Continental Shelf Environments Research Articles

Estimates of source range using horizontal multipath in continental shelf environments

In a continental shelf environment, energy from an acoustic source propagating obliquely upslope repeatedly reflects from the sea surface and sloping seafloor with a consequent change in direction with each bottom reflection. Measurements of this type of effect can be observed in beamformed data from a horizontal line array. The arrivals from a single source are seen on two beams: a direct path with bearing angle corresponding to the source location and a refracted path with a bearing angle inshore of the source. In this work, the horizontal multipath effects are exploited to estimate the location of an acoustic source. Using the hybrid modeling approach of vertical modes and horizontal rays, rays are traced in the horizontal plane with refraction determined by the modal phase speed. Invoking reciprocity, the rays orientate from the center of the array and have launch angles equal to the estimated bearing angles of the direct and refracted paths. The location of the source in the horizontal plane is estimated from the point where the rays intersect. The technique is applied to data recorded on a horizontal line array located about 12 km east of the southern coast of Florida. [Work supported by ONR.]

Three-Dimensional Sound Propagation and Scattering in Two-Dimensional Waveguides

A coupled-mode method for three-dimensional acoustic propagation and scattering in two-dimensional waveguides is presented. This method synthesizes the three-dimensional field solution by using Fourier transform techniques based on a sequence of two-dimensional problems, each of which is solved by a numerical model recently developed by Luo et al. [Chin. Phys. Lett. 29 (2012) 014302]. Numerical results indicate that the present model is remarkably accurate, and thus can serve as benchmark against other numerical models. In addition, this model can be applied to realistic problems, and can also be used to analyze horizontal refraction in some range-dependent waveguides in reality, such as the continental shelf environment, ridge-like bathymetry, and underwater trenches.

Temporal coherence of the acoustic field forward propagated through a continental shelf with random internal waves

An analytical model derived from normal mode theory for the accumulated effects of range-dependent multiple forward scattering is applied to estimate the temporal coherence of the acoustic field forward propagated through a continental-shelf waveguide containing random three-dimensional internal waves. The modeled coherence time scale of narrow band low-frequency acoustic field fluctuations after propagating through a continental-shelf waveguide is shown to decay with a power-law of range to the -1/2 beyond roughly 1 km, decrease with increasing internal wave energy, to be consistent with measured acoustic coherence time scales. The model should provide a useful prediction of the acoustic coherence time scale as a function of internal wave energy in continental-shelf environments. The acoustic coherence time scale is an important parameter in remote sensing applications because it determines (i) the time window within which standard coherent processing such as matched filtering may be conducted, and (ii) the number of statistically independent fluctuations in a given measurement period that determines the variance reduction possible by stationary averaging.

Giant dune morphologies and dynamics in a deep continental shelf environment: Example of the banc du four (Western Brittany, France)

The aim of this paper is to improve the knowledge of spatio-temporal variability of very large to giant dunes in deep tide dominated environment. Their growth mechanisms and dynamic evolution still remain in debate and constitute a significant scientific challenge in regard to difficulties to achieve accurate measures of migrations. This motivated the realization of three recent swath bathymetry surveys across the wide dune field of the Banc du Four located offshore the western Brittany. The Banc du Four field is composed of more than 500 large dunes exhibiting a great diversity of morphologies ranging from 2D to 3D shapes and reaching the largest sizes of such sedimentary structure ever described (wavelength over 1000 m and height over 30 m). The analysis of the entire ensemble of dune parameters offers the opportunity to discuss the relative influence of forcings and the validity of the equations predicting the bedform geometry in shallow waters. Our results show a good height-spacing power law correlation but do not strike the usual statistical regression observed in previous studies. The steeper positive slope of the equation and the outstanding values of the height parameter reveal the complexity of local hydrodynamic regime, interacting with bed load transport and sediment supply. No clear relationship has been observed between asymmetry and size parameters. Water depth can not be considered as a major factor controlling the size of dunes. Otherwise, the vertical sedimentary variation suggests the action of storm waves on dune crests and the horizontal migration rates of dunes in the eastern field of the Banc du Four were found to range from 3 to 20 m.yr- 1. Such velocities were only mentioned before by Van Landeghem et al. (2012) on deep continental shelves (> 70 m deep) and attest of the still present morphodynamical equilibrium of the large dunes. Furthermore, as was previously found for similar dunes in the Irish Sea, no power-law relationship between dune migration rates and dune size can be deduced. Similarly, no strong correlation is established between migration rates and the degree of asymmetry. Nevertheless, the dune asymmetry can be used to predict the migration direction. As for dimension parameters, the evolution asymmetry depends on the study area and migration rates. More precisely, when the crest lines of dunes form straight lines and their migration rates remain below 8-10 m.yr- 1, the degree of dune asymmetry increases with the migration rate. This trend is reversed for the dunes with higher migration velocities and crests which are sinuous. These results relativize the migratory predictions based on punctual geometrical observations.

Use of mode subspace projections for depth discrimination with a horizontal line array: Theory and experimental results

A solution to the problem of acoustic source depth discrimination in a downward refracting, shallow-water ocean waveguide is presented for the case of a horizontal line array at endfire. The approach exploits the phenomenon of mode trapping, wherein a shallow acoustic source cannot excite the lowest order waveguide modes due to its evanescent amplitude dependence near the surface. The important implication of this "trapping" behavior is that, given sufficient spatial aperture, it provides a mechanism for differentiating a shallow acoustic noise source from one at depth. The method does not require array cant, or physical vertical aperture of any kind, but instead relies only on the sensitivity of a line array at endfire to differences in horizontal wave number to resolve low and high order mode subspace excitations. The only inputs to the algorithm are an approximate sound speed profile, water depth, and bottom type. The theoretical basis for the test statistic is first reviewed, followed by discussion of key requirements, and illustration of the concept using results from a RAM PE simulation for a downward refracting environment. Finally, the algorithm is experimentally demonstrated using data from a bottom-mounted HLA deployed in the moderately cluttered continental shelf environment of the Florida Straits.

Three-dimensional localization of multiple sources in an uncertain ocean environment

This paper develops a Bayesian focalization approach to simultaneous three-dimensional localization of multiple sources in shallow water with uncertain environmental properties, for application to horizontal line array (HLA) data. The algorithm maximizes the posterior probability density over unknown environmental (seabed and water column) and source parameters (includes source-bearing) using an adaptive hybrid optimization algorithm. Maximum-likelihood expressions for source strengths and noise variance are used that allows these parameters to be sampled implicitly rather than explicitly. An extension to the algorithm that optimizes for a priori unknown number of sources, based on minimizing the Bayes information criterion, is developed and presented. The algorithm is applied to simulated multi-frequency data in a continental shelf environment.

Review paper: Instrumentation for marine magnetotelluric and controlled source electromagnetic sounding

ABSTRACTWe review and describe the electromagnetic transmitters and receivers used to carry out magnetotelluric and controlled source soundings in the marine environment. Academic studies using marine electromagnetic methods started in the 1970s but during the last decade these methods have been used extensively by the offshore hydrocarbon exploration industry. The principal sensors (magnetometers and non‐polarizing electrodes) are similar to those used on land but magnetotelluric field strengths are not only much smaller on the deep sea‐floor but also fall off more rapidly with increasing frequency. As a result, magnetotelluric signals approach the noise floor of electric field and induction coil sensors (0.1 nV/m and 0.1 pT) at around 1 Hz in typical continental shelf environments. Fluxgate magnetometers have higher noise than induction coils at periods shorter than 500 s but can still be used to collect sea‐floor magnetotelluric data down to 40–100 s. Controlled source transmitters using electric dipoles can be towed continuously through the seawater or on the sea‐bed, achieving output currents of 1000 A or more, limited by the conductivity of seawater and the power that can be transmitted down the cables used to tow the devices behind a ship. The maximum source‐receiver separation achieved in controlled source soundings depends on both the transmitter dipole moment and on the receiver noise floor and is typically around 10 km in continental shelf exploration environments. The position of both receivers and transmitters needs to be navigated using either long baseline or short baseline acoustic ranging, while sea‐floor receivers need additional measurements of orientations from compasses and tiltmeters. All equipment has to be packaged to accommodate the high pressure (up to 40 MPa) and corrosive properties of seawater. Usually receiver instruments are self‐contained, battery powered and have highly accurate clocks for timekeeping, even when towed on the sea‐floor or in the water column behind a transmitter.

Marine spatial planning in practice

Multiple competing uses of continental-shelf environments have led to a proliferation of marine spatial planning initiatives, together with expert guidance on marine spatial planning. This study provides an empirical review of marine spatial plans, their attributes, and the extent to which the expert guidance is actually being followed. We performed a structured review of 16 existing marine spatial plans and created an idealized marine spatial plan from the steps included in recent expert papers. A cluster analysis of the yes/no answers to 28 questions was used to ordinate the 16 marine spatial plans and to compare them with the idealized plan. All the plans that have been implemented have a high-level government mandate and the authority to implement spatial planning vested in existing institutions. Almost all the plans used data with clear criteria for data inclusion. Stakeholders were included in almost all the plans; they did not participate in all stages of the planning process but their roles were generally clearly defined. Decision-support tools were applied inconsistently across plans and were seldom used dynamically over time. Most spatial planning processes did not select specific outcomes, such as preferred use scenarios. Success is defined inconsistently across plans; in half the cases there are no metrics of success with reference benchmarks. Although monitoring is included in the majority of plans, only in some cases do monitoring results feed back into management decisions. The process of marine spatial planning had advanced in that some of the more recent plans were developed more quickly and contain more desirable attributes than earlier plans. Even so, existing marine spatial plans are heterogeneous—there are essential ingredients, but no single recipe for success.

High Benthic Foraminiferal Diversity in Polluted Busan North Port (Korea)

Surface sediments were collected from Busan North Port (southern coast of Korea) and the outer port area (OPA) in February and August 2009 to ascertain the distribution of benthic foraminifera and assess controlling environmental factors. Ninety-eight species (81 calcareous and 17 agglutinated), 40 of which had live representatives, were identified. Busan North Port can be divided into distinctive regions according to different depositional environments and to the degree of influence from pollution sources, such as nearby streams and industrial facilities around the inside north port (INP) and outside north port (ONP). The OPA is characterized by inner continental-shelf environments. These areal distinctions corresponded well to cluster- and principal component-analysis results based on foraminiferal-assemblage and heavy-metal data. The degree of organic enrichment and heavy-metal concentration in the sediments were major factors explaining the distributions of the foraminiferal assemblage. The low abundance of benthic foraminifera in the INP compared to the OPA seems to be directly related to heavymetal toxicity caused by high Cu, Zn, and Pb concentrations. The dominant occurrence of pioneer species Cribroelphidium excavatum and Trochammina hadai appears to indicate the adverse effects of heavy metals and high levels of organic matter on benthic foraminiferal distribution in the INP. Species diversity, however, was unusually high in Busan North Port across all pollution levels, and can possibly be explained in two ways. Short water-residence time in the inner port and maintenance of high salinity through active seawater exchange may reduce the effects of heavy metals and possibly make the area hospitable to a variety of species. High diversity may also be attributed to “rare short-lived” species that favor unstable conditions. The response of the benthic foraminiferal community to the unstable environment seems to be a succession of these “rare short-lived” species rather than a predominance of a few opportunistic ones. The uncoupling of species diversity and pollution level observed in Busan North Port might not be uncommon, and caution should be exercised when benthic foraminiferal diversity is used as an index of relative environmental health. Presence of indicator species and/or size distribution in the foraminiferal assemblage should be considered as well as species diversity to correlate the community structure with environmental change and pollution level.

Orbital cycles analysis and its genesis significance for the sequence hierarchy: A case study of Carboniferous Karashayi Formation, Central Tarim basin

The Carboniferous Karashayi (卡拉沙依) Formation of Tarim basin formed in the epi- continental sea shelf environment where the evolution of basins is pediocratic. As an important reservoir-forming combination, the Carboniferous Karashayi Formation consist of clastic rocks, car- bonate rocks and gypsum rocks, which show clear sedimentary cyclicity. According to paleontology research and stratigraphic correlation, Karashayi Formation corresponds to Visean and Serpukhovian in international stratigraphic chart with a time limit about ±27 Ma. The sequence stratigraphic study for the Karashayi Formation is rough because of the difficulty to identify the unconformity surfaces. The current study mostly divides it into three or four 3rd sequences. However, this partition cannot meet the geological crossing correlation. At the theory aspect of sequence stratigraphy, the genesis of the 3rd sequence is also a disputed problem. Cyclostratigraphy study of the Natural Gamma-Ray Log, Spontaneous Potential Log and other logging curves has been taken out by spectrum analysis and wavelet analysis, etc.. For Well Shun6 and Well Zhong1 in Tazhong (塔中) and neighbouring area, combined core observation and base level analysis, the Karashayi Formation was divided into 9 or 11 sequence stratigraphic units. Continuous constraints of each sequence is about ±2.40 Ma, correspond- ing to astronomical cycle formed by three parameters of Earth orbit. Accompanied by the historical division of 3rd sequences, the formation mechanism of 3rd sequences was possibly forced by the 2.4 Ma astro-

Complex habitat generated by marine bryozoans: a review of its distribution, structure, diversity, threats and conservation

ABSTRACTHabitat‐forming bryozoans are here defined as extant, heavily‐calcified species which regularly attain sizes over 50 mm in three‐dimensions and which contribute significantly to benthic habitat structure as living colonies.Records of habitat‐forming bryozoans were collated and mapped, together with information about the location, environment, habitat‐forming species, the nature and size of the habitat formed, any associated fauna, and relevant threats and/or conservation measures.Records collated here indicated that habitat‐forming bryozoans occurred from ~59°N to 77°S, but that they did not occur frequently in the tropics, being found most commonly in temperate continental shelf environments, on stable substrata in places where water movement was relatively fast and consistent.Habitat‐forming bryozoans are particularly abundant and diverse in New Zealand, where 27 species, a quarter of which are cyclostomes, provide habitat over hundreds of square kilometres of sea floor. Other areas where they are particularly rich and/or abundant include Antarctica (Weddell, Lazarev and Ross Seas), the North Pacific around Japan, the northern Mediterranean and Adriatic, and along the southern edge of the North Sea, through the English Channel and around the United Kingdom.Large bryozoans provide habitat for diverse associated assemblages, particularly for other bryozoans, molluscs, annelids, arthropods, cnidarians, sponges, echinoderms and macroalgae.Protected areas which include habitat‐forming bryozoans occur throughout the distribution of this frequently unrecognized habitat type, but despite this they are prone to damage by anthropogenic impacts including pollution and bottom fishing. Copyright © 2012 John Wiley & Sons, Ltd.

Monitoring of Benthic Reference Sites: Using an Autonomous Underwater Vehicle

We have established an Australia-wide observation program that exhibits recent developments in autonomous underwater vehicle (AUV) systems to deliver precisely navigated time series benthic imagery at selected reference stations on Australia's continental shelf. These observations are designed to help characterize changes in benthic assemblage composition and cover derived from precisely registered maps collected at regular intervals. This information will provide researchers with the baseline ecological data necessary to make quantitative inferences about the long-term effects of climate change and human activities on the benthos. Incorporating a suite of observations that capitalize on the unique capabilities of AUVs into Australia's integrated marine observation system (IMOS) [1] is providing a critical link between oceanographic and benthic processes. IMOS is a nationally coordinated program designed to establish and maintain the research infrastructure required to support Australia's marine science research. It has, and will maintain, a strategic focus on the impact of major boundary currents on continental shelf environments, ecosystems, and biodiversity. The IMOS AUV facility observation program is designed to generate physical and biological observations of benthic variables that cannot be cost effectively obtained by other means.

Modeling three-dimensional propagation in a continental shelf environment

An acoustic propagation model is applied to predict measurements of three-dimensional (3-D) effects recorded off the southeast coast of Florida. The measured signal is produced by a low frequency source that is towed north parallel to the shelf from a fixed receiving array. The acoustic data show the direct path arrival at the bearing of the tow ship and a second refracted path arrival as much as 30° inshore of the direct arrival. Notably, the refracted arrival has a received level more than 25 dB greater than that of the direct arrival. A geoacoustic model of the environment is created to explain the data. It is shown that the topography of the seafloor plays the largest role in controlling horizontal refraction effects, whereas the range-dependent sediment properties have the most influence on the received level. The modeling approach is based on a 3-D adiabatic mode technique in which the horizontal refraction equation is solved using a parabolic equation in Cartesian coordinates. A modal decomposition of the field provides insight into the variability in the arrival angle and received level of the measured signal.

Geology of the Fuding inlier in southeastern China: Implication for late Paleozoic Cathaysian paleogeography

The age and composition of the basement rocks underlying the Mesozoic volcanic rocks in southeastern China have been long debated. New field investigation, stratigraphical and sedimentological studies of the Fuding inlier in eastern Cathaysian block have identified the presence of three stratigraphic units. The Upper Carboniferous carbonate unit, composed of silicified limestone, deposited on carbonate platform in a continental shelf environment. The Lower Permian siliciclastic unit, consisting of slate and phyllite with minor arenites, deposited on shallow semi-restricted shelf. The third unit is a suite of Jurassic conglomerates and sandstones deposited in alluvial fan environment. Provenance data indicate that the Fuding inlier was part of the Wuyishan terrane in the eastern Cathaysian block. The U–Pb age and Hf isotope of detrital zircons indicate that extensive magmatic activity happened during 280–360Ma in the source area. The εHf values of detrital zircons point to magmatic mixing of juvenile material and Precambrian crust. The similarities in rock compositions and detrital zircon ages among the Yeongnam massif in the Korean Peninsula, the Tananao complex in Taiwan and the Fuding inlier have led to the conclusion that these three regions most likely belonged to the Wuyishan terrane, eastern Cathaysia during the late Paleozoic.

Bayesian histogram filter localizer performance in a multiple contact, continental shelf environment.

Model-based recursive Bayesian state estimation provides a straight forward way of including a priori environmental knowledge in a localization framework. Previous work by Jemmott et al. demonstrates successful range and depth localization of an acoustic source using this method and data from a single hydrophone. However, that work utilized a relatively benign environment from the SWellEx 96 experiment in which only two source signals were present. This talk will extend the work of Jemmott et al. and discuss the performance of the localization scheme in a multiple contact, continental shelf environment. [Work supported by ONR Undersea Signal Processing.]

Oxygen respiration rates of benthic foraminifera as measured with oxygen microsensors

Oxygen respiration rates of benthic foraminifera are still badly known, mainly because they are difficult to measure. Oxygen respiration rates of seventeen species of benthic foraminifera were measured using microelectrodes and calculated on the basis of the oxygen fluxes measured in the vicinity of the foraminiferal specimens. The results show a wide range of oxygen respiration rates for the different species (from 0.09 to 5.27nlcell−1h−1) and a clear correlation with foraminiferal biovolume showed by the power law relationship: R=3.9810−3BioVol0.88 where the oxygen respiration rate (R) is expressed in nl O2h−1 and in μm3 biovolume (BioVol) (n=44, R2=0.72, F=114, p<0.0001). The results expressed per biovolume unit (1.82 to 15.7nlO210−8μm−3h−1) allow us to compare our data with the previous published data showing similar ranges. A comparison with available data for other microbenthos groups (nematodes, copepods, ostracods, ciliates and flagellates) suggests that benthic foraminifera have a lower oxygen respiration rates per unit biovolume. The total contribution of benthic foraminifera to the aerobic mineralisation of organic matter is estimated for the studied areas. The results suggest that benthic foraminifera play only a minor role (0.5 to 2.5%) in continental shelf environments, which strongly contrasts with their strong contribution to anaerobic organic matter mineralisation, by denitrification, in the same areas.

Scattering from extended targets in a range-dependent fluctuating ocean waveguide from theory and experiments where the sonar equation is invalid.

Bi-static, long-range measurements of acoustic scattered returns from vertically extended, air-filled cylindrical targets were made during three separate ONR-sponsored field experiments in fluctuating continental shelf environments. We show that the sonar equation estimates of mean scattered intensity from these targets lead to large errors, differing by an order of magnitude from both our measurements and waveguide scattering theory. This is because the sonar equation approximation is not applicable to targets large compared to the acoustic wavelength in an ocean waveguide. We find that use of the Ingenito scattering model also leads to significant errors in estimating mean target scattered intensity in our field experiments because they were conducted in range-dependent ocean environments with large variations in sound speed structure over the depth of the targets, scenarios that violate the basic assumptions of the Ingenito model. Here we describe scattering from extended cylindrical targets in a range-dependent fluctuating ocean waveguide using a Green’s theorem based full-field model that takes into account nonuniform sound speed structure over the target’s depth extent. We show that this model accurately describes the statistics of the acoustic field scattered by the targets in all three field experiments.

End-Permian mass extinction and palaeoenvironmental changes in Neotethys: Evidence from an oceanic carbonate section in southwestern Tibet

This paper documents a new Permian–Triassic carbonate sequence which recorded the end-Permian mass extinction in the isolated oceanic setting of Neotethys in southwestern Tibet, China. The sequence is over 350 m thick and consists of the Gyanyima and the Lower Lanchengquxia formations in ascending order. The Lopingian (Late Permian) Gyanyima Formation is composed of fossiliferous reddish carbonates dominated by Colaniella grainstone and reef facies including fenestrate/sponge/coral framestone and bafflestone. 156 species are recognized from the Lopingian Gyanyima Formation. Composite ranges of brachiopods, ostracods, rugose corals and foraminifers at the Gyanyima Section suggest that evolution and diversification of Permian marine organisms continued to the end-Permian preceding a major faunal extinction close to the Permian–Triassic boundary (PTB), coincident with a 2–3‰ negative shift of δ 13C carb. The timing and accelerating extinction pattern and the negative δ 13C carb excursion are largely comparable with those reported from many previously-documented sections on continental shelf environments. Based on a detailed lithofacies analysis, the latest Permian reefal facies is sharply replaced by ostracod/crinoid packstone/grainstone with abrupt abundant occurrences of Early Triassic conodonts at the Gyanyima Section. This is then followed by thrombolitic microbialite, stromatolite, packstone containing abundant spherical microbes, and bivalve/ammonoid packstone of tidal and intertidal facies. This distinct lithofacies and biofacies shift would, therefore, suggest a dramatic faunal community and environmental change across the PTB. Distinct palaeoclimate fluctuations through the P– T interval are also indicated by the alternation of warm- and cool-water faunas through the uppermost part of the succession. The lower part of the Gyanyima Formation is characterized by a warm condition as indicated by Cathaysian-dominated fossils. This was then followed by a mild cooling event as suggested by the occurrence of many distinct peri-Gondwanan elements and absence of compound corals. The latest Changhsingian is characterized exclusively by warm-water faunal elements of Cathaysian affinities, together with the negative shift of δ 13C carb, suggesting a rapid warming event at the very end of Permian in association with much more widespread volcanic activities than we thought before.

Comparison of fading statistics for shallow and deep acoustic sources in a continental shelf environment.

Previous work has shown a noticeable difference in the effects of the ocean environment on signals from shallow and deep moving sources. These effects are seen in received amplitude statistics, and such statistics can be used in passive acoustic depth classification. This talk presents a statistical analysis of signals from a September 2007 shallow water acoustic transmission test performed along the continental shelf off the coast of southeast Florida. The data used include low frequency (between 25 and 450 Hz), continuous-wave signals from a towed source at 100 m depth, as well as tones from surface ships in the area. Using statistical class models in a Minimum Hellinger Distance Classifier, the usefulness of received signal amplitude statistics for passive acoustic source-depth classification is discussed. [Work supported by ONR Undersea Signal Processing.]

Information theory application to inversions of acoustic data from a continental shelf environment.

This research examines the uncertainty in waveguide parameter values obtained from acoustic inversion of data collected in range-dependent environments. To account for range-dependence in the environmental parameters, the number of inversion parameters must be increased. Generally, from a fixed data sample the uncertainty in the solution for individual parameter values increases as the number of inversion parameters increases, especially if the structure of the propagated acoustic field is sensitive to the additional parameters. This effect can be offset by prior information that reduces the volume of the parameter search space and to a limited extent by larger inversion data samples. How an inversion algorithm accounts for prior information is thus an essential issue to be addressed for range-dependent waveguides. Measurements made on the New Jersey shelf in 70–90 m of water provide an opportunity to study the effects of environmental range dependence on sound propagation and inversion processing. Inversions based on pre-existing geophysical information are combined with a maximum entropy approach to quantify the amount of information on the waveguide parameters that can be extracted from measured propagation over range-dependent tracks. Both towed sources emitting CW lines and impulsive sources are employed. [Work supported by ONR.]