Scan Sonar Research Articles

Detailed bathymetry and seafloor image datasets, using multi-beam echo sounder and side scan sonar, around Seonam, Ilsan beach, Ulsan

We obtained detailed bathymetry and seafloor image datasets using multi-beam echo sounder and side scan sonar around Seonam at Ilsan beach in Ulsan. The survey was conducted from May 9 to May 12, 2018, using a small vessel. The equipment used for the survey was R2Sonic's Sonic 2020 (Multi-beam Echo Sounder) and EdgeTech’s 4125 (Side Scan Sonar). The detailed bathymetry of the study area shows slightly different topographical features for each part around Seonam. The eastern and southern parts have steep slopes and the depth of the water in the areas deepens away from the surroundings of the submerged bedrock area adjacent to Seonam. The water depth in the western and northern part is shallower with a relatively gentle slope, compared to the eastern and southern parts. The rocky bottom zones are mainly distributed in the eastern part area with the water depth range of 3 to 9 m around the Seonam. Some rocky areas also appear in the western region. Seafloor backscatter image shows irregular, complex reflection patterns of acoustic intensity mainly in the eastern and western adjacent areas around Seonam, and in the northern and southern coastal waters, which are almost identical to the underwater rock zones with shallow water depth. The detailed bathymetry and seafloor image datasets around Seonam, using the multi-beam echo sounder and the side scan sonar, can be used as an essential data for environment habitat mapping.

Freespan Analysis for Subsea Pipeline Integrity Management Strategy

. Over a rough seabed or on seabed subject to scour, freespans can occur when contact between a subsea pipeline and the seabed is lost over an acceptable distance. When this exceeds the allowable freespan length, design stresses can be exceeded, and a vortex induced vibration (VIV) response can be initiated, resulting in the risk of fatigue failure. If this is not predicted and controlled properly, it will affect pipeline integrity, leading to expensive rectification and intervention work. Freespan analysis consisted primarily of a screening check in which the as-found freespans from Remotely Operated Vehicle (ROV) or multibeam Side Scan Sonar (SSS) inspection survey were compared against the design allowable lengths and determine the expected fatigue life of a freespan that may be experiencing Vortex Induced Vibration (VIV). Freespans are considered acceptable if the calculated fatigue life exceeds the design life criteria. This paper describes the freespan analysis that has been developed to perform detailed freespan engineering assessments, incorporating the latest survey and as-laid conditions. This analysis follows a methodology in standard code DNVGL RP F105 that has been accepted and used by operators to produce more accurate and less conservative freespan analysis results, leading to a subsea pipeline integrity management strategy with fewer unnecessary interventions and greater cost benefits.

Калибровка интерферометрического гидролокатора бокового обзора по данным съемки на участке дна с известным рельефом

The paper considers algorithms for calculating the parameters of the interferometric side scan sonar based on the processing of acoustic survey data of the seabed surface with a known relief. The algorithms were tested on mathematical models of signals reflected from a flat horizontal surface. The possibilities of the proposed algorithms are demonstrated on experimental phase-difference measurements of interferometric side scan sonar obtained on a site with a flat seabed. The effectiveness of algorithms for calibration of interferometric side scan sonars is shown.

Morpho-bathymetric acoustic surveys as a tool for mapping traces of anthropogenic activities on the seafloor: The case study of the Taranto area, southern Italy

In this study, an investigation procedure for mapping the traces of anthropogenic activities on the seafloor is proposed. Analyses are based on the interpretation of acoustic data (Multibeam Echosounder and Side Scan Sonar) acquired in the Taranto coastal area. Specific GIS tools supported the acoustic data analysis, interpretation, and mapping. These analyses highlighted that the seafloor of both coastal basins included in the investigated area is affected by a high distribution of traces related to different anthropogenic activities such as dredging, shipping, and mussel farming activities. Such kind of traces resulted efficiently detectable from morpho-bathymetric acoustic data. In particular, groove traces resulted highly distributed in both basins, while sunken mussel farm facilities are widely distributed in the Mar Grande basin. The results highlight as acoustic surveys represent a useful tool for orienting effective coastal management actions. This study points out how geophysical surveys support the geo-environmental characterization of highly urbanized coastal sectors.

Development and preservation of transgressive sandy versus rocky shorelines: Observations from the SE African shelf

Relict rocky shorelines are seemingly rarely preserved in the geological record, often underrepresented in the literature and the reasons for their scarcity, as well as the conditions needed for preservation are not well understood. On the “Wild Coast” shelf along the subtropical east coast of South Africa a series of submerged shorelines are present. High-resolution multibeam bathymetry, side scan sonar, ultra-high-resolution seismic profiling and legacy seismic reflection data show several fluvial channels incised into the shelf, associated with the Last Glacial Maximum (LGM) lowstand. Palaeo-shorelines are preserved at −85, −75, −60 and −40 m. The −85 and −75 m shorelines reflect sediment abundance and depositional features preserved as lagoonal depressions, lithified barrier-spits and coastal barrier dunes with limited bedrock influence. The shallower shorelines at −60 and −40 m occur on a steeper shelf gradient and reflect the development of rocky headlands, palaeo-cliffs and rock shore platforms. Their occurrence implies increasing bedrock control, erosion and sediment bypass during transgression.The preservation of rocky shoreline signatures on the inner shelf is aided by competency of the bedrock lithologies, in addition to their depth relative to sea-level stillstands and rapid rises in sea level associated with meltwater pulses (MWPs) (notably the −60 m shorelines and MWP1B). MWPs may have helped facilitate the overstepping and eventual preservation of these shoreline landforms on the seabed. Preservation of the −40 m shoreline is considered to correlate with accelerated rates in sea- level rise prior to MWP1C. The preservation of the −85 and −75 m aeolianite shorelines are not linked to meltwater pulses, however, their substantial sediment volume precluded total loss through transgressive erosion and this, coupled with early cementation, promoted their partial preservation. Such deposits rarely survive subaerial exposure but could be preserved if covered by marine sediments. Conversely, the existence of well-preserved, Pleistocene-age rocky shorelines on the shelf points to their high potential for inclusion into the longer-term geological record under subsequent high or low sedimentation. The formation of distinctive rocky shorelines, however, may reflect alternating slow and fast rates of sea-level rise.

Semantic Segmentation of Side-Scan Sonar Images with Few Samples

Underwater sensing and detection still rely heavily on acoustic equipment, known as sonar. As an imaging sonar, side-scan sonar can present a specific underwater situation in images, so the application scenario is comprehensive. However, the definition of side scan sonar is low; many objects are in the picture, and the scale is enormous. Therefore, the traditional image segmentation method is not practical. In addition, data acquisition is challenging, and the sample size is insufficient. To solve these problems, we design a semantic segmentation model of side-scan sonar images based on a convolutional neural network, which is used to realize the semantic segmentation of side-scan sonar images with few training samples. The model uses a large convolution kernel to extract large-scale features, adds a parallel channel using a small convolution kernel to obtain multi-scale features, and uses SE-block to focus on the weight of different channels. Finally, we verify the effect of the model on the self-collected side-scan sonar dataset. Experimental results show that, compared with the traditional lightweight semantic segmentation network, the model’s performance is improved, and the number of parameters is relatively small, which is easy to transplant to AUV.

Marine priority habitat mapping in a Mediterranean conservation area (Gyaros, South Aegean) through multi-platform marine remote sensing techniques

The aim of this study is to present the results of the first complete marine habitat mapping through marine remote sensing techniques in Gyaros Island, a remote island in the Cyclades archipelago with a great historical and ecological value. Gyaros Island is of great biological importance and, for this reason, since 2011, is part of the NATURA 2000 network and, by 2019, is characterised as a marine protected area (MPA). More than 80 km2 of seafloor area were inspected through a multi-platform marine remote sensing technique survey through two individual expeditions in 2014 and 2017. The remote sensing survey was conducted utilising side scan sonar, sub bottom profiler and multi- and single-beam echosounders. In addition to the remote sensing survey, an extensive ground truth network was established utilising a underwater towed camera and VanVeen sediment grabber. These resulted in the area being fully covered with high-quality data. Through these, a classification of the area was performed based on three distinct seafloor habitat mapping schemes (Expert, European Nature Information System and NATURA HD). The survey and the classification revealed that over 50% of the seafloor is covered by Posidonia oceanica meadows and coralligenous and other calcareous bio-concretion priority habitats. Based on the results of this work, Gyaros MPA was also established and different protections and conservation zones were set.

Islands of the Tectonic Origin in the Shatt Al-Arab River, Southern Iraq

Several factors control the formation of river islands, such as tectonic factors, sedimentation, erosion, and human activities. This research is concerned with islands in the Shatt Al-Arab channel, which are likely to be traced back to tectonic factors in their formation. The islands of the Shatt Al-Arab River are distributed along its axis starting from the islands of Sindbad and Muhammadiyah, 15 km to the north of the city center of Basra, to the islands of Shatit, Al-Dawasir and Minauhi, 70 km south of the city center of Basrah. This study area is part of the Mesopotamia Plain, southern Iraq. The fieldwork period extended from the beginning of September 2012 to June 2013, which included making subsurface sections (longitudinal and transverse) of the bottom of the Shatt Al-Arab channel using the Sub-bottom profile Strata box device, imaging the topography of the river bottom in a side scan sonar, collecting the bottom sediments by a grab sampler. The volumetric analysis method uses the Master Sizer device based on the Folk classification to calculate the volumetric gradient of sediments. The study showed that the tectonic factor has a prominent role in the growth island Maino and as a result of the presence of sub-surface structures for the structure of the Siba, which appeared to affect the subsurface layers of the river bed near the island as it appeared in the survey sections of the Strata box device. There are some islands that more than one factor emerged as a reason for their formation and development, as in the islands of Muhammadiyah and Umm al-Rasas, where tectonic and sedimentary factors interfered in their formation and development due to the influence of sub-surface structures as well as the influence of nearby rivers such as the Karma and Karun Rivers.

Hydrographic surveys as an art of delineating the impact of climate change on the coastal environment

Marine acoustic techniques are quite efficient in remote seabed classifications and hydrographic investigations by providing highly detailed information about broad areas of the seafloor and the subsurface layers in a short period of time. Climatic change is considered as the forcing factor for the eustatic sea-level-rise along the Mediterranean coast, leading to the large coastal inundation and the subsidence of the ancient maritime installations, these ancient sites can be used as indicators for the relative changes in sea-level through later times. Marine surveys were carried out to delineate the coastal geomorphological changes associated with sea level rises and natural hazards along some areas on the coast of Alexandria during the last two millennia. Hydrographic and seismic surveys were implemented in the study area by using a multi-beam echo-sounder, side scan sonar, and sub-bottom profiler, then the acoustic data were calibrated with dated core samples and ROV camera images. Multi-beam is considered the most commonly used tool in harbor surveys utilizing the returned acoustic signals to measure the depth of the seafloor, while the side scan sonar instrument provides high-resolution images for the elevated structures from the seafloor depending on the backscatter strength of the signal, and the sub-bottom profiler uses chirp waves that deliver high resolution vertical images for the subsurface sediment situation. Results of sonar imaging and bathymetric mapping outlined submerged margins of archaeological remains related to ancient Ptolemaic and Greek ports. Seismic interpretations revealed significant changes in the coastal geomorphology, where the massive burial of the port structure indicated the occurrence of sudden natural hazards originated from seismic waves; therefore, a destructive tsunami wave accompanied by sediment slumping that followed by eustatic sea-level-rise seems to be the dominant factors for this dramatic burial. The research results showed the potentiality of hydrographic surveys in detecting climate change indicators.

A Multidisciplinary Approach for A Better Knowledge of the Benthic Habitat and Community Distribution in the Central and Western English Channel

About 80% of the seabed of the English Channel (EC) is covered by coarse sediment, from coarse sand to pebbles. Quantitative data on the benthic macrofauna in these types of sediment remains are rare due to the difficulty of using grab corers in such hard substrates. The deepest central part of the EC (45–101 m depth) was prospected during two VIDEOCHARM surveys in June 2010 and June 2011 to increase knowledge of such sublittoral coarse sediment benthic habitats. Sampling focussed on a longitudinal transect in the deepest part of the EC (13 boxes), extending from the western approach to the Greenwich meridian. Both indirect (side scan sonar, Remote Operated Vehicule) and direct (grab sampling with benthos determination, and grain-size analyses) approaches were used and combined, permitting description of the benthic habitats and communities using seven methods. Five benthic EUNIS habitats (European Nature Information System) were reported: MC3215, MD3211, MC4, MC3212 and MC4215, of which two extended main habitats (MC3211 and M23212) corresponded to an eastern/western gradient from sandy gravel to sandy gravel and pebbles sediment. Three other spatially discrete habitats were associated with poor coarse sand and gravel habitats as well as sandy gravel and pebbles with the presence of the brittle star Ophiothrix fragilis. Taxonomic richness of both extended habitats was on the same order of magnitude as the coarse sand habitat reported elsewhere in the EC, whilst the abundances were among the lowest in deeper areas with low nutrient input and low primary production. The epifauna appeared relatively homogenous in this type of sediment at the scale of the sampling area and was not determined to assign a EUNIS habitat/class. ROV footage illustrated the presence of large epifauna and provided valuable information to ground truth in other sampling methods such as side scan sonar mosaic. Grab photos showing surface sediment was relevant to determine the sediment type, whilst granulometric analyses gave additional information on fine particles content (typically very low).

Morpho-stratigraphic evolution of a tectonically controlled canyon-channel system in the Gioia Basin (Southern Tyrrhenian Sea)

The Gioia-Mesima Canyon-Channel System (GMS) incises the northern part of the Gioia Basin, a post-Tortonian intra-slope basin located along the south-eastern Tyrrhenian margin. Integration of multibeam, side scan sonar and seismic reflection data allowed to reconstruct the morpho-stratigraphic evolution of the GMS and the main factors controlling its development and maintenance. The GMS drains the entire continental margin and displays a complex morphology consisting in two main courses: the Gioia-Petrace Canyon and the Mesima Canyon-Channel. These courses run parallel to each other, changing from narrow straight to meandering geometry until they merge into a single, relatively straight lower reach, which debouches in the Stromboli Valley. Seismic reflection profiles and morphometric analysis highlight a strong tectonic control on GMS location and morphology throughout its course. At large scale, tectonic structures cross cutting the GMS caused both the formation of a mini-basin in the upper slope and a larger confined sub-basin downslope. These features promote abrupt changes in slope gradients that in turn are reflected on the plan-view morphology and stratigraphic architecture (seismic facies and sediment depocenters) of the GMS. Particularly, the progressive infilling of the confined sub-basin caused changes in the GMS base level, determining a polyphased evolution of the system encompassing repeated erosional and depositional events. The complete infill of the sub-basin promoted a by-pass of turbiditic flows in the lower slope, leading to the development of the lower reach of the GMS and its connection with the Stromboli Valley. This connection favoured a base level lowering of the GMS, causing its rejuvenation by retrogressive upslope erosion. This stage was recorded only in the shelf-indenting Gioia Canyon, characterized by a narrow and entrenched course, with irregular longitudinal profile and multiple knickpoints. Based on these results, we propose an evolutionary architectural model of the GMS to point out how erosional and depositional processes of channel/canyon system in a tectonically controlled margin can rapidly change in space and time in relation to multiple factors.

High-Throughput Superresolved Focal Imaging Based on a Phase-Modulated Acoustic Superoscillatory Lens

Breaking the Abbe-Rayleigh diffraction limit and realizing acoustic superdiffraction focusing and imaging is of great significance in acoustic science and engineering. While a conventional focusing lens cannot even reach the Abbe-Rayleigh resolution limit due to the cutoff of spatial-frequency components of the regular lens. Superoscillation technology paves a way to generate superdiffraction for acoustic superresolution imaging. However, high-transmission superresolution is still a challenge issue in the imaging field of acoustics. Here, we propose a high-throughput phase-modulated acoustic-superoscillatory-lens (ASOL) focal-imaging technology. The ASOL is made up of many subwavelength spiral metamaterial elements for superimposing different spatial-frequency components to generate a superdiffraction focal spot. For proof of concept, the design and fabrication of the phase-modulated ASOL, a 0.326\ensuremath{\lambda} focal spot with a 0.9 transmission coefficient, is achieved through both simulation and experiment. Moreover, a superresolution focal-imaging experiment is successfully demonstrated by the Chinese traditional window-grille structure with a slit width of 0.33\ensuremath{\lambda}. It verifies that the proposed ASOL has the great technical potential to improve acoustic-focal-imaging resolution. The proposed method also opens up opportunities for developing high-throughput acoustic superresolution imaging, and it will reduce the reflection effect and improve the signal-to-noise ratio. The proposed ASOL has significant potential applications in underwater sonar scanning, acoustic diagnosis, and acoustic imaging technology.

Assessing the optimal location for Sipora wave energy power generator site placement from a geological perspective

One of the potential sources of renewable energy in the Mentawai region is ocean wave energy. The western Sumatra sea has a wave energy potential of up to 20 kW/m. The wave energy potential modeling conducted by the Marine Geological Institute (P3GL) ESDM revealed that western Mentawai waters are locations that produce annual average wave energy of around 20 kW/m. The geological survey of the seabed for power generating wave energy turbine sites using acoustic equipment including the Echosounder Odec, SBP Stratabox, and Star Fish Side Scan Sonar. The mapping of the seabed allows for the identification of undersea morphology, slopes, structural elements, and seafloor objects. Ocean depth sounding and bottom surface imaging are used in this research to determine the placement of the turbine site from the wave energy generator connection grid. The existence of underwater corals and fluvial plain units, continental shelf with shallow seabed and a gentle slope with an average depth of 30 meters was recognized as the most important factor in determining the site's location in this research. Based on the weighting and scoring results, optimal locations for potential power plant sites are in coastal areas located in offshore near Batu putih coast.

PAVOT: FIRST DISCOVERED SUNKEN SHIP FROM 1ST WORLD WAR AT ISKENDERUN BAY

Iskenderun Bay was an important role in providing logistic support to the occupiying that invaded the Anatolian lands during the First World War. Five minesweepers of the allied powers had come to the İskenderun Bay in order to the clear the sea bad from torpedos. One of them, PAVOT which was bought by the French Navy in 1916, struck a mine and sank. Pavot's coordinates taken from the ship with side scan sonar and GPS are 36o 35, 982' N – 36o 07, 843' E (DMS 36o 35' 59'' N; 36o 07' 47'' E). Moreover, the angle with the Iskenderun Lighthouse is 289o. The distance to it is 1.41 nautical miles (2.46 km) the nearest point to the shore is 0.95 nautical miles (1.95 km). Diving to the sunken ship has been carried out by three instructor divers teaching at the Iskenderun Technical University (İSTE) Marine Vocational School Underwater Technologies Program. The ship named R/V İSTE1 belonging to İSTE was used during the divings. The divings have been performed with SCUBA equipments. The Pavot sunken ship was identified for the first time in the Iskenderun Bay with this study.

Remote inspection method for water conveyance tunnel based on single-beam scanning sonar

Water conveyance tunnels perform an important role in water-diversion projects. With a water conveyance tunnel transferring water, the conflict between urban crowded ecology and agricultural water can be relieved. Research shows that there is a significant need for a more objective and accurate inspection of water conveyance tunnels during operation. Hence, we investigate the use of a single-beam scanning sonar to detect a water conveyance tunnel and generate a three-dimensional point cloud model, which can be used to make objective predictions for surface defects. First, a cobweb-like grid matrix was proposed to grid discrete points to establish the topological relationship of the point cloud. Second, a denoising method fusing acoustic properties and intensity was used to filter the noise in the acoustic point cloud. Third, an optimal echo extraction technology based on a second-order differential model was proposed to obtain a high-precision point cloud of the tunnel. Experiments show that the proposed denoising method and optimal echo extraction method are superior to traditional algorithms commonly used in ground point clouds, with good results achieved at real test sites.

Rapid Assessment Survey Using Side Scan Sonar: International Obligations and Role of Hydrography in Ensuring Safety of Navigation – Case Study of the Aftermath of MV X- Press Pearl Incident

The International Maritime Organization (IMO) was established in 1948 as a specialized agency of the United Nations (UN). They provide international standards on the safety, security, and environmental performance of ships involved in international trade. International shipping is effective only if the regulations and standards are agreed upon, adopted and implemented on an international basis. IMO Instruments Implementation Code (III code) is one such attempt where it looks into states’ capacities of implementing obligations. Here, we present in what way the hydrographic rapid assessment survey support the above obligation of ensuring safety of navigation for ships visiting Colombo Port in the aftermath of MV X-Press Pearl accident in West coast of Sri Lanka. The comprehensive Side Scan Sonar (SSS) survey that was conducted over the area found some of the fallen containers and debris lying on the seabed. Due to the extent and the nature of the incident, and due to the ocean current dynamics, a larger area had to be investigated and a number of relevant agencies in the maritime field were involved. As a country, the responsible authorities have taken timely actions to disseminate the safety related information to the mariners and subsequent measures were taken to update the relevant nautical charts by conducting necessary systematic hydrographic surveys to fulfill the obligations of the International Convention for the Safety of Life at Sea ( SOLAS) of IMO.

Sub-Bottom Profiler and Side Scan Sonar investigations, with the assistance of hydrochemical and isotopic analysis of Sawa Lake, Al-Muthana Governorate, Southern Iraq

A marine geophysical survey are including Sub Bottom Profilers and Side Scan Sonar integrating with under-water imagery, as well as hydrochemical and isotopic investigations have been carried out for Sawa Lake, one of the unique, shallow and closed water body in Iraq. The results showed that the depth of the Lake is shallow, ranged between 1 m in areas near the edges of the Lake and 2 m in the Lake center. The Recharge site has been detected exactly (this is a first recording (Nov. 2012)), as a hole. It has a longitudinal an axis of 50 m with NW-SE tren, which resembles the trending of the longitudinal axis of the Lake and Abu-Jir fault trending. The maximum width of this hole ranging from 23.0-29.5 m. The hole depth about 20 m, but may be greater then, because the emergence of a high proportion of noise in the geophysical sections. The Lake bottom is homogeneity and does not has any diacritical features. Also, the results showed discontinuous layers on either side of the hole due to the existance a number of caves under the bottom of the Lake extends into several meters with different dimensions and forms. Hydrochemical analysis of the water samples collected from different depths inside the hole by scientific diving with specefic equibments and the result showed that the water is NaCl type, with domination of Mg and Cl over Ca and SO4. The Stiff diagram, detected that very high TDS arising mainly from sodium and chloride, it is mostly typical marine water. Values of δ18O and δ2H in water samples of Sawa Lake, when plotted along the global meteoric water line shows that the three collected water samples from the lake, which have a similar isotopic composition, indicating that all of them are receiving water from the same flow zones, with diffuse recharge type. Also, the isotopic parameter showed that the Sawa lake water suffering from strong evaporation.

Mapping of Seafloor and Delineating Coral Reefs of Southern Iraq/Northwest Arabian Gulf by Side Scan Sonar Technique

High-resolution Side Scan Sonar data, under-water observation (video and photograph) and bottom samples (rocks and sediments) were integrated to map the seafloor of the Palinurus shoal which is one of the submerged shoal site in the Iraqi territorial marine, Northwest region of the Arabian Gulf. The Side Scan Sonar field survey was carried out along with eight parallel profile extending in SE-NW direction, at a length of 2.8 km for each line. Four backscatter patterns were mapped within the study area. The pattern reflects various seafloor lithology. The average depth of sea bottom was determined at 7 - 10 m. Coral Reefs were mainly found at two parts of the shoal site in the formed assemblages and sporadic in central and northern parts of the site respectively. These coral take different shapes, sizes and colors. Also, they are live in some places and dead in others.

Morpho-stratigraphic characterization of the southern shelf of Porto Santo Island (Madeira Archipelago): Insights for small-scale instability processes and post-LGM sedimentary architecture

Shelves surrounding volcanic islands represent only a small portion of the entire submarine edifice that can extend to a few thousand meters below sea level. Despite their small areas, shelves show a significant morphologic imprint made by the erosional, depositional, tectonic and volcanic processes when compared to those left on slopes or the submarine bases of islands. Therefore, shelf morphology can be used to improve our knowledge about the island evolution, especially the more recent events. In this study, we use multibeam bathymetry, side scan sonar and seismic reflection profiles of the southern shelf of Porto Santo Island in Madeira Archipelago to map its seabed morphology and seismic stratigraphy from the nearshore to the shelf edge. Based on morphological and seismo-stratigraphic differences, the study area has been divided in three sectors (Western, Central and Eastern). In general, the shelf consists of an erosive rocky surface mostly covered by sediments, and locally outcropping on the seafloor, especially on the Western and Eastern sectors. On these sectors two sets of submerged erosional terraces have been mapped at different depths (35–45 m, 50–75 m). The depth range of the terraces and their absence in the Central sector suggests that these were formed/modified at least by the Marine Isotope Stages (MIS) 5a-5d stillstands or even by older relative sea-level stillstands. Dykes, which are more resistant to erosion than the surrounding outcrops, have orientations that are similar to the volcanic and structural features present on the island (NNE-SSW and NW-SE). The sedimentary cover on the shelf is highly varied both in thickness and internal architecture, mainly due to different onshore sediment supply and available accommodation space. Two main seismic units have been recognized: the lower one (U1) is interpreted as a transgressive deposit, formed during the sea level rise after the Last Glacial Maximum (LGM); the upper one (U2) is interpreted as a modern highstand deposit. The Western sector is practically sediment-starved, except for a local thin sedimentary cover of U2 on the inner and outer shelf. The Central sector shows U2 as a sigmoidal-oblique progradational clinoform characterized by inner prograding geometry on top of U1. The Eastern sector shows U2 as sigmoidal clinoform extending over a large area and having a more aggradational component. The shelf edge shows an overall arcuate shape that strongly matches the coastline configuration, suggesting the occurrence of an old large-scale landslide event (LS1) prior to the shelf formation. Further incisions in the shelf edge of the Central sector are interpreted as resulting from mass-movements, classified as LS2 and LS3. LS2 is at least older than seismic unit U1 and LS3 formed during the late Holocene. The volume of the LS2 event (0.6 km3) suggests that it might have caused a hazardous tsunami if it failed as a single event. The recent LS3 events, due to their small magnitude were probably not hazardous. These observations provided a more comprehensive understanding of the recent evolution of Porto Santo insular shelf in terms of mass-movement processes and post-LGM sedimentary architecture.

Leakage Rate Modeling in Depleted Gas Fields Safeguards Well Integrity, CO2 Storage

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 205537, “Safeguarding CO2 Storage by Restoring Well Integrity Using Leakage Rate Modeling Along the Wellbore in Depleted Gas Fields Offshore Sarawak,” by Parimal Patil, SPE, Prasanna Chidambaram, SPE, and M. Syafeeq B. Ebining Amir, Petronas, et al. The paper has not been peer reviewed. Ensuring the long-term integrity of existing plugged and abandoned (P&A) and active wells that penetrated a studied CO2 storage reservoir was key to reduction of leakage risks along the well path for long-term containment sustainability. With a goal of developing depleted gas fields as carbon dioxide (CO2) storage sites in offshore Sarawak, determining the complexity involved in restoring the integrity of these P&A wells, as well as the development wells, is critical. Leakage-rate modeling (LRM) was performed to identify and evaluate the associated risks for designing the remedial action plan. CO2 Storage Challenges in Central Luconia Storage sites in this study include three major depleted gas reservoirs in the Central Luconia gas field offshore Sarawak. Hydrocarbons from deeply buried reservoirs were produced through drilling of vertical and deviated wells. Thirty-eight wells were drilled in these three depleted gas reservoirs, of which 11 are exploration wells that had been P&A; 27 wells include active producers, water injectors, and idle wells. By estimating the performance of well-barrier elements in the legacy P&A wells and existing development wells, an assessment of well integrity analysis was performed. The casing, as well as the cement, may degrade with time depending on the downhole temperature, pressure, stress conditions, and formation fluids. Degradation of wellbore cement in the presence of carbonized acid fluids poses a risk of creating leakage pathways. Understanding the local stress conditions acting on the cement/casing/formation sheath is important to increase knowledge regarding leakage-pathway creation, together with geochemical and geomechanics processes. CO2 Storage Well Integrity Risk Assessment Once the locations of the existing P&A wells were identified, site-inspection data helped define the preparation required to assess possible gas seepage or the existence of leakage pathways along these wellbores. The measured leakage amount of gas/CO2 could then be used to backfeed the simulation model to identify the leakage system along the wellbore. The method uses desktop modeling studies together with site surveys using a remotely operated vehicle (ROV) or side scan sonar. The modeling helps simulate various leakage scenarios, while actual field measurements for any bubbling at the seabed provide the critical inputs to refine the modeling work to define the risk associated with each well. To gauge the integrity of storage sites for each well, a comprehensive understanding of critical factors and dominant criteria had to be considered as part of the data-evaluation process. Tables 1 and 2 of the complete paper define the risk-assessment elements for the existing P&A exploration and appraisal wells and development wells, respectively, used for the risk-assessment exercise. The risk-assessment process includes identification of risk, risk analysis, and risk evaluation. All wells that penetrated the CO2 storage reservoir were benchmarked as a baseline for a corrosive storage environment. Such data are useful for forecasting the possible leakage rates at the CO2 sequestration site.