Amplitude Migration Research Articles

Steeply dipping structural target oriented viscoacoustic prismatic reverse time migration in frequency domain and its application

AbstractSteeply dipping structural imaging is a great challenge due to its poor illumination. Conventional migration methods are unable to produce an accurate image of complex steeply dipping structures. The prismatic wave can improve the illumination of steeply dipping structures and is often used to improve the imaging results of such structures. Traditional elastic wave theory assumes that seismic waves do not attenuate when propagating through subsurface media. However, during seismic wave propagation, the wave energy decays exponentially due to the absorption and attenuation of the ground layer. Subsurface attenuation leads to amplitude loss and phase distortion of seismic waves, resulting in blurring of migration amplitudes when this attenuation is not taken into account during imaging. To address this issue, a frequency‐domain Q‐compensated prismatic reverse time migration method is proposed, which derives Q‐compensated prismatic wavefield propagation operators. In the proposed frequency‐domain Q‐compensated prismatic reverse time migration, Q attenuation is fully compensated along three propagation paths and two propagation types of prismatic waves. The optimized four‐order mixed 25‐point difference format and LU decomposition method are used to solve the Q‐compensated prismatic wavefield propagation equations with high computational efficiency. Numerical and field data examples demonstrate that the proposed frequency‐domain Q‐compensated prismatic reverse time migration method can compensate for deep attenuation energy and improve the imaging resolution of steeply dipping structures.

Use of polarization properties of seismic waves to improve Fresnel Volume Migration of three-component subsurface seismic data

Long-range exploration in mines contributes significantly to work safety, raw material extraction and storage of materials in the subsurface. The distribution and number of source and receiver points for seismic surveys depend on the available gallery passages in the mine or underground buildings. Due to the resulting limited aperture, migration artifacts, spatial ambiguities, and geometrically induced amplitude focusing can occur in migration images. In addition, background noise is often very interfering, and reflectors smear in migration images. It has been shown that along drifts that have directional changes and thus corners or chambers, the signal-to-noise ratio is low due to scattered waves. We have incorporated directional and polarization filters into the workflow for pre-processing subsurface seismic data before starting Fresnel Volume Migration (FVM). This significantly reduces background noise and geometrically induced amplitude focusing as well as migration amplitude smearing effects. By applying directional and polarization filters beforehand, we improve the 3D seismic imaging. This allows us to highlight previously hidden or smeared reflectors when calculating migrations of reflected P and S waves and suppress amplitude focusing effects along drifts. In this paper, we present step-by-step this novel methodological approach that improves 3D seismic imaging for large-scale structure exploration in mining.

Deep learning framework for true amplitude imaging: Effect of conditioners and initial models

ABSTRACTWe propose a workflow to correct migration amplitudes by estimating the inverse Hessian operator weights using a neural network–based framework. We train the network such that it learns the transformation between the migration output and true amplitude reflectivity constrained by different conditioners. We analyse the network output with a velocity model and with source illumination as a conditioner. Compared to the velocity model, source illumination as a conditioner performs better because source illumination encodes the geometrical spreading information and accounts for non‐stationarity. We further use the output of the deep neural network as a starting model for accelerating the convergence of iterative least‐squares reverse time migration. Using a deep learning framework, the proposed method combines the model domain and data domain least‐squares migration approaches to recover images with interpretable amplitudes, attenuated migration artefacts, better signal‐to‐noise ratio and improved resolution. We compare the output of the proposed algorithm with conventional least‐squares output and show that the proposed workflow is more robust, especially in the areas with weak illumination.

A Climatological Study of the Mechanisms Controlling the Seasonal Meridional Migration of the Atlantic Warm Pool in an OGCM

The tropical Atlantic Warm Pool is one of the main drivers of the marine intertropical convergence zone and the associated coastal Northeast Brazilian and West-African monsoons. Its meridional displacement is driven by the solar cycle, modulated by the atmosphere and ocean interactions, whose nature and respective proportions are still poorly understood. This paper presents a climatological study of the upper ocean and lower atmosphere contributions to the warm pool seasonal migration, using an Ocean General Circulation Model (OGCM). First, we provide quantitative, albeit simple, pieces of evidence on how the large amplitude of migration in the west, compared to the east, is mainly due to the strong east–west contrast of the background meridional SST gradient intensities, which is maintained by equatorial and eastern tropical upwellings. Our main results consist first in identifying a diagnostic equation for the migration speed of the two meridional boundary isotherms of the Warm Pool, expressed in terms of the various mixed-layer heat fluxes. We then evidence and quantify how, in general, the migration is forced by air–sea fluxes, and damped by ocean circulation. However, remarkable controls by the ocean are identified in some specific regions. In particular, in the northwestern part of the Warm Pool, characterized by a large temperature inversion area, the boreal spring northward movement speed depends on the restitution of the solar heating by the thermocline. Additionally, over the southern part of the Warm Pool, our study quantifies the key role of the equatorial upwelling, which, depending on the longitude, significantly accelerates or slows down the summer poleward migration.

Active Carbon Flux of Mesozooplankton in South China Sea and Western Philippine Sea

The active carbon flux mediated by diel vertical migration (DVM) of zooplankton is an important component of the downward carbon flux in the ocean. However, active fluxes transported by zooplankton DVM are poorly known in the South China Sea (SCS) and the Western Philippine Sea (WPS). In this study, active carbon fluxes in the SCS and WPS were evaluated on the basis of the data of mesozooplankton community and DVM at two stations of these areas. The mesozooplankton community in the SCS was obviously different from that in the WPS, and higher species number and abundance in the SCS were observed, which may be related to the higher chlorophyll a (Chl a) concentration and the wide gradients of temperature and salinity in this sea. Moreover, shallow depth Chl a maximum and strong thermocline were detected in the SCS, causing lower migration amplitudes of mesozooplankton in the SCS than those in the WPS. However, the migrant biomass of mesozooplankton in the SCS was 98.40 mg C m–2, higher than that in the WPS at 25.12 mg C m–2. The mesozooplankton active carbon flux in the SCS (4.64 mg C m–2⋅d–1) was also higher than that in the WPS (1.80 mg C m–2⋅d–1). The mesozooplankton active fluxes were equivalent to 8.3 and 8.1% of the total flux (active flux plus passive flux) of the SCS and WPS, respectively, and they play an important role in the biological pump functioning in the two regions.

A nonlinear perturbation study of a shoreface nourishment on a multiply barred beach

The 1D (cross-shore) nonlinear nourishment-as-a-perturbation model of Chen and Dodd (2019) is extended to include wave streaming and depth-varying mean return flow. This is shown to lead to physically consistent values for the migration rate of small amplitude bed perturbations in the whole nearshore region. The model is also extended to arbitrary wave incidence angle, and an alternative energy dissipation description is included. The model is applied to a background profile that includes multiple nearshore bars, and which also allows comparison with observations of nourishment development and migration. The chosen profile is that of the Terschelling shoreface nourishment. Over the initial nourishment region the linearised bed evolution equation predicts onshore small-amplitude bed perturbation migration rates in the troughs (where waves shoal) and offshore rates on the bars (where waves break). Sensitivity studies using the fully nonlinear model reveal wave height to be the primary controlling parameter for the resultant perturbed sediment flux. Perturbed sediment fluxes are onshore up to a threshold wave height; beyond this offshore fluxes predominate. Wave period and angle of incidence, and tide level are all secondary controlling factors, variation in which yields no change in perturbed sediment flux sign (for fixed wave height). Recorded wave and tide signals are used to drive the model to simulate six months of the initial nourishment evolution. It is shown that in calmer conditions the small amplitude migration rate of bed disturbances is a highly accurate predictor of the fully nonlinear migration rate (as described by the centre of mass of the nourishment); in storm periods the two predictors diverge. The observed overall migration rate is well described by the model. The details of the profile evolution show a moderately good reproduction of those observed (Brier Skill Score =0.52).

Morphological characteristics and dynamic changes of seif dunes in the eastern margin of the Kumtagh Desert, China

The seif dune field over the gravel desert surface in the eastern margin of the Kumtagh Desert is a valuable experimental site for the observation of dune formation and dynamics. We used high-resolution remote sensing and station observation approaches, combined with wind and grain size data, to study the characteristics of the aeolian environment and the morphologies of and dynamic changes in seif dunes. We observed the ratio of the resultant drift potential (RDP) to the drift potential (DP), which was 0.37, associated with an obtuse bimodal wind regime. The drift potentials in the west-northwest (WNW) and east-northeast (ENE) directions were dominant, and the angle between the two primary DP directions was 135.00°. The dune orientations ranged from 168.75°–213.75°, which were parallel to the resultant drift direction (186.15°). The dune lengths ranged from 51.68 to 1932.11 m with a mean value of 344.91 m. The spacings of the dunes ranged from 32.34 to 319.77 m with a mean value of 93.39 m. The mean grain size of the sediments became finer, and the sorting became better from upwind tail to downwind tip, which indicated that the sediment of the seif dunes in the study region may be transported from northward to southward. The rate of increase in the length, the mean longitudinal migration rate of the dune tail, and the mean longitudinal extension rate of the dune tip (also called elongation rate) were 4.93, 4.63, and 9.55 m/a, respectively. The mean lateral migration vector of the seif dunes was approximately 0.11 m/a towards the west (–0.11 m/a), while the mean amplitude of lateral migration was 0.53 m/a, ignoring the direction of lateral migration. We found that the seif dune field formed first beside seasonal rivers, which can provide sediment, and then expanded downwind.

Diel vertical migration of dominant planktonic crustaceans in the south branch of Yangtze Estuary, China

Zooplanktons are important food organisms of fishes, and their spatial and temporal distribution pattern has an important influence on the growth and reproduction of fishes in local habitats. The Yangtze River Estuary of China is rich in fishery resources, but the knowledge on the distribution pattern of zooplankton in this area is still very limited. The diel vertical migration (DVM) patterns and seasonal variations of the crustacean community, and their relationship with environmental factors were investigated in the south branch of the Yangtze Estuary, from June 2017 to May 2018. The results showed that the dispersion patterns of dominant crustacean taxa in the Yangtze Estuary were mostly aggregated. Migration amplitudes of crustacean were greater in spring and summer than in autumn and winter, and different migratory patterns occurred in different seasons. Sinocalanus dorrii, Schmackeria inopinus, Mesmackeria leuckarti, Bosmina longirostris, and B. fatalis, unlike S. forbesi, showed a significant DVM. The DVM patterns of crustaceans was possibly influenced by light dentisy, temperature, and total dissolved solids in the water column, and the fish predation effect; whereas, the nutrient concentration and pH were not supposed to be key factors.

Representatives of the genus Eurytemora Giesbrecht, 1881 (Calanoida, Temoridae) in aquatic ecosystems of Belarus

Abstract The main biological features of two species of the genus Eurytemora have been studied: the relict E. lacustris (Poppe, 1887) and the alien E. velox (Lilljeborg, 1853), which can be found in natural environments on the territory of the Republic of Belarus. The relict species E. lacustris has been found only in two mesotrophic dimictic lakes belonging to the Baltic Sea basin: Volchin (maximum depth 32.9 m) and Vecheliye (35.9 m). E. velox has now colonized the main rivers of the south of Belarus and their accessory reservoirs belonging to the basins of the Baltic and Black seas. E. lacustris is characterized by an uneven distribution in the water column and location in deep water layers with temperatures below 13°C. Seasonal and daily vertical migrations have been established for E. lacustris. With age, the amplitude of migrations increases, reaching values of more than 10 m of depth for adults. E. velox is recorded in the coastal biotopes of watercourses and shallow water bodies. In the recent two years, the average density of E. lacustris in Lake Vecheliye has been established as about 2000 ind./m3, while their percentage in the zooplankton was 6%. In Lake Volchin, its density has been 2-5 times lower in the same period, and there this species’ percentage in the plankton did not exceed 2%. The density of E. velox varies from 20 to 12 000 ind./m3, their average density is 1631 ind./m3, and their percentual share in the zooplankton does not exceed 1%. For E. lacustris, the body length at all stages of development has been determined. It has been established that males have a greater body length in comparison with females, due to the relatively longer male abdomen, which is not typical for other species of freshwater planktonic copepods. In Belarus, E. lacustris is a bicyclic species, whereas E. velox is polycyclic.

Contrasting sand-mud transition migrations in estuarine and bay beaches and their potential morphological responses

Morphological variations in a sandy-muddy transitional beach (SMT-beach) are expected to coincide with sand-mud transition (SMT) boundary migrations. SMT migrations of SMT-beaches should also differ in river estuaries and semi-enclosed bays due to their contrasting dynamic sedimentary environments. However, the anticipated diversity of SMT migrations of estuarine and bay beaches and their potential morphodynamic responses are still poorly understood. Thus, four measurements of SMT spatial distributions and typical profiles of 6 estuarine beaches and 6 bay beaches located along the macrotidal coasts of the Taiwan Strait, East Asia are presented, and these sites were studied in April and August of 2018 and 2019, respectively. The results show that the SMT boundary of the estuarine beaches continuously migrates landward with increased elevations. Simultaneously, the SMT boundary of bay beaches synchronously remains relatively stable with minor fluctuations and relatively stable elevations. The SMT migration amplitudes present a significantly positive linear correlation with SMT elevation changes, which are strongly controlled by the sediment supply. Following the SMT migration processes, both estuarine and bay beaches display clear erosion during the summer monsoon period that is mainly associated with strong river discharge and monsoon rainfall, respectively. In contrast, all beaches present distinct accretion and recovery during the winter monsoon period that are mainly due to the decreased monsoon rainfall and river discharge and/or the strengthened onshore sediment transport. Despite limitations, the results of this study have clarified the diverse SMT migrations of estuarine and bay beaches and aid in a better understanding of their possible morphodynamic responses.

SUPPORTING IMMIGRATION FOR AN ENHANCED AGRICULTURAL SUSTAINABLE DEVELOPMENT WITHIN THE EUROPEAN UNION

Given the amplitude of migration in Europe and its potential to encourage or hinder the agricultural development, the general objective of this paper is to assess this possible potential on the long term, within a sustainable development frame of reference. Along these lines, we have built up several scenarios that focus on the agricultural results attained by ten EU Member States mainly targeted by immigrants, throughout the 2020-2025 period (sustainable development extrapolation). We have elaborated a set of indicators and within a panel in order to implement the spatial analysis and structural equation modelling (SEM), as methodological endeavour. The results obtained, verified by testing four hypotheses, show that a positive tendency in terms of increased government agri-innovation support is revealed on the long run, through the economic (labour) migration. Our findings outline that conclusive results of labour immigration could reverse generating unbalances in the agricultural sector. Thus, the need to develop accurate tailored policies is more than necessary by acknowledging the complex problems of the rural areas and those of international migration, as well as the major discrepancies among countries and stronger socio-economic interconnections.

Spatial variation in the trophic structure of micronekton assemblages from the eastern tropical North Atlantic in two regions of differing productivity and oxygen environments

The ecology of vertically migrating mesopelagic micronekton is affected by physical properties of their environment. Increased light attenuation in particle-rich productive waters, as well as low oxygen conditions decrease the migration amplitude. This likely has implications on the trophic organisation of micronekton communities, which are predominantly governed by niche partitioning in the vertical dimension. We investigated trophic structures of pelagic communities in the eastern tropical North Atlantic by comparing micronekton species assemblages from the low-oxygen region influenced by Mauritanian upwelling between 8° and 11° N (LO) and the less productive and more oxygenated equatorial area between 0 and 4°N (EQ). We analysed stable isotopes of carbon (δ13C) and nitrogen (δ15N) in body tissues of 35 species of mesopelagic fishes, four species of cephalopods, two species of cnidarians, and two species of decapods and used these values as a proxy for their trophic niche and correlated them with the traits feeding guild, migration pattern, mean depth of occurrence and body size. Our results demonstrate significant regional differences in the food web structure and vertical trophic interactions of the investigated micronekton assemblages. Diurnally migrating fishes that predominantly feed on copepods exhibited higher δ15N values in the LO (9.6‰) than in the EQ (8.9‰), reflecting changes in baseline values of pelagic tunicates. Contrary, all other Feeding – Migrator guilds show lower or similar δ15N values in the LO compared to the EQ, indicating reduced isotopic enrichment between trophic levels (TL) in the LO compared to the EQ. Further, a generally lower δ15N enrichment between TL3 – TL4 compared to TL2 – TL3 was observed (LO: TL2 – TL3: ~2.2‰, TL3 – TL4: ~1.2‰; EQ: TL2 – TL3: ~3.5‰, TL3 – TL4: ~2.2‰). Quantitative isotopic niche metrics suggest enhanced competition in trophic niche space, whereas relative isotopic niche positions indicate an increased importance of food from lower trophic levels (non-crustacean and/or gelatinous prey resources) for fishes from the LO compared to the EQ. The absence of a depth-related increase in δ15N values of partial- and nonmigrators of the LO is contrary to results from the EQ and previously published data. Low δ15N values in partially and nonmigrating micronektonivores of the LO in comparison with those of the EQ could be due to feeding on lower trophic prey components in the LO, as is indicated by an overlap in isotopic niche with that of partially and nonmigrating mixed crustacean feeders in the LO. Alternatively, driving mechanisms could be the consumption of prey from shallower waters, regional differences in δ15N enrichment, species-specific ecological differences or a combination of these processes. Each of these explanations is likely tightly correlated to a vertical biogeochemical structuring effect of low oxygen midwater layers fuelled by high nitrate inputs from the Mauritanian upwelling region. Our study provides crucial ecological insights for a better understanding of large-scale gradients in micronekton migration patterns.

Zooplankton diel vertical migration and downward C flux into the oxygen minimum zone in the highly productive upwelling region off northern Chile

Abstract. Diel vertical migration (DVM) can enhance the vertical flux of carbon (C), and so contributes to the functioning of the biological pump in the ocean. The magnitude and efficiency of this active transport of C may depend on the size and taxonomic structure of the migrant zooplankton. However, the impact that a variable community structure can have on zooplankton-mediated downward C flux has not been properly addressed. This taxonomic effect may become critically important in highly productive eastern boundary upwelling systems (EBUSs), where high levels of zooplankton biomass are found in the coastal zone and are composed by a diverse community with variable DVM behavior. In these systems, presence of a subsurface oxygen minimum zone (OMZ) can impose an additional constraint to vertical migration and so influence the downward C export. Here, we address these issues based on a vertically stratified zooplankton sampling at three stations off northern Chile (20–30∘ S) during November–December 2015. Automated analysis of zooplankton composition and taxa-structured biomass allowed us to estimate daily migrant biomass by taxa and their amplitude of migration. We found that a higher biomass aggregates above the oxycline, associated with more oxygenated surface waters and this was more evident upon a more intense OMZ. Some taxonomic groups, however, were found closely associated with the OMZ. Most taxa were able to perform DVM in the upwelling zone withstanding severe hypoxia. Also, strong migrants, such as eucalanid copepods and euphausiids, can exhibit a large migration amplitude (∼500 m), remaining either temporarily or permanently within the core of the OMZ and thus contributing to the release of C below the thermocline. Our estimates of DVM-mediated C flux suggested that a mean migrant biomass of ca. 958 mg C m−2 d−1 may contribute with about 71.3 mg C m−2 d−1 to the OMZ system through respiration, mortality and C excretion at depth, accounting for ca. 4 % of the net primary production, and so implies the existence of an efficient mechanism to incorporate freshly produced C into the OMZ. This downward C flux mediated by zooplankton is however spatially variable and mostly dependent on the taxonomic structure due to variable migration amplitude and DVM behavior.

Poststack impedance inversion considering the diffractive component of the wavefield

Seismic diffraction waveform energy contains important information about small-scale subsurface elements, and it is complementary to specular reflection information about subsurface properties. Diffraction imaging has been used for fault, pinchout, and fracture detection. Very little research, however, has been carried out taking diffraction into account in the impedance inversion. Usually, in the standard inversion scheme, the input is the migrated data and the assumption is taken that the diffraction energy is optimally focused. This assumption is true only for a perfectly known velocity model and accurate true amplitude migration algorithm, which are rare in practice. We have developed a new approach for impedance inversion, which takes into account diffractive components of the total wavefield and uses the unmigrated input data. Forward modeling, designed for impedance inversion, includes the classical specular reflection plus asymptotic diffraction modeling schemes. The output model is composed of impedance perturbation and the low-frequency model. The impedance perturbation is estimated using the Bayesian approach and remapped to the migrated domain by the kinematic ray tracing. Our method is demonstrated using synthetic and field data in comparison with the standard inversion. Results indicate that inversion with taking into account diffraction can improve the acoustic impedance prediction in the vicinity of local reflector discontinuities.

A stable and effective strategy for angle gather extraction from reverse time migration using traveltime gradient

ABSTRACTThe generation of angle-domain common imaging gathers (ADCIGs) from reverse time migration (RTM) is essential for velocity updates, migration quality control and amplitude variation with angle (AVA) inversion. However, computational stability and efficiency challenges remain with ADCIGs extracted from RTM. Here, we develop a stable and effective strategy to produce ADCIGs during RTM based on the traveltime gradient. The first arrival traveltime is calculated efficiently using the dynamic programming scheme, and the source-side propagating direction of the local plane-wave can be obtained using the traveltime field gradient. Compared with propagation directions calculated using the local slowness analysis method, the traveltime gradient can also provide stable and accurate source-side directions. Benefitting from the migrated images, we can collect normal direction information for the reflector from the stacked images effectively using the omnidirectional plane-wave destruction (OPWD) method. Construction of subsurface scattering angles is straightforward via combination of the source-side propagation and reflector normal directions. Subsequently, we apply an effective imaging condition based on the excitation criterion in the angle domain to generate ADCIGs. Thanks to conversion from a wavefield-based to a traveltime-based calculation, the proposed method provides more stable and reliable wave propagation directions in complicated areas compared with the Poynting vector (PV) method which is based on the full wavefield. Because traveltime can be calculated efficiently beforehand, the proposed method can be adapted economically to the conventional RTM imaging frame, which reveals significant implementation potential for practical seismic exploration. We demonstrate the stability and effectiveness of this approach for both synthetic and field data.

Light comfort zone in a mesopelagic fish emerges from adaptive behaviour along a latitudinal gradient

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 623:161-174 (2019) - DOI: https://doi.org/10.3354/meps13024 Light comfort zone in a mesopelagic fish emerges from adaptive behaviour along a latitudinal gradient Tom J. Langbehn1,*, Dag L. Aksnes1, Stein Kaartvedt2, Øyvind Fiksen1, Christian Jørgensen1 1Department of Biological Sciences, University of Bergen, Bergen 5020, Norway 2Department of Biosciences, University of Oslo, Oslo 0316, Norway *Corresponding author: tom.langbehn@uib.no ABSTRACT: Throughout the oceans, small fish and other micronekton migrate between daytime depths of several hundred meters and near-surface waters at night. These diel vertical migrations of mesopelagic organisms structure pelagic ecosystems through trophic interactions, and are a key element in the biological carbon pump. However, depth distributions and migration amplitude vary greatly. Suggested proximate causes of the migration such as oxygen, temperature, and light often correlate and therefore the causal underpinnings have remained unclear. Using mesopelagic fishes and the Norwegian Sea as a study system, we developed a dynamic state variable model that finds optimal migration patterns that we validate with acoustic observations along a latitudinal gradient. The model describes predation risk and bioenergetics, and maximizes expected energy surplus, a proxy for Darwinian fitness. The model allows us to disentangle the drivers of migration and make predictions about depth distribution and related fitness consequences along a latitudinal trajectory with strong gradients in environmental drivers and vertical distribution of scattering layers. We show that the model-predicted vertical migration of mesopelagic fishes matches that observed along this transect. For most situations, modelled mesopelagic fish behaviour can be well described by a light comfort zone near identical to that derived from observations. By selectively keeping light or temperature constant, the model reveals that temperature, in comparison with light, has little effect on depth distribution. We find that water clarity, which limits how deeply light can penetrate into the ocean, structures daytime depths, while surface light at night controlled the depth of nocturnal ascents. KEY WORDS: Antipredation window · Deep scattering layer · Photoperiodic constraint hypothesis · High-latitude · Myctophidae · Twilight zone · Norwegian Sea · Benthosema glaciale Full text in pdf format Supplementary material PreviousNextCite this article as: Langbehn TJ, Aksnes DL, Kaartvedt S, Fiksen Ø, Jørgensen C (2019) Light comfort zone in a mesopelagic fish emerges from adaptive behaviour along a latitudinal gradient. Mar Ecol Prog Ser 623:161-174. https://doi.org/10.3354/meps13024 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 623. Online publication date: July 30, 2019 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2019 Inter-Research.

Frequency-domain reflection-based full waveform inversion for short-offset seismic data

The full waveform inversion (FWI) is a high-resolution algorithm used to invert accurate subsurface velocity models. However, inverting accurate velocity models from field data using the conventional FWI without an accurate long-wavelength starting velocity model is difficult. This difficulty occurs because of the band-limited frequency of the field seismic data and the acquisition geometry of the field seismic exploration. The low-frequency components and long-offset seismic data are essential for the inversion of the long-wavelength velocity model using the FWI. However, low-frequency signals are difficult to record from the field seismic exploration, and the maximum offset of the streamer is usually not sufficient in length. Therefore, the conventional FWI cannot easily invert the long-wavelength velocity from field seismic data but can invert the migration-like short-wavelength velocity, and it is subject to the problem of severe local minima. To invert the long-wavelength velocity from reflection-dominant, short-offset field seismic data, reflection-based full waveform inversion (RFWI) which decomposes the FWI gradient into high- and low-wavenumber components, is suggested. However, the conventional RFWI also contains high-wavenumber components, which obstruct long-wavelength velocity updates in the deep part of the model. Moreover, true amplitude migration and preprocessing to extract reflection signals from observed data are necessary for the conventional RFWI. In this study, a new frequency-domain RFWI algorithm, which uses wavefield separation and a two-step approach, is proposed. The wavefield separation divides the wavefield into up/down-going waves to remove the high-wavenumber component of the gradient and the two-step approach alternately updates the short- and long-wavelength velocities to reduce the computational cost. The effectiveness of the proposed algorithm is verified using the reflection-dominant, short-offset Marmousi synthetic seismic data and Tonga field seismic data.

Artificial evolution of behavioral and life history strategies of high-latitude copepods in response to bottom-up and top-down selection pressures

Strong seasonality of resources and predation risk act as bottom-up and top-down selection pressures in high-latitudes, under which numerous behavioral and life history strategies evolve. Although such seasonal strategies are well-documented among high-latitude marine zooplankton, it is difficult to separate the role of bottom-up and top-down selection pressures in the evolution of seasonal strategies. Here, we present a model that allows partitioning of bottom-up (i.e. food availability and temperature) and top-down (i.e. visual predation risk) selection pressures to study how behavioral and life history strategies of high-latitude copepods evolve. In the model, timing, amplitude and ontogenetic trajectories of diel and seasonal vertical migrations (DVM and SVM) were defined as behavioral strategies. Body size, generation time and birth time comprised the life history strategy. Numerous combinations of behavioral and life history strategies were hardwired to copepods representing three model species. In a given model environment, strategies were evaluated for growth, survival and reproductive performances using a fitness estimate, which was heuristically maximized using a Genetic Algorithm. Model simulations were performed in three seasonality regimes representing various levels of visual predation risk from low- to high-Arctic. At lower visual predation risk, species-specific behavioral and life history strategies were largely influenced by food availability and temperature. As visual predation risk increased, the influence of bottom-up selection pressures diminished, and irrespective of the modelled latitude, all model species employed largely similar strategies to counter the predation risk. Modest increase of visual predation risk stimulated the diel vertical migration behavior. Further increase of visual predation risk was associated with decrease of body size, which created a significant impact on the observed behavioral and life history strategies through allometric processes. Our findings suggest that top-down selection pressures play a significant role in the evolution of behavioral and life history strategies of high-latitude copepods.

Fully developed and transient concentration profiles of particulate suspensions sheared in a cylindrical Couette cell

We experimentally investigate particle migration in a non-Brownian suspension sheared in a Taylor–Couette configuration and in the limit of vanishing Reynolds number. Highly resolved index-matching techniques are used to measure the local particulate volume fraction. In this wide-gap Taylor–Couette configuration, we find that for a large range of bulk volume fraction,$\unicode[STIX]{x1D719}_{b}\in [20\,\%{-}50\,\%]$, the fully developed concentration profiles are well predicted by the suspension balance model of Nott & Brady (J. Fluid Mech., vol. 275, 1994, pp. 157–199). Moreover, we provide systematic measurements of the migration strain scale and of the migration amplitude which highlight the limits of the suspension balance model predictions.

Investigating the cotton ring spun yarn structure using micro computerized tomography and digital image processing techniques

The micro-level structure of staple yarn and the fiber arrangement inside it has a decisive influence on its physical properties. This study aims to introduce a combined method based on micro computerized tomography (micro CT) and digital image processing techniques to probe the ring yarn structure in a non-invasive manner. Two micro CT systems at different CT settings were applied to achieve optimal quality CT images of cotton ring yarns. Three image processing algorithms were proposed to enhance and process the yarn CT images in order to extract yarn structural information. The proposed method was also applied on two yarn specimens, which varied significantly in terms of their tensile strength, to study differences in their underlying structures. The results showed that the longitudinal arrangement of fibers in terms of their migratory behavior had a decisive influence on the tensile properties of the yarn. The stronger yarn showed a higher value of the amplitude and intensity of fiber migration compared to the weaker yarn, suggesting that the protocol established in this study can effectively reveal fiber arrangements within a staple yarn structure in a non-invasive manner.