ice flow patterns and glacial transport in the eastern hudson bay region implications for the late quaternary dynamics of the laurentide ice sheet recent field surveys in the eastern hudson bay region have led to the discovery of regional ice flow sequences that require a significant reassessment of the late quaternary dynamics of the laurentide ice sheet two regional ice flow phases can be recognized from till compositional data and from crosscutting relationships observed on striated bedrock surfaces the oldest is directed toward the northwest and north northwest while the youngest is directed toward the west and includes a late glacial deflection toward the southwest the wide regional distribution of striae formed during the early northwestward glacial movement together with the recognition of palimpsest glacial dispersal trains associated with this phase suggest that it was a long lived time transgressive regional event the ensuing glacial movement is a regionally dominant westward ice flow phase during which several large glacial dispersal trains were formed downglacier from distinctive bedrock sources the largest of these trains extends

detrital carbonate peaks on the labrador shelf a 13 7 ka template for freshwater forcing from the hudson strait outlet of the laurentide ice sheet into the subpolar gyre abstract the laurentide ice sheet lis was a large dynamic ice sheet in the early holocene the glacial events through hudson strait leading to its eventual demise are recorded in the well dated labrador shelf core md99 2236 from the cartwright saddle we develop a detailed history of the timing of ice sheet discharge events from the hudson strait outlet of the lis during the holocene using high resolution detrital carbonate ice rafted detritus ird 18o and sediment color data eight detrital carbonate peaks dcps associated with ird peaks and light oxygen isotope events punctuate the md99 2236 record between 11 5 and 8 0 ka we use the stratigraphy of the dcps developed from md99 2236 to select the appropriate r to calibrate the ages of recorded glacial events in hudson bay and hudson strait such that they match the dcps in md99 2236 we associate the eight dcps with h0 gold cove advance noble inlet advance initial retreat of the hudson strait ice stream hsis from hudson strait opening of the tyrrell sea and drainage of glacial lakes agassiz and ojibway the opening of foxe channel and retreat of glacial ice from foxe basin are represented by a shoulder in the carbonate data r of 350 years applied to the radiocarbon ages constraining glacial events h0 through the opening of the tyrell sea provided the best match with the md99 2236 dcps r values and ages from the literature are used for the younger events a very close age match was achieved between the 8 2 ka cold event in the greenland ice cores dcp7 8 15 ka bp and the drainage of glacial lakes agassiz and ojibway our stratigraphic comparison between the dcps in md99 2236 and the calibrated ages of hudson strait bay deglacial events shows that the retreat of the hsis the opening of the tyrell sea and the catastrophic drainage of glacial lakes agassiz and ojibway at 8 2 ka are separate events that have been combined in previous estimates of the timing of the 8 2 ka event from marine records sw iceland shelf core md99 2256 documents freshwater entrainment into the subpolar gyre from the hudson strait outlet via the labrador north atlantic and irminger currents the timing of freshwater release from the lis hudson strait outlet in md99 2236 matches evidence for freshwater forcing and lis icebergs carrying foreign minerals to the sw iceland shelf between 11 5 and 8 2 ka the congruency of these records supports the conclusion of the entrainment of freshwater from the retreat of the lis through hudson strait into the subpolar gyre and provides specific time periods when pulses of lis freshwater were present to influence climate

aligned vertical fractures hti reservoir symmetry and thomsen seismic anisotropy parameters for polar media certain crack influence parameters of sayers and kachanov are shown to be directly related to thomsen s weak anisotropy seismic parameters for fractured reservoirs when the crack fracture density is small enough these results are then applied to the problem of seismic wave propagation in polar reservoirs i e those anisotropic reservoirs having two axes that are equivalent but distinct from the third axis especially for horizontal transversely isotropic seismic wave symmetry due to the presence of aligned vertical fractures and resulting in azimuthal seismic wave symmetry at the earth s surface the approach presented suggests one method of inverting for fracture density from wave speed data a significant fraction of the technical effort in the paper is devoted to showing how to predict the angular location of the true peak or trough of the quasi sv wave for polar media and especially how this peak is related to another angle that is very easy to compute the axis of symmetry is always treated here as the x 3 axis for either vertical transversely isotropic symmetry due for example to horizontal cracks or horizontal transversely isotropic symmetry due to aligned vertical cracks then the meaning of the stiffnesses is derived from the fracture analysis in the same way for vertical transversely isotropic and horizontal transversely isotropic media but for horizontal transverse isotropy the wave speeds relative to the earth s surface are shifted by 90 in the plane perpendicular to the aligned vertical fractures skempton s poroelastic coefficient b is used as a general means of quantifying the effects of fluids inside the fractures explicit biot gassmann consistent formulas for thomsen s parameters are also obtained for either drained or undrained fractures resulting in either vertical transversely isotropic or horizontal transversely isotropic symmetry of the reservoir

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

advantages of borehole vector gravity in density imaging abstractborehole gravity is an effective method for directly imaging anomalous mass distributions at large depths and it has broad applications in mineral and petroleum exploration and production the method is presently limited to vertical gravity data only the anticipated development of vector gravimeters may change this scenario dramatically there was a need to understand the potential capability of vector gravimetry and to develop corresponding modeling and interpretation techniques we introduced a set of numerical modeling tools for vector gravity and examined the relative information content of vertical and vector gravity data in borehole environments through simulations we illustrated that vector gravity was superior to vertical gravity in applications such as locating and characterizing anomalous mass and in tracking the movement of fluid injection in reservoirs

hudson bay hudson strait jokulhlaups and heinrich events a hypothesis abstract abrupt temperature changes in the northern north atlantic occurred frequently throughout the last glaciation as shown by proxy records from greenland ice cores deepsea cores and norway speleothems many of these variations occurred in irregular sawtooth cooling cycles which sometimes ended with the deposition of thick layers of ice rafted debris ird in the deep ocean that are known as heinrich events the lithologies of most of the ird deposits are consistent with widespread surges of the laurentide ice sheet that may have resulted from the accumulation of deformable sediments in portions of the ice sheet bed an alternative conceptual model proposed here to explain the sawtooth coolings and the surges involves repetitive jokulhlaups from a hudson bay lake dammed by ice at the mouth of hudson strait the slow sawtooth coolings may be explained by storm track diversion due to progressive coverage of the lake by icebergs and the surges by abrupt losses of water pressure and buttressing ice shelves at all ice sheet fronts in the lake when the ice dams failed

40ar 39ar and pb pb study of individual hornblende and feldspar grains from southeastern baffin island glacial sediments implications for the provenance of the heinrich layers lead isotope analysis of individual feldspar grains and 40ar 39ar analysis of individual hornblende grains from three samples taken from baffin island ice proximal till deposits are consistent with a provenance recording a two stage geologic history including formation of continental crust in the late archean and partial isotopic resetting and or reequilibration during major paleoproterozoic regional metamorphism and magmatism significantly although the populations are small and the two stage archean paleoproterozoic history of their sources are similar the pb isotope compositions of samples from baffin island do not match the results from heinrich layer h2 in core hu87 033 009 from northeast of the hudson strait in the labrador sea specifically in h2 samples from hu87 033 009 the 207pb 204pb is high for a given value of 206pb 204pb additionally the range in 206pb 204pb is greater in hu87 033 009 samples and there are two distinct trajectories of 208pb 204pb the steeper of the 208pb 204pb versus 206pb 204pb trends is not seen in the studied baffin island glacial sediment samples samples of feldspar grains from heinrich layers h1 h2 h4 and h5 from core v28 82 in the eastern north atlantic have a 207pb 204pb versus 206pb 204pb relation similar to that of baffin island till samples but there is also a high 208pb 204pb that we have not captured in this suite of glacial sediment samples our results highlight the strength of a combined radiogenic isotope approach to characterizing the geological history of sediment sources and provide a first step in assessing the variability in isotope character in this region of dynamic glacial events during the late quaternary period

rare earth elements and nd and pb isotopes as source indicators for labrador sea clay size sediments during heinrich event 2 elemental abundances and nd and pb isotope ratios were determined on samples from the carbonate free clay size fractions of sediments from intervals above within and below heinrich event 2 h 2 in core hu87 9 from the northwest labrador sea slope in hu87 9 rare earth element ree distributions and elemental concentrations within the h 2 event are distinct from those outside this event end 0 and 206 pb 204 pbdata also indicate different values for sediments deposited within and outside the h 2 event comparisons of ree patterns from the h 2 interval with those from bedrock units in baffin island northern quebec and labrador indicate that the lake harbour group lhg which crops out on the north side of the hudson strait is the most probable bedrock source of the clay size fraction found within the h 2 interval in hu87 9 the tasiuyak gneiss tg and lac lomier complex llc have ree patterns including a negative eu anomaly similar to those found in h 2 sediments however the la ybratios of these units are smaller than those associated with h 2 sediments the nd and pbisotope data support and complement ree based interpretations of provenance i e the nd pb signatures of sediments deposited at the hu87 9 site during the h 2 event are similar to nd pbsignatures ob tained on diamicts from the western end of hudson strait published by elsevier science ltd

final stages in the collapse of the laurentide ice sheet hudson strait canada nwt 14c ams dates seismics stratigraphy and magnetic susceptibility logs hudson strait is a key element in the history of ice sheet ocean interactions during the deglaciation of the laurentide ice sheet data from over 30 giant piston cores supplemented by ca 40 14c ams dates on foraminifers and shells from sites within and adjacent to hudson strait are used to describe the timing and sediment processes associated with the final complex phases of deglaciation in hudson strait each core is placed in a regional seismic architectural context through examination of deep tow huntec profiles most cores sample a thin postglacial unit and extend several meters into glacial marine sediment mainly silty clays 14c dates on cores from the adjacent shelf indicate that deglaciation occurred by 13 ka we have not been unable to confirm that hudson strait was ice free by 11 12 ka although this is a probable scenario whole core magnetic susceptibility wcms data are used as the primary signature of changes in the style of sedimentation and provenance and to correlate between cores wcms indicates that there is a regional differentiation in the ms signal associated with bedrock provenance changes in contribution of detrital carbonate and style of sedimentation one low wcms event is present in cores from both the western and eastern basins it dates from 8 0 ka and provides an important regional stratigraphic marker probably associated with the final deglaciation of hudson bay and hudson strait sediment from a southerly source ungava bay continued to be contributed into the 900 m deep eastern basin of hudson strait 8 and 7 ka and thick postglacial sequences were piled within ungava bay between 6 and 7 ka

southern laurentide ice lobes were created by ice streams des moines lobe in minnesota usa abstract regional mapping in southern minnesota has illuminated a suite of landforms developed by the des moines lobe that delimit the position of the lobe at its maximum and at lesser readvances the ice lobe repeatedly advanced discharged its subglacial water and subsequently stagnated recent glaciological research on antarctic ice streams has led some glacial geologists to postulate that ice streams drained parts of the marine based areas of the laurentide ice sheet i postulate that such ice streams may develop in land based areas of an ice sheet as well and that the des moines lobe 200 km wide and 900 km long was an outlet glacier of an ice stream it appears to have been able to advance beyond the laurentide ice sheet as long as adequate water pressure was maintained however the outer part of the lobe stagnated because subglacial water that facilitated the flow was able to drain away through tunnel valleys stagnation of the lobe is not equivalent to stoppage of the ice stream because ice repeatedly advanced into and onto the stagnant margins stacking ice and debris similar landforms are also seen in other lobes of the upper midwestern united states

the use of vertical line sources in electrical prospecting for hydrocarbon conventional electrical prospecting can be extended to the search for deep seated hydrocarbon deposits by using the steel casings of drill holes as vertical line sources these sources produce at depth a density of current higher than the density created by point sources located at the ground surface several tests have shown that the contrast of conductivity between resistive hydrocarbon deposits and the surrounding salt water produces relevant anomalies on a resistivity map obtained with vertical line sources especially where there exists a superficial masking effect caused by a highly resistive layer in a survey carried out in the ussr combined measurements were performed both with line source and with surface point sources the detected residual resistivity anomaly roughly delineates the contours of the known hydrocarbon deposit

lateral plucking as a mechanism for elongate erosional glacial bedforms explaining megagrooves in britain and canada megagrooves are kilometre scale linear topographic lows carved in bedrock separated by ridges typically in areas of largely devoid of till they have been reported from several areas covered by pleistocene glaciations such as canadian nw territories michigan and nw scotland here we report two previously undocumented megagroove fields from ungava canada and northern england and present new analyses of the megagrooves from nw scotland this paper seeks to determine the nature of the lithological and structural controls on the occurrence and formation of megagrooves analysis of both geomorphological and bedrock properties shows that megagrooves are generally a confined to well stratified or layered bedrock such as meta sedimentary rocks with closely spaced joints and tend not to occur on massive rocks such as gneiss or granite or thick bedded sedimentary rocks b subparallel to palaeo ice flow and the strike of the strata and tend not to occur where palaeo ice flow is at high angles to the strike of strata c produced by significant glacial erosion by sustained unidirectional ice flow detailed analysis of megagrooves in nw scotland shows that neither glacio fluvial erosion nor differential abrasion was the dominant mechanism of formation a mechanism here termed lateral plucking is suggested that involves block plucking on rock steps parallel to ice flow removal of joint bounded blocks from such rock steps involves a component of rotation along a vertical axis block removal may be enhanced by a direct component of shear stress onto the vertical stoss sides the lateral plucking mechanism results in horizontal erosion at right angles to the ice flow and enhances the groove ridge topography megagrooves are potentially useful as palaeo ice flow indicators in areas devoid of till and can thus complement the palaeo ice stream datasets which are presently largely based on soft sediment landform studies

isotope stratigraphy sedimentation rates deep circulation and carbonate events in the labrador sea during the last 200 ka high resolution stable isotope and accelerator mass spectrometry 14c measurements in 25 box and piston cores from the labrador sea allow the reconstruction of major paleoceanographical changes during the last 200 ka in this basin and also document the links between the interior of the laurentide ice sheet and the north atlantic ocean during this interval two deep circulation regimes are indicated by contrasting sedimentation rates on the slopes i the modern situation is characterized by a gyre of the north atlantic deep water nadw components into the basin which is responsible for the high energy western boundary undercurrent this regime also prevailed during the former true interglacials and possibly during shorter warm intervals e g bolling allerod ii a more frequent nadw free situation during glacial and interstadial intervals is marked by the presence of a vertically homogeneous water mass in the basin sedimentological records also indicate that carbonate events i e pulses of

marine episodes in hudson strait and hudson bay canada during the wisconsin glaciation abstract twelve reworked shell fragments were recovered from five till samples collected on nottingham island in western hudson strait canada total hydrolysate amino acid ratios d alloisoleucine l isoleucine for each fragment range from 0 043 to 0 154 and are significantly higher than ratios for radiocarbon dated holocene shells which average 0 022 one shell fragment with an aile ile ratio of 0 065 yielded an ams radiocarbon age of 44 200 2300 yr b p aa 3254 absolute age estimates based on the amino acid ratios are constrained by reasonable limits on the estimated effective diagenetic temperature of the fossils on this basis the shell fragments are correlated with oxygen isotope substages 5e and 5a and stage 3 in the preferred hypothesis theses results independently support hypotheses calling for at least partial deglaciation and marine incursion in hudson strait and hudson bay at these times

catastrophic ice shelf breakup as the source of heinrich event icebergs 1 heinrich layers of the glacial north atlantic record abrupt widespread iceberg rafting of detrital carbonate and other lithic material at the extreme cold culminations of bond climate cycles both internal glaciologic and external climate forcings have been proposed here we suggest an explanation for the iceberg release that encompasses external climate forcing on the basis of a new glaciological process recently witnessed along the antarctic peninsula rapid disintegrations of fringing ice shelves induced by climate controlled meltwater infilling of surface crevasses we postulate that peripheral ice shelves formed along the eastern canadian seaboard during extreme cold conditions would be vulnerable to sudden climate driven disintegration during any climate amelioration ice shelf disintegration then would be the source of heinrich event icebergs

electrical efficiency a pore geometric theory for interpreting the electrical properties of reservoir rocks knowledge of the way pore geometry affects the conductivity of rocks is important for the evaluation of sedimentary rocks for hydrocarbon potential virtually all formation evaluation from logs however uses the empirical archie equation that has never been given a satisfactory physical basis as an alternative we have formulated a new model based on the efficiency with which water bearing rocks conduct electric currents as determined by the conducting phase geometry electric current densities are not uniform throughout the conducting phase in the pore system of a rock numerical model studies show high current density in pore throats and low current density in nearly stagnant volumes in isolated parts of the pore system the electrical efficiency of a rock is the ratio of the average power developed in all water bearing parts of the pore system to the power developed in a straight tube with the same length and water volume as the rock it is inherently independent of the bulk volume of water in the ro

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

numerical modelling of the laurentide ice sheet in the baffin island region the role of a cumberland sound ice stream a numerical model reconstruction was made of the northeastern laurentide ice sheet in the baffin island foxe basin region using geophysical terrestrial and marine geologic evidence for initial

the physiography of high arctic cross shelf troughs abstract a comprehensive inventory of high arctic cross shelf troughs is compiled from international bathymetric chart of the arctic ocean ibcao bathymetric data v 3 0 the location of 75 cross shelf troughs is presented alongside a synthesis of their key physiographic characteristics and available glacial geological evidence the troughs are interpreted to have been occupied intermittently and eroded by marine terminating ice streams that traversed the shelf during at least one and often many quaternary full glacial periods considerable variation in cross shelf trough physiography exists in the high arctic trough lengths range between 35 and 1400 km widths from 12 to 260 km and maximum depths from 200 to 1000 m the longest cross shelf troughs extend through inter island channels on the beaufort sea queen elizabeth islands and barents kara sea margins the gradient of the upper slope beyond high arctic glacial troughs which ranges between 0 3 and 13 is shown to have a negative relationship with palaeo ice stream drainage basin area and trough length glacial sedimentary depocentres or trough mouth fans tmfs are inferred on the basis of bathymetric and where available seismic evidence to exist beyond the majority of high arctic cross shelf troughs evidence of shelf progradation or tmf development is absent from the slope beyond several high arctic troughs probably as a result of limited sediment supply to the margin and or sediment by passing of the upper slope on the south greenland continental margin it is likely that steep slopes prevented the development of significant glacial sedimentary depocentres beyond cross shelf troughs or that glacigenic debris has been removed from the upper slope by submarine slope failure

seismic characterization of multiple fracture sets does orthotropy suffice as geophysicists rely increasingly on effective media theories to characterize naturally fractured reservoirs they become more and more interested in evaluating the accuracy of different theories estimating their limits of applicability and assessing their usefulness for practice with this in mind we compare two popular seismological theories of hudson and schoenberg with the theory of kachanov developed in the context of mechanics of materials by performing finite element simulations of effective media for models that contain several sets of nonintersecting circular vertical fractures embedded in otherwise isotropic host rock we examine the accuracy of these theories our numeric study reveals that predictions of both the first and second order hudson s theories are typically inferior to those of others especially when the fractures are dry while on average the theories of schoenberg and kachanov fit finite element computations with comparable accuracy the latter appears to be more useful

finite element analysis of top casing electric source method for imaging hydraulically active fracture zones author s um es kim j wilt mj commer m kim ss abstract 2019 society of exploration geophysicists imaging hydraulically active fracture zones hafz is of paramount importance to subsurface resource extraction geologic storage and hazardous waste disposal we have developed advanced 3d finite element fe electrical imaging algorithms for hafz in the presence of a steel cased well the algorithms use tetrahedral fe meshes in the simulation domain and coarse rectangular finite difference meshes in the imaging domain this heterogeneous dual mesh approach is well suited to modeling the multiscale earth model due to steel cased wells we find that the algorithms accurately and efficiently simulate surface electric field measurements over a 3d hafz at depth when one end point of a surface electric source is connected to a wellhead for brevity this configuration is called the top casing electric source method by replacing a hollow cased well with a solid prism we improve our computational efficiency without affecting the solution accuracy the sensitivity of the top casing source method to hafz highly depends on the continuity of a steel cased well because it makes currents preferentially flow to hafz the sensitivity also depends on conductivity structures around the well because they control current leaking from the steel cased well we find that the method can image a localized hafz and detect changes in its width and height the imaging results are improved when a volume of the imaging domain is constrained from geomechanical perspectives a primary advantage of the method is the fact that the sources and receivers are placed on the surface thus not interrupting the well operation

palaeo ice streams abstract the location and behaviour of ice streams is one of the most important controls on ice sheet configuration and stability in order to reconstruct former ice sheets we need to know ice stream location and timing once identified the beds of palaeo ice streams provide an unprecedented opportunity to glean information about their basal environment something that remains very difficult under contemporary ice streams this paper represents the first synthesis and discussion of palaeo ice stream research from a variety of former ice sheets and includes new insights relating their configuration and activity to the evidence they leave behind the ambiguous use of the term ice stream is addressed and four possible configurations for ice streams are presented diagnostic landforms and landform assemblages that we would expect an ice stream to produce are discussed and the temporal context of bedform generation is described as either a rubber stamped or smudged bedform imprint resulting from isochronous or time transgressive landform generation in particular we focus on the configuration of terrestrially terminating ice streams for which there are no modern analogues technological advances in marine geophysics are helping to generate a convergence of interest between antarctic glaciology and palaeo glaciology it is suggested that investigations around the fringes of contemporary ice sheets i e west antarctica can provide evidence that directly links the geomorphological record of palaeo ice streams with their contemporary counterparts in addition computational advances and modelling adaptations have permitted the incorporation of ice streams within ice sheet models we argue that data from palaeo ice stream beds are invaluable to ice sheet stream modelling experiments and will help us understand ice stream operation and the linkages between climate perturbations and both palaeo and contemporary ice sheets

les ecoulements et le transport glaciaires dans la partie septentrionale du nunavik quebec in northern nunavik orientation of ice flow features and dispersal patterns of lithologic indicators suggest three major regional ice flow phases the oldest ice flow phase is related to the buildup of an outflow centre the ungava centre over the high plateaux located north of the povungnituk hills during that event probably of early wisconsinan age ice flows towards the south and southeast carried cape smith belt rock fragments over a maximum distance of 65 km the second phase is characterized by ice flow mainly towards the north from an outflow ice centre located south of the studied area payne centre the most recent ice flow phase is related to the presence of the northern extension of the new quebec ice divide from that northwest trending ice divide extending between ivujivik and lake nantais ice was flowing outward on the peninsula the orientation of most of the ice flow features and dispersal patterns is associated with that last ice flow north of the cape smith belt glacial transport was at least 70 km in hudson strait ice flow and glacial transport data support the existence of an eastward flowing ice stream

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

dansgaard oeschger events is there a signal off the hudson strait ice stream abstract it is generally accepted that the dramatic heinrich h events were principally sourced from the major hudson strait drainage basin of the laurentide ice sheet but it is not known whether the higher frequency dansgaard oeschger d o events have an imprint on sediments deposited during the inter h intervals in ice proximal slope sediments this paper examines this question by an analysis of data from cores on the se baffin slope because the proxies for heinrich events are so massive in scale we removed the major h events h 1 h 2 and h 4 from the records so as to better examine the structure of data in the intervening hemipelagic sediments the period of interest extends between 15 and 40 cal ka interstadial events 2 8 resolution varies with the proxy to between 210 and 700 yrs per sample in hu87033 009 from the lower slope a total of twelve variables for example 18o mass magnetic susceptibility calcite dolomite ratio number of ird 2 mm show a variety of multi millennial oscillations although not all of the same amplitude nor frequency several parameters show little correlation with the gisp2 18o record however a comparison of cie sediment color l from hu87033 009 and hu97048 007 from the upper slope with the gisp2 oxygen isotope record suggests that a substantial correlation exists between the records such changes might be associated with either detrital carbonate content or foraminiferal concentrations analysis suggests however that the first possibility is correct hence suggestive of a glaciological response of the hudson strait ice stream with d o events in the nw labrador sea the sediment records from around h 3 time 30 cal ka show excursions which might be correlative within this event but its amplitude is not unusual compared to events both before and after this reaffirms the argument that h 3 represents a different glaciological response a significant issue discussed in this paper is the inherent ambiguity of correlating records whose errors in the time domain approach that of a dominating periodicity in this case ca 1 5 kyr

deglacial reservoir age and implications foxe peninsula baffin island marine reservoir age is reported for foxe basin canada during deglaciation of the laurentide ice sheet radiocarbon 14c measurements were made on pairs of contemporaneous molluscs and salix willow macrofossils yielding a mean marine reservoir age of 985 10 14c a mean regional r is calculated to be 615 20 14c a relative to a mean global reservoir age of ca 400 14c a previous studies in the canadian arctic that have been conducted on modern pre bomb molluscs give r values of ca 100 300 14c a the difference between modern and deglacial reservoir ages indicates that r in foxe basin has changed over time which has implications for the reconstruction of past events based on marine 14c dates we recalculate the timing of deglaciation of hudson strait with this new reservoir age and relate this to the 8 2 cal ka event we recommend that local r be determined for the time period being examined whenever possible and that in cases where this is impossible a r value of 615 14c a be applied when calibrating marine samples from foxe basin and vicinity during deglaciation copyright 2010 john wiley sons ltd

csem performance in light of well results during the past several years we have seen an increasing focus on the use of csem technology for hydrocarbon exploration in marine environments and recently a number of success stories have been published the technology has been demonstrated to aid both detection and delineation of hydrocarbon filled reservoirs

transient electromagnetic finite difference time domain earth modeling over steel infrastructure including highly conductive steel infrastructure into electromagnetic em earth modeling is motivated by the fact that long metal cased boreholes have the potential to be used as boosting antennas that enable larger source dipole moments and greater signal penetration depths unfortunately geophysical algorithms designed to simulate em responses over rather regional scales are complicated by material property contrasts and structure geometries that are more typical for em engineering applications hence the great majority of earth modeling algorithms that consider em responses from steel cased boreholes use integral equation methods to be able to model complex casing scenarios we revisited the finite difference time domain fdtd method to advance the modeling of transient em field responses from steel cased boreholes a time dependent function that allows for larger fdtd time steps in the dufort frankel method was developed alleviating the generally large computational overhead we compared our method against three different kinds of benchmark solutions to demonstrate the reliability of the fdtd field solutions these test cases were carried out to check the feasibility of a final hydraulic fracturing study images of the electric current distribution in a sheetlike rock fracture were calculated for the cases with and without the presence of a connecting borehole casing demonstrating the casing s potential of illuminating deep target zones

geomorphological criteria for identifying pleistocene ice streams ice streams are critical regulatory mechanisms in contemporary ice sheets it has been inferred that they also had a significant effect on the dynamics of former ice sheets subsequently many people have invoked their widespread occurrence from a variety of formerly glaciated areas hypothesised locations however have often outweighed meaningful evidence this paper addresses the problem using the characteristics of contemporary ice streams as a basis for their identification from former ice sheet beds a convergence of knowledge gained from contemporary ice stream research coupled with theories of glacial geomorphology allows several geomorphological criteria to be identified as suggestive signatures of ice stream activity it is envisaged that the geomorphological criteria developed here will introduce a more objective approach to the study of former ice streams the criteria are used to construct conceptual land system models of the beds of former ice streams and it is hoped such models can provide an observational template upon which hypotheses of former ice streams can be better based

the magnetic signature of rapidly deposited detrital layers from the deep labrador sea relationship to north atlantic heinrich layers rock magnetic parameters from deep labrador sea piston core p 094 are useful for recognizing rapidly deposited detrital layers some of which correlate with north atlantic heinrich layers variations in magnetic properties and lithology distinguish two classes of rapidly deposited detrital layers during the last ice age 1 seven detrital carbonate dc layers have high carbonate content and mean magnetite grain diameters about 4 times greater than the background sediments 2 six low detrital carbonate ldc layers have very similar magnetic properties to dc layers but low carbonate content similar to the background sediments the magnetite associated with dc and ldc layers is well sorted and relatively uniform within and between layers ice rafted detritus ird recognized by increased percentage of the 125 micron grain size fraction and by high magnetic susceptibility is associated with most dc and ldc layers but is not the dominant detrital constituent the correspondence of dc and ldc layers with increased grain size of well sorted magnetite but not with coarse fraction content precludes ice rafting as the primary depositional mechanism dc and ldc layers may have been deposited from suspended sediment derived from turbidite activity in the nearby northwest atlantic mid ocean channel namoc this interpretation is supported by a piston core p 013 from a greenland rise site largely but not completely outside the influence of the namoc the association of dc layers with ird and the apparent age correlation of dc layers to north atlantic heinrich layers suggest that the ice advances which produced the ird in p 094 and in correlative north atlantic heinrich layers also triggered turbiditic flows down the namoc several dc and ldc layers however do not correlate with recognized heinrich layers but appear to be coeval with recently discovered high frequency ird rich layers in the north atlantic

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

location based parameterization of controlled source electromagnetic data inversion for the monitoring of hydraulic fracturing with magnetically enhanced proppantsparametric inversion for hf monitoring

sensitivity analysis for the appraisal of hydrofractures in horizontal wells with borehole resistivity measurements abstractwe numerically evaluate the possibility of using borehole electromagnetic measurements to diagnose and quantify hydraulic fractures that have been artificially generated in a horizontal well hydrofractures are modeled as thin disks perpendicular to the well and filled with either sand based or electrically conductive proppant the study focuses on the effect of thickness and length radius of hydrofractures to assess their effects on specific configurations of borehole resistivity instruments numerical results indicate that several measurements e g those obtained with low and high frequency solenoids could be used to assess the thickness of a fracture however only low frequency measurements performed with electrodes and large spacing between transmitter and receivers 18 m exhibit the necessary sensitivity to reliably and accurately estimate the length of long hydrofractures up to 150 m in open hole wells in the case of steel cased wells the casing acts as a long electrode whereby

discrete dual porosity model for electric current flow in fractured rock the identification of fractures and the characterization of their properties are of critical importance in a wide variety of research fields and applications to this end geophysical methods are of significant interest as they can provide information regarding the spatial distribution of a number of subsurface physical properties in a rapid and noninvasive manner electrical resistivity surveying in particular has been shown in several previous investigations to exhibit sensitivity to the presence of fractures suggesting that geoelectrical experiments may contain important information regarding how fractures are distributed and connected in the subsurface however a lack of suitable numerical modeling tools for electric current flow in fractured media has prevented a detailed and systematic exploration of this concept to address this issue we present a novel discrete dual porosity modeling approach that is specifically tailored to the electrical resistivity problem with our approach an analytical formulation for fracture matrix current flow exchange at the fracture scale is integrated into a discrete fracture network model which is then combined with a block scale finite volume representation of the rock matrix our methodology allows for low cost and accurate simulation of electric current flow through both the fractures and matrix and is readily applicable to complex fracture networks at relatively large scales although formulated here in two dimensions this work represents an important first step toward investigating the effect of fracture network characteristics on bulk electrical properties as well as toward the simulation of geoelectrical survey data in realistic fractured rock environments

anomalous electrical resistivity anisotropy in clean reservoir sandstones we report novel laboratory measurements of the full electrical resistivity tensor in reservoir analogue quartzose sandstones with clay contents less than 1 5 we show that clean homogeneous visually uniform sandstone samples typically display between 15 and 25 resistivity anisotropy with minimum resistivity normal to the bedding plane thin section petrography analysis of fabric anisotropy and comparison to finite element simulations of grain pack compaction show that the observed anisotropy symmetries and magnitudes can be explained by syn depositional and post depositional compaction processes our findings suggest that electrical resistivity anisotropy is likely to be present in most clastic rocks as a consequence of ballistic deposition and compaction compaction may be deduced from measurements of electrical anisotropy and the anisotropy observed at larger scales in well logging and controlled source electromagnetic data with maximum resistivity normal to bedding is most likely the result of meso scale 10 1 m 101 m periodic layering of electrically dissimilar lithologies

finite element analysis for model parameters distributed on a hierarchy of geometric simplices abstracta resurgence of interest in the problem of electrical or electromagnetic scattering from thin conductors either on a line or in a plane has been motivated in recent years by time lapse fracture monitoring in the near surface enhanced geothermal reservoirs and unconventional hydrocarbon plays however finite element modeling of small electrical features in large computational domains results in a disproportionately large number of elements concentrated in a volumetrically insignificant fraction of the mesh and it focuses computational resources away from areas of interest elsewhere such as receiver locations we have developed a novel hierarchical electrical model is proposed for unstructured tetrahedral finite element meshes in which the usual volume based conductivity on tetrahedra is augmented by facet and edge based conductivity on the infinitesimally thin regions between elements doing so allows a slender borehole casing of arbitrary shape to be coarsely approximated by set of connec

accuracy and effectiveness of three dimensional controlled source electromagnetic data inversions abstractunconstrained 3d inversion of marine controlled source electromagnetic data csem data sets produces resistivity volumes that have an uncertain relationship to the true subsurface resistivity at the scale of typical hydrocarbon reservoirs furthermore csem scale resistivity is an ambiguous indicator of hydrocarbon presence not all resistivity anomalies are caused by hydrocarbon reservoirs and not all hydrocarbon reservoirs produce a distinct resistivity anomaly we have developed a method for quantifying the effectiveness of resistivities from csem inversion in detecting hydrocarbon reservoirs our approach uses probabilistic rock physics modeling to update information from a preexisting prospect assessment based on uncertain resistivities from csem the result is an estimate the probability of hydrocarbon presence that accounts for uncertainty in the resistivity and in rock properties examples using synthetic and real csem data sets demonstrate that the effectiveness of csem inversion in id

multi proxy records of environmental changes in hudson bay and strait since the final outburst flood of lake agassiz ojibway abstract in this study postglacial continuous high resolution data from five marine sediment cores from hudson bay and hudson strait are presented up to now studies in this area focused on the distinction between large scale climatic variations like the glacial postglacial transition or most recent processes however as demonstrated in this study using sedimentological geochemical and geophysical parameters postglacial sediments provide provenance signals distinguishing between proterozoic dubawnt group sediments paleozoic limestone sources underlying parts of hudson bay and strait and precambrian granitoids from the canadian shield for example it is shown that redistribution of dubawnt group sediments is of minor importance after the outburst flood of lake agassiz paleoproductivity indicators also give insights into paleotemperature trends these are in phase with terrestrial reconstructions from that area colder conditions in hudson strait until ca 4500 cal bp followed by a signal of warmth until ca 4000 cal bp are suggested subsequently a cool interval lasting until approximately 2000 cal bp occurred thereafter a gradual decrease in temperature was observed reaching lowest values during the little ice age period and increasing again afterwards

a method of moments approach to modeling the electromagnetic response of multiple steel casings in a layered earth in oil and gas production environments controlled source electromagnetics can be used to aid brownfield exploration development and reservoir monitoring efforts however such environments typically have many highly conductive steel cased wells in the area of interest we present a modeling algorithm using a method of moments approach to calculate the electromagnetic response of multiple 3d steel cased wells of arbitrary geometry in a layered earth conductivity model this approach involves dividing each casing into a collection of segments each carrying a uniform current density a matrix is computed that describes how the casing segments interact with each other electromagnetically then we solve a linear system for the current within each casing segment given a transmitter of arbitrary frequency and location from these currents we are able to solve for the secondary electromagnetic fields produced solely by the casings at any point in our layered model and add these to the primary fields produce

pore fluid effects on seismic waves in vertically fractured earth with orthotropic symmetry for elastically noninteracting vertical fracture sets at arbitrary orientation angles to each other a detailed model is presented in which the resulting anisotropic fractured medium generally has orthorhombic symmetry overall some of the analysis methods and ideas of schoenberg are emphasized together with their connections to other similarly motivated and conceptually related methods by sayers and kachanov among others examples show how parallel vertical fracture sets having hti horizontal transversely isotropic symmetry transform into orthotropic fractured media if some subsets of the vertical fractures are misaligned with the others and then the fractured system can have vti vertical transversely isotropic symmetry if all of the fractures are aligned randomly or half parallel and half perpendicular to a given vertical plane an orthotropic example having vertical fractures in an otherwise vti earth system studied previously by schoenberg and helbig is compared with the other examples treate

abrupt deglaciation events and holocene palaeoceanography from high resolution cores cartwright saddle labrador shelf canada the uppermost quaternary sediments in cartwright saddle labrador shelf are acoustically laminated with reflectors that can be traced over long distances two piston cores from the saddle record changes in sediment and meltwater delivery from the northeast margin of the laurentide ice sheet lis during deglaciation variations in sediment properties indicate a similar history of sediment accumulation over the last 12 kyr the temporal sampling interval reaches decadal resolution in the last deglacial period 7 9 ka analyses of total carbonate content sediment magnetic variables foraminiferal species and stable isotope measurements on planktic foraminifers show that abrupt changes occurred ca 10 9 9 2 8 8 7 9 and 7 ka with 450 yr correction there was no distinct change in sediment character during much of the younger dryas chronozone in the 18o record the 8 8 ka event is a dramatic 1 shift toward lower values which we associate with the noble inlet glacial event within hudson strait we do not see the pronounced low 18o event at 7 1 ka reported off nova scotia but surprisingly neither the nova scotia records nor other records in the labrador sea capture the impressive 8 8 ka change serious consideration must be given to the final collapse of the lis as the cause of the 8 2 cal ka cold event recorded in greenland and northwest europe copyright 1999 john wiley sons ltd

3d modeling of a deepwater em exploration survey analysis of a current offshore prospect employed 3d numerical modeling of a controlled source electromagnetic csem exploration system the analysis considers the sensitivity of data presentations to assumptions about the background model the numerical simulations show that false anomalies and significant distortion to anomaly magnitude can be caused by normalization of the observed electric fields by reference fields calculated from an incorrect or oversimplified background model bathymetry effects on the measured electric fields if not accounted for can produce anomalies as large as those of target sands the maximum sensitivity to oil water contacts or other strong lateral variations within the modeled channel sands is achieved by profiling along the length of the channel profiles run offset from a simulated oil water contact by as little as 2 km show a response below the expected noise levels good background models can be constructed by taking advantage of the magnetotelluric data recorded by m

glacial geology of western hudson strait canada with reference to laurentide ice sheet dynamics hudson strait is a large marine trough that connected the marine based interior of the laurentide ice sheet with the ocean field investigations of the glacial geology of western hudson strait have helped to define the role of the strait in laurentide ice sheet dynamics in general ice converged on western hudson strait from the north west and south upon reaching the strait ice flowing from foxe and ungava peninsulas turned eastward within less than 10 km of the coast to parallel eastwardly flowing ice in the strait glacial erosion of western hudson strait is identified as slight the texture carbonate content ph and the presence of several types of erratics distinguish till on nottingham salisbury and mill islands in western hudson strait from till on southern foxe peninsula and northern ungava peninsula till on the islands is characterized by higher ph and higher proportions of silt clay and carbonate than till on either adjacent peninsula in addition the presence of red sandstones porphyritic intrusives stromatolites omarolluks and iron formation are restricted to the islands with few exceptions that can be explained by ice rafting carbonate is present in the pebble and boulder fractions of till on foxe peninsula and in all fractions of till on the islands but is absent from till on ungava peninsula these data suggest that ice from distant and widely distributed source regions affected western hudson strait and along with evidence for a rapid change in ice flow direction along the margins of hudson strait suggest that the strait was occupied by a relatively fast moving ice stream that had a substantial impact on the interior of the ice sheet through downdraw

late glacial ice margins and deglacial chronology for southeastern baffin island and hudson strait eastern canadian arctic radiocarbon dates from marine piston cores and from onshore raised marine stratigraphic sections in the hudson strait region were used to reconstruct deglacial isochrons for 9900 9500 8800 8500 and 8000 bp at the culmination of the gold cove readvance 9900 bp labrador ungava ice flowed northeastward across hudson strait and outer frobisher bay and stood for the last time on the baffin island continental shelf subsequent retreat by calving was rapid and profound opening the entire hudson strait marine trough by 9500 bp at this time ice dispersal from foxe basin labrador ungava and local ice on meta incognita peninsula supported tidewater margins along much of the coastline with the exception of northernmost ungava peninsula where the ice margin stabilized onshore this onshore margin remained in place throughout the cockburn substage while a major northeastward readvance of ungava bay ice the noble inlet readvance from 8800 to 8500 bp crossed outer hudson strait grounding on the hudson st

subglacial sediments as a control on the onset and location of two siple coast ice streams west antarctica laterally continuous subglacial sediments are a necessary component for ice streaming in the modern onset regions of the ice streams draining the siple coast of west antarctica on the basis of new seismic data combined with previous results we present geophysical results from seismic reflection and refraction experiments in the upper reaches of ice streams c and d that highlight continuous sedimentary basins within and upstream of the current onset regions of both ice streams with streaming ice overlying these sedimentary packages the subglacial environment changes from no sediment to discontinuous sediment to continuous sediment cover along a longitudinal profile from the ice sheet to tributary c1b along this same profile we observe a speedup of ice flow and then full development of the ice stream tributary ice stream d flows above a thick sedimentary package with an uppermost low seismic velocity zone indicative of soft till and the upglacier and lateral extensions of ice stream d are tightly constrained by the extent of continuous sediments the inland termination of these sediments suggests that future migration of high velocity low shear stress ice flow in these regions appears unlikely

a bayesian model for gas saturation estimation using marine seismic ava and csem data we develop a bayesian model to jointly invert marine seismic amplitude versus angle ava and controlled source electromagnetic csem data for a layered reservoir model we consider the porosity and fluid saturation of each layer in the reservoir the bulk and shear moduli and density of each layer not in the reservoir and the electrical conductivity of the overburden and bedrock as random variables we also consider prestack seismic ava data in a selected time window as well as real and quadrature components of the recorded electrical field as data using markov chain monte carlo mcmc sampling methods wedraw a large number of samples from the joint posterior distribution function with these samples we obtain not only the estimates of each unknown variable but also various types of uncertainty information associated with the estimation this method is applied to both synthetic and field data to investigate the combined use of seismic ava and csem data for gas saturation estimation results show th

terrestrial ice streams a view from the lobe abstract the glacial landforms of minnesota are interpreted as the products of the lobate extensions of ice streams that issued from various ice sheds within the laurentide ice sheet low relief till plains trough shaped lowlands boulder pavements and streamlined forms make up the subglacial landsystem in minnesota that is interpreted as having been formed by streaming ice extremely uniform tills are created subglacially in a way that remains somewhat mysterious at the ice margins thrust moraines and hummocky stagnation topography are more common than single crested simple moraines if the ice lobes had repeated advances subglacial drainage features are obscure up ice but are present down ice in the form of tunnel valleys eskers spooner hills and associated ice marginal fans ice streaming may occur when basal shear stress is lowered as a result of high subglacial water pressure subglacial conditions that allow the retention of water will allow an ice lobe to extend far beyond the ice sheet as long as the ice shed also supports the advance by supplying adequate ice even with adequate ice flux however the advance of an ice lobe may be terminated at least temporarily if the subglacial water is drained through tunnel valleys or perhaps a permeable substrate thrust moraines and ice stagnation topography will result from sudden drainage although climate change is ultimately responsible for the accumulation of ice in the laurentide ice sheet the asynchronous advances and retreats of the ice lobes in the mid continent are strongly overprinted by the internal dynamics of individual ice streams as well as the interaction of ice sheds which obscure the climate signal

a regional stratigraphic isochron ca 800014c yr b p from final deglaciation of hudson strait sedimentologic rock magnetic and x ray fluorescence data from two marine sediment cores in hudson strait suggest that a red hematite rich clay layer was deposited throughout the strait during the final collapse of the laurentide ice sheet in the vicinity of northern hudson bay and western hudson strait this layer which can be recognized by its reddish pink color 10yr6 2 to 5yr4 2 and relatively high hematite proportions low magnetic susceptibility and magnetite to hematite ratio is dated from 8000 to 790014c yr b p at both ends of the strait the dubawnt group a proterozoic bedrock unit in northern hudson bay is the most likely source of this stratigraphic isochron in eastern hudson strait the recognition of this red unit and other distal glaciomarine sediments from 8400 to 790014c yr b p indicates that little sediment from the nearby labrador dome reached eastern hudson strait during this 500 yr interval this time interval immediately postdates the noble inlet advance a northward flow of labrador ice across eastern hudson strait onto southern baffin island from ca 8900 to 840014c yr b p one explanation for the lack of labrador sediments is that the northern margin of the labrador dome was cold based for up to 500 yr following the noble inlet advance

stochastic inversion of magnetotelluric data using a sharp boundary parameterization and application to a geothermal site we developed a bayesian model to invert magnetotelluric mt data using a 2d sharp boundary parameterization we divided the 2d cross section into layers and considered the locations of interfaces and resistivity of the regions formed by the interfaces as random variables we assumed that those variables are independent in the vertical direction and dependent along the lateral direction whose spatial dependence is described by either pairwise difference or multivariate gaussian priors we used a parallel adaptive finite element algorithm to rapidly forward simulate frequency domain mt responses of the 2d resistivity structure and used markov chain monte carlo methods to draw many samples from the joint posterior probability distribution we applied the bayesian model to a synthetic case that mimics a geothermal exploration scenario our results demonstrated that the developed method is effective in estimating the resistivity and depths to interfaces and in quantifying uncertainty on the estimates we also applied the developed method to the field mt data collected from the darajat geothermal site in indonesia we compared our inversion results with those obtained from a deterministic inversion of 3d mt data they are consistent even if the two inversion methods are very different and the amount of information used for inversion is different

modelling electrical conductivity for earth media with macroscopic fluid filled fractures modeling electrical conductivity for earth media with macroscopic fluid filled fractures james g berryman 1 and g michael hoversten 2 lawrence berkeley national laboratory one cyclotron road ms 90r1116 berkeley ca 94740 usa e mail jgberryman lbl gov chevron energy technology co 6001 bollinger canyon road san ramon ca 94583 e mail hovg chevron com

hydro frac monitoring using ground time domain electromagnetics as motivation for considering new electromagnetic techniques for hydraulic fracture monitoring we develop a simple financial model for the net present value offered by geophysical characterization to reduce the error in stimulated reservoir volume calculations this model shows that even a 5 improvement in stimulated reservoir volume for a 1 billion barrel bbl field results in over 1 billion u s dollars us in net present value over 24 years for us 100 bbl oil and us 0 5 billion for us 50 bbl oil the application of conductivity upscaling often used in electromagnetic modeling to reduce mesh size and thus simulation runtimes is shown to be inaccurate for the high electrical contrasts needed to represent steel cased wells in the earth finescale finite difference modeling with 12 22 mm cells to capture the steel casing and fractures shows that the steel casing provides a direct current pathway to a created fracture that significantly enhances the response compared with neglecting the steel casing we consider conductively enhanced proppant such as coke breeze coated sand and a highly saline brine solution to produce electrically conductive fractures for a relatively small frac job at a depth of 3 km involving 5 000 bbl of slurry and a source midpoint to receiver separation of 50m the models show that the conductively enhanced proppant produces a 15 increase in the electric field strength in line with the transmitter in a 10 m background in a 100 m background the response due to the proppant increases to 213 replacing the conductive proppant by brine with a concentration of 100 000 ppm nacl the field strength is increased by 23 in the 100 m background and by 2 3 in the 10 m background all but the 100 000 ppm nacl brine in a 10 m background produce calculated fracture induced electric field increases that are significantly above 2 a value that has been demonstrated to be observable in field measurements

geoelectrical resistivity variations and lithological composition in coastal gypsum rocks a case study from the lesina marina area apulia southern italy abstract the lesina marina village apulia southern italy lies on an exotic rocky body basically composed of triassic gypsum surrounded and covered by thin quaternary sandy deposits during the last two decades instability phenomena connected to gypsum dissolution and cover suffusion sinkholes occurred mainly along the acquarotta canal and in the urbanized area in this study electrical resistivity tomographies field and petrographic observations have been carried out just ca 300 m to the south of the residential area the east west striking geoelectrical profile has a length of 1900 m intercepts two boreholes and crosscuts the acquarotta canal close to the boreholes higher resolution geoelectrical profiles both perpendicular and parallel to the former were performed the two boreholes highlight the presence from the top to the bottom of quaternary sandy deposits coarse grained gypsum and layered finer grained gypsum in correspondence electrical resistivities are very low for the wet sandy deposits up to 5 m increase for coarse grained gypsum 20 120 m and reach the maximum values for the layered finer grained gypsum greater than 300 m the relationship between the electrical resistivity and the lithological composition of the studied rocks is strongly controlled by their fracturing and water saturation degree thus considering the main geological features here recovered are the impermeable bedrock and the above rocky body involved by mineral transformations and karstification phenomena additionally the anhydrite gypsum transformation and the gypsum dissolution are related to the groundwater circulation and localized within massive coarse grained gypsum rocks therefore the hazard related to karst processes involves both an area and a depth greater than those considered up to now

palaeo ice streams in the foxe baffin sector of the laurentide ice sheet in this paper we describe mapping of palaeo ice streams in the foxe baffin sector of the laurentide ice sheet by means of geomorphological interpretation of high resolution satellite images our interpretations were guided by a glaciological inversion scheme aided by digital elevation models publicly available sonar surveys and field studies as a result we produced a map depicting the location geometry and relative temporal changes of palaeo ice streams and analysed their palaeoglaciological implications for the foxe baffin sector we conclude that in the period between the last glacial maximum and approximately 7 0 kyr bp the foxe baffin sector was largely drained by topographically controlled outlet glaciers and ice streams which were organized in a relatively stable pattern during this time large areas of melville peninsula and central baffin island were subject to cold based conditions between 7 0 and 6 0 kyr bp the foxe baffin sector collapsed catastrophically in the foxe basin after which its remnant portion became confined to baffin island during this collapse rapid successions of small transient ice streams occurred along wide loosely defined topographical corridors in two sectors of baffin island the presently available landform archive on emerged land is not sufficient to conclusively support the existence of fully scaled ice streams along the whole length of hudson strait as an alternative solution we propose that topographically controlled ice streams might have occurred along the deepest parts of hudson strait with attendant cold based ice zones on marginal areas and islands at the head of the strait

comparison of rev size and tensor characteristics for the electrical and hydraulic conductivities in fractured rock the representative elementary volume rev is a critically important concept in fractured rock investigations as it tells us at what scale the fractured domain can be represented by an anisotropic tensor as opposed to requiring the details of each individual fracture for modelling purposes whereas the rev size and corresponding tensor characteristics for the hydraulic conductivity k in fractured rock have been the subject of numerous previous investigations no studies to date have focused on the electrical conductivity this is despite the fact that geoelectrical measurements are arguably the most popular means of geophysically investigating fractured rock typically via azimuthal resistivity surveying where the observed electrical anisotropy is commonly used to infer hydraulic characteristics in this paper we attempt to fill this void and present a systematic numerical study of the impacts of changes in fracture network properties on the rev size and equivalent tensor characteristics for both the electrical and hydraulic conductivities we employ a combined statistical and numerical approach where the size of the rev is estimated from the conductivity variability observed across multiple stochastic fracture network realizations for various domain sizes two important differences between fluid and electric current flow in fractured media are found to lead to significant differences in the rev size and tensor characteristics for and k these are the greater importance of the matrix in the electrical case and the single power instead of cubic dependence of electric current flow upon aperture specifically the rev for the electrical conductivity will always be smaller than that for the hydraulic conductivity and the corresponding equivalent tensor will exhibit less anisotropy often with notably different principal orientations these findings are of key importance for the eventual interpretation of geoelectrical measurements in fractured rock where we conclude that extreme caution must be taken when attempting to make the link to hydraulic properties

cosmogenic radionuclides from fiord landscapes support differential erosion by overriding ice sheets the interpretation of differentially weathered mountainous areas along the fringes of pleistocene ice sheets is fundamental for determining ice sheet behavior and thickness during the last glaciation two existing interpretations are either that highly weathered uplands remained as nunataks while freshly eroded troughs held outlet glaciers during the last glaciation or that uplands and lowlands were equally covered by ice but that it was differentially erosive as a function of its spatially variable basal thermal regime cosmogenic radionuclide measurements from 33 bedrock samples and 27 upland erratics from differentially weathered fi ord landscapes on northeastern baffi n island shed light on laurentide ice sheet lis dynamics and thickness tors on weathered upland surfaces have minimum 10 be ages between ca 50 and ca 170 ka n 12 whereas the majority of erratics perched in the uplands range from ca 10 to ca 13 ka n 14 indicating that the whole landscape was glaciated during the last glacial maximum lgm glacially sculpted bedrock n 7 near sea level refl ects the age of deglaciation and increasing amounts of isotopic inheritance in highelevation sculpted bedrock n 3 higher elevation intermediately weathered bedrock n 11 and highest elevation intensely weathered bedrock refl ect the weakening of erosive power as the ice sheet transitioned from fi ords to interfi ord plateaus cold based glaciation of uplands was contemporaneous with warm based glaciation in fi ords suggesting the presence of ice streams 26 al and 10 be concentrations measured in 10 bedrock samples indicate that tors have experienced a complex history of alternating periods of exposure burial and limited glacial erosion the paired isotope data reveal a minimum duration of burial of 80 500 k y presumably by cold based ice and indicate that the tors have an age of at least 150 580 ka these data together with other reconstructions of ice streams in the eastern canadian arctic suggest that ice streams were a large infl uence on northeastern lis dynamics throughout the late quaternary thus the northeastern lis was sensitively tied with north atlantic thermohaline circulation and abrupt climate change

postglacial emergence of ungava peninsula and its relationship to glacial history a series of 178 radiocarbon dates of late glacial and postglacial age from raised marine terraces on the hudson strait hudson bay and ungava bay coasts permit a new synthesis of deglaciation h

meltwater and deglaciation se baffin shelf ne margin laurentide ice sheet between 13 5 and 7 ka from o and c stable isotopic data stable isotopic values on planktonic foraminifera in a suite of cores from basins across the se baffin shelf are used to extract a record of meltwater events during termination i deglaciation resolution and hatton basins lie on the se baffin shelf at water depths 500 m seaward of major conduits for ice drainage from the eastern sector of the laurentide ice sheet lis accelerator mass spectrometry 14c dates are used to constrain our chronology of events in ten cores in resolution basin three cores have 14c ams dates on foraminifera of 20 ka at their bases whereas hatton basin cores terminate in sediments 13 kyr sedimentation rates varied between 0 1 to 4 5 m ka stable oxygen and carbon isotopic ratios were obtained on 146 samples of the planktonic foraminifera neogloboquadrina pachyderma ehrenberg sinistral from seven of the ten cores no evidence was found to indicate that test morphology or size affected 18o between 7 and 13 5 ka the surface water on the shelf was on average 1 lower than the open ocean signal significant temporal variations were found in both 18o and 13c evidence for significant low 18o events occurred between 13 and 8 ka the 13c record from the planktonic foraminifera suggests a threefold division of events between 13 and 7 ka with positive values between 10 8 and 13 0 ka negative values between 9 and 10 8 ka and positive values from 7 to 9 ka the 18o data suggest the presence of meltwater on the shelf some 3 000 years prior to the first late glacial dates on terrestrial deglaciation at circa 10 4 ka hudson strait must be the real key to the importance of the calving process during deglaciation because it is potentially the largest marine outlet for the laurentide ice sheet and because it leads into the very center of the ice sheet the rates of calving through hudson strait during the period of initial 18o rise unfortunately are unknown w f ruddiman 1987 p 151

seismic waves in rocks with fluids and fractures seismic wave propagation through the earth is often stronglyaffected by the presence of fractures when these fractures are filledwith fluids oil gas water co2 etc the type and state of the fluid liquid or gas can make a large difference in the response of theseismic waves this paper summarizes recent work on methods ofdeconstructing the effects of fractures and any fluids within thesefractures on seismic wave propagation as observed in reflection seismicdata one method explored here is thomsen s weak anisotropy approximationfor wave moveout since fractures often induce elastic anisotropy due tononuniform crack orientation statistics another method makes use ofsome very convenient fracture parameters introduced previously thatpermit a relatively simple deconstruction of the elastic and wavepropagation behavior in terms of a small number of fracture parameters whenever this is appropriate as is certainly the case for small crackdensities then the quantitative effects of fluids on thesecrack influence parameters are shown to be directly related to skempton scoefficient b of undrained poroelasticity where b typically ranges from0 to 1 in particular the rigorous result obtained for the low crackdensity limit is that the crack influence parameters are multiplied by afactor 1 b for undrained systems it is also shown how fractureanisotropy affects rayleigh wave speed and how measured rayleigh wavespeeds can be used to infer shear wave speed of the fractured medium higher crack density results are also presented by incorporating recentsimulation data on such cracked systems

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

effective elasticity of rocks with closely spaced and intersecting cracks the noninteraction approximation nia is the simplest effective media theory that describes the overall elasticity of fractured rocks if the nia is used for fracture characterization its accuracy and the range of applicability must be estimated we do it by performing a series of 3d finite element simulations of effective elasticity for models that contain several sets of fractures embedded in otherwise isotropic host rock we intentionally place the cracks close to each other to create strong interactions in their local stress fields in addition we allow the cracks to intersect in such a way that they do not break a rock specimen apart perhaps surprisingly we find that fracture interactions and intersections have little influence on the effective elasticity and the nia performs well in all cases while it has a tendency to slightly underestimate the effective stiffnesses the incurred errors are small their typical magnitudes are just a few percent in the entire range of the crack densities expe