seismic imaging of reservoir flow properties time lapse amplitude changes asymptotic methods provide an efficient means by which to infer reservoir flow properties such as permeability from time lapse seismic data a trajectory based methodology much like ray based methods for medical and seismic imaging is the basis for an iterative inversion of time lapse amplitude changes in this approach a single reservoir simulation is required for each iteration of the algorithm a comparison between purely numerical and the trajectory based sensitivities demonstrates their accuracy an application to a set of synthetic amplitude changes indicates that they can recover large scale reservoir permeability variations from time lapse data in an application of actual time lapse amplitude changes from the bay marchand field in the gulf of mexico we are able to reduce the misfit by 81 in twelve iterations the time lapse observations indicate lower permeabilities are required in the central portion of the reservoir

seismic monitoring of pressure depletion evaluated for a united kingdom continental shelf gas reservoir the possibility of using 4d seismic data for monitoring pressure depletion in the low porosity tight gas bearing rotliegende sandstones of the uk southern gas basin is investigated the focus here is on whether fractures in the upper part of the reservoir known to enhance productivity can also enhance the time lapse seismic response the study uses laboratory data to evaluate core plug stress sensitivity published data for the stress behaviour of the fractures followed by petro elastic and 4d seismic modelling of both the fractured and unfractured formation the magnitude of the resultant 4d signatures suggests that production induced changes in the unfractured sands are unlikely to be observed except perhaps with highly repeatable time lapse surveys on the other hand the presence of fractures could render production effects visible in dedicated 4d acquisition or prestack parallel processed data if present however the signature will be sporadic as fractures in the area are known to exist in clusters the 4d signature may be enhanced further by certain classes of vertical geological variability and also areas of high reservoir pressure the strongest evidence of depletion is expected to be time shifts seen at the base of the rotliegende reservoir

response of laterally loaded rectangular and circular piles in soils with properties varying with depth continuum based analyses for laterally loaded piles with rectangular and circular cross sections are presented using solutions that can be obtained quickly without requiring any elaborate inputs for the geometry and numerical mesh the analysis is developed by solving the differential equations governing the displacements of the pile soil system derived using the variational principles of mechanics parametric studies are performed to investigate the influence of the pile cross sectional shape soil layering pile slenderness ratio and pile soil modulus ratio on the response of laterally loaded piles in heterogeneous soil in which the soil shear modulus varies continuously or discretely with depth the results show that piles with the same second moment of inertia have similar lateral load response the lateral responses of piles in two layer systems were mainly affected by the thickness and stiffness of the top soil layer soil layering also influences the lateral response of piles in three layer soil deposits consisting of two thin layers overlying the third layer algebraic equations for estimating the pile head deflection and maximum bending moment are proposed that can be readily used in design a user friendly spreadsheet program is developed as a tool to perform calculations of pile response using the analysis numerical examples demonstrating the use of the analysis are provided

tectonic erosion of the peruvian forearc lima basin by subduction and nazca ridge collision 1 subsidence of lima basin part of the peruvian forearc is controlled by tectonic erosion by the subducting nazca plate multichannel seismic reflection data coupled with age and paleowater depth constraints derived from ocean drilling program odp coring now allow the rates of erosion to be reconstructed through time in trenchward locations the forearc has experienced limited recent relative uplift 700 850 m likely due to preferential basal erosion under the center of lima basin long term subsidence driven by basal tectonic erosion dominates and is fastest closest to the trench since 47 ma eocene up to 148 km of the plate margin have been lost at an average rate of up to 3 1 km myr 1 appoximately 110 km of that total appears to be lost since 11 ma implying much faster average rates of trench retreat 10 km myr 1 since collision of the nazca ridge with the lima basin at 11 ma although there is no clear subsidence event at odp site 679 during the time at which nazca ridge was subducting beneath this part of the forearc 4 11 ma the more trenchward odp sites 682 and 688 show significant deepening after 11 ma indicating that subduction of the ridge accelerates tectonic erosion long term rates of crustal erosion in the region of lima basin are greater than estimates of regional arc magmatic productivity implying that such margins are net sinks of continental crust

middle miocene to present plate tectonic history of the southern central american volcanic arc abstract new mid miocene to present plate tectonic reconstructions of the southern central american volcanic arc cava reveal that the inception of cocos ridge subduction began no earlier than 3 ma and possibly as late as 2 ma the cocos ridge has been displaced from the malpelo ridge to the southeast since 9 ma along the panama fracture zone pfz system ambiguous pfz and coiba fracture zone cfz interaction since 9 ma precludes conclusively establishing the age of initial cocos ridge subduction detailed reconstructions based on magnetic anomalies offshore reveal several other variations in subduction parameters beneath southern central america that preceded subduction of the cocos ridge including southeastward migration of the nazca cocos caribbean triple junction along the middle america trench mat from 12 ma to present and subduction of 2 km high scarps both parallel and perpendicular to the trench from 6 to 1 ma the timing of changes in subduction processes has commonly been determined by and correlated with geologic changes in the upper plate however reliable 40 ar 39 ar dating of these events has become available only recently abstr programs geol soc am 2002 these new dates better constrain the magmatic and structural history of southern costa rica observations from this data set include a gap in the volcanic record from 11 to 6 ma which coincides temporally with emplacement of most plutons in southern costa rica normal arc volcanism ceased after 3 5 ma in southern costa rica and pliocene mostly 1 5 ma adakite volcanism was widely distributed from central panama to southern costa rica though volumetrically insignificant this new data reveals that many geologic phenomena commonly attributed to subduction and underplating of the buoyant cocos ridge in fact precede inception of cocos ridge subduction and seem to correlate more favorably in time with earlier tectonic events adakite volcanic activity corresponds in space and time with the subduction of a large scarp associated with a tectonic boundary off southern panama regional unconformities and an 11 6 ma gap in arc volcanism match temporally with oblique subduction of the nazca plate beneath central and southern costa rica cessation of volcanic activity low temperature cooling of plutons in the cordillera de talamanca ct and rapid increases in sedimentation in the fore arc and back arc basins coincide with passage of the nazca cocos caribbean triple junction and initiation of subduction of rough crust associated with cocos nazca rifting 3 5 ma closely followed by initial subduction of the cocos ridge 2 3 ma none of the aforementioned geologic events occurred at a time that would allow for underplating by the cocos ridge rather they are probably related to complex interactions with subduction of complicated plates offshore all of the aforementioned events indicate that the southern central american subduction system has been in flux since at least 12 ma

fluid accumulation along the costa rica subduction thrust and development of the seismogenic zone in 2011 we acquired an 11 55 km 3 d seismic reflection volume across the costa rica margin nw of the osa peninsula to accurately image the subduction thrust in 3 d to examine fault zone properties and to infer the hydrogeology that controls fluid accumulation along the thrust following processing to remove water column multiples noise and acquisition artifacts we constructed a 3 d seismic velocity model for kirchhoff prestack depth migration imaging images of the plate boundary thrust show high reflection amplitudes underneath the middle to lower slope that we attribute to fluid rich poorly drained portions of the subduction thrust at 5 km subseafloor beneath the upper slope the plate interface abruptly becomes weakly reflective which we interpret as a transition to a well drained subduction thrust mineral dehydration during diagenesis may also diminish at 5 km subseafloor to reduce fluid production and contribute to the downdip change from high to low amplitude there is also a layered fabric and systems of both thrust and normal faults within the overriding plate that form a plumbing system faults commonly have fault plane reflections and are presumably fluid charged the faults and layered fabric form three compartmentalized hydrogeologic zones 1 a shallow ne dipping zone beneath the slope 2 a steeply sw dipping zone beneath the shelf slope break and 3 a ne dipping zone beneath the shelf the more direct pathway in the middle zone drains the subduction thrust more efficiently and contributes to reduced fluid pressure elevates effective stress and creates greater potential for unstable coseismic slip

discrimination between pressure and fluid saturation changes from time lapse seismic data explicit expressions for computing saturation and pressure related changes from time lapse seismic data have been derived and tested on a real time lapse seismic data set necessary input is near and far offset stacks for the baseline seismic survey and the repeat survey the method has been tested successfully in a segment where pressure measurements in two wells verify a pore pressure increase of 5 to 6 mpa between the baseline survey and the monitor survey estimated pressure changes using the proposed relationships fit very well with observations between the baseline and monitor seismic surveys 27 of the estimated recoverable hydrocarbon reserves were produced from this segment the estimated saturation changes also agree well with observed changes apart from some areas in the water zone that are mapped as being exposed to saturation changes which is unlikely saturation changes in other segments close to the original oil water contact and the top reservoir interface are also estimated and conf

uncertainties in quantitative time lapse seismic analysis most seismic time lapse studies so far have been of a qualitative nature identification of areas with minor or no seismic changes has been used to plan new infill drilling targets increased accuracy in seismic acquisition methods in both conventional streamer surveys and newer methods such as multicomponent sea bed seismic and permanent sensors opens possibilities for the next step quantitative time lapse analysis quantitative methods here mean the estimation of for instance a change in fluid saturation from 20 water to 90 water or the estimation of a pore pressure change of 5 mpa explicit expressions for the uncertainties associated with estimated changes in for instance reservoir pressure and fluid saturation are derived these formulae can be used to compare relative uncertainties between estimated parameters as well as to identify the critical factors in various estimation techniques the importance of accurate rock physics input as well as that of highly repeatable time lapse seismic data is emphasized furthermore uncertainty analysis can be used to find optimal weight factors when the same parameter e g saturation change is estimated by two or three different techniques

late miocene to recent plate tectonic history of the southern central america convergent margin new plate reconstructions constrain the tectonic evolution of the subducting cocos and nazca plates across the southern central american subduction zone from late miocene to recent because of the strong relationships between lower and upper caribbean plate dynamics along this margin these constraints have wide ranging implications for the timing and growth of upper plate deformation and volcanism in southern central america the reconstructions outline three important events in the neogene history of this margin 1 the coeval development of the panama triple junction with the initiation of oblique subduction of the nazca plate at 8 5 ma 2 the initiation of seamount and rough crust subduction beginning at 3 4 ma and 3 cocos ridge subduction from 2 to 3 ma a comparison of these events with independent geologic geomorphic volcanic and stratigraphic data sets reveals that the timing rates and origin of subducting crust directly impacted the neogene growth of upper plate deformation and volcanism in southern central america these analyses constrain the timing geometry and causes of a number of significant tectonic and volcanic processes including rapid plio quaternary arc fore arc contraction due to cocos ridge subduction the detachment of the panama microplate at 1 3 ma and the late miocene cessation of mantle wedge derived volcanism across 300 km of the subduction zone

relationship between outer forearc subsidence and plate boundary kinematics along the northeast japan convergent margin 1 tectonic erosion along convergent plate boundaries whereby removal of upper plate material along the subduction zone interface drives kilometer scale outer forearc subsidence has been purported to explain the evolution of nearly half the world s subduction margins including part of the history of northeast japan here we evaluate the role of plate boundary dynamics in driving forearc subsidence in northeastern japan a synthesis of newly updated analyses of outer forearc subsidence the timing and kinematics of upper plate deformation and the history of plate convergence along the japan trench demonstrate that the onset of rapid fore arc tectonic subsidence is contemporaneous with upper plate extension during the opening of the sea of japan and with an acceleration in convergence rate at the trench in plio quaternary time relative uplift of the outer forearc is contemporaneous with contraction across the arc and a decrease in plate convergence rate the coincidence of these changes across the forearc arc backarc system appears to require an explanation at the scale of the entire plate boundary similar observations along other western pacific margins suggest that correlations between forearc subsidence and major changes in plate kinematics are the rule rather than the exception we suggest that a significant component of forearc subsidence at the northeast japan margin is not the consequence of basal tectonic erosion but instead reflects dynamic changes in plate boundary geometry driven by temporal variations in plate kinematics if correct this model requires a reconsideration of the mass balance and crustal recycling of continental crust at nonaccretionary margins

seismic imaging of reservoir flow properties resolving water influx and reservoir permeability methods for geophysical model assessment in particular the computation of model parameter resolution indicate the value and the limitations of time lapse data in estimating reservoir flow properties a trajectory based method for computing sensitivities provides an effective means to compute model parameter resolution we examine the common situation in which water encroaches into a reservoir from below as caused by the upward movement of an oil water contact though the techniques described are not limited to this case we treat the situation in which the time lapse response is primarily caused by changes in saturation using straightforward techniques we find that by including reflections off the top and bottom of a reservoir tens of meters thick we can infer reservoir permeability based upon time lapse data we find that for the case of water influx from below using multiple time lapse snapshots does not necessarily improve the resolution of reservoir permeability an application to time lapse data from the norne field in the north sea illustrates that we can resolve the permeability near a producing well using reflections from three interfaces associated with the reservoir

effects of pile shape and pile end condition on the lateral response of displacement piles in soft clay a series of lateral tests conducted in a centrifuge on displacement piles in kaolin is used to examine the effects of pile shape pile end condition and clay overconsolidation on the lateral load t

analysis of normal fault populations in the kumano forearc basin nankai trough japan 1 multiple orientations and generations of faults from 3 d coherency mapping 1 analyses of normal faults in the kumano forearc basin of the nankai trough reveal multiple normal fault populations in a region generally thought to be under compression most faults have offsets of less than 20 m and dips of 60 70 and show no growth structures indicating that the faults were active for short periods of time the oldest generation of faults is older than 0 9 ma and strikes 50 60 the next oldest faults strike 160 170 are older than 0 44 ma and are related to local uplift along the western edge of the region the youngest faults cut the seafloor shallow faults near the se margin of the basin curve from 100 in the middle of the survey area to 145 at the se corner of the area the pattern of the two youngest fault populations is consistent with the regional stress pattern maximum horizontal stress subparallel to the trench orientations of older fault populations are caused by uplift of the underlying accretionary prism implying that the forearc basin region is not as stable as previously thought reconstruction of displacements on the youngest faults shows that the overall horizontal extension is less than 2 concentrated near the seaward edge of the basin the active normal faults distributed throughout the basin support the idea that the horizontal stress parallel to the plate convergence direction does not reach the critical stress to activate or form thrust faults and produce horizontal shortening within the shallow portion of the inner wedge

the 4d seismic signature of oil water contact movement due to natural production in a stacked turbidite reservoir combined time lapse reservoir simulation and seismic modelling has been performed on both 1d and 3d models of a channelized turbidite reservoir the models have been built using core log laboratory and seismic data from the nelson field central north sea as a template oil and water movement in the main channels channel margins and interchannel regions is investigated with a particular focus being the effect of poor net to gross the analysis confirms that saturation effects dominate the response whilst stress sensitivity effects play a minor role the trough peak signature in the seismic difference volumes formed by the sweep of the water can be continued and mapped slightly further than the channel margins this characteristic 4d signature remains roughly intact despite the complicated depositional architecture and accurately delineates the area of moved fluid but it needs additional calibration to combat the detrimental influence of the low net to gross signal strength is largely dependent on reservoir quality but is also compromised by the net to gross fluid distribution and more critically by the exact timing of the seismic survey for example a region of bypassed oil zone remains undetected as it forms early during the production this work demonstrates that to understand fully the 4d signature at a quantitative level requires adequate knowledge of the fluid properties but also more critically the geology

reservoir characterization based upon the onset of time lapse amplitude changes abstracttime lapse geophysical monitoring has potential as a tool for reservoir characterization that is for determining reservoir properties such as permeability onset times the calendar times at which geophysical observations begin to deviate from their initial or background values provide a useful basis for such characterization we found that in contrast to time lapse amplitude changes onset times were not sensitive to the exact method used to related changes in fluid saturation to changes in seismic velocities as a consequence of this we found that an inversion for effective permeability based upon onset times was robust with respect to variations in the rock physics model in particular inversions of synthetic onset times calculated using voigt and reuss averaging techniques but inverted using sensitivities from hill s averaging method resulted in almost identical misfit reductions and similar permeability models all solutions based on onset times recovered the large scale resolvable f

utilizing the onset of time lapse changes a robust basis for reservoir monitoring and characterization time lapse monitoring is useful for imaging changes in geophysical attributes due to fluid flow in trying to use time lapse data for reservoir characterization difficulties often arise when relating changes in geophysical observations to changes in fluid saturation and pressure as an alternative approach for reservoir monitoring and characterization we introduce the idea of an onset time the time at which a measured quantity such as a seismic traveltime or a reflection amplitude begins to deviate from its background value we illustrate the idea and demonstrate its utility through the consideration of traveltimes recorded by the continuous active source seismic monitoring system at the frio pilot site near houston texas the system which transmits an elastic wave every 15 min is used to monitor the movement of carbon dioxide injected into a permeable sand formation from these data we can estimate the onset of changes in seismic traveltimes to six receivers in an adjacent borehole numerical simulation and synthetic tests indicate that the onset times are not very sensitive to the method used to compute the effective fluid bulk modulus and correspondingly the seismic compressional velocity rather the onset times are strongly influenced by saturation changes within the formation specifically by the break through time of the injected fluid phase by means of an iterative inversion algorithm we use the onset times to estimate permeability variations between the boreholes at the frio pilot site the authors 2014 published by oxford university press on behalf of the royal astronomical society

a recent phase of accretion along the southern costa rican subduction zone abstract in 2011 we acquired a 3d seismic reflection volume across the costa rica margin nw of the osa peninsula to investigate the complex structure and the development of the seismogenic zone within the costa rican subduction zone in the vicinity of recent international ocean drilling program iodp drilling in contrast to previous interpretations these newly acquired seismic images show that the margin wedge is composed of a layered fabric that is consistent with clastic sediments similar to materials recovered from iodp drilling that have been thrust and thickened into thrust bounded folded sequences these structures are consistent with a balanced sequence that has been frontally accreted in the context of an accretionary model we interpret these sequences as sediment originally deposited on the subducting crust in a trench basin created by the southward migration of the cocos nazca caribbean triple junction and accreted during recent margin subduction that also accelerated with passage of the triple junction the margin is composed of relatively rapidly accreted sediment that was added to the margin during a phase of accretion within the last 5 ma that was probably preceded throughout the neogene by periods of non accretion or erosion

active thrusting in the inner forearc of an erosive convergent margin pacific coast costa rica 1 structural and geomorphic analyses of the fila costena thrust belt in southwest costa rica indicate active thrusting within the inner forearc the fila costena exposes three major thrust faults that imbricate the late tertiary forearc basin sequence of the terraba basin the frontal thrust of the fila costena marks the boundary between an uplifting inner forearc and a subsiding outer forearc with only local uplift astride the indenting cocos ridge on the basis of surface constraints a cross section across the thrust belt suggests that all three thrusts flatten into parallelism with a low angle decollement horizon near the contact between the basement and the cover sequence of the terraba basin this decollement lies at a depth of 4 km the minimum shortening recorded by restoration of fault related folds is 17 km or 45 observations of late tertiary marine sediments tilted and faulted late quaternary fluvial terraces and raised holocene marine terraces indicate that fila costena uplift was likely initiated in the quaternary and is ongoing given that the coastal mountains are separated from the talamanca range by a valley the decollement that delaminates the forearc basin from the underthrusting forearc must continue as a flat beneath the valley but must link with the plate boundary along a crustal scale ramp system a structural geometry that has resulted in uplift of the talamanca range the highest peaks in central america the dichotomy between uplift in the inner forearc and subsidence in the outer forearc is explained in terms of the response of an arcward thickening wedge to rough subducting crust index terms 8150 tectonophysics plate boundary general 3040 8123 tectonophysics dynamics seismotectonics 8010 structural geology fractures and faults 8015 structural geology local crustal structure 8005 structural geology folds and folding

structure of the costa rica convergent margin offshore nicoya peninsula we present the results of a seismic refraction survey conducted offshore costa rica near the nicoya peninsula a dip profile and three strike profiles were carried out over 22 ocean bottom hydrophones and seismographs and were also recorded by land receivers these data are used to construct a crustal structure model of the convergent margin from 20 km seaward of the middle america trench onto the nicoya peninsula the best constrained portion of our model is the velocity at the top of the margin wedge immediately below the slope apron velocities increase from 3 5 to 4 2 to 4 6 km s at distances of 10 20 and 30 50 km landward of the trench these velocities are higher than observed within margin wedges at other well studied convergent margins but lower than the velocities within the adjacent nicoya complex which are 5 5 km s at similar depths below the surface we interpret the margin wedge velocities as indicating that material similar to the nicoya complex extends seaward to near the lower slope but that fracturing alteration or accretion processes have lowered the velocity of the margin wedge with respect to the nicoya complex the seismic refraction data cannot constrain the exact thickness or velocity of a possible low velocity zone lvz overlying the subducting plate however geologically reasonable structures are only produced with a lvz 400 m thick velocities in the upper part of oceanic layer 2 are 3 5 4 0 km s within the subducting slab these velocities are unusually low for oceanic crust of this age and may correlate with a proposed highly permeable zone at the top of the subducting crust the top of the subducted slab is well resolved and deepens from 5 km depth at the trench to 15 16 km depth at the nicoya peninsula coastline the dip angle of the subducting plate increases from 6 to 13 at a distance of 30 km from the trench interplate seismicity appears to become common 55 km from the trench where the plate boundary is at 14 km depth

rapid pulses of uplift subsidence and subduction erosion offshore central america implications for building the rock record of convergent margins integrated ocean drilling program iodp expedition 334 to southern costa rica central america documented unprecedented subduction erosion in an area of active seismic slip widespread subduction erosion of the upper plate initiated when the cocos ridge an overthickened aseismic ridge arrived at the middle america trench subduction erosion was coeval with the rapid formation of deposition centers on the forearc of the upper plate the completely recovered shelf sequence constrains a short 2 m y interval of extreme subsidence 1200 m with a rapid pulse occurring during the first 0 3 m y this event removed an estimated 1 2 106 km3 of forearc material at a rate of 1690 km3 m y km of trench during a time of rapid 1035 m m y shelf sediment accumulation at this erosive margin a sediment starved trench persisted in spite of abundant sediment supply because subduction erosion led to the creation of forearc basins similar rapid pulses of subduction erosion may punctuate the evolution of many margins contributing disproportionately to crustal recycling at subduction zones with implications for the evolution of continental crust and mountain belts and recycling of continental material into the mantle

magnetic anomaly interpretation across the southern central andes 32 34 s the role of the juan fernandez ridge in the late tertiary evolution of the margin marine and terrestrial magnetic surveys have been integrated to study the tectonic structure of the convergent margin of chile between 32 34 s three magnetic domains have been identified oceanic continental margin and subaerial the oceanic domain has seafloor spreading anomalies 16 37 ma to 18 39 5 ma disturbed by anomalies of the juan fernandez hot spot chain in the continental margin the most prominent fabric are e w anomalies in the upper slope corresponding to onshore e w anomalies of large intrusive bodies onshore a n s lineament of short wavelength anomalies defines the roots of a cretaceous volcanic arc a resembling lineament offshore indicates a submerged older volcanic arc and that continental basement extends to 50 km landward of the trench absolute cenozoic plate motion for nazca and south american plates and dating of the juan fernandez chain provide a kinematic model of ridge continent collision the reconstruction indicates rapid southward migration of the collision point along 1400 km of the margin from 20 to 11 ma 20 cm yr 1 from 11 ma to present the collison point has migrated at a slower rate along 375 km of the margin 3 5 cm yr 1 the predicted location of the subducted portion of the juan fernandez chain coincides with the south edge of the southward migrating flat slab segment of the subducted lithosphere and with a cluster of deep earthquakes indicating a causal relationship in the last 10 myr the ridge has separated a sediment starved trench to the north where subduction erosion may dominate from a sediment filled trench to the south where recent sediment accretion dominates these observations indicate that subduction of the juan fernandez chain plays a major role in arc forearc tectonics

crustal accretion in the manila trench accretionary wedge at the transition from subduction to mountain building in taiwan abstract new marine seismic reflection and coincident wide angle ocean bottom seismometer data acquired offshore taiwan provide high resolution constraints on the crustal structure of an incipient mountain belt during the earliest stage of arc continent collision the new seismic reflection image and travel time tomography velocity model show evidence for crust of the distal southern chinese continental margin being subducted eastward beneath the manila trench and underplated to the accretionary wedge before collision with the southern chinese continental shelf the distal margin crust consists of highly extended continental crust interspersed with volcanic bodies and a high velocity lower crustal layer of likely magmatic intrusions the distal margin crust is 10 14 km thick outboard of the trench but thins to 6 km thick beneath the lower slope of the manila trench accretionary wedge along the lower slope of the accretionary prism we image westward verging imbricate thrusts and folded strata up to 10 km thick a sharp decrease in bathymetry marks the transition from lower to upper slope where we observe a fast 6 0 km s seismic velocity anomaly at the base of the wedge that we interpret as structurally underplated crust from the distal continental margin our results support a model of arc continent collision in taiwan where the accretionary wedge is first thickened by structural underplating of distal margin crust prior to collision with the continental shelf the crustal rocks exposed throughout the central range in taiwan may be similarly derived from subducted and structurally underplated crust from the highly extended distal continental margin

4d seismic for oil rim monitoring in the central north sea gannet a field a 50 ft oil rim is overlain by a gas cap of variable thickness oil is produced from horizontal wells which initially produced dry oil but as the field became more mature a significant water cut was seen in several wells a dedicated 4d seismic monitor survey was acquired in order to assess the remaining distribution of oil reserves by forward modelling the synthetic seismic response to parameters such as contact movement and residual saturations using 2d and 3d wedge models and comparing the results with real seismic data we are able to decipher the contact movements across the field it is shown that in one part of the field the increased water cut is caused primarily by the vertical displacement of the entire oil rim into the initial gas cap this oil rim displacement produces a very different 4d seismic response from the case of a static gas oil contact and rising oil water contact normal production as a result of these observations we are able to optimize field production by both re perforation of existing wells and by drilling sidetracks into the displaced rim a brown field development opportunity that might otherwise be missed

evidence and mechanisms for forearc extension at the accretionary costa rica convergent margin seismic reflection data across the upper trench slope off the nicoya peninsula costa rica reveal a wide zone of nearly trench parallel normal faults although work in the last decade has shown that normal faults are present at many convergent margins most examples e g japan peru chile and guatemala have been associated with margins experiencing subduction erosion or non accretion in contrast extension in the costa rica study area apparently is coeval with frontal accretion and underplating the normal faults across the costa rica forearc are striking in seismic section due to the well layered 2 km thick upper slope apron fault plane reflections and reflector terminations show that the faults extend through the sedimentary apron and apparently into the underlying accretionary prism indicating a deep seated deformation process the zone of extension is from the midslope area to within 10 km of the shelf edge a minimum width of about 20 km the estimated extension across the zone is at least 1 5 to 3 km within the apron section spacing between the faults is generally 200 500 m and nominal fault dip is 20 40 and predominantly landward activity on the normal faults appears to have occurred over a significant period of time based on increased displacement with depth and on fault controlled sedimentary thickening at least some of the faults may be presently active shallow reflectors and possibly the seafloor are displaced by faulting contemporary sediment accretion is documented by the same seismic reflection profiles showing offscraping and underplating near the toe of the wedge and out of sequence thrusting primarily below the midslope area the consistent landward normal fault dip may be influenced by structural anisotropy in the prism and possible extensional reactivation of earlier thrust faults associated with accretion processes with the available data it is not possible to conclusively determine the cause of the stress field leading to the upper prism and apron extension however the three most likely causes are underplating changes in basal shear stress or a brief episode of subduction erosion

extraction of permeability from time lapse seismic data two formulae are developed for estimating horizontal permeability directly from maps of 4d seismic signatures the choice of the formula used depends on whether the seismic is dominated by changes of pressure or saturation however pressure derived from time lapse seismic or seismic amplitudes controlled predominantly by pressure are to be preferred for optimal illumination of the reservoir the permeability is predicted to be dependent on porosity but weighted by a 4d term related to the magnitude and spatial gradient of the 4d signature tests performed on model based synthetic seismic data affirm the validity and accuracy of this approach application to field data from the uk continental shelf reveals a large scale permeability variation similar to the existing simulation model but with additional fine scale detail the technique thus has the potential of providing extra information with which to update the simulation model the resultant permeability estimates have been successfully ground truthed against the results of two well tests as non repeatable noise in the time lapse seismic data diminishes with improved 4d related acquisition it will become increasingly possible to make robust permeability estimates using this approach

crustal structure of the middle american trench off costa rica from wide angle seismic data off costa rica seismic wide angle experiments were made using marine seismic sources of high shot density and ocean bottom hydrophones obh and land stations for recording three profiles perpendicular to the middle american trench mat and one parallel to it are presented here the coverage from closely spaced obh and structural constraints from the coincident near vertical reflection seismic survey provided detailed velocity depth distributions over 4 0 km s velocities were found directly underlying the thick continental margin sediment in a margin wedge that extends from below the coastal area seaward within 10 km from the mat axis the interpretation of the wide angle data required a low velocity zone lvz between the upper and lower plate indicating its nature as subducted and underplated sediment the velocity depth distributions support the hypothesis that the continental margin wedge off costa rica is most likely an offshore extension of the nicoya complex ophiolitic rocks which are exposed along the coast the subduction of the sediments seems very efficient and the accretionary prism appears to be confined to the lower slope

the response of flexible piles to lateral loading the majority of piles encountered in practice may be regarded as flexible under lateral loading that is the induced deformations and bending moments are confined to the upper part of the pile and the overall length of the pile does not significantly affect the response of the pile the results of a parametric study conducted using the finite element method and treating the soil as an elastic continuum with a linearly varying soil modulus are presented as simple algebraic expressions these expressions are similar in form to those which arise from a winkler idealization of the soil enabling immediate estimates to be made of the active length of the pile the ground level deformations and the maximum bending moment down the pile in addition expressions are presented quantifying the effect of interaction between neighbouring piles from which the behaviour of groups of piles subjected to lateral loading may be deduced application of the solutions is demonstrated by the analysis of lateral loading