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

gas migration into gas hydrate bearing sediments on the southern hikurangi margin of new zealand we present multichannel seismic data from new zealand s hikurangi subduction margin that show widespread evidence for gas migration into the field of gas hydrate stability gas migration along stratigraphic layers into the hydrate system manifests itself as highly reflective segments of dipping strata that originate at the base of hydrate stability and extend some distance toward the seafloor the highly reflective segments exhibit the same polarity as the seafloor reflection indicating that localized gas hydrate precipitation from gas charged fluids within relatively permeable layers has occurred high density velocity analysis shows that these layer constrained gas hydrate accumulations are underlain by thick up to 500 m free gas zones which provide the source for focused gas migration into the hydrate layer in addition to gas being channeled along layers we also interpret gas migration through a fault zone into the field of hydrate stability in this case a low velocity layer within the hydrate stability zone extends laterally away from the fault which might indicate that gas charged fluids have also migrated away from the fault along strata at this site where both dipping strata and faulting seem to influence fluid migration we observe anomalously high velocities at the base of hydrate stability that we interpret as concentrated gas hydrates our results give insight into how shallow fluid flow responds to permeability contrasts between strata fault zones and perhaps also the gas hydrate system itself ultimately these relationships can lead to gas migration across the base of hydrate stability and the precipitation of concentrated hydrate deposits

natural gas hydrates on the southeast u s margin constraints from full waveform and travel time inversions of wide angle seismic data strong bottom simulating reflectors bsr have been mapped over a region of approximately 50 000 km2 on the southeastern u s margin and have been associated with possible abundance of natural gas hydrates in june 1992 coincident single channel seismic and wide angle ocean bottom seismic data were acquired in the region focusing on the blake ridge and the carolina rise wide angle reflections from the bsrs were clearly observed at offsets up to 6 km joint travel time inversion was conducted with wide angle and vertical incidence data in order to explore possible regional variation and the resultant two dimensional average velocity models imply higher background velocities on the carolina rise full waveform inversion was then performed to determine the seismic origin of the bsrs the best fit model shows a similar low velocity 1 4 km s beneath the bsr at both sites indicating trapped free gas with low saturation 10 the inversion results also indicate that a thin high velocity wedge with a maximum velocity of 2 3 km s is present just above the blake ridge bsr sediment reflectivities were also calculated and higher reflectivities are observed on the carolina rise an increase in reflectivity below the bsr seems to correspond to the gas bearing zone at both sites concentration of hydrates were estimated based on these velocity models whereas average hydrate concentration of 3 of the total sediment volume is suggested for the lower half of the hydrate stability zone at the blake ridge only a very low average concentration of hydrate can be expected at the carolina rise the hydrates seem to be concentrated near the base of the hydrate stability zone and the maximum hydrate concentration is estimated as 20 at the blake ridge and 7 at the carolina rise both of which are too high to be explained by in situ biogenic activity only and require some secondary accumulation mechanism it is suggested that hydrate recycling caused by the stability field migration may have effectively condensed hydrates at both sites additional enhancement by upward fluid expulsion may also be viable for the blake ridge

field appraisal and accurate resource estimation from 3d quantitative interpretation of seismic and csem data the key questions in field appraisal are what is the hydrocarbon volume and how are the hydrocarbons distributed in the field the ability to answer these questions accurately is critical for deciding whether to produce a field and for developing a production plan wells drilled during the appraisal phase provide well and flow test data which are combined with structural knowledge from seismic surveys to map the extent of the field and generate a reservoir model the cost for appraising an offshore field can exceed us 100 million and it is desirable to obtain the information required with fewer wells if possible quantitative interpretation of surface geophysical data provides reservoir properties between well locations and can therefore significantly reduce appraisal costs a quantitative analysis of seismic data using well log information will typically determine reservoir rock porosity other important parameters are hydrocarbon saturation permeability and net to gross ratio quantitative inte

estimation of sorptivity from disc permeameter measurements the use of disc permeameter in the field often requires the placement of a layer of sand in order to provide good contact between the soil surface and disc supply membrane the effect of the contact sand on water infiltration into the soil and the estimate of sorptivity s was investigated a numerical study using a loam and clay and a field experiment on heavy clay was conducted placement of the sand significantly increased the cumulative infiltration but showed small differences in the final infiltration rate estimation of s based on philip s two term algebraic model using different methods was also examined conventionally s is estimated from the early time infiltration rate when the capillary force is dominating but results showed that this method always overestimated s and give the largest error a numerical study indicated that differentiating the cumulative infiltration with respect to the square root of time provided the closest estimate of s compared to the true value and furthermore showed small variations between treatments with contact sand field experiment revealed that the error in infiltration measurements followed the pattern of the infiltration curve the absolute rate of infiltration without placement of sand was considerably lower because of the poor contact between the disc and soil surface thus contact sand is recommended although care must be exercised with the interpretation of the data

geophysical signatures for low porosity can mimic natural gas hydrate an example from alaminos canyon gulf of mexico natural gas hydrate in sand sediments can increase both the measured compressional velocity and resistivity the same geophysical signatures occur however in low porosity sand we investigate the possible occurrence of natural gas hydrate in a sand interval in alaminos canyon block 21 ac 21 in the gulf of mexico drilled by the u s gas hydrate joint industry project the sand interval has an increase in resistivity 2 2 m and a strong peak and trough at the top and bottom of the sand on exploration seismic which has been interpreted as natural gas hydrate we reexamine the logging data and construct a new reservoir model that matches the measured resistivity the high density sublayers in the sand and the surface seismic trace our model shows that the sand interval in ac 21 is most likely water saturated and the slight increase in resistivity higher measured density and the seismic amplitudes are caused by a reduction in porosity to 30 in the sand interval relative to a porosity of 42 in the surrounding marine muds more broadly we show that the average depth where the porosity of marine muds becomes lower than sand sediment is 900 mbsf though it could be as shallow as 600 mbsf for high porosity sands in any case the similar geophysical signatures for water saturated sand and low saturations of natural gas hydrate in sand probably occur throughout the gas hydrate stability zone at most sites worldwide

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

velocity structure of a gas hydrate reflector seismic reflection profiles across many continental margins have imaged bottom simulating reflectors bsrs parallel to the seabed these are often interpreted as the base of a zone in which methane hydrate ice is stable waveform inversion of seismic reflection data can be used to estimate from seismic data worldwide the velocity structure of a bsr and its thickness a test of this method at a drill site of the ocean drilling program predicts that sediment pores beneath the bsr contain free methane for approximately 30 meters the hydrate and underlying gas represent a large global reservoir of methane which may have economic importance and may influence global climate

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

the free gas zone beneath gas hydrate bearing sediments and its link to fluid flow 3 d seismic imaging offshore mid norway abstract hydrate recycling and vertical migration of fluids from deep sources are processes evoked as controllers of the formation and stability of the free gas zone fgz beneath the base of the gas hydrate stability zone bghsz these processes have been often investigated through analytical and numerical modeling at some locations in continental margins however the seismic response of sediments deformed by such mechanisms has been poorly investigated due to the lack of reliable seismic resolution to clearly image anomalies confined to the fgz the present study is dedicated to the seismic characterization of the fgz beneath the bghsz at a hydrate site offshore mid norway using high resolution p cable 3d seismic data dim amplitude anomalies showing mainly lineations in a polygonal pattern of distribution 50 80 m vertical extension 150 300 m lateral extension and up to a few kilometers long are interpreted as evidence of sediment remobilization possibly aided by hydrate dissociation at focused fluid flow zones below former bases of the ghsz the exclusive occurrence of the anomalies within a depth range comprising estimated paleo bghszs during the last 200 kyr suggests that the anomalies are associated to paleo hydrate bearing sediment seals emplaced at depths controlled by overburden it also supports recently reported indications of main periods of fluid expulsion towards the seafloor occurring between 200 and 130 ka bp

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

positive degree day factors for ablation on the greenland ice sheet studied by energy balance modelling ice ablation is related to air temperature by the positive degree day factor variations of the positive degree day factor in west greenland are studied using an energy balance model to simulate ablation under different conditions degree day factors for simulated and measured ice ablation at nordbogletscher and qaman rssup sermia agree well with values around 8 mm d 1 c 1 degree day factors for snow are less than half those for ice energy balance modelling shows that degree day factors vary with summer mean temperature surface albedo and turbulence but there is only evidence of large positive degree day factors at lower temperatures and with low albedo 0 3 the greatest effect of albedo variations 0 3 0 7 is at lower temperatures while variations in turbulence have greater effect at higher temperatures current models may underestimate runoff from the greenland ice sheet by several tenths because they use a degree day factor for melting ice that is too small for the colder parts of the ice sheet i e the upper ablation area and the northerly margin

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

assessment of gas hydrate saturations in the makran accretionary prism using the offset dependence of seismic amplitudes we estimate the saturations of gas hydrate and free gas based on measurements of seismic reflection amplitude variation with offset avo for a bottom simulating reflector coupled with rock physics modeling when we apply the approach to data from a seismic line in the makran accretionary prism in the arabian sea the results reveal lateral variations of gas hydrate and free gas saturations of 4 29 and 1 7 5 respectively depending on the rock physics model used to relate seismic velocity to saturation our approach is simple and easy to implement

complex pockmarks with carbonate ridges off mid norway products of sediment degassing abstract a new type of pockmark has been discovered off mid norway they contain up to 10 m high ridges of methane derived authigenic carbonate mdac rock the pockmarks were investigated using rovs remotely operated vehicles in 2003 and 2004 the complex pockmarks are located well within the gas hydrate pressure temperature stability field and are associated with a regional bsr bottom simulating reflector our high resolution seismic data demonstrate that the near surface sedimentary bedding dips into vertical pipe structures underlying the pockmarks and that the pipes are deeply rooted the pockmarks are circular in plan view and contain up to 190 m long and 40 m wide ridges formed by up to 4 m wide carbonate rocks piled on top of each other in irregular piles other key features of the complex pockmarks include 1 a distinct fauna with local bacterial mats small tubeworms stalked crinoids and pycnogonids sea spiders and 2 the presence of light hydrocarbon gases c 1 c 5 in clay rich sediments the methane 13 c values range from 54 to 69 pdb suggesting the presence of both bacterial and thermogenic gases during our two day observation periods only micro seepage occurred without ebullition or other visual fluid flow petrography and geochemistry of carbonate blocks from the pockmarks suggest precipitation of methane derived aragonite within the sediments 13 c 52 to 58 pdb we suggest that the complex pockmarks formed by sudden catastrophic fluid flow and that the large carbonate ridges represent dislodged mdacs originally formed subsurface prior to the event subsequent micro seepage is still active and may indicate that the pipes represent fluid flow conduits which transport both bacterial and thermogenic hydrocarbons to the water column our data do not suggest pressure buildup and do not predict future eruptions from the complex pockmarks thus our data rather suggest that the catastrophic formation of the complex pockmarks may be a one time event

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

comparison and implications from strikingly different authigenic carbonates in a nyegga complex pockmark g11 norwegian sea authigenic carbonates sampled in a complex pockmark g11 from the nyegga area south western voring basin reveal strikingly different morphologies a comparative study of macro and micro features of these neighbouring samples was approached in order to attempt a reconstruction of the pockmark dynamics the carbonate morphologies include thin 1 cm platy carbonate crusts blocky and massive carbonate ridges up to several metres in size and irregularly shaped carbonate deposits consisting of interconnected tubular and uneven intervals displaying high porosity our observations indicate that three distinct mechanisms of fluid seepage and carbonate precipitation occurred within the pockmark petrography studies reveal that the samples contain three generations of predominantly aragonitic cements in addition zoned crystals with coexisting mg calcite and dolomite are abundant in one sample suggesting a trend of gradual increase of the sulphate concentrations in the pore fluids during carbonate precipitation isotope measurements on bulk carbonate cements show consistent values with 13 c as low as 52 1 18 o as high as 6 4 and 87 86 sr of 0 7092 anaerobic oxidation of methane close to seafloor conditions can explain these isotope data the carbonates have a common carbon source despite the differences in morphology methane probably originated from the dissolution of gas hydrates as supported from calculated 18 o enrichment in the pore fluids and the methanogenic carbon we propose a model where the differences in bulk carbonate morphology are related to local sediment properties depth of carbonate precipitation and the flow rate of the seeping fluids

evidence from three dimensional seismic tomography for a substantial accumulation of gas hydrate in a fluid escape chimney in the nyegga pockmark field offshore norway in recent years it has become evident that features commonly called gas chimneys provide major routes for methane to pass through the methane hydrate stability zone in continental margins and escape to the ocean one of many such chimneys lying beneath pockmarks in the southeastern voring plateau off norway was investigated with a high resolution seismic experiment employing a 2 d array of sixteen 4 component ocean bottom seismic recorders at approximately 100 m separation and a dense network of shots to define the 3 d variation of the chimney s structure and seismic properties the tomographic model derived from p wave travel times shows that p wave velocity inside the chimney is up to 300 m s higher than in the surrounding strata within the methane hydrate stability zone the zone of anomalously high velocity about 500 m wide near its base narrowing to about 200 m near the seabed extends to a depth of 250 m below the seafloor the depth extent of this zone and absence of high velocity beneath the base of the methane hydrate stability field make it more likely that it contains hydrate rather than carbonate if a predominantly fracture filling model is appropriate for the formation of hydrate in low permeability sediment the maximum hydrate concentration in the chimney is estimated to be 14 27 by total volume depending on how host sediment properties are affected by hydrate formation doming of the strata penetrated by the chimney appears to be associated with the emplacement of hydrate accompanying the invasion of the gas hydrate stability zone by free gas

hydrate dissolution as a potential mechanism for pockmark formation in the niger delta received 17 february 2010 accepted 9 march 2010 published 11 august 2010 1 based on acquired geophysical geological and geotechnical data and modeling we suggest hydrate dissolution to cause sediment collapse and pockmark formation in the niger delta very high resolution bathymetry data acquired from the niger delta reveal the morphology of pockmarks with different shapes and sizes going from a small ring depression surrounding an irregular floor to more typical pockmarks with uniform depression geophysical data in situ piezocone measurements piezometer measurements and sediment cores demonstrate the presence of a common internal architecture of the studied pockmarks inner sediments rich in gas hydrates surrounded by overpressured sediments the temperature pressure and salinity conditions of the studied area have allowed us to exclude the process of gas hydrate dissociation gas hydrate turns into free gas water mixture as a trigger of the observed pockmarks based on numerical modeling we demonstrate that gas hydrate dissolution gas hydrate becomes mixture of water and dissolved gas under a local decrease of the gas concentration at the base of the gas hydrate occurrence zone ghoz can explain the excess pore pressure and fluid flow surrounding the central hydrated area and the sediment collapse at the border of the ghoz the different deformation or development stages of the detected pockmarks confirm that a local process such as the amount of gas flow through faults rather than a regional one is at the origin of those depressions

assessment of gas hydrate and free gas distribution on the south shetland margin antarctica based on multichannel seismic reflection data summary processing and interpretation of a grid of intermediate resolution multichannel seismic reflection profiles collected on the ne sector of the south shetland continental margin allowed us to map the lateral extent of a bottom simulating reflector bsr the margin an accretionary wedge located off the northern tip of the antarctic peninsula consists of two distinct and superimposed tectonic regimes an older regime is related to mesozoic middle cenozoic subduction related tectonism a younger one is associated with a mainly extensional tectonic phase and related to the oligocene development of the western scotia sea the occurrence of the bsr appears to be controlled by the geological structure of the margin the bsr lacks continuity near basement structures main geological discontinuities and faults on the other hand the amplitude and continuity of the bsr are not affected by the presence of folded structures and undeformed sedimentary layering we found that the bsr is mostly confined to the ne sector of the south shetland margin where propagation of faulting associated with the shackleton fracture zone may have driven migration of natural gas towards the surface and created the conditions for a bsr to appear the application of reflection tomography techniques allowed us to reconstruct the averaged seismic velocity field between the seafloor and bsr in order to map the depth of bsr by averaging the observed velocity structure above and below the bsr and applying a theoretical model of elastic wave propagation in porous media we attempted as rigorous a quantitative assessment as possible of the natural gas present as gas hydrate above the bsr and as free gas between the bsr and the base of gas reflector bgr

origin of bottom simulating reflectors geophysical evidence from the cascadia accretionary prism vertical seismic profile vsp data from two drill sites on the cascadia margin show low velocity zones indicative of free gas be neath a bottom simulating reflector bsr offshore oregon at ocean drilling program odp site 892 velocities drop from an average of 1750 m s above the bsr to less than 1250 m s below it sonic logs confirm that seismic velocity in the sediments adjacent to the borehole is less than that of water for at least 50 m beneath the depth of the bsr at this site similarly at odp site 889 offshore vancouver velocities range from 1700 to 1900 m s in the 100 m above the bsr and drop abruptly to 1520 m s in the 15 m just beneath it the low velocities observed beneath the bsr are strong evidence for the presence of 1 5 free gas by volume the bsr at these two sites results from the contact between gas free sediments containing a small quantity of hydrate above the bsr and a low velocity free gas zone beneath it although the bsr is associated with the base of the hydrate stability field hydrate appears to account for relatively little of the velocity contrast that produces the bsr velocity above the bsr at site 889 is only about 100 m s greater than that expected for sediments of similar porosity sediments above the bsr at site 892 appear to have normal velocity for their porosity and may contain little hydrate

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

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

repeated fluid expulsion through sub seabed chimneys offshore norway in response to glacial cycles abstract focused fluid flow through sub seabed sediments is a common phenomenon on continental margins worldwide however the governing controls and timing of this fluid release have been difficult to understand in particular for fluid flow features buried beneath sub surface sediments a link between fluid flow activity ensuing pockmark formation and the last glacial maximum has been hypothesized on the formerly glaciated norwegian margin new high resolution p cable 3d seismic data from the nyegga area on the mid norwegian margin reveal at least two more periods of fluid expulsion from sub seabed sediments the 3d seismic data show depositional patterns within chimney features expressed as truncations of seismic horizons against the flanks of the chimneys the truncations are interpreted by analogy with present day observations as evidence for buried carbonate mounds and or sediment wash out during formation of pockmarks in the past the truncations are hence an indicator and a chronological marker for fluid expulsion in the past the classification of chimneys results in three major groups 1 chimneys that have been formed and consecutively reactivated one or two times during the last 200 kyr and that have a fluid flow expression at the present seafloor 2 chimneys that are approx 125 160 ka bp old without any associated fluid flow expression at the present day seafloor 3 chimneys with no stratigraphical evidence for reactivation formed after the last glacial maximum 18 25 ka bp the observations suggest that each activity period was likely related to the last stages of maximum glaciations in the region the emplacement of thick sequences of glacigenic debris flow deposits during these maximum stages most likely caused rapid increase in overpressure and subsequently the formation of focused fluid flow features piercing through sediments of the naust formation at nyegga

sensitivity studies applied to a two dimensional resistivity model from the central andes summary long period magnetotelluric mt data have been collected along an e w profile across the central andes in northern chile and southern bolivia a 2 d resistivity model explaining these data was found by inverse modelling the model includes a prominent conductivity anomaly in the backarc of the south american subduction zone below the bolivian altiplano another comparatively localized anomaly occurs beneath the precordillera in the forearc the enhanced conductivity in the backarc correlates with reflective zones low seismic velocities and high seismic attenuation in contrast no correlation can be found in the forearc a strong seismic reflector in context with the down going slab and another reflector in the middle crust below the pre and western cordillera do not coincide with anomalous conductivity other 2 d models may also be consistent with the data we applied different techniques to find the range of relevant models one simple but useful approach is the direct use of the sensitivity matrix containing the partial derivatives of the data with respect to the model parameters the sensitivity of the model parameters to the data or data subsets can be visualized by calculating columnwise sums from the sensitivity matrix these sensitivities may be used to indicate model parameters that are less resolved by the data and thus should not form part of an interpretation another approach is to perform systematic studies to establish the validity of geologically important features of the model these studies comprise forward modelling and the use of a priori information combining all approaches generates a more complete assessment of the principal model features we conclude that a minimum depth extent of the altiplano conductor can be specified as approximately 50 km low sensitivity in the forearc beneath the longitudinal valley indicates a limited structure resolution and therefore may explain the absence of a conductive slab in the model the existence of the localized conductor below the precordillera can be established by high sensitivity finally we included the two seismic reflectors in the forearc as a priori information in the inversion process as a result the absence of a conductive slab in the model is probably a result of low sensitivity but cannot be totally excluded a conductive zone coinciding with the strong reflector below the pre and western cordillera is not manifested by the mt data

a seismic study of methane hydrate marine bottom simulating reflectors multichannel seismic reflection data have been analyzed from an area of clear bottom simulating reflectors bsrs on the northern cascadia subduction zone margin off vancouver island the reflector at a depth of about 300 m subbottom is interpreted to represent the base of a layer of methane hydrate or clathrate the shallow water depth of 1300 m and the 3600 m long hydrophone array have allowed bsr amplitude versus offset and high resolution velocity analysis as well as modelling of vertical incidence data the results of all three types of analysis can be best explained by a 10 to 30 m thick high velocity layer located immediately above the bsr about 300 m below the seafloor having a sharp base and transitional top in the layer about one third of the sediment pore spaces must be filled with hydrate ice there is no seismically detectable free gas beneath the bsrs these results put important constraints on models for the distribution and formation of bsr hydrate

a new marine controlled source electromagnetic receiver with an acoustic telemetry modem and arm folding mechanism we developed a new marine controlled source electromagnetic receiver for detecting methane hydrate zones and oil and gas reservoirs on the seafloor which is not imaged well by seismic reflection surveys to determine the seafloor structure the electromagnetic receiver should have low noise power consumption clock drift error and operating costs while being highly reliable because no suitable receiver was available in our laboratory we developed a new marine controlled source electromagnetic receiver with these characteristics the receiver is equipped with acoustic telemetry modem and an arm folding mechanism to facilitate deployment and recovering operations to demonstrate the applicability of our new receiver we carried out a field experiment offshore of guangzhou in the south china sea where methane hydrates have been discovered we successfully obtained controlled source electromagnetic data along a profile about 13 km long all six new receivers were recovered and high quality electromagnetic data were obtained relatively high apparent resistivity values were detected the results of the offshore field experiment support the claim that the electromagnetic data obtained using the new receiver are of sufficient quality for the survey target

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

review paper instrumentation for marine magnetotelluric and controlled source electromagnetic sounding we review and describe the electromagnetic transmitters and receivers used to carry out magnetotelluric and controlled source soundings in the marine environment academic studies using marine electromagnetic methods started in the 1970s but during the last decade these methods have been used extensively by the offshore hydrocarbon exploration industry the principal sensors magnetometers and nonpolarizing electrodes are similar to those used on land but magnetotelluric field strengths are not only much smaller on the deep sea floor but also fall off more rapidly with increasing frequency as a result magnetotelluric signals approach the noise floor of electric field and induction coil sensors 0 1 nv m and 0 1 pt at around 1 hz in typical continental shelf environments fluxgate magnetometers have higher noise than induction coils at periods shorter than 500 s but can still be used to collect seafloor magnetotelluric data down to 40 100 s controlled source transmitters using electric dipoles can be towed continuously through the seawater or on the sea bed achieving output currents of 1000 a or more limited by the conductivity of seawater and the power that can be transmitted down the cables used to tow the devices behind a ship the maximum source receiver separation achieved in controlled source soundings depends on both the transmitter dipole moment and on the receiver noise floor and is typically around 10 km in continental shelf exploration environments the position of both receivers and transmitters needs to be navigated using either long baseline or short baseline acoustic ranging while sea floor receivers need additional measurements of orientations from compasses and tiltmeters all equipment has to be packaged to accommodate the high pressure up to 40 mpa and corrosive properties of seawater usually receiver instruments are self contained battery powered and have highly accurate clocks for timekeeping even when towed on the sea floor or in the water column behind a transmitter

marine csem of the scarborough gas field part 2 2d inversion we explored the application of 2d inversion of marine controlled source electromagnetic and marine magnetotelluric data to image an ambiguous target the scarborough gas reservoir off the west coast of australia lies in close repose to a layer in the overburden of similar resistivity thickness product and also is not far above the resistive basement making it a difficult electric target we found that the standard 2d smooth inversion method yielded models that were unable to resolve this ambiguous structural configuration we solved this problem by developing a two step workflow in which we first invert for a coarse background resistivity model e g anisotropic layers then invert for the minimum deviation from this background using a much finer model discretization the main purpose of our two stage workflow is to inject the knowledge into the inversion that the subsurface is composed of self similar geologic domains though the resulting models did not resolve fine scale structural details they might still be used to map the overall extent and bulk qualities of a target in an otherwise confounding setting

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

estimation of gas hydrate concentration from multi component seismic data at sites on the continental margins of nw svalbard and the storegga region of norway high resolution seismic experiments employing arrays of closely spaced four component ocean bottom seismic recorders were conducted at a site off western svalbard and a site on the northern margin of the storegga slide off norway to investigate how well seismic data can be used to determine the concentration of methane hydrate beneath the seabed data from p waves and from s waves generated by p s conversion on reflection were inverted for p and s wave velocity vp and vs using 3d travel time tomography 2d ray tracing inversion and 1d waveform inversion at the nw svalbard site positive vp anomalies above a sea bottom simulating reflector bsr indicate the presence of gas hydrate a zone containing free gas up to 150 m thick lying immediately beneath the bsr is indicated by a large reduction in vp without significant reduction in vs at the storegga site the lateral and vertical variation in vp and vs and the variation in amplitude and polarity of reflectors indicate a heterogeneous distribution of hydrate that is related to a stratigraphically mediated distribution of free gas beneath the bsr derivation of hydrate content from vp and vs was evaluated using different models for how hydrate affects the seismic properties of the sediment host and different approaches for estimating the background velocity of the sediment host the error in the average vp of an interval of 20 m thickness is about 2 5 at 95 confidence and yields a resolution of hydrate concentration of about 3 if hydrate forms a connected framework or about 7 if it is both pore filling and framework forming at nw svalbard in a zone about 90 m thick above the bsr a biot theory based method predicts hydrate concentrations of up to 11 of pore space and an effective medium based method predicts concentrations of up to 6 if hydrate forms a connected framework or 12 if hydrate is both pore filling and framework forming at storegga hydrate concentrations of up to 10 or 20 were predicted depending on the hydrate model in a zone about 120 m thick above a bsr with seismic techniques alone we can only estimate with any confidence the average hydrate content of broad intervals containing more than one layer not only because of the uncertainty in the layer by layer variation in lithology but also because of the negative correlation in the errors of estimation of velocity between adjacent layers in this investigation an interval of about 20 m thickness equivalent to between 2 and 5 layers in the model used for waveform inversion was the smallest within which one could sensibly estimate the hydrate content if lithological layering much thinner than 20 m thickness controls hydrate content then hydrate concentrations within layers could significantly exceed or fall below the average values derived from seismic data