Spatial modulation experiments in the underwater acoustic channel

A modulation technique for increasing the reliable data rate achievable by an underwater acoustic communication system is presented and demonstrated. The technique, termed spatial modulation, seeks to control the spatial distribution of signal energy such that the single physical ocean channel supports multiple parallel communication channels. Given a signal energy constraint, a communication architecture with access to parallel channels will have increased capacity and reliability as compared to one with access to a single channel. Results from two experiments demonstrate higher obtainable data rates and power throughput for a system employing spatial modulation than for one that does not. The demonstrated benefits were characterized by an equivalent SNR gain of over 5 dB in the first experiment. In the second experiment, using two element source and receiver arrays with apertures of 0.9 m, a coherently modulated signal was shown to offer nearly 50% greater capacity by using spatial modulation than by using temporal modulation alone.     

Nitrate loss and transformation in 2 vegetated headwater streams

The processes of nitrate removal from the stream waters of 2 headwater catchments were studied. In study stretches where the stream channels were vegetated with thick mats of grass (Gly‐ceria fluitans), nitrate removal processes were particularly active. In such grassed stream channels, c. 75% of the nitrate removal was attributable to plant uptake and the remainder to denitrifica‐tion. Both of these nitrate removal processes were linearly dependent on nitrate concentration, resulting in an exponential decline of nitrate level from the springs along the stream's length. Regeneration of nitrogen, by plant decay within the stream channel, results in export of dissolved organic nitrogen and paniculate nitrogen from the catchments. The overall impact of vegetated stream channels in modifying nitrogen exports from catchments is discussed.     

Multichannel processing of broad-band multiuser communicationsignals in shallow water acoustic channels

High-throughout multiple-access communication networks are being considered for use in underwater acoustic channels. Bandwidth limitations of underwater acoustic channels require receivers to process broad-band communications signals in the presence of several active users. To deal with the resulting multiple-access interference in addition to high intersymbol interference, the spatial variability of ocean multipath is exploited in a multichannel multiuser receiver. Two configurations of such a receiver, a centralized and a decentralized one, are presented in fully adaptive modes of operations. While greatly reducing intersymbol and multiple-access interference, spatial diversity implies high increase in adaptive multiuser receiver complexity. To reduce the complexity of the optimal multichannel combiner, spatial structure of multipath is exploited. The complexity of resulting adaptive decentralized multichannel multiuser receiver is reduced at almost no cost in performance. Comparison of proposed multichannel receivers in an experimental shallow water channel demonstrates superior performance of spatial signal combining. The use of multiple input channels is shown to provide high level of tolerance for the near-far effect in both centralized and decentralized receivers. Decentralized receiver with reduced-complexity combining is found to satisfy the performance/complexity trade-off required for practical receiver realization in shallow water networks     

Morphodynamic modelling of open-sea tidal channels eroded into a sandy seabed, with reference to the channel systems on the China coast

In this contribution, the morphodynamics of open-sea tidal channels eroded into sandy seabed in regions of flow constriction is simulated by a one-dimensional model using the Bagnold formula for bedload transport rate, and accounting for the effect of bed slope. The results show that equilibrium conditions for such channels can be reached over a period of 10²–10³ years. Sediment eroded from the channel floor is transported in the direction of the dominant current, and deposited beyond the regions of flow constriction where the current looses competence due to spreading. In this way, the material remobilized from older strata in the channels is deposited in younger sand banks near the channel heads. Where several successive channels are incised along the current axis, they interact in their morphological evolution. The morphodynamic equilibrium of a tidal channel is reached once the combined interacting sedimentological and hydrodynamic factors, such as sediment particle diameter, tidal current velocity and flood/ebb dominance, are balanced. The model output shows that the equilibrium shape of the tidal channels appears to be related mainly to flow field characteristics and, to a lesser extent, to particle size. A positive correlation exists between the depth of the channels and their response times. The equilibrium water depths of the channels are more sensitive to current speed than to either particle size or the time–velocity asymmetry of the flow field. The response times for overall morphological equilibrium are sensitive to all of the above-mentioned parameters. In particular, sediment characteristics associated with critical current velocities have far-reaching effects on the morphodynamic behaviour of tidal channels.     

Quantifying the influence of channel sinuosity on the depositional mechanics of channelized turbidity currents: A laboratory study

Here we present results from a suite of laboratory experiments that highlight the influence of channel sinuosity on the depositional mechanics of channelized turbidity currents. We released turbidity currents into three channels in an experimental basin filled with water and monitored current properties and the evolution of topography via sedimentation. The three channels were similar in cross-sectional geometry but varied in sinuosity. Results from these experiments are used to constrain the run-up of channelized turbidity currents on the outer banks of moderate to high curvature channel bends. We find that a current is unlikely to remain contained within a channel when the kinetic energy of a flow exceeds the potential energy associated with an elevation gain equal to the channel relief; setting an effective upper limit for current velocity. Next we show that flow through bends induces a vertical mixing that redistributes suspended sediment back into the interiors of depositional turbidity currents. This mixing counteracts the natural tendency for suspended sediment concentration and grain size to stratify vertically, thereby reducing the rate at which sediment is lost from a current via deposition. Finally, the laboratory experiments suggest that turbidity currents might commonly separate from channel sidewalls along the inner banks of bends. In some cases, sedimentation rates and patterns within the resulting separation zones are sufficient to construct bar forms that are attached to the channel sidewalls and represent an important mechanism of submarine channel filling. These bar forms have inclined strata that might be mistaken for the deposits of point bars and internal levees, even though the formation mechanism and its implications to channel history are different.     

Multicarrier Communication Over Underwater Acoustic Channels With Nonuniform Doppler Shifts

Underwater acoustic (UWA) channels are wideband in nature due to the small ratio of the carrier frequency to the signal bandwidth, which introduces frequency-dependent Doppler shifts. In this paper, we treat the channel as having a common Doppler scaling factor on all propagation paths, and propose a two-step approach to mitigating the Doppler effect: 1) nonuniform Doppler compensation via resampling that converts a “wideband” problem into a “narrowband” problem and 2) high-resolution uniform compensation of the residual Doppler. We focus on zero-padded orthogonal frequency-division multiplexing (OFDM) to minimize the transmission power. Null subcarriers are used to facilitate Doppler compensation, and pilot subcarriers are used for channel estimation. The receiver is based on block-by-block processing, and does not rely on channel dependence across OFDM blocks; thus, it is suitable for fast-varying UWA channels. The data from two shallow-water experiments near Woods Hole, MA, are used to demonstrate the receiver performance. Excellent performance results are obtained even when the transmitter and the receiver are moving at a relative speed of up to 10 kn, at which the Doppler shifts are greater than the OFDM subcarrier spacing. These results suggest that OFDM is a viable option for high-rate communications over wideband UWA channels with nonuniform Doppler shifts.      

Seismic expression of channel outcrops: Offset stacked versus amalgamated channel systems

Forward seismic models of two ‘seismic scale’ outcrops of different style channel systems have been made to investigate their seismic signature. These two outcrops illustrate the geometric end members of channel stacking architecture in response to low- and high-accommodation space. The Eocene Nohut Tepe channel system of the Elaziğ Basin in eastern Turkey was deposited in an area of high accommodation resulting in an aggradational geometrical offset stacking of channels up against a slope. The Eocene Ainsa II Channel system of the Tremp-Pamplona Basin in the Spanish Pyrenees was deposited in an area of low accommodation resulting in a tabular, compound sheet geometry, with amalgamated channel bodies separated by clay drapes.Depth models were drawn from outcrop photos and converted to impedance models by assigning acoustic impedance properties to the sand filled channels and surrounding and interbedded mud and clay layers. These were the input for the forward seismic models, which constructed various frequency synthetic seismic sections of the two outcrops. Analysis of the outcrop synthetic seismic identified three distinct reflection configurations. Type I is characterised by a strong black peak and white trough reflection, which is due to a discrete channel body. Type II is characterised by multiple offset, time ‘stepped’ black peak reflections that are underlain by one continuous, strong white trough reflection, which is due to offset stacked channel bodies. Type III is characterised by strong black peaks which onlap an underlying, continuous white trough reflection, caused by the lateral amalgamation of channel bodies.These three types of reflection configurations observed on the outcrop synthetic seismic can also be found on actual seismic from channelised turbidite systems, which aids in interpreting channel stacking architecture, accommodation space prediction and depositional styles from the actual seismic data. Channel stacking architecture is clearly an important aspect which needs to be considered when making channel system interpretations based on seismic data.     

琼东南盆地陆坡区深水浊积水道的地震相特征

水道-天然堤体系作为油气储集圈闭日益引起沉积学家和勘探家的重视。地震相特征是识别深水水道的有效途径,本文基于高分辨率2D、3D地震资料的地震相分析,在琼东南盆地陆坡区深水盆地中识别出早中新世、上新世和第四纪多期深水水道体系。早中新世深水水道在地震剖面上具有强、弱振幅交替反射和相互叠置的地震反射特征,局部具有杂乱反射特点;上新世水道整体表现为强振幅,横向上连续或者半连续,纵向上为强振幅的叠加;第四纪水道在地震剖面上具有典型下切反射特点,该水道整体振幅相对较弱,但其水道轴部充填具有典型的强反射特征,这与世界典型地区的水道轴部粗粒充填强振反射一致。这几期深水水道都发育于低水位时期,为上部物源搬运引起的浊流事件而形成。     

Impacts of maintenance channel dredging in a northern Adriatic coastal lagoon. II: Effects on macrobenthic assemblages in channels and ponds

Coastal lagoons are ephemeral habitats whose conservation requires human intervention, such as maintenance dredging of inner channels. Dredging can reduce the abundance of benthic species due to the removal of individuals with the sediment, modify sediment properties, and resuspend fine sediment, nutrients and pollutants, which can lead to eutrophication, hypoxic events and increasing toxicity. Both direct effects in the dredged channel and possible indirect effects in surrounding shallow areas could be expected. This study assesses the effects of the channel maintenance dredging, performed between October 2004 and August 2005, on the invertebrate assemblages both in channels and adjacent ponds in the northern Adriatic coastal lagoon of Pialassa Baiona. The lagoon is affected by eutrophication, chemical and thermal pollution from wastewater treatment and power plants. Three impacted sites were located in the dredged channel and three in the adjacent interconnected shallow water ponds, while three non-impacted sites were located in a channel and in a pond far from the dredged area. Replicate samples were collected from each site one time before and one time after the dredging operations. Despite the extent of the intervention, effects of the dredging on macrobenthic assemblages were detected only within the dredged channel, while in the surrounding ponds no clear and unequivocal effects were found. In particular the dredging could have promoted the increase of the abundance of the polychaete Streblospio shrubsolii in the southern and central parts of the dredged channel and the increase in abundance of the amphipod Corophium insidiosum in the northern side, compared to the controls. Instead, species diversity was reduced in the central and northern parts of the dredged channel. These effects on the macrobenthic invertebrate assemblages could be related to the observed changes of sediment characteristics, contamination and toxicity. Overall, direct effects on benthic assemblages in the dredged channels were more detectable than the possible secondary effects in the surrounding shallow ponds, where the higher spatial heterogeneity can mask any relevant effects.     

Wigner/cycle spectrum analysis of spread spectrum and diversitytransmissions

A high-resolution t-ω estimator, termed the Wigner distribution (WD), is shown to form a sound basis for representing nonstationary acoustic returns. Signal returns are modeled as the output of a time-variant random filter where the WD of the nonstationary signal return defines a random process whose expectation reduces to the instantaneous power spectral density defined for dispersive communication channels. From the WD, a set of relations describing time-variant channel effects on spread-spectrum and diversity transmissions are developed. These relations are shown to be useful in comparing spreading techniques under differing channel conditions and for estimating channel-imposed bounds on the spreading parameters required for effective transmission. A mapping from the Wigner distribution to the cycle spectrum is shown to produce cyclic correlations characteristic of the modulation rate. The WD-based formulation is applied to an example of spread-spectrum transmission through a reverberation-limited channel     

3D seismic images of a low-sinuosity slope channel and related depositional lobe (West Africa deep-offshore)

In this paper a channel levee system and the associated depositional lobe are described. The proposed example derives from a recently acquired 3D survey in the West Africa deep-offshore. It is mainly based on a detailed 3D seismic reconstruction, attribute map interpretation and log data from a single well to calibrate seismic responses and sedimentary facies.The stratigraphic section under consideration, informally named the A 100 Sequence, is about 80 ms TWT thick, and of early Pliocene age. The attribute maps focused on this interval clearly show the presence of two narrow (up to 250 m wide) low-sinuosity slope channels that can be followed for more than 32 km in an E–W direction (down slope to the W) as far as the western border of the 3D acquisition. Over most of their length both channels are characterized by the low amplitude aspect of the axial belt and by brighter responses in the flanks (presence of thin-bedded sands in the levee areas). This character, associated with the common convex-down geometry of the reflections lying just above the channel axis, suggests a predominant fine-grained infilling of the thalwegs.One of these channels, termed the Southern Channel, shows a high amplitude lobe-shaped zone in the middle part of its course. The ‘anomalous’ development of this depositional element has been related to a local reduction of the slope gradient, probably induced by the synsedimentary growth of an adjacent mud-cored anticline.Because of hydrocarbon occurrence, the lobe area and the associated feeder channel have been investigated in detail through careful picking of all the mappable reflections inside the channel-lobe system. The resulting physical-stratigraphic framework and the related attribute maps suggest that channel development occurred through distinct growth stages. The lower stage (Stage 1) is expressed by symmetric levees flanking the main channel to the east and by a depositional lobe/lobe fringe area to the west. Between the levee belt and the lobe' a transitional zone occurs where the presence of isolated bypassing bars has been inferred. The upper stage (Stage 2) seems to record a phase of overall bypassing of flows within the channel conduit, producing the westward propagation of the channel and consequent dissection of the previous stage lobe. A contemporaneous lateral spillover from the channel axis of low-density turbidites constructed prominent gull-wing shaped levees that uniformly covered the stage 1 elements.     

Application of multi-seismic attributes analysis in the study of distributary channels

During the exploration of stratigraphic reservoirs, the key to locating these reservoirs is to identify the sandstone distribution. Seismic data can be used to recognize large-scale distributary-channel sedimentary bodies; however, depiction of sedimentary bodies in small distributary channels using traditional interpretation data from seismic profiles is extremely difficult. In the Upper Cretaceous first sandstone bed of the third member of the Nenjiang Formation (called the Nen 3 member for simplicity) in the XB area of the southern Songliao basin, distributary channels are the main sedimentary facies. The distributary channels migrate frequently; therefore, it is difficult to precisely depict the morphology of the channel and the distribution of sandstone. In this study, we investigated the deposition of distributary channels using equal-scale formation plane seismic attributes such as post-stack amplitude, instantaneous amplitude and seismic waveform classification, analyzed well logging data from target formations, and mapped the distribution of distributary channels. This study shows that seismic sedimentological study and stacking multiple attribute analysis can identify the distribution of distributary channels precisely and effectively. The attributes of equal-scale strata slices are evidently superior to those of time and horizon slices; Attribute extraction and selection of equal-scale formation slices is an essential step. The comprehensive selection of seismic attributes that show a good correlation to a single-well can be used to clearly depict channel bodies. The overlap of the 40-Hz single-frequency energy and the RMS amplitude depicts the sedimentary characteristics and shapes of sandstone bodies in main channels as well as small-scale distributary channels more clearly than a single attribute. These attributes show that the first sandstone bed of the Nen 3 member was from distributary channels with complex shapes. The distributary channels cut across each other vertically and laterally. It indicates that, during the deposition of the Nen 3 member in XB area, the direction of the main channel is from north to south, and the provenance could be from north and east. The method of this paper may provide some helpful suggestions to the geologist using seismic attributes to undertake research on sedimentary environment at other places in the world.     

Diversion and morphology of submarine channels in response to regional slopes and localized salt tectonics,Levant Basin

In the Levant Basin, submarine channels are abundant around the Nile deep-sea fan (NDSF), an area which is also affected by salt tectonics related to the Messinian salt giant. Here we focus on the relationship between submarine channels and obstacles formed by salt tectonics. Initially, we use methods developed for terrestrial morphological analysis and quantify channel sinuosity, width and slope in search for consistent relationships between morphometric parameters and channel response to obstacles. However, this traditional analysis did not yield robust conclusions. Then, we apply two new morphometric parameters suggested here to express the distortion of channels by obstacles: incident angle (α), defined as the acute angle between the regionally influenced channel direction and the strike of the tectonic obstacle and diversion angle (Ω), defined as the angle between the direction of the regional bathymetric slope and the average direction of the channel. These parameters illustrate the influence of the regional-scale basin geometry and the superimposed tectonic-influenced seabed patterns, on channel development. We found hyperbolic relationships between incident angle (α) and diversion angle (Ω) in which channels flowing approximately parallel (α ≈ 0°) to tectonic folds are (obviously) not diverted; channels nearly orthogonal (α ≈ 90°) to obstacles, crosscut them right through and, again, not diverted much. In contrast, channels with a general direction diagonal to the obstacles (α ≈ 40°), are diverted by ten degrees (Ω ≈ 10°). This diversion accumulates along large distances and significantly influences the regional development of channels around the NDSF. Noteworthy, this phenomenon of channel diversion, indirectly deteriorate normal slope-sinuosity relationships known from terrestrial studies. In light of these findings, we suggest that these new parameters can be applied to other basins, where submarine channels interact with seabed obstacles.     

Turbidite deposition and the development of canyons through time on an intermittently glaciated continental margin: The Bonanza Canyon system,offshore eastern Canada

Bonanza Canyon is a complex canyon system on the slope from the intermittently glaciated Grand Bank on the south side of Orphan Basin. A 3D seismic reflection volume, 2D high-resolution seismic reflection profiles and ten piston cores were acquired to study the evolution of this canyon system in relation to glacial processes on the continental shelf and the effects of different types of turbidity currents on the development of deep water channels. Mapped reflector surfaces from the 3D seismic volume show that the Bonanza Canyons developed in a depression created by a large submarine slide of middle Pleistocene age, coincident with the onset of glacigenic debris flows entering western Orphan Basin. Two 3–5 km wide, flat-floored channels were cut into the resulting mass-transport deposit and resemble catastrophic glacial meltwater channels elsewhere on the margin. Both channels subsequently aggraded. The eastern channel A became narrower but maintained a sandy channel floor. The western channel, B, heads at a spur on the continental slope and appears to have been rather passively draped by muds and minor sands that have built 1500-m wave length sediment waves.Muddy turbidites recorded by piston cores in the channel and on the inter-channel ridges are restricted to marine isotope stage (MIS) 2 and were deposited from thick, sheet-like, and sluggish turbidity current derived from western Orphan Basin that resulted in aggradation of the channels and inter-channel ridges. Sandy turbidites in channels and on inner levees were deposited throughout MIS 2–3 and were restricted to the channels, locally causing erosion. Some coincide with Heinrich events. Channels with well-developed distributaries on the upper slope more readily trap the sediments on Grand Bank to form sandy turbidity currents. Channel B dominated by muddy turbidity currents has wide and relatively smooth floor whereas channel A dominated by sandy turbidity currents has a sharp geometry.     

Slope channel sedimentary processes and stratigraphic stacking,Cretaceous Tres Pasos Formation slope system,Chilean Patagonia

The Cretaceous Tres Pasos Formation of southern Chile records a slope system characterized by >800 m of paleo-bathymetric relief. Channel deposits are exposed in an outcrop 2.5 km long by 125 m thick and are located in proximity to the toe of a slope clinoform. Exquisite exposures of channel strata offer a unique opportunity for high-resolution analyses of channel stacking patterns and provide insight into the evolution of conduits that transport sediment from continents to the deep ocean.Eighteen slope channels, or channel elements, are present in the strata studied. They are 6–15 m thick and comprised of stacked turbiditic sedimentation units. Channel fills are characterized by a gradational transition from amalgamated sandstone-rich facies in the channel axes to thinly interbedded sandstone and siltstone at the channel margins over distances of 10–30 m. These elements are generally considered to be ∼300 m wide and were formed by punctuated periods of incision and sedimentary bypass, followed by in-filling by collapsing turbidity currents. Out-of-channel deposits consist primarily of fine-grained facies, which are typically covered by vegetation in the study area.The channel strata of the mapped portion of the Tres Pasos Formation can be grouped into three channel complexes 25–70 m thick. Complexes are differentiated based on the preservation of siltstone-dominated deposits (bypass drapes and channel margin), which persist across the entire outcrop belt and coincide with shifts in channel stacking pattern. The oldest four channel elements (channel complex 1) are characterized by the highest lateral offsets, relative to one another. These are interpreted to record the most unconfined channel-stacking pattern present. As the channel system evolved (channel complexes 2 and 3), channel elements began to stack on top of one another, due to the increased confinement imparted on the slope channel system. The amount of vertical offset between successive channel elements preserves the record of channel aggradation as well as erosional degradation. The greatest vertical offset observed is associated with the oldest channels; as the system matured, vertical offset decreased. This decrease in vertical offset is coincident with the decrease in lateral offset of channels. The lateral offset decrease is attributed to establishment of constructional confinement and is the consequence of increased focusing of successive channel-initiating gravity flows. As confinement establishes, channels are predisposed toward underfilled conditions upon abandonment. The capture of channel-initiating currents along channel abandonment relief fairways focused incision and resulted in increased erosion and decreased vertical offset. The consequence of these conditions is an upward increase in channel element amalgamation.The organized stacking of slope channels observed in the Tres Pasos Formation is comparable to that of seismically imaged channel-levee or entrenched slope valley systems. By analogy to these 3-dimentionally constrained systems, a portion of the poorly exposed out-of-channel facies in the Tres Pasos Formation is attributed to aggradational internal levee deposits. The facies insight derived from the studied outcrop provides insight into analogous hydrocarbon-bearing units from numerous continental margins.     

Modeling the influence of hydroperiod and vegetation on the cross-sectional formation of tidal channels

The evolution of the cross section of a salt-marsh channel is explored using a numerical model. Deposition on the marsh platform and erosion and deposition in the channel affect the tidal prism flowing through the cross section, such that the model captures the evolution of the stage–discharge relationship as the channel and marsh platform evolve. The model also captures the growth of salt-marsh vegetation on the marsh platform, and how this vegetation affects flow resistance and the rate of sedimentation. The model is utilized to study the influence of hydroperiod and vegetation encroachment on channel cross section. Numerical results show that a reduction in hydroperiod due to the emergence of the marsh platform causes an infilling of the channel. Vegetation encroachment on the marsh surface produces an increase in flow resistance and accretion due to organic and mineral sedimentation, with important consequences for the shape of the channel cross section. Finally, modeling results indicate that in microtidal marshes with vegetation dominated by Spartina alterniflora, the width-to-depth ratio of the channels decreases when the tidal flats evolve in salt marshes, whereas the cross-sectional area remains proportional to the tidal peak discharge throughout channel evolution.     

Impact of dredging and dumping on the stability of ebb–flood channel systems

The impact of dredging and dumping on the morphologic stability of the tidal channels is investigated using morphologic field observations for the Westerschelde estuary dating back to 1955. The results are used to verify the theoretical concept presented by Wang and Winterwerp (2001). This concept states that a critical threshold for the amount of sediment dumping exists above which a channel system in equilibrium may become unstable and degenerate. The value of this threshold amounts to 5–10% of the total sediment transport capacity. Verification of this concept using field observations is not straightforward as the morphology of tidal channel often changes as a result of both natural processes and human interferences, i.e. the channels are not in equilibrium. In addition, the morphological timescales associated with channel degeneration are large (decades to centuries). Verification of the theory thus requires a careful analysis of abundant morphological data and numerical modeling of sediment transports. The results of such analyses presented in this study confirm the existence and the approximate magnitude of the critical level for dumping that follows from theory. Refined guidelines are derived to use the theoretical concept as an engineering tool for the evaluation and design of strategies for dumping in estuarine multi-channel systems. In the absence of the required morphological data the indicative theoretical level of 5–10% can be used to obtain a first estimate of the dump capacity in two-channel systems.     

Graphical determination of the height of surge waves

This study deals with the propagation of surge waves in open channels. The governing equations are derived for an arbitrary cross-section and specific solutions are given for rectangular, parabolic, and trapezoidal cross-sections. The method can be applied in each case, where the relation between the level and the area is known in advance. The hypotheses are the same as in classical treatises of surge waves, but the derived equations in dimensionless parameters show the possibility of plotting graphs for the given profile, which allows the finding of the solution in each case without a trial-and-error procedure, as with the classical method.The same equations show that (a) in the case of positive waves generated at the upstream side of a channel, two of the three defined parameters can have any prescribed value and the third can readily be found; (b) in the case of positive waves generated at the downstream end, an inequality must hold in order that the third parameter has a physical interpretation.     

Interpretation of channelized architecture using three-dimensional photo real models, Pennsylvanian deep-water deposits at Big Rock Quarry, Arkansas

Mapping geological details and interpreting three-dimensional geometries in a highly heterogeneous outcrop such as the exposure at Big Rock Quarry has been a continuous challenge especially because high vertical cliffs make access to most of the rocks difficult for direct geological observations. Previous interpretations of facies architecture were derived from gamma-ray profiles, a core and measurements made on two-dimensional photomosaics. This paper represents the first attempt of three-dimensional interpretation of the geometry and facies pattern of the Jackfork nested channel complex deposited at the base-of-slope.Examination of the photo real model of the outcrop with assigned lithologies allowed extraction of accurate 3-D qualitative, as well as quantitative (channel dimensions) geometric information. This facilitated interpretation and reconstruction of the submarine channel complex architecture making possible correlations of strata exposed on the two sides of the quarry.Most of the exposed vertically and laterally stacked channels are large, aggradational with well-defined axial regions overlain by matrix-supported breccia which grades upward into amalgamated sandstones. The thickness of the sandstone decreases toward the southeastern end of the quarry where more shale is present. The channel infill consists of thin-bedded sandstones interlayered with shale which overlain the breccia. The upper part of the quarry is made up of smaller, lateral migrating channels.Significant channel width and thickness variation can be recognized at outcrop scale. Thirty-eight identified channels are characterized by a relatively low aspect ratio (4:1 to 32:1) with channel dimensions ranging from 25 m to 314 m wide and 2 m-24 m deep. Bed thickness distributions of various facies show that the sandstone comprises a significant proportion (83%) of the total channel thickness, while shale and breccia represent about 8%, and 17% respectively. This yields a high net-to gross ratio of more than 80%.Compared to previous reconstructions our 3-D photo real model is more accurate and it can be used to calibrate simulation of processes in deep-water environments.     

Migration–aggradation history and 3-D seismic geomorphology of submarine channels in the Pleistocene Benin-major Canyon,western Niger Delta slope

Several laterally offset and aggradational sinuous submarine channels are contained within a 54 km long segment of the Benin-major Canyon. Axial channel deposits produce high amplitude reflections on three-dimensional (3-D) seismic profiles. Some seismic reflections have U- or V-shaped cross-sectional motifs that were correlated with confidence along linear to meandering paths for distances up to 70 km. They are referred to here as channel-forms (CFs), and are believed to be the axial parts of submarine channels preserved during overall channel floor aggradation. A total of 15 separate CFs were mapped allowing thalweg-gradients, dimensions, and morphology to be studied spatially and through time, providing insight into how submarine canyons fill. Their planform geometry evolved predominantly in a stepwise fashion through alternating periods of cut-and-fill, but more gradual channel migrations are also observed. The largest offsets in successive channel floor position occur after periods of significant vertical CF fill (‘thalweg plugging’—with deposits commonly consisting of lower amplitude, transparent to chaotic seismic reflections). The passage of erosive flows after such periods of fill caused abrupt shifts in channel position, particularly at meander bends, with increased potential for the formation of pseudo meander loop cut-offs. Significant spatial differences in the stacking architecture of CFs are attributed to local slope deformation and perhaps also to a recent channel avulsion just west of the study area. Abrupt channel straightening in the western study area coincides with a period of increased valley-gradient associated with amplification of an underlying anticlinal fold. The youngest CFs in this area show limited aggradation and are characterized by repeated episodes of headward erosion causing knickpoint migration as the recent channel floor tried, unsuccessfully, to establish a smooth graded depth profile. This is in stark contrast to the time-equivalent predominantly aggradational CFs in the eastern study area that show a progressive increase in sinuosity through time.