observations of currents and water properties in the amazon frontal zone measurements of currents and water properties were obtained at various locations in the frontal zone of the amazon river outflow plume during four cruises in 1989 1991 as part of a multidisciplinary amazon shelf sediment study a salinity front resembling a salt wedge is found approximately 150 km seaward of the river mouth it continues northwestward more than 400 km along the shelf between the 10 and 20 m isobaths tidal currents which are oriented principally in the cross shelf direction reach 200 cm s 1 during spring tides and drop to less than 80 cm s 1 during neaps in the vicinity of the river mouth the nontidal flow is strongly sheared with near surface speeds of up to 100 cm s 1 and weak near bottom velocities the direction of the near surface flow is generally northwestward except near the river mouth where river discharge leads to a significant offshore component of flow the strong velocity shears are stabilized by strong stratification by salinity and by highly concentrated suspended sediment or fluid mud near the seabed variations in stratification and frontal position are due principally to spring neap variations in tidal mixing and seasonal variations in runoff

electromagnetic induction in a fully 3 d anisotropic earth the bulk electrical anisotropy of sedimentary formations is a macroscopic phenomenon which can result from the presence of porosity variations laminated shaly sands and water saturation accounting for its effect on induction log responses is an ongoing research problem for the well logging community since these types of sedimentary structures have long been correlated with productive hydrocarbon reservoirs such as the jurassic norphlet sandstone and permian rotliegendes sandstone presented here is a staggered grid finite difference method for simulating electromagnetic em induction in a fully 3 d anisotropic medium the electrical conductivity of the formation is represented as a full 3 3 tensor whose elements can vary arbitrarily with position throughout the formation to demonstrate the validity of this approach finite difference results are compared against analytic and quasi analytic solutions for tractable 1 d and 3 d model geometries as a final example we simulate 2c 40 induction tool responses in a crossbedded aeolian sandstone to illustrate the magnitude of the challenge faced by interpreters when electrical anisotropy is neglected

hydraulic control of a highly stratified estuarine front abstract observations at the mouth of the fraser river british columbia canada indicate an abrupt frontal transition between unstratified river outflow and a highly stratified river plume with differences in salinity greater than 25 psu across a few meters in the vertical direction and several hundred meters in the horizontal direction the front roughly follows a natural break in the bathymetry crossing the channel at an angle of approximately 45 and is essentially stationary for a period of approximately 3 5 h centered on the low tide following the larger of two daily ebbs the location of the front is coincident with observations of significantly supercritical internal froude numbers at the front based on velocities in the along flow direction this observation contradicts the one dimensional theory which indicates that the froude number should be 1 however because the front is oriented obliquely to the outflow a coordinate system can be selected that is normal to the front and for which a c

stratigraphic numerical modelling of a carbonate platform on the romanche transverse ridge equatorial atlantic abstract the romanche transverse ridge equatorial atlantic is located in the northern flank of the fracture zone opposite the eastern ridge transform intersection rti it constitutes a major positive topographic anomaly that reaches a height of over 4 km above the level predicted by the thermal subsidence curve a series of e w aligned peaks located on the crest of the transverse ridge were at sea level during early and middle miocene times they are presently capped by a 300 m thick shallow water carbonate platform that grew on a subsiding oceanic crust basement surface flattened by erosion at sea level these structures are now about 1 km below sea level high resolution seismic reflection surveys and multibeam morphobathymetry as well as study of samples recovered from the carbonate platform allowed a reconstruction of the paleoenvironment and of the vertical movements of the peaks starting from the lower miocene ages derived from microfossils suggest that the base of the carbonate platform dates from 17 25 m y ago and the sinking of the platform started between 18 and 13 m y ago these data were included in a numerical simulation model that takes into account thermal and tectonic subsidence growth potential of the carbonates subaerial and submarine erosion rates and mio pliocene absolute sea level fluctuations the results outline the subsidence history of the romanche transverse ridge and suggest post miocene subsidence faster than that predicted by the thermal cooling model

seismic wave modeling for seismic imaging many scientific applications require accurate modeling of seismic wave propagation in complex media these objectives can include fundamental understanding of seismic wave propagation of the earth on a global scale including fluid envelopes mitigation of seismic risk with better quantitative estimates of seismic hazard and improved exploitation of the natural resources in the crust of the earth accurate quantification is a continual quest and benchmark protocols have been designed for model definitions and comparison of solutions through this quest an impressive number of numerical tools have been developed ranging from efficient finite difference methods to more sophisticated finite element methods and including the so called pseudospectral methods see wu and maupin for a review the main motivation behind these permanent developments has been to improve the efficiency and accuracy of forward modeling to achieve this one systematic choice for both finite difference and finite element methods has involved explicit time stepping integration to avoid matrix inversion

three dimensional magnetotelluric responses for arbitrary electrically anisotropic media and a practical application electrical anisotropy in earth media increases the complexity of magnetotelluric responses magnetotelluric models based on anisotropic media must be developed to fully understand observed data this paper presents a three dimensional algorithm for calculating magnetotelluric responses of arbitrary anisotropic media in the frequency domain using a staggered grid finite difference method the model space is discretized into rectangular blocks with electric fields on the edges of each block and the magnetic fields normal to the faces of each block the electric field helmholtz vector equation that considers a full 3 3 conductivity tensor is calculated numerically under two orthogonal polarizations in calculating the boundary values on the four sides of the three dimensional anisotropic model we adopt different procedures for calculating the two dimensional responses on the sides in the x and y directions the responses for a layered anisotropic model and a three dimensional isotropic model calculated with this algorithm are compared with the corresponding analytical and numerical solutions respectively the comparisons show that the algorithm s approximations are highly precise for a wide frequency band a typical two dimensional anisotropic model and a general three dimensional anisotropic model were also constructed and their responses were calculated these anisotropic models have ordinary structures but can produce phase rolling out of quadrant magnetotelluric responses which indicates that considering electrical anisotropy may improve our interpretation of observed data using this algorithm we can model the observed data from the northern qaidam basin in northern tibet where ultrahighpressure metamorphic rocks are exposed along an old suture and seismic anisotropy was indicated in neighbouring areas the phase tensors of the magnetotelluric sites at this location show large skew angles and the corresponding phase splits are distinct in the off diagonal impedance elements although the isotropic three dimensional electrical structure can model the profile data well the structure shows a sequence of conductive and resistive bodies in the mid lower crust of the north qaidam basin which is very spatially inhomogeneous and a simple intrinsic anisotropic body can also produce similar surficial responses using the three dimensional anisotropic algorithm we found an equivalent anisotropic replacement for this area the results of the three dimensional anisotropy modelling of the magnetotelluric data from the northern tibetan plateau show the valuable applicability of the threedimensional anisotropic algorithm in testing the qualitative presumption of electrical anisotropy

efficient 3 d frequency domain mono parameter full waveform inversion of ocean bottom cable data application to valhall in the visco acoustic vertical transverse isotropic approximation computationally efficient 3 d frequency domain full waveform inversion fwi is applied to ocean bottom cable data from the valhall oil field in the visco acoustic vertical transverse isotropic vti approximation frequency domain seismic modelling is performed with a parallel sparse direct solver on a limited number of computer nodes a multiscale imaging is performed by successive inversions of single frequencies in the 3 5 10 hz frequency band the vertical wave speed is updated during fwi while density quality factor qp and anisotropic thomsen s parameters and are kept fixed to their initial values the final fwi model shows the resolution improvement that was achieved compared to the initial model that was built by reflection traveltime tomography this fwi model shows a glacial channel system at 175 m depth the footprint of drifting icebergs on the palaeo seafloor at 500 m depth a detailed view of a gas cloud at 1 km depth and the base cretaceous reflector at 3 5 km depth the relevance of the fwi model is assessed by frequency domain and time domain seismic modelling and source wavelet estimation the agreement between the modelled and recorded data in the frequency domain is excellent up to 10 hz although amplitudes of modelled wavefields propagating across the gas cloud are overestimated this might highlight the footprint of attenuation whose absorption effects are underestimated by the homogeneous background qp model qp 200 the match between recorded and modelled time domain seismograms suggests that the inversion was not significantly hampered by cycle skipping however late arrivals in the synthetic seismograms computed without attenuation and with a source wavelet estimated from short offset early arrivals arrive 40 ms earlier than the recorded seismograms this might result from dispersion effects related to attenuation the repeatability of the source wavelets inferred from data that are weighted by a linear gain with offset is dramatically improved when they are estimated in the fwi model rather than in the smooth initial model the two source wavelets estimated in the fwi model from data with and without offset gain show a 40 ms time shift which is consistent with the previous analysis of the time domain seismograms the computational efficiency of our frequency domain approach is assessed against a recent time domain fwi case study performed in a similar geological environment this analysis highlights the efficiency of the frequency domain approach to process a large number of sources and receivers with limited computational resources thanks to the efficiency of the substitution step performed by the direct solver this efficiency can be further improved by using a block low rank version of the multifrontal solver and by exploiting the sparsity of the source vectors during the substitution step future work will aim to update attenuation and density at the same time of the vertical wave speed

historical changes in terrestrially derived organic carbon inputs to louisiana continental margin sediments over the past 150 years 1 major rivers and associated deltaic environments provide the dominant pathway for the input of terrestrial derived organic carbon in sediments toct to the ocean natural watershed processes and land use changes are important in dictating the amount and character of carbon being buried on continental margins seven core sites were occupied on the louisiana continental margin aboard the r v pelican in july 2003 along two major sediment transport pathways south and west of the mississippi river mouth lignin profiles in these age dated cores 210pb geochronology indicate artificial reservoir retention as a primary control on organic carbon quantity and quality reaching the margin post 1950 whereas pre 1950 sediments may reflect soil erosion due to land clearing and farming practices lignin 8 concentrations range 0 2 to 1 7 also indicate that toct delivery rates decay processes have probably remained relatively consistent from proximal to distal stations along transects the down core profile at the canyon station seems to be temporally linked and connected to inner shelf deposition suggestive of rapid cross shelf transport sources of terrestrially derived organic carbon were reflective of mixed angiosperms over the last 150 years in cores west and south of the mississippi river delta the lignin phenol vegetation index lpvi range 130 0 to 510 proved to be a sensitive indicator of source changes in these sediments and eliminated some of the variability compared to c v range 0 01 to 0 4 and s v range 0 9 to 2 1 ratios stochastic events such as hurricanes and large river floods have a measurable albeit ephemeral effect on the shelf toct record burial of toct on the river dominated louisiana continental margin is largely driven by anthropogenic land use alterations in the last 150 years land use changes in the mississippi river basin and river damming have likely affected carbon cycling and toct burial on the louisiana continental margin over a large spatial extent as observed by similar trends in cores from across and along the margin

world wide delivery of river sediment to the oceans new data and new estimates from old data show that rivers with large sediment loads annual discharges greater than about 15 times 10 6 tons contribute about 7 times 10 9 tons of suspended sediment to the ocean yearly extrapolating available data for all drainage basins the total suspended sediment delivered by all rivers to the oceans is about 13 5 times 10 9 tons annually bedload and flood discharges may account for an additional 1 2 times 10 9 tons about 70 of this total is derived from southern asia and the larger islands in the pacific and indian oceans where sediment yields are much greater than for other drainage basins

the ramesses experiment iii controlled source electromagnetic sounding of the reykjanes ridge at 57 45 n summary a controlled source electromagnetic sounding survey centred on an axial volcanic ridge avr segment of the reykjanes ridge at 57 45aen was performed as part of the ramesses experiment low frequency 0 35 11 hz electromagnetic signals were transmitted through the crust to an array of horizontal electric field recorders at the seafloor to ranges of 15 km from the source which was a 100 m long horizontal electric dipole towed at heights of 50 80 m from the seafloor coincident seismic and magnetotelluric studies were conducted during the rest of the ramesses experiment data were interpreted using a combination of 1 d forward modelling and inversion and iterative forward modelling in two dimensions on the axis of the avr the resistivity at the seafloor is 1 v m there is a steep resistivity gradient in the upper few hundred metres of the crust with the resistivity reaching approximately 10 v ma t a depth of 500 m in order to explain the low resistivities the upper layer of the crust must be heavily fractured and saturated with sea water the resistivity increases with distance from the axis as the porosity decreases with increasing crustal age the most intriguing feature in the data is the large diverence in amplitude between fields transmitted along and across the avr axis a significant zone of low resistivity material is required at approximately 2 km depth beneath the ridge crest in order to explain this diverence it is coincident with the low velocity zone required by the seismic data and has a total electrical conductance in excellent agreement with the results of the magnetotelluric study the low resistivity zone can be explained by the presence of a body of partially molten basalt in the crust taken together these results provide the first clear evidence for a crustal magma chamber at a slow spreading mid ocean ridge the data constrain the melt fraction within the body to be at least 20 per cent with a melt volume suycient to feed crustal accretion at this segment of the ridge for of the order of 20 000 years since this body would freeze in the order of 1500 years this finding lends support to the hypothesis that at slow spreading rates crustal accretion is a cyclic process accompanying periodic influxes of melt from the mantle to a crustal melt reservoir

effects of surface scattering in full waveform inversion in full waveform inversion of seismic body waves often the free surface is ignored on grounds of computational efficiency asyntheticstudywasperformedtoinvestigatetheeffectsofthissimplification intermsofsizeandfrequency the testmodelanddataconformtoareallong offsetsurveyofthe upper crust across the san andreas fault random fractal variations are superimposed on a background model with strong lateral and vertical velocity variations ranging from 1200to6800m s syntheticdatawerecomputedandinverted forthismodelanddifferenttopographies afullyviscoelastic time domain code was used to synthesize the seismograms and a viscoacoustic frequency domain code was utilized to invert them the inversion was focused on early arrivals which are dominated by p waves but also contain strong p rayleigh wave conversions from the near field of the receiver resulting waveform models show artifacts and a loss of resolution from neglecting the free surface in the inversion but the inversions are stable and they still improve the resolution of kinematic models the extent of deterioration dependsmoreonthesubsurfacethanonthesurfacestructure inversion results were improved at no additional expense by introducing a weak contrast along a staircase function above shotsandreceivers

controlled source electromagnetic sounding of the oceanic lithosphere the attenuation of ionospheric signals in the frequency range 0 06 24 hz by sea water effectively precludes using the magnetotel luric method to study the electrical structure of the upper oceanic lithosphere we have carried out a dipole dipole electromagnetic sounding in the north pacific by injecting electromagnetic signals into the ocean and sea bed the crust at the site is 25 myr old and has a thin sediment cover the technique similar to that used in earlier work1 2 involves dragging a horizontal dipole antenna along the sea floor the electric fields that propagated through the resistive basement were detected by seafloor receivers at ranges of 10 65 km as the ambient electric field is very small varying from 10 18 v2 m 2 hz 1 at 0 1 hz to 10 24 v2 m2 hz 1 above 1 hz ref 3 the controlled source signals could be easily monitored our data are consistent with a simple one dimensional earth model consisting of a 3 7 km thick crustal layer of moderate conductivity 0 001 s m 1 underlain by a thicker region of very low conductivity 2 10 5 s m 1 the results suggest an upper mantle water content of at most 0 1 by volume

buoyant surface discharges into water bodies ii jet integral model the near field region of a buoyant surface discharge into water bodies often displays significant jet like motions in form of free jets shoreline attached jets and wall jets respectively as classified by the cormix3 expert system see jones et al 2007 paper i a new jet integral model corsurf has been developed that addresses in a single formulation this entire spectrum of jet motions in both deep or shallow environments the model employs an entrainment closure approach for the separate contributions of entrainment resulting from transverse shear buoyant damping advected puff motions frontal mixing and interfacial mixing due to lateral spreading it also contains a quadratic law turbulent drag force mechanism an alternative model formulation applies to the two dimensional bottom attached form of the jet this formulation contains a deflecting pressure force mechanism as well as the bottom shear force specific criteria describe bottom attachment and detachment processes finally a number of confinement effects on the jet dynamics due to shallow water and or lateral boundaries are included the model has been validated under a wide range of geometric and dynamic conditions using in particular hitherto unavailable high resolution laboratory data

transient electromagnetic responses in seafloor with triaxial anisotropy summary electrical anisotropy of young oceanic crust at mid ocean ridges is detectable by observation of the rate and geometry of the diffusion of electromagnetic fields the anisotropy in electrical properties arises from the presence of conductive seawater in an interconnected network of mostly ridge parallel cracks in this paper we first justify the choice of a triaxial model to represent young oceanic crust with three distinct electrical conductivities in the vertical strike and spreading directions we then present an algorithm to calculate the transient electromagnetic responses generated by an electric dipole source over such a triaxially anisotropic seafloor we show that if the transient passages are measured with three distinct electric dipole dipole configurations it is possible to discern all three unknown conductivities independently of each other

cylindrical fdtd analysis of lwd tools through anisotropic dipping layered earth media electrical logging while drilling lwd tools are commonly used in oil and gas exploration to estimate the conductivity resistivity of adjacent earth media in general earth media exhibit anisotropic conductivities this implies that when lwd tools are used for deviated and horizontal drilling the resulting borehole problem may include dipping layered media with dipping beds having full 3 times 3 conductivity tensors to model this problem we describe a 3 d cylindrical finite difference time domain fdtd algorithm extended to fully anisotropic conductive media and implemented with cylindrical perfectly matched layers to mimic open domain problems the 3 d fdtd algorithm is validated against analytical results in simple formations showing good agreement and used to simulate the response of lwd tools through anisotropic dipping beds for various values of anisotropic conductivities and dipping angles

adaptive finite element modelling of two dimensional magnetotelluric fields in general anisotropic media summary an adaptive unstructured mesh finite element fe procedure is presented for improving the quality of numerical solutions to the magnetotelluric forward problem in a general 2 d anisotropic conductivity structure we implement a self adaptive goal oriented grid refinement algorithm in which a finite element analysis is performed on a sequence of refined meshes the mesh refinement process is guided by a dual error estimate weighting to bias refinement towards elements that affect the solution at the em receiver locations we validate the finite element code against a layered 1 d model with a sea water layer further we compare the fe results with those obtained by a finite difference fd scheme for both a block seamountain and a sea bottom hill model both fe and fd schemes show very good agreement for the block seamountain model for the sea bottom hill model however only on the flat seafloor segments both the fe and fd solutions fit very well but on the seafloor slope fd results are oscillating due to a simplistic staircase approximation of the bathymetric undulations the fd scheme for 2 d anisotropic conductors developed primarily for the modelling of magnetotelluric data on a flat earth surface is thus not an adequate tool for dealing with structures with sloping bathymetry and topography whereas the fe method with adapting mesh can easily handle such structures at almost any level of complexity

marine controlled source electromagnetic sounding 2 the pegasus experiment the marine controlled source electromagnetic sounding method developed over the past 15 years at scripps institution of oceanography employs a towed seafloor electric dipole transmitter of moment 4 104 am and multiple free vehicle seafloor electric field recorders a survey of 40 ma normal oceanic lithosphere in the northeast pacific using frequencies of 0 25 to 24 hz and synchronous stacking of 0 25 to 12 hour duration detected signals at transmitter receiver ranges between 5 and 95 km one dimensional electrical conductivity structure is recovered from the data using the occam process of nonlinear regularized inversion repeated inversion of a model terminated with an essentially infinite conductor or resistor demonstrates that the maximum depth of inference for this experiment is about 30 km well into the upper mantle with bounds placed on conductivity to depths of 60 km structure shallower than about 1 km is comparable to that obtained by a similar experiment on the east pacific rise and by borehole logging with a sharp increase in resistivity at depths of 600 800 m although strictly our experiment is sensitive only to integrated square root of conductivity or total attenuation in the surface layers the lower crust and upper mantle has a resistivity between 2 and 7 104 m and a transverse resistance of at least 109 m2 suggesting at most 0 3 volume fraction of free water in the lower crust and some form of conductivity enhancement over mineral conductivity in the uppermost mantle although resolution is weak below 30 km our data are compatible with a dry olivine model of mantle conductivity temperature

toward a robust workflow for deep crustal imaging by fwi of obs data the eastern nankai trough revisited crustal scale imaging by the full waveform inversion fwi of long offset seismic data is inherently difficult because the large number of wavelengths propagating through the crust makes the inversion prone to cycle skipping therefore efficient crustal scale fwi requires an accurate starting model and a stable workflow minimizing the nonlinearity of the inversion here we attempt to reprocess a challenging 2d ocean bottom seismometer obs dataset from the eastern nankai trough the starting model is built by first arrival traveltime tomography fat which is fwi assisted for tracking cycle skipping we iteratively refine the picked traveltimes and then reiterate the fat until the traveltime residuals remain below the cycle skipping limit subsequently we apply laplace fourier fwi in which progressive relaxation of time damping is nested within frequency continuation to hierarchically inject more data into the inversion these two multiscale levels are complemented by a layer stripping approach implemented through offset continuation the reliability of the fwi velocity model is assessed by means of source wavelet estimation synthetic seismogram modeling ray tracing modeling dynamic warping and checkerboard tests although the visco acoustic approximation is used for wave modeling the synthetic seismograms reproduce most of the complexity of the data with a high traveltime accuracy the revised fwi scheme produces a high resolution velocity model of the entire crust that can be jointly interpreted with migrated images derived from multichannel seismic data this study opens a new perspective on the design of obs crustal scale experiments amenable to fwi however a further assessment of the optimal obs spacing is required for reliable fwi

the detection of lateral anisotropy of the ocean floor by electromagnetic methods summary in a typical marine controlled source electromagnetic csem experiment one or more self contained receivers capable of measuring small variations in the electromagnetic field are located in an array on the sea floor a remote horizontal electric dipole is towed close to the sea floor by a surface vessel variations in the current through the dipole cause correlated variations in the electric and magnetic fields at the receivers the variations contain information about the distribution of electrical conductivity of the subsurface rocks the electrical conductivity is related to such critical physical parameters as porosity temperature and fluid content published studies of the response of the sea floor to electromagnetic excitation have not included any systematic variation in the sea floor conductivity with horizontal direction a horizontally isotropic oceanic lithosphere is not an appropriate assumption particularly in the vicinity of the ridge here for example in the near surface layers basalts are extruded onto the sea floor within the very narrow fissured neo volcanic zone the fissures may be filled with sea water and sediment which are better conductors than typical basalt alternatively as time progresses they may become sealed with hydrothermal mineral deposits which can be less conductive than the host rocks in both instances the electrical conductivity of the composite material in the direction oriented along the fisures parallel to the ridge will be greater than that in the transverse direction measures of anisotropy on sea floor of varying age are an indication of the history of the tectonic activity at the ridge the electromagnetic effects of lateral anisotropy are examined through the study of the response of a simple double half space model an upper half space representing sea water is connected to a lower half space representing anisotropic crustal material a general theory describing the excitation of this model by any transient electromagnetic source is developed and the step on responses for systems composed of a horizontal electric dipole transmitter and horizontal magnetic and electric field receivers are computed the results of previous work indicate that following a change in current in the transmitter two distinct changes in horizontal electric field strength separated in time are observed at the receiver the first is caused by the diffusion of the electromagnetic field through the relatively resistive sea floor the second is caused by diffusion through the relatively conductive sea water the times at which the two events occur are robust measures of the isotropic conductivity of the sea floor and the sea water respectively the results of this study show that shape and time of the initial change in the electric field varies systematically with both the average conductivity and the anisotropy of the sea floor for some orientations of the transmitter and the receiver the effect of increasing the anisotropy is similar to the effect of increasing the average conductivity the initial event is delayed however there are orientations for which increasing the anisotropy actually results in an earlier arrival natural science and engineering research council in canada

geomorphic tectonic control of sediment discharge to the ocean the importance of small mountainous rivers analysis of data from 280 rivers discharging to the ocean indicates that sediment loads yields are a log linear function of basin area and maximum elevation of the river basin other factors controlling sediment discharge e g climate runoff appear to have secondary importance a notable exception is the influence of human activity climate and geology on the rivers draining southern asia and oceania sediment fluxes from small mountainous rivers many of which discharge directly onto active margins e g western south and north america and most high standing oceanic islands have been greatly underestimated in previous global sediment budgets perhaps by as much as a factor of three in contrast sediment fluxes to the ocean from large rivers nearly all of which discharge onto passive margins or marginal seas have been overestimated as some of the sediment load is subaerially sequestered in subsiding deltas before the proliferation of dam construction in the latter half of this century rivers

a stochastic formulation for erosion of cohesive sediments the linear formulation for erosion e m b c often applied in engineering applications has two properties which do not always comply with field and laboratory observations they are as follows 1 the erosion rate is zero below the critical bed shear stress c and increases linearly with bed shear stress b when exceeding the critical bed shear stress incipient motion b simeq c is poorly represented 2 the erosion rate is constant in time for constant values of m and c whereas observations often suggest time dependency in this paper we analyze the process of incipient motion and time dependency by using a stochastic forcing bed shear stress and a stochastic bed strength critical bed shear stress it is well known that the bed shear stress is not constant but varies due to turbulence this stochastic nature of the turbulent motion is accounted for by a probability density distribution for the bed shear stress which is based on the formulation of hofland and battjes 2006 this distribution is implemented in the linear erosion formulation an analytical solution for the erosion rate is obtained which only depends on the mean bed shear stress a parametrization is made for efficient application in numerical models the sediment in the bed is considered to be nonuniform therefore it is subdivided into several classes distinguished by the critical bed shear stress and not necessarily by the grain size the variability of the critical bed shear stress is treated in a discritized way sediment balance equations are solved for each class considering different classes the total erosion rate becomes time dependent as the erosion depends on the availability of sediment the model is applied to two annular flume data sets jacobs 2009 and amos et al 1992a the results show that with a proper choice of the required parameters the time dependence of the erosion rate and the concentration can be reproduced we conclude that the occurrence of incipient motion can be explained from a stochastic forcing time varying erosion rates can be explained from a stochastic bed strength distribution or from a vertical gradient in bed strength the latter is however not likely and not measurable in the top layers of dense consolidated cohesive sediment beds

controlled electromagnetic sources for measuring electrical conductivity beneath the oceans 1 forward problem and model study exact closed form expressions for the electromagnetic induction fields produced by vertical and horizontal current sources in the conducting ocean overlying a one dimensional earth are derived from the maxwell equations numerical methods for the evaluation of the solutions are given including correction for the finite size of real sources simple models of the electrical conductivity structure of the ocean crust and lithosphere are deduced from geologic petrologic and laboratory data and their electromagnetic response is modeled horizontal electric dipole sources produce much larger field amplitudes than their vertical counterparts for a given frequency and range and the horizontal electric field offers superior received signal performance reflections of electromagnetic waves from the sea surface and thermocline must be considered for low enough frequencies or long ranges estimates of the ambient noise level from natural electromagnetic sources in the frequency range 0 01 10 hz are presented the ability of controlled sources to determine features of the conductivity of the ocean crust and upper mantle especially low conductivity zones is demonstrated if the mantle conductivity is low enough horizontal ranges of 50 km and conductivity estimates to over 20 km depth can be achieved

three dimensional modelling of controlled source electromagnetic surveys using an edge finite element method with a direct solver a fully three dimensional finite element algorithm has been developed for simulating controlled source electromagnetic surveys to exploit the advantages of geometric flexibility frequency domain maxwell s equations of the secondary electric field were discretised using edge based finite elements while the primary field was calculated analytically for a horizontally layered earth model the resulting system of equations for the secondary field was solved using a parallel version of direct solvers the accuracy of the algorithm was successfully verified by comparisons with integral equations and iterative solutions and the applicability to models containing large conductivity contrasts was verified against published data the advantages of geometry conforming meshes have been demonstrated by comparing different mesh systems to simulate an inclined sheet model a comparison of the performance between direct and iterative solvers demonstrated the superior efficiency of direct solvers particularly for multisource problems

numerical modeling of eccentered lwd borehole sensors in dipping and fully anisotropic earth formations logging while drilling lwd borehole sensors are used to provide real time resistivity data of adjacent earth formations for hydrocarbon exploration this allows for a proactive adjustment of the dipping angle and azimuth direction of the drill and hence geosteering capabilities the analysis of borehole eccentricity effects on lwd sensor response in full 3 3 anisotropic earth formations is important for correct data interpretation in deviated or horizontal wells in this paper we present a cylindrical grid finite difference time domain model to tackle this problem the grid is aligned to the sensor axis to avoid staircasing error in the sensor geometry but in general misaligned to the eccentered borehole formation interface a locally conformal discretization is used to compute effective conductivity tensors of partially filled grid cells at those interfaces involving an isotropic medium borehole and a full 3 3 anisotropic medium in general dipped earth formation the numerical model is used to compute the response of eccentered lwd sensors in layered earth formations with anisotropic dipping beds

electric dipole fields over an anisotropic seafloor theory and application to the structure of 40 ma pacific ocean lithosphere summary seismic anisotropy has been detected in the oceanic crust and upper mantle and likewise it is geologically reasonable to expect that a certain amount of lateral anisotropy exists in sea oor electrical properties anisotropy in earth properties can often lead to surprising eiects on geophysical responses that are not anticipated from simple isotropic theories here we investigate the eiects of lateral anisotropy on the frequency domain controlled source electromagnetic csem response of a uniaxially conducting non magnetic sea oor excited by a horizontal electric dipole whose moment is oriented obliquely with respect to the electrical strike direction a paradox of anisotropy is observed in which the sea oor electric eld strength is enhanced in the most conductive direction of the sea oor this enhancement is opposite to what one would expect based on naive isotropic theory we also show that it is possible in certain circumstances to extract the along strike electrical conductivity from marine controlled source electromagnetic data using only isotropic modelling the extraction of across strike conductivity however requires full anisotropic modelling the physical insight into electromagnetic induction in uniaxial media that is presented here should greatly assist the geological interpretation of marine csem experimental data applying our algorithm to the pegasus data set csem data collected over 40 ma paci c ocean lithosphere produces a model with conductivity in the fossil spreading direction that is seven times greater than the conductivity perpendicular to spreading strain aligned mineralogical fabric as predicted by tectonic modelling would explain our result with enhanced conductivities caused by hydrogen conduction along the olivine a axis or connected accumulations of trace conductors such as graphite or magnetite

facies and evolution of the modern brazos delta texas wave versus flood influence abstract in order to better define the facies architecture and processes controlling the evolution of wave dominated deltas a detailed sedimentary and geomorphologic study was undertaken on the brazos delta texas the delta has formed since 1929 when the brazos river was diverted by the u s army corps of engineers the brazos delta is composed primarily of fine grained sediments prodelta clay constitutes more than half of the sediment volume thick sands are restricted to the narrow delta front environment whereas back bar lagoonal clays are a significant component of the delta plain sequence the facies architecture is not representative of the classic strandplain model for wave dominated deltas this is due to the strong influence of floods on deltaic evolution in early 1992 statewide flooding gave rise to a major constructional phase of the delta significant quantities of fine grained sediments were deposited in the prodelta one year after the onset of flooding a channel mouth bar emerged offshore of the river mouth and enabled progradation of the delta similar flood events that occurred during 1941 1957 and 1965 were recorded as ridge trough pairs in the delta headland the brazos delta is therefore fluvially influenced during floods and wave influenced during intervening periods

sediment flux and the anthropocene data and computer simulations are reviewed to help better define the timing and magnitude of human influence on sediment flux the anthropocene epoch impacts on the earth surface processes are not spatially or temporally homogeneous human influences on this sediment flux have a secondary effect on floodplain and delta plain functions and sediment dispersal into the coastal ocean human impact on sediment production began 3000 years ago but accelerated more widely 1000 years ago by the sixteenth century societies were already engineering their environment early twentieth century mechanization has led to global signals of increased sediment flux in most large rivers by the 1950s this sediment disturbance signal reversed for many rivers owing to the proliferation of dams and sediment load reduction below pristine conditions is the dominant signal today a delta subsidence signal began in the 1930s and is now a dominant signal in terms of sea level for many coastal environments overwhelming even the global warming imprint on sea level humans have engineered how most water and sediment are discharged into the coastal ocean hyperpycnal flow events have become more common for some rivers and less common for other rivers bottom trawling is now widespread suggesting that even continental shelves have received a significant but as yet quantified anthropocene impact the anthropocene attains the level of a geological climate event such as that seen in the transition between the pleistocene and the holocene

teleseismic imaging of subaxial flow at mid ocean ridges traveltime effects of anisotropic mineral texture in the mantle summary deformation of peridotite caused by mantle flow beneath an oceanic spreading centre can result in the development of seismic anisotropy traveltime anomalies and shearwave splitting will develop as seismic energy propagates through such an anisotropic region thus providing a signature of the deformation field at depth in this study we investigate the nature of deformation associated with mantle upwelling for two models of flow in the upper 100 km of the mantle the finite strain fields of the passive upwelling model versus the buoyancy enhanced upwelling model are quite different this suggests that mineral aggregates deform differently in the two models thus developing seismic signatures that are distinguishable numerical estimates of the corresponding mineral textures are made using polycrystal theory for olivine with four operative slip systems the activation of a slip system is determined for each grain on the basis of the local critical resolved shear stress the computed grain deformation reflects a balance between stress equilibrium for the aggregate as a whole and strain continuity between neighbouring grains within the aggregate this approach enables a direct link to be made between the model flow fields and the resulting texture development given these mineral orientation distributions elastic parameters are calculated and wavefronts are propagated through the anisotropic structure traveltimes for teleseismic body waves are computed using ray theory and amplitudes are estimated for an across axis profile extending 100 km from the ridge axis relative p wave residuals of up to 1 s are predicted for the buoyant model with on axis arrivals being earliest since near vertical velocities are fastest beneath the axis on axis p wave arrivals for the passive model are half a second earlier than arrivals 60 km off axis and relative delays continue to increase slowly as distance from the ridge increases s wave splitting of almost a second is predicted for the buoyant model whereas less than a half second of splitting is determined for the passive model

3d modeling of new generation 1999 2010 resistivity logging tools electromagnetic em logging is an im portant method of formation evaluation because of its sensitivity to resistivity which is a function of fluid saturation and fluid properties conventional induction and propagation resistivity logging tools excite the geological formation by axial magnetic dipole transmitter s axial re ceivers measure the formation response reflecting the medium resistivity such axial measurement has no azimuthal sensitivity and is insensitive to the anisotropy and many other details of the formation

focused source electromagnetic survey versus standard csem 3d modeling in complex geometries a novel focused source electromagnetic fsem method focusestheemfieldintheverticaldirectiontoprovidedeepreadingresistivitydata fsemoffersbetterspatialresolution and greater depth of investigation than the conventional controlled source electromagnetic csem method for land and marine em surveys we have proven the high efficiency of fsem by analyzing 3d models of various complex geologic formations in the presence of seafloor bathymetry shallow resistive gas hydrate overburdens and secondary gas reservoirs formed above deeper oil reservoirs combining the powerofourfocusingtechniquewiththepowerofour3dnumerical modeling method we have developed exceptionally challenging test cases to conclude that fsem automatically cancels unwanted shallow effects and allows simple visual interpretationofdeepreservoirresponses inaddition fsem is insensitive to imperfections in the setup geometry we achievetheseadvantagesusingapropercombinationofmeasurementsacquiredinthereceiverexcitedbytransmitterssituated at different space points the method is promising in anisotropicformationsaswell

event driven deltaic sedimentation on a low gradient low energy shelf the brazos river subaqueous delta northwestern gulf of mexico abstract the brazos river delta is located in the northwestern gulf of mexico sedimentation on the brazos river subaqueous delta has been largely understudied swath bathymetry data collected across the subaqueous delta reveals a noticeable asymmetry with much deeper water depths and greater cross shelf slopes along the eastern up drift side of the delta than along the western down drift side end member sampling reveals that brazos river derived sediment is red red brown or brown in color exhibits increased bulk density and is enriched in calcium a function of carbonate rich areas of the watershed whereas the gulf of mexico inner shelf muds are gray to black and have a relatively lower calcium content analyses of a series of 33 submersible vibra cores collected across the subaqueous delta reveal alternating beds of brazos river derived sediment and gulf of mexico shelf dominated sediment excess 210pb geochronologies of select cores reveal that sediment accumulation has been a combination of fluvial flood event deposition and deposition of remobilized sediment from the proximal shelf and older portions of the delta rather than steady state accumulation because the brazos river is susceptible to local climatic cycles fluvial flood events that create the event layers are intermittent between prolonged periods where marine processes drive accumulation results also show that the primary depocenter of the subaqueous delta has migrated southwestward over time and the erosion of earlier depocenters and proximal shelf sediment has contributed to accumulation on the delta over the past 50 years the subaqueous delta has reached an equilibrium where flood event beds can be completely remobilized within a year and net accumulation approximates relative sea level rise this study highlights relatively rapid clinoform migration due to natural and anthropogenic changes to the system and demonstrates subaqueous delta growth through a combination of direct fluvial and marine derived sediment input

the influence of a salt wedge intrusion on fluvial suspended sediment and the implications for sediment transport to the adjacent coastal ocean a study of the lower brazos river tx usa abstract the brazos river is second only to the mississippi river in terms of sediment load discharged to the gulf of mexico gom this study focused on the interaction between a salt wedge and suspended sediment in the lower brazos river and the implications for sediment export to the coastal ocean salinity and turbidity optical backscatter sensor data with depth swath bathymetry and chirp sub bottom profiling data were collected from 2007 to 2012 over various river discharge stages sampling locations included the lower most 15 km river reach and the coastal ocean proximal to the mouth results indicated that during low to moderate river discharges a salt wedge was present in the lower river but absent during elevated fluvial discharge quantitative analysis showed the salt wedge within the river modulated sediment export to the gom by trapping sediment along the lower most 9 km reach within this reach the trapped sediment formed a decimeter thick mud layer observed in the chirp and bathymetric data at elevated fluvial discharges the salt wedge was no longer observed in the river and the lower river reach transitioned from depositional to erosional based on the observations and analysis from this study a fluvial discharge threshold where the salt wedge is not present in the river and sediment export to the coastal ocean was maximized was estimated this threshold has been met or exceeded only 10 of time since the 1960s these conditions imply that sediment transport to the coastal ocean is highly variable and occurs during irregular pulsed events this study highlights how estuarine conditions at the river mouth can impact terrestrial material flux to the coastal ocean

triangulated finite difference methods for global scale electromagnetic induction simulations of whole mantle electrical heterogeneity long period global scale electromagnetic induction data have long been recognized as containing potentially useful information for constraining our understanding of the dynamics and physiochemical state of earth s deep interior a key component to realizing the potential of this data set is the ability to compute the induction response of a fully 3 d earth mantle in global spherical geometries in this contribution a novel finite difference method is described which preserves many algorithmic advantages of staggered grid discretizations in cartesian computational electromagnetics but is generalized for the sphere by introducing a hybrid tensor product mesh consisting of concentrically nested triangulated spherical shells topologically connected node for node in the radial direction such a discretization allows for a flexible distribution of mesh nodes in the lateral sense and avoids the problems associated with excessive node density near the poles which results from conformally mapping the standard cartesian mesh modified finite difference templates for spherical geometries are derived and presented as is a comparison with integral equation solutions for a simple double hemisphere test model as a first step toward applying the algorithm for mantle induction studies results of a conservative thermally activated mantle heterogeneity model confirm that mantle structure remains observable in the presence of the strongly conductive ocean and may manifest important diagnostic signatures in the orientation of the horizontal component of induced magnetic field

estimating river sediment discharge to the ocean application to the eel margin northern california abstract hydrotrend a synthetic river discharge numerical model can simulate the natural variability daily annually and longer in the flux of water and sediment delivered to the coastal ocean the numerical model is designed to make discharge predictions based on climate even though field measurements of river flow are not available provided that appropriate assumptions are made regarding past climate the model can predict how a river may have behaved in the geological past sediment load is determined from a river s rating coefficients when applied to the flood dominated eel river northern california hydrotrend captures the intermonthly and interannual variability of discharge and loads over a century or more hydrotrend is applied to the ice age paleo eel river when sea level was lower and climate much cooler and wetter the paleo eel river at 18 000 year bp would see its hinterland runoff increase but not the magnitude of the peak discharges the post dicted sediment load of the paleo eel river was 25 greater than today s although basin precipitation was 45 greater and the hinterland area 4 greater through sea level lowering a method is provided to obtain sediment discharge rating coefficients for unmonitored modern rivers and paleo rivers the relationship q s g 1 2 h 3 2 a 1 2 provides for an estimate of a river s sediment load q s to within a factor of 3 over 7 orders of magnitude from knowledge of basin relief h and basin area a rating coefficients are determined from this estimate of q s and the power law rating curve q s aq b 1 where q is discharge

marine magnetotellurics for petroleum exploration part i a sea floor equipment system induction in electrically conductive seawater attenuates the magnetotelluric mt fields and coupled with a minimum around 1 hz in the natural magnetic field spectrum leads to a dramatic loss of electric and magnetic field power on the sea floor at periods shorter than 1000 s for this reason the marine mt method traditionally has been used only at periods of 103 to 105 s to probe deep mantle structure rarely does a sea floor mt response extend to a 100 s period to be useful for mapping continental shelf structure at depths relevant to petroleum exploration however mt measurements need to be made at periods between 1 and 1000 s this can be accomplished using ac coupled sensors induction coils for the magnetic field and an electric field amplifier developed for marine controlled source applications the electrically quiet sea floor allows the attenuated electric field to be amplified greatly before recording in deep 1 km water motional noise in magnetic field sensors appears not to be a proble

two triaxial induction tools sensitivity to radial invasion profile i study the responses of two different triaxial induction tools to invaded dipping anisotropic formations i show that the triaxial measurements have generally higher sensitivity to the radial invasion profile compared to the conventional induction measurements this enables accurate interpretation of both the anisotropic formation properties and the invasion parameters multi spacing and single spacing multifrequency triaxial induction tools can both be used for this purpose failure to take the invasion properties into account may lead to misinterpretation of the vertical formation resistivity symmetrization of the apparent conductivity matrix opens ways for a visual interpretation of triaxial induction logs for the formation and the invasion zone properties this technique enables simpler and faster inversion algorithms i study how the effect of a conductive annulus forming around the invasion zone couples with effects of the dipping anisotropy and the dipping boundaries and show when these effects are additive thus a visual detection of log parts affected by a conductive annulus becomes possible the key tool for interpretation in complex 3d scenarios is efficient modelling software i use a 3d finite difference modelling approach to simulate responses of induction logging tools of the new generation its high efficiency enables simultaneous multi spacing and multi frequency computing of the tool responses to arbitrary 3d anisotropic formations that made the study possible

evidence of recent flood deposits within a distal shelf depocenter and implications for terrestrial carbon preservation in non deltaic shelf settings abstract extreme episodic events have the capacity to transport large amounts of terrestrial material to the coastal ocean while estuaries and deltas are typically thought to trap most of this material some escapes these coastal features and is transported to distal depocenters along the continental shelf distal shelf depocenters can act as the ultimate sink for event deposits yet event sedimentation and the impact of terrestrial carbon burial within these shelf deposits remains understudied therefore this study investigated event sedimentation within the texas mud blanket tmb a distal shelf depocenter of the brazos river located in the northwestern gulf of mexico using a core that was collected from the tmb during a months long elevated discharge event in late 2015 to early 2016 we applied a multi proxy approach to delineate the event deposit we utilized physical sediment properties fabric texture and bulk density radioisotopes 7be and 210pb and organic geochemical properties 13c c n and lignin phenols to identify a 10 cm event layer at the surface of the core within this 10 cm layer we also distinguished two sub layers associated with separate extreme discharge periods in the winter and spring respectively using these event layer characteristics we identified three other earlier potential event deposits in the core that we have attributed to events in 2010 2007 and 2000 2001 from these results this study has demonstrated that the tmb may be an archive for event deposits over multi decadal time periods in particular the transition from periods of prolonged drought to extreme wetness associated with the brazos river this event dominated sedimentation regime can enhance long term carbon burial potentially increasing current estimates of carbon burial significantly in the tmb and other similar distal shelf depocenters ultimately this study highlights the potential for distal shelf depocenters as organic carbon burial hotspots and postulates that this role may expand in the future as climate change increases the intensity and frequency of extreme events

the geochemistry of trace metals in the brazos river estuary abstract a seasonal study of trace metal behavior and transport in the brazos river estuary was conducted in the winter spring and fall of 1981 surface water was analyzed for total dissolved pb cu mn and fe and particulate cd pb cu mn and fe ancillary data included river discharge total suspended matter ph major ions nutrients dissolved and particulate organic carbon and humic acid the major ions were generally conservative across the river water seawater mixing zone however a significant input of alkalinity and ca was measured in the spring attributable to cation exchange on clays and sediment diagenetic processes distinct concentration maxima were found in the 3 10 chlorinity range for dissolved pb 0 06 0 9 n m cu 6 42 n m mn 2 290 n m and fe 5 80 n m particulate metal concentrations showed significant seasonal and spatial variations cd 0 09 0 57 ppm pb 15 43 ppm cu 12 32 ppm mn 250 1990 ppm and fe 0 5 5 4 reducing conditions leading to diagenetic remobilization in estuarine sediments are proposed to be an important source of dissolved metals for estuarine water calculations of dissolved riverine metal transport to the ocean that include estuarine metal input are as much as 40 times greater than when estuarine processes are ignored

free carbon electrical conductivity in the earth s mantle laboratory measurements indicate that solid carbon either amorphous or as graphite enhances the electrical conductivity of rocks carbon in quantities of only a few ppm providing it forms an interconnected phase could give rise to electrical conductivities of the order seen in the high conductivity layers in the earth s mantle this possibility is consistent with carbon petrology in mantle rocks and puts the formation and migration of abiogenic carbon compounds into a broader context

subspace inversion of electromagnetic data application to mid ocean ridge exploration controlled source electromagnetic csem surveys have the ability to provide tomo graphic images of electrical conductivity within the earth the interpretation of such data sets has long been hampered by inadequate modelling and inversion techniques in this paper a subspace inversion technique is described that allows electric dipole dipole data to be inverted for a 2 d electrical conductivity model more efficiently than with existing techniques the subspace technique is validated by comparison with conventional inversion methods and by inverting data collected over the east pacific rise in 1989 a model study indicates that with adequate data a variety of possible mid ocean ridge conductivity models could be distinguished on the basis of a csem survey

on the electrical nature of the axial melt zone at 13 n on the east pacific rise the first controlled source electromagnetic experiment directly on a ridge with the potential to identify the presence of an axial melt body beneath a fast spreading center was conducted at 13on on the east pacific rise epr in 1989 transmission for 36 hours was achieved by a deep towed horizontal electric dipole source of moment 6000 am operating at frequencies between 1 4 and 8 hz signals from the source were recorded by seven seafloor electric field receivers positioned both along the ridge crest and 5 km to the east on 100 000 year old crust data above ambient noise levels were obtained at ranges of up to 10 kin the results of modeling observed electric field amplitudes reveal that resistivities in the uppermost crust are very low 1 tim indicating a heavily fractured high porosity surficial layer below this topmost layer the upper 2 km of crust is found to be moderately resistive 100 tim we find no evidence for a large conductive axial melt body with dimensions on the order of kilometers in the middle or upper crust if a partial melt body is present which is continuous along strike and which comprises a connected and therefore conductive melt texture it must be of very limited volumetric extent this picture is consistent with recently proposed models of a thin sill like melt lens with across strike dimensions of no more than 1 km and probably with smaller vertical extent the larger region below the sill characterized by low seismic velocities must contain at best a very small melt fraction distributed in isolated pockets providing further evidence that the epr at 13on is currently in a state of relative magmatic quiescence

from the highest to the deepest the gaoping river gaoping submarine canyon dispersal system abstract there are many different source to sink dispersal systems around the world and the gaoping river gpr gaoping submarine canyon gpsc provides an example especially as a canyon captured system the gpr a small mountainous river having an average gradient of 1 150 and the gpsc which links the river catchment to the deep sea basin represent two major topographic features around sw taiwan together they constitute a terrestrial to marine dispersal system that has an overriding impact on the source to sink transport of sediment in this region the gpsc extents from the mouth of the gpr through the shelf and slope and into the northeastern manila trench a distance of about 260 km it is a major conduit for the transport of terrestrial sediment and carbon to the south china sea and the landward transport of particles of marine and biological origin in the gpsc the dominant mode of suspended sediment transport is tidal oscillations and the net direction is up canyon in contrast sediment transport associated with episodic gravity driven events is down canyon the steady sedimentation of the tidal regime results in hemipelagic mud across the canyon floor whereas the gravity driven hyperpycnal regime causes turbidite erosion and deposition along the canyon thalweg typhoon induced river floods often lead to hyperpycnal plumes at the river mouth which directly and indirectly ignite hyperpycnal turbidity currents in the canyon forming an effective agent for transporting large amounts of terrestrial organic material modern and fossil carbon to the south china sea basin therefore the gpr gpsc represents a source to sink system in which terrestrial sediment in a mountainous catchment is promptly removed and transported to the deep sea by episodic gravity flows this is also a pathway by which modern terrestrial organic carbon is quickly and effectively delivered to the deep sea

maximal two layer exchange over a sill and through the combination of a sill and contraction with barotropic flow the analysis of two layer exchange flow through contractions with a barotropic component treated by armi farmer 1986 is extended to include exchange flows over sills and through a combination of a sill and contraction it is shown that exchange over a sill is fundamentally different from exchange through a contraction control at the sill crest acts primarily through the deeper layer into which the sill projects and only indirectly controls the surface layer this asymmetry in the control results in asymmetrical flows the interface depth above the crest is not one half the total depth as assumed in other studies by analogy with flow through contractions but is somewhat deeper the maximal exchange rate is less than for flow through a contraction of equal depth when both a sill and a contraction are present the contraction influences control at the sill crest only if it lies between the sill and the source of denser water the response to barotropic flow is also asymmetrical the transition to single layer flow occurs at much lower speeds for a barotropic component in one direction than the other results of the analysis are applied to exchange flow through the strait of gibraltar which includes both a sill and a contraction it is shown that maximal exchange conditions apply throughout part of the tidal cycle and observations illustrate several of the analytical predictions for barotropic flows including the formation of fronts single layer flow submaximal exchange and reverse flow

a time domain electromagnetic survey of the tag hydrothermal mound a conductivity survey of the tag mound was made using innovative transient electric dipole dipole instrumentation the receiver was deployed on the sulfide mound 30 m south of the central black smoker while the submersible alvin carried the transmitter on a survey path around the receiver at a radius of approximately 70 m conductivity measurements of the seafloor between the transmitter and receiver were made at 5 minute intervals the survey lasted 4 hours measurements made at 12 sites are initially interpreted in terms of an apparent conductivity of a uniform seafloor a more complex model in which the seafloor has two layers is then used apparent conductivities range from 1 4 to 15 9 s m generally higher than that of seawater the results suggest possible focusing of hydrothermal convection in the northern quadrant of the mound adjacent zones of anhydrite accumulation and fluid convection in the north eastern quadrant heterogeneity and layering in the kremlin zone and unexpected spatial variability in the western quadrant the data from several sites show evidence of distortion attributed to 3 d anomalies

a porosity mapping survey in hecate strait using a seafloor electro magnetic profiling system abstract a newly developed marine electromagnetic e m system was used to create apparent porosity maps of the uppermost 20 m of sediments on the seafloor measurements of electrical conductivity were interpreted to give the porosity of the bottom sediments and underlying units the data were collected continuously in a surveying mode using two receivers at different spacings to improve the resolution of porosity as a function of depth the variations in apparent porosity over the area correlate well with information obtained from cores and acoustic profiles acoustic profiles indicated the presence of biogenic gas in the central region of the survey area the absence of distinct changes in apparent porosity over regions of gas allows us to conclude that the concentration of gas trapped within the queen charlotte muds does not exceed about 4 of the sediment by volume if we assume that the gas displaces fluid in the sediment a buried lower porosity layer was detected its position reflecting changes in the depth to the glaciomarine layer mapped variations in apparent porosity are well correlated with features on the acoustic record this illustrates the system s ability to obtain continuous profiles of apparent porosity over seafloor features the marine e m survey provided rapid areal coverage and combined with the acoustic profiles information on the porosity of deeper units

estuarine controls on fine grained sediment storage in the lower mississippi and atchafalaya rivers observations of the lower mississippi river channel in 1999 2003 with side scan and subbottom seismic profilers multibeam bathymetry core radiotracers 7be 137cs 210pb and 234th and water column profiles of fluid and suspended sediment properties are utilized to constrain the timing location and intensity of the seasonal storage of fine grained sediments in the estuarine channel reach below venice louisiana unlike earlier studies that suggested little or no estuarine sedimentation was taking place in the river the present study demonstrates that the river channel above the head of passes seasonally stores as much as 10 of the annual suspended sediment discharge during periods of falling to low mississippi water discharge associated with salt stratification in the channel thalweg mud deposits are up to 3 2 m thick with 7be derived sediment deposition rates of up to 8 9 mm day water column profiling and 234th 7be ratios suggest that 1 particles are transported and settled mainly as flocs and 2 riverine flocs are settling through the strong salt wedge halocline and are protected from re erosion by the salt stratification with onset of the spring freshet and the corresponding seaward retreat of the saltwater wedge the entire volume of fine grained material stored in the lower river except in low freshet years is remobilized and transported to the gulf of mexico in the turbid freshwater plume this tidal hysteresis strongly influences the timing of sediment supply to the ocean high turbidities in the shelf plume during rising water discharge spikes of 1 to 2 week duration are the product of local channel floor remobilization as well as increased supply from the drainage basin pulsed resupply of sediments and the pore water products of early diagenesis potentially have major implications for issues such as continental margin carbon budgets and coastal eutrophication companion surveys conducted in the lowermost 12 km reach of the atchafalaya distributary of the mississippi show no evidence of saltwater wedge intrusion at low discharge or seasonal fine grained sediment storage likely due to the shallow 7 m river mouth sill that impedes the upstream movement of gulf water

quantitative analysis of the stratigraphic architecture of incised valley fills a global comparison of quaternary systems abstract facies models of the internal fills of incised valleys developed in shelf and coastal settings during cycles of relative sea level change are largely conceptual descriptive and qualitative in form moreover they are commonly bespoke to individual examples here a database driven quantitative statistical analysis of 87 late quaternary incised valley fills ivfs has been undertaken to assess the general validity and predictive value of classical facies models for ivfs and to investigate the relative importance of possible controls on their stratigraphic organization based on datasets from the published literature stored in a sedimentological database the geometry and proportion of systems tracts and of architectural elements of different hierarchies within ivfs are quantified these variables were analysed to assess how they vary in relation to parameters that represent potential controlling factors relative sea level stage continental margin type drainage basin area valley geometry basin physiography and shoreline hydrodynamics the stratigraphic organization of the studied coastal plain ivfs is generally consistent with that represented in facies models the primary control being the rate and magnitude of relative sea level change however results from this study demonstrate significant variability in the stratigraphic architectures of ivfs which is not accounted for by existing models variations in the facies architecture of coastal plain and cross shelf valley fills can be attributed to controls other than sea level and expressed in relationships with continental margin type basin physiography catchment area river system size and shoreline hydrodynamics the following primary findings arise from this research i compared to their counterparts on passive margins coastal plain ivfs hosted on active margins contain on average a higher proportion of fluvial deposits and a lower proportion of central basin estuarine deposits estuarine deposits tend however to be thicker this suggests a control on ivf stratigraphic architecture exerted by distinct characteristics of the tectonic setting of the host continental margins notably basin physiography rates and mode of sediment supply and nature of sediment load ii the thickness and proportion of lowstand systems tract are positively correlated with coastal plain ivf dimensions likely reflecting the role of drainage basin area in dictating the scale of the fluvial systems that carved and infilled the valleys iii positive correlations are observed between the thickness of fluvial deposits bayhead delta deposits and central basin estuarine deposits versus coastal plain ivf dimensions and valley catchment area this suggests a control exerted by the river system scale on sediment supply rates and on the accommodation determined by valley size iv positive correlations between the thickness and proportion of barrier complex deposits within cross shelf ivfs versus mean shelf gradient indicate that the geometry of the shelf might control the establishment and preservation of barrier island environments in incised valleys located on the shelf v correlations between the width of coastal plain ivfs and present day mean tidal range at the shoreline indicate that tidal dynamics may contribute to the widening of the incised valleys positive correlation is observed between the proportion of tide dominated elements in highstand ivf deposits and ivf width suggesting possible effects of interplays between hydrodynamic conditions and the geometry of incised valleys on their infills this study highlights the complexity of the internal fills of incised valleys which must be considered when attempting the application of facies models of ivfs to rock record interpretations or as predictive tools in subsurface studies

on the theory of sea floor conductivity mapping using transient electromagnetic systems the electrical conductivity of the sea floor is usually much less than that of the seawater above it a theoretical study of the transient step on responses of some common controlled source electromagnetic systems to adjoining conductive half spaces shows that two systems the horizontal in line electric dipole dipole and horizontal coaxial magnetic dipole dipole are capable of accurately measuring the relatively low conductivity of the sea floor in the presence of seawater for these systems the position in time of the initial transient is indicative of the conductivity of the sea floor while at distinctly later times a second characteristic of the transient is a measure of the seawater conductivity the diagnostic separation in time between the two parts of the transient response does not occur for many other systems including several systems commonly used for exploration on land a change in the conductivity of the sea floor produces a minor perturbation in what is essentially a seawater response some transient responses which could be observed with a practical deep towed coaxial magnetic dipole dipole system located near the sea floor are those for the half space the layer over a conductive or resistive basement and the half space with an intermediate resistive zone the system response to two adjoining half spaces representing seawater and sea floor respectively is derived analytically the solution is valid for all time provided the conductivity ratio is greater than about ten or less than about one tenth the analytic theory confirms the validity of numerical evaluations of closed form solutions to these layered earth models a lateral conductor such as a vertical infinite conductive dike outcropping at the sea floor delays the arrival of the initial crustal transient response the delay varies linearly with the conductance of the dike this suggests that time delay could be inverted directly to give a measure of the anomalous integrated conductance of the sea floor both between and in the vicinity of the transmitter and the receiver dipoles

observations of distribution and flocculation of suspended particulate matter in the minjiang river estuary china abstract in this study we described the distribution pattern of suspended particulate matter spm and its influencing factors in the waters around the minjiang river estuary further we investigated the flocculation process of spm and its underlying mechanism in the study area the objectives were achieved using water temperature salinity turbidity fluorescence and particle size distribution of spm data measured in august 2009 as influenced by dilution from the minjiang river water the water column structure was significantly stratified in the estuary with an obvious thermocline and halocline the turbidity was relatively low in the surface water ssc 15 mg l fluorescence was high up to 10 l l in the surface water which was markedly lower and more uniform in the lower water layer the total volume concentration and particle size of the spm in the upper water layer were significantly higher than those of the lower water layer the in situ particle size frequency distribution of spm markedly changed in the different water layers whereas the dispersed particles were fine with little difference in frequency distribution the mean effective density of spm was generally 3 in the surface and benthic water and the values were slightly higher in the benthic water layer than in the surface water layer with a significant increase in the middle water layer the significant differences between the distribution trends of the total volume concentration and water turbidity were controlled by the size distribution of spm the concentration distribution of spm was mainly influenced by source re suspension and thermocline among other factors flocs widely occurred in the study area and the flocculation mechanism of spm varied in the different water layers in the upper water layer intense biological activity promoted the formation of stable macroflocs in the benthic water layer the biological activity was limited the flocculation of sediment particles was mainly caused by physico chemical attraction of inorganic particles and the hydrodynamic conditions and high settling velocity were also unfavourable to the formation of macroflocs

structure variability and salt flux in a strongly forced salt wedge estuary this researchwas funded by national science foundation grant oce 0452054 ralston also received partial support from the penzance endowed fund in support of assistant scientists and the john f and dorothy h magee fund in support of scientific staff at woods hole oceanographic institution