Search Results (28)

The high-resolution seismic method can provide acoustic reflectivity images of shallow marine geology structures. In South Vietnam, the demand for construction materials like sand is high; therefore, the exploration of its deposits is considered in this study. This study investigated an around 200-square-kilometer area offshore Bac Lieu using 2D seismic sub-bottom lines. We employed the processed seismic amplitude and its seismic attributes as mean and variance textures to interpret the data. The processed seismic amplitude and its attributes can represent the young Holocene sediments (i.e., sand, silt, clay, and their mixtures) thanks to their different seismic patterns. Our interpretation result consists of 3D horizons of the seabed, Holocene silt, and sand sediments, which are compatible with the prior geological information, including three nearby drill holes. The seabed gradually descends from 10.0 m to 19.0 m over a horizontal distance of around 11 km. Moreover, the interpreted results show that the sand sediments reside in the center of survey area, with a maximum thickness of around 12.0 m. Interestingly, a fill sediment channel effectively separates two different zones of young Holocene sand and silt sediments. The findings provide valuable information for Vietnamese government officers to develop sustainable policies and regulations for marine mineral exploitation and exploration. Full article

Marine sand, in addition to oil and gas resources, is the second-largest marine mineral resource. The rational development and utilization of marine sand resources are conducive to the growth of the marine economy. In the process of marketing marine sand in China, local authorities are required to delineate auctioned sand mining areas after a general survey, commonly referred to as preliminary exploration. Marine sand can be categorized into surface marine sand and buried marine sand. Buried marine sand deposits are buried beneath the sea floor, making it challenging to locate them due to their thin thickness. Consequently, there exist numerous technical difficulties associated with marine sand exploration. We conducted the preliminary research work in the waters off Guangdong Province of the South China Sea, employing a reduced drilling and identifying a potentially extensive deposit of marine sand ore. In this study, various geophysical methods such as sub-bottom profile survey, single-channel seismic survey, and drilling engineering were employed in the northern offshore waters of the South China Sea. As a result, two distinct marine sand bodies were delineated within the study area. Additionally, five reflective interfaces (R₁, R₂, R₃, R₄, and R₅) were identified from top to bottom. These interfaces can be divided into five seismic sequences: A₁, B₁, C₁, D₁, and E₁, respectively. Three sets of strata were recognized: the Holocene Marine facies sediment layer (Q₄^m), the Pleistocene alluvial and pluvial facies sediment layer (Q₃^al+pl), as well as the Pleistocene Marine facies sedimentary layer (Q₃^m). In total, two placers containing marine sand have been discovered during this study. We estimated the volume of marine sand and achieved highly favorable results of the concept that we are proposing a geologic exploration approach that does not involve any previous outcropping analogue study. Full article

The Amapá margin, part of the Brazilian Equatorial Margin (BEM), is a key region that plays a strategic role in the global climate balance between the North and South Atlantic Ocean as it is strictly tied to equatorial heat conveyance and the fresh/salt water equilibrium with the Amazon River. We performed a new scientific expedition on the Amapá continental shelf (ACS, northern part of the Amazon continental platform) collecting sediment and using instrumental observation at an unstudied site. We show here the preliminary outcomes following the applied methodologies for investigation. Geophysical, geological, and biological surveys were carried out within the ACS to (1) perform bathymetric and sonographic mapping, high-resolution sub-surface geophysical characterization of the deep environment of the margin of the continental platform, (2) characterize the habitats and benthic communities through underwater images and biological sampling, (3) collect benthic organisms for ecological and taxonomic studies, (4) define the mineralogical and (5) elemental components of sediments from the study region, and (6) identify their provenance. The geophysical data collection included the use of bathymetry, a sub-bottom profiler, side scan sonar, bathythermograph acquisition, moving vessel profiler, and a thermosalinograph. The geological data were obtained through mineralogical, elemental, and grain size analysis. The biological investigation involved epifauna/infauna characterization, microbial analysis, and eDNA analysis. The preliminary results of the geophysical mapping, shallow seismic, and ultrasonographic surveys endorsed the identification of a hard substrate in a mesophotic environment. The preliminary geological data allowed the identification of amphibole, feldspar, biotite, as well as other minerals (e.g., calcite, quartz, goethite, ilmenite) present in the substrata of the Amapá continental shelf. Silicon, iron, calcium, and aluminum composes ~85% of sediments from the ACS. Sand and clay are the main fraction from these sediments. Within the sediments, Polychaeta (Annelida) dominated, followed by Crustacea (Arthropoda), and Ophiuroidea (Echinodermata). Through TowCam videos, 35 taxons with diverse epifauna were recorded, including polychaetes, hydroids, algae, gastropods, anemones, cephalopods, crustaceans, fishes, and sea stars. Full article

Some seismo-stratigraphic evidence on the occurrence of wave-cut marine terraces in the Licosa promontory (Southern Tyrrhenian Sea, Italy) based on Sub-bottom Chirp seismic sections is herein presented. Such evidence is provided by marine terraced surfaces situated at various water depths below sea level and etched into the rocky acoustic basement, which are extensively extending in the seaward extension of the Licosa promontory. It is possible that the isotopic stratigraphy and the terraced marine surfaces are connected, so they can be attributed and dated indirectly. The geologic study of seismic profiles has pointed to the prominence of the acoustic basement, extending to the seabed close to the coast and subsiding seawards under the Quaternary marine succession. Ancient remains of marine terraces, found at a range of water depths between 5 m and 50 m, have documented the major morphological changes of the acoustic basement during the Late Quaternary. Full article

The Patras Gulf Pockmark field is located in shallow waters offshore Patras City (Greece) and is considered one of the most spectacular and best-documented fluid seepage activities in the Ionian Sea. The field has been under investigation since 1996, though surveying was partially sparse and fragmentary. This paper provides a complete mapping of the field and generates new knowledge regarding the fluid escape structures, the fluid pathways, their origin and the link with seismic activity. For this, data sets were acquired utilising high-resolution marine remote sensing techniques, including multibeam echosounders, side-scan sonars, sub-bottom profilers and remotely operated vehicles, and laboratory techniques focusing on the chemical composition of the escaping fluids. The examined morphometric parameters and spatial distribution patterns of the pockmarks are directly linked to tectonic structures. Acoustic anomalies related to the presence of gas in sediments and in the water column document the activity of the field at present and in the past. Methane is the main component of the fluids and is of microbial origin. Regional and local tectonism, together with the Holocene sedimentary deposits, appear to be the main contributors to the growth of the field. The field preserves evidence that earthquake activity prompts the activation of the field. Full article

This study combines surface heat flow, multi-channel seismic reflection profiles, and ocean-bottom seismometer (OBS) profiles to determine the thermo-rheological structure of the Qiongdongnan Basin (QDNB) and Pearl River Mouth Basin (PRMB), with the aim of researching the west–east variation of the passive continental margin rifting. Based on the initial lithospheric rheological model of a jelly sandwich-1 (JS-1) regime, the current architecture of the continental margin is identified to be the result of a non-uniform extension. Due to the decoupled crust–mantle relationship caused by the weak lower crust, the non-uniform extension led to the rupture of the mantle lithosphere before the crust. The central Xisha Trough falls into the JS-2 regime with only one brittle load layer, which is close to the rigid oceanic lithosphere of the Northwest Sub-basin (NSB). The high-velocity layers (HVLs) and detachment faults beneath the Xisha Trough are considered to be the result of the cooling of a thinned lower crust with mantle underplating during the middle stage of continental margin rifting. A seaward-increasing trend of lithospheric rheological strength is exhibited across the PRMB, from the crème brûlée-1 (CB-1) regime at the continental shelf to the JS-2 regime at the NSB. Unlike the HVLs of the Xisha Trough, the lower crustal HVLs beneath the eastern PRMB formed during the late stage of continental margin rifting due to the mantle lateral flow. The absence of HVLs beneath the western PRMB may indicate that the mantle lateral flow demonstrates a limited impact. Full article

High-resolution seismoacoustic data represent a useful tool for the investigations of gas-charged sediments occurring beneath the seabed through the identification of the diagnostic intrasedimentary features associated with them. Acoustic blanking revealed shallow gas pockets in the seismostratigraphic units of the inner shelf off the Northern Cilento promontory. Six main seismostratigraphic units were recognized based on the geological interpretation of the seismic profiles. Large shallow gas pockets, reaching a lateral extension of 1 km, are concentrated at the depocenter of Late Pleistocene–Holocene marine sediments that are limited northwards by the Solofrone River mouth and southwards by the Licosa Cape promontory. A morphobathymetric interpretation, reported in a GIS environment, was constructed in order to show the main morphological lineaments and to link them with the acoustic anomalies interpreted through the Sub-bottom chirp profiles. A newly constructed workflow was assessed to perform data elaboration with Seismic Unix software by comparing and improving the seismic data of the previously processed profiles that used Seisprho software. The identification of these anomalies and the corresponding units from the offshore Cilento promontory represent a useful basis for an assessment of marine geohazards and could help to plan for the mitigation of geohazards in the Cilento region. Full article

Marine acoustic sources are widely used for geophysical imaging, oceanographic sensing, and communicating with and tracking objects or robotic vehicles in the water column. Under the U.S. Marine Mammal Protection Act and similar regulations in several other countries, the impact of controlled acoustic sources is assessed based on whether the sound levels received by marine mammals meet the criteria for harassment that causes certain behavioral responses. This study describes quantitative factors beyond received sound levels that could be used to assess how marine species are affected by many commonly deployed marine acoustic sources, including airguns, high-resolution geophysical sources (e.g., multibeam echosounders, sidescan sonars, subbottom profilers, boomers, and sparkers), oceanographic instrumentation (e.g., acoustic doppler current profilers, split-beam fisheries sonars), and communication/tracking sources (e.g., acoustic releases and locators, navigational transponders). Using physical criteria about the sources, such as source level, transmission frequency, directionality, beamwidth, and pulse repetition rate, we divide marine acoustic sources into four tiers that could inform regulatory evaluation. Tier 1 refers to high-energy airgun surveys with a total volume larger than 1500 in³ (24.5 L) or arrays with more than 12 airguns, while Tier 2 covers the remaining low/intermediate energy airgun surveys. Tier 4 includes most high-resolution geophysical, oceanographic, and communication/tracking sources, which are considered unlikely to result in incidental take of marine mammals and therefore termed de minimis. Tier 3 covers most non-airgun seismic sources, which either have characteristics that do not meet the de minimis category (e.g., some sparkers) or could not be fully evaluated here (e.g., bubble guns, some boomers). We also consider the simultaneous use of multiple acoustic sources, discuss marine mammal field observations that are consistent with the de minimis designation for some acoustic sources, and suggest how to evaluate acoustic sources that are not explicitly considered here. Full article

The paper discusses the problem of localizing zones of high pore pressure in sub-bottom sediments (first tens of meters under the seafloor). Prediction of the overpressure zones in the near-surface is required for the mitigation of risks at the early stages of the offshore hydrocarbon field exploration and development. The results of seismic data interpretation generally serve as the main source of information for this kind of problems, yet there are other methods to predict overpressure zones in the subsurface. The paper presents the results of the overpressure zone prediction using a set of methods including empirical ones, and the approach based on rock-physics modeling that features the soft-sand model of unconsolidated media effective properties. While the rock-physics modeling grants the most reliable result, it is also the most demanding method to the input data. Hence, it can be used to verify other methods of the overpressure zone prediction. We present the results of the overpressure zone prediction at the research site on the Black Sea shelf. The mitigation of the drilling risks via changing the drilling conditions is discussed in detail. As the drilling through the overpressure zones is often a necessity, the engineering solutions proposed in the paper can be applied elsewhere when facing similar problems typical for offshore exploration. Full article

Koločep bay is a 30 km elongated karst basin located between the Elafiti Islands and the mainland with a NW–SE general direction. The bay lies within the seismically active southern Dalmatia zone. Irregular grid sub-bottom profiles and two legacy reflection seismic profiles have been used to determine the overall morphology of the bay and to establish the seismic stratigraphy of its sedimentary infill. Three major seismic–stratigraphic units have been identified in the upper ~50 m of the ~120-meter-thick sedimentary sequence that lies above the karstified limestone bedrock. The karst polje basin was flooded due to sea-level rise before 12.1 kyr BP. The morphology of the bay implies complex influences of Late Glacial meltwater discharges, aeolian sand deposition, the existence of paleo–ponor/karst spring zones and buried Pleistocene river channels. The Pleistocene seismotectonic units are deformed in the NW and SE parts of the basin. The central part of the basin has no signatures of intensive tectonic activity during the Holocene. A major erosion event was identified that led to the formation of a basin within the older sedimentary infill. In the southern part of the basin, we have evidence of Holocene tectonic activity with the formation of erosional scarps on the seafloor of the bay. Full article

In this study, we extracted three source wavelets of a Chirp sub-bottom profiler (SBP), which is widely used for high-resolution marine seismic exploration, using a MATLAB-based graphical user interface tool for computational processing. To extract the source wavelet for general seismic exploration data processing techniques, including that for Chirp SBPs, we first evaluated source wavelet extraction techniques using an adaptive machine learning filter. Subsequently, we performed deterministic deconvolution by extracting the optimal source wavelet from the raw data of Chirp SBP. This source wavelet was generated by applying an adaptive filter. Various methods have been studied to solve the multivariate optimization problem of minimizing the error; in this study, a least-mean-square algorithm was selected owing to its suitability for application to geophysical time-series data. Extracting the source wavelet is a crucial part of high-resolution marine seismic wave exploration data processing. Our results highlight the effectiveness of performing deterministic deconvolution by extracting source wavelets using adaptive filters, and we believe that our method is useful for marine seismic exploration data processing. Full article

Seabed surveying is the basis of engineering development in shallow waters. At present, geophysical survey methods mainly utilize sonars for qualitative surveying, which requires the calibration of the results found through in situ drilling and sampling. Among them, the parameters required for engineering designs are obtained from either in situ tests or laboratory experiments of soil samples retrieved from drilling. However, the experience from onshore applications shows that the physical quantities obtained through quantitative geophysical survey methods for shallow waters can be indirectly used to estimate engineering parameters or directly as parameters for engineering evaluation, which has high application potential. This review analyzes various geophysical survey methods for nearshore site characterization (i.e., side-scan sonar, single/multi- beam sonar, sub-bottom profiler, seismic reflection method, and underwater magnetometer) and challenges in their application, and introduces quantitative geophysical survey methods (including the underwater seismic refraction method, seismic surface wave method and underwater electrical resistivity tomography) that are worth focusing on for future development. Three application difficulties have been identified, namely, the lack of operational efficiency, appropriate operational equipment and systems, and sufficient guidance for experimental shallow sea applications. It is hoped that comprehensive discussion of these challenges will increase awareness leading to engineering improvements in the surveying and measuring capabilities in shallow waters, further reducing the risk of geotechnical hazards. Full article

We compare sediment vertical methane flux off the Mahia Peninsula, on the Hikurangi Margin, east of New Zealand’s North Island, with a combination of geochemical, multichannel seismic and sub-bottom profiler data. Stable carbon isotope data provided an overview of methane contributions to shallow sediment carbon pools. Methane varied considerably in concentration and vertical flux across stations in close proximities. At two Mahia transects, methane profiles correlated well with integrated seismic and TOPAS data for predicting vertical methane migration rates from deep to shallow sediment. However, at our “control site”, where no seismic blanking or indications of vertical gas migration were observed, geochemical data were similar to the two Mahia transect lines. This apparent mismatch between seismic and geochemistry data suggests a potential to underestimate gas hydrate volumes based on standard seismic data interpretations. To accurately assess global gas hydrate deposits, multiple approaches for initial assessment, e.g., seismic data interpretation, heatflow profiling and controlled-source electromagnetics, should be compared to geochemical sediment and porewater profiles. A more thorough data matrix will provide better accuracy in gas hydrate volume for modeling climate change and potential available energy content. Full article

Seafloor geoacoustic properties are important in determining sound propagation in the marine environment, which broadly affects sub-sea activities. However, geoacoustic investigation of the deep seafloor, which is required by the recent expansion of deep-water operations, is challenging. This paper presents a methodology for estimating the seafloor sound speed, $c_0$, and a sub-bottom velocity gradient, $K$, in a relatively deep-water-compacting (~1000 m) passive-margin setting, based on standard commercial 2D seismic data. Here we study the seafloor of the southeastern Mediterranean margin based on data from three commercial seismic profiles, which were acquired using a 7.2 km-long horizontal receiver array. The estimation applies a geoacoustic inversion of the wide-angle reflections and the travel times of the head waves of bending rays. Under the assumption of a constant positive $K$, the geoacoustic inversion converges to a unique set of parameters that best satisfy the data. The analysis of 24 measurement locations revealed an increase in the average estimates of $c_0$ from 1537 $\pm$ 13 m s⁻¹ to 1613 $\pm$ 12 m s⁻¹ for seafloor depths between ~1150 m and ~1350 m. $K$ ranged between 0.75 and 0.85 m s⁻¹ with an average of 0.80 $\pm$ 0.035 s⁻¹. The parameters were consistent across the different locations and seismic lines and they match the values that were obtained through depth-migration-velocity analysis and empiric relations, thereby validating our estimation methodology. Full article

Lakes are integrators of past climate and ecological change. This information is stored in the sediment record at the lake bottom, and to make it available for paleoclimate research, potential target sites with undisturbed and continuous sediment sequences need to be identified. Different geophysical methods are suitable to identify, explore, and characterize sediment layers prior to sediment core recovery. Due to the high resolution, reflection seismic methods have become standard for this purpose. However, seismic measurements cannot always provide a comprehensive image of lake-bottom sediments, e.g., due to lacking seismic contrasts between geological units or high attenuation of seismic waves. Here, we developed and tested a complementary method based on water-borne electrical-resistivity tomography (ERT) measurements. Our setup consisted of 13 floating electrodes (at 5 m spacing) used to collect ERT data with a dipole–dipole configuration. We used a 1D inversion to adjust a layered-earth model, which facilitates the implementation of constraints on water depth, water resistivity, and sediment resistivity as a priori information. The first two parameters were readily obtained from the echo-sounder and conductivity-probe measurements. The resistivity of sediment samples can also be determined in the laboratory. We applied this approach to process ERT data collected on a lake in southern Mexico. The direct comparison of ERT data with reflection seismic data collected with a sub-bottom profiler (SBP) showed that we can significantly improve the sediment-thickness estimates compared to unconstrained 2D inversions. Down to water depths of 20 m, our sediment thickness estimates were close to the sediment thickness derived from collocated SBP seismograms. Our approach represents an implementation of ERT measurements on lakes and complements the standard lake-bottom exploration by reflection seismic methods. Full article