evaluation of possible gas microseepage mechanisms petroleum microseepage anomalies over petroleum accumulations are commonly explained by rapid vertical migration of colloidal gas bubbles through fracture networks this article is a theoretical anal ysis of this mechanism and of continuous gas phase flow in frac tures the gas bubble ascent mechanism is much slower than re ported microseepage velocities so it cannot account for observed microseepage in contrast continuous gas phase flow through frac tures can equal or exceed reported microseepage velocity while maintaining total flux low enough so that petroleum accumulations can exist for geological lengths of time fracture entry pressure for bubbles is more than twice that of a continuous gas phase so con tinuous gas phase migration also requires a lower pressure threshold before initiating seepage vertical microseepage is therefore best explained by the same mechanism interpreted for macroseepage although this article provides a theoretically justified mecha nism for microseepage it also shows why interpretation of surface microseepage signals is problematic fracture geometry controls seepage velocity and flux so geochemical anomalies may indicate an increase in fracture aperture as well as possible subsurface ac cumulations larger fractures require very low gas capillary entry pressures so in some settings surface seepage could result from fractures over stratal migration pathways as well as over petroleum accumulations

deltaic morphodynamics and stratigraphic evolution of middle barataria bay and middle breton sound regions louisiana usa implications for river sediment diversions abstract river sediment diversions have been recognized as a key strategy for offsetting land loss in coastal louisiana recently much attention has been focused on sediment capture from the main stem river and conveyance of that material through diversion outflows yet the performance and long term feasibility of diversions as land building agents also hinges upon the subsurface stratigraphic architecture of receiving basins a concept that has largely been unexplored a major core collection and analysis program was undertaken to study the geological properties of substrates in the middle barataria bay and middle breton sound diversion receiving basins louisiana usa over a region of 200 square kilometers in both basins 50 vibracores up to 5 5 m in length were collected in spring and summer of 2015 cores were logged for bulk density and imaged with a geotek multi sensor core logger split cores were then subsampled for granulometry organic content and 14c dating both receiving basins are characterized by 1 2 m of organic rich surficial strata underlain by 2 m of mineral rich sand and silt beds that display a greater bulk density additionally some cores contain deeper peaty strata intercalated within muds and sands 14c dates of these buried peats range between 1910 and 3203 calendar years before present demonstrating that relatively old deltaic strata exist at shallow depths age comparisons of stratigraphically similar peat beds from this study and previous delta lobe chronological models indicate that the st bernard lobe prograded eastward at a time averaged rate of 75 m yr from a geotechnical standpoint the surficial uncompacted

sedimentation patterns in the selenga river delta under changing hydroclimatic conditions the selenga river delta russia is a large 600km 2 fluvially dominated fresh water system that transfers water and sediment from an undammed drainage basin into lake baikal a united nation

the estimation of spectra from time frequency transforms for use in attenuation studies we introduce the signal dependent time frequency distribution which is a time frequency distribution that allows the user to optimize the tradeoff between joint time frequency resolution and suppression of transform artefacts the signaldependent time frequency distribution as well as the short time fourier transform stockwell transform and the fourier transform are analysed for their ability to estimate the spectrum of a known wavelet used in a tuning wedge model next the signal dependent time frequency distribution and fixed and variable window transforms are used to estimate spectra from a zero offset synthetic seismogram attenuation is estimated from the associated spectral ratio curves and the accuracy of the results is compared the synthetic consisted of six pairs of strong reflections based on real well log data with a modeled intrinsic attenuation value of 1000 q 20 the signal dependent time frequency distribution was the only time frequency transform found to produce spectra that estimated consistent attenuation values with an average of 1000 q 26 2 results from the fixed and variable window transforms were 24 17 and 39 10 respectively finally all three time frequency transforms were used in a pre stack attenuation estimation method the pre stack q inversion algorithm applied to a gather from a north sea seismic dataset to estimate attenuation between nine different strong reflections in this case the signal dependent time frequency distribution produced spectra more consistent with the constant q model of attenuation assumed in the pre stack attenuation estimation algorithm the average l1 residuals of the spectral ratio surfaces from the theoretical constant q expectation for the signal dependent time frequency distribution short time fourier transform and stockwell transform were 0 12 0 21 and 0 33 respectively based on the results shown the signal dependent time frequency distribution is a time frequency distribution that can provide more accurate and precise estimations of the amplitude spectrum of a reflection due to a higher attainable time frequency resolution

modeling flow and sediment transport dynamics in the lowermost mississippi river louisiana usa with an upstream alluvial bedrock transition and a downstream bedrock alluvial transition implications for land building using engineered diversions the lowermost mississippi river defined herein as the river segment downstream of the old river control structure and hydrodynamically influenced by the gulf of mexico extends for approximately 500 km this segment includes a bedrock or more precisely mixed bedrock alluvial reach that is bounded by an upstream alluvial bedrock transition and a downstream bedrock alluvial transition here we present a one dimensional mathematical formulation for the long term evolution of lowland rivers that is able to reproduce the morphodynamics of both the alluvial bedrock and the bedrock alluvial transitions model results show that the magnitude of the alluvial equilibrium bed slope relative to the bedrock surface slope and the depth of bedrock surface relative to the water surface base level strongly influence the mobile bed equilibrium of low sloping river channels using data from the lowermost mississippi river the model is zeroed and validated at field scale by comparing the numerical results with field measurements the model is then applied to predict the influence on the stability of channel bed elevation in response to delta restoration projects in particular the response of the river bed to the implementation of two examples of land building diversions to extract water and sediment from the main channel is studied in this regard our model results show that engineered land building diversions along the lowermost mississippi river are capable of producing equilibrated bed profiles with only modest shoaling or erosion and therefore such diversions are a sustainable strategy for mitigating land loss within the mississippi river delta

robust prestack q determination using surface seismic data part 1 method and synthetic examples the accurate determination of seismic attenuation or 1 q is useful for signal enhancement and reservoir characterization to arrive at the necessary accuracy however a number of issues must be addressed in the measurement technique specifically spectral interference from closely spaced reflections is a major concern in addition to the assumptions and errors associated with the raypath geometries of the reference and measured reflections we have developed a robust method for measuring attenuation from prestack surface seismic gathers that helps minimize these issues in our prestack q inversion technique the presence of spectral interference was first reduced by making use of a variable window time frequency transform to minimize the effects of the remaining interference we then made use of an inversion scheme operating simultaneously in the frequency and traveltime difference coordinates a by product of this inversion was a collection of the frequency independent amplitude changes which in the

river base level change in mouth channel evolution the case of the yellow river delta china abstract the base level is one of the key factors controlling river evolution in the yellow river mouth a base level rise takes place not only because of a relative sea level rise but also as a consequence of the increasing of river channel length associated with the large progradation rate of the delta a change in river length results in a change in the gradient slope of the mouth channel which further leads to a riverbed aggradation degradation or a water level change at the deltaic apex this process is principally associated with the morphodynamic backwater effect the consequent water level change can be considered as a base level change of the yellow river in this work a relation was constructed between the water level at the deltaic apex and the riverine water and sediment inputs as well as with the gradient of the mouth channel in the progradation period of each deltaic lobe also for disclosing the importance of the base level change due to variations of mouth channel length it was estimated and compared with the relative sea level change in the past decades and for predicting the water level at the deltaic apex the relation between mouth channel length and cumulative riverine sediment input was investigated it was found that in the period 1953 2016 the base level changes associated with variations of the mouth channel length could be larger than 1 m or even larger than 2 m in one year with a conservatively estimated annual mean of 34 8 mm a which is much larger than the relative sea level change of 7 0 mm a in the yellow river mouth the mouth channel length is closely and linearly related to the cumulative sediment discharge of the yellow river the relation for calculating the water level at the deltaic apex was found to be capable of giving a reliable result with the riverine water and sediment inputs the relative sea level and the mouth channel length estimated from cumulative riverine sediment discharge this relation was also used to estimate the equilibrium gradient of the mouth channel for sediment transport the results show that after the year 2000 the equilibrium gradient has remained smaller than the actual gradient of the mouth channel and both gradients decreased continuously with the decrease of riverine sediment inputs but their values were becoming closer to each other it is highly possible that the water level at the deltaic apex will be raised by the increase in sediment discharge of the river in the future

defining the ecogeomorphic succession of land building for freshwater intertidal wetlands in wax lake delta louisiana abstract land building in deltaic environments occurs when sediment discharged from a river mouth is deposited subaqueously and transitions to subaerial land the transition from subaqueous deposition to subaerial land is a critical process that marks the creation of relatively stable land yet it is unclear what controls the speed and style of this transition we define how this transition herein termed the land building succession varies in time and space for the freshwater intertidal wetlands in wax lake delta la using remote sensing and field data we classify land cover into sediment water or vegetation classes at maximum and minimum biomass we see two succession patterns within wax lake delta deltaic islands near the apex are initially covered by sediment and open water through time open water and sediment coverage decreases as vegetation coverage increases on the other hand distal islands show little sediment exposure through time in both cases all deltaic islands become covered with vegetation by 2015 as vegetation colonizes the island the topography organizes into two platforms vertically separated by 0 35 m the lower intertidal platform occurs in the island interiors and is commonly inundated by water and dominated by subaqueous or floating vegetation the upper subaerial platform occurs along island edges and is dominated by a variety of vegetation species including salix nigra colocasia esculenta and polygonum punctatum it takes an average of 10 years for the intertidal platform to transition to the subaerial platform these two platforms are separated by the tidal range measured in atchafalaya bay and the different vegetation communities occupying each platform suggest they are a manifestation of multiple stable states and arise due to vegetation and sedimentation feedbacks

drowning of the mississippi delta due to insufficient sediment supply and global sea level rise global sea level rise reduced sediment supply and subsidence threaten the stability of the mississippi delta calculations of riverine sediment load and storage indicate that 10 000 13 500 km2 of the delta could be submerged by ad 2100

the lowermost mississippi river a mixed bedrock alluvial channel in this study the distribution of channel bed sediment facies in the lowermost mississippi river is analysed using multibeam data complemented by sidescan sonar and compressed high intensity radar pulse seismic data as well as grab and core samples of bed material the channel bed is composed of a discontinuous layer of alluvial sediment and a relict substratum that is exposed on the channel bed and sidewalls the consolidated substratum is made up of latest pleistocene and early holocene fluvio deltaic deposits and is preferentially exposed in the deepest thalweg segments and on channel sidewalls in river bends the exposed substratum commonly displays a suite of erosional features including flutes that are quantitatively similar in form to those produced under known laboratory conditions a total of five bed facies are mapped three of which include modern alluvial deposits and two facies that are associated with the relict substratum a radius of curvature analysis applied to the mississippi river centreline demonstrates that the reach scale distribution of channel bed facies is related to river planform from a broader perspective the distribution of channel bed facies is related to channel sinuosity higher sinuosity promotes greater substratum exposure at the expense of alluvial sediment for example the ratio of alluvial cover to substratum is ca 1 5 1 for a 45 km segment of the river that has a sinuosity of 1 76 and this ratio increases to ca 3 1 for a 120 km segment of the river that has a sinuosity of 1 21 the exposed substratum is interpreted as bedrock and given the relative coverage of alluvial sediment in the channel the lowermost mississippi river can be classified as a mixed bedrock alluvial channel the analyses demonstrate that a mixed bedrock alluvial channel boundary can be associated with low gradient and sand bed rivers near their marine outlet

punctuated sand transport in the lowermost mississippi river 1 measurements of sand flux and water flow in the mississippi river are presented for a portion of the system 35 50 km upstream from the head of its subaerial delta these data are used to provide insight into how nonuniform flow conditions present in the lower reaches of large alluvial rivers affect the timing and magnitude of sand transport near the river outlet field surveys during both low and high water discharge include 1 sequential digital bathymetric maps defining mobile river bottom topography which were used to estimate bed material flux 2 multiple water velocity profiles and 3 multiple suspended sediment profiles collected using a point integrated sampler these data show that total sand transport increases by two orders of magnitude over the measured range in water discharge 11 300 to 38 400 m3 s 1 during low water discharge no sand is measured in suspension and sand discharge via bed form migration is minimal during high water discharge 54 of the sand discharge is measured in suspension while 46 of the sand discharge is part of bed form migration the component of boundary shear stress associated with moving this sediment is estimated using a set of established sediment transport algorithms and values for the total boundary shear stress are predicted by fitting logarithmic velocity functions to the measured profiles the estimates of boundary shear stress using measurements of suspended sand transport bed form transport and downstream oriented velocity profiles are internally consistent moreover the analyses show that boundary shear stress increases by nearly 10 fold over the measured water discharge range we show how this increase in shear stress is consistent with backwater flow arising where the river approaches its outlet the hydrodynamic properties of backwater flow affect the timing and magnitude of sand flux and produce punctuated sand transport through the lowermost mississippi river our field data are used to evaluate the influence of this sand transport style on development of the mixed bedrock alluvial channel for the lowermost mississippi river

experimental comparison between spectral ratio and rise time techniques for attenuation measurement two techniques for the measurement of attenuation spectral ratio and rise time techniques were tested and compared in the laboratory the spectral ratio technique proved to be reliable and easy to implement for intermediate values 5 50 and high attenuations the spectral ratio technique is inaccurate calculating the rise time on simulated signals we found a relation between rise time and the ratio travel time to quality factor t q which could be approximated in intervals by the linear relation c t q the constants and c depend on the absolute value of t q and on the initial source signal the rise time technique performed on the first quarter period of the signal enables high attenuations q 5 to be measured the determination of the relation between and t q is possible if one knows the initial source we theoretically approximate this relation through a simulation using a realistic propagation model with laboratory measurements made on fontainebleau sandstone we show that the rise time technique using the theoretical relation t q gives comparable values of q to those obtained from the spectral ratio technique in borehole seismics where it is often difficult to remove undesired signals the rise time technique applied with the right t q relation is the best method to use

bedform transport rates for the lowermost mississippi river 1 new methods of data collection and processing are developed to provide quantitative reach scale measurements of bedform transport mass within the tidally influenced mississippi river a multibeam swath profiler was used to collect daily bathymetry over a range of water discharges and bed elevation changes induced by dune migration are measured these values are coupled with bulk physical properties of the bed sediment to constrain mass flux and annual bedform transport is estimated at 2 2 x 10 6 metric tons mt the total annual sand flux from the mississippi river calculated by combining measured bedform transport rates and suspended sediment flux is estimated to be 20 x 10 6 mt survey data also provide information about the spatial distribution of dunes across the channel bottom straight reach segments are commonly mantled by dunes for the entire cross section while bends are typically areas of focused scour devoid of bedforms presumably any sediments associated with migrating dunes are propelled into suspension within bends before redepositing in the subsequent straight reach movement via suspension is therefore an important component for the downriver transport of bed materials in the lower mississippi river

degradation of the plaquemines sub delta and relative sea level in eastern mississippi deltaic coast during late holocene abstract the stratigraphic architecture of plaquemines sub delta lobe is identified from sedimentary cores and sub bottom seismic data collected at lower breton sound louisiana usa four lithofacies were identified in cores from bottom to top 1 organic poor mud 2 organic poor silty sand 3 massive mud 4 organic rich peat and root rich soft mud overall the lithofacies displays a typical progradational deltaic deposit at bottom with upward coarsening feature and then an upward fining deposit overlying on top stratigraphic position of the organic rich peat facies indicates that it is formed after the fluvial sediment source moves away from this region and it is recognized as a termination of progradation of the plaquemines sub delta dated peat samples show that sub delta progradation stops between 876 and 1384 yr ad during which a 20 70 cm relative sea level rise is detected the geological framework reported in this study provides critical information for future modeling on river diversions the stratigraphic architecture underlying lower breton sound implicates that top three facies of root rich soft mud organic rich peat and massive mud are likely to be eroded away before the development of new crevasse splay or sub delta when sediment diversion is in operation the deeper organic poor silty sand lithofacies however can serve as a firm foundation to build new land

suspension of bed material over sand bars in the lower mississippi river and its implications for mississippi delta environmental restoration 1 understanding specific pathways for sand transport in the lower reaches of large rivers including the mississippi is a key for addressing multiple significant geologic problems such as delta building and discharge to the oceans and for environmental restoration efforts in deltaic environments threatened by rising sea levels field studies were performed in the mississippi river 75 100 km upstream of the gulf of mexico outlet in 2010 2011 to examine sand transport phenomena in the tidally affected river channel over a range of discharges methods included mapping bottom morphology multibeam sonar cross sectional and longitudinal measurements of water column velocity and acoustic backscatter suspended sediment sampling and channel bed sampling substantial interaction was observed between the flow conditions in the river boundary shear stress channel bed morphology size and extent of sandy bedforms and bed material sand transport quantity transport mode and spatial distribution a lateral shift was observed in the region of maximum bed material transport from deep to shallow areas of subaqueous sand bars with increasing water discharge bed material was transported both in traction and in suspension at these water discharges and we posit that the downriver flux of sand grains is composed of both locally and drainage basin sourced material with distinct transport pathways and relations to flow conditions we provide suggestions for the optimal design and operation of planned river diversion projects

q estimation from vertical seismic profile data and anomalous variations in the central north sea four different methods of estimating q from vertical seismic profile vsp data based on measurements of spectral ratios pulse amplitude pulse width and zeroth lag autocorrelation of the attenuated impulse are described the last procedure is referred to as the pulse power method practical problems concerning nonlinearity in the estimating procedures uncertainties in the gain setting of the recording equipment and the influence of structure are considered in detail vsp data recorded in a well in the central north sea were processed to obtain estimates of seismic attenuation these data revealed a zone of high attenuation from approximately 4 900 ft to 5600 ft with a value of 25 3 results of the spectral ratio analysis show that the data conform to a linear constant q model in addition since the pulse width measurement is dependent upon the dispersive model adopted it is shown that a nondispersive model cannot possibly provide a match to the real data no unambiguous evidence is presented that e

measurements of earth attenuation from downhole and surface seismic recordings frequency dependent attenuation of compressional waves within the earth has been estimated in the vicinity of wells from 1 spectral power ratios of the coherent events in separate time gates on the seismic section 2 matching a broadband synthetic trace with seismic data at the well and 3 determining the operator that transforms one down up going pulse recorded in the well into another recorded at a deeper shallower level the accuracy of estimation of all three methods was insufficient to estimate attenuation over small depth intervals and it was not possible to distinguish between the contribution due to internal multiples and that of genuine absorption with much confidence spectral ratios from 1 showed a smoother variation with frequency and one more consistent with other estimates when they were compensated for the spectra of the reflectivities over the time gates employed but they did not provide more than a broad indication of attenuation over a substantial depth interval approach 2 was hampered by the restricted durations over which synthetic trace and seismic data can be reliably matched approach 3 gave the best results here matching is a much more powerful tool than the spectral ratio techniques that are commonly applied since it can yield the form of the attenuation operator i e both its amplitude and phase response together with properly defined measures of its accuracy while at the same time it minimizes the influence of noise and local interference effects at each recording level for seismic target depths where internal multiple activity was low the logarithms of the amplitude responses of the estimated attenuation operators decreased approximately linearly with frequency and the phase responses showed no significant dispersion application of approach 3 to downgoing and upgoing waves estimated from a vertical seismic profile revealed the importance of changes in frequency dependent geophone coupling and their effect on values of q determined from downgoing pulses only

current and future mudflat losses in the southern huanghe delta due to coastal hard structures and shoreline retreat abstract coastal wetlands especially broad mudflats are facing an unprecedented crisis due to combined influence of intensified coastal development activities drastic river sediment reduction and rapid relative sea level rise however it is still a huge challenge to accurately assess the evolution of mudflats for the lack of reliable shoreline indicators on satellite images especially the indicators for shoreline at low tide this study based on instantaneous shoreline positions derived from 430 landsat satellite images during 1983 2019 uses sea level correction and statistical analysis methods to study the evolution of laizhou mudflat and the intertidal mudflat loss in laizhou mudflat was accurately assessed and the effects of multiple factors especially the effects of various coastal hard structures on the evolution of laizhou mudflat were quantitatively estimated for the first time the results show that the intertidal mudflat loss caused by the construction of cofferdams was 98 km2 93 of the total intertidal mudflat loss although the previous intertidal mudflat loss caused by shore parallel structures was less the shore parallel structures will cause great mudflat loss in the future and the broad laizhou mudflat will disappear in 110 years owing to the barrier effect of the widespread shore parallel structures if the current rapid shoreline retreat is not improved the construction of shore normal structures can slow the shoreline retreat of adjacent mudflat but accelerate downstream shoreline retreat as far as coastal protection is concerned the construction of shore normal structures in laizhou mudflat is inadvisable diverting 13 m ton year of huanghe sediment can totally offset the mudflat loss on the upstream of dongying pier however the mudflat located on the downstream of dongying pier is hard to restore owing to the block effect of the shore normal structures to the alongshore transported sediment another option is demolishing the shore parallel structures to recover the landward movement of shoreline avoiding the mudflat loss at the expense of land loss

a review of sediment diversion in the mississippi river deltaic plain abstract one of the proposed methods for restoring the disappearing mississippi delta is sediment diversion which uses channels and structures to divert water and sediment from the mississippi and atchafalaya rivers into adjacent basins this study presents a comprehensive review of geological and physical aspects of sediment dynamics in the mississippi river deltaic plain mrdp with special reference to diversion studies over the past two decades we synthesize these studies present the current understanding of sediment diversions in the context of sediment dynamics identify multiple key knowledge gaps and make recommendations for future studies to maximize net land building in the mrdp management strategies should be focused on a enhancing river sediment delivery both mud and sand b increasing sediment retention in receiving basins and c minimizing erosion in bays and estuaries compared with extensive studies of land building there have been relatively fewer studies of erosional processes a heterogeneous coastal geological framework cohesive sediment erodibility and subsidence together play complicated yet critical roles in future sediment dynamics in bays and estuaries of the mrdp sediment retention rates are highly sensitive to spatial and temporal scales types of sediments and delivery season sediment diversions to seaward receiving basins provide more surge protection but tend to have lower sediment retention due to active coastal processes structures and devices that improve sediment retention trap sediments dissipate waves and build living shorelines should be explored and cost to benefit analysis is needed long term planning should consider more landward diversions strategic community relocation and nonlinear response of the complex sedimentary system of the mrdp

geomorphology flood built land the southeastern us coastline is under threat as land subsides and sea level rises measurements of the 2011 mississippi river flood suggest that the river carries enough sandy sediment to offset some of this coastal drowning

mitigating land loss in coastal louisiana by controlled diversion of mississippi river sand the bonnet carre spillway diverts floodwaters from the mississippi river to lake pontchartrain and was opened for 42 days during the 2011 flood according to measurements of the newly deposited sediments at least 31 46 of the river s sand load was diverted into the spillway at this time suggesting that such diversions can help mitigate coastal wetland loss

soil gas co 2 ch 4 and h 2 distribution in and around las canadas caldera tenerife canary islands spain abstract diffuse degassing of co 2 ch 4 and h 2 was investigated at the surface environment of canadas caldera canary islands during the gas survey carried out in the summer of 1995 soil co 2 concentration varied significantly from atmospheric levels to 30 while soil ch 4 and h 2 contents ranged from 5 to 851 ppm and from 0 5 to 620 ppm respectively soil co 2 ch 4 and h 2 distribution suggests that high diffuse degassing at canadas caldera is volcanic structurally controlled anomalous soil h 2 concentrations were identified at the summit of teide and outside caldera boundaries where the most recent eruption of tenerife island occurred 13 c co 2 data showed a magmatic mixed magmatic biogenic and biogenic origin while a biogenic origin is suggested for soil ch 4 at canadas caldera and its surroundings by coupling the co 2 3 he ratio with the 3 he 4 he ratio of fumarolic gas samples from the summit of teide we propose three possible sources for carbon morb type organic carbon and carbonate

seismic q stratigraphy or dissipation the basic problems encountered in extracting estimates of seismic dissipation from data recorded on vertical seismic profiles are analyzed because anomalous dissipation in the subsurface is likely to be associated with conditions or lithologies of limited vertical extent a knowledge of the factors which influence the spatial resolution of an attenuation measurement is of considerable importance by introducing a statistical perspective it is possible to simulate multiple measurements in an inhomogeneous interval and to draw conclusions which apply to an entire class of impedance structures theoretical seismograms are analyzed to demonstrate that for small receiver separations neither a single measurement nor the mean value determined from multiple measurements is likely to give a good estimate of the attenuation for an inhomogeneous depth interval for small receiver separations the attenuation computed from the amplitude ratios method is much more strongly influenced by the local stratigraphy in the

sand as a stable and sustainable resource for nourishing the mississippi river delta dams have starved the lower mississippi river of sediment over recent decades suggesting that the drowning of the delta is inevitable analysis of the rivers suspended sediment load and morphodynamic modelling suggest that the amount of sand essential for land building has not significantly decreased since dam construction with sand remaining available for several centuries

estimation of q from surface seismic reflection data reliable estimates of the anelastic attenuation factor q are desirable for improved resolution through inverse q deconvolution and to facilitate amplitude analysis q is a useful petrophysical parameter itself yet q is rarely measured estimates must currently be made from borehole seismology this paper presents a simple technique for determining q from conventional surface seismic common midpoint cmp gathers it is essentially the classic spectral ratio method applied on a trace by trace basis to a designatured and nmo stretch corrected cmp gather the variation of apparent q versus offset qvo is extrapolated to give a zero offset q estimate studies on synthetics suggest that for reasonable data quality s n ratios better than 3 1 shallow 5 dips and stacking velocity accuracy 5 source to reflector average q is recoverable to within some 3 and q for a specific interval depending on its two way time thickness and depth is recoverable to 15 20 three case studies are reported first

hydrodynamic and geomorphic controls on mouth bar evolution 1 while river deltas are one of the major repositories for sediments and carbon on earth there exists a paucity of field data on the formation of distributary mouth bars one of their key features here we present results from an experiment that tested a model of mouth bar development using hydroacoustic optical sedimentary and geochemical tools on a mouth bar in a crevasse splay near the mouth of the mississippi river our results validate an existing model for mouth bar development which we extend to explain mouth bar stratigraphy we propose that changes across a hydrological cycle are important for mouth bar development resulting in a stratigraphy that has alternating fine and coarse grain sediments results also indicate that sand is carried up to 6 km from the main stem of the mississippi river despite repeated channel bifurcations which has important implications for our interpretation of the rock record understanding of coastal sedimentary systems and the restoration of large deltas

post glacial sea level change along the pacific coast of north america abstract sea level history since the last glacial maximum on the pacific margin of north america is complex and heterogeneous owing to regional differences in crustal deformation neotectonics changes in global ocean volumes eustasy and the depression and rebound of the earth s crust in response to ice sheets on land isostasy at the last glacial maximum the cordilleran ice sheet depressed the crust over which it formed and created a raised forebulge along peripheral areas offshore this combined with different tectonic settings along the coast resulted in divergent relative sea level responses during the holocene for example sea level was up to 200 m higher than present in the lower fraser valley region of southwest british columbia due largely to isostatic depression at the same time sea level was 150 m lower than present in haida gwaii on the northern coast of british columbia due to the combined effects of the forebulge raising the land and lower eustatic sea level a forebulge also developed in parts of southeast alaska resulting in post glacial sea levels at least 122 m lower than present and possibly as low as 165 m on the coasts of washington and oregon as well as south central alaska neotectonics and eustasy seem to have played larger roles than isostatic adjustments in controlling relative sea level changes

the determination of the seismic quality factor q from vsp data a comparison of different computational methods1 ten methods for the computation of attenuation have been investigated namely amplitude decay analytical signal wavelet modelling phase modelling frequency modelling rise time pulse amplitude matching technique spectral modelling and spectral ratio in particular we have studied the reliability of each of these methods in estimating correct values of q using three synthetic vsp seismograms for plane p waves with different noise contents the investigations proved that no single method is generally superior rather some methods are more suitable than others in specific situations depending on recording noise or geology the analytical signal method has been demonstrated to be superior if true amplitude recordings are available otherwise spectral modelling or in the noise free case the spectral ratio method is optimal finally two field vsps in sediments are investigated only in the case of the highest quality vsp can significant information be deduced from the computed attenuation

model for hydrocarbon microseepage and related near surface alterations despite recent advances in surface and near surface geochemical methods there continues to be a need for improved understanding of the scientific basis relating these methods to petroleum deposits we have reviewed the literature pertinent to the several methods we have used and integrated that information with some 100 man years of our experience to contribute to that end evidence indicates that hydrocarbon micro seepage from reservoirs most probably involves buoyant colloidal size microbubbles of light hydrocarbons principally methane through the butanes ascending relatively rapidly through a water filled network of fractures joints and bedding planes bacterial consumption of these hydrocarbons creates carbon dioxide and hydrogen sulfide which may alter sediments producing detectable modifications in geomorphic seismic magnetic and radiometric properties we have used combinations of 1 airborne microwave sensing and laboratory analyses of soil gas hydrocarbons 2 shallow source aeromagnetic and soil magnetic susceptibility measurements 3 aerial and surface gamma ray measurements and 4 geomorphology to find productive oil and gas deposits results suggest that using these methods can substantially increase the probability of wildcat success and reduce finding costs in selected geologic settings however these surface methods cannot reveal depth size or quality of reservoirs or even if producible hydrocarbons will be found

morphodynamics of the erosional phase of crevasse splay evolution and implications for river sediment diversion function abstract despite being a primarily depositional landform a crevasse splay experiences an initial evolutionary phase that is primarily erosional as sediment laden river water spills from a main river channel and incises a new route through the river banks and levee into an interdistributary basin or floodplain this phase sets the dimensions and the conveyance properties of the crevasse which in turn influences the continued expansion or closure of the crevasse channel however little is known about the controlling morphodynamics or how the erosional processes transition to depositional processes during this phase the objective of this study is to investigate these phenomena at the west bay sediment diversion louisiana usa using coupled field observations and numerical modeling the west bay diversion was cut into a lower mississippi river levee to mimic the function of a crevasse splay i e to divert river water and sediment to an adjacent receiving basin for land building purposes bathymetric measurements show that the diversion channel experienced significant natural morphologic evolution during the initial decade 2004 2014 hydrodynamic and sediment transport modeling suggests that this evolution initially increased the discharge of flow and sediment through the crevasse as the channel became wider and deeper and altered its orientation relative to the main river flow direction after 5 years the model results predict that further evolution led to monotonically reduced diversion discharges during this time natural and engineered sediment deposition in the receiving basin decreased predicted basin flow velocities and promoted a backwater effect that reduced the sediment transport capacity of the diversion channel observations during the final 2 years show that much of the initial erosion around the diversion had abated indicating that diversion morphology may have stabilized a modeling sensitivity analysis confirmed that the observed changes to channel geometry and orientation likely promoted flows of water and sediment through the diversion while increases in basin bed elevation would have had a contrary effect the morphodynamic evolution of the west bay diversion documented in this study presents a model indicative of the erosional phase of crevasse splay evolution in a deltaic distributary fluvial network study results offer an analogue on how an engineered river sediment diversion constructed for coastal restoration may function during its first years of operation and suggest that the desired land building processes may take time to become established

geochemistry and field spectrometry for detecting hydrocarbon microseepage remote sensing detection of hydrocarbon reserves would be a valuable addition to conventional methods used currently tests were made in a drilled proven hydrocarbon area predominantly covered by wheat field spectroradiometers measured spectral characteristics of wheat and soil associated with hydrocarbon microseepage soil gas and secondary carbonate produced only by hydrocarbon microseepage were at anomalously high levels at the oil water or gas water boundaries kaolinite and illite in the soil have higher percentages above petroleum reservoirs the inflection point positions of spectra on oil field wheat were shifted 7 nm to longer wavelengths than off field wheat samples and can be positively correlated with the delta carbonate and soil gas the absorption of soil spectra at 2 346 mm is at least 0 2 higher over oil fields than off fields this research proves that remote sensing can be used in an monocultural agricultural area to detect hydrocarbon microseepage

predictions of oil or gas potential by near surface geochemistry recent advances in surface geochemical prospecting have enabled age old seep detection technology to be used to determine the gas versus oil character of a potential fairway extensive field work has demonstrated that the chemical compositions of near surface hydrocarbon soil gases measured by flame ionization gas chromatography are largely determined by the hydrocarbons in nearby underlying reservoirs by using compositions and ratios of the light hydrocarbons methane ethane propane and butane one may predict whether oil or gas is more likely to be discovered in the prospect area near surface hydrocarbons are best represented by normalized histograms of composition data these histograms are strongly correlative with those of reservoir gas and with compositions o gas from shows recorded in downhole mud logging this correspondence with the actual formation gases suggests that the upward migration of reservoired light hydrocarbons into near surface soils represents a viable mechanism allowing surface geochemical exploration to be utilized for regional hydrocarbon evaluations geochemical profiles over known production areas are shown for the sacramento and san joaquin basins in california and for the utah wyoming overthrust belt geochemical predictions were documented by subsequent drilling near the pineview field in utah the data imply that the pineview field should extend westward into an area containing a dry hole in addition a new gas discovery on the intersection of a landsat lineament and a large methane anomaly was made 6 5 km 4 mi northeast of the pineview field by amoco in 1981 most of the geochemical examples reported show direct anomalies over known fields however seeps can be laterally displaced in certain geologic settings in addition geochemical investigations indicate that seep magnitudes depend on tectonic activity to aid gas migration along faults and fractures which appear to provide the major migration pathways this fault association suggests that diffusion is of secondary importance geochemical prospecting must be used with caution and only in conjunction with geologic and geophysical tools because the location and shape of many geochemical anomalies are governed more by the local tectonic structure of the region than by the position and shape of the actual deposit regional groundwater flow is less significant thus geochemical prospecting when used alone cannot predict whether a particular soil gas anomaly is associated with a commercial deposit it can only be used to verify the presence of petroleum hydrocarbons and to predict whether gas or oil is likely to occur in a potential structure geochemical prospecting yields excellent regional evaluations of hydrocarbon potential

in situ investigation of seismic body wave attenuation in heterogeneous media field experiments have been carried out to study the nature and magnitude of seismic wave attenuation for a variety of lithologies in each survey two three component sets of geophones with wall clamping mechanisms were lowered down boreholes and signals originating from surface compressional and shear wave sources were recorded the data collected were corrected for spherical divergence and analyzed to determine intrinsic attenuation for situations of anomalous wavefront expansion and in cases where multiple reflection losses may be significant corrections were supplied by a synthetic seismogram programme to improve the estimates of intrinsic attenuation values of attenuation were obtained for pure sandstone sandstone marl sequences and fissured unfissured chalk sequences these formations were all near surface and relatively porous significant differences in the relative values of compressional and shear wave attenuation in the various lithologies were noted in particular compressional absorption in the fissured zones of chalk was twice the absorption associated with the seismic velocities and the absorption of shear waves fluctuated much less a large contrast in p wave attenuation was also observed between pure bunter sandstone and the sandstone marl sequence absorption was over three times as small in the former a smaller contrast was noted for shear attenuation the results obtained suggest that for the relatively homogeneous formations such as pure sandstone and unfissured chalk shear absorption was dominant over bulk absorption in contrast bulk absorption was larger than shear absorption for other formations e g the fissured chalk sequences and a partially saturated chalk section attenuation was found to be approximately proportional to frequency in all experiments

measurements of attenuation from vertical seismic profiles values of attenuation have been extracted from five detailed velocity surveys significant amounts of attenuation are observed in all five wells measured values of attenuation vary by a factor of 10 ranging from less than 0 1 to 0 9 db wavelength correlation of attenuation with lithology is good considering the resolution limits of the surveys one of the surveys was performed in west texas and the other four were carried out in the gulf coast area the total number of recording depths in each survey varied from 31 to 208 with downhole recordings taken every 10 or 20 ft over key stratigraphic zones in all five surveys a monitor geophone was placed near the well to check for source consistency our procedure for measuring attenuation involved constructing the spectral ratio between each downhole pulse and one particular reference pulse a value of cumulative attenuation ca was then extracted from each spectral ratio and plotted as a function of geophone depth this method allowed us to use many m

forecasting landscape effects of mississippi river diversions on elevation and accretion in louisiana deltaic wetlands under future environmental uncertainty scenarios abstract large sediment diversions are proposed and expected to build new wetlands to alleviate the extensive wetland loss 5000 km 2 affecting coastal louisiana during the last 78 years current assessment and prediction of the impacts of sediment diversions have focused on the capture and dispersal of both water and sediment on the adjacent river side and the immediate outfall marsh area however little is known about the effects of sediment diversions on existing wetland surface elevation and vertical accretion dynamics in the receiving basin at the landscape scale in this study we used a spatial wetland surface elevation model developed in support of louisiana s 2012 coastal master plan to examine such landscape scale effects of sediment diversions multiple sediment diversion projects were incorporated in the model to simulate surface elevation and vertical accretion for the next 50 years 2010 2060 under two environmental moderate and less optimistic scenarios specifically we examined landscape scale surface elevation and vertical accretion trends under diversions with different geographical locations diverted discharge rates and geomorphic characteristics of the receiving basin model results indicate that small diversions 3 s 1 tend to have limited effects of reducing landscape scale elevation loss 1500 m 3 s 1 are required to achieve landscape level benefits to promote surface elevation via vertical accretion to keep pace with rising sea level

overbank sedimentation from the historic a d 2011 flood along the lower mississippi river usa in this study we document sedimentary characteristics of overbank flood deposits associated with the epic a d 2011 flood along the lower mississippi river southern usa and directly compare the findings to sedimentation from a comparable flood event in 1973 with the general purpose of understanding how extreme floods contribute to floodplain depositional patterns and accretion rates of embanked fluvial systems the thicknesses of the 2011 flood deposits averaged 138 mm along natural levee crests 9 mm on meander scrolls and 3 mm in backswamps these thicknesses are considerably less than those documented for the 1973 flood sampled at the same locations we contend that less sedimentation in 2011 occurred because the flood was not supplied with much upstream sediment from the missouri river further the 2011 sediments are coarser than in 1973 indicating that the higher 2011 flood levels were associated with more energetic overbank flows that flushed fine grained sediments downstream within the narrow embanked floodplain corridor the largest recorded flood in north american history is only marginally preserved in the embanked floodplain stratigraphy of the alluvial valley of earth s third largest fluvial system