coarse grained deep water sedimentation along a border fault margin of lake malawi africa seismic stratigraphic analysis lake malawi africa serves as an important modern analog for understanding depositional processes in an active rift setting this paper explores the depositional processes and stacking patterns of coarse grained facies across a large fan delta and associated sub lacustrine fan system offshore of the south rukuru river using single channel seismic data coupled with deep water vibracores and gravity cores a companion paper wells et al 1999 discusses the details of the depositional processes of the system inferred from core data using 2900 km of single channel seismic data eight genetic facies were defined based on seismic reflection character external geometry accompanying core data and location within the modern geomorphic system five sand dominated facies include 1 mouth of canyon fan facies 2 progradational fan delta facies 3 channel fill facies 4 canyon fill facies and 5 basin plain fill facies these facies occur as surficial deposits in water depths of more than 600 m suggesting that coarse grained sedimentation characterizes certain localities in even the deepest parts of lake malawi climatically driven lake level changes influenced the internal character of these facies their thickness and to some extent their location within the south rukuru study area however climate was generally subordinate to tectonism in controlling the temporal and spatial distribution of the facies tectonic controls can be categorized as 1 orientation of the first order border fault and intrabasinal faults 2 syndepositional footwall uplift and rotation and 3 temporal changes in the rate and location of maximum subsidence these controls affect the location and stacking of the mouth of canyon fans and the presence and orientation of sub lacustrine canyons and the zone of maximum sediment accumulation and depositional gradients

single well acoustic imaging in anisotropic formations the effect of formation anisotropy on single well acoustic imaging was analyzed to provide a simple and effective correction method for the imaging application we used analytical and synthetic modeling to analyze acoustic reflection moveout in a transversely isotropic ti formation for borehole configuration far borehole reflections from a bed generally have small reflection angles to the bed thus the reflection moveout away from the borehole is controlled largely by the velocity along the ti symmetry axis consequently reflection imaging using measured velocity along the borehole can cause errors in the imaged reflector position by correcting the anisotropy effect in the velocity reflectors can be imaged correctly a simple correction is to use the velocity along the ti symmetry axis which effectively maps the bed reflector to its correct position application of the correction method to synthetic and field data examples demonstrate the anisotropy effect on reflector imaging and the effectiveness of the method

imaging near borehole structure using directional acoustic wave measurement directional acoustic measurements made in the borehole are used for imaging a near borehole geological formation structure and determination of its orientation four component cross dipole data set measured in a deviated borehole in combination with the directionality of the compressional waves in the dipole data give the orientation of bed boundaries crossing the borehole the low frequency content 2 3 khz of the data allows for imaging the radial extent of the formation structure up to 15 m greatly enhancing the penetration depth as compared to that obtained using conventional monopole compressional wave data a combination monopole dipole arrangement of sources and receivers may also be used for imaging of bed boundaries

seismic anisotropy in sedimentary rocks part 2 laboratory data part one of this paper presents a method for measuring seismic velocities and transverse isotropy in rocks using a single core plug this method saves at least two thirds of the time for preparing core samples and measuring velocities in transversely isotropic ti rocks using this method we have measured velocity and anisotropy of many shale and reservoir rocks from oil and gas fields around the world we present some of the data in this paper which include seismic velocity and anisotropy in 17 brine saturated shale samples 1 gas and brine saturated coal sample 8 brine saturated sands 12 gas saturated sands 32 gas saturated carbonate samples and 25 brine saturated carbonate samples the results show that clays and fine layering in sedimentary rocks are the main causes of seismic anisotropy very little intrinsic anisotropy exists in unfractured reservoir rocks such as sands sandstones and carbonates under reservoir conditions in contrast all shales were found seismically anisotropic anisotr

two paradigms of baroclinic wave life cycle behaviour two idealized baroclinic wave 6 life cycles examined here suggest a framework of opposite extremes a in which to view the behaviour of real synoptic scale disturbances in middle latitudes and b in which to interrelate the synoptic and wave theoretic viewpoints using the saturation propagation saturation sps picture of wave 6 life cycle behaviour the two life cycles dented by lc1 and lc2 are higher resolution versions of the simmons hoskins basic and anomalous cases showing strong and weak late decay of eddy kinetic energy eke they illustrate in varying degrees two extreme types of behaviour here disignated anticyclonic and cyclonic and epitomized by strongly contrasting upper air trough behaviour anticyclonic behaviour dominates the late stages of lc1 and is characterized by backward tilted thinning troughs being advected anticyclonically and equatorward as in the commoner cases of planetary scale mid stratospheric rossby wave breaking cyclonic behaviour dominates lc2 and is characterized by forward tilted broadening troughs wrapping themselves up cyclonically and poleward producing major cut off cyclones in high latitudes these morphologies are visualized by upper air maps of potential temperature on the nominal tropopause defined as a constant potential vorticity surface some atmospheric mid latitude disturbances examined here using the same visualization applied to operational analyses show the same two extreme types of trough behaviour together with intermediate cases the sps picture is re examined using eliassen palm and refractive index cross sections it is shown in particular by reference to a wave activity theorem of haynes that the late stages of lc2 can be looked upon as a remarkably clear and morphologically novel large amplitude counterpart of the nonlinear reflection scenario of rossby wave critical layer theory the late stages of lc1 by constrast look more akin to an nonlinear critical layer absorption scenario lc2 exhibits region of largely undular pv contours adjacent to a nonlinear of irreversiby deformen pv contous in the latter region pv rearrangement and hence absorption of rossby wave activity has largely ceased this accounts for the more persistent ekee in the lc2 case

investigating fault shadows in the niger delta the stratigraphic evolution of the niger delta is dominated by a continuous stepwise progradation occurring during a major tertiary regressive phase that continues to the present the successive phases of the delta growth form transient depobelts that are bounded by mega structural growth faults

imaging salt with turning seismic waves turning seismic waves which first travel downward and then upward before and after reflection have been recorded in a 3 d seismic survey conducted over an overhanging salt dome careful processing of these turning waves enables the imaging of the underside of the salt dome and of intrusions of salt into vertical faults radiating from the dome when seismic wave velocity increases with depth waves that initially travel downward are reflected and may turn so as to travel upward before reflection a simple geometrical argument suggests that these turning waves are likely to exhibit abnormal moveout in com mon midpoint cmp gathers in that reflection time decreases with increasing source receiver offset this abnormal moveout and the attenuation of turning waves by most migration methods suggest that conventional seismic processing does not properly image turning waves the most important step in imaging turning waves assuming that they have been recorded is the migration process simple and inexpensive modifications to the conventional phase shift migration method enable turning waves to be imaged for little additional computational cost the examples provided in this paper suggest that these and other such modifications to conventional processing should be used routinely when imaging salt domes

documentation and quantification of velocity anisotropy in shales using wireline log measurements shales are regarded as anisotropic during low strain dynamic elasticity behavior this means that in seismic analysis we must assume that the commonly used seismic properties like modulus velocity and impedance are anisotropic in shales the electromagnetic property resistivity important in recent controlled source electromagnetic surveying is also assumed to be anisotropic due to the nature of shales the seismic and electromagnetic properties are assumed to behave as a vertical transverse isotropic vti medium they are then isotropic in the plane orthogonal to the symmetry axis normally being the vertical direction seismic amplitudes created by lithology contrasts involving at least one shale layer are then controlled by this anisotropy especially at larger incidence angles or offsets seismic waves in shales travel with different velocities at zero and nonzero incidence angles making anisotropy a controlling factor also in seismic processing the overwhelming presence of shales in many b

acoustic imaging of reservoir structure from a horizontal well recent advances in well placement technology now allow a borehole to be steered with considerable precision inside the reservoir with modern completion technology the subsequent flow of hydrocarbons within the reservoir can be controlled to a greater extent than ever before these developments have further emphasized the need for an improved ability to image structures such as caprock stringers faults fractures and pore fluid contacts on the reservoir scale

climate variability across the last deglaciation in nw iberia and its margin abstract the direct comparison between marine and terrestrial data from the nw iberian margin core md03 2697 allows us to accurately evaluate without chronological ambiguity the vegetation response to north atlantic climate events across the last deglaciation comparison of md03 2697 data with other marine and terrestrial records from a vast area stretching from the azores to western w france iberia and its margin the w mediterranean and nw africa reveals the importance of enhanced winter north atlantic westerlies episodes in driving a heterogeneous regional climatic signal during particular events of the last deglaciation heinrich stadial 1 hs1 oldest dryas is a complex event marked by three synchronous main phases a extremely cold relatively wet b cool dry c relatively warmer increasing moisture availability in regions directly influenced by the north atlantic while it is characterized by a single phase cold and dry in most inland and high altitude areas changes in the strength and position of north atlantic westerlies could explain the variability in moisture during hs1 from w pyrenees to w mediterranean the bolling allerod b a event is marked by a synchronous progressive increase of ocean and atmospheric temperatures and precipitation from the bolling to the allerod in w iberia and w pyrenees contrasting with the greenland temperature pattern mid to high latitudes thermal contrast and the gradual strengthening of the atlantic meridional overturning circulation amoc triggered the continuing enhancement of westerlies and moisture along this period finally the younger dryas yd stadial is characterized by a land ocean synchronous returning cold conditions but not so cold as hs1 and although dry conditions persisted during this period extreme winter precipitation occurrences might have affected the most coastal areas of north and central w iberia and pyrenees in contrast dry conditions persisted during the yd in the western mediterranean regions the westerlies migrated further north following the displacement of the polar front during the deglaciation precluding the input of moisture in the w mediterranean region which persisted very dry during the yd stadial

processing array acoustic logging data to image near borehole geologic structures imaging near borehole structures using acoustic logging data depends on the data processing method used we demonstrate that extracting the up and downgoing reflections from the data and using those separately for the imaging can significantly improve the image quality a parametric wave separation is first applied to array data to separate direct and reflection waves in particular we use a transmitter array gathered from successive source positions to extract the upgoing reflection reflection data can also be enhanced by stacking data along the reflection moveout in array using approximate structural dip information implementation of this method to acoustic logging data processing improves imaging quality making it possible to image near borehole geologic structures using conventional array acoustic logging a potentially important application of drilling steering is using logging while drilling acoustic measurements

past circulation along the western iberian margin a time slice vision from the last glacial to the holocene abstract fifteen iberian margin sediment cores distributed between 43 12 n and 35 53 n have been used to reconstruct spatial and temporal sub surface circulation along the iberian margin since the last glacial period time slice maps of planktonic foraminiferal derived summer sea surface temperature sst and export productivity pexp were established for specific time intervals within the last 35 ky the holocene recent and last 8 ky younger dryas yd heinrich stadials hs 1 2a 2b 3 and the last glacial maximum lgm the sst during the holocene shows the same latitudinal gradient along the western iberian margin as present day with cold but productive areas that reflect the influence of coastal upwelling centers the lgm appears as a slightly less warm but more productive period relative to the holocene and present day conditions suggesting that sea level minima forced a westward displacement of the coastal upwelling centers possibly accompanied by a strengthening of northward winds during the yd a longitudinal thermal front is depicted at 10 w with cold polar waters offshore and warmer subtropical waters inshore suggesting that the subtropical paleo iberian poleward current more likely flowed at a more inshore location masking the local sst signal and amplitude of variation a substantial cooling and drop in productivity is observed during all hs in particular hs1 and hs3 reflecting the penetration of icebergs derived meltwater these most extreme southward extensions of very cold waters define a strong sst gradient that marks a possible paleo azores front higher production south of this front was likely fed by frontal nutrient advection

kirchhoff time migration for transversely isotropic media an application to trinidad data using a newly developed nonhyperbolic offset mid point traveltime equation for prestack kirchhoff time migration instead of the conventional double square root dsr equation results in overall better images from anisotropic data specifically prestack kirchhoff time migration for transversely isotropic media with a vertical symmetry axis vti media is implemented using an analytical offset midpoint traveltime equation that represents the equivalent of cheop s pyramid for vti media it includes higher order terms necessary to better handle anisotropy as well as vertical inhomogeneity application of this enhanced kirchhoff time migration method to the anisotropic marmousi data set demonstrates the effectiveness of the approach further application of the method to field data from trinidad results in sharper reflectivity images of the subsurface with the faults better focused and positioned than with images obtained using isotropic methods the superiority of the anisotropic time migration is evident

anisotropy correction for deviated well sonic logs application to seismic well tie borehole sonic logs acquired in deviated wells penetrating the hrz and colville shales in the niakuk field in alaska s north slope are seen to be significantly faster than vertical well logs these differences are attributed to shale anisotropy an iterative inversion scheme was created to invert for shale anisotropy parameters using multiple wells penetrating shale sections at different angles the inversion involves fitting the sonic log data at a range of borehole angles to the compressional wave group velocity surface the result is an estimate of the anisotropy parameters e and and the vertical p wave velocity the results show that the shales are strongly anisotropic with compressional wave anisotropy thomsen s parameter e on the order of 40 and the anisotropy parameter relates vertical velocity to short offset nmo velocity around 10 this large anisotropy can affect seismic imaging avo and time depth calculations a procedure was created to estimate the anisotropy corrected vertical s

a dynamic link between high intensity precipitation events in southwestern north america and europe at the last glacial maximum abstract both hydroclimate proxies and modeling studies alike present compelling evidence for abundant precipitation in southwestern north america and europe at the last glacial maximum lgm however model simulations exhibit an apparent disagreement between changes in precipitation and the orientation of the midlatitude jet streams relative to modern this is surprising as it is well known from modern observations that the jet streams and storm tracks closely follow one another in the time mean the present study shows that a substantial fraction between 35 and 50 of the precipitation along the western seaboards of the northern hemisphere continents at the lgm is due to short lived high intensity events rather than a permanent rearrangement of the circulation moreover these precipitation events are shown to be dynamically linked by a wavenumber 5 stationary wave packet that is trapped in a midlatitude waveguide when the wave energy is concentrated in the eastern north pacific an atmospheric river is excited that makes landfall along the north american west coast days later when the wave packet has reached europe the north atlantic jet stream assumes a zonally oriented state that brings abundant precipitation to the western mediterranean sector understanding the dynamics of this teleconnection pattern is not only key for interpreting proxy data signals in past climates but may also be important for improving predictions of storm track dynamics and water resources availability in the face of climate variability and change

hydraulic stimulation of natural fractures as revealed by induced microearthquakes carthage cotton valley gas field east texas we produced a high resolution microseismic image of a hydraulic fracture stimulation in the carthage cotton valley gas field of east texas we improved the precision of microseismic event locations four fold over initial locations by manually repicking the traveltimes in a spatial sequence allowing us to visually correlate waveforms of adjacent sources the new locations show vertical containment within individual targeted sands suggesting little or no hydraulic communication between the discrete perforation intervals simultaneously treated within an 80 m section treatment i e fracture zone lengths inferred from event locations are about 200 m greater at the shallow perforation intervals than at the deeper intervals the highest quality locations indicate fracture zone widths as narrow as 6 m similarity of adjacent source waveforms along with systematic changes of phase amplitude ratios and polarities indicate fairly uniform source mechanisms fracture plane orientation and sense of slip over

the generation of global rotational flow by steady idealized tropical divergence abstract tropical convective heating is balanced on the large scale by the adiabatic cooling of ascent the horizontal divergence of the wind above this heating may be viewed as driving the upper tropospheric rotational wind field a vorticity equation model is used to diagnose this relationship it is shown that because of the advection of vorticity by the divergent component of the flow the rossby wave source can be very different from the simple fd source often used in particular an equatorial region of divergence situated in easterly winds can lead to a rossby wave source in the subtropical westerlies where it is extremely effective this part of the source can be relatively insensitive to the longitudinal position of the equatorial divergence a divergence field which is asymmetric about the equator can lead to a quite symmetric rossby wave source for a steady frictionless flow the rossby wave source averaged over regions within closed streamfunction or absolute vorticity contours is under cert

imaging of near borehole structure using full waveform sonic data the full waveforms recorded by an array of receivers in a modern borehole sonic tool contain secondary arrivals that are reflected from near borehole structural features these arrivals are used to form an image of the near borehole structural features in a manner similar to seismic migration possible uses of this technique include horizontal well logging structural dip and contour determination fault salt dome pinnacle reef and fracture zone imaging and eor steam flood monitoring since both the source and the receivers pass through structures that cross the borehole the downdip structure and the updip structure can be imaged separately the technique involves a backprojection of the recorded data into a matrix of accumulation bins representing distances radially out from the borehole and along the borehole axis separate matrices are formed for the updip and for the downdip raypaths the basic technique is illustrated with synthetic data generated to approximate the case of a sonic tool logging

interactions between stationary waves and ice sheets linear versus nonlinear atmospheric response this study examines the mutual interaction between topographically forced atmospheric stationary waves and continental scale ice sheets using a thermomechanical ice sheet model coupled to a linear

fault related folding a review of kinematic models and their application abstract folding that is directly related to fault activity is an important deformation feature that occurs all over the world in mountain belts accretionary wedges fold and thrust belts and intra plate settings in either strike slip compressional or extensional regimes due to their widespread occurrence knowledge about the development of these structures is important to a broad spectrum of geoscience sub disciplines such as structural geology seismology geomorphology petroleum geology and quaternary geology fault related folding has been analyzed intensively over the last 140 years for the sake of this review we divide the folds according to the way the faults and the folds form that is into detachment fault bend and fault propagation folds all fault related folds are caused by changes in fault parameters the simplest method to produce folds is to transport material along faults that have stepped flat ramp flat geometries fault bend fold alternatively the slip can decrease along the length of the fault and depending on whether the fault remains within a detachment layer or steps up through mechanical stratigraphy either a detachment fold or a fault propagation fold is formed respectively detachment folding was first investigated in the early 20th century whereas the full significance of fault propagation folds was recognized quite late in the 1980s seminal work on fault related folding was carried out in the 1930s but quantitative kinematic models have only been available in the last 30 years these models are extremely valuable because they allow a comprehensive understanding of the evolution of fault related folds and lead to more accurate predictions of the sub surface structure from the mid 1990s onwards numerical simulations have been used to identify how fault parameters such as dip and fault bend angle propagation to slip ratio and shape of the trishear zone influence the geometry of the related folding this is directly applicable to the analysis of the shape of anticlines produced however this does not mean that fold geometry is uniquely related to fault geometry on the contrary different kinematic approaches can lead to a similar fold shape

numerical simulation of radiation reflection and reception of elastic waves from a borehole dipole source abstracta recent advance in single well reflection imaging is the use of a dipole acoustic system in a borehole to radiate and receive elastic waves to and from a remote geologic reflector in formation this dipole acoustic imaging technology is evaluated by numerically simulating the radiation and reflection of the wavefield generated by the borehole dipole source and analyzing the receiving sensitivity of the dipole system to the incoming reflected waves the analyses show that a borehole dipole source can radiate a compressional wave p wave and two types of shear waves i e sv and sh waves into the formation the sh wave has wide radiation coverage and the best receiving sensitivity and is most suitable for dipole shear imaging in an acoustically slow formation the dipole generated p wave has strong receiving sensitivity and can also be used for reflection imaging an important feature of dipole imaging is its sensitivity to reflector azimuth which results from the directivity of the dipole s

effect of latitudinal variations in low level baroclinicity on eddy life cycles and upper tropospheric wave breaking processes abstract an analysis of the potential vorticity gradient and the refractive index in quasigeostrophic qg flows on the sphere reveals that the absolute vorticity and the stretching parts have two contradictory effects on the horizontal shape of the baroclinic waves when the full variations of the coriolis parameter are taken into account in each term the absolute vorticity effect favors the anticyclonic southwest northeast tilt and anticyclonic wave breaking awb and is stronger in the upper troposphere in contrast the stretching effect promotes the cyclonic northwest southeast tilt and cyclonic wave breaking cwb and is more efficient at lower levels a positive eddy feedback acting on the latitudinal variations of the zonal winds is deduced because the absolute vorticity and the stretching effects are respectively more and less efficient with increasing latitude a more northward southward jet renders awb more less probable and cwb less more probable the jet is pushed or maintained mor

pluvial lakes in the great basin of the western united states a view from the outcrop abstract paleo lakes in the western united states provide geomorphic and hydrologic records of climate and drainage basin change at multiple time scales extending back to the miocene recent reviews and studies of paleo lake records have focused on interpretations of proxies in lake sediment cores from the northern and central parts of the great basin in this review emphasis is placed on equally important studies of lake history during the past 30 years that were derived from outcrop exposures and geomorphology in some cases combined with cores outcrop and core records have different strengths and weaknesses that must be recognized and exploited in the interpretation of paleohydrology and paleoclimate outcrops and landforms can yield direct evidence of lake level facies changes that record details of lake level fluctuations and geologic events such as catastrophic floods drainage basin changes and isostatic rebound cores can potentially yield continuous records when sampled in stable parts of lake basins and can provide proxies for changes in lake level water temperature and chemistry and ecological conditions in the surrounding landscape however proxies such as stable isotopes may be influenced by several competing factors the relative effects of which may be difficult to assess and interpretations may be confounded by geologic events within the drainage basin that were unrecorded or not recognized in a core the best evidence for documenting absolute lake level changes lies within the shore nearshore and deltaic sediments that were deposited across piedmonts and at the mouths of streams as lake level rose and fell we review the different shorezone environments and resulting deposits used in such reconstructions and discuss potential estimation errors lake level studies based on deposits and landforms have provided paleohydrologic records ranging from general changes during the past million years to centennial scale details of fluctuations during the late pleistocene and holocene outcrop studies have documented the integration histories of several important drainage basins including the humboldt amargosa owens and mojave river systems that have evolved since the miocene within the active tectonic setting of the great basin these histories have influenced lake levels in terminal basins many pre late pleistocene lakes in the western great basin were significantly larger and record wetter conditions than the youngest lakes outcrop based lake level data provide important checks on core based proxy interpretations we discuss four such comparisons in some cases such as for lakes owens and manix outcrop and core data synthesis yields stronger and more complete records in other cases such as for bonneville and lahontan conflicts point toward reconsideration of confounding factors in interpretation of core based proxies