three dimensional topography correction applied to magnetotelluric data from sikkim himalayas abstract magnetotelluric mt method is one of the powerful tools to investigate the deep crustal image of mountainous regions such as himalayas topographic variations due to irregular surface terrain distort the resistivity curves and hence may not give accurate interpretation of magnetotelluric data the two dimensional 2 d topographic effects in transverse magnetic tm mode is only galvanic whereas inductive in transverse electric te mode thus tm mode responses is much more important than te mode responses in 2 d in three dimensional 3 d the topography effect is both galvanic and inductive in each element of impedance tensor and hence the interpretation is complicated in the present work we investigate the effects of three dimensional 3 d topography for a hill model this paper presents the impedance tensor correction algorithm to reduce the topographic effects in mt data the distortion caused by surface topography effectively decreases by using homogeneous background resistivity in impedance correction method in this study we analyze the response of ramp distance from topographic edges conductive and resistive dykes the new correction method is applied to the real data from sikkim himalayas which brought out the true nature of the basement in this region

groundwater electromagnetic imaging in complex geological and topographical regions a case study of a tectonic boundary in the french alps very low frequency vlf audiomagnetotelluric amt and water geochemistry surveys were performed on the sur fretes ridge in the french alps to evaluate the groundwater circulation system within the ridge at this site temporal variations of the electric field have been observed in association with water level variations of neighboring artificial lakes the sur fretes ridge is 1 km wide and trends east west water samples were collected at 52 points distributed throughout the area vlf soundings were carried out along three parallel east west profiles and 7 amt soundings were carried out along an east west profile on the top of the ridge this site is characterized by a rugged topography of the ridge where geological and topographic trends are almost perpendicular making the structure fully three dimensional we constructed a 2 d resistivity model of the ridge from 2 d and 3 d analyses of the vlf and amt data associating geology and topography models when combined with the water chemistry data a m

a magnetotelluric study of the alpine fault new zealand summary magnetotelluric soundings have been made at seven locations on a 4 km profile crossing the alpine fault in the south island of new zealand the distortion techniques of groom bailey 1989 and lilley 1998a b have been used to derive regional apparent resistivity and phase curves that correspond to electromagnetic induction in orientations parallel and perpendicular to the fault 2 d inversion of the regional responses reveals that a narrow 1 km wide conductive zone is associated with the alpine fault this conductor is most probably related to the heating of deep circulating meteoric water in a region in which enhanced temperatures occur at shallow depth due to the tectonic uplift of the southern alps

correction for the static shift in magnetotellurics using transient electromagnetic soundings shallow inhomogeneities can lead to severe problems in the interpretation of magnetotelluric mt data by shifting the mt apparent resistivity sounding curve by a scale factor which is independent of frequency on the standard log apparent resistivity versus log frequency display the amount of parallel shift commonly referred to as the mt static shift can not be determined directly from conventionally recorded mt data at a single site one method for measuring the static shift is a controlled source measurement of the magnetic field unlike the electric field the magnetic field is relatively unaffected by surface inhomogeneities the controlled source sounding which may be a relatively shallow sounding made with lightweight equipment can be combined with a deep mt sounding to obtain a complete undistorted model of the earth inversions of the static shift corrected mt data provide a much closer match to well log resistivities than do inversions of the uncorrected data the particular controlled source magnetic field sounding which we used was a central induction transient electromagnetic tem sounding correction for the static shift in the mt data was made by jointly inverting the mt data and the tem data a parameter which allowed vertical shifts in the mt apparent resistivity curves was included in the computer inversion to account for static shifts a simple graphical comparison between the mt apparent resistivities and the tem apparent resistivities produced essentially the same estimate of the static shift within 0 1 decade as the joint computer inversion central induction tem measurements were made adjacent to over 100 mt sites in central oregon the complete data base of over 100 sites showed an average static shift between 0 and 0 2 decade however in the rougher topography and more complex structure of the cascade mountain range the majority of the sites had static shifts of the order of 0 3 to 0 4 decade the static shifts in this area are probably due to a combination of topography and surficial inhomogeneities the tem apparent resistivity which is used to estimate the unshifted mt apparent resistivity does not necessarily agree with either the transverse electric te or the transverse magnetic tm mt polarization tem apparent resistivity may occur between the two or may agree with one of the two polarizations or may lie outside the mt polarizations

sea effect correction in magnetotelluric mt data and its application to mt soundings carried out in jeju island korea summary when magnetotelluric mt data are obtained in the vicinity of the coast the surrounding sea makes it difficult to interpret subsurface structures especially at the deep parts of the subsurface we apply an iterative method to remove the sea effect the iterative method was originally developed to remove the distortion due to topographic changes from mt data recorded on the seafloor the iterative sea effect correction method is carried out in two steps the first corrects the sea effect whereas the second inverts the sea effect corrected responses the two steps are alternatively carried out until the criterion for either the inversion or the seaeffect correction is satisfied because the surrounding 3 d sea bathymetry is only incorporated into forward modelling for the sea effect correction it can be more robust than the method that directly incorporates 3 d sea bathymetry into a model space for inversion the synthetic examples show that the sea effect correction method yields an inverted model comparable to the true model by applying the sea effect correction method to real field data acquired in jeju island korea we also demonstrate that the sea effect correction method effectively removes the sea effects from the 1 d and 2 d real field data which helps enhance the inversion results on the basis of these results it may be concluded that the iterative sea effect correction method can be used as a promising technique for recovering the true response of the subsurface in mt data suffering from sea effects

3 d magnetotelluric inversion including topography using deformed hexahedral edge finite elements and direct solvers parallelized on smp computers part i forward problem and parameter jacobians

diagnosis of the record minimum in arctic sea ice area during 1990 and associated snow cover extremes the arctic sea ice cover exhibited its record minimum area during 1990 characterized by extensive ice free conditions during august along the siberian coast these reductions are consistent with warm windy conditions in may and continued warmth in june promoting early melt and reductions in ice concentration followed in august by strong coastal winds forcing a final breakup and retreat of the pack ice the unusually warm arctic conditions in 1990 are part of a larger scale temperature anomaly pattern linking the sea ice anomaly to accompanying record minima in eurasian snow cover

a case study of the anomalous arctic sea ice conditions during 1990 insights from coupled and uncoupled regional climate model simulations 1 the regional coupled atmosphere ice ocean model hirham mom has been developed to simulate the arctic climate the model has been applied for the year 1990 which was characterized by anomalous ice retreat along the eastern arctic coasts many observed features of the anomalous atmospheric circulation and sea ice concentration pattern during spring to early summer are reproduced by the model it was even able to generate the extreme summer sea ice conditions in the eurasian arctic and the associated high surface air temperatures the model shows less success in simulating the large ice retreat in the siberian arctic in late summer this is explained mainly by deviations in the simulated atmospheric circulations results from an ocean ice alone model confirm that the atmospheric forcing dominates the simulated ice retreat on the other hand a sensitivity study with the atmosphere alone model shows the impact of the sea ice conditions on the atmospheric circulation only with the accurate satellite derived sea ice data is the model able to reproduce the anomalous late summer atmospheric pressure patterns

three dimensional terrain corrections in resistivity surveys a three dimensional finite element computer algorithm which can accommodate arbitrarily complex topography and subsurface structure has been developed to model the resistivity response of the earth the algorithm has undergone extensive evaluation and is believed to provide accurate results for realistic earth models testing included comparison to scale model measurements analytically calculated solutions and results calculated numerically by other independent means computer modeling experiments have demonstrated that it is possible to remove the effect of topography on resistivity data even under conditions where such effects are large with respect to the subsurface responses this can be done without resorting to lengthy and costly trial and error computer modeling after correction the data can be interpreted as if the anomalies are due only to subsurface structure the results of case studies on field data measured in high relief topography indicate the following 1 pre or postsurvey 3 d compu

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

understanding lotem data from mountainous terrain long offset transient electromagnetic lotem data from the vesuvius volcano in italy show that the em response of the topography is a potential cause of data distortions a modeling study was carried out to simulate the effect of mountainous terrain on vertical magnetic field time derivatives using a 3 d finite difference code the objectives were to assess the importance of topographic effects and to help identify them in existing field data the total effect of topography on the lotem response can be considered as a combination of four distortions of the corresponding responses for a flat terrain first the receiver is at some height above the flat surface second the mountain acts as a conductive body displacing air third large loop receivers are nonhorizontal and sense a combination of horizontal and vertical magnetic fields finally the electromagnetic coupling between the mountain and deeper lying structure modifies the structure response each of the effects can be identified in field data

two circulation regimes of the wind driven arctic ocean the major goal of this paper is to demonstrate the existence in the arctic ocean of two regimes of wind forced circulation we simulated the vertically averaged currents sea level heights and ice drift in the arctic ocean from 1946 to 1993 using a two dimensional wind forced barotropic model that includes frictional coupling between the ocean and ice the model has a spatial resolution of 55 5 km and is driven by winds river runoff and an imposed but realistic sea level slope between the pacific and the atlantic oceans there is a good agreement between velocities from observed buoy motions and velocities of modeled ice drift even though the model lacks ocean baroclinicity and ice thermodynamics the results indicate that wind driven motion in the central arctic alternates between anticyclonic and cyclonic circulation with each regime persisting for 5 7 years based upon our analysis of the modeled sea level and ice motion anticyclonic wind driven motion in the central arctic appeared during 1946 1952 1958 1963 1972 1979 and 1984 1988 and cyclonic motion appeared during 1953 1957 1964 1971 1980 1983 and 1989 1993 shifts from one regime to another are forced by changes in the location and intensity of the icelandic low and the siberian high the two regimes may help explain the significant basin scale changes in the arctic s temperature and salinity structure observed recently the great salinity anomaly and the variability of ice conditions in the arctic ocean

pathways and modification of the upper and intermediate waters of the arctic ocean 1 the purpose of this study is to investigate the pathways and the ventilation of source water masses of the upper and intermediate waters of the arctic ocean for the arctic and subarctic domain a coupled ice ocean general circulation model is set up to be integrated for several decades it is driven by a climatological seasonal cycle of monthly mean atmospheric data from 1980 to 1989 and by restored sea surface salinities passive tracers are used to visualize and interpret the modeled flow and to compare it with observations the model is able to reproduce known features of the arctic ocean circulation like the inflow of two branches of atlantic origin via the fram strait and the barents sea and their subsequent passage at middepths in several cyclonic circulation cells the fate of these atlantic source water masses river water and bering strait inflow water in the model are studied the branch crossing the barents sea is subject to an intense heat loss and ice formation as a result water of this branch leaves the shelf toward the central part of the arctic ocean not only at the surface but also in denser varieties which finally feed the central arctic at halocline and middepths the lightest part turns northward and finally westward joining the transpolar drift the densest part 200 1000 m depth moves eastward along the continental slope a similar path is taken by the atlantic water branch from the fram strait the inflowing branch over the barents sea turns out to be the dominant source for the lower atlantic water layer in the arctic ocean in this investigation atlantic tracers starting in fram strait need 6 years to reach the northern laptev sea slope travel times to return to fram strait are 15 20 years along the lomonossov ridge and about 30 years along the continental slope of the canadian basin tracers that mark the pacific water or the mackenzie river water flow eastward and leave the arctic mainly via the canadian archipelago the siberian river water tracers at the surface penetrate far into the canadian basin before they join the transpolar drift the travel times of the river water from the river mouths are 2 3 years to the shelf edge and 12 14 years to fram strait

environmental signal propagation in sedimentary systems across timescales abstract earth surface processes operate across erosionally dominated landscapes and deliver sediment to depositional systems that can be preserved over a range of timescales the geomorphic and stratigraphic products of this source to sink sediment transfer record signals of external environmental forcings as well as internal or autogenic dynamics of the sedimentary system here we evaluate environmental signal propagation across sediment routing systems with emphasis on sediment supply qs as the carrier of up system forcings we review experimental numerical and natural examples of source to sink sediment routing and signal propagation during three timescales 1 historic which includes measurement and monitoring of events and processes of landscape change and deposition during decades to centuries 2 centuries to several millions of years referred to as intermediate timescale and 3 deep time we discuss issues related to autogenic dynamics of sediment transport transient storage and release that can introduce noise lags and or completely mask signals of external environmental forcings we provide a set of conceptual and practical tools for evaluating sediment supply within a source to sink context which can inform interpretations of signals from the sedimentary record these tools include stratigraphic and sediment routing system characterization sediment budgets geochronology detrital mineral analysis e g thermochronology comparative analog approaches and modeling techniques to measure calculate or estimate the magnitude and frequency of external forcings compared to the characteristic response time of the sediment routing systems

from mountain source to ocean sink the passage of sediment across an active margin waipaoa sedimentary system new zealand abstract the joint us nz source to sink initiative aims to unravel the tectonic climatic and other processes that shape the landscape from the mountains to deep ocean of two focus areas chosen world wide the waipaoa sedimentary system wss of north eastern new zealand was identified as an example of a mid latitude high sediment input system it resides within the westerly wind belt and extends across the convergence zone of the australian and pacific plates accordingly the tectonic and climatic drivers are prominent and their influences on the wss are decipherable as is documented in the suite of source to sink studies presented in this special issue of marine geology the wss is forced at several temporal and spatial scales which collectively appear as short term events storms earthquakes and volcanic eruptions superimposed on long term behavioural shifts tectonic uplift and eustatic sea level change overall the wss is undergoing uplift which has been contributing to valley incision since the last glacial maximum uplift is both gradual and intermittent the latter relating to fault and subduction related earthquakes such seismic activity destabilises the landscape and together with frequent storms generates large amounts of sediment as this sediment passes through the system part is captured in traps formed by local subsidence of the coastal plain and middle continental shelf where large thicknesses of holocene and probably older sediment are sequestered e g 200 m under the coastal plain nevertheless substantial sediment still reaches the continental slope to accumulate in canyon heads and slope basins with other deposits redistributed down slope in mass flows major volcanic eruptions also affect the sediment flux via direct input from ash fall and by enhanced erosion following volcanic destruction of terrestrial vegetation frequent storms of varying intensity play a key role in the generation and transfer of sediment major events such as cyclone bola of 1988 inundated the shelf with sediment but identifiable flood deposits were preserved only locally attesting to the effectiveness of shelf physical and biological mixing regimes despite the storms earthquakes and volcanic eruptions the largest perturbation of the wss at least in the last 2400 years has been the human deforestation of the landscape the resultant influx of sediment overwhelmed the shelf s storage capacity instead of capturing 90 the fluvial input as in pre human times the modern shelf retains only 25 the remainder escaping to the slope and elsewhere

results of the sea ice model intercomparison project evaluation of sea ice rheology schemes for use in climate simulations a hierarchy of sea ice rheologies is evaluated on the basis of a comprehensive set of observational data the investigations are part of the sea ice model intercomparison project simip four different sea ice rheology schemes are compared a viscous plastic rheology a cavitating fluid model a compressible newtonian fluid and a simple free drift approach with velocity correction the same grid land boundaries and forcing fields are applied to all models as verification data there are 1 ice thickness data from upward looking sonars uls 2 ice concentration data from the passive microwave radiometers smmr and ssm i 3 daily buoy drift data obtained by the international arctic buoy program iabp and 4 satellite derived ice drift fields based on the 85 ghz channel of ssm i all models are optimized individually with respect to mean drift speed and daily drift speed statistics the impact of ice strength on the ice cover is best revealed by the spatial pattern of ice thickness ice drift on different timescales daily drift speed statistics and the drift velocities in fram strait overall the viscous plastic rheology yields the most realistic simulation in contrast the results of the very simple free drift model with velocity correction clearly show large errors in simulated ice drift as well as in ice thicknesses and ice export through fram strait compared to observation the compressible newtonian fluid cannot prevent excessive ice thickness buildup in the central arctic and overestimates the internal forces in fram strait because of the lack of shear strength the cavitating fluid model shows marked differences to the statistics of observed ice drift and the observed spatial pattern of ice thickness comparison of required computer resources demonstrates that the additional cost for the viscous plastic sea ice rheology is minor compared with the atmospheric and oceanic model components in global climate simulations

the effects of 3d topography on controlled source audio frequency magnetotelluric responses we are investigating the 3d topographic effects on controlled source audio frequency magnetotelluric data two 3d topographic models are considered a trapezoidal hill model and a trapezoidal valley model different responses are generated including the amplitude of electric field amplitude of magnetic field apparent resistivity and phase data the responses distorted by the 3d topography are simulated for the source located both next to and on the hill valley our study indicates that all the electric field magnetic field apparent resistivity and phase data are influenced by 3d topography but to different extents these topographic effects depend on the transmission receiver topography geometry the transmission frequency earth resistivity and the roughness of the surface the effects in the near field generated by topography in the survey area are quite different from those in the far field because of the existence of the source compared to those in the far field zone the magnetic field and pha

numerical simulation of tropical cyclone mora using a regional coupled ocean atmospheric model tropical storms ts are highly sensitive to the behavior of nearby oceans and the only way to improve their predictability is to include all the possible factors contributing to their genesis and motion the present study evaluates the response of air sea coupling on ts mora which occurred during the pre monsoon season of 2017 mora is simulated using a non hydrostatic regional atmospheric modeling system coupled to a regional ocean modeling system model the observations show a low level rossby wave in the atmosphere during the storm genesis there was also evidence of increased sea surface height simultaneously in the form of a downwelling kelvin wave at the west coast of the bay of bengal the present study emphasizes the relationship between near surface oceanic and atmospheric circulation patterns we have tried to elucidate the process with the help of dynamical equations and model solutions we also analyze the pivotal role of the atmospheric rossby wave in the formation of the coastal kelvin wave the kelvin wave seems to produce a warm pool required to form a low pressure system which further gave rise to the tropical storm mora the changes in the temperature and salinity profile within the ocean subsurface layers during storm passage are further used for inferences regarding the vertical structure of the upper ocean due to wind generated mixing the model analysis reveals that the ocean and atmospheric counterparts responsible for storm genesis are well recorded in the coupled model when compared to the observations it was found that air sea coupling in the model made it possible to realistically capture ocean and atmospheric responses

3 dimensional magnetotelluric inversion including topography using deformed hexahedral edge finite elements and direct solvers parallelized on symmetric multiprocessor computers part ii direct data space inverse solution

spatial variability in sedimentary processes on the eel continental slope the northern california continental margin receives episodically large quantities of sediment from the eel river which seasonally discharges to the shelf regional circulation broadcasts this sediment widely not only affecting sedimentation on the shelf but on the slope as well a suite of 60 cores has been collected to examine the processes of sediment accumulation and sediment delivery to the eel continental slope in waters 800 m deep spatial surveys of accumulation rate 0 2 1 3 g cm 2 year 1 and surficial sediment grain size 4 8 5 demonstrate that sediment is accumulating throughout the eel margin reflecting a rapid and widespread redistribution of the fluvial discharge downcore grain size profiles of heterogeneous sediments suggest that hemipelagic and other episodic sediment delivery processes may be important for the slope in areas proximal to the river in more distal areas downcore profiles reveal relatively homogeneous sediments delivered dominantly by hemipelagic processes a budget for fine grained sediment shows that this upper portion of the slope contains at a maximum 20 of the river s annual discharge a combined budget for the shelf and upper slope suggests that at least 60 of the annual sediment load is not accounted for in these areas the eel continental slope because of the large terrigenous sediment input the narrow width of the adjacent shelf and its energetic environment is a good analogue for slopes during transitional stands of sea level and for modern tectonically active settings processes typically associated with these geological settings i e supply of sediment to the slope and downslope mass movement are actively occurring 1999 elsevier science b v all rights reserved

modern accumulation rates and a sediment budget for the eel shelf a flood dominated depositional environment the northern california continental margin is periodically impacted by geologically significant storms which have a marked influence on terrigenous sediment supply flood deposition and long term accumulation of fine grained sediment on the eel shelf accumulation of eel river muds on the adjacent shelf was investigated using 210pb and 137cs geochronologies in order to understand the fate of sediment discharged by the eel river and to relate patterns of net sediment accumulation 100 yr time scale to sediment dynamics 210pb data demonstrate that modern accumulation of river mud occurs from the 50 m isobath seaward across shelf accumulation rates decrease from maximum mid shelf values of 0 6 1 7 g cm 2 yr 1 to values of 0 2 0 4 g cm 2 yr 1 at the shelf break with a spatially weighted mean of 0 5 g cm 2 yr 1 0 4 cm yr for the entire shelf 210pbxs sediment depth profiles from the region of highest accumulation rate are characterized by subsurface intervals of low and uniform activity which are produced by flood deposition in some cores particular 210pbxs activity intervals may be associated with major eel river floods of 1955 1964 and 1974 it is postulated that because of the coincidence of high river flow events and southerly winds during cyclonic winter storms net northward transport allows for preferential deposition of fine grained sediment north of the river mouth over the past 100 years fluvial sediment input combined with marine dispersal processes have produced a mid shelf depocenter evident by both the spatial distribution of 210pb accumulation rates and by clay rich flood layers partially preserved in shelf deposits a fine grained sediment budget for the dispersal system based on hydrological data and 210pb geochronologies demonstrates that a maximum of 20 3 109 kg yr of the mean annual supply of fluvial mud 14 109 kg yr is trapped on the shelf the results of this study demonstrate that 1 short term sedimentation processes associated with floods can influence sediment accumulation on longer time scales and 2 a major fraction of fine grained sediment supplied to tectonically active margins by flood prone mountainous rivers bypasses narrow continental shelves

application of the 3d magnetotelluric inversion code in a geologically complex area the wsinv3dmt code makes the implementation of 3d inversion of magnetotelluric data feasible using a single pc audio magnetotelluric data were collected along two profiles in a cu ni mining area in xinjiang china where the apparent resistivity and phase curves the phase tensors and the magnetic induction vectors indicate a complex 3d conductivity structure 3d inversions were carried out to reveal the electrical structure of the area the final 3d model is selected from the inversion results using different initial lagrange values and steps the relatively low root mean square rms misfit and model norm indicate a reliable electrical model the final model includes four types of low resistivity areas the first ones coincide with the known location of an orebody and further forward modelling indicates that they are not in full connectivity to form a low resistivity zone the second ones are not controlled by magnetotelluric sites and embody little information of the observed data they are considered as tedious structures the third one is near to the regional kangguer fault and should be treated carefully considering the effect of the fault the last ones are isolated and existing at a limited level as the first ones they should be paid more attention to

dynamic monitoring of coalbed methane reservoirs using super low frequency electromagnetic prospecting abstract dynamic monitoring of coalbed methane cbm reservoirs plays an important role in reservoir evaluation production estimation exploitation and development planning in order to efficiently operate producing wells this paper proposes a passive super low frequency slf electromagnetic prospecting and monitoring method which helps us derive electromagnetic radiation emr anomalies from reservoirs to directly identify and dynamically analyze cbm reservoirs the modeling study shows that the slf magnetic responses are sensitive to high resistivity layers these responses turn out to be approximately stationary and can be seen as simply a component of the background field this stationary background field can be clearly distinguished and then dynamic anomaly extraction would be completed in order to suppress cultural noise and high frequency hf random noise the methods of empirical mode decomposition emd and wavelet transform are used in data processing the reconstructed curves are employed to identify emr anomalies at corresponding depths of reservoirs and subsequently help directly interpret and dynamically monitor reservoirs the slf prospecting method is validated using the field data observed from cbm wells in the years from 2007 to 2013 in qinshui basin china the results present that the high emr wave packets contribute conceivably to the cbm reservoir identification compared with audio magnetotelluric amt inversion results the slf identification resolution is greatly improved the dynamic characteristics of producing reservoirs are revealed using emr anomalies and agree with production histories and other surveys

on the record reduction in 1998 western arctic sea ice cover during summer 1998 record reductions in ice cover occurred in the beaufort and chukchi seas open water formed earlier than in prior years and september ice extent in this region was 25 percent less than the previous minimum for 1953 1997 seven percent of the arctic basin that had been perennially ice covered was ice free in 1998 this reduction in western arctic ice extent can be attributed in part to preconditioning by light ice cover in autumn 1997 and to atmospheric circulation patterns during the following winter through autumn that favored southerly and easterly winds such decreases in northerly winds and the associated weakening or displacement of the beaufort gyre is found to be typical of winters that precede years with below normal ice extent in the western arctic

comment on eel river margin source to sink sediment budgets revisited by j a warrick marine geology 351 2014 25 37

three dimensional topography corrections of magnetotelluric data summary topographic effects due to irregular surface terrain may prevent accurate interpretation of magnetotelluric mt data three dimensional 3 d topographic effects have been investigated for a trapezoidal hill model using an edge finite element method the 3 d topography generates significant mt anomalies and has both galvanic and inductive effects in any polarization this paper presents two different correction algorithms which are applied to the impedance tensor and to both electric and magnetic fields respectively to reduce topographic effects on mt data the correction procedures using a homogeneous background resistivity derived from a simple averaging method effectively decrease distortions caused by surface topography and improve the quality of subsurface interpretation nonlinear least squares inversion of topography corrected data successfully recovers most of structures including a conductive or resistive dyke

new geological and hydrogeological implications of the resistivity distribution inferred from audiomagnetotellurics over la fournaise young shield volcano reunion island abstract an audiomagnetotelluric survey has been performed along an inactive flank of la fournaise volcano massif in reunion island to study the subsurface resistivity structure one dimensional modelling of the amt data at each site revealed an extensive low resistivity less than 10 m zone at a few hundred meters below the surface the significance of this unexpected conductive substratum is discussed in relation with the proposed impact of volcano tectonic processes caldera and landslides collapses and a new interpretation of the geological structure is proposed moreover it is likely that these conductors are poorly permeable argilaceous materials then they coincide with a limit in permeability and determine groundwater behavior

mineral exploration using csamt data application to longmen region metallogenic belt guangdong province china abstracta controlled source audio frequency magnetotelluric csamt survey has been carried out to investigate potential iron fe and polymetallic pb zn cu deposits in longmen region which is one of the main metallogenic belts in southern china conducting geophysical surveys in this area is quite difficult due to mountainous terrain dense forest and thick vegetation cover a total of 560 csamt soundings were recorded along twelve surveying lines two dimensional occam s inversion scheme was used to interpret these csamt data the resulting electric resistivity models showed that three large scale highly conductive bodies exist within the surveying area by integrated interpretation combined with available geologic geophysical and geochemical data in this area three prospective mineral deposits were demarcated based on the csamt results a borehole penetrating approximately 250 m depth was drilled at the location of 470 m to the northwest end of line 06 defined with a massive pyrite from the de

advances in the combined interpretation of seismics with magnetotellurics1 five examples obtained during exploration for hydrocarbons in the pannonian basin of hungary are used to show how the interpretation of seismic sections can be usefully complemented by results from mt surveys selection of the most appropriate mt quantities considered to be proper mt attributes for the purpose of visualization as well as recognition of the subsurface structures and the different inversions of mt data is essential for practical integration of seismic and mt surveys a new technique providing a semiquantitive mt attribute pseudosection for the purpose of visualization of the subsurface structures is proposed the procedure utilizes derivative functions of the phase of mt impedance for visualization and derives estimated depths from the bostick transformation of cagniard apparent resistivities on the basis of the mt attribute pseudosections constructed from the phase derivatives and transformed resistivity data depths are estimated for interfaces between geological formations with significant resistivity contrast in particular examples the interface between the tertiary sediments and the older basement rocks as well as tectonic fracture zones with decreased resistivity can be resolved

two dimensional terrain correction in magnetotelluric surveys field distortions caused by topography hamper the interpretation of magnetotelluric mt data topographic features that can be simulated by two dimensional models seriously affect the h polarization results a technique to reduce those effects in mt data uses a finite element program to compute correction coefficients after correction the resulting data can be interpreted as if they were obtained over a flat surface and depended only on the subsurface structure the technique is applied to four examples representative of mt survey targets in high relief terrain results indicate that terrain correction removes the misleading topographic anomalies and improves the quality of subsurface interpretation in regions where the surface relief is two dimensional the correction technique yields some geometrical distortion of the original subsurface structure but the distortion is usually of small importance in practice telluric dipoles of realistic length do not smooth out topographic effects having waveleng

mechanisms in the development of anomalous sea ice extent in the western arctic a case study interannual cycles and trends in arctic ice cover are dominated by strong regional variability arctic sea ice extent exhibited a record minimum within the arctic basin during september 1990 dominated by a rapid retreat in the chukchi east siberian and laptev seas simulations using a coupled regional model reproduced the enhanced cyclonic activity and enhanced ice melt which led to this large retreat in sea ice cover sensitivity experiments showed that accurate initial ice conditions are crucial for a realistic simulation of the ice anomaly pointing to the need for adequate spin up in coupled model experiments it was also found that thermodynamic melt can account for the bulk of the total loss in ice mass over the summer season in 1990 but without dynamics the spatial patterns characteristic of the ice reductions are not reproduced well also transport of ice was crucial to initiate the formation of the anomaly even if the formation was delayed until later in the season the ice albedo feedback is important in allowing the continued formation of the ice anomaly throughout the summer interannual variations in atmospheric circulation that yield regional differences in ice thickness provide a preconditioning that significantly affected development of the ice anomaly this suggests that circulation modes play an important role in determining ice severity along the siberian coast

evidence for a recent change in the link between the north atlantic oscillation and arctic sea ice export evidence for a recent change in the link between the north atlantic oscillation nao and arctic sea ice export through fram strait during wintertime djfm is presented from the analysis of simulated arctic sea ice and observations for the period 1958 1997 whereas no correlation between the two time series is found from 1958 to 1977 r 0 1 the correlation increased significantly thereafter r 0 7 1978 1997 the increased coherency between the nao and arctic sea ice export through fram strait during the last two decades resulted from anomalous meridional wind components near fram strait which were associated with the more easterly position of the nao s centers of interannual variability compared to 1958 1977

sea ice transport a highly variable link between arctic and north atlantic the variability of the ice volume flux into the northeast atlantic is investigated with an optimized dynamic thermodynamic sea ice model using 40 years 1958 1997 of atmospheric forcing fields derived from the ncep ncar reanalysis project the simulated sea ice export from the arctic exhibits considerable interannual to decadal variability and is primarily a linear response to sea level pressure anomalies over greenland and over the barents and kara seas our model results suggest that ice export anomalies such as in 1968 which supposedly caused the so called great salinity anomaly in the northern north atlantic are not unique but rather frequent events as part of the variability of the arctic climate system

geoelectric investigation of the hellenic subduction zone using long period magnetotelluric data abstract the strongest evidence up to date for a subduction zone in the hellenic region is a clearly identified wadati benioff zone below the central aegean sea to a maximum depth of 180 km alternative seismic tomography models suggest that subduction process continues deeper than the wadati benioff zone to a maximum depth of at least 600 km so far the lack of deep electrical studies in the region impeded scientists from imposing other control factors than seismic to the proposed models for the hellenic subduction zone hsz a long period magnetotelluric lmt study was carried out in the southern part of the greek mainland to study the deep electrical characteristics of the hsz and examine whether prominent modelled features correlate with structures identified by the seismic methods the study comprised collection processing and modelling of magnetotelluric mt data in the period range 100 10000 s from ten sites located along a 250 km ne sw trending profile the dimensionality of the data was examined at a pre modelling stage and it was found that they do not exhibit three dimensional 3 d features the latter enabled to construct both one dimensional 1 d and two dimensional 2 d models the proposed geoelectric model for hsz was based on 2 d modelling since it had better maximum depth resolution of about 400 km and revealed structures not detected by 1 d modelling attempts the model structure which was related to the african and euro asian lithosphere is relatively resistive 800 m and has an average thickness of 150 170 km although the bottom of the lithosphere is adequately resolved the wadati benioff zone that delineates the top of the subducting lithospheric slab is not identified by any electrical feature the modelled structure associated with the subducting part of the african lithosphere penetrates a relatively conductive

arctic warming evolution and spreading of the 1990s warm event in the nordic seas and the arctic ocean 1 observations in the arctic ocean revealed changes in oceanic temperature salinity and ice cover of the 1990s as compared with earlier data with a numerical model we favorably reproduce the development and subsequent propagation of temperature anomalies in water of atlantic origin in the 1980s and 1990s these propagated into the arctic ocean via the barents sea and the fram strait two warm anomalies entered the arctic ocean through these passages while the first smaller anomaly only warmed up the western eurasian basin the second large anomaly spread far into the eastern eurasian basin and across the lomonossov ridge into the western arctic basins intensified boundary currents during the high nao state in the first half of the 1990s significantly influenced the amplitude and speed of propagation of the temperature anomalies inside the arctic ocean in contrast to the notion of a continuous warming process during the 1990s our model results suggest the warming of the atlantic layer in the arctic ocean occurred in the form of events the event with the largest anomalous heat input during the modeled period entered the arctic between 1989 and 1994 it is possible to trace back the additional heat input into the arctic to an increased volume inflow via the faroer scotland passage and reduced heat loss to the atmosphere in the early 1990s after a weaker warm inflow in the second half of the 1990s the most recent observations and the model results point to a recurring warm anomaly in the inflow from 1999 onward

a new technique for the incorporation of seafloor topography in electromagnetic modelling summary we describe a new technique for incorporating seafloor topography in electromagnetic modelling it is based on a transformation of the topographic relief into a change in electrical conductivity and magnetic permeability within a flat seafloor since the technique allows us to model arbitrary topographic changes without extra grid cells and any restriction by vertical discretization we can model very precise topographic changes easily without an extra burden in terms of computer memory or calculation time the reliability and stability of the technique are tested by comparing the magnetotelluric responses from two synthetic seafloor topography models with three different approaches these incorporate the topographic change in terms of 1 the change in conductance using the thin sheet approximation 2 a series of rectangular block like steps and 3 triangular finite elements the technique is easily applied to any modelling method including 3d modelling allowing us to model complex structure in the earth while taking full account of the 3d seafloor topography

two dimensional topographic responses in magnetotellurics modeled using finite elements we simulate the magnetotelluric response to two dimensional earth topography using finite elements linear interpolation of the secondary field parallel to strike over triangular elements allows accurate modeling of inclined resistivity boundaries including topographic surfaces to avoid discontinuities in field derivatives or resistivity care must be taken that the nodal values of the field parallal to strike used to obtain the auxiliary secondary fields are kept within uniform earth media the nodal locations may be shifted but the derivatives still are evaluated at the field points of interest correct values may be returned at gentle breaks in slope as well as along straight surfaces the finite element program is verified by comparison with the analytic transverse magnetic response of a hemicylindrical depression and with rayleigh scattering and transmission surface results for transverse electric and transverse magnetic polarization agreement with the other methods generally is excellent with t

3d magnetotelluric modelling including surface topography an edge finite element method has been applied to compute magnetotelluric mt responses to three dimensional 3d earth topography the finite element algorithm uses a single edge shape function at each edge of hexahedral elements guaranteeing the continuity of the tangential electric field while conserving the continuity of magnetic flux at boundaries we solve the resulting system of equations using the biconjugate gradient method with a jacobian preconditioner the solution gives electric fields parallel to the slope of a surface relief that is often encountered in mt surveys the algorithm is successfully verified by comparison with other numerical solutions for a 3d 2 model for comparison of modelling methods for em induction and a ridge model we use a 3d trapezoidal hill model to investigate 3d topographic effects which are caused mainly by galvanic effects not only in the zxy mode but also in the zyx mode if a 3d topography were approximated by a two dimensional topography therefore errors occurring in the transverse electric mode would be more serious than those in the transverse magnetic mode

on the improvement of sea ice models for climate simulations the sea ice model intercomparison project experiments with dynamic thermodynamic sea ice models indicate a strong dependence of the net freezing rate sea ice transport and variability on dynamic model parameters although current dynamic thermodynamic sea ice models show relatively good agreement with observations an optimization seems to be necessary especially for the parameterizations of dynamic processes presently only a few coupled climate models use dynamic thermodynamic sea ice models in order to promote by means of coordinated numerical experiments the development of an optimal sea ice model for climate research the sea ice ocean modelling panel of the arctic climate system study acsys a project of the world climate research programme has initiated the sea ice model intercomparison project simip the first results from this model hierarchy approach are presented

storm induced sediment gravity flows at the head of the eel submarine canyon northern california margin 1 as part of the strataform program a bottom boundary layer bbl tripod was deployed at 120 m depth in the northern thalweg of the eel canyon during winter 2000 increases of the near bottom suspended sediment concentrations ssc recorded at the canyon head were not directly related to the eel river discharge but were clearly linked to the occurrence of storms bbl measurements revealed that during intensifications of the wave orbital velocity sediment transport at the head of the canyon occurred as sediment gravity flows directed down canyon observational evidence for near bed sediment gravity flow transport included an increase toward the bed of the down canyon component of wave averaged velocity and high estimated ssc at higher sampling frequencies 1 hz the current components during these events fluctuated at the same periodicity as the pressure reflecting a clear influence of the surface wave activity on the generation and maintenance of the sediment gravity flows the origin of such flows is not related to the formation of fluid muds on the shelf or to intense wave current sediment resuspension around the canyon head region rather liquefaction of sediment deposited at the head of the canyon induced by wave load excess pore water pressures during storms combined with elevated slopes around the canyon head appear to be the mechanisms initiating sediment transport the resulting fluidized sediment layer can easily be eroded entrained into the water column and transported down canyon as a sediment gravity flow results from this study reveal that storm induced sediment gravity flows occur periodically in the eel canyon head and suggest that this kind of sediment transport process can occur in other submarine canyons more frequently than previously expected

macro anisotropy of the electrical conductivity of the crust a magnetotelluric study of the german continental deep drilling site ktb summary the interpretation of magnetotelluric mt data collected in the vicinity of the german continental drilling project ktb has revealed two major structures in the earth s crust first a large scale mid crustal layer of enhanced conductivity is required to fit the regional observations of the anomalous vertical magnetic field second the mt impedances of the area close to the ktb namely the crustal unit of the zone erbendorf vohenstraus zev support a model with a highly anisotropic upper to middle crust the extremely high values of anisotropy derived from the mt measurements exceed the values of anisotropy observed on rock samples in the laboratory by an order of magnitude and therefore exclude intrinsic features as a source for the observed anisotropy the regional mid crustal conductor is in agreement with central europe wide observations of high conductivity layers in the crust while the proposed model of well conducting subvertical dykes within a resistive host rock explains the mt observations and agrees with the steeply dipping seismic reflectors the downhole and core observations from the drilling and the model of the tectonic development of this area however the superposition of both structures leaves some unanswered questions

shallow karst exploration using mt vlf and dc resistivity methods1 a geophysical test was carried out over a well located and fairly embedded karstic conduit the mt vlf method was selected because of its high resolution and its ability to provide a resistivity parameter sensitive to water and clay this method was used together with dc resistivity methods which allow vertical adjustment of the vlf data and show consistency between the investigation and target depths after correcting the deformations due to the polarization of the primary field the mt vlf data show clearly in the central part of the site that the conduit does not coincide with an anomaly axis but coincides with the boundary between a conductive area and a resistive area 2d modelling confirms that direct detection of the conduit is not feasible and that the conduit is located close to a conductive zone corresponding to a completely clay filled fractured zone this situation was observed on the whole site and the conduit seemed systematically to thread its way between the conductive zones to join the outlet the distribution of the conductive fractured zones and the direction of the hydraulic gradient were two important elements in predicting the location of the conduits a 3d approach would increase the probability of finding the conduits in such a case

adaptive unstructured grid finite element simulation of two dimensional magnetotelluric fields for arbitrary surface and seafloor topography summary we present an adaptive unstructured triangular grid finite element approach for effectively simulating plane wave diffusive electromagnetic fields in 2 d conductivity structures the most striking advantage of irregular grids is their potential to incorporate arbitrary geometries including surface and seafloor topography adaptive mesh refinement strategies using an a posteriori error estimator yield most efficient numerical solutions since meshes are only refined where required we demonstrate the robustness of this approach by comparison with analytical solutions and previously published numerical simulations maximum errors may systematically be reduced to for example 0 8 per cent for the apparent resistivity and 0 2 in the phase an additional accuracy study of the thickness of the air layer in e polarization suggests to keep a minimum thickness depending on lateral conductivity contrasts within the earth furthermore we point out the new quality and flexibility of our simulation technique by addressing two marine magnetotelluric applications in the first case we discuss topographic effects associated with a synthetic sinusoidal sea bottom model and in the second case we show a close to reality scenario using real bathymetry data from the east pacific rise at 17 s

basement involved blind thrusting in the eastern cordillera oriental southern bolivia evidence from cross sectional balancing gravimetric and magnetotelluric data abstract a neogene to recent thrust belt forms the subandean ranges and cordillera oriental along the eastern slope of the central andes in bolivia over much of its length there is an area of intermediate topographic height and structural level between the subandean zone and the cordillera oriental proper cross sectional balancing suggests that in southern bolivia the basal detachment of this tightly folded zone links up with a blind thrust that dips westward beneath the high standing eastern margin of the cordillera oriental and rides piggy back on a large buried thrust sheet which forms the backstop for the thin skinned subandean belt magnetotelluric and gravimetric data indicate that this thrust sheet is made up of basement rocks probably including intrusives and higher grade metamorphics such blind basement thrusts in a relatively external position may characterize much of the eastern andean thrust belt and their geometry is possibly related to preexisting structural patterns linked to the configuration of early palaeozoic basins and their margins

a strong topographic valley effect in amt and vlf r measurements summary the topographic effects of sharply embanked deep and narrow valleys cut in resistive rock are very strong on mt amt and vlf r soundings when the widths of the valley floors are of the same order as the skin depths the electromagnetic responses of such 2 d valley models have been calculated by means of a finite element numerical method for both e and h polarization field configurations the most striking effects occur in the h polarization geometry where the entire current flow must round the bottom of the valley in concentrated bundles thereby creating large electric fields and high apparent resistivities ph with e polarization there is a sharp reduction of both electric and magnetic fields at the centre of the valley but the reduction is stronger for the electric field so the apparent resistivity pe decreases below the matrix resistivity when the bottom of the valley contains a fill of conducting material like sediments or alluviums it is found that both ph and pe are dramatically reduced to values below even the fill resistivity these effects are studied for a variety of ratios between skin depth and width of valley floor

near synchronous and delayed initiation of long run out submarine sediment flows from a record breaking river flood offshore taiwan subsea fiber optic telecommunication cables can break under fast sediment flows that travel 100s of kilometers through the deep ocean in response to earthquakes and submarine landslides similar flows are inferred to form from major river floods whose sediment laden waters plunge and travel along the seabed however the complex initiation of flood related flows and their hazard potential have not been observed until now here we use cable fault data from the gaoping canyon manila trench off taiwan to show that a major river flood formed during typhoon morakot 2009 generated two long run out destructive sediment flows one during peak flood and the other 3 days later the latter flow was more damaging with speeds and run out similar to that of landslide triggered turbidity currents formed in the same catchment if the second flow was due to remobilized canyon sediment it occurred during low earthquake mw 2 0 activity suggesting other triggering mechanisms

the open boundary condition in the united kingdom fine resolution antarctic model abstract the united kingdom fine resolution antartic model fram project it a community program to study the southern ocean central to this is an eddy resolving three dimensional primitive equation ocean general circulation model the open boundary condition at the northern boundary is described here the boundary condition is based on that of stevens 1990

eel river margin source to sink sediment budgets revisited abstract the eel river coastal margin has been used as a representative source to sink sediment dispersal system owing to its steep high sediment yield river and the formation of sedimentary strata on its continental shelf one finding of previous studies is that the adjacent continental shelf retains only 25 of the eel river fine grained sediment less than 63 m discharged over time scales of both individual floods and the 20th century thus suggesting that the eel shelf trapping efficiency is uniquely lower than other similar systems here i provide data and analyses showing that sediment discharge relationships in the eel river have varied strongly with time and include substantial decreases in suspended sediment concentrations during the latter 20th century including these trends in margin wide sediment budgets i show that previous eel river sediment discharge rates were overestimated by a factor of two thus revised sediment budgets shown here reveal that the eel shelf retained 50 of the discharged river fine grained suspended sediment during intensively sampled events of 1995 97 and over the 20th century in light of this hypotheses about high rates of sediment export away from the primary shelf depocenter should be reevaluated

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

the effect of seafloor topography on magnetotelluric fields an analytical formulation confirmed with numerical results summary magnetotelluric fields and impedances are distorted at undulating interfaces an analytical formulation is presented to calculate magnetotelluric effects in the presence of a sinusoidal interface in contrast to previous analytical approaches this formulation is not based on perturbation theory it is applicable to observations both on land and on the seafloor electric and magnetic fields as well as apparent resistivities and phases are calculated on the interface the topographic distortion on land mainly influences the tm mode data where the electric field is perpendicular to the geological strike both the tm mode and the orthogonal te mode data are distorted on the seafloor systematic parameter tests indicate which modes are independent of the period and the conductivity contrast provided that the induction depths are large relative to the amplitude of the topography the differing physics of the seafloor and land surface is illustrated by plotting current streamlines for the land model contour lines diverge below a hill and converge below a valley for the seafloor model electric currents mainly flow in the conductive sea water contour lines converge above a hill and diverge above a valley numerical results derived from finite element modelling support the analytical solutions streamlines of the electric current derived from a model for the central andes illustrate the connection between a graphical display of electromagnetic fields and an algebraic sensitivity analysis of the magnetotelluric impedance tensor