tectonically driven underfilled overfilled cycles the middle cretaceous in the northern cordilleran foreland basin abstract it is shown that the middle cretaceous succession in the northern cordilleran foreland basin consists of several million year tectonically driven cycles comprising two components strata deposited in an underfilled basin with a prominent forebulge zone and strata deposited in an overfilled basin lacking evidence of a forebulge the episodic thrusting of the cordilleran orogenic wedge and its rich sediment supply to the basin are two main controlling factors for the formation of these cycles a qualitative model of several million year tectonically driven underfilled overfilled cycle for migration and stratigraphic fill in this basin is proposed during the early underfilled period orogenic loading period due to orogenic loading of emplaced thrust sheets flexural subsidence is created in the region proximal to the mountain belt and a prominent forebulge is developed during the late underfilled period early orogenic unloading period as the cratonward migration of the subsidence center of sediment loading in the foredeep zone forebulge zones and backbulge zones migrate cratonwards forming a diachronic erosion surface in the central basin during the overfilled period late orogenic unloading period a prominent erosion forms in the proximal basin and a peripheral sag develops above the forebulge area of the previous underfilled period this model may provide a pattern to subdivide sedimentary successions in the cordilleran foreland basin using this model alternative interpretations are suggested for some important but controversial stratigraphic phenomena in the cretaceous cordilleran foreland basin traditionally defined eustatic highstands wide sedimentation area of the basin erosion surfaces and widespread subtle topographic uplifts in the central basin high frequency coarsening up cycles extensively distributed erosive based sandstones and conglomerates enclosed in marine mudstones

field test of sub basalt hydrocarbon exploration with marine controlled source electromagnetic and magnetotelluric data the recent use of marine electromagnetic technology for exploration geophysics has primarily focused on applying the controlled source electromagnetic method for hydrocarbon mapping however this technology also has potential for structural mapping applications particularly when the relative higher frequency controlled source electromagnetic data are combined with the lower frequencies of naturally occurring magnetotelluric data this paper reports on an extensive test using data from 84 marine controlled source electromagnetic and magnetotelluric stations for imaging volcanic sections and underlying sediments on a 128 km long profile the profile extends across the trough between the faroe and shetland islands in the north sea here we focus on how 2 5d inversion can best recover the volcanic and sedimentary sections a synthetic test carried out with 3d anisotropic model responses shows that vertically transverse isotropy 2 5d inversion using controlled source electromagnetic and magnetotelluric data provides the most accurate prediction of the resistivity in both volcanic and sedimentary sections we find the 2 5d inversion works well despite moderate 3d structure in the synthetic model triaxial inversion using the combination of controlled source electromagnetic and magnetotelluric data provided ac onstant resistivity contour that most closely matched the true base of the volcanic flows for the field survey data triaxial inversion of controlled source electromagnetic and magnetotelluric data provides the best overall tie to well logs with vertically transverse isotropy inversion of controlled source electromagnetic and magnetotelluric data a close second vertical transverse isotropy inversion of controlled source electromagnetic and magnetotelluric data provided the best interpreted base of the volcanic horizon when compared with our best seismic interpretation the structural boundaries estimated by the 20 mc ontour of the vertical resistivity obtained by vertical transverse isotropy inversion of controlled source electromagnetic and magnetotelluric data gives a maximum geometric location error of 11 with a mean error of 1 2 compared with the interpreted base of the volcanic horizon both the model study and field data interpretation indicate that marine electromagnetic technology has the potential to discriminate between low resistivity prospective siliciclastic sediments and higher resistivity non prospective volcaniclastic sediments beneath the volcanic section

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

geochronology and tectonic implications of magmatism and metamorphism southern kootenay arc and neighbouring regions southeastern british columbia part i jurassic to mid cretaceous k ar dates and u pb zircon dates define three periods of igneous activity in the southern kootenay arc 1 emplacement of late synkinematic to post kinematic granodioritic plutons in mid jurassic time 170 165 ma accompanying amphibolite facies regional metamorphism 2 emplacement of post kinematic granitic plutons in mid cretaceous time 100 ma and 3 emplacement of small bodies of syenite in eocene time 50 ma in the western part of the area micas from mid jurassic plutons that yield the oldest k ar dates 158 166 ma also yield plateau shaped 40ar 39ar age spectra age spectra for biotites younger than these but older than 125 ma reflect thermal overprinting in southeastern british columbia the kootenay arc marks the transition from the north american rocks of the cordilleran miogeocline to the tectonic collage of allochthonous terranes that have been accreted to it deformation metamorphism and plutonism recorded in rocks of the southern kootenay arc commenced in mid jurassic time as a co

biostratigraphic units and tectonism in mid cretaceous foreland of wyoming colorado and adjoining areas abstract chronolithologic units and unconformities in mid cretaceous formations of the central rocky mountains region indicate widespread marine transgressions and regressions as well as recurrent deformation of the foreland in the western interior during cenomanian turonian and coniacian times 88 96 ma the stratigraphic record of the widely recognized cenomanian and early turonian transgression middle turonian end page 856 regression and late turonian and coniacian transgression was modified in several areas by episodes of slight uplift and attendant erosion the most evident tectonism was in western montana during the middle to late cenomanian 93 94 ma in western wyoming and adjoining areas during the early turonian to earliest middle turonian 90 91 ma in north central colorado eastern wyoming and northwestern wyoming in the early late turonian 89 8 ma and in northeastern colorado wyoming and southwestern montana in the late late turonian 89 3 ma crestlines of most of the swells trend generally either northwest or northeast the tectonism of the mid cretaceous foreland corresponds in age to displacements of thrusts in the sevier orogenic belt of southwestern wyoming and southeastern id ho furthermore much of the foreland deformation probably reflects episodes of eastward thrusting in the thrust belt end of article last page 857

resolution of reservoir scale electrical anisotropy from marine csem data a combination of 1d and 3d forward and inverse solutions is used to quantify the sensitivity and resolution of conventional controlled source electromagnetic csem data collected using a horizontal electric dipole source to transverse electric anisotropy located in a deep water exploration reservoir target because strongly anisotropic shale layers have a vertical resistivity that can be comparable to many reservoirs we examined how csem can discriminate confounding shale layers through their characteristically lower horizontal resistivity forward modeling indicated that the sensitivity to reservoir level anisotropy is very low compared with the sensitivity to isotropic reservoirs especially when the reservoir is deeper than about 2 km below the seabed however for 1d models where the number of inversion parameters can be fixed to be only a few layers both vertical and horizontal resistivity of the reservoir can be well resolved using a stochastic inversion we found that the resolution of horizonta

direct reservoir parameter estimation using joint inversion of marine seismic ava and csem data a new joint inversion algorithm to directly estimate reservoir parameters is described this algorithm combines seismic amplitude versus angle ava and marine controlled source electromagnetic csem data the rock properties model needed to link the geophysical parameters to the reservoir parameters is described errors in the rock properties model parameters measured in percent introduce errors of comparable size in the joint inversion reservoir parameter estimates tests of the concept on synthetic one dimensional models demonstrate improved fluid saturation and porosity estimates for joint ava csem data inversion compared to ava or csem inversion alone comparing inversions of ava csem and joint ava csem data over the north sea troll field at a location with well control shows that the joint inversion produces estimated gas saturation oil saturation and porosity that is closest as measured by the rms difference l1 norm of the difference and net over the interval to the logged values whereas csem inversion provides the closest estimates of water saturation

on the exploration of a marine aquifer offshore israel by long offset transient electromagnetics abstract the existence of aquifers extending from land beneath the sea floor up to a distance of several kilometres has been observed and examined all over the world the coastal aquifer of israel is a heavily used groundwater reservoir which has to be constantly monitored to ensure the drinking water supply former land based electromagnetic measurements show that it is in several places blocked to seawater intrusion and is consequently a candidate for submarine extension multicomponent long offset transient electromagnetic measurements were carried out offshore on the coast of israel we deployed a 400 m long grounded dipole as transmitter and several electric and magnetic receivers on the sea floor up to a distance of 4 8 km from the coast altogether we deployed 8 transmitter positions and received data sets at 14 receiver stations onshore and offshore with offsets of mostly 400 and 800 m in this paper we present the survey and 1d occam and marquardt inversions of the offshore horizontal electric components in the broadside and inline configuration in addition the vertical magnetic component in the broadside position is also considered only single inversions both single offset and single component were used to detect the aquifer under sea sediments we prove the submarine existence of the israeli coastal aquifer up to a distance to the coast of approximately 3 2 to 3 6 km using all measured long offset transient electromagnetic components in addition we present modelling studies with synthetic data derived from a subsurface model adjusted to our survey area with very shallow water from 10 to 50 m as part of the planning before the survey a parameter study of the expected subsurface the examination of the airwave phenomenon and the justification for our 1d inversion strategy are shown more detailed eigen parameter analyses are added to explain the measured data

multi method virtual electromagnetic experiments for developing suitable monitoring designs a fictitious co2 sequestration scenario in northern germany we present a numerical study for 3d time lapse electromagnetic monitoring of a fictitious co2 sequestration using the geometry of a real geological site and a suite of suitable electromagnetic methods with different source receiver configurations and different sensitivity patterns all available geological information is processed and directly implemented into the computational domain which is discretized by unstructured tetrahedral grids we thus demonstrate the performance capability of our numerical simulation techniques the scenario considers a co2 injection in approximately 1100 m depth the expected changes in conductivity were inferred from preceding laboratory measurements a resistive anomaly is caused within the conductive brines of the undisturbed reservoir horizon the resistive nature of the anomaly is enhanced by the co2 dissolution regime which prevails in the high salinity environment due to the physicochemical properties of co2 the affected portion of the subsurface is laterally widespread but very thin we combine controlled source electromagnetics borehole transient electromagnetics and the direct current resistivity method to perform a virtual experiment with the aim of scrutinizing a set of source receiver configurations with respect to coverage resolution and detectability of the anomalous co2 plume prior to the field survey our simulation studies are carried out using the 3d codes developed in our working group they are all based on linear and higher order lagrange and n ed elec finite element formulations on unstructured grids providing the necessary flexibility with respect to the complex real world geometry we provide different strategies for addressing the accuracy of numerical simulations in the case of arbitrary structures the presented computations demonstrate the expected great advantage of positioning transmitters or receivers close to the target for direct current geoelectrics 50 change in electric potential may be detected even at the earth s surface monitoring with inductive methods is also promising for a well positioned surface transmitter more than 10 difference in the vertical electric field is predicted for a receiver located 200 m above the target our borehole transient electromagnetics results demonstrate that traditional transient electromagnetics with a vertical magnetic dipole source is not well suited for monitoring a thin horizontal resistive target this is due to the mainly horizontal current system which is induced by a vertical magnetic dipole

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

determining the orientation of marine csem receivers using orthogonal procrustes rotation analysis electromagnetic receivers deployed to the seafloor for csem surveys can have unknown orientations because of the unavailability of compass and tilt recordings in such situations only the orientation independent parameters derived from the measured csem field vector can be interpreted and this may result in less structural resolution than possible when the sensor orientations are known an orthogonal procrustes rotation analysis opra technique can be used to estimate the full 3d receiver orientation for inline and off line csem receivers the generality of this method allows it to be easily embedded into nonlinear csem inversion routines so that they iteratively search for both the receiver orientation and a seafloor electrical conductivity model compatible with the data synthetic tests using the opra method jointly with a 1d inversion demonstrate that it can recover the rotation and tilt angles to about one degree accuracy for 1d data and to within a few degrees for 2d data application of this method to real survey data shows good agreement with a previous orientation method that is suitable only for determining the horizontal rotation of inline receivers csem data collected over the pluto gas field offshore the northwest coast of australia were used to demonstrate how the opra method can be used to orient csem receivers prior to inversion of only the inline electric and crossline magnetic field components

faults of the central part of the lewis and clark line and fragmentation of the late cretaceous foreland basin in west central montana the lewis and clark line is a prominent zone of strike slip dip slip and oblique slip faults that extends from near wallace idaho to east of helena montana faults of this zone have been intermittently active from middle proterozoic to holocene time and because of numerous tectonic overprints controversy continues about displacement directions and times of displacement along specific faults geologic mapping shows evidence that many principal faults of the lewis and clark line such as the st marys helena valley bald butte ninemile and osburn faults had right separation or slip that ranged between 28 and 11 km and this displacement probably occurred during late cretaceous time other faults such as the elevation mountain placer creek and ranch creek faults have late cretaceous right separations that range between 8 and 3 2 km and the mount sentinel fault zone has between 6 5 and 3 km of right separation of probable late cretaceous age subsidiary structures of the lewis and clark line postdate paleozoic and lower cretaceous rocks and predate late cretaceous stocks at some places subsidiary faults and folds that have age constraints have slip directions compatible with right slip along adjacent principal faults sedimentation patterns of lower and upper cretaceous rocks indicate that faults of the lewis and clark line fragmented the foredeep region of the foreland basin into separate northern and southern basins in late cretaceous time the lower and upper cretaceous blackleaf formation albian and lower cenomanian was deposited in a continuous foredeep basin that extended across the lewis and clark line from north of the canadian border to southwestern montana a distance of about 450 km north of the lewis and clark line middle and upper cenomanian rocks are absent and a thin sequence of uppermost cenomanian to campanian rocks was deposited in a marine environment that changed to a strand line and continental environment in early campanian time south of the lewis and clark line middle and upper cenomanian deposits are also absent but a thick sequence of turonian to campanian rocks was deposited in brackish water and strand line environments and during later campanian time in a continental environment in the region between the st marys helena valley and bald butte faults a barrier may have formed that served as a local sediment source between foredeep regions in the northern and southern foreland basin during the period 91 to 75 ma south of the bald butte fault an extensional tectonic regime contributed to a higher sediment accumulation rate in the foredeep region along the north border of the southern basin 30 cm 1 000 yr as compared to lower sediment accumulation rates 6 9 and 7 8 cm 1 000 yr in the foredeep region of the south part of the northern basin

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

flexural subsidence and basement tectonics of the cretaceous western interior basin united states the flexural subsidence history recorded in cenomanian to early campanian 97 to 80 ma strata in the cretaceous u s western interior basin was studied with two dimensional flexural backstripping techniques results indicate that the flexural subsidence resulting from thrust loading was superimposed on epeirogenic subsidence in the foreland basin the flexural component exhibits significant spatial and temporal variations along both the strike and dip relative to the sevier thrust belt the greatest cumulative subsidence occurred in southwestern wyoming and northern utah concurrent subsidence in northwestern montana and southern utah was insignificant temporal trends in subsidence also show a distinct regional pattern from the cenomanian to late turonian 97 to 90 ma subsidence rates were high in utah and much lower in wyoming and montana in contrast during the coniacian and santonian 90 to 85 ma subsidence accelerated rapidly in wyoming increased slightly in montana and decreased in utah we suggest that these spatially and temporally varying subsidence patterns reflect the interplay of several geodynamic factors including 1 temporal and spatial variation in emplacement of the thrust loads 2 segmentation of the basement into adjacent blocks with different rheological properties 3 reactivation of basement fault trends and 4 regional dynamic topographic effects

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

simulation of foreland basin stratigraphy using a diffusion model of mountain belt uplift and erosion an example from the central alps switzerland foreland basin stratigraphy can be considered as the result of three interacting processes thrust deformation which builds the tectonic load sedimentary and erosional processes which redistribute that load and the flexural response of the lithosphere the resultant stratigraphy of foreland basins is commonly composed of a small number of shallowing and coarsening upward cycles bounded by regional unconformities to understand the development of these unconformities we present a simple model of these three processes coupling an evolving thrust wedge on a linear elastic plate with erosion and sedimentation defined by the diffusion equation applied to topography our model demonstrates the development of regional unconformities without recourse to either eustasy or complex viscoelastic models for the continental lithosphere the model describes the thrust wedge foreland basin system in terms of four parameters 1 the effective elastic thickness of the foreland plate te 2 the sediment transport coefficient k 3 the thrust wedge advance rate 4 the surface slope of the thrust wedge the model is applied to the oligocene miocene north alpine foreland basin nafb of eastern switzerland the stratigraphy of the nafb can be simplified into two large scale shallowing upward cycles separated by an unconformity at the base of the burdigalian 22 ma geological information is taken from the nafb to estimate suitable values for the parameters listed above assuming a linear elastic lithospheric rheology the te value is estimated at 10 5 km from decompacted sediment columns data to constrain the sequential development of the thrust wedge come from structural geology the early stages 40 24 ma of compression involved a relatively low angle thrust wedge with an advance rate of approximately 2 4 mm yr at about 24 ma the wedge advance slowed down and thickened by underplating crystalline basement of the foreland plate the value for the transport coefficient has been estimated from previous studies prior to attempting to simulate the broad scale geometry of the nafb the role of each parameter was assessed individually the values for te and k are held constant throughout the simulation of the nafb at 7 5 km and 500 m yr respectively the geometry of the base burdigalian unconformity is reproduced by variations in the parameters describing the thrust wedge the surface slope angle of the wedge is increased from 2 5 to 4 over 0 2 m y and the rate of thrust advance is decreased from 2 mm yr to 0 2 mm yr during the thickening event between 24 and 23 8 ma the rejuvenation of the internal parts of the thrust load causes backtilting of the foreland basin sediments and after a time lag erosion of the distal stratigraphy over the forebulge so simulating the base burdigalian unconformity

use of marine controlled source electromagnetic sounding for sub basalt exploration the problem of imaging sedimentary structure obscured by high velocity layers such as carbonate basalt or salt using conventional seismic techniques is well known when this problem is encountered in offshore areas marine electromagnetic data can provide valuable alternative and complementary constraints on the structure we concentrate on the use of marine controlled source electromagnetic csem sounding in the frequency domain the feasibility of mapping sub basalt sedimentary structure using this technique is explored by means of modelling studies which mimic a type of survey which could readily be employed in practice as a starting point the problem is addressed in terms of 1d resistivity structure we show that sub basalt sediments can be detected and their depth of burial quantified to within 200 m in the examples shown the presence of small scale inhomogeneities in the basalt which cause much of the scattering in seismic data is shown to have no appreciable effect on the ability of the csem data to detect the sediments the modelling is then extended to 2 5 dimensions again the presence of sub basalt sediments can be readily detected and their properties and geometry quantified especially if the electromagnetic data are combined with constraints from complementary geophysical techniques

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

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

structural configuration of the early cretaceous cordilleran foreland basin system and sevier thrust belt utah and colorado abstract stratigraphic and provenance data from lower cretaceous rocks in utah and colorado and structural evidence from the sevier belt in west central utah allow recognition of the ties between early thrust belt evolution and foreland basin system development regional isopach patterns of lower cretaceous strata define foreland basin system depozones lower cretaceous strata in west central utah that thicken westward from 100 m to 1 2 km were deposited within the flexurally subsiding foredeep depozone correlative units in eastern utah and western colorado 100 m thick were deposited in the back bulge depozone these thickness trends indicate the lower cretaceous foreland basin system consisted of a foredeep 100 180 km wide and forebulge 260 460 km wide a computer generated flexural model of the foreland basin system produces a reasonable match of the observed basin geometry using a 400 km wide l

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

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

migration and stratigraphic fill of an underfilled foreland basin middle late cenomanian belle fourche formation in southern alberta canada abstract the geometry of a foreland basin is mainly a product of a complex dynamic balance between the orogenic loading erosion and sedimentation and lithospheric flexural response to improve our understanding of the relationships between these processes and the foreland basin stratigraphy a detailed stratigraphic and sedimentologic study of the middle late cenomanian belle fourche formation in the subsurface of southern alberta is presented in this paper the middle cenomanian lower belle fourche formation and the late cenomanian upper belle fourche formation were deposited in underfilled conditions showing prominent forebulge zones it is demonstrated that during the late cenomanian with the cratonward migration of the uplifting forebulge zones the axes of backbulge zones also migrated cratonwards and a wide uplifted forebulge zone was formed during the last stage of the underfilled condition the foreland basin had a relatively flat topography and fine sands were deposited along the flank of the uplifting forebulge a qualitative model for the migration and stratigraphic fill of an underfilled foreland basin is proposed this model provides a qualitative pattern of the cratonward migration of the forebulge zone and backbulge zone within a several million year tectonically driven cycle it is suggested that during the early underfilled period the basin geometry is mainly controlled by the orogenic loading and a prominent forebulge zone with an approximately fixed location is formed during the late underfilled period the basin geometry is mainly controlled by the sediment loading and the forebulge zone move very fast along with the rapid cratonward shift of the sediment loading center over a longer time period and between different tectonically driven cycles along with the cratonward propagation of the thrusting wedge the orogenic loading plays a more important role than the sediment loading for the migration of the forebulge zone

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

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

geochronology and tectonic implications of magmatism and metamorphism southern kootenay arc and neighbouring regions southeastern british columbia part ii mid cretaceous to eocene a north south trending belt of amphibolite facies regional metamorphism parallels the purcell trench transects the kootenay arc and is in part fault bounded towards the axis of this belt progressively higher pressure metamorphic mineral assemblages are exposed in the contact aureoles of post kinematic mid cretaceous 100 ma plutons and in metapelites contours of k ar biotite dates for plutonic rocks 55 95 ma are regular are broadly conformable with metamorphic isograds appear to cross internal intrusive contacts of post kinematic plutons and young towards the highest pressure and temperature regional metamorphic zones within the sillimanite zone most micas yield k ar dates between 40 and 55 ma rb sr muscovite dates for deformed and undeformed pegmatites and for muscovite bearing monzogranite and granodiorite fall between 53 and 84 ma u pb zircon dates for the kaniksu batholith and nearby gneiss of uncertain origin yield a lower concordia intercept of 94 ma micas from mid jurassic and mid

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

sharp boundary inversion of 2d magnetotelluric data we consider 2d earth models consisting of laterally variable layers boundaries between layers are described by their depths at a set of nodes and interpolated laterally between nodes conductivity within each layer is described by values at a set of nodes fixed within each layer and is interpolated laterally within each layer within the set of possible models of this sort we iteratively invert magnetotelluric data for models minimizing the lateral roughness of the layer boundaries and the lateral roughness of conductivities within layers for a given level of data misfit this stabilizes the inverse problem and avoids superfluous detail this approach allows the determination of boundary positions between geological units with sharp discontinuities in properties across boundaries while sharing the stability features of recent smooth conductivity distribution inversions we compare sharp boundary inversion results with smooth conductivity distribution inversion results on a numerical example and on inversion of field data from the columbia river flood basalts of washington state in the synthetic example where true positions and resistivities are known sharp boundary inversion results determine both layer boundary locations and layer resistivities accurately in inversion of columbia flood basalt data sharp boundary inversion recovers a model with substantially less internal variation within units and less ambiguity in both the depth to base of the basalts and depth to resistive basement

applicability of 1d and 2 5d marine controlled source electromagnetic modelling we present two and a half dimensional 2 5d and three dimensional 3d integral equation modelling of the marine controlled source electromagnetic method we implement 2 5d modelling using a point source and a two dimensional reservoir and compare the results with point source responses from one dimensional and three dimensional reservoir models these methods are based on an electric field domain integral equation formulation we show how the 2 5d method performs in terms of both accuracy and computing speed with different configurations we compare the results from 1d 2 5d and 3d modelling for a symmetrically placed reservoir and the in line acquisition configuration as a function of different reservoir sizes in the cross line direction thickness and for different frequencies and depths depending on the model s parameters 2 5d modelling can be considered as an accurate and fast method for marine controlled source electromagnetic acquisition optimization and interpretation if the thickness of the reservoir is less than one fifth of the skin depth of the embedding and if the depth of the reservoir is two or more times the skin depth of the embedding the largest amplitude difference between two dimensional and three dimensional reservoirs is less than 10 when the source is above the centre of the reservoir in this paper we discuss supporting examples with different configurations where the 2 5d results lead to an optimistic detection estimate phase differences between 2 5d and 3d modelling are even smaller and the 2 5d solution can be used to assess the ability to detect the reservoir with a given acquisition configuration

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

useful characteristics of shallow and deep marine csem responses inferred from 3d finite difference modeling hydrocarbon reservoirs can be mapped if sufficient resistivity contrastsexistbetweenthemandtheirconfininglayers butpractical problems remain in target discrimination in deep and shallowwaters especiallyinthepresenceofheterogeneousoverburden wehavedevelopedanefficient3dstaggered gridfinite difference controlled source electromagnetic csem modeling code that enables study of the physics underlying some practical problems we undertook a comparative analysis of reservoir detection in 2000 m and 300 m deep waters using the simulated electric and magnetic field responses of a simple 3d reservoir we examined the effect of two types of near surface heterogeneitymimickingdisconnectedgascloudsand orpatchygeochemicalalterationhalosonthe3dreservoirresponse wefoundthat small scale shallowheterogeneitiescausedistortionsthatarealmost independent of the source frequency these persist at all source receiver offsets in the electric amplitude response in deep and shallow waters and phase response in shallow water they decrease in magnitude with increasing offset in deepwater phase response large scale near surface heterogeneities distort the horizontal electric field response more significantly than the small scale ones but the near surface response gets smaller in amplitude as the offset increases the distortions in shallow water are much smaller in magnitude than those for the deepwater case so that the reservoir signatures still are visible on the responseprofiles thismightbeconsideredasapositivefeaturefor shallow water inline electric field exploration the magnetic field responses for the orthogonal direction provide diagnostic target signatures that are similar to the inline electric field responsesindeepwaterbutthataredifferentinshallowwater the magnetic responses are affected by the airwave in a different manner from the electric field suggesting that combined 3d electric and magnetic field analysis might be vital for handling theairwaveproblem

evolution of the northern cordilleran foreland basin during the middle cretaceous a detailed study of the middle late cenomanian belle fourche formation in southern alberta helps to elucidate the evolution of the cordilleran foreland basin during the middle cretaceous by combining isopach data from the study area with previously published research on middle cretaceous stratigraphy in adjacent areas of northern alberta and montana it is possible to defi ne the position of the proximal foredeep forebulge and backbulge depozones from late albian to cenomanian time the foredeep probably lay to the west of the present fold thrust belt in western montana from the late albian to middle cenomanian with the forebulge extending northwestward through the northwest corner of wyoming and curving northeastward through the present day location of calgary thick dunvegan deposits middle cenomanian of northwestern alberta are interpreted as foredeep deposits palinspastic reconstruction of the southern rocky mountains of alberta is consistent with a location of the foredeep within the area of the present fold thrust belt at the end of the middle cenomanian the forebulge retreated westward within alberta defi ning a nearly straight northwest southeast trend through the present position of the foothills and front ranges of alberta and northwestern montana the change in trend of the foreland basin at the end of the middle cenomanian may refl ect the change of convergence vectors along the western margin of north america during the middle cretaceous

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

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

mapping offshore sedimentary structure using electromagnetic methods and terrain effects in marine magnetotelluric data summary marine magnetotelluric mt and marine controlled source electromagnetic csem soundings can be used to study sedimentary structure offshore in an example of this application we collected mt and csem data in the 1 km deep water of the san diego trough california the trough is a pull apart basin and part of the complex pacific north american tectonic plate boundary and is flanked by the thirtymile bank to the west and the coronado bank to the east our mt data are highly distorted by seafloor topography and the coast effect which is largely 2 d and can be modelled using 2 d finite element codes the distortion includes a strong several orders of magnitude static depression of tm mode resistivities electric field perpendicular to structure upward cusps in the te mode resistivities electric field parallel to structure and negative te mode phases the depressed tm mode resistivity is a well known consequence of galvanic interruption of coastperpendicular electric fields the te mode distortion is an inductive effect associated with currents flowing along the edge of the deep ocean basins steepening the magnetic field and even causing a phase reversal in the horizontal field used for mt impedance calculations and thus generating negative phases the land side enhanced vertical magnetic field is well known as the geomagnetic coast effect but the ocean side consequences have been less well documented although the mt data are dominated by coast effect and topographic distortion inclusion of accurate bathymetry in the inversion model s finite element mesh allows the subseafloor geological structure to be recovered this shows the trough sediments to be about 3 km thick bounded to the west by resistive basement but to the east by conductive clastic sediments forming coronado bank amplitudes and phases of five frequencies of csem data from 0 1 to 1 0 hz collected along the axis of the trough are well fit with a simple 1 d layered model indicating that sediment resistivities increase with depth from 1 5 to 2 3 m and are no more than 3300 m thick thinning to the north in good agreement with the mt model an existing density model generated by fitting surface and deep towed gravity is in good agreement with the em interpretations in particular combining sediment densities and csem resistivities allows us to estimate pore water conductivity and temperature which follows a geothermal gradient of 25 4 8kk m 1

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

thrust loads and foreland basin evolution cretaceous western united states two dimensional modeling of loading during the formation of the idaho wyoming thrust belt shows that regional isostatic compensation by flexure of an elastic lithosphere is sufficient to control the formation of a foreland basin the flexural rigidity of the lithosphere is inferred to have been approximately 1023 nm 1030 dyne cm on the basis of palinspastic comparison of predicted downwarping due to the thrust plate loads to the shape of the sedimentary wedge on the west side of the cretaceous western interior seaway erosion of part of the uplifted thrust plates redistributed the load depositing it farther to the east thereby causing subsidence over a much wider area than could have been accomplished only by the loading by thrust plates paleotopography after major cretaceous thrust events was calculated the resulting mountainous terrain gentle alluvial plain and flat sea floor correspond well with the topography of the modern foreland thrust belt and basin system in the andes of south america and to paleogeographic reconstructions in the western united states thrust belt topography is controlled by the subsurface geometry of the thrust faults particularly the positions of ramp zones and by isostatic subsidence

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

time scales of assembly and thermal history of a composite felsic pluton constraints from the emerald lake area northern canadian cordillera yukon knowledge of the time scales of emplacement and thermal history during assembly of composite felsic plutons in the shallow crust are critical to deciphering the processes of crustal growth and magma chamber development detailed petrological and chemical study of the mid cretaceous composite emerald lake pluton from the northern canadian cordillera yukon territory coupled with u pb and 40 ar 39 ar geochronology indicates that this pluton was intruded as a series of magmatic pulses intrusion of these pulses produced a strong petrological zonation from augite syenite hornblende quartz syenite and monzonite to biotite granite our data further indicate that multiple phases were emplaced and cooled to below the mineral closure temperatures over a time scale on the order of the resolution of the 40 ar 39 ar technique v1 myr and that emplacement occurred at 94 3 ma simple thermal modelling and heat conduction calculations were used to further constrain the temporal relationships within the intrusion these calculations are consistent with the geochronology and show that emplacement and cooling were complete in less than 100 kyr and probably 70 5 kyr these results demonstrate that production transport and emplacement of the different phases of the emerald lake pluton occurred essentially simultaneously and that these processes must also have been closely related in time and space by analogy these results provide insights into the assembly and petrogenesis of other complex intrusions and ultimately lead to an understanding of the processes involved in crustal development 2002 elsevier science b v all rights reserved

a synthetic stratigraphic model of foreland basin development the processes of erosion and deposition must be included in foreland basin models to predict correctly basin geometry and stratigraphy we present a synthetic stratigraphic model of the development of nonmarine foreland basins that predicts progressive geometry topography and facies patterns in the model steady crustal shortening occurs according to a wedgethickening model erosion and deposition follow a diffusive process and the lithosphere is compensated elastically erosion and deposition are controlled by the transport coefficients r of the diffusion equation for a range of thrust velocities and lithospheric rigidities transport coefficients are of order 102 103 m yr in the mountain belt these values are much higher than those derived from the study of scarp degradation in the sedimentary basin transport coefficients of order 104 m2 yr are appropriate and are compatible with previous studies of fluvial and deltaic deposition rapid thrusting results in a narrow underfilled basin while slow thrusting results in a wide overfilled basin in addition by varying the erosional and depositional transport coefficients while holding other parameters constant we generate both overfilled and underfilled basins these results suggest that changes in the rate of thrust loading the climate or the source rock lithology can create stratigraphic signatures that have been interpreted to record viscoelastic relaxation of the lithosphere clearly to understand either the long term behavior of the lithosphere or to interpret orogenic history from preserved foreland strata the manner in which a basin was filled must be considered we apply the model to the evolution of the modern sub andean foreland and find that an erosional transport coefficient of 3000 m2 yr and a depositional transport coefficient of 20 000 m2 yr successfully predict the observed basin geometry

the marine controlled source electromagnetic response of a steel borehole casing applications for the neptune canada gas hydrate observatory gas hydrates are a potential energy resource a possible factor in climate change and an exploration geohazard the university of toronto has deployed a permanent seafloor time domain controlled source electromagnetic csem system offshore vancouver island within the framework of the neptune canada underwater cabled observatory hydrates are known to be present in the area and due to their electrically resistive nature can be monitored by 5 permanent electric field receivers however two cased boreholes may be drilled near the csem site in the near future to understand any potential distortions of the electric fields due to the metal we model the marine electromagnetic response of a conductive steel borehole casing first we consider the commonly used canonical model consisting of a 100 m 100 m thick resistive hydrocarbon layer embedded at a depth of 1000 m in a 1 m conductive host medium with the addition of a typical steel production casing extending from the seafloor to the resistive zone results show that in both the frequency and time domains the distortion produced by the casing occurs at smaller transmitter receiver offsets than the offsets required to detect the resistive layer second we consider the experimentally determined model of the offshore vancouver island hydrate zone consisting of a 5 5 m 36 m thick hydrate layer overlying a 0 7 m sedimentary half space with the addition of two borehole casings extending 300 m into the seafloor in this case results show that the distortion produced by casings located within a 100 m safety zone of the csem system will be measured at 4 of the 5 receivers we conclude that the boreholes must be positioned at least 200 m away from the csem array so as to minimize the effects of the casings

two phase stratigraphic model of foreland basin sequences application of flexural models to nonmarine foreland basin evolution indicates that two different stratigraphic styles of basin fill may develop over time basin subsidence is most rapid during times of thrust load emplacement associated sedimentation is coarse grained immediately adjacent to the thrust front and grades rapidly into fine grained deposits that cover most of the basin the distal part of the basin may also contain deposits derived from streams that flow from beyond the basin toward the thrust belt subsequent removal of the thrust load by erosion and other processes results in flexural rebound of the thrust belt and adjacent foreland basin during this postorogenic phase of adjustment a regional unconformity develops in the proximal part of the foreland basin proximal deposits along with thrust derived sediment are redeposited in the distal foreland basin and beyond this two phase model of foreland sedimentation predicts that coarsening upward sequences in the proximal and distal parts of the basin have reciprocal significance the proximal sequence represents thrust belt advance whereas the distal sequence represents thrust belt cessation

large scale stratigraphic architecture eustatic variation and unsteady tectonism a theoretical evaluation we incorporate a process based depositional model with basin subsidence models to predict stratigraphic records this allows us to investigate the importance of subsidence geometry on coastal stratigraphy and thus to characterize and compare the stratigraphic architecture of two categories of tectonic basins the models demonstrate that the correlation of stratigraphic sequences to eustatic cycles is not the same in passive margin basins as in foreland basins and that in a foreland basin the record of episodic tectonism is distinct from that of eustatic sea level change in the model sediment transport is approximated by slope controlled diffusion nonmarine transport is treated as more efficient diffusion than marine transport three different subsidence and sediment supply models are examined a simple passive margin basin a simple foreland basin and then a foreland basin for which vertical motions are driven by thrust shortening that is compensated flexurally and for which sediment supply is related to relief predicted passive margin stratigraphies for cases of varying eustatic sea level are similar to those of natural basins and include progradational packages and subaerial unconformities which are used to define sequence boundaries that form during sea level fall we explore the timing relationships between stratigraphic features and a sinusoidal sea level history showing that the phase relationship depends on subsidence sediment flux efficiency of sediment transport and period and amplitude of sea level when the basic geometry of the basin is inverted placing the sediment supply on the side with maximum subsidence as is the case in foreland basins the sequence character changes markedly subaerial erosion does not generate unconformities in the models of a dynamic foreland basin sediment supply and subsidence are linked to the structure of the flanking thrust belt and are not necessarily constant for steady thrusting and variable sea level unconformities that define sequence boundaries form only on the distal or forebulge side of the basin and the ages of the sequence boundaries correlate to limes of rising sea level in cases of constant sea level but variable thrusting subaerial unconformities are cut locally on both the proximal margin of the basin and the distal margin of the basin yet the ages of the proximal margin and distal margin unconformities are out of phase in the tectonic cycle erosion is most pronounced during quiescence on the proximal side and during thrusting on the distal side

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

the effect of paleotopography on the late albian and cenomanian sea level record of the canadian cretaceous interior seaway in western canada a major paleoenvironmental change at the albian cenomanian boundary is related to a eustatic sea level rise overprinted by a relative sea level fall in conjunction with preexisting topography within the basin this paper shows the lateral variability of sedimentology paleoecology and biostratigraphy of the latest albian to cenomanian interval along the canadian western margin of the cretaceous interior seaway this paper 1 provides an integrated depositional and paleoecological examination of the albian and cenomanian lithologic units in the northern western interior seaway 2 demonstrates the effect of antecedent paleotopography within the basin at the time of the transgression 3 documents the variability in sedimentary facies and paleoecology controlled by paleorelief and 4 demonstrates the diachroneity lateral extent and extremity of the multiple unconformities controlled by this paleorelief an enigmatic aspect of the lowermost cenomanian fish scales formation has been the regional occurrence of chert quartz and bioclastic pebbles associated with black anoxic shale a regional north south traverse across alberta provides insight into this problem in southwestern alberta chert and quartzite pebble conglomerate and sandstone that are equivalent to the fish scales formation known as barons sandstone subsurface and blairmore grits outcrop represent proximal shelf sedimentation this coarse grained sediment was flushed out during sea level lowstand and then reworked by a subsequent transgression associated with the belle fourche formation a large paleohigh existed in southwesternmost alberta at this time northward the fish scales formation is bounded top and bottom by unconformities and conglomerate indicating multiple sea level fluctuations and deposition in a wave influenced shelf environment in west central alberta preserved deposits of the fish scales formation indicate deposition in a nearshore setting farther to the north and most distally in northwestern alberta and elsewhere to the east the coarse component of the fish scales formation is predominantly gravel sized fish and other vertebrate debris with significantly less siliciclastic detritus regionally the unconformity at the base of fish scales barons becomes more pronounced southward and westward where the underlying shale of the westgate formation or westgate member has been eroded or was never deposited the regional paleogeographic setting for the barons sandstone and fish scales formation indicates greater amounts of erosion and coarser grained deposition in the southwest associated with the paleohigh northward and eastward there was greater accommodation space less erosion associated with the unconformity and finer grained sediment

vetusols and paleosols natural versus man induced environmental change in the atlantic coastal region of morocco abstract surface terrae rossae rhodoxeralfs and a terra fusca petrocalcic palexeralf developed on mid quaternary calcarenites in the moister region of rabat show substantial pedogenic formation of clay minerals particularly kaolinite mainly in the fraction m for the most part the pedogenic kaolinites show poor crystallinity of the fireclay type they are mainly formed from feldspars and perhaps also amphiboles and pyroxenes in the fractions 2 m and from smectites inherited from the calcarenite residual loam in the coarse 2 0 2 m and fine clay fractions m in one terra rossa soil formation was interrupted twice by deposition of eolian materials and it is regarded as a polycyclic pedocomplex eolian input was much less important during development of the other surface terrae rossae on younger calcarenites 100 ka near the coast three rendzinas typic calcixerolls show only slight mineral weathering and pedogenic clay formation in spite of a varied spectrum of primary minerals in one example kaolinite formation has just started the calcixerolls were probably recalcified by addition of eolian material as a result of anthropogenic thinning of the vegetation and or due to the vicinity of the coast these results suggest that at least in this region the younger terrae rossae of the coastal area were formed over much of the brunhes epoch in areas farther from the coast they have been developing for almost all of the brunhes epoch because climatic fluctuations in the quaternary were generally slight in this region the direction of soil formation was constant therefore we regard the rhodoxeralfs in the northern part of our study area as vetusols according to our expanded definition vetusols are surface soils which underwent almost constant processes of soil formation under an almost constant constellation of soil forming factors especially climate and vegetation over a long period of time including at least some part of the pleistocene consequently the present patchy distribution of terrae rossae is hardly a result of climatically induced periods of geomorphological activity and stability but the result of the removal of an earlier dense forest cover followed by severe soil erosion the human impact on the ecosystem is much more serious than any climatic fluctuations in the southwestern part of the study area between el jadida and agadir the soil moisture regime is dry xeric to aridic and terrae rossae occur only in small areas mostly in depressions however two selected rhodoxeralfs in the vicinity of tamanar show considerable pedogenic mineral weathering and clay mineral formation though hardly any kaolinite the strong weathering and clay illuviation must have been preceded by decalcification implying a distinctly moister climate here in the past however micromorphological features of recent recalcification are common in these rhodoxeralfs suggesting that their main properties are not in equilibrium with the present climate and the soils should be regarded as nonburied paleosols or relict paleosols

correlation of physical surfaces bentonites and biozones in the cretaceous colorado group from the alberta foothills to southwest saskatchewan and a revision of the belle fourche second white specks formational boundary new allostratigraphic correlations in west central alberta show that flooding surfaces in the sunkay and basal vimy members of the blackstone formation can be traced eastward from burnt timber creek for 500 km to western saskatchewan and for 350 km northward into the dunvegan and kaskapau formations at burnt timber creek a miliammina manitobensis microfauna in the lowest 1 m of the sunkay member indicates equivalence to the westgate formation and an overlying 70 cm conglomerate is correlative with the fish scales formation overlying mudstones contain foraminifera of the verneuilinoides perplexus zone indicative of the middle cenomanian belle fourche formation the base of the second white specks formation as currently defined in core in eastern alberta equates in outcrop with the lower part of the sunkay rather than the base of the vimy member as previously thought the base of the second white specks formation should be placed about 35 m higher in the stratotype core a bentonite located 9 3 m

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

monitoring geothermal reservoir developments with the controlled source electro magnetic method a calibration study on the reykjanes geothermal field abstract surface geophysical monitoring techniques are important tools for geothermal reservoir management as they provide unique information on the reservoir development away from boreholes for magmatic environments electromagnetic em methods are attractive monitoring tools as they allow to characterize the reservoir and hence potentially monitor changes related to fluid injection production indeed the electrical resistivity of reservoir rocks is highly dependent on the volume chemistry and phase of the in situ geothermal brine e g liquid vapor supercritical passive em techniques e g magnetotellurics or mt are traditionally used for geothermal exploration and a few recent studies have demonstrated its potential for monitoring reservoir development one of the main challenges is though the presence of cultural noise and or variability of the earth magnetic field that can obfuscate the em signals of interest we have investigated the benefits and drawbacks of active em surveying controlled source em or csem to tackle this challenge first with a synthetic study and subsequently with an actual time lapse survey acquired in 2016 and 2017 over the reykjanes geothermal field in iceland before baseline and after monitor the thermal stimulation of the supercritical rn 15 iddp 2 geothermal well the synthetic study showed that for geothermal fields having a resistivity structure similar to the reykjanes field i e a conductive caprock overlying a more resistive higher temperature reservoir csem and mt measurements can both detect resistivity changes within the deep resistive reservoir provided that measurement errors are small variations in many survey parameters e g errors in receiver position orientation differences in recording devices variations of near surface conditions external noise can create significant time lapse csem measurement errors our actual time lapse survey showed that when similar csem equipment is used during the baseline and monitor surveys and systematically d gps positioned the remaining key parameter controlling the survey repeatability is the level of external noise since the influence of external noise on csem data can be artificially reduced e g by increasing the transmitter dipolar moment it offers the possibility to adapt the survey design to increase the chance of detecting the time lapse signals of interest on the contrary little control is possible on the mt signal to noise ratio and hence repeatability the time lapse em survey acquired over the reykjanes geothermal reservoir showed indeed that a high csem survey repeatability can be achieved with electric field measurements within a few percent but that time lapse mt survey is a challenging task because of the high level of cultural noise in this industrialized environment to assess the quality of our csem dataset we inverted the data and confronted the resulting resistivity model with the resistivity logged in the rn 15 iddp 2 well we obtained a good match up to 2 3km depth i e enough to image the caprock and the liquid dominated reservoir but not deep enough to image the reservoir in supercritical conditions to obtain such an image we had to jointly invert legacy mt data with our csem data on the monitoring aspects the analysis of changes in electric fields did not allow to identify any csem signal related to the thermal stimulation of the rn 15 iddp 2 well one possible explanation is the weakness of the time lapse csem signal compared the achieved csem survey repeatability as a result of a limited resistivity change over a limited volume within the reservoir future reservoir developments in the supercritical reservoir e g hydraulic stimulation long term fluid circulation will most likely generate stronger resistivity variations over a larger volume than during the thermal stimulation of the well this calibration study provides the basic information for deciding when and how an em monitor survey must be performed for the monitoring of the reykjanes geothermal reservoir but also for the definition of the monitoring strategy of similar high enthalpy geothermal reservoirs

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

geochemistry geochronology and tectonic implications of two quartz monzonite intrusions purcell mountains southeastern british columbia the reade lake and kiakho stocks are posttectonic mesozonal quartz monzonite porphyries that intrude dominantly middle proterozoic purcell supergroup rocks in southeastern british columbia k ar dates of hornblende from the reade lake stock range from 103 to 143 ma however a u pb date of 94 ma from zircon concentrates is interpreted to be the age of emplacement of the stock suggesting the range and older k ar dates are due to excess 40ar a k ar date of 122 ma for the hornblende from the kiakho stock is believed to be a more reliable intrusive age both stocks cut across and apparently seal two faults that have played roles in the tectonic evolution of the purcell anticlinorium and rocky mountain thrust belt the reade lake stock cuts the st mary fault an east trending reverse thrust that crosses the rocky mountain trench and links with thrusts in the rocky mountains the kiakho stock cuts the cranbrook fault an older east trending normal fault hence the 94 ma date on the reade lake stock constrain

sedimentary controls on the sequence stratigraphic architecture in intra cratonic basins an example from the lower permian shanxi formation ordos basin northern china intra cratonic basins are characterized by stable tectonic regimes closed depositional systems finite sizes of receiving basins and extremely low morphological gradients this paper examines the effect of sediment accumulation on the sequence stratigraphic architecture and quantitatively evaluates its controls on the development of intra cratonic sedimentary sequences using numerical modelling a well documented intra cratonic sedimentary sequence the lower permian shanxi formation in the ordos basin northern china was used to illustrate a sequence stratigraphic model developed for intra cratonic sedimentary basins the studied sequence is characterized by a typical backstepping or source ward retrogradation a 2 d simulation software sedpak and a 3 d simulation software sedsim were used to model the intra cratonic sequence the modelling results indicated that sediment accumulation alone can produce the classical retrogradational stratigraphic stacking patterns the sediment accumulation can 1 increase the retrogradational range of the original retrogradational stacking sequences 2 change an original aggradational stacking sequence to a retrogradational stacking one and 3 decrease the progradational range of an original progradational stacking sequence or 4 even change an original progradational stacking sequence to a retrogradational stacking sequence understanding the relationship between the sediment accumulation and the stratigraphic development in an intra cratonic basin is essential for interpreting the sequence stratigraphic framework and stacking patterns and for predicting the distribution of potential reservoir sand bodies within such basins this work enriches the classic sequence stratigraphic models by providing a new model for intra continental basins and offers new insight on hydrocarbon exploration in intra cratonic basins

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

inversion of magnetotelluric data for 2d structure with sharp resistivity contrasts we have developed a linearized algorithm to invert noisy 2 d magnetotelluric data for subsurface conductivity structures represented by smooth boundaries defining sharp resistivity contrasts we solve for both a fixed number of subsurface resistivities and for the boundary locations between adjacent units the boundary depths are forced to be discrete values defined by the mesh used in the forward modeling code the algorithm employs a lagrange multiplier approach in a manner similar to the widely used occam method the main difference is that we penalize variations in the boundary depths rather than in resistivity contrasts between a large number of adjacent blocks to reduce instabilities resulting from the breakdown of the linear approximation we allow an option to penalize contrasts in the resistivities of adjacent units we compare this boundary inversion method to the smooth occam inversion for two synthetic models one that includes a conductive wedge between two resistors and another that includes a resistive wedge between two conductors the two methods give good agreement for the conductive wedge but the solutions differ for the more poorly resolved resistive wedge with the boundary inversion method giving a more geologically realistic result application of the boundary inversion method to the resistive gemini subsalt petroleum prospect in the gulf of mexico indicates that the shape of this salt feature is accurately imaged by this method and that the method remains stable when applied to real data

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

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

cretaceous albian turonian foraminiferal biostratigraphy and paleontology of northern montana and southern alberta abstract in albian turonian time the interior of north america was flooded by a seaway extending from the present gulf of mexico to the arctic ocean transgressive phase greenhorn cyclothem detailed studies of this interval north and south of the international border have usually not been integrated the present foraminiferal biostratigraphic study includes a 38 000 km2 area straddling the alberta montana border from the lewis thrust in the west to the sweetgrass hills in the east including the sweetgrass arch stratigraphic cross sections and isopach maps of 6 albian early turonian stratigraphic units prepared from 57 surface and subsurface sections demonstrate that sedimentation was controlled primarily by 1 spasmodic volcanism to the west 2 tectonic activity coincident with the present location of the sweetgrass arch the occurrence of the late cenomanian planktonic foraminifer rotalipora cushmani in association with three other keeled species suggests an east west marine connection between the eastern pacific and the western interior at the maximum greenhorn transgression this is consistent with the facies model described by kauffman 1967 1969 1977 the paleogeographic model developed in the present study reported gastropod paleo biogeographic data and reevaluation of pelecypod and ammonite paleo zoogeographic interpretations

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

tectono sedimentary control on modern sand deposition on the forebulge of the western taiwan foreland basin abstract whether the formation of the isolated sand body deposition in the forebulge area of a foreland basin system is structure or deposition controlled has puzzled geologists for decades although sand body deposition is generally believed to be indicative of the position of the flexural forebulge in a foreland basin the formation of a modern sand body in the forebulge area is thus examined by multi scale geophysical observations based on combined reflection seismic profiles and compressed high intensity radar pulse chirp profiles across the sand deposition along the forebulge of the western taiwan foreland basin wtfb which is a late miocene present foreland basin in the overfilled stage these profiles suggest that the accumulation of the sand deposits along the forebulge of the wtfb is not directly associated with forebulge faultings the relief map of the forebulge deposit substratum shows a northwestward tilting slope and the isopach of the forebulge sand body indicates that a large part of the sand body accumulated along the axis of the taiwan strait and the subdued forebulge of the wtfb the difference between the prevailing directions of tidal currents between the taiwan strait and the east china sea reflects the probable sedimentary influence of the cratonward migrating fold thrust belt within a foreland shelf we suggest that the formation and distribution of the sand deposits along the forebulge of the wtfb are generally controlled not only by the transverse downslope sedimentation but also longitudinal hydrodynamic processes at distal parts of the foreland basin our explanation provides a plausible tectono sedimentary cause of the sand body deposition in the forebulge area in an overfilled foreland basin the sedimentary dynamics of the sand body in the taiwan strait may be applicable for understanding the formation of isolated sand bodies in the distal part of the cretaceous western interior foreland basin

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