fine materials and coarse Pleistocene materials. Province IV is dominated by relict sediments characterized by Pleistocene coarse sandy materials. Mean grain size, velocity, and wet bulk density increase from Province I (>7.4o, 1515 m/s, and 1.52 g/cm 3 , respectively) through Province II (5.4o, 1597 m/s, and 1.69 g/cm 3 ) to Province III (2.9o, 1766 m/s, and 2.0 g/cm 3 ). Porosity, on the other hand, exhibits an opposite trend, decreasing from Province I (71.3%) to Province II (60.3%) and decreasing significantly in Province III (44.6%). Province IV is characterized by the highest velocity (1979 m/s average) and the coarsest material (2.1o). Nevertheless the values of physical properties such as wet bulk density, porosity, and density lie between Province II and III.

High resolution seismic profiles (chirp and sparker system) were analyzed for the interpretation of nearshore sedimentary environments of Suyeong Bay, Busan. The sedimentary sequence is classified into three seismic units (SU1a, SU1b, and SU2), overlying acoustic basement, and each units can be defined as erosional and disconformable strata. The lowermost SU1a is characterized by the acoustically parallel and prolonged inner reflections, compared with the upper SU1b displays irregular internal reflectors. The uppermost unit, SU2, is acoustically transparent. The acoustic basement is incised with channels, probably due to the active erosion during the early period of transgression. The acoustic basement deepens eastward in the study area, suggesting primary association with the Suyeong River. The upper SU1a and SU1b units constitute lowland-fill strata. SU2 is widely distributed over the study area. High resolution seismic profiles of Suyeong Bay provide significant information crucial to the interpretation of sedimentary environmental history, which is closely related to the sea level change, estuarine environment and influx of terrestrial sediments from the adjacent rivers.

High-resolution (Chirp and Sparker system) seismic profiles were analyzed to investigate the sedimentary sequence and distribution pattern of the late Holocene deposits in Gyunggi Bay, the Yellow Sea. The bay is located in the western part of Korea, east of the Yellow Sea. The sedimentary sequence divided into three units bounded by erosional bounding surface: (1) acoustically parallel to subparallel reflectors with cross bedding structures (Unit 1); (2) confused inner reflectors and top of unit exposed partially at the seafloor (Unit 2); and (3) approximately parallel reflections and regressive to transgressive incision-fills (Unit 3). On the basis of seafloor morphology, surface bedforms, and subbotom acoustic characters, echo types in the study area were identified following the schemes of Chough et al. (2002); (1) flat seafloor with sharp bottom echoes (echo types 1-1, 1-2 and 1-3; transgressive sediment sheets or relict sands), (2) mounded seafloor with either smooth surface or superposed bedforms (echo types 2-1 and 2-2; tidal ridges), and (3) various-scale eroded seafloor (echo types 3-1 and 3-2; channels). Suspect features of acoustic turbid zones which is related to gas charged sediment are reported.

High-resolution seismic survey and sediment core sampling were conducted to investigate acoustic characteristics of gas-bearing sediments in Jinhae Bay, the southeast of Korea. The sediment in Jinhae Bay is mostly homogenous mud deposited after the Holocene transgression. Along with the 410 km of chirp seismic profiling, five piston core samples were collected on the track lines. Gassy sediments are common and occur widely in the bay. Core samples were analyzed for sediment texture, physical properties (porosity, water content, bulk density, and grain density), acoustic properties (compressional wave velocity and attenuation), and electrical resistivity. X-radiograph image analysis was also performed to observe the shape of degassing cracks. There is no significant downcore variation on physical and sediment textures regardless of existence of gas bubbles. However, compressional wave velocity dramatically decreases from average 1480 to 1380∼739 m/s for the cores that penetrate the gas-bearing zones. This ...

During Ocean Drilling Program Leg 138, eleven sites were drilled. The sediments recovered are mainly composed of carbonate-rich pelagic ooze including minor diatom and radiolarian ooze. Average carbonate contents of Sites 846 and 850 are about 59% and 75%, respectively. The porosity and velocity measured in laboratory were corrected to in situ condition using empirical equation (Hamilton, 1976) and delta velocity (log value-laboratory value). The corrected laboratory porosity matches well with logging data, but it is deviating in the intervals of low carbonate content. The correlation between log porosity and the corrected porosity shows a similar trend. Depth profiles between the corrected velocity and logging data do not agree in the intervals of low carbonate content, but the intervals characterized by high carbonate content are relatively matched. This suggests that Hamilton's equation is not appropriate for diatom-rich sediments. The correlation between log velocity and the corrected velocity is also...

Six sites were drilled on ODP Legs 127 and 128. One of the significant results of this drilling was the detection of an opal amorphous silica (opal-A) and opal cristobalite (opal-CT) transformation at all the drill sites. Laboratory measurements of electrical resistivity and physical properties (e.g., velocity, porosity, and density) were conducted on the sediments and rocks sampled from the four sites (Sites 794, 795, 796, and 797). Electrical resistivity and physical properties data show significant change at the opal-A/opal-CT and the opal-CT/Quartz boundaries, suggesting the effects of silica diagenesis. Electrical resistivity reflects the change of physical properties (esp., velocity, wet bulk density, and porosity) well throughout the sites. The transformation boundary is easily recognized in the electrical and physical properties. Electrical resistivity increases by about 50% and 950% across the opal-A/opal-CT and opal-CT/Quartz boundaries, respectively. Thus, electrical resistivity as well as physical properties may be used to identify lithology and diagenetic boundaries such as opal-A/CT, opal-CT/Quartz, and to estimate the reservoir character (e.g., permeability and pore geometry) of sediments and rocks.

Two sites (Sites 1150 and 1151) located in the deep-sea terrace on the landward side of the Japan Trench were drilled during the Ocean Drilling Program (ODP) Leg 186. Measurements of sediment physical and acoustic properties were made on whole-round core sections and on the least-disturbed portions of split-core sections. Compressional wave velocity generally increases with depth. But the data below 600 meters below seafloor (mbsf) is more scattered due to velocity variation caused by microstructures than that of the upper section (

Two sites (Sites 1150 and 1151) located in the landward deep-sea terrace of the Japan Trench were drilled during the Ocean Drilling Program (ODP) Leg 186. Site 1150 is seismically active, whereas Site 1151, aseismic. Measurements of sediment physical properties were made on both whole-round core and split-core sections. Bulk density is relatively uniform with similar variation with depth, although its absolute values are slightly different. It is most likely attributable to coring disturbance. Magnetic susceptibility is characterized by the highest value at the lithologic unit I. Depth variation of water content and porosity shows gradual decreasing pattern, except for lithologic unit I, similar to that of natural Gamma radiation. The abnormal characteristics of unit I are likely related to the under-consolidation caused by rapid deposition (100–200 m/myr) as well as impermeable nature of the sediment preventing dewatering. Grain density is higher in the unit I, but below which is relatively constant with depth. P-wave velocity gradually increases from 1500 to 2000 m/s with depth, caused by sediment compaction and/or consolidation. Sediment physical properties of both sites are very similar, although p-wave velocity is slightly higher (≈20 m/s on the average) at Site 1151. This indicates that sediment source, mineral composition and sediment diagenesis are not significantly different at both sites, implying that seismic activities such as earthquake do not likely cause marked changes of sediment physical properties.

Shallow gas in the Korea Strait shelf mud (KSSM) off SE Korea, revealed by high-resolution subbottom profiles, is associated with acoustic blanking, acoustic turbidity, seepages with plumes in the water column, and seafloor depressions. The acoustic blanking, characterized by strong, consistent top reflection and wipeout below, is most dominant. The seaward edge of the acoustic blanking zone generally coincides with the 100-m water-depth contour, suggesting that the water depth (the pressure) may control the distribution of shallow gas. The acoustic turbidity, characterized by diffuse top reflection, is a dark smear, partially blanking the data below. The seepages with plumes, characterized by vertical smearing and disturbed seafloor, are seen only along the shallowest, landward edge of the acoustic blanking zone. This may suggest that the decreased gas solubility at shallow water depths, caused by the lowered pressure, increases the volume of free gas in the sediments, facilitating the gas escape. The se...

High-resolution profiles (chirp sonar, 2–7 kHz) acquired from the Ulleung Basin, East Sea (Sea of Japan) revealed several discrete echo types, suggesting deposits formed mainly by mass flow processes. The Ulleung Basin plain sediments are characterized by turbidite/hemipelagic deposits, while slide, slump, and debrite deposits are mainly found in the slope area. Rock fall deposits also appear near the Ulleung Island. The core sediments show distinctive downslope variations in most physical properties studied. Over the plain and the slope, mass physical properties (water content, porosity, wet bulk density, shear strength, and velocity) show considerably different values in spite of the nearly constant sediment texture. However, the relationships among the physical properties have relatively good correlations, although the absolute values are different. Grain density in the plain is relatively lower than the slope, probably because of a tephra and calcite compensation depth (CCD). Therefore, we suggest that the slope sediments may be affected by sedimentological changes resulting from diagenesis such as a certain degree of compaction and/or consolidation.

Variations in the calcite compensation depth (CCD) in the East Sea (Sea of Japan) since the early middle Miocene were inferred from data on age-depth relationships, sediment carbonate, and benthic foraminifers from ODP sites 794, 795, and 797. The CCD remained relatively shallow during much of the middle Miocene–Pliocene, and deepened sharply at the beginning of the Pleistocene. Since then it has fluctuated rapidly, possibly in relation to the onset of the northern hemisphere glacial cycles in the late Pliocene. The average CCD has deepened since at least the early middle Miocene, coinciding with the long-term drop in eustatic sea level.

Foraminiferal assemblage and stable isotopic data are presented for three Quaternary piston cores from Ulleung Basin, East Sea of Korea ((ESK) Japan Sea) near the Korean Peninsula. Major changes in both temperature and salinity strongly affected surface and deep waters of the ESK during the transition from the Last Glacial Maximum (LGM) to the middle Holocene. Local environmental effects dominated during the LGM and the Bolling/Allerod (B/A) when the ESK became semi-isolated from the Pacific Ocean. Regional/global influences dominated following the B/A, after sufficient reconnection with the Pacific. This is reflected in the foraminiferal δ18O record which was largely salinity-controlled before the Younger Dryas (YD) and temperature-controlled after the YD. Paleoceanographic changes in the ESK during the last deglaciation reflect sequential reconnection with the Pacific Ocean, through gateways, first (B/A) in the north (Tsugaru Strait) and later (Holocene) in the south (Korea Strait).

High-resolution (chirp, 2–7 kHz) reflection profiles from the southern continental slope and basin plain of the Ulleung Basin reveal three echo types. Sediment texture and geotechnical properties were analyzed using 22 piston cores. Echo type 1 shows a sharp bottom echo with discrete, continuous subbottom reflectors with a flat or undulatory upper surface interpreted as turbidite/hemipelagic sediment. It is dominant in the basin plain. Echo type 2 is characterized by laterally wedged, an acoustically transparent masses (debrites). This type occurs in the southern lower slope and base-of-slope area. Echo type 3 shows irregular blocky/lumpy or hyperbolic masses with various amount of internal deformation, (interpreted as slide/slump deposits). It occurs in the middle to upper slope area. These echo types show a zonal distribution.

High-resolution seismic profiles were analysed to investigate the sequence stratigraphy of late Quaternary deposits in the southeastern Yellow Sea of Korea. Approximately 1500 line-km data of chirp and sparker profiles were acquired. We also used deep drill cores to interpret the sedimentary sequence. High-resolution seismic profiles reveal the Holocene mud deposits and the complex sedimentary structure. The late Quaternary deposits in the study area can be divided into four sedimentary units (units A, B, C, and D from oldest to youngest) bounded by an erosional surface and internal seismic reflector: (1) incised channel fill/lowstand deltaic wedge (unit A); (2) transgressive sand ridges and sand sheets (unit B); (3) estuarine/deltaic or shallow-water mud (unit C); and (4) distal mud patch (unit D). Unit C is divided into two sedimentary subunits on the basis of high amplitude and unconformity: (i) estuarine/deltaic mud (unit C-a); and (ii) mud redeposited by sea-level rise and strong tidal currents (unit C-b). The evolution of units C-a and C-b is closely related to postglacial sea-level changes, sediment erosion, and reworking. These late Quaternary deposits have been controlled by sea-level change, tidal currents, and sediment erosion.

Laboratory measurements of ultrasonic velocity (VP, VS) and attenuation (Q−1P, Q−1S) in deep-sea carbonate sequences at DSDP Sites 288, 289 and 316 in the equatorial Pacific were made in conjunction with studies of sediment density, porosity and pore geometry in order to investigate the role of diagenesis in the development of physical properties. Bulk porosity decrease appears to be related more significantly to depth of burial than to age of strata. Both depth of burial and age, however, are important factors controlling the modal pore diameter. In deep-burial diagenesis the modification of pore geometry is influenced by the presence of silica during diagenesis. In carbonate sequences at the three DSDP sites studied, shear wave attenuation anisotropy ( Q −1 SHH Q −1 SHV ) correlates with the shear wave velocity anisotropy. Pore orientation, resulting from overburden pressure and other deep-burial diagenetic processes, is an important factor controlling the increase of VP anisotropy with age and depth of burial. On the basis of observed minor changes in anisotropy values with increasing pressure for some samples, other contributions to VP anisotropy such as grain orientation and bedding lamination cannot be ruled out.

Laboratory studies have been carried out to determine the causes of velocity and attenuation anisotropy in pelagic carbonate sediments from Deep Sea Drilling Project site 288 on the Ontong-Java Plateau. Compressional velocity Vp was measured under independently controlled pore pressure and confining pressure. Velocity anisotropy for compressional waves (horizontal velocity > vertical velocity) is found to decrease with increasing effective pressure, i.e., with closure of pores. Previous studies have suggested that velocity anisotropy in pelagic carbonate sequences is due largely to preferred vertical orientation of calcite c axes. The present results indicate that preferred horizontal orientation of grain contacts and flat pores is also a significant cause of the observed anisotropy for many samples. Significant anisotropy in ultrasonic attenuation for compressional (Qp−1) and shear (Qs−1) waves is observed such that the losses are greatest for propagation modes that can generate relative motion across horizontal grain contacts or flat horizontal pores. The results are discussed in terms of the possible mechanisms of dissipation.