Continental Shelf Environments Research Articles

The Western Arctic Shelf–Basin Interactions (SBI) project, volume II: An overview

The Western Arctic Shelf–Basin Interactions (SBI) project is a 10-year Arctic environmental change program. A major field campaign occurred in the spring, summer and fall seasons between 2002 and 2004. The SBI program was developed to investigate the production, transformation and fate of carbon on shelf and slope regions of the Chukchi and Beaufort seas and shelf–basin interactions with the Arctic Basin. This 2nd special issue documents many of the key findings, ranging from sea ice and hydrographic changes to water column and sediment dynamics and trophic level interactions during the core Phase II field program. The integration of these results with ongoing synthesis and modeling activities within the SBI program are illustrating the critical importance of this continental shelf environment to understand shelf–basin interactions and the ongoing changes being observed and forecasted in the marine Amerasian Arctic.

Measurements of three-dimensional propagation in a continental shelf environment

Although a significant amount of theoretical and numerical modeling effort has been put into the study of three-dimensional (3D) acoustic propagation on a coastal wedge, including the development of the ASA 3D benchmark problem set, there have been few observations of the predicted 3D propagation effects. Significant horizontal multipath arrivals were observed in a pair of acoustic transmission tests on the continental shelf off the east coast of Florida in September 2007 and February 2008. For many transmissions, arrivals were received coming from nearly the global positioning system (GPS) bearing of the ship, as well as up to 30 deg inshore of the true bearing. The inshore path was up to 25 dB stronger than the direct path in some cases. The experimental waveforms transmitted included continuous-wave transmissions ranging in frequency from 24 to 415 Hz as well as wideband linear frequency modulation pulses (20-420 Hz). Horizontal multipath arrivals were observed for source ranges from 10 to 80 km, source depths of 20 and 100 m, and along several different bearings (inshore and along the 250 m isobath). It is a conclusion of this paper that the bearing bias and multiple horizontal arrivals are the result of 3D propagation due to the local shoaling bathymetry.

Statistics of the forward field propagated through three-dimensional random internal waves in an ocean waveguide.

Internal waves randomize the forward propagated acoustic field in both continental shelf and deep water environments. This has implications in underwater communications and remote sensing using wide area sonar systems. Here, we derive compact analytic expressions for the mean, variance, mutual intensity, spatial covariance, and temporal coherence of the acoustic field propagated through three‐dimensional (3‐D) random internal waves. These take into account multiple forward scattering effects and are derived in terms of the statistics of the internal waves. For typical continental shelf environments, we show that when internal wave height exceeds the acoustic wavelength, the acoustic field becomes so randomized that the expected total intensity is dominated by the field variance beyond moderate ranges. This leads to an effectively saturated field that decays monotonically and no longer exhibits the periodic range‐dependent modal interference structure present in nonrandom waveguides. 3‐D scattering effects become important when the Fresnel width exceeds the cross‐range coherence length of the internal wave field. Our derivation makes it possible to predict the coherence time scale of field fluctuations in ocean‐acoustic measurements from knowledge of the oceanography. It is used to explain the time scale of acoustic field fluctuations observed at megameter ranges in various deep ocean‐acoustic transmission experiments.

Geomorphological Mapping along the Upper Southeast Florida Atlantic Continental Platform; I: Mapping Units, Symbolization and Geographic Information System Presentation of Interpreted Seafloor Topography

Abstract Continental shelf environments along the Florida Atlantic southeast coast offshore Palm Beach County were mapped by interpreting seafloor topography derived from airborne laser bathymetry. The study area covered approximately 270 km2 along 75 km of coast, extending from the shore to about 55 m water depth beyond the shelf break to the upper Florida-Hatteras Slope. Forty-two mapping units were used to depict seafloor features that ranged from bedrock outcrops and coral reefs to different types of morphosedimentary bodies. Background theory and protocols of geomorphological symbolization are provided as a rationale for the seafloor mapping. Geomorphological mapping symbols were developed specifically for this project and collected in a symbol library as part of an ArcInfo GIS extension that was used to produce the maps in a GIS environment. Spatial distribution patterns of landform assemblages evident in the color-pattern mapping units permitted regionalization of shelf environments into three prim...

Geochemistry of basal Cambrian black shales and cherts from the Northern Tarim Basin, Northwest China: Implications for depositional setting and tectonic history

Black shales and thin-bedded cherts in the basal Cambrian are widespread worldwide and they carry important information on the formation of sedimentary basins and on the tectonic history. We studied the geochemical signatures of the early Cambrian black shales and bedded cherts from the Northern Tarim Basin, China, with the objectives of understanding the depositional setting of these rocks and inferring the tectonic history in the region. Twenty two black shales, ten cherts, and two nodular phosphorites were collected from two outcrops at Xiaoerbulake and Sugaitebulake in the Northern Tarim Basin, spanning vertical sections of 8.8 and 7.5 m, respectively. A suite of techniques, including field investigations, X-ray diffraction, electron microscopy, trace element, rare earth element (REE), and isotope geochemistry, were employed to characterize the geochemical signatures of these rocks. Field evidence indicates that the black shales and bedded cherts are over- and underlain by dolomites, suggesting a shallow marine depositional environment. Mineralogical and trace element data suggest that the Tarim black shales and cherts were deposited in a suboxic continental shelf environment, and hydrothermal activity may have extracted certain trace elements from mafic continental crust and concentrated them in the sedimentary basin. REE characteristics for the cherts are very similar to those that are known to be deposited in pelagic ocean floor settings, suggesting that the hydrothermal fluids may be derived from the infant southern Tianshan Ocean in the north of the Tarim Basin. Os isotope signatures at the time of deposition ( 187Os/ 188Os i = 1.1–2.7) are typical of crustal signatures, and the radiogenic Os isotope signatures rule out the mantle as a possible source of Os and other metals. A positive correlation between 187Os/ 188Os and ε Nd is consistent with upper crust-derived basin sediments that contain a variable contribution of hydrothermal fluids possibly derived from ancient mafic continental crust. These trace element, REE, and isotope systematics collectively suggest that incorporation of hydrothermal fluids derived from ancient, mafic continental crust combined with deposition in relatively reducing conditions may have controlled the chemical and isotopic compositions of these rocks. We infer that the hydrothermal fluid was carried to the continental shelf by upwelling during the initial stages of formation of the southern Tianshan Ocean, where the fluid interacted with thinned, mafic crustal basement lithologies and was subsequently incorporated into the black shales and bedded cherts in the Northern Tarim Basin. This study provides important geochemical evidence for the creation of the Tianshan Ocean, which is a result of break-up of the Rodinia Supercontinent during the early Cambrian.

Shelf Geomorphology along the Southeast Florida Atlantic Continental Platform: Barrier Coral Reefs, Nearshore Bedrock, and Morphosedimentary Features

Differentiation of continental shelf morphology along the southeast Florida Atlantic coast was based on interpretation of airborne laser bathymetry. The 600-km2 shelf study area, which had a shoreline extent of about 160 km and extended up to 10 km offshore, displayed a diverse range of seafloor morphologies that were characteristic of four main alongshore reaches. Reach I (sand flats and karst topography) in the northern part of the study area is terminated southward by the Bahamas Fracture Zone, a major morphotectonic feature. Reach II (sand flats and coral reefs) is characterized by sand flats with diabathic channel fields leeward or shoreward of the Florida Reef Tract, the seaward margin of which occurs along the shelf break on the upper part of the continental slope. Reach III (sand flats, hardgrounds, and coral reefs) is characterized by extensive nearshore rock outcrops that are exposed as bare rock surfaces on the seafloor or are variously mantled by thin veneers of sand that are not thick enough to disguise the underlying rock structure. Reach IV (tidal sand flats and ridges, hardgrounds, and coral reefs) is dominated by tidal features that notably include fields of tidal sand ridges in the lee of the Florida Reef Tract. The barrier reef on the southeast Florida Atlantic coast, which transitions to Florida Keys shelf environments southward, grades northward to drowned karst topography that is overlain by sand sheets and sand waves. Tidal channels and associated bars, deltas, and shoals occur on the interface between Biscayne Bay and the Atlantic Ocean. This reconnaissance-level characterization of continental shelf environments into morphological reaches in a geographic information system platform provides a basis for quantifying spatial distribution patterns of discrete landform units.

High-Pressure/High-Temperature Consolidated Completion in the Gulf of Mexico

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper OTC 18976, "High- Pressure, High-Temperature Consolidated Completion in the Continental Shelf Environment of the Gulf of Mexico: Case History," by Richard E. Patterson, SPE, and Trevor J. Willms, SPE, El Paso E&P, and Keith Foley, SPE, and Jason Edwards, SPE, Halliburton, prepared for the 2007 Offshore Technology Conference, Houston, 30 April-3 May. The paper has not been peer reviewed. Decades after the first completion in the Gulf of Mexico (GOM) continental shelf, the logical expansion of these mature assets has extended into reservoirs that are deeper, hotter, and higher-pressured than wells completed previously. These high-pressure/high-temperature (HP/HT) wells can cause extreme completion challenges. HP/HT formations also tend to have low permeabilities, which is the opposite of most GOM reservoirs. To make these low-permeability formations economical in an offshore environment, it is imperative that stimulation treatments be completely effective. Introduction The West Cameron 62 field, on the continental shelf just south of Louisiana at a water depth of 35 ft, had its first well completed more than 20 years ago. Since that time, more than 30 different intervals have been completed at depths ranging from a few thousand feet to more than 10,000 ft. In recent years, it has become necessary to focus on reservoirs that exist in the 18,000 to 20,000 ft true vertical depth (TVD) range. At these depths, the pressures and temperatures of the formation trend toward the limit of what current technology allows when completion equipment and fluids are considered. The West Cameron 62 Well A-2 was completed at these depths in the Cris R formation. The Cris R sand is located between 17,843 and 18,021 ft TVD, which correlates to 19,789 and 19,976 ft measured depth (MD), respectively. The initial bottomhole pressure (BHP) for the Cris R formation was measured at 16,500 psi at midperforation. The bottomhole temperature (BHT) at midperforation was 356°F. Contrary to conventional GOM reservoirs, which typically have average permeabilities greater than 50 md, the permeability of the Cris R sand averaged only 0.64 md. The 18% porosity also was significantly lower than that of the well-sorted formations found at shallower depths. In fact, along with permeability and porosity, the rock mechanics falls more in line with typical south Texas "hard rock" reservoirs such as the Wilcox and Frio formations. Despite the low permeability and porosity, the production of the well before the hydraulic-fracturing treatment was 12.0 MMscf/D and 396 B/D of condensate at a 8,572-psi flowing tubing pressure (FTP). Use of fracture models and nodal analysis led to the belief that the production rates could be increased by at least two-fold through implementation of a hydraulic fracture. However, very little information on HP/HT fracturing treatments in the GOM was available.

Range-dependent waveguide scattering model calibrated for bottom reverberation in a continental shelf environment

An analytic model is developed for scattering from random inhomogeneities in range-dependent ocean waveguides using the Rayleigh-Born approximation to Green's theorem. The expected scattered intensity depends on statistical moments of fractional changes in compressibility and density, which scatter as monopoles and dipoles, respectively, and the coherence volume of the inhomogeneities. The model is calibrated for ocean bottom scattering using data acquired by instantaneous wide-area ocean acoustic waveguide remote sensing (OAWRS) and geophysical surveys of the ONR Geoclutter Program. The scattering strength of the seafloor on the New Jersey shelf, a typical continental shelf environment, is found to depend on wave number k, medium coherence volume V(c), and seabed depth penetration factor F(p) following a 10 log(10)(F(p)V(c)k(4)) dependence. A computationally efficient numerical approach is developed to rapidly compute bottom reverberation over wide areas using the parabolic equation by exploiting correlation between monopole and dipole scattering terms and introducing seafloor depth penetration factors. An approach is also developed for distinguishing moving clutter from statistically stationary background reverberation by tracking temporal and spatial fluctuations in OAWRS intensity images.

Fate of persistent organic pollutants in the water column: Does turbulent mixing matter?

The effect of vertical turbulent mixing on the dynamics of persistent organic pollutants has long been overlooked and its role is still hardly understood. Here we present the first comprehensive analysis of the role of turbulent diffusion on the distribution of those contaminants and its interplay with sinking fluxes. To this end, a 1D dynamic coupled hydrodynamic-contaminant model has been developed and applied to a Mediterranean continental shelf environment. The hydrodynamic sub-model is adapted from COHERENS, the contaminant sub-model is an improvement from the BIODEP model and considers the contaminant in 3 phases: dissolved-colloidal-particulate. The simulation highlights the role of turbulence in determining the POP distribution and variability in the water column. In short, turbulent flux of contaminants strengthens the upward diffusion of sediment entrained contaminants and determines the extent to which inputs from the atmosphere mix into the water column. It acts in parallel with degradation and sinking fluxes, the combined effect yielding a surface enriched – depth depleted – benthic layer enriched region distribution, which presents similarities to reported experimental measures.

Seabed sediment transport pathway investigations: review of scientific approach and methodologies

Abstract An understanding of the geomorphological evolution of the coastal and inner continental shelf environments, in response to natural and/or anthropogenic forcings, depends upon a comprehensive knowledge of the prevailing sediment dynamics. In this contribution, selected methods/tools that can be used to study seabed mobility/transport patterns, are described and these all reviewed in relation to their inherent limitations. The geological, morphological, hydrodynamic and sedimentary information (the environmental framework) necessary to develop a general understanding of the areas investigated are identified. This preliminary information prescribes the investigative tools to be utilized subsequently, i.e. the collection/analysis of data; these can define geomorphological and sedimentological indicators of sediment transport, hydrodynamic/sediment transport field observations and numerical simulations. The results derived from the various methods can be used then independently or, preferably in combination, to establish conceptual models of seabed sediment dynamics over the investigated area. The coherence between the outputs of the different methods/tools controls the confidence level (high, medium or low) of the models. This approach is illustrated in relation to case studies from the southern UK inner continental shelf.

Fish population and behavior revealed by instantaneous continental‐shelf scale imaging

Fish populations in continental shelf environments are instantaneously imaged over thousands of square kilometers and continuously monitored by a remote sensing technique in which the ocean acts as an acoustic waveguide [Science 311, 660–663 (2006)]. The technique, ocean acoustic waveguide remote sensing (OAWRS), has revealed the instantaneous horizontal structural characteristics and volatile short‐term behavior of very large fish shoals, containing tens of millions of fish and stretching for many kilometers. The former follows a fractal or power‐law spectral process, indicative of structural similarity at all scales. The latter features compressional waves of fish population density that travel roughly an order of magnitude faster than fish can swim. Here we present an overview of OAWRS imagery documenting fish activity over a 2‐week period in the Spring of 2003 at the edge of the continental shelf, roughly 200 km south of Long Island, NY. Until now, continental shelf environments have been monitored with highly localized line‐transect methods from slow‐moving research vessels. These methods significantly undersample fish populations in time and space, leaving an incomplete and ambiguous record of abundance and behavior.

Mechanism for sea bottom reverberation in long range sonar in a range-dependent continental shelf environment

The statistically stationary background reverberation in long-range sonar data acquired in the New Jersey Continental shelf during the ONR 2003 Main Acoustic Clutter Experiment is mainly due to seafloor scattering. We compare three mechanisms or models for scattering from the sea bottom in a range-dependent waveguide with the experimental data. They are the Rayleigh-Born volume scattering model, a rough surface scattering model, and an empirical Lambertian model. Each of these models has a different decay rate as a function of range and different frequency dependence. These models are compared to the measured background reverberation as a function of range from the sonar system and at various frequency bandwidths from 300 to 1500 Hz to determine which mechanism or model for seafloor scattering best describes the data.

Benthic foraminifera as bio-indicators of drill cutting disposal in tropical east Atlantic outer shelf environments

We present a study of benthic foraminiferal faunas from the outer continental shelf off Congo (tropical West Africa), with the aim to determine the impact of the discharge of oily drill cuttings on the sea floor environment, to judge the regenerating capacity of the benthic ecosystem, and to investigate the possibility to develop an environmental monitoring method for open marine continental shelf environments, based on benthic foraminifera. We studied the spatial distribution and microhabitats of living and dead foraminiferal faunas, sampled in April 2003, 4 years after the end of disposal activities, in the upper 3 cm of the sediment at 9 stations (about 180 m depth) offshore Congo, that were subject to various degrees of pollution by oily cuttings from 1993 until 1999. Our results describe a zonation of foraminiferal faunas in the 750 m around the former disposal sites. At the immediate vicinity of the discharge points (within 70 m), faunas are characterized by low foraminiferal densities. Faunas between 70 m and 250 m of the disposal sites have very high foraminiferal densities, with high percentages (about 80%) of opportunistic taxa such as Bulimina aculeata, Bulimina marginata, Textularia sagittula, Trifarina bradyi and Bolivina spp. Between 250 and 750 m from the disposal site, foraminiferal densities decrease, and the percentages of opportunistic species are lower (40–60% of indicator species). These results show that 4 years after the cessation of oily cutting disposal, strong environmental impact is limited to the 250 m around the disposal sites. We used these data to develop a quantitative pollution index, values of which are strongly correlated to distance to the disposal site. This foraminiferal index offers the possibility to quantify the impact of anthropogenic eutrophication in continental shelf environments, but its validity must be tested in other continental shelf environments.

Temporal and cross-range coherence of sound traveling through shallow-water nonlinear internal wave packets

Expressions governing coherence scales of sound passing through a moving packet of nonlinear internal waves in a continental shelf environment are presented. The expressions describe the temporal coherence scale at a point, and the horizontal coherence scale in a plane transverse to the acoustic path, respectively. Factors in the expressions are the wave packet propagation speed, wave packet propagation direction, the fractional distance from the packet to the source, and the spatial scale S of packet displacement required to cause acoustic field decorrelation. The scale S is determined by the details of coupled mode propagation within the packet and the waveguide. Here, S is evaluated as a function of frequency for one environment, providing numerical values for the coherence scales of this environment. Coherence scales derived from numerical simulation of coupled mode acoustic propagation through moving wave packets substantiate the expressions.

Assessing fish populations and enhancing biological detection in continental shelf environments with range-dependent scattering and reverberation models

Fish populations in continental shelf environments can be continuously imaged over thousands of square kilometers using acoustic waveguide remote sensing techniques [Makris et al., Science, Feb. (2006)]. A calibrated range-dependent scattering and reverberation model [Ratilal et al., J. Acoust. Soc. Am. 114, 2302 (2003)] based on the parabolic equation has been applied to assess population densities of fish by inverting long-range acoustic data collected on the New Jersey continental shelf. This model is now applied to predict the types of fish species and zooplankton that are detectable in a general range-dependent continental shelf environment, including the resolution and accuracy that can be expected in estimating fish population densities and for differentiating fish species. We consider different geometries of the source and receiving array to enhance biological detection and reduce background reverberation in highly range-dependent environments. Using multiple source frequencies, the possibility of distinguishing fish species based on their differing scattering characteristics and resonance frequencies will be examined.

Mineralogy, geochemistry and petrology of an amphibolite-facies aluminum-phosphate and borosilicate (APB)-bearing quartzite from the Mesoproterozoic Itremo Group (Central Madagascar)

The present paper reports the whole rock and mineral chemistry, textural relations and the mineral reactions under greenschist to amphibolite facies conditions of the Aluminum-Phosphate and -Borosilicate (APB)-bearing Mesoproterozoic quartzite of the Itremo Group (central Madagascar) and discusses the genesis of the protolith as well as the metamorphic P-T-conditions. The Itremo Group consists from bottom to top of a micaschist formation (metapelite and metasiltstone) (approximate to 500 m), a quartzite (approximate to 1000 m), and a stromatolitic dolomitic marble formation (> 1000 m). In the quartzite cross bedding and ripple marks document a deposition in a shallow continental shelf environment. Lazulite, augelite, trolleite, svanbergite, goyazite, crandallite, berlinite, tourmaline, apatite, celestine, anhydrite, muscovite and dumortierite with the minor constituents zircon, monazite and xenotime in addition to quartz are the rock-forming minerals. Boron and Sr have been supplied to the protolith of the quartzite by submarine hydrothermal fluids or by rivers fed by B and Sr-rich thermal springs. Using textures, stability data for augelite, trolleite, berlinite, lazulite, scorzalite, dumortierite, muscovite, kyanite and temperature data from the literature the P-T conditions of Panafrican metamorphism, which produced the prograde APB-parageneses can be approximated as 3.8 kbar and about 475 degrees C. Deposition and metamorphism of the protolith can be summarized as: i.) deposition of clay-bearing sand in a shallow continental shelf environment, ii.) chemical precipitation of phosphates and of phosphorus-rich iron hydroxides by oxidizing marine pore waters, iii.) reductive dissolution of the Fe-oxyhydroxides with crystallization of authigenic Al-phosphates in early diagenesis, iv) metamorphic overprint and crystallization during the Panafrican tectonothermal event, v) hydration reactions, element mobilisation and consequent retrograde resetting of prograde mineral equilibrium by postorogenic fluid circulation at the end of the Panafrican tectonothermal event.

Fish Population and Behavior Revealed by Instantaneous Continental Shelf-Scale Imaging

Until now, continental shelf environments have been monitored with highly localized line-transect methods from slow-moving research vessels. These methods significantly undersample fish populations in time and space, leaving an incomplete and ambiguous record of abundance and behavior. We show that fish populations in continental shelf environments can be instantaneously imaged over thousands of square kilometers and continuously monitored by a remote sensing technique in which the ocean acts as an acoustic waveguide. The technique has revealed the instantaneous horizontal structural characteristics and volatile short-term behavior of very large fish shoals, containing tens of millions of fish and stretching for many kilometers.

Mean and variance of the forward field propagated through three-dimensional random internal waves in a continental-shelf waveguide

The mean and variance of the acoustic field forward propagated through a stratified ocean waveguide containing three-dimensional (3-D) random internal waves is modeled using an analytic normal mode formulation. The formulation accounts for the accumulated effects of multiple forward scattering. These lead to redistribution of both coherent and incoherent modal energies, including attenuation and dispersion. The inhomogeneous medium’s scatter function density is modeled using the Rayleigh-Born approximation to Green’s theorem to account for random fluctuations in both density and compressibility caused by internal waves. The generalized waveguide extinction theorem is applied to determine attenuation due to scattering from internal wave inhomogeneities. Simulations for typical continental-shelf environments show that when internal wave height exceeds the acoustic wavelength, the acoustic field becomes so randomized that the expected total intensity is dominated by the field variance beyond moderate ranges. This leads to an effectively saturated field that decays monotonically and no longer exhibits the periodic range-dependent modal interference structure present in nonrandom waveguides. Three-dimensional scattering effects can become important when the Fresnel width approaches and exceeds the cross-range coherence length of the internal wave field. Density fluctuations caused by internal waves are found to noticably affect acoustic transmission in certain Arctic environments.

Range-dependent 3D scattering and reverberation in the continental shelf environment from biology, geology and oceanography

Several unified scattering and reverberation models were developed in support of the ONR Acoustic Clutter Program. They include a range-dependent model based on the parabolic equation that can be used to efficiently model scattering from a random spatial distribution of random targets that obey the sonar equation in the waveguide and a similar but range-independent waveguide model based on normal modes for scattering from extended objects. Both these models are bistatic and fully 3D, and the latter model also accounts for modal coupling between propagation and scattering caused by extended objects in the waveguide. These models are applied to examine both coherent and diffuse scattering measured after beamforming and match-filtering on an array from schools of fish, plankton, volume inhomogeneities in the sea bottom, roughness on the seafloor, extended seafloor and sub-bottom features such as river channels and reflective strata, and internal and surface waves. We provide a review of the dominant sources of clutter as well as background reverberation for long range active sonar based on comparison of model predictions with measured data from the Acoustic Clutter Experiments of 2001 and 2003.

Long range acoustic imaging of the continental shelf environment: The Acoustic Clutter Reconnaissance Experiment 2001

An active sonar system is used to image wide areas of the continental shelf environment by long-range echo sounding at low frequency. The bistatic system, deployed in the STRATAFORM area south of Long Island in April-May of 2001, imaged a large number of prominent clutter events over ranges spanning tens of kilometers in near real time. Roughly 3000 waveforms were transmitted into the water column. Wide-area acoustic images of the ocean environment were generated in near real time for each transmission. Between roughly 10 to more than 100 discrete and localized scatterers were registered for each image. This amounts to a total of at least 30000 scattering events that could be confused with those from submerged vehicles over the period of the experiment. Bathymetric relief in the STRATAFORM area is extremely benign, with slopes typically less than 0.5 degrees according to high resolution (30 m sampled) bathymetric data. Most of the clutter occurs in regions where the bathymetry is locally level and does not coregister with seafloor features. No statistically significant difference is found in the frequency of occurrence per unit area of repeatable clutter inside versus outside of areas occupied by subsurface river channels.