high resolution resistivity imaging of marine gas hydrate structures by combined inversion of csem towed and ocean bottom receiver data we present high resolution resistivity imaging of gas hydrate pipe like structures as derived from marine controlled source electromagnetic csem inversions that combine towed and ocean bottom electric field receiver data acquired from the nyegga region offshore norway two dimensional csem inversions applied to the towed receiver data detected four new prominent vertical resistive features that are likely gas hydrate structures located in proximity to a major gas hydrate pipe like structure known as the cne03 pockmark the resistivity model resulting from the csem data inversion resolved the cne03 hydrate structure in high resolution as inferred by comparison to seismically constrained inversions our results indicate that shallow gas hydrate vertical features can be delineated effectively by inverting both ocean bottom and towed receiver csem data simultaneously the approach applied here can be utilised to map and monitor seafloor mineralisation freshwater reservoirs co2 sequestration sites and near surface geothermal systems

the fallacy of the shallow water problem in marine csem exploration the recent explosion of activity in offshore controlled source electromagnetic csem exploration has shown both theoretically and in practice for the time harmonic case that shallow water environments depths less than say 300 m can pose a significant challenge for detection and characterization of thin resistive targets in the subsurface because the weakly attenuated atmospheric response overprints the weaker target response as an alternative the transient csem experiment is considered to explore the nature of the airwave signature when viewed from the perspective of csem time series the signature of a thin isolated resistive horizon is computed by using the quasi analytic solution for a 1d earth in response to excitation by a horizontal electric dipole antenna that signature clearly seen in the transient data generally lies in the time interval between the arrivalof the airwave and the late time seawater response the fact that either decreasing the water depth or increasing the source r

adjustment of the electromagnetic field distorted by 3 d heterogeneities summary the 3 d adjustment distance of electromagnetic fields distorted by lateral contrasts of conductivity was studied with applications to conductivity anomalies occurring near coasts or islands investigations of the electrical conductivity structure of the mantle depend critically on whether the distortion of the electromagnetic data is caused by local or by remote contrasts of conductivity with the help of simple models it is shown that the actual adjustment distance of the electric field along the off strike direction to the conductivity contrast is a balance between an adjustment distance related to the galvanic coupling of the upper layer to the substratum and a distance describing the horizontal scale of the electric currents deviated by the contrast in conductivity it is shown that this adjustment distance cannot exceed the effective dimension of the conductivity contrast consequently published adjustment distances of several thousand kilometres are unlikely to exist as a result sea floor electromagnetic soundings carried out at large distances from coastlines are useful for probing the local conductivity structure of the oceanic mantle the 2 d interpretation of the electric field in the offshore direction at stations perturbed by the coastal effect has a high probability of bias because the field is affected by the combination of the 2 d galvanic coupling and the effect of the horizontal short cut by the closest conductive channels of the electric current flowing in the ocean

electromagnetic induction studies recent developments in electromagnetic induction studies of the lithosphere and the asthenosphere are reviewed attention is given to geoelectrical studies of active tectonic areas in terms of the major zones of crustal extension the basin and range province along western regions of north america and the rio grande rift studies have also been performed of tectonic activity around iceland the salton trough and cerro prieto and the subduction zones of the cascade mountains volcanic belt where magnetotelluric and geomagnetic variation studies have been done geomagnetic variations experiments have been reported in the central appalachians and submarine electromagnetic studies along the juan de fuca ridge controlled source electromagnetic and dc resistivity investigations have been carried out in nevada hawaii and in the adirondacks mountains laboratory examinations on the conductivity of representative materials over a broad range of temperature pressure and chemistry are described

mechanics of dual mode dilative failure in subaqueous sediment deposits abstract we introduce dual mode dilative failure with flume experiments dual mode dilative failure combines slow and steady release of sediments by breaching with periodic sliding which rapidly releases an internally coherent wedge of sediments it occurs in dilative sandy deposits this periodic slope failure results from cyclic evolution of the excess pore pressure in the deposit sliding generates large transient negative excess pore pressure that strengthens the deposit and allows breaching to occur during breaching negative excess pore pressure dissipates the deposit weakens and ultimately sliding occurs once again we show that the sliding frequency is proportional to the coefficient of consolidation we find that thicker deposits are more susceptible to dual mode dilative failure discovery of dual mode dilative failure provides a new mechanism to consider when interpreting the sedimentary deposits linked to submarine slope failures

no evidence for extreme long term warming in early pliocene sediments of the southern ocean abstract during the past 10 years there has been considerable debate with respect to the response of the antarctic ice sheets to early pliocene warmth interpretations generally fall into one of three categories 1 there was a major drawdown of the east antarctic ice sheet eais 2 drawdown of eais was relatively small 3 eais was largely unaffected by early pliocene warmth here this question is approached by using biogenic opal and carbonate in deep sea sediments as proxy for paleoceanographic change during the present day these measures are largely dictated by the position of the antarctic polar front apf with biogenic carbonate exhibiting an increase north of the apf and biogenic opal increasing to the south that the carbonate opal transition zone is a reliable feature in deep sea sediments is evidenced by the fact that it is recorded north of its present position during the last glacial maximum lgm and to the south of it during oxygen isotope stage 11 a warmer than present interglacial when these proxies are applied to southern ocean sediments high frequency glacial interglacial changes are seen in the pliocene and early pleistocene what appears to be a major glacial is recorded at about 4 5 myrs b p three short duration carbonate highs up to 18 suggesting a southward movement of apf occur at discrete intervals between about 4 6 4 8 myrs b p however these disappear or become reduced in amplitude toward the continent such data do not support any appreciable warming of surface waters near the antarctic continent neither does it support any claims of high circa 20 c sea level air temperatures over the eais it is concluded that there is no evidence for significant drawdown of eais during early pliocene warmth

increasing the sensitivity of controlled source electromagnetics with synthetic aperture controlled source electromagnetics csem has been used as a derisking tool in the hydrocarbon exploration industry we apply the concept of synthetic aperture to the lowfrequency electromagnetic field in csem synthetic aperture sources have been used in radar imaging for many years using the synthetic aperture concept big synthetic sources can be constructed by adding the response to small sources building blocks in different ways and consequently big sources with different radiation patterns can be created we show that the detectability of hydrocarbons is significantly enhanced by applying synthetic aperture to csem data more challenging targets such as deep reservoirs 4km belowsea floor can be detected the synthetic aperture technique also increases the sensitivity of the field to subsurface targets in the towing streamer acquisition we also show that a pseudovertical source orthogonally distributed dipole pairs can be constructed synthetically and that the detection capability of this pseudovertical source is increased by applying field steering the synthetic aperture concept opens a new line of research in csem with the freedom to design suitable synthetic aperture sources for a given purpose

splitting marine controlled source electromagnetic responses into sea and subseafloor contributions grounding the air wave abstractwe have found how the effects of the air wave in marine controlled source electromagnetic csem methods gradually vanish in the sea for shallow waters and how at the same time they gradually grow below the seafloor in an effort to comprehend existing detectability definitions the transition from sea to land is smooth because the sea becomes a thin conductive layer when the water depth is smaller than the skin depth in the sea we consider the problem of detecting resistive layers at depth associated with hydrocarbon reservoirs particularly in shallow water explorations and specifically on how the air wave affects detection our analysis is based on an integral representation of the electric field in terms of its sensitivity to changes in the electrical conductivity of a 1d profile two dimensional images of the integrands are obtained by plotting the integrand as a function of depth for different offsets results include the expected growth of the inhibiting effect of the sea as the water d

joint electromagnetic and seismic inversion using structural constraints we have developed a frequency domain joint electromagnetic em and seismic inversion algorithm for reservoir evaluation and exploration applications em and seismic data are jointly inverted using a cross gradient constraint that enforces structural similarity between the conductivity image and the compressional wave p wave velocity image the inversion algorithm is based on a gauss newton optimization approach because of the ill posed nature of the inverse problem regularization is used to constrain the solution the multiplicative regularization technique selects the regularization parameters automatically improving the robustness of the algorithm a multifrequency data weighting scheme prevents the high frequency data from dominating the inversion process when the joint inversion algorithm is applied in integrating marine controlled source electromagnetic data with surface seismic data for subsea reservoir exploration applications and in integrating crosswell em and sonic data for reservoir monitoring and evaluation applications results improve significantly over those obtained from separate em or seismic inversions

lithofacies analysis and 40ar 39ar geochronology of ice volcano interactions at mt murphy and the crary mountains marie byrd land antarctica

electromagnetic induction fields in the deep ocean off california oceanic and ionospheric sources summary a 56 day long measurement of seafloor electromagnetic field fluctuations is separated into parts coherent and incoherent with the horizontal magnetic field at a terrestrial observatory 1700 km distant but at the same geomagnetic latitude least squares methods show that the m2 and n2 ocean tides are significant in the seafloor data but not on land spectra of the remaining data show high seafloor electric to seafloor magnetic field coherences but markedly lower seafloor to land values especially for the horizontal electric field model studies show that the correlated parts of the seafloor spectra are consistent with induction by broad scale ionospheric sources while the uncorrelated parts are explained by a combination of a barotropic long wave continuum and an internal wave component the oceanic electromagnetic field is discussed in terms of its modal structure and wavelength dependence and implications for seafloor magnetotellurics may be profound

a feasibility study of land csem reservoir monitoring in a complex 3 d model summary we carried out a series of numerical simulations in a complex 3 d resistivity model to investigate the feasibility of using controlled source electromagnetics on land for monitoring changes in a hydrocarbon reservoir during production displacement of oil by saline water injection changes the resistivity the modelling allows a comparison of the measured time lapse em signal to various sources of noise that can be expected in a field experiment for instance magnetotelluric signals repeatability errors and near surface resistivity changes caused by seasonal variations our estimates show that land csem monitoring should be feasible though not easily for the example considered here a thick reservoir at a depth of about 1 km the trade off between signal strength and repeatability errors requires the source to be located at some distance from the reservoir measurements in a monitoring well suffer less from surface noise measuring the vertical electric component in a well placed at some distance from the reservoir provides the best result

ten years of marine csem for hydrocarbon exploration marine controlled source electromagnetic csem surveying has been in commercial use for predrill reservoir appraisalandhydrocarbonexplorationfor10 years althougha recent decrease has occurred in the number of surveys and publications associated with this technique the method has become firmly established as an important geophysical tool intheoffshoreenvironment thisisaconsequenceoftwoimportant aspects associated with the physics of the method first it is sensitive to high electrical resistivity which although not an unambiguous indicator of hydrocarbons is an important property of economically viable reservoirs second although the method lacks the resolution of seismic wave propagation it has a much better intrinsic resolution than potential field methods such as gravity and magnetic surveying which until now have been the primary nonseismicdatasetsusedinoffshoreexploration althoughbymany measures marine csem is still in its infancy the reliability and noise floors of the instrument systems have improved significantly over the last decade and interpretation methodology has progressed from simple anomaly detection to 3d anisotropic inversion of multicomponent data using some of theworld sfastestsupercomputers researchdirectionspresently include tackling the airwave problem in shallow water by applying time domain methodology continuous profiling tools and the use of csem for reservoir monitoring during production

magnetotelluric results opposing magma origin of crustal conductors in the rio grande rift although conductive intracrustal layers appear to be characteristic of continental rift zones they also have been detected in stable crustal environments the low resistivity layers are often explained by free water or crustal magma however alternative explanations are possible purely geometrical effects can be mistaken as conductive layers at depth in the interpretation of magnetotelluric or geomagnetic depth soundings certain minerals such as graphite and hydrated minerals e g serpentine may be important thermally activated processes associated with ductile flow mechanisms may also enhance conductivity two dimensional analysis of a portion of 25 new magnetotelluric stations in the central rio grande rift supports the presence of a conductive zone at 10 km depth or less however very significant differences are apparent in the crustal sections at two close localities mid crustal magma lenses well defined by seismic data cannot explain these differences in fact the crust is modeled to be more conductive by at least an order of magnitude where magma is not detected compared to where it occurs at shallow and intermediate levels only the possible effects of regional three dimensional current channeling could conceivably alter this conclusion to explain the unexpected result it is suggested that a conductive horizon occurs where an impermeable ductile cap traps pore fluids beneath the cap may be nearly an order of magnitude more conductive 100 s m than the dry brittle crust above the zone of trapped pore fluids much more conductive by more than an order of magnitude 10 m where active magma injection destroys the integrity of the ductile cap trapped fluids may escape and produce an overall decrease in conductivity the final electrical signature with this dynamic concept depends on the thermal gradient the relative impermeability of the cap the extent of pore fluids beneath and the amount and frequency of magma intrusion the temporal and spatial distribution of earthquake foci in our study area supports the existence of a ductile layer at about 10 km depth and the hypothesis of magma injection

electromagnetic study of the atlantic continental margin using a section of a transatlantic cable the induction of currents in an approximately 1200 km length of the tat 7 transatlantic telecommunications cable by geomagnetic field fluctuations is studied analyses of the induced voltage drop across the essentially geomagnetically east west oriented cable and of magnetic field data obtained at field sites at the cable head tuckerton new jersey and in new hampshire show a very high correlation over a wide frequency range between the induced currents in the cable and the east west magnetic field fluctuations the observations do not correspond to the classical ocean coast effects discussed in the literature the results of the analyses suggest that there are telluric currents flowing in an approximately north south direction off the east coast of the united states the electrical current flow could be primarily confined in the ocean water or channeled in a conductivity structure within the earth s crust and upper mantle if the latter situation exists as may be likely because of the wide range of frequencies over which the effect is seen then the region of the induced current flow could be related to the east coast magnetic anomaly ecma in this case these results would indicate that in addition to being a static magnetic anomaly the ecma is a structure with significant electrical characteristics such as higher conductivity

magnetotelluric exploration of the north pacific progress report and preliminary soundings near a spreading ridge abstract intensive sea floor magnetotelluric field work has been carried out over the north pacific for the past four years including a total of six experiments each involving the deployment of up to eighteen instruments in clusters or arrays because sea floor magnetotelluric exploration today is still subject to a broad range of often stringent limitations an important objective of this program is to assess the potential contribution of this technique to submarine geophysics by checking the consistency of its predictions with more generally accepted knowledge a progress report is presented on this subject in one of the recent experiments which took place on the cocos plate in the immediate vicinity of the pacific rise crest at 12 n the electrical conductivity is shown to rise rapidly at a depth as shallow as 20 km and to remain relatively high for at least another 150 km this finding is in harmony with earlier electrical conductivity models of the upper mantle proposed for two widely different provinces of the north pacific the experiment reported here includes two stations located 35 and 120 km to the east of the axis of the spreading center and with crustal ages of 0 8 and 2 7 my respectively the conductivity profiles are remarkably similar at both stations with only a slight decrease in the total conductance in the upper 200 km for the eastern station the one most distant from the ridge crest this lateral conductivity evolution is consistent with the progressive cooling of the new lithosphere and with the increased distance from ascending magma another feature common to both soundings is the presence of a significant though small anisotropy of the sea floor impedance that does not seem explainable in terms of the coastal effect a similarity of origin with seismic velocity anisotropy observed on the cocos plate is not excluded the conductivity structure proposed for this pacific rise area appears remarkably similar to that of the atlantic ridge crest beneath northeastern iceland

an application of thin sheet electromagnetic modelling to the tasman sea abstract the method of thin sheet approximation is invoked for the ocean layer to model and interpret magnetotelluric mt data observed on the floor of the tasman sea between australia and new zealand a technique to remove 3d distortion from the observed seafloor mt data is developed from the thin sheet model mt responses the de distorted data then appear isotropic and are reinverted using 1d procedures this approach thus assists interpretation through a combination of 3d forward modelling procedures and 1d inversion a seafloor sediment layer is shown to have a strong effect on observed seafloor mt data also an exercise is carried out to compare the results given by the vertical gradient sounding vgs method with seafloor mt data the electrical conductivity structure beneath the tasman sea is analysed in terms of an upper mantle lithosphere and asthenosphere and a highly conducting lower mantle the observed anisotropy in seafloor mt data is diagnostic of a lithospheric conductivity of less than 10 4 s m 1 conductivity rises by two orders of magnitude below 80 km to greater than 10 2 s m 1 and is probably related to an asthenospheric layer in the upper mantle seismic interpretations similarly place a low velocity zone below a depth of 70 km at a depth of approximately 400 km the conductivity is approximately 1 s m 1 consistent with global estimates for the lower mantle there is little evidence for major change with age in the structure across the tasman sea

2d marine controlled source electromagnetic modeling part 2 the effect of bathymetry marine controlled source electromagnetic csem data are strongly affected by bathymetry because of the conductivity contrast between seawater and the crust below the seafloor we simulate the marine csem response to 2d bathymetry using our new finite element fe code and our numerical modeling shows that all electric and magnetic components are influenced by bathymery but to different extents bathymetry effects depend upon transmission frequency seabed conductivity seawater depth transmitter receiver geometry and roughness of the seafloor topography bathymetry effects clearly have to be take into account to avoid the misinterpretation of marine csem data sets

hydrodynamics of turbid underflows ii aggradation avulsion and channelization a two dimensional depth averaged finite volume model is used to study the hydrodynamics of turbid underflows a sensitivity analysis is performed with regard to model input parameters and the initial bed properties simulations are then performed to illustrate aggradation and avulsion in an existing submarine channel the conditions under which channelization of deposits occur are also examined the effect of numerical dissipation on flow ignition is examined and the high resolution properties of the model are found to produce satisfactory results the model is applied to historical field scale events including the reserve fan in lake superior and rupert inlet in british columbia

flow dynamics at the origin of thin clayey sand lacustrine turbidites examples from lake hazar turkey turbidity currents and their deposits can be investigated using several methods i e direct monitoring physical and numerical modelling sediment cores and outcrops the present study focused on thin clayey sand turbidites found in lake hazar turkey occurring in eleven clusters of closely spaced thin beds depositional processes and sources for three of those eleven clusters are studied at three coring sites bathymetrical data and seismic reflection profiles are used to understand the specific geomorphology of each site x ray thin sections and ct scan imagery combined with grain size geochemical and mineralogical measurements on the cores allow characterization of the turbidites turbidites included in each cluster were produced by remobilization of surficial slope sediment a process identified in very few studies worldwide three types of turbidites are distinguished and compared with deposits obtained in flume studies published in the literature type 1 is made of an ungraded clayey silt layer issued from a cohesive flow type 2 is composed of a partially graded clayey sand layer overlain by a mud cap attributed to a transitional flow type 3 corresponds to a graded clayey sand layer overlain by a mud cap issued from a turbulence dominated flow while the published experimental studies show that turbulence is damped by cohesion for low clay content type 3 deposits of this study show evidence for a turbulence dominated mechanism despite their high clay content this divergence may in part relate to input variables such as water chemistry and clay mineralogy that are not routinely considered in experimental studies furthermore the large sedimentological variety observed in the turbidites from one coring site to another is related to the evolution of a sediment flow within a field scale basin made of a complex physiography that cannot be tackled by flume experiments

seismoelectric and electroseismic responses to a point source in a marine stratified model abstract we investigate the seismoelectric electroseismic wavefields excited by a point source in an air seawater three layered porous medium configuration containing a hydrocarbon layer the results show that if an explosive source for excitation is used receivers at seafloor can record the coseismic electromagnetic fields accompanying the p s fluid acoustic waves and the interface responses converted from the acoustic waves at seafloor interface and from the seismic waves at the interfaces beneath the seafloor employing a vertical electric dipole source shows that with the exception of the interface responses converted from electromagnetic waves at seafloor the interface responses converted from transmitted electromagnetic waves at the interfaces beneath the seafloor can also be identified given that the strength of the explosive source is within excitation capability of industry air guns the generated interface responses from the hydrocarbon layer can be detected by current electromagnetic sensors considering the low ambient noise at the seafloor our results demonstrate the feasibility of the seismoelectric method applied to marine hydrocarbon exploration electroseismic modelling results suggest that it is not practical to employ this method to prospect marine hydrocarbon layer due to the weak interface response signal unless a much larger current is injected into seafloor

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

electromagnetic detection of plate hydration due to bending faults at the middle america trench water plays an important role in the processes occurring at subduction zones since the release of water from the downgoing slab impacts seismicity and enhances arc volcanism geochemical indicators suggest that the nicaraguan slab is anomalously wet yet the mechanism of slab hydration remains poorly constrained extensional bending faults on the incoming oceanic plate of the middle america trench offshore nicaragua have been observed to penetrate to mantle depths suggesting a permeable pathway for hydration of the crust and serpentinization of the upper mantle low seismic velocities observed in the uppermost mantle of the incoming plate have been explained as serpentinization due to deep fluid penetration but could also be explained by intrinsic anisotropy and fractures in the absence of fluid circulation here we use controlled source electromagnetic imaging to map the electrical resistivity of the crust and uppermost mantle along a 220 km profile crossing the trench offshore nicaragua along the incoming plate our data reveal that crustal resistivity decreases by up to a factor of five directly with the onset of the bending faults furthermore a strong azimuthal anisotropy compatible with conductive vertical fault planes is observed only on the faulted trench seafloor the observed resistivity decrease and anisotropy can be explained by a porosity increase along vertical fault planes which we interpret as evidence that the lithospheric bending faults provide the necessary permeable fluid pathways required for serpentinization of the uppermost mantle this implies that most serpentinisation happens at the trench with the width of the faulting region and the density of fractures controlling the extent of upper mantle alteration this observation explains why the heavily faulted trench offshore nicaragua is associated with an anomalously wet slab whereas other sections of the middle america trench containing fewer bending faults have less fluid flux from the subducting slab

experiments on upstream migrating submarine knickpoints abstract laboratory studies are commonly used to improve our understanding of physical processes that are very difficult or almost impossible to investigate in their natural settings erosion impacts on large scale submarine morphology are good examples we present the results of a preliminary laboratory study focused on submarine knickpoints which are bed depressions bounded by steep walls upstream and a gentle slope downstream originated by the passage of a turbidity current over an erodible bed eight experiments using different materials were performed in a facility capable of running continuous turbidity currents the sediments used in these experiments were kaolin clay two different silica flours and fine sand silica flours and fine sand were each used as single sediments in four experiments the other four experiments were performed using a mixture of these sediment types the bed deposit was formed by a net depositional turbidity current the total sediment concentration by volume in the experiments was 5 in one experiment and 10 in the other seven the discharge ranged from 0 009 to 0 055 l s each experiment started with an initial slope in the channel bed of 15 8 53 as the experiments progressed the bed deposit inside the flume built up knickpoints were observed in four of the eight experiments knickpoints formed under a wide variety of conditions they were found when one material or a combination of materials was used in the tests and when the turbidity current was partially and totally distributed over the channel measured width of the bed depressions ranged from 2 to 4 cm length ranged from 3 to 6 cm and the estimated maximum depth was close to 0 5 cm calculations of the speed of upstream migration indicate values in the vicinity of 0 5 mm s upscaled geometric dimensions based on these laboratory experiments fairly reproduce bed depressions observed on a field scale in earlier studies these laboratory observations suggest that the body of the turbidity current is responsible for the upstream migration of the knickpoint

quaternary mud turbidites from the south shetland trench west antarctica recognition and implications for turbidite facies modelling a piston core from the basinal part depth of 5188 m of the south shetland trench west antarctica yielded a terrigenous mud section 11 m long which can be subdivided with great precision into turbidite and hemipelagite layers mud turbidites mean bed thickness 44 cm alternate regularly with and are best distinguishable from their hemipelagite host mean bed thickness 17 cm by the following features i sharp basal contacts ii terrigenous sand free textures except basal well sorted silt laminae and the absence of outsized ice rafted components iii a laminated little to non bioturbated internal structure iv distinct textural and compositional grading and v marked steps on water content and sediment density logs mud turbidites recovered from the south shetland trench differ from an earlier model mud turbidite sequence by their i excessive about six times larger bed thickness ii complex internal organization manifested in multiple repetitions up to four of the same structural interval s in sequential or nonsequential order iii distinctive very fine grained cap of highly porous clay rich in fragments of siliceous biogenics iv widespread zones of penesyndepositional deformation and v evidence of flow reversals these features are interpreted to record deposition from large muddy turbidity currents subjected to flow transformations including soliton and or seiche related reversals induced by ponding and interactions of the flow with the topographical confinements of the trench it is concluded that contained muddy turbidites cannot be adequately modelled using published sequences differentiation of single model and contained mud turbidites offers obvious advantages in basin analysis and in understanding the plethora of turbidity current related depositional mechanisms of deep sea mud

conductivity structure of the lithosphere asthenosphere boundary beneath the eastern north american margin tectonic plate motion and mantle dynamics processes are heavily influenced by the characteristics of the lithosphere asthenosphere boundary lab yet this boundary remains enigmatic regarding its properties and geometry the processes involved in rifting at passive margins result in substantial alteration of the lithosphere through the transition from continental to oceanic lithologies here we employ marine magnetotelluric mt data acquired along a 135 km long profile offshore martha s vineyard new england usa to image the electrical conductivity structure beneath the new england continental margin for the first time we invert the data using two different mt 2 d inversion algorithms and present a series of models that are obtained using three different parameterizations fully unconstrained unconstrained with an imposed lab discontinuity and a priori constrained lithosphere resistivity this suite of models infers variability in the depth of the lab with an average depth of 115 km at the eastern north america passive margin models robustly detect a 350 m lithospheric anomalous conductivity zone lacz that extends vertically through the entire lithosphere our preferred conductivity model is consistent with regional p to s receiver function data shear wave velocity gravity anomalies and prominent geological features we propose that the lacz is indicative of paleo lithospheric thinning either resulting from kimberlite intrusions associated with rifting and the new england great meteor hotspot track or from shear driven localized deformation related to rifting

geology of mount murphy volcano an 8 m y history of interaction between a rift volcano and the west antarctic ice sheet volcanic rocks glaciogenic sediments and recycled marine microfossils at mount murphy provide evidence for large scale fluctuations in the mass of the west antarctic ice sheet during neogene time mount murphy is a large shield volcano with an atypical structure its basal unit consists of alternating subaquatic and subaerial rock types through a stratigraphic interval of at least 300 m suggesting that ice level changed several times during shield building much larger changes are suggested by an outcrop of glacial lake sediment near the volcano summit 1 300 m above present ice level it contains an assemblage of recycled marine microfossils derived from marine basins in the interior of west antarctica they suggest multiple intervals of near complete deglaciation in west antarctica between about 24 and 3 5 ma the microfossils were probably deposited by meltwater from the ice sheet during a much higher stand of ice in late pliocene time it has long been suspected that the west antarctic ice sheet is unstable and vulnerable to greenhouse warming because it is grounded more than 1 000 m below sea level our results provide field evidence consistent with that proposal more than the west antarctic ice sheet may be involved however the nature of the evidence and timing of events at mount murphy are similar to the record in east antarctica suggesting continent wide synchroneity of the major glacial and interglacial intervals this conflicts with interpretations of marine data which suggest a shorter history for the west antarctic ice sheet and a more stable history for the entire ice sheet

vulcan a deep towed csem receiver we have developed a three axis electric field receiver designed to be towed behind a marine electromagnetic transmitter for the purpose of mapping the electrical resistivity in the upper 1000 m of seafloor geology by careful adjustment of buoyancy and the use of real time monitoring of depth and altitude we are able to deep tow multiple receivers on arrays up to 1200 m long within 50 m of the seafloor thereby obtaining good coupling to geology the rigid body of the receiver is designed to reduce noise associated with lateral motion of flexible antennas during towing and allows the measurement of the vertical electric field component which modeling shows to be particularly sensitive to near seafloor resistivity variations the positions and orientations of the receivers are continuously measured and realistic estimates of positioning errors can be used to build an error model for the data during a test in the san diego trough offshore california inversions of the data were able to fit amplitude and phase of horizontal electric fields at three frequencies on three receivers to about 1 in amplitude and 1 in phase and vertical fields to about 5 in amplitude and 5 in phase the geological target of the tests was a known cold seep and methane vent in 1000 m water depth which inversions show to be associated with a 1 km wide resistor at a depth between 50 and 150 m below seafloor given the high resistivity 30 m and position within the gas hydrate stability field we interpret this to be massive methane hydrate

sedimentation and mixing of a turbulent fluid suspension a laboratory study in many geological and industrial situations the mixing that occurs across the moving interface between a turbulent fluid layer and a quiescent layer of different density is of fundamental importance much of our knowledge of this process has been obtained from laboratory experiments in mixing boxes where the turbulent motions are generated by the vertical oscillation of a horizontal grid located in the interior of the stirred layer we report here some experiments in which the turbulent layer contains a suspension of small dense particles with the stirring grid located at the base of the tank in order to simulate the generation of turbulence by stresses acting at the rough bottom over which it traverses we compare the data with experiments using dense but particle free fluid layers in the same geometry the results are distinctly different since the particles responsible for the density difference between the two layers can also migrate through and fall out of the suspension layer as it changes in thickness we find that the following equilibrium conditions are attained sequentially as the frequency of stirring and hence the intensity of turbulence is increased the particles eventually all precipitate or only some precipitate while others are held indefinitely in suspension or all the particles are suspended by the turbulent motions both the last two cases produce a stable self maintained suspension layer separated from the overlying fluid by a sharp density interface the results cannot be explained simply in terms of particles falling away from an entraining interface the work done in keeping the particles in suspension must be taken explicitly into account the relationship between these experiments and results previously obtained when the interstitial fluid in the lower sediment layer is less dense than the upper layer and can rise across the interface to drive convective motions above is also discussed briefly our results will be applicable to various sediment laden flows including crystal rich magma layers pyroclastic flows avalanches river outflows turbidites and the deposition of industrial wastes

deposition in an esker bead and fan complex lanark ontario canada abstract a glaciofluvial landform complex comprising a single esker ridge proximally multiple ridges and beads in the middle section and a complex of ridges and fans distally is mapped and described we present detailed successions in the associated sediment showing sedimentary structure grain size and palaeocurrent estimates sediment in the esker is mainly coarse grained and varies from boulders with a fine sand matrix deposited at maximum discharges to diamicton interpreted as a product of ice pressing under low discharges early stage deposition in the esker at high flow was from suspension and bed load intermediate stage deposition was mainly from bed load and late stage deposition was from hyperconcentrated suspension the esker rests in a bedrock channel which functioned as a nye channel prior to esker formation the channel surface carries a variety of fluvial erosional marks short distributary ridges of the ridges and beads complex contain mainly cobbles and boulders but beads are mainly in sand with rapid down flow fining and change in sedimentary structure faulting and overfolding of bead sediment suggest a subglacial origin deposition of the esker fan complex was highly variable in space and time down fan sedimentary changes with transitions from gravel to cross laminated fine sand in a few tens of metres indicate rapid loss in competence fan location lateral to the esker indicates a broadening of the zone of subglacial meltwater with time deposition from suspension and steep sided scours are interpreted to result from deposition and erosion by hyperconcentrated flows a model of underflows with submerged hydraulic jumps spilling laterally from the esker tunnel into subglacial cavities explains the morphological and sedimentary characteristics of the fans detailed analysis of small scale fining upwards sequences in the fans and beads indicates deposition by a large number of flow events these events are explained by glacier hydraulics whereby periodic separation of the glacier from its bed caused water with a high suspended sediment concentration to spill out from the esker tunnel into adjacent subglacial cavities evolution of the subglacial drainage system indicates a complexity of subglacial sedimentation near to a grounding line which is not readily apparent from observations at modern glaciers terminating in relatively deep water

what determines the downstream evolution of turbidity currents abstract seabed sediment flows called turbidity currents form some of the largest sediment accumulations deepest canyons and longest channel systems on earth only rivers transport comparable sediment volumes over such large areas but there are far fewer measurements from turbidity currents ensuring they are much more poorly understood turbidity currents differ fundamentally from rivers as turbidity currents are driven by the sediment that they suspend fast turbidity currents can pick up sediment and self accelerate ignite whilst slow flows deposit sediment and dissipate self acceleration cannot continue indefinitely and flows might reach a near uniform state autosuspension here we show how turbidity currents evolve using the first detailed measurements from multiple locations along their pathway which come from monterey canyon offshore california all flows initially ignite typically initially faster flows then achieve near uniform velocities autosuspension whilst slower flows dissipate fractional increases in initial velocity favour much longer runout and a new model explains this bifurcating behaviour however the only flow during less stormy summer months is anomalous as it self accelerated which is perhaps due to erosion of surficial mud layer mid canyon turbidity current evolution is therefore highly sensitive to both initial velocities and seabed character

numerical model of turbidity currents with a deforming bottom boundary a numerical model of turbidity currents with a deforming bottom boundary has been developed the model predicts the vertical structure of the flow velocity and concentration as well as change in the bed level due to erosion and deposition of suspended sediment the reynolds averaged navier stokes equations for dilute suspension have been solved using a finite volume method the bottom boundary and the grid system are allowed to adjust in response to sediment deposition and entrainment during the computation the model has been applied to simulate the evolution of a conservative saline density current and turbidity currents along an 11 6 m long flume that includes a slope followed by a horizontal bed the model successfully simulates the evolution of the currents model results have been compared with the experimental data good similarity profiles of velocity and excess density or suspended sediment concen tration are obtained at both the upstream supercritical and the downstream subcritical flow regions a turbulent schmidt number larger than one has been found to be appropriate for providing a good match with the experimental data changes in bed level predicted by the model have also been found to be in agreement with the experiment data

an introduction to marine controlled source electromagnetic methods for hydrocarbon exploration early development of marine electromagnetic methods dating back about 80 years was driven largely by defense military applications and use for these purposes continues to this day deepwater frequency domain electric dipole dipole controlled source electromagnetic csem methods arose from academic studies of the oceanic lithosphere in the 1980s and although the hydrocarbon exploration industry was aware of this work the shallow water environments being explored at that time were not ideally suited for its use low oil prices and increasingly successful results from 3d seismic methods further discouraged investment in costly alternative geophysical data streams these circumstances changed in the late 1990s when both statoil and exxonmobil began modeling studies and fieldtrials of csem surveying in deep water around 1000 m or deeper specifically for characterizing the resistivity of previously identified drilling targets trials offshore angola in 2000 2002 by both these companies showed that

signal detectability of marine electromagnetic methods in the exploration of resistive targets we compare selected marine electromagnetic methods for sensitivity to the presence of relatively thin resistive targets e g hydrocarbons gas hydrates fresh groundwater etc the study includes the conventional controlled source electromagnetic method the recently introduced transient electromagnetic prospecting with vertical electric lines method and the novel marine circular electric dipole method which is still in the stage of theoretical development the comparison is based on general physical considerations analytical mainly asymptotic analysis and rigorous one dimensional and multidimensional forward modelling it is shown that transient electromagnetic prospecting with vertical electric lines and marine circular electric dipole methods represent an alternative to the conventional controlled source electromagnetic method at shallow sea where the latter becomes less efficient due to the air wave phenomenon since both former methods are essentially short offset time domain techniques they exhibit a much better lateral resolution than the controlled source electromagnetic method in both shallow sea and deep sea the greatest shortcoming of the transient electromagnetic prospecting with vertical electric lines and marine circular electric dipole methods comes from the difficulties in accurately assembling the transmitter antenna within the marine environment this makes these methods significantly less practical than the controlled source electromagnetic method consequently the controlled source electromagnetic method remains the leading marine electromagnetic technique in the exploration of large resistive targets in deep sea however exploring laterally small targets in deep sea and both small and large targets in shallow sea might require the use of the less practical transient electromagnetic prospecting with vertical electric lines and or marine circular electric dipole method as a desirable alternative to the controlled source electromagnetic method

synthetic aperture controlled source electromagnetics controlled source electromagnetics csem has been used as a de risking tool in the hydrocarbon exploration industry although there have been successful applications of csem this technique is still not widely used in the industry because the limited types of hydrocarbon reservoirs csem can detect in this paper we apply the concept of synthetic aperture to csem data synthetic aperture allows us to design sources with specific radiation patterns for different purposes the ability to detect reservoirs is dramatically increased after forming an appropriate synthetic aperture antenna consequently the types of hydrocarbon reservoirs that csem can detect are significantly extended because synthetic apertures are constructed as a data processing step there is no additional cost for the csem acquisition synthetic aperture has potential for simplifying and reducing the cost of csem acquisition we show a data example that illustrates the increased sensitivity obtained by applying synthetic aperture csem source

conditions for the ignition of catastrophically erosive turbidity currents abstract the basic impediment to a clear understanding of eroding and depositing turbidity currents has been the lack of a proper formulation of bed sediment entrainment this problem is addressed herein a detailed analysis indicates that the bagnold criterion is necessary but insufficient for self sustaining turbidity currents the analysis reveals two possible equilibrium states a relatively low velocity ignitive state and a high velocity dense catastrophic state a stability analysis indicates that the ignitive state is unstable flows below it invariably die out and flows above it ignite i e accelerate and entrain sediment to the catastrophic state which is stable the ignitive state thus defines the criterion for a self sustaining turbidity current estimates of the catastrophic state suggest that it is highly erosive and competent to scour out submarine canyons

controlled source electromagnetic three dimensional grid modelling based on a complex resistivity structure of the seafloor effects of acquisition parameters and geometry of multi layered resistors a comprehensive controlled source electromagnetic csem modelling study based on complex resistivity structures in a deep marine geological setting is conducted the study demonstrates the effects of acquisition parameters and multi layered resistors on csem responses three dimensional 3d finite difference time domain fdtd grid modelling is used for csem sensitivity analysis interpolation of normalized csem responses provides attributes representing relative sensitivity of the modelled structures modelling results show that fine grid 1 1 km receiver spacing provides good correlations between csem responses and the modelled structures irrespective of source orientation the resolution of csem attributes decreases for receiver spacing 2 2 km when using only in line data broadside data in the grid geometry increase data density by 100 approximately 200 by filling in in line responses and improve the resolution of csem attributes optimized source orientation i e oblique to the strike of an elongated resistor improves the structural definition of the csem anomalies for coarse grid geometries receiver spacing 3 3 km the study also shows that a multi resistor anomaly is not simply the summation but a cumulative response with mutual interference between constituent resistors the combined response of constituent resistors is approximately 50 higher than the cumulative response of the multi resistor for 0 5 hz at 4000 m offset a gradual inverse variation of offset and frequency allows differentiation of csem anomalies for multi layered resistors similar frequency offset variations for laterally persistent high resistivity facies show visual continuity with varying geometric expressions 3d grid modelling is an effective and adequate tool for csem survey design and sensitivity analysis

mapping thin resistors and hydrocarbons with marine em methods insights from 1d modeling the use of marine controlled source electromagnetic em csem sounding to detect thin resistive layers at depths below the seafloor has been exploited recently to assess the resistivity of potential hydrocarbon reservoirs before drilling we examine the sensitivity of the csem method to such layers with forward and inverse modeling in one and three dimensions the 3d modeling demonstrates that if both source and receivers are over a tabular 3d target 1d modeling predicts the observed response to very high accuracy experimental design can thus be based on 1d analysis in which hundreds of range and frequency combinations can be computed to find the optimal survey parameters for a given target structure modeling in three dimensions shows that the vertical electric field response is largest over the edges of a 3d target the 3d modeling also suggests that a target body needs to have a diameter twice the burial depth to be reliably seen by csem sounding a simple air wave model energy propagating from source to receiver via the atmosphere allows the effects of the target layer and atmosphere to be separated and shows where sensitivity to the target is diminished or lost because of finite water depth as a function of range frequency and seafloor resistivity unlike dc resistivity sounding the marine csem method is not completely t equivalent and in principle can resolve resistivity and thickness separately smooth inversion provides an estimate of the method s resolving power and highlights the fact that although the radial csem fields contain most of the sensitivity to the thin resistive target inverted alone they produce only increasing resistivity with depth inclusion of the radial mode csem data forces the recovery of the thin resistor but magnetotelluric data can be used more effectively to achieve the same result

deglaciation of pope glacier implies widespread early holocene ice sheet thinning in the amundsen sea sector of antarctica abstract the amundsen sea sector of the antarctic ice sheet presently dominates the contribution from antarctica to sea level rise several large ice streams that currently drain the sector have experienced rapid flow acceleration grounding line retreat and thinning during the past few decades however little is known of their longer term millennial scale retreat history despite the reliance of several ice sheet and glacial isostatic adjustment models on such data for improving sea level prediction from this critical region this study investigates the timing and extent of surface lowering of one of those ice streams pope glacier since the last glacial maximum lgm using glacial geological evidence for former ice cover we present a new deglacial chronology for the glacier derived from surface exposure dating of glacially deposited cobbles and ice scoured bedrock from mount murphy and its surrounding peaks cosmogenic 10be exposure ages from 44 erratic cobbles and 5 bedrock samples and in situ 14c exposure ages from one erratic and 8 bedrock samples are predominantly in the range 5 5 16 ka although 10be inheritance from prior exposure is prevalent in some erratics and probably all bedrock samples none of the ages pre date the lgm from these results we infer that the surface of pope glacier lowered by 560 m during the early to mid holocene 9 6 ka at an average rate of 0 13 0 09 0 04 m yr 1 the lowering coincided with a period of enhanced upwelling of warm circumpolar deep water onto the continental shelf in the region a reduction in buttressing facilitated by such upwelling by an ice shelf that is thought to have spanned the embayment until 10 6 cal kyr bp could have triggered simultaneous early holocene thinning of pope glacier and glaciers elsewhere in the amundsen sea embayment

lithospheric and upper mantle structures results of electromagnetic soundings in europe abstract information on electrical conductivity of the subsurface at a crustal scale is inferred mainly from magnetotelluric mt and magnetometer data the latter include both magnetometer array studies mv and geomagnetic depth soundings gds locally controlled source data e g data from d c and very low frequency vlf resistivity surveys frequency soundings and airborne electromagnetic mappings provide information on near surface structures e g on exposures of deeper conductors the variety of electromagnetic methods provides an efficient zooming ability for structures from a regional scale of hundreds of kilometres to small local details of some metres thus both the mapping of large scale crustal conductivity structures and a detail study of the anomalous structures are possible the electrical conductivity of the crust and upper mantle is described within the plate tectonic framework examples of structures which become electrically conductive at different phases of the process are shown using the results of electromagnetic soundings in various tectonic environments in europe it is shown how the crustal conductors can be interpreted as ancient and modern tectonic markers these include structures from the fennoscandian shield precambrian extensional basins and collisional zones the carpathians scotland and ireland suture zones and the pannonian basin extensional basin and thrust zones as well as conductors detected along the european geotraverse e g the terrain boundaries and the decollement of the variscan thrusting an example which considers the lower crustal conductivity from the fennoscandian shield shows rather large lateral variations in conductance of the lower crust varying from 1 s to several hundreds of siemens estimates of the depth of the asthenospheric conducting layer within the european territory also vary considerably shallowest depths are detected beneath extensional regions the pannonian basin and the rhinegraben 40 80 km whereas depths of 200 km or over or the absence of a conducting layer are detected beneath the alps and old precambrian cratons among the key areas where electromagnetic investigations should play an essential role are the deep structure of the east european platform boundaries of archaean and proterozoic terrains the structure of the teisseyre tornquist zone the nature of the present day continental lower crust and the characteristics of the electrical asthenosphere beneath europe

on the electrical conductivity of the oceanic lithosphere abstract below the bottom of the sediments the oceanic lithosphere is difficult to sound electromagnetically because the resistivity is high compared to over and under lying structures there are three methods which can contribute information 1 1 magnetotelluric methods provide the impedance as a function of frequency for signals of sufficiently low frequency to penetrate through the ocean in the deep sea the upper frequency limit is a few cph 2 2 a quasi static electric field can be measured at the ocean floor the field is generated by charges built up by the motion of ocean water currents through the geomagnetic field by providing a current path below the ocean the lithospheric conductivity tends to reduce this field in order to interpret the measurements one needs to know the spatial character of the water motions 3 3 an artificial em source which transmits at frequencies near 1 hz can be used at the ocean floor energy propagation through the lithosphere has been detected on the ocean floor to horizontal distances of 18 km detection is aided by the very low noise level at the high frequencies concerned estimates of the typical conductivity distribution throughout the ocean crust are given