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

on the resource evaluation of marine gas hydrate deposits using sea floor transient electric dipole dipole methods methane hydrates are solid nonstoichiometric mixtures of water and the gas methane they occur worldwide in sediment beneath the sea floor and estimates of the total mass available there exceed 1016kg since each volume of hydrate can yield up to 164 volumes of gas offshore methane hydrate is recognized as a very important natural energy resource the depth extent and stability of the hydrate zone is governed by the phase diagram for mixtures of methane and hydrate and determined by ambient pressures and temperatures in sea depths greater than about 300 m the pressure is high enough and the temperature low enough for hydrate to occur at the seafloor the fraction of hydrate in the sediment usually increases with increasing depth the base of the hydrate zone is a phase boundary between solid hydrate and free gas and water its depth is determined principally by the value of the geothermal gradient it stands out on seismic sections as a bright reflection the diffuse upper boundary is not as well mar

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

estimates of the anomaly threshold in controlled source electromagnetics controlled source electromagnetics csem has been commercially applied for hydrocarbon exploration for more than 10 years the normalized amplitude response nar a common measure for csem anomaly strength is the ratio of the csem response at the target to the background response use of this measure allows anomaly strength to be quantified but leaves open the question of how strong the nar level needs to be before it can be considered significant for hydrocarbon exploration we apply three statistical approaches to determine the appropriate nar threshold for defining a csem anomaly and for estimating the overall significance of the csem method in hydrocarbon exploration the analysis is based on the published data in hesthammer et al 2010a 2010b referred to here as the hesthammer database this database reports nar levels and hydrocarbon exploration outcomes versus the depth to the target for a set of drilled prospects the database authors used an nar level of 1 15 or 15 for their anomaly t

mapping 3d salt using the 2d marine magnetotelluric method case study from gemini prospect gulf of mexico the dominant salt body at gemini prospect gulf of mexico has been analyzed by seismic methods revealing a complex 3d salt volume at depths 1 to 5 km beneath the mud line because of the high contrast in electrical conductivity between the salt and surrounding sediments gemini is an attractive target for electromagnetic interrogation using a broadband magnetotelluric mt sensor package developed at the scripps institution of oceanography data in the period band of 1 to 3000 s were collected at 42 sites in a series of profiles over gemini one of which was directly over a linear ridgelike salt feature striking roughly northwest southeast and another orthogonal to it these two profiles reveal that the strongest mt response arises when the electric field is oriented northeast southwest we test the suitability of 2d inversion of these data for recovering the true salt structure by examining inversions of both actual data and synthetic 3d mt responses derived from the seismically inferred salt volume occam inversions of the northeast southwest component result in resistivity images that generally agree with the seismic data whereas inversions of the complementary component yield significantly poorer fidelity disagreement is greatest 1 2 km along the salt sides and base depth errors for top of salt are less than 500 m although thin deep salt 1k m thick at 5 km depth is not well resolved the inversions reveal a resistive basement and a shallow subseabed environment rich in electrical heterogeneity that is weakly if at all suggested by the seismic data a notable exception is a correlation between a previously uninterpreted seismic reflector and the base of a shallow resistivity anomaly whose presence is consistent with gas accumulation near the hydrate stability zone

polymetallic sulfide exploration on the deep sea floor the feasibility of the mini moses experiment the recent discoveries of polymetallic sulfide deposits on the deep sea floor have created an interest in geophysical techniques for mapping them a magnetometric resistivity method mini moses has been developed for detecting the distribution of sub sea floor electrical resistivity a vertical long wire bipole feeds electric current into the ocean some of the current enters the sea floor and its magnetic field is measured at various distances by a low frequency induction coil magnetometer the method is sensitive to conductive and resistive zones at depth and information about three dimensional resistivity structures can be obtained the small load resistance of an electric transmitter grounded in the ocean allows high current to be driven by a battery powered instrument the induction coil receiver is more sensitive and less noisy than conventional flux gate magnetometers over the selected frequency band this receiver is therefore well adapted to the almost noise free deep sea floor environment b

3d synthetic aperture and steering for controlled source electromagnetics controlled source electromagnetics csem is a geophysical electromagnetic method used to detect hydrocarbon reservoirs in marine settings used mainly as a derisking tool by the industry the applicability of csem is limited by the size of the target low spatial resolution and depth of the reservoir synthetic aperture a technique that increases the size of the source by combining multiple individual sources has been applied to csem fields to increase the detectability of hydrocarbon reservoirs we apply synthetic aperture to a 3d synthetic csem field with a 2d source distribution to evaluate the benefits of the technique the 2d source allows steering in the inline and crossline directions we present an optimized beamforming of the 2d source which increases the detectability of the reservoir with only a portion of three towlines spaced 2 km apart we enhance the anomaly from the target by 80 we also demonstrate the benefits of using the poynting vector to view csem fields in 3d synthetic apert

on electric fields produced by inductive sources on the seafloor abstractthe transient electromagnetic tem method has recently been proposed as a tool for mineral exploration on the seafloor similar to airborne tem surveys conducted on land marine tem systems can use a concentric or coincident wire loop transmitter and receiver towed behind a ship such towed loop tem surveys can be further augmented by placing additional stationary receivers on the seafloor throughout the survey area we examine the electric fields measured by remote receivers from an inductive source transmitter within a 1d layered earth model at sea it is conceivable to deploy either a horizontal transmitter such as the analogous standard airborne configuration or a more exotic vertical transmitter therefore we study and compare the sensitivity of the vertical and horizontal towed loop systems with a variety of seafloor conductivity structures our results indicate that the horizontal loop system is more sensitive to the thickness of a buried conductive layer and would be advantageous over

first results of the moses experiment sea sediment conductivity and thickness determination bute inlet british columbia by magnetometric offshore electrical sounding a static electrical method has been developed to determine the electrical resistivity of crustal rock beneath the sea the transmitter is a vertical long wire bipole extending from the sea surface to the sea floor a commutated current generated on the ship is fed to two large electrodes one near the sea surface the other at the end of a long insulated wire the return path for the current is through the sea and the subjacent crust the receiver is a self contained remote microprocessor controlled magnetometer which is deployed from the ship to the sea floor and subsequently recovered the data are measurements of the azimuthal component of the magnetic field as a function of transmitter receiver horizontal separation the acronym moses has been coined for the method the choice of the name moses is appropriate because the system geometry is carefully arranged to remove many of the adverse effects of the relatively conductive sea water in particular accurate estimates of sea floor resistivity ar

marine csem of the scarborough gas field part 1 experimental design and data uncertainty we describe the planning processing and uncertainty analysis for a marine csem survey of the scarborough gas field off the northwest coast of australia consisting of 20 transmitter tow lines and 144 deployments positioned along a dense 2d profile and a complex 3d grid the purpose of this survey was to collect a high quality data set over a known hydrocarbon prospect and use it to further the development of csem as a hydrocarbon mapping tool recent improvements in navigation and processing techniques yielded high quality frequency domain data data pseudosections exhibit a significant anomaly that is laterally confined within the known reservoir location perturbation analysis of the uncertainties in the transmitter parameters yielded predicted uncertainties in amplitude and phase of just a few percent at close ranges these uncertainties may however be underestimated we introduce a method for more accurately deriving uncertainties using a line of receivers towed twice in opposite directions comparing the residuals for each line yields a gaussian distribution directly related to the aggregate uncertainty of the transmitter parameters constraints on systematic error in the transmitter antenna dip and inline range can be calculated by perturbation analysis uncertainties are not equal in amplitude and phase suggesting that inversion of these data would be better suited in these components rather than in real and imaginary components one dimensional inversion showed that the reservoir and a confounding resistive layer above it cannot be separately resolved even when the roughness constraint is modified to allow for jumps in resistivity and prejudices are provided indicating that this level of detail is beyond the single site csem data further when range dependent error bars are used the resolution decreases at a shallower depth than when a fixed error level is used

and the geophysicist replied which model do you want abstractmarine controlled source electromagnetic csem and magnetotelluric mt soundings were carried out between 1997 and 2003 over the gemini prospect in the gulf of mexico during early development of marine instrumentation the resulting data sets provide a good test bed for examining the effect of the data type and misfit choice on regularized inversion solutions we inverted the data sets individually and jointly for isotropic and anisotropic resistivity at a variety of data misfits we found that multifrequency csem inversions vastly improved structural resolution over single frequency inversions suggesting that variation in skin depth added significant information joint mt and csem inversion appeared to improve resolution over mt only inversions at depths considerably deeper than the csem data can resolve probably by constraining shallow structure in the parts of the model to which the mt data were sensitive the addition of model anisotropy improved data fit but introduced an arbitrary scali

mapping the resistivity structure of walker ridge 313 in the gulf of mexico using the marine csem method abstract a marine controlled source electromagnetic csem campaign was carried out in the gulf of mexico to further develop marine electromagnetic techniques in order to aid the detection and mapping of gas hydrate deposits marine csem methods are used to obtain an electrical resistivity structure of the subsurface which can indicate the type of substance filling the pore space such as gas hydrates which are more resistive results from the walker ridge 313 study wr 313 are presented in this paper and compared with the gulf of mexico gas hydrate joint industry project ii jip2 logging while drilling lwd results and available seismic data the hydrate known to exist within sheeted sand deposits is mapped as a resistive region in the two dimensional 2d csem inversion models this is consistent with the jip2 lwd resistivity results csem inversions that use seismic horizons provide more realistic results compared to the unconstrained inversions by providing sharp boundaries and architectural control on the location of the resistive and conductive regions in the csem model the seismic horizons include 1 the base of the gas hydrate stability zone bghsz 2 the top of salt and 3 the top and bottom of a fine grained marine mud interval with near vertical hydrate filled fractures to constrain the csem inversion model the top of salt provides improved location for brines water saturated salt and resistive salt inversions of the csem data map the occurrence of a halo of conductive brines above salt the use of the bghsz as a constraint on the inversion helps distinguish between free gas and gas hydrate as well as gas hydrate and water saturated sediments

using csem techniques to map the shallow section of seafloor from the coastline to the edges of the continental slope many important processes occur within the shallow section of the seafloor on the continental shelf and slope yet conventional geophysical constraints on the physical properties within this critical boundary layer are limited some of the key constraints involve quantification of fluids within the seafloor which can be provided by electrical methods this paper reviews the application of a towed em system to map the uppermost 20 m of seafloor in a variety of settings ranging from nearshore regions in water depths of approximately 10 m on the continental shelf out to water depths of 1300 m the system is a mapping tool that provides areal maps of seafloor resistivity and has been used for a variety of purposes including sedimentary characterization and facies mapping evaluation of groundwater discharge and mapping seafloor mounds in the gulf of mexico thought to contain massive deposits of gas hydrate

marine magnetotellurics for petroleum exploration part ii numerical analysis of subsalt resolution in areas where seismic imaging of the base of salt structures is difficult seaborne electromagnetic techniques offer complementary as well as independent structural information numerical models of 2 d and 3 d salt structures demonstrate the capability of the marine magnetotelluric mt technique to map the base of the salt structures with an average depth accuracy of better than 10 the mapping of the base of the salt with marine mt is virtually unaffected by internal variation within the salt three dimensional anticlinal structures with a horizontal aspect ratio greater than two can be interpreted adequately via two dimensional inversions marine mt can distinguish between salt structures which possess deep vertical roots and those which do not one measure of the relative accuracy of mt and seismic methods can be made by considering the vertical and lateral position errors in the locations of interfaces caused by neglecting velocity anisotropy in migration for the shallow part of the section where two way travel times are on the order of 1 s the vertical and lateral position errors in the locations of salt sediment interfaces from 2 d mt inversion is more than twice the expected migration error in reflectors in transversely isotropic sediments such as those in the gulf of mexico deeper in the section where two way times are on the order of 4 s lateral position errors in migration become comparable to those of the mt inverse whereas seismic vertical position errors remain more than a factor of two smaller than mt errors this analysis shows that structural mapping accuracy would be improved using mt and seismic together

electrical resistivity structure of the valu fa ridge lau basin from marine controlled source electromagnetic sounding summary in december 1995 we carried out a comprehensive controlled source electromagnetic survey of the valu fa ridge at 22u25k si n the lau basin the valu fa ridge is ab ack arc spreading centre of intermediate spreading rate and is a site of extensive hydrothermal activity seismic studies have imaged a melt lens at an average depth of 3 2 km below the seafloor surrounded by a zone of lowered seismic velocity interpreted as a region of partial melt in the crust the electromagnetic experiment was part of a multidisciplinary study which included wide angle and reflection seismics bathymetry and potential field measurements electromagnetic signals at frequencies between 0 25 and 40 hz were transmitted from a horizontal electric dipole towed close to the seafloor and were recorded by an array of 11 sea bottom receivers at ranges of up to 20 km from the source over 80 hr of data consisting of the magnitude of the horizontal electric field at the seafloor were collected these data have extremely low scatter compared to similar data from previous surveys the data were interpreted using a combination of 1 and 2 d forward modelling and inversion the vertical resistivity gradient in the upper crust at the valu fa ridge is abnormally low with resistivities of less than 10 v m observed throughout layer 2 of the crust to a depth of 3 km this is significantly more conductive at depth than the axis of the slow spreading reykjanes ridge at 57u45kn and the fastspreading east pacific rise at 13un where similar data sets have been collected in the past although the structure of layer 2 is well constrained by the electromagnetic data its extremely low resistivity causes rapid attenuation of electromagnetic signals diffusing through it and hence the data are not sensitive to the structure in layer 3 in particular the structure of the melt lens or surrounding low velocity zone the seismic velocity structure of the valu fa ridge determined from the coincident wide angle seismic study is similar to that observed at other mid ocean ridges with a steep seismic velocity gradient through layer 2 although overall velocities are slightly lower the seismic velocity anomaly calculated relative to an average off axis structure is also small this suggests that the very low resistivities observed at the axis are not caused by an upper crust of abnormally high porosity however hot and or saline fluids permeating the crust can explain the low resistivities without affecting the seismic velocity since the conductive region extends unbroken from 3 km depth to the seafloor it is probable that these fluids circulate to or close to the magma chamber itself

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

electromagnetic surveying of seafloor mounds in the northern gulf of mexico seafloor controlled source electromagnetic data probing the uppermost 30 m of seafloor sediments have been collected with a towed magnetic dipole dipole system across two seafloor mounds at approximately 1300 m water depth in the northern gulf of mexico one of these mounds was the focus of a recent gas hydrate research drilling program rather than the highly resistive response expected of massive gas hydrate within the confines of the mounds the em data are dominated by the effects of raised temperatures and pore fluid salinities that result in an electrically conductive seafloor this structure suggests that fluid advection towards the seafloor is taking place beneath both mounds similar responses are seen at discrete locations away from the mounds in areas that might be associated with faults further suggesting substantial shallow fluid circulation raised temperatures and salinities may inhibit gas hydrate formation at depth as has been suggested at other similar locations in the gulf of mexico

seismically regularized controlled source electromagnetic inversion marine controlled source electromagnetic csem data can be highly sensitive to the presence of resistive hydrocarbon bearing layers in the subsurface yet due to the relatively poor depth resolution of csem data and the smoothness constraints imposed by electromagnetic em inversion methods the resulting resistivity models are often highly smoothed out typically underestimating the reservoir resistivity and overestimating its thickness conversely seismic full waveform inversion fwi can accurately recover the depths of seismic velocity changes yet is relatively insensitive the presence of hydrocarbons in spite of their low depth resolution csem data have been shown to be highly sensitive to the resistivity thickness product of buried resistive layers suggesting that if the thickness of a target layer can be constrained a priori very accurate resistivity estimates may be obtained we developed a method for leveraging the high depth resolution of fwi into a standard csem inversion algorithm so

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

bayesian inversion of marine csem data with a trans dimensional self parametrizing algorithm summary the posterior distribution of earth models that fit observed geophysical data convey information on the uncertainty with which they are resolved from another perspective the non uniqueness inherent in most geophysical inverse problems of interest can be quantified by examining the posterior model distribution converged upon by a bayesian inversion in this work we apply a reversible jump markov chain monte carlo method to sample the posterior model distribution for the anisotropic 1 d seafloor conductivity constrained by marine controlled source electromagnetic data unlike conventional gradient based inversion approaches our algorithm does not require any subjective choice of regularization parameter and it is self parametrizing and trans dimensional in that the number of interfaces with a resistivity contrast at depth is variable as are their positions a synthetic example demonstrates how the algorithm can be used to appraise the resolution capabilities of various electromagnetic field components for mapping a thin resistive reservoir buried beneath anisotropic conductive sediments a second example applies the method to survey data collected over the pluto gas field on the northwest australian shelf a benefit of our bayesian approach is that subsets of the posterior model probabilities can be selected to test various hypotheses about the model structure without requiring further inversions as examples the subset of model probabilities can be viewed for models only containing a certain number of layers or for models where resistive layers are present between a certain interval as suggested by other geological constraints such as seismic stratigraphy or nearby well logs

rapid resistivity imaging for marine controlled source electromagnetic surveys with two transmitter polarizations an application to the north alex mud volcano west nile delta to image the internal resistivity structure of the north alex mud volcano offshore egypt the marine electromagnetics group at the helmholtz centre for ocean research kiel geomar developed and conducted a novel transient marine controlled source electromagnetic experiment the system which was specifically developed to image the mud volcano is also generally suitable for surveys of other small seafloor targets such as gas hydrate reservoirs fluid flow features and submarine massive sulfide deposits an electric bipole antenna is set down by a remotely operated vehicle on the seafloor sequentially in two perpendicular polarizations at each transmission station two orthogonal horizontal electric field components are recorded on the seabed by an array of independently deployed nodal receivers rxs with two transmitter polarizations the unique acquisition geometry of the system provides a very rich data set however for this geometric setup conventional marine electromagnetic interpretation schemes such as normalized magnitude variation with offset plots have been difficult to implement we have developed a simple imaging technique which can be used for a first step mapping of seafloor apparent resistivity with the geomar system images can be produced in just a few minutes on a regular laptop computer and the robustness of the approach was demonstrated using two synthetic data sets from simple seafloor models the method was then applied to the real data acquired at the north alex mud volcano in 2008 results found increased apparent sediment resistivities of up to 4 omega m near the center of the mud volcano occurring at source rx offsets greater than 500 m which mapped to apparent depths of greater than 150 m this may be caused by large quantities of free gas or freshwater in the sediment pore space

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

asymptotic airwave modeling for marine controlled source electromagnetic surveying marine controlled source electromagnetic csem surveying also called seabed logging is a technique for finding resistive layers in the subsurface which can be hydrocarbon saturated reservoirs the em signal from the horizontal electric dipole source induces an airwave along the air sea surface that interferes with the signal from the subsurface and dominates at larger offsets the airwave commonly denotes the energy that propagates from the source via the atmosphere to the receiver on the seabed the airwave component is especially problematic in shallow waters where it is less attenuated during its up and down propagation in the water column than for deeper waters an asymptotically derived space domain equation that describes the airwavecomponent in a water half space bounded by air is well known from the em literature we demonstrate that by taking into account the reflections and reverberations of the airwave component in the water column between the seabed and the sea surface at both the source

marine controlled source electromagnetic of the scarborough gas field part 3 multicomponent 2d magnetotelluric controlled source electromagnetic inversionsmulticomponent inversion

offshore electrical exploration of sedimentary basins the effects of anisotropy in horizontally isotropic layered media our electrical method for exploring beneath the sea consists of a long vertical bipolar ac source extending downward from the sea surface to the bottom of the sea and a remote encapsulated microprocessor controlled magnetometer located on the sea floor the amplitude and phase of the magnetic field are measured over a range of suitable frequencies and transmitter magnetometer separations at the low frequency static limit apparent resistivity curves similar to standard schlumberger resistivity sounding curves are constructed as an aid in the direct interpretation of isotropic crustal resistivity an intermediate relatively resistive or relatively conductive zone is detectable when the transmitter receiver separation exceeds the order of twice the depth to the zone the physical property resolved by the method in an anisotropic crust which has different horizontal and vertical resistivities is the geometric mean of the two independent resistivities the thickness of a layer is indeterminate a la

probabilistic 3 d time lapse inversion of magnetotelluric data application to an enhanced geothermal system summary surface based monitoring of mass transfer caused by injections and extractions in deep boreholes is crucial to maximize oil gas and geothermal production inductive electromagnetic methods such as magnetotellurics are appealing for these applications due to their large penetration depths and sensitivity to changes in fluid conductivity and fracture connectivity in this work we propose a 3 d markov chain monte carlo inversion of time lapse magnetotelluric data to image mass transfer following a saline fluid injection the inversion estimates the posterior probability density function of the resulting plume and thereby quantifies model uncertainty to decrease computation times we base the parametrization on a reduced legendre moment decomposition of the plume a synthetic test shows that our methodology is effective when the electrical resistivity structure prior to the injection is well known the centre of mass and spread of the plume are well retrieved we then apply our inversion strategy to an injection experiment in an enhanced geothermal system at paralana south australia and compare it to a 3 d deterministic time lapse inversion the latter retrieves resistivity changes that are more shallow than the actual injection interval whereas the probabilistic inversion retrieves plumes that are located at the correct depths and oriented in a preferential north south direction to explain the time lapse data the inversion requires unrealistically large resistivity changes with respect to the base model we suggest that this is partly explained by unaccounted subsurface heterogeneities in the base model from which time lapse changes are inferred

first results from a marine controlled source electromagnetic survey to detect gas hydrates offshore oregon 1 submarine gas hydrate is a hazard to drilling a potential hydrocarbon resource and has been implicated as a factor in both submarine slope stability and climate change bulk in situ electrical resistivities evaluated from electromagnetic surveys have the potential to provide an estimate of the total hydrate volume fraction more directly than by using seismic and well log data we conducted a marine controlled source electromagnetic sounding at hydrate ridge oregon usa in august 2004 electromagnetic fields transmitted by a deep towed horizontal electric dipole source were measured by a linear array of 25 seafloor electromagnetic receivers positioned 600 m apart to produce a dense coverage in the recorded electric field data results are presented in simple form by apparent resistivity pseudosections which produce an approximate image of lateral resistivity variations across the study region resistivity values are consistent with those from well logs collected in the area and pseudosection features are correlated with seismic reflectors archie s law based on pseudosection apparent resistivities predicts volumetric hydrate concentrations vary from 0 30 across the ridge

application of the perfectly matched layer in 3 d marine controlled source electromagnetic modelling in this study the complex frequency shifted perfectly matched layer cfs pml in stretching cartesian coordinates is successfully applied to three dimensional 3d frequency domain marine controlled source electromagnetic csem field modelling the dirichlet boundary which is usually used within the traditional framework of em modeling algorithms assumes the electric or magnetic field values are zero at the boundaries this requires the boundaries be sufficiently far away from the sources in the area of interest to mitigate the boundary artifacts a large modelling area may be necessary even though cell sizes are allowed to grow toward the boundaries due to the diffusion of the electromagnetic wave propagation compared with the conventional dirichlet boundary the pml boundary is preferred as the modelling area of interest could be restricted to the target region and only a few absorbing layers surrounding can effectively depress the artificial boundary effect without losing the numerical accuracy furthermore for joint inversion of seismic and marine csem data if we used the pml for csem field simulation instead of the conventional dirichlet the modeling area for these two different geophysical data collected from the same survey area could the same which is convenient for joint inversion grid matching we apply the cfs pml boundary to 3d marine csem modelling by using the staggered finite difference sfd discretization numerical test indicates that the modeling algorithm using the cfs pml also shows good accuracy compared to the dirichlet furthermore the modeling algorithm using the cfs pml shows advantages in computational time and memory saving than that using the dirichlet boundary for the 3d example in this study the memory saving using the pml is nearly 42 and the time saving is around 48 compared to using the dirichlet

marine magnetotellurics for base of salt mapping gulf of mexico field test at the gemini structure a sea floor magnetotelluric mt survey was conducted over the gemini subsalt petroleum prospect in the gulf of mexico gom to demonstrate that the base of salt can be mapped using marine magnetotelluric mmt methods the high contrast in electrical resistivity between the salt and the surrounding sediments provides an excellent target for mmt the gemini salt body located at 28 46 n 88 36 w is a relatively complex shape buried 2 5 km below the sea floor in 1 km deep water its geometry has been previously determined using 3 d seismic prestack depth migration with well log control in order not to confuse limitations in interpretation technique with limitations in data acquisition numerical forward and inverse modeling guided the survey design to locate a profile that would be amenable to 2 d inversion even though the body was clearly 3 d the seismic imaging of the base of salt along the chosen profile is considered good thus providing a good control for testing the mt method in many other ar

the assessment of marine gas hydrates through electrical remote sounding hydrate without a bsr marine gas hydrates prevalent in offshore sediments in canada and japan are a possible hydrocarbon resource a hazard to drilling and a source of a major greenhouse gas quantitative estimates of hydrate concentrations in deep sea sediment are difficult to obtain by conventional methods we have sought novel techniques specifically designed for assessment such as transient electric dipole dipole electromagnetics the latter method is based on the assumption of a reduced electrical conductivity in hydrate rich zones field trails of new apparatus on the cascadia margin have proven successful excellent data were collected and analysed using a differential phase method to reduce systematic error apparent resistivities collected on three lines demonstrate that the resistivity of the seafloor is remarkably uniform over the whole survey area the average hydrate concentration deduced with the aid of a reference model based on the electrical logs of odp holes 888 and 889 is about 17 26 of pore space 9 13 of sediment volume in the 100 m interval above the bsr the values are consistent with those obtained by other analyses further the presence of hydrate is predicted in a region to the east of 889b where there is no visible bsr

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

remote sensing of hydrocarbon layers by seabed logging sbl results from a cruise offshore angola detecting and assessing hydrocarbon reservoirs without the need to drill test wells is of major importance to the petroleum industry seismic methods have traditionally been used in this context but the results can be ambiguous another approach is to use electromagnetic sounding methods that exploit the resistivity differences between a reservoir containing highly resistive hydrocarbons and one saturated with conductive saline fluids modeling presented by eidesmo et al 2002 demonstrates that by using seabed logging sbl a special application of frequency domain controlled source electromagnetic csem sounding the existence or otherwise of hydrocarbon bearing layers can be determined and their lateral extent and boundaries can be quantified such information provides valuable complementary constraints on reservoir geometry and characteristics obtained by seismic surveying

determination of effective grain geometry for electrical modeling of sedimentary rocks accurate electrical modeling of sedimentary rocks is crucial for the interpretation of electrical survey data and must take into account the geometric information of the rock grains we have developed an incremental model for the multiphase electrical conductivity of sedimentary rocks on the theoretical basis of differential effective medium models the developed incremental model was first validated on clean and clay rich sandstones and then applied to simulate the conductivity offive digitally created rock samples with different spectra of grain aspect ratios the conductivity is calculated as a function of water conductivity at full water saturation and an effective grain aspect ratio for a two phase medium is determined to reproduce the electrical behavior of the digital rocks it was found that the effectivegrain aspect ratio can either be determined analytically or estimated from the aspect ratio spectrum a link between the obtained effective grain aspect ratio and the cementation coefficient widely used in petrophysics has been established the cementation coefficient increases with the decrease of the grain aspect ratio increasing the z axis depolarization factor and grain conductivity and the increase of sample porosity it showed that the cementation coefficient can be interpreted as a measure of the connectivity of pores as well as the conductivity of the grains our results gave us new insights into the physical meanings of the grain aspect ratio and cementation coefficient which could be used to facilitate the electrical modeling or interpretation of reservoir rocks

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

petro electric modeling for csem reservoir characterization and monitoring the controlled source electromagnetic csem method has been successfully applied to petroleum exploration however less effort has been made to highlight the applicability of this technique for reservoir monitoring this work appraises the ability of time lapse csem data to detect the changes in fluid saturation during water flooding into an oil reservoir we simulated a poorly consolidated shaly sandstone reservoir based on a prograding near shore depositional environment starting with an effective porosity model simulated by gaussian geostatistics dispersed clay and dual water models were efficiently combined with other well known theoretical and experimental petrophysical correlations to consistently simulate reservoir properties the constructed reservoir model was subjected to numerical simulation of multiphase fluid flow to predict the spatial distributions of fluid pressure and saturation a geologically consistent rock physics model and a modified archie s equation for shaly sandstones were th

the feasibility of reservoir monitoring using time lapse marine csem monitoring changes in hydrocarbon reservoir geometry and pore fluid properties that occur during production is a critical part of estimating extraction efficiency and quantifying remaining reserves we examine the applicability of the marine controlled source electromagnetic csem method to the reservoir monitoring problem by analyzing representative 2d models these studies show that csem responses exhibit small but measureable changes that are characteristic of reservoir depletion geometry with lateral flooding producing a concave up depletion anomaly curve and bottom flooding producing a concave down depletion anomaly curve lateral flooding is also revealed by the spatial temporal variation of the csem anomaly where the edge of the response anomaly closely tracks the retreating edge of the flooding reservoir measureable changes in csem responses are observed when 10 of the resistive reservoir is replaced by conductive pore fluids however to avoid corrupting the relatively small signal changes associated with depletion the acquisition geometry must be maintained to a fraction of a percent accuracy additional factors such as unknown nearby seafloor inhomogeneities and variable seawater conductivity can mask depletion anomalies if not accounted for during repeat monitoring measurements although addressing these factors may be challenging using current exploration csem practices straightforward solutions such as permanent monuments for seafloor receivers and transmitters are available and suggest the method could be utilized with present day technology

two types of marine controlled source electromagnetic transmitters marine controlled source electromagnetic methods are used to derive the electrical properties of a wide range of sub seafloor targets including gas hydrate reservoirs in most marine controlled source electromagnetic surveys the deep tow transmitter is used with a long horizontal electric dipole being towed above the seafloor which is capable of transmitting dipole moments in the order of up to several thousand amperemetres the newly developed deployed transmitter uses two horizontal orthogonal electrical dipoles and can land on the seafloor it can transmit higher frequency electromagnetic signals can provide accurate transmission orientation and can obtain higher signal stacking which compensates for the shorter source dipole length in this paper we present the study key technologies and implementation details of two new marine controlled source electromagnetic transmitters the deep tow transmitter and the deployed transmitter we also present the results of a marine controlled source electromagnetic experiment conducted from april to may 2014 in the south china sea using both the deep tow transmitter and the deployed transmitter which show that the two types of marine transmitters can be used as effective source for gas hydrate exploration

bayesian inversion of marine controlled source electromagnetic data offshore vancouver island canada this paper applies nonlinear bayesian inversion to marine controlled source electromagnetic csem data collected near two sites of the integrated ocean drilling program iodp expedition 311 on the northern cascadia margin to investigate subseafloor resistivity structure related to gas hydrate deposits and cold vents the cascadia margin off the west coast of vancouver island canada has a large accretionary prism where sediments are under pressure due to convergent plate boundary tectonics gas hydrate deposits and cold vent structures have previously been investigated by various geophysical methods and seabed drilling here we invert time domain csem data collected at sites u1328 and u1329 of iodp expedition 311 using bayesian methods to derive subsurface resistivity model parameters and uncertainties the bayesian information criterion is applied to determine the amount of structure number of layers in a depth dependent model that can be resolved by the data the parameter space is sampled with the metropolis hastings algorithm in principal component space utilizing parallel tempering to ensure wider and efficient sampling and convergence nonlinear inversion allows analysis of uncertain acquisition parameters such as time delays between receiver and transmitter clocks as well as input electrical current amplitude marginalizing over these instrument parameters in the inversion accounts for their contribution to the geophysical model uncertainties one dimensional inversion of time domain csem data collected at measurement sites along a survey line allows interpretation of the subsurface resistivity structure the data sets can be generally explained by models with 1 to 3 layers inversion results at u1329 at the landward edge of the gas hydrate stability zone indicate a sediment unconformity as well as potential cold vents which were previously unknown the resistivities generally increase upslope due to sediment erosion along the slope inversion results at u1328 on the middle slope suggest several vent systems close to bullseye vent in agreement with ongoing interdisciplinary observations

a numerical comparison of time and frequency domain marine electromagnetic methods for hydrocarbon exploration in shallow water in shallow water the frequency domain controlled source electromagnetic method is subject to airwave saturation that strongly limits the sensitivity to resistive hydrocarbon targets at depth it has been suggested that time domain csem may offer an improved sensitivity and resolution of these deep targets in the presence of the airwave in order to examine and test these claims this work presents a side by side investigation of both methods with a main focus on practical considerations and how these effect the resolution of a hydrocarbon reservoir synthetic noisy data for both time domain and frequency domain methods are simulated using a realistic frequency dependent noise model and frequency dependent scaling for representative source waveforms the synthetic data studied here include the frequency domain response from a compact broadband waveform the time domain step response from a low frequency square wave and the time domain impulse response obtained from pseudo random binary sequences these data are used in a systematic resolution study of each method as a function of water depth relative noise and stacking length the results indicate that the broadband frequency domain data have the best resolution for a given stacking time whereas the time domain data require prohibitively longer stacking times to achieve similar resolution

co2 storage in the high arctic efficient modelling of pre stack depth migrated seismic sections for survey planning the sequestration of co2 in subsurface reservoirs constitutes an immediate countermeasure to reduce anthropogenic emissions of co2 now recognized by international scientific panels to be the single most critical factor driving the observed global climatic warming to ensure and verify the safe geological containment of co2 underground monitoring of the co2 site is critical in the high arctic environmental considerations are paramount and human impact through for instance active seismic surveys has to be minimized efficient seismic modelling is a powerful tool to test the detectability and imaging capability prior to acquisition and thus improve the characterization of co2 storage sites taking both geological setting and seismic acquisition set up into account the unique method presented here avoids the costly generation of large synthetic data sets by employing point spread functions to directly generate pre stack depth migrated seismic images we test both a local target approach using an analytical filter assuming an average velocity and a full field approach accounting for the spatial variability of point spread functions we assume a hypothetical co2 plume emplaced in a sloping aquifer inspired by the conditions found at the university of svalbard co2 lab close to longyearbyen svalbard norway constituting an unconventional reservoir cap rock system using the local target approach we find that even the low to moderate values of porosity 5 18 measured in the reservoir should be sufficient to induce significant change in seismic response when co2 is injected the sensitivity of the seismic response to changes in co2 saturation however is limited once a relatively low saturation threshold of 5 is exceeded depending on the illumination angle provided by the seismic survey the quality of the images of five hypothetical co2 plumes of varying volume differs depending on the steepness of their flanks when comparing the resolution of two orthogonal 2d surveys to a 3d survey we discover that the images of the 2d surveys contain significant artefacts the co2 brine contact is misplaced and an additional reflector is introduced due to the projection of the point spread function of the unresolvable plane onto the imaging plane all of these could easily lead to a misinterpretation of the behaviour of the injected co2 our workflow allows for testing the influence of geological heterogeneities in the target aquifer igneous intrusions faults pervasive fracture networks by utilizing increasingly complex and more realistic geological models as input as more information on the subsurface becomes available

a land based controlled source electromagnetic method for oil field exploration an example from the schoonebeek oil field controlled source electromagnetic csem data are sensitive to the subsurface resistivity distribution but 3d inversion results are ambiguous and in depth interpretation is challenging resolution and sensitivity analysis as well as the influence of noise on resolution have been used to quantify 3d inversion performance based on these numerical studies a land based csem survey was designed and carried out at the schoonebeek oil field the netherlands the acquired data were processed and subsequently inverted for the resistivity distribution the 1d and 3d inversion of horizontal electric field data show the reservoir at the right depth matching well log data without using a priori knowledge about the actual reservoir depth we used a 1d model with fine layering as a starting model for 3d inversion synthetic data inversions and sensitivity tests demonstrate that resistive or conductive bodies inside the reservoir zone may be well detectable with our limited acquisition geometry spatial variations in the reservoir resistivity are visible in the measured data and after inversion by assuming good knowledge of the background resistivity distribution the reservoir resistivity and size however have to be interpreted with care considering the intrinsically low resolution of electromagnetic em which is further reduced by manmade em noise

preliminary interpretation of electromagnetic heat flow seismic and geochemical data for gas hydrate distribution across the porangahau ridge new zealand porangahau ridge located offshore the wairarapa on the hikurangi margin is an active ocean continent collision region in northeastern new zealand coastal waters bottom simulating reflections bsrs in seismic data indicate the potential for significant gas hydrate deposits across this part of the margin beneath porangahau ridge a prominent high amplitude reflection band has been observed to extend from a deep bsr towards the seafloor review of the seismic data suggest that this high amplitude band is caused by local shoaling of the base of gas hydrate stability due to advective heat flow and it may constitute the location of elevated gas hydrate concentrations during r v tangaroa cruise tan0607 in 2006 heat flow probing for measurements of vertical fluid migration sediment coring for methane concentrations and additional seismic profiles were obtained across the ridge in a subsequent 2007 expedition on r v sonne cruise so191 a controlled source electromagnetic csem experiment was conducted along the same seismic geochemical and heat flow transect to reveal the electrical resistivity distribution csem data highlight a remarkable coincidence of anomalously high resistivity along the western landward flank of the ridge which point to locally higher gas hydrate concentration above the high amplitude reflection band measured sediment temperature profiles also along the western flank consistently show non linear and concave geothermal gradients typical of advective heat flow geochemical data reveal elevated methane concentrations in surface sediments concomitant with a rapid decline in sulfate concentrations indicating elevated methane flux and oxidation of methane in conjunction with sulfate reduction at the landward ridge base together these data sets suggest that the western rim of porangahau ridge is a tectonically driven zone of rising fluids that transport methane and cause an upward inflection of the base of gas hydrate stability and the formation of locally enriched gas hydrate above the reflective zone

a first application of a marine inductive source electromagnetic configuration with remote electric dipole receivers palinuro seamount tyrrhenian sea we study a new marine electromagnetic configuration which consists of a ship towed inductive source transmitter and a series of remote electric dipole receivers placed on the seafloor the approach was tested at the palinuro seamount in the southern tyrrhenian sea at a site where massive sulfide mineralization has been previously identified by shallow drilling a 3d model of the palinuro study area was created using bathymetry data and forward modeling of the electric field diffusion was carried out using a finite volume method these numerical results suggest that the remote receivers can theoretically detect a block of shallowly buried conductive material at up to 100 m away when the transmitter is located directly above the target we also compared the sensitivity of the method using either a horizontal loop transmitter or a vertical loop transmitter and found that when either transmitter is located directly above the mineralized zone the vertical loop transmitter has sensitivity to the target at a farther distance than the horizontal loop transmitter in the broadside direction by a few 10s of meters furthermore the vertical loop transmitter is more effective at distinguishing the seafloor conductivity structure when the vertical separation between transmitter and receiver is large due to the bathymetry as a horizontal transmitter is logistically easier to deploy we conducted a first test of the method with a horizontal transmitter apparent conductivities are calculated from the electric field transients recorded at the remote receivers the analysis indicates higher apparent seafloor conductivities when the transmitter is located near the mineralized zone forward modeling suggests that the best match to the apparent conductivity data is obtained when the mineralized zone is extended southward by 40 m beyond the zone of previous drilling our results demonstrate that the method adds value to the exploration and characterization of seafloor massive sulfide deposits

trans dimensional bayesian inversion of controlled source electromagnetic data in the german north sea this paper presents the first controlled source electromagnetic survey carried out in the german north sea with a recently developed seafloor towed electrical dipole dipole system i e hydra ii controlled source electromagnetic data are measured processed and inverted in the time domain to estimate an electrical resistivity model of the sub seafloor the controlled source electromagnetic survey targeted a shallow phase reversed seismic reflector which potentially indicates free gas to compare the resistivity model to reflection seismic data and draw a combined interpretation we apply a trans dimensional bayesian inversion that estimates model parameters and uncertainties and samples probabilistically over the number of layers of the resistivity model the controlled source electromagnetic data errors show time varying correlations and we therefore apply a non toeplitz data covariance matrix in the inversion that is estimated from residual analysis the geological interpretation drawn from controlled source electromagnetic inversion results and borehole and reflection seismic data yield resistivities of 1 m at the seafloor which are typical for fine grained marine deposits whereas resistivities below 20 mbsf increase to 2 4 m and can be related to a transition from fine grained holocene age to unsorted coarse grained and compacted glacial sediments pleistocene age interface depths from controlled source electromagnetic inversion generally match the seismic reflector related to the contrast between the different depositional environments resistivities decrease again at greater depths to 1 m with a minimum resistivity at 300 mbsf where a seismic reflector that marks a major flooding surface of late miocene age correlates with an increased gamma ray count indicating an increased amount of fine grained sediments we suggest that the grain size may have a major impact on the electrical resistivity of the sediment with lower resistivities for fine grained sediments concerning the phase reversed seismic reflector that was targeted by the survey controlled source electromagnetic inversion results yield no indication for free gas below it as resistivities are generally elevated above the reflector we suggest that the elevated resistivities are caused by an overall decrease in porosity in the glacial sediments and that the seismic reflector could be caused by an impedance contrast at a thin low velocity layer controlled source electromagnetic interface depths near the reflector are quite uncertain and variable we conclude that the seismic interface cannot be resolved with the controlled source electromagnetic data but the thickness of the corresponding resistive layer follows the trend of the reflector that is inclined towards the west

detection and imaging sensitivity of the marine csem method we have developed a formalism to systematically study the sensitivity of the marine controlled source electromagnetic method for hydrocarbon exploration taking into account measurement errors we utilize error propagation to estimate the data uncertainty and find that contributions that scale by the transmitter dipole moment give qualitatively different behavior than noise contributions that are independent of the dipole moment the uncertainty is used in quantitative criteria to determine if a hydrocarbon reservoir can be detected and whether it can be successfully imaged our formalism can be used to make detailed feasibility studies incorporating equipment accuracy limitations and to identify which experimental component that must be improved to ensure overall improvement in sensitivity the criteria can be applied to identify the limiting factors for detection and imaging of targets with increasing burial depth we have defined experimental accuracy conditions for next generation csem equipment to successfully image a large hydrocarbon reservoir at 5 km burial depth

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

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

controlled source electromagnetic sounding in shallow water principles and applications the marine controlled source electromagnetic csem method is being applied to the problem of detecting and characterizing hydrocarbons in a variety of settings until recently its use was confined to deepwater water depths greater than approximately 300 m because of the interaction of signals with the atmosphere in shallower water depths the purpose of this study was to investigate using a simple 1d analytical analysis the physics of csem in shallow water this approach demonstrates that it is difficult to simply decouple signals that have interacted with the earth from those that have interacted with the air using either frequency domain or time domain methods stepping away from wavelike approaches which if applied without care can be misleading for the diffusive fields of csem we demonstrate an effective way to mitigate the effect of the air in shallow water surveys by decomposing the em signal into modes and using only the mode least affected by interaction with the atmosphere such decompos

3d marine magnetotelluric inversion a hybrid impedance and direct field approach abstractthe measured electromagnetic field in magnetotellurics mt is composed of the natural source field and its subsurface response commonly the data are represented as impedances the complex ratio between the horizontal electric and magnetic fields this measure is independent of the source distribution because the impedance tensor estimation contains a deconvolution operator we have used a gauss newton type 3d mt inversion scheme to compare impedance data inversion with an inversion using the recorded electric field directly the use of the observed electric field is beneficial to the inversion algorithm because it simplifies the estimation of the sensitivities the direct field approach permits the use of the observed data without processing but it presumes knowledge of the source distribution a method to estimate the time variable strength and polarization of the incoming plane wave source is presented and tested on synthetic and real data examples the direct field inversion is successfully

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

controlled source electromagnetic monitoring of reservoir oil saturation using a novel borehole to surface configuration to advance and optimize secondary and tertiary oil recovery techniques it is essential to know the areal propagation and distribution of the injected fluids in the subsurface we investigate the applicability of controlled source electromagnetic methods to monitor fluid movements in a german oilfield bockstedt onshore northwest germany as injected brines highly saline formation water have much lower electrical resistivity than the oil within the reservoir the main focus of this study is on controlled source electromagnetic simulations to test the sensitivity of various source receiver configurations the background model for the simulations is based on two dimensional inversion of magnetotelluric data gathered across the oil field and calibrated with resistivity logs three dimensional modelling results suggest that controlled source electromagnetic methods are sensitive to resistivity changes at reservoir depths but the effect is difficult to resolve with surface measurements only resolution increases significantly if sensors or transmitters can be placed in observation wells closer to the reservoir in particular observation of the vertical electric field component in shallow boreholes and or use of source configurations consisting of combinations of vertical and horizontal dipoles are promising preliminary results from a borehole to surface controlled source electromagnetic field survey carried out in spring 2014 are in good agreement with the modelling studies

the practical application of 2d inversion to marine controlled source electromagnetic data an algorithm is presented for the inversion of marine controlled source electromagnetic csem data that uses a 2d finite difference fd forward driver this code is demonstrated by inverting a csem data set collected at hydrate ridge oregon consisting of 25 seafloor sites recording a 5 hz transmission frequency the sites are located across a bathymetric high with variations in water depth of 300 m along the 16 km profile to model this complex seafloor bathymetry accurately the fd grid was designed by careful benchmarking using a different 2d finite element fe forward code a comparison of the fe and fd forward model solutions verifies that no features in the inversion are due to inaccuracies of the fd grid the inversion includes the local seawater conductivity depth profile as recorded by the transmitter s conductivity temperature depth gauge because seawater conductivity is known to have a significant effect on the csem responses an apparent resistivity pseudosection of the csem data resemb

navigating marine electromagnetic transmitters using dipole field geometry the marine controlled source electromagnetic csem technique has been adopted by the hydrocarbon industry to characterize the resistivity of targets identified from seismic data prior to drilling over the years marine controlled source electromagnetic has matured to the point that four dimensional or time lapse surveys and monitoring could be applied to hydrocarbon reservoirs in production or to monitor the sequestration of carbon dioxide marine controlled source electromagnetic surveys have also been used to target shallow resistors such as gas hydrates these novel uses of the technique require very well constrained transmitter and receiver geometry in order to make meaningful and accurate geologic interpretations of the data current navigation in marine controlled source electromagnetic surveys utilize a long base line or a short base line acoustic navigation system to locate the transmitter and seafloor receivers if these systems fail then rudimentary navigation is possible by assuming the transmitter follows in the ship s track however these navigational assumptions are insufficient to capture the detailed orientation and position of the transmitter required for both shallow targets and repeat surveys in circumstances when acoustic navigation systems fail we propose the use of an inversion algorithm that solves for transmitter geometry this algorithm utilizes the transmitter s electromagnetic dipole radiation pattern as recorded by stationary close range 1000 m receivers in order to model the geometry of the transmitter we test the code with a synthetic model and validate it with data from a well navigated controlled source electromagnetic survey over the scarborough gas field in australia

methane hydrates in controlled source electromagnetic surveys analysis of a recent data example data from a recently acquired sea bed logging deep water survey are analysed for resistive bodies at depths below mudline shallower than about 300 m a model consistent with known methane hydrate properties is found to explain near offset structures over an offset scale of a few hundred metres observed in the data the lateral near seabed resolution of the sea bed logging method was determined to less than 100 m for source frequencies of up to 10 hz the importance of accurate hydrate maps to improve data processing is demonstrated by placing synthetic reservoirs below hydrates and observing their effects on reference model processing the phase is shown to be less perturbed by shallow resistors than the amplitude which is an important quality control of standard anomaly maps while patchy shallow resistors can generally be mapped with simple normalized magnitude versus offset and phase versus offset difference analyses large area distributions of hydrates over kilometres are hard to distinguish from deeper structures using controlled source electromagnetic data only short of conducting a full 3d inversion of a sufficiently large survey beyond the study confirms the applicability of controlled source electromagnetic techniques in general to map shallow resistive structures for drilling hazards and possible future exploration of methane hydrates as an energy source

exploration of permafrost with audiomagnetotelluric data for gas hydrates in the juhugeng mine of the qilian mountains chinagas hydrate exploration using amt

alternating joint inversion of controlled source electromagnetic and seismic data using the joint total variation constraint an alternating joint inversion method for controlled source electromagnetic csem and seismic data is developed the structural constraint is used for correlating and constraining the electromagnetic em resistivity and seismic velocity parameters during the inversion the structural coupling used is the joint total variation jtv constraint which is incorporated into the objective function of the individual em or seismic inversion to enforce the structural similarity between the resistivity and velocity in this paper the conventional cross gradient constraint is not preferred as it can only be used for enforcing structural similarity for 2 d or 3 d case since it is always zero for 1 d joint inversion the jtv constraint can be applied for 1 d joint inversion as well as for the 2 d 3 d case which is of a broader interest the improved gauss newton gn is used for minimizing the objective function and for reconstructing the subsurface resistivity and velocity the alternating joint inversion algorithm is applied for integrating land csem data with cross well seismic data for subsurface reservoir evaluation and water oil identification numerical examples show that the developed joint inversion can improve the inversion results significantly over those from the separate em or seismic inversion

decomposition in upgoing and downgoing fields and inversion of marine csem data abstractwe describe a data driven method to estimate the top formation resistivity using marine controlled source electromagnetic data the procedure exploits the fact that the airwave contributions are traveling up or down with a poynting vector close to the vertical axis the proposed method is based on forming an impedance in the same way as it is done in magnetotelluric processing the top formation resistivity is used to perform up down decomposition of the electric field below the seabed this procedure suppresses the contribution from the airwave in the resulting upgoing electric field and increases the data sensitivity to the subsurface resistivity distribution the upgoing electric field is used in the misfit kernel for an inversion scheme this is a method that is intended for use in shallow water inversion using the upgoing electric field means that the observed electric and magnetic fields contribute to the data misfit the optimization procedure seeks to reduce the misfit between the observe

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

sensitivity study of marine csem data for reservoir production monitoring using numerical and analytical modeling we assess the feasibility of marine controlled source electromagnetic csem data for monitoring the flooding front during water flooding of an oil reservoir we discuss the ability of time lapse csem data to resolve changes in the electric conductivity small enough to be of interest for monitoring purposes measurement and modeling errors are discussed briefly and analytical calculations comparing time lapse signals with a certain type of time lapse modeling errors are performed numerical calculations performed with a volume integral equation method are used to study the effect on the electromagnetic em fields of relevant conductivity changes the numerical investigation includes robustness with respect to survey parameter values discrimination between different front shapes and ability to overcome time lapse error simulated signals are found to be strong enough to overtake typical measurement errors and are fairly robust toward perturbations of survey parameters it is found analytically and numerically that certain modeling errors experience a high degree of time lapse cancellation

decomposition of electromagnetic fields into upgoing and downgoing components this paper gives a unified treatment of electromagnetic em field decomposition into upgoing and downgoing components forconductiveandnonconductivemedia wheretheelectromagneticdataaremeasuredonaplaneinwhichtheelectricpermittivity magneticpermeability andelectricalconductivityareknown constants with respect to space and time above and below the plane of measurement the medium can be arbitrarily inhomogeneousandanisotropic in particular the proposed decomposition theory applies to marine em low frequency data acquired for hydrocarbon mapping where the upgoing components of the recordedfield guided and refracted from the reservoir that are of interest for the interpretation the direct source field the refracted airwave induced by the source the reflected field from the sea surface and most magnetotelluric noise traveling downward just below the seabed are field components that are considered to be noise in electromagneticmeasurements the viability and validity of the decomposition method is demonstrated using modeled and real marine em data also termed seabed logging sbl data the synthetic data are simulated in a model that is fairly representative of the geologic area wheretherealsblwerecollected theresultsfromthesynthetic data study therefore are used to assist in the interpretation of the realdatafromanareawith320 mwaterdepthaboveaknowngas province offshore norway the effect of the airwave is seen clearly in measured data after field decomposition just below the seabed the upgoing component of the recorded electric field has almost linear phase indicating that most of the effect of the airwavecomponenthasbeenremoved

elastic velocities of partially gas saturated unconsolidated sediments abstract fluid in sediments significantly affects elastic properties of sediments and gas in the pore space can be identified by a marked reduction of p wave velocity or a decrease of poisson s ratio the elastic properties of gas saturated sediments can be predicted by the classical biot gassmann theory bgt however parameters for the bgt such as the biot coefficient or moduli of dry frame of unconsolidated and high porosity sediments are not readily available dependence of velocities on differential pressure or porosity for partially gas saturated sediments is formulated using properties derived from velocities of water saturated sediments laboratory samples for unconsolidated and consolidated sediments and well log data acquired for unconsolidated marine sediments agree well with the predictions however because the p wave velocity depends highly on how the gas is saturated in the pore space such as uniform or patch the amounts of gas estimated from the p wave velocity contains high uncertainty the modeled vp vs ratio of partially gas saturated sediment using the patch distribution is usually greater than 1 6 whereas the ratio modeled assuming a uniform distribution is about 1 6 thus poisson s ratio or vp vs ratio may be used to differentiate patch from uniform saturation but differences between various models of patch saturation cannot be easily identified

hypersaline benthic foraminifera from the shuaiba lagoon eastern red sea saudi arabia their environmental controls and usefulness in sea level reconstruction abstract the shuaiba lagoon is a fossil back reef hypersaline small basin located 80 km south of jeddah city on the eastern red sea coast saudi arabia the surface sediments of the lagoon were investigated for their benthic foraminiferal content in order to correlate in general with environmental factors such as temperature salinity ph sediment grain size organic matter and in particular with tidal elevations to develop a training set for predicting sea level changes in the lagoon hierarchical cluster analysis divided the benthic foraminifera in the shuaiba lagoon into four distinct faunal assemblages quinqueloculina cf q limbata assemblage 1 and monalysidium acicularis assemblage 2 assemblages dominated the intertidal high subtidal areas 0 3 to 0 5 m lat the peneroplis planatus sorites orbiculus assemblage 3 occurred abundantly at all subtidal elevations 0 to 1 m lat whereas the quinqueloculina costata spiroloculina communis elphidium striatopunctatum assemblage 4 dominated the lowest elevations

detecting hydrocarbon reservoirs from csem data in complex settings application to deepwater sabah malaysia controlled source electromagnetic csem field surveys offer a geophysical method to discriminate between high and low hydrocarbon saturations in a potential reservoir however the same geological processes that create the possible hydrocarbon reservoir may also create topography and near surface variations of resistivity e g shallow gas or hydrates that can complicate the interpretation of csem data in this paper we discuss the interpretation of such data over a thrust belt prospect in deepwater sabah malaysia we show that detailed modeling of the key scenarios can help us understand the contributions of topography near surface hydrates and possible hydrocarbons at reservoir depth complexity at the surface and at depth requires a 3d electromagnetic modeling code that can handle realistic ten million cell models this has been achieved by using an iterative solver based on a multigrid preconditioner finite difference approach with frequency dependent grid adaptation

a marine electromagnetic survey to detect gas hydrate at hydrate ridge oregon gas hydrates are a potential energy resource and hazard for drilling and infrastructure yet estimates of global volume vary by over three orders of magnitude hydrates are electrically resistive compared to water saturated sediment and so electromagnetic methods provide an additional tool to seismic surveys and drilling for determining hydrate saturations a marine electromagnetic survey was carried out at hydrate ridge oregon usa with the aim of testing the use of controlled source electromagnetic csem and magnetotelluric mt methods to map gas hydrate and free gas below the gas hydrate stability zone a 2 d csem inversion supports the scenario deduced from previous seismic and drilling results which indicate two mechanisms of hydrate emplacement a transport dominated and reaction dominated regime a prominent resistive region of 2 5 4 m at a depth of about 130 mbsf near the seismic bottom simulating reflector bsr suggests that 27 to 46 per cent of the bulk volume is filled with hydrate depending on whether archie s law or the hashin strikman bounds are used this is representative of a reaction dominated regime for hydrate emplacement and where a significant low velocity zone exists based on a seismic tomography inversion suggests large quantities of free gas below the bsr electrical resistivity logging while drilling lwd data show general agreement with the csem inversion model except for a csem derived resistive region at seismic horizon a known to transport free gas into the gas hydrate stability zone inversion of mt data collected simultaneously during the csem survey provides a complimentary low resolution image of the shallow sediments and shows folding in the accretionary complex sediments similar to that imaged by a tomographic seismic velocity model

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

estimation of seafloor electromagnetic receiver orientation abstractif the orientation of a seafloor electromagnetic em instrument is unknown it has to be estimated afterwards relatively early in the processing here we present a method to estimate the horizontal and vertical orientation of seafloor electromagnetic receivers three component magnetic recordings of the naturally occurring magnetotelluric field are utilized to find the tilt pitch and azimuth angle relative to a given reference estimation of angles is controlled by minimum difference criteria precautionary measures are necessary to eliminate the subsurface response in the recordings of the em field components the proposed method is applied to synthetic and real data demonstrating that even in difficult geologic settings the simple characteristics of the incident natural em field allow us to quickly estimate the orientation of the instruments

modelling the low frequency electrical properties of pyrite bearing reservoir sandstones abstract marine controlled source electromagnetic sounding has developed rapidly in recent years to complement conventional seismic method for better discrimination of pore fluid to obtain quantitative and reliable interpretation of the csem data a robust rock physics model is vital to link the bulk rock electrical conductivity to the electrical properties and microstructure of the rock constituents especially when some of the constituents exhibit extreme electrical behaviours e g pyrite a common mineral associated with reservoir rocks based on the multi phase incremental model validated on published experimentally measured electrical conductivity of pyrite bearing sandstones the effects of key parameters i e pyrite content porosity pyrite conductivity grain aspect ratio and water saturation on the electrical conductivity of pyrite bearing sandstones and their corresponding csem responses were comprehensively studied the results are expected to assist in the csem data interpretation when a pyrite bearing sandstone reservoir is encountered in the future

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

glacier and lake variations in the yamzhog yumco basin southern tibetan plateau from 1980 to 2000 using remote sensing and gis technologies glacier and lake variations in the yamzhog yumco basin in southern tibet were studied by integrating series of spatial data from topographic maps and landsat images at three different times 1980 1988 90 and 2000 the results indicate that the total glacier area has decreased from 218 km 2 in 1980 to 215 km 2 in 2000 a total reduction of 3 km 2 i e a 1 5 decrease glacier recession rates were clearly larger in the 1990s than the 1980s due to the warmer climate the total lake area decreased by about 67 km 2 during 1980 90 and increased by 32 km 2 during 1990 2000 it is suggested that change of lake area in the basin was rapid and most likely caused primarily by the change in precipitation and evaporation in the basin and secondarily by the increased water supply from melting glaciers