High-resolution Seismic Survey Research Articles

Near-surface geophysical studies in the Ulsan Fault Zone of Korea

Recent earthquakes near nuclear power plants in Korea have triggered public concerns about possible seismicity of the Ulsan Fault Zone in the south-eastern part of the Korean peninsula. To reveal subsurface structures of this fault zone, we conducted high-resolution seismic refraction and reflection surveys, and closely spaced gravity measurements in the Dongchon River valley north of Ulsan, Korea. Here alluvium covers the north–south trending fault zone in a 1-km wide valley. Both source points and receivers were spaced at 5-m intervals for the 24-channel seismic refraction and reflection methods, along two profiles of 835 m and 415 m length. Gravity data were also measured along these profiles at 131 stations using a 10-m interval. Synergetic interpretation of seismic refraction, high-resolution seismic reflection, and gravity surveys across the valley indicates that the Ulsan Fault Zone was formed by apparent north–south strike-slip motions during the Cretaceous, and that some faults may have been reactivated by east–west compressional or transpressional stresses during the Tertiary or Quaternary.

紀伊半島南東沖南海トラフでの高分解能3次元反射法地震探査と重合前深度マイグレーション処理

IODP南海トラフ地震発生帯掘削計画として紀伊半島沖東南海地震（マグニチュード8.1）の震源域掘削が2007年秋頃から始まる。それに先立ち，我々は2006年3月，紀伊半島南東沖南海トラフ付近における地殻構造の高精度イメージングのため，深海調査研究船「かいれい」のマルチチャンネル反射法地震探査システムを用いた高分解能3次元反射法地震探査を行った（KR06-02航海）。「かいれい」3次元探査域は3ヶ所の掘削サイトをカバーしており，本調査には長さ約5kmのストリーマーケーブル（204チャンネル）と，約100 m 離れた2式の震源アレイを用いた。高分解能調査のため用いた各々の震源アレイはGガン2基とGIガン1基の組合せである。特に，ストリーマーケーブル1本のみを曳航する本調査では，左右震源アレイを交互に発震するFlip-flop方式を導入することで，1 sail line につき2 CMP line のデータ取得が可能となり，データ取得作業の効率が倍増した。最終的な3次元データ取得範囲は3.5×52 km となった。データ記録長は10秒,サンプリング間隔は1 msec である。また，震源アレイとストリーマーケーブルの曳航深度は，それぞれ5mと8mに制御した。発震点および受振点の測位のため, SPECTRAとREFLEXを使用した。調査期間中に船上QCなどの結果，良好なデータ取得が確認できた。調査終了後，陸上での3次元ビンニングなどの前処理を終えたCMPデータを用い，3次元重合前深度マイグレーション処理を行った。最終的に，3次元区間速度モデルと高分解能の地殻構造イメージが得られた。速度不確定性を推定するために行った3D PSDM速度テストの結果より，最終速度モデルは，約6kmの深度において最大±5％の速度不確定性を持つことがわかった。得られた3次元地殻構造の解釈の結果，南海トラフ底で沈み込んでいる，3つの音響ユニットから成る四国海盆堆積層の層厚変化が明らかとなった。特に，最上位のユニットCは，トラフ底から陸側への有意義な層厚増加や背斜構造によって特徴付けられ，また，ユニットCの中央には強振幅の反射面Rの存在が認められる。この反射面Rは斜めスリップ断層面として解釈され，このスラスト断層運動によって，ユニットCが重なり，陸側へ厚くなっていることが考えられる。

Colluvial wedge imaging using traveltime and waveform tomography along the Wasatch Fault near Mapleton, Utah

SUMMARY Four high-resolution seismic surveys were conducted across the Wasatch Fault Zone near Mapleton, Utah. The objective was twofold: (1) To use velocity tomograms and reflection images to delineate fault structures and colluvial wedges to more than twice the depth of the Mapleton Megatrench excavated by URS personnel, (2) to assess the strengths and limitations of traveltime and waveform tomography by synthetic studies and comparison of the tomogram to the ground truth seen in the Megatrench log. Four out of the five faults within the trench area are accurately identified in the migrated image and in the tomograms, and the main fault's dip angle is estimated to be between 71 and 80°. Two additional faults are interpreted outside the trench. The faults can be delineated down to 30 m below the surface, which is 20 m deeper than the excavated trench. Five out of six colluvial wedges found in the trench log were seen as low-velocity zones (LVZs) in the tomogram, however the biggest colluvial wedge could not be identified by either tomography method. Waveform tomography prevailed over ray-based traveltime tomography by more clearly recovering the faults and LVZs. A newly discovered LVZ at a depth of 18–21 m below the surface possibly represents a colluvial wedge and is estimated to be less than 21 000 years old. If this LVZ is a colluvial wedge, the earthquake history obtained by trenching can be extended from 13 500 to 21 000 yr with seismic tomography. Our results further demonstrate the capability of tomography in identifying faults, and show that waveform tomography more accurately resolves colluvial wedges compared to traveltime tomography. However, despite the successful recovery of most faults and some, but not all, colluvial wedges, both tomography methods show many more LVZs besides the wedges, so that an unambiguous interpretation cannot be made. A major part of the ambiguity in the tomograms is due to the many major faults, which result in an uneven raypath coverage as our synthetic studies show. Hence, seismic trenching will be more successful at simple fault geometries. However, even under optimal conditions there will be some ambiguity in the interpretation, so that detected LVZs should be drilled, cored, and dated to determine the history of the ancient earthquakes.

High Resolution Seismic Reflection and Radar for Hydrogeology: The Gnangara Mound, Perth Basin, Western Australia

Curtin University Department of Exploration Geophysics in collaboration with Water Corporation of Western Australia have undertaken a comprehensive geophysical program with the overall objective of resolving hydraulic properties and boundaries below the Gnangara mound. Initial results have been highly encouraging. Radar transects, more than 15 km in length, reveal up to three shallow potentially ``water retentive' layers above the regional water table for the superficial aquifer. Both shallow sedimentation such as buried dunes and post deposition layering can be readily interpreted over large areas from the radar data. Interpretation of the radar data is assisting in developing a large scale infiltration distribution models for the Gnangara Mound. The high resolution seismic reflection surveys aim to resolve hydrostratigraphy from surface to 1500m depth. Of particular interest is large scale hydraulic separation between Perth?s major aquifers, which include the Superficial, Leederville and Yarragadee. Previous seismic surveys in this area produced low-resolution discontinuous events that could not be used for such purposes. Recently we acquired high-resolution seismic profiles using our telemetric system and a high-power impact source. High CMP fold combined with careful target oriented data processing produced quality images. These enable interpretation of up to 20 layers including several unconformities in the top 1500m. Further advance in acquisition, particularly application of high-power, high-frequency seismic sources and data processing are hoped to enable routine application of reflection seismic methods in hydrogeology. Current results demonstrate significant advances have been achieved with respect to the past trials even at these early stages of the Gnangara Mound research program. Results from the seismic reflection and radar surveys will feed into the PRAMS hydraulic flow model for the Perth Basin.

High resolution seismic survey of the proposed Beenyup wastewater injection site

The Beenyup wastewater treatment plant is situated in close proximity to residential properties approximately 20 km north of the central business district of Perth, Western Australia. It is proposed that treated wastewater from the plant will be injected into the sub-surface in preference to releasing it to the ocean. In order for this to occur over a long period of time a good understanding of the connectivity of the sub-surface aquifers in the vicinity of the injection site is required. High resolution surface seismic together with vertical seismic profiling data were collected at the site using a high-power impact source. The resulting sub-surface images correlate well with geophysical well-logs and clearly show the injection zone and confining seals.

Land Seismic Acquisition Repeatability for Time-Lapse Monitoring of CO2 Sequestration

Time-lapse surface and borehole seismic surveys are planned for monitoring the injection of CO2 in the CO2CRC Otway Basin Pilot Project in Victoria. Critical to the success of this is to ensure optimum repeatability of acquisition parameters and ground conditions. In order to assess the relative influences of source types and environmental conditions, a series of repeated test surveys have been undertaken. The study utilised repeated high-resolution seismic surveys along the same 2D line. The first test line was acquired with mini-vibroseis (6000 lb) in wet conditions when the top soil and the weathered layers were fully saturated. The line was subsequently re-recorded in dry conditions where we utilised the same mini-vibrator, but in addition repeated the line using a free-fall weight drop. Both sources have similar total energy output but a vibrator is a controlled frequency source while a weight drop is not. Despite differences in the frequency content and phase of the signal generated by these two sources, and positioning differences for source and receiver locations of up to 1m, almost identical stacked sections were obtained after phase matching and scaling of the two datasets. Far greater differences in total energy, frequency content and phase of the signal were observed between the two vibroseis lines recorded at different times of the year (wet and dry periods). Our results clearly demonstrate that near surface conditions has a first order effect on repeatability of land seismic surveys. A common belief that deployment of the same seismic source and positioning errors are crucial for successful time-lapse seismic needs to be reexamined in light of our results, which show that these factors are of secondary importance when it comes to land seismic surveys.

Evolution of collision-related basins in the eastern end of the Kurile Basin, Okhotsk Sea, Northwestern Pacific

A densely-meshed 9400 km long high-resolution seismic survey revealed the evolution of sedimentary basins in the southeastern rim of the Kurile Basin, offshore of eastern Hokkaido, NW Pacific. Hokkaido is situated in a collision zone not only of Okhotsk and Eurasian plates and also of the Kurile and NE Japan arcs. Offshore eastern Hokkaido is overlain by N–S-trending basins: Hamatonbetu-oki, Monbetu-oki, and Kitami-Yamato Sedimentary Basins (HOSB, MOSB and KYSB, respectively), and Abasiri-oki Trough (AOT), separated by zones of uplift. Depositional patterns of the HOSB and deformation of the adjacent Uplift Zone suggest the pull-apart origin of the basin related to dextral motions along the Central Sakhalin Fault, which extends to Hokkaido and binds the Okhotsk and Eurasian plates. The Uplift Zone adjacent to the MOSB suggests the initiation of subsidence in the Middle Miocene within a splay-fault basin with subsequent transformation into a 40 km wide graben in the Late Miocene and into a half graben in the Pliocene. The pre-late Miocene unconformity is widespread. The KYSB and the AOT began as a concatenated basin in the south of MOSB in the Middle Miocene. In the Pliocene, the Kitami’Yamato Bank (KYB) was uplifted and divided the KYSB–AOT basin into KYSB and AOT. The KYB uplift was probably induced by the Kurile forearc sliver movement. The interpretation of structures suggests that the Kurile Basin may have opened in a N–S-direction, from dextral shearing along a collision zone in the Middle Miocene. In the Pliocene, the kinematics changed to a NE–SE spreading direction; because of southwestward migration of the Kurile Arc with respect to the NE Japan Arc. Compressional tectonics after the Pliocene may suggest the destruction of the Kurile Basin.

The Lago Cardiel Basin, Argentina (49°S): Origin and evolution revealed by high-resolution multichannel seismic reflection studies

A multichannel, high-resolution seismic reflection survey was conducted with an airgun source in the closed lake system of Lago Cardiel, Argentina (49°S). As part of an ongoing study of interhemispheric correlations in climate change, it was intended to determine the maximum age and depth of the lake sediments and better understand the origin and evolution of the lake basin. In agreement with previous single-channel, 3.5-kHz surveys, six major seismic sequences throughout the lake’s subsurface were mapped. The stronger seismic source of this study allows the deepest sequence (Sequence VI) to be interpreted as folded Cretaceous marls with a highly eroded surface. Further examination of this sequence reveals various stratigraphic signatures and mechanisms of the generally east–west orientation of tectonic compression. The nature and continuity of Sequence VI do not indicate a volcanic or impact-related origin of the lake. The original basin has a maximal depression of 154 ms (∼114 m) deep beneath the modern lake surface and is overlain by an alluvial fan (Sequence V) of likely Pleistocene age in the western portion and late Quaternary lacustrine sediments (Sequences I–IV) everywhere else. The data confirm that southern Patagonia was dominated by extremely contrasting climatic conditions during the Quaternary.

Lithology of the Profundal Sediments in Słupiańska Bay (Wigry Lake, NE Poland) - Introduction to Interdisciplinary Study

In this paper, a lithological study taking into account trace metals of the profundal sediments from Słupiańska Bay in Wigry Lake is presented. A profile 5.26 m long has sandy muds at the bottom and moving upwards layers of calcareous gyttja, lacustrine chalk, and again calcareous gyttja are present. The profile is continuous, typical for the sediments of NE Poland formed from Allerøde to recent time. The results of the lithological analysis were correlated with results of high-resolution seismic survey. The results together with radiocarbon dating are part of interdisciplinary complex research comprising paleobiological study of pollen, Cladocera, diatom successions and human activity, which will be published in successive papers.

Strategies for waveform processing in sparker data

A sparker is a marine seismic impulsive source used for high-resolution seismic surveys. Sparker sources were very popular during the late 1960s and 1970s before being supplanted by small volume airguns. However, in the last 10 years there has been renewed interest in sparker technology because (1) it can be easily deployed at relatively low costs and (2) in certain areas the use of small airguns is restricted for environmental purposes. In this study a sparker source was used to assess the seismic stratigraphy of Quaternary deposits and to image the sediment/bedrock interface. Three different inverse filtering methods were tested (i.e., spiking deconvolution, match-filtering and vertical seismic profile (VSP) deconvolution) to correct the poor shot-to-shot repeatability of the source and to compress its reverberations. Results show that the matched-filter and VSP deconvolution methods, which design and apply one operator for each shot, produced comparable results, whereas the spiking deconvolution that used the same operator on all traces failed to compress the source signature properly.

High-resolution seismic imaging of gas accumulations and seepage in the sediments of the Ria de Aveiro barrier lagoon (Portugal)

Methane is a powerful greenhouse gas and an important energy source. The global significance and impact in coastal zones of methane gas accumulation and seepage in sediments from coastal lagoon environments are still largely unknown. This paper presents results from four high-resolution seismic surveys carried out in the Ria de Aveiro barrier lagoon (Portugal) in 1999, 2002 and 2003. These comprise three chirp surveys (RIAV99, RIAV02, RIAV02A) and one boomer survey (RIAV03). Evidence of extensive gas accumulation and seepage in tidal channel sediments from the Ria de Aveiro barrier lagoon is presented here for the first time. This evidence includes: acoustic turbidity, enhanced reflections, acoustic blanking, domes, and acoustic plumes in the water column (flares). The stratigraphy and structural framework control the distribution and extent of gas accumulations and seepage in the study area. In these shallow systems, however, tidal altitude variations have a significant impact on gas detection using acoustic methods, by changing the raw amplitude of the enhanced seismic reflections, acoustic turbidity, and acoustic blanking in gas-prone areas. Direct evidence of gas escape from drill holes in the surrounding area has shown that the gas present in the Ria de Aveiro consists of biogenic methane. Most of the gas in the study area was probably generated mainly in Holocene lagoon sediments. Evidence of faults affecting the Mesozoic limestones and clays underlying some of the shallow gas occurrences, and the presence of high-amplitude reflections in these deeper units raise the possibility that some of this gas could have been generated in deeper sedimentary layers, and then migrated upward through the fractured Mesozoic strata.

High-resolution, deep tow, multichannel seismic and sidescan sonar survey of the submarine mounds and associated BSR off Nicaragua pacific margin

Bathymetric and conventional multichannel seismic surveys offshore Nicaragua and Costa Rica have revealed numerous mud mounds beneath which the generally widespread BSR is not well imaged. However, many of the mounds are partially capped by patches of authigenic carbonate crusts, so it was not clear if the semitransparent seismic facies and the apparent gaps in the BSR beneath the mounds are real or due to poor normal-incidence seismic penetration through the cap rocks. To address these problems, a high-resolution seismic survey was carried out over the continental slope of the Nicaraguan Pacific margin using a deep towed multichannel seismic streamer (DTMCS) along with a sidescan sonar system (DTS) to image submarine mud mounds and the associated BSR. The proximity of the very short (39 m active length) but high-resolution 17 channel streamer to the seafloor of the deep towed system allows greatly improved lateral resolution whereas the relatively large source–receiver offset allows the undershooting of the cap rocks. For the first time our data show that the BSR in many cases continues but rises beneath the mounds. This is consistent with the advection of deep warm fluids and thus increased heat flow through the mounds. The occurrence of mud mounds seems to be controlled by the locations of faults.

Fusion of 3D seismic exploration and seafloor geochemical survey for methane hydrate exploration

The MH21 Research Consortium has conducted a high-resolution 3D seismic survey and a seafloor geochemical survey, to explore methane hydrate reservoirs in the eastern Nankai Trough, offshore Japan. Excellent geological information about shallow formations was obtained from the high-resolution 3D seismic survey, which was designed to image the shallow formations where methane hydrates exist. The information is useful in constructing a geological and geochemical model, and especially to understand the complex geology of seafloor, including geochemical manifestations and the structure of migration conduits for methane gas or methane-bearing fluid.By comparing methane seep sites observed by submersibles with seismic sections, some significant relationships between methane hydrate reservoirs, free gas accumulations below the seafloor, and seafloor manifestations are recognised. Bathymetric charts and seafloor reflection amplitude maps, constructed from seismic reflections from the seafloor, are also useful in understanding the relationships over a vast area. A new geochemical seafloor survey targeted by these maps is required.The relationships between methane hydrate reservoirs and seafloor manifestations are becoming clearer from interpretation of high-resolution 3D seismic data. The MH21 Research Consortium will continue to conduct seafloor geochemical surveys based on the geological and geochemical model constructed from high-resolution 3D seismic data analysis.In this paper, we introduce a basis for exploration of methane hydrate reservoirs in Japan by fusion of 3D seismic exploration and seafloor geochemical surveys.

Three-Dimensional Seismic Architecture of Fluvial Sequences on the Low-Gradient Sunda Shelf, Offshore Indonesia

Abstract The sequence stratigraphy of fluvial–deltaic strata deposited near the center of the wide, gradually subsiding Sunda Shelf differs from standard highstand examples because low accommodation and low depositional gradients restricted the thickness of fluvial–deltaic sequences and the depth of lowstand fluvial incision. In this setting sea-level fluctuations may have had less influence on fluvial depositional patterns, and fluvial stratigraphy is more likely to be defined by changes in river discharge during gradual aggradation. The upper Muda Formation in West Natuna Basin, offshore Indonesia, is a 225-m-thick, dominantly fluvial succession that prograded onto the middle part of the Sunda shelf from Pliocene to Holocene. The succession changes upward from offshore shelf deposits, through a thin interval of deltaic deposits, a succession of fluvial deposits, and finally a thin transgressive succession directly beneath the modern sea floor. A high-resolution seismic survey (17 km by 40 km in area) of Belida Field near the middle of the Sunda shelf provides exceptional images of changing fluvial architecture within five erosionally based, fluvial sequences. Sequences, each several tens of meters thick, are characterized by low-relief basal incisions that extend laterally onto interfluves cut by a network of gullies. Although a regressive stacking pattern of successive sequences is defined to some extent by an increase in the relief of basal erosion and a decrease in average sequence thickness, it is best defined by progressive increase in the extent of gully formation onto interfluve areas (indicating longer periods of interfluve exposure). Channel size tends to decrease upward within individual sequences, recording both a decrease in the size of the largest channels and greater preservation of smaller tributary and floodplain drainage channels. The largest channels preserved directly above the better developed sequence boundaries tend to be more sinuous and to have more extensive inner-bend accretion deposits than those that cut down from horizons higher within sequences. This suggests that incised channels had more stream power to erode their banks relative to the sediment loads they carried. Although mechanisms controlling the episodic shallow incision and then aggradation of these fluvial systems are difficult to constrain from variations observed in this small study area, the consistent thickness of the Muda Formation observed in regional 2D seismic sections is evidence against significant syndepositional tectonic deformation or differential subsidence. Low regional gradients and great distance of the study area from the paleo-shelf edge (~ 1000 km) are evidence against direct sea-level influence. Larger channel sizes directly above sequence boundaries supports temporal changes in channel discharge, perhaps related to climatic changes within the drainage basin. Recognition of episodic channel incision and bypass alternating with periods of floodplain aggradation influences predictions of channel-deposit connectivity based on channel proportion.

Coast of Ancient Kaulonia (Calabria, Italy): Its Submergence, Lateral Shifts, and Use as a Major Source of Construction Material

Divers in the 1980s discovered a large concentration of archaeological material on the seafloor off Kaulonia, a Greek colony on the Ionian coast of southern Italy. It has been proposed that a small hook-shaped cape, now submerged, was present off the site during settlement from about 700 to 389 BC. Since 1992, however, most of Kaulonia's seafloor sector has been buried by a thick (to 4 m) sand cover. This investigation resumes study of Kaulonia's margin by high-resolution seismic survey, analysis of sediment in cores and excavated sections buried along the coast, and petrologic examination of rock used for construction of a temple, the site's major structure. The sector, located in the tectonically active Calabrian arc, has been subject to considerable elevation rise of land onshore, and an equivalent rate of seafloor subsidence (∼1 mm/yr). A distinct Holocene sequence, composed of beachrock-type sandstone and fluvial-marine–transported conglomerate, in sediment sections landward of the beach can be traced seaward to depths of ∼7 m about 300 m from the present shoreline. Correlation of the sequence helps define changes of coastline position from before Greek settlement to the present. Rather than a generally progressive migration landward of the shoreline in response to relative sea-level rise, the coast has experienced more complex shifts (sea-to-land, land-to-sea, and return to land) due to up-and-down motion in the sector between the alluvial plain behind the beach and innermost shelf. By the mid-Holocene, the shoreline had advanced seaward, resulting in coastal emergence, and it did not return landward (resubmerging the margin) until after the site's abandonment; submergence was complete by late Roman time. We now recognize a broad arcuate headland, rather than a small hooked cape, seaward of the site ∼2500 years ago. Without a lagoon, sufficiently deep lake, or man-made port available for protected anchorage, vessels sailing to Kaulonia for trade were either beached or anchored close to shore and, perhaps seasonally, within the mouth of the Assi River. Composition of beachrock-type sandstone in Holocene sections on land is comparable to that offshore and also to sandstone used for temple base construction. Extensive sandstone exposures along this coast may have been one reason why Greeks selected the site for settlement.

Time-lapse seismic reservoir monitoring technology in China

Digital surveys of the T block in China's Daqing oil field began in 1988, and the first high-resolution seismic survey in the area was conducted from the winter of 1998 to the spring of 1999. These 2D seismic data sets of 1988 and 1999 have been used in a time-lapse seismic reservoir monitoring study. Figure 1 shows the quality of the seismic data is good, and three events can be clearly seen in the seismic profiles.

Tectonic control over topography and channel sedimentation across the forearc slope of the southern Kurile Trench

A high-resolution seismic survey covering more than 2,000 km2 has revealed the processes responsible for the slope morphology and channel sedimentation across the forearc slope-basin of the Kurile Arc–NE Japan Arc collision zone, offshore from Tokachi (Hokkaido, Japan). The dominant slope contours parallel the trench but, in the middle and lower reaches of the southern slope, contours are convex-shaped with an offshore trend. This sector of the slope is traversed diagonally by the Hiroo submarine channel. The offshore-trending convex contours and the channel course have developed through the interplay of tectonic and sedimentary processes, including the development of anticlines, anticline-induced lobe sedimentation and channel avulsion. In its upper reaches, the channel is restricted by a topographic low associated with NNW–SSE-trending anticlines which developed within the upper and middle slope sectors during late Miocene uplift. The uplift timing and trend of these anticlines indicate that they resulted from collision, the channel sedimentology and slope morphology of the middle and lower slopes having been influenced by Pliocene uplift of NE–SW-trending anticlines. The trends of these anticlines parallel those of the Kurile Trench. The Pliocene and early Pleistocene strata of the middle and lower slopes consist of ponded lobe sediments deposited along the palaeo-Hiroo submarine channel on the landward side of the anticlines. As a lobe pile accumulated, the channel thalweg shifted to the north of the stack, allowing the channel to bypass the topographic high formed by the growing stack. Thick levee deposits built up along the channel course during the late Pleistocene and Holocene. These levees, along with the Pliocene and early Pleistocene lobes, are reflected in the present-day sigmoid-shaped, convex offshore-trending contours. Thus, the interplay of subduction- and collision-related anticlines, tectonic-related channel ponding, and avulsion has contributed to the slope morphology of the southern Kurile Trench.

Late Quaternary depositional history of the North Evvoikos Gulf, Aegean Sea, Greece

A high-resolution seismic reflection survey was carried out in the North Evvoikos Gulf, a deep (450 m) semi-enclosed basin in east-central Greece connected to the Aegean sea via a 45-m sill to the north and a 30-m wide, 10-m deep straight to the south. This active extensional basin is bound by major normal faults, that switch polarity along strike, thereby controlling the asymmetric shape of the basin. The survey records show a continuous upper unit in the marine strata deposited since the post-glacial transgression on an erosion surface generated during the last glacial. The upper unit can be traced downslope into the basin plain where it merges into a unit of laterally continuous interbedded turbidites and hemipelagic deposits. The buried erosion surface has many terraces distributed over several depth zones, between 65 m and 165 m below present sea-level that are evidence for a fluctuating water level when, in the last glacial interval, the sill depth in the north was about 55 m below today's sealevel and the gulf became isolated from the Aegean Sea. Thus during the last 70 kyr the water level of the lowstand lakes fluctuated in correspondence with the Oxygen Isotope Stages (OIS) and the climatic conditions. In the dryer stages (OIS 4, 2) the lake water level was lower, in the wetter stages (OIS 1, 3) the lake water level was higher. These events are also correlated with sediment budgets input from the surrounding land to the basin. Researches carried out in many other gulfs of variable depth in the Ionian and Aegean seas indicated presence of sills at today's depths between − 50 and − 70 m. The generation of these features is postulated as sedimentary as many sea level fluctuations ranged in this depth interval.

Shallow gas accumulation in sediments of the Patos Lagoon, southern Brazil

A high resolution seismic survey was conducted in the Patos Lagoon, southern Brazil, aboard of the research vessel LARUS of the Fundação Universidade Federal do Rio Grande (FURG). Around 400 km of 3.5 kHz seismic profiles were collected, which provided acoustic signals of good penetration depth and resolution. Seismic anomalies, including turbidity and pocket gas, revealed that gas-charged sediments are common in several areas of the lagoon. The gas accumulations in the Patos Lagoon are controlled by the spatial distribution of the sedimentary facies. Either in 'curtains' or in 'acoustic turbid zones', the main gas accumulations occur in areas with paleotopographic lows related to fluvial channels and valleys developed in the Rio Grande do Sul coastal plain during regressive/transgressive events of the Quaternary.

High resolution seismic tomography of a Strombolian volcanic cone

We determine the 3D velocity structure of the Puy des Goules, a small, 1 km wide, Strombolian volcano that erupted 10 ky ago in central France, through a high resolution seismic survey. One major goal for this experiment was to develop methods to reach a high resolution focused on the plumbing system. This has raised different problems such as: mixing active sources (explosive, vibroseis) with different signal properties; inverting traveltime residuals of the order of 10 ms which requires the corresponding accuracy on the Digital Elevation Model (DEM), source and sensor locations (300 sites) and traveltime computations. The results of the traveltime tomography reveal three main bodies of high velocity embedded within scoria layers. These bodies can be interpreted as the central chimney and two complex feeding zones that compare quite well with the Puy de Lemptégy, a neighboring cone that was quarried and removed, showing its underlying feeding conduits and dykes. These results represent a first step toward our objective that is to determine geological structures related to natural hazards with a high resolution.