Negative Reflection Coefficients Research Articles

Monitoring the seasonal changes of an englacial conduit network using repeated ground-penetrating radar measurements

Abstract. Englacial conduits act as water pathways to feed surface meltwater into the subglacial drainage system. A change of meltwater into the subglacial drainage system can alter the glacier's dynamics. Between 2012 and 2019, repeated 25 MHz ground-penetrating radar (GPR) surveys were carried out over an active englacial conduit network within the ablation area of the temperate Rhonegletscher, Switzerland. In 2012, 2016, and 2017 GPR measurements were carried out only once a year, and an englacial conduit was detected in 2017. In 2018 and 2019 the repetition survey rate was increased to monitor seasonal variations in the detected englacial conduit. The resulting GPR data were processed using an impedance inversion workflow to compute GPR reflection coefficients and layer impedances, which are indicative of the conduit's infill material. The spatial and temporal evolution of the reflection coefficients also provided insights into the morphology of the Rhonegletscher's englacial conduit network. During the summer melt seasons, we observed an active, water-filled, sediment-transporting englacial conduit network that yielded large negative GPR reflection coefficients (<-0.2). The GPR surveys conducted during the summer provided evidence that the englacial conduit was 15–20 m±6 m wide, ∼0.4m±0.35m thick, ∼250m±6m long with a shallow inclination (2∘), and having a sinusoidal shape from the GPR data. We speculate that extensional hydraulic fracturing is responsible for the formation of the conduit as a result of the conduit network geometry observed and from borehole observations. Synthetic GPR waveform modelling using a thin water-filled conduit showed that a conduit thickness larger than 0.4 m (0.3× minimum wavelength) thick can be correctly identified using 25 MHz GPR data. During the winter periods, the englacial conduit no longer transports water and either physically closed or became very thin (<0.1 m), thereby producing small negative reflection coefficients that are caused by either sediments lying within the closed conduit or water within the very thin conduit. Furthermore, the englacial conduit reactivated during the following melt season at an identical position as in the previous year.

Pulse wave propagation along human aorta: a model study

The problem of the pulse wave propagation and reflection along human aorta and the diagnostic importance of the pressure and flow signals are studied. The geometrical model is based on the detailed postmortem measurements on the systemic arterial trees. The aortic model consists of 36 aortic segments and 57 side branches of the aorta including the larger and medium vessels. It was shown most of the branches have zero wave reflection coefficients but the large branches like celiac, renal and iliac arteries could produce noticeable wave reflections. The smaller branches possess negative wave reflection coefficient and, thus, contribute to the blood suction effect and lower aortic resistance to the blood flow. Mathematical model is based on the axisymmetric incompressible Navier-Stokes equations for blood and the momentum equations for incompressible viscoelastic arterial wall. The Windkessel and structured tree outflow boundary conditions at the outlets of the branches have been used. The solution has been found as superposition of the forward and backward running waves. The blood flow curves measured in vivo by Doppler ultrasound in the larger systemic arteries of healthy volunteers have been used for identification of the model parameters. It is shown, the individual geometry plays an essential role in the location of the positive and negative wave reflection sites along the aorta and, thus, in the pressure and flow patterns and blood distribution into the branches. The influence of occlusion of the iliac arteries, low/high wall rigidity, and total length of aorta are studied on different individual geometries. The model can be used for determination of the individual parameters for patient-specific cardiovascular models and further modeling of the outcomes of the surgical and therapeutic procedures.

Investigation of Pore Pressure on Seismic Amplitude Response: A Well-Based Modeling Case Study of Sojuko Field, Niger Delta

The Sojuko field was discovered in 2001 in the eastern shallow offshore area of the Niger Delta, Nigeria. Three (3) exploration wells have so far been drilled in the field, two (2) of which are reasonably vertical and the third highly deviated. Three (3) key reservoirs which are laterally continuous across the wells have been identified with proven oil and gas reserves. Pore pressure data from repeat formation test (RFT) measurements acquired in the deviated well show that the wells are entirely hydrostatic to true depth (TD). This research focuses on investigating how seismic amplitudes change with offset/angle of incidence in relation to varying pore pressure regimes at the shale-hydrocarbon sand and shale-brine sand interfaces using well data. The aim is to aid quantitative interpretation in an on-going field-wide exploration drive to de-risk hydrocarbon exploration in the deeper plays in the area which are below TD, and are expected to be overpressured. The study is hinged on end-member shale elastic parameter substitution in which the shales are subjected to varying overpressure regimes while keeping the reservoirs (sands) at in situ (hydrostatic) condition. The end-member shale property substitution simulated shale compaction dis-equilibrium as the main overpressure generation mechanism in this study. The results show that top gas sands, top oil sands and top brine sands would be visible on seismic in the deeper plays where pore pressures are expected to be very high, but with distinctive seismic amplitude with offset/angle behavior. The top gas sands are visible as blue loop with small positive reflection coefficients at the near offsets/angles, but with polarity reversal to red loop with negative reflection coefficients which become more and more negative at the far angles at hard overpressure regimes. Top oil sands are recognized as blue loop with large positive reflection coefficients at the near angles; the coefficients becoming less and less positive at the far angles/offsets. The top oil sands may not be detected on seismic at the far angles/offsets unless at very hard overpressures. Brine sands have similar seismic response as oil sands at hard overpressures, but can be distinguished from oil sands based on their much higher amplitudes over the entire offset/angle range. The study is also aimed at removing uncertainty in seismic-based pore pressure quantification at the deeper targets where there is absence of well data for calibrating pore pressure effects at varying conditions.

Investigating deep geological reservoirs using seismic reflection and well logs, Tawila oil field, Yemen: Implications for structural setting and reservoir properties

Seismic reflection data were used to evaluate the deep geological structures in Tawila oil field, Masila rift basin, Yemen. A combined set of 2D seismic profiles and geophysical well logs were used to produce structural mapping at the top of the clastic Qishn reservoir and the several overlying and underlying horizons. We have investigated the distribution of reservoir fluids in relation to the identified structures. The seismic horizons with their intersecting faults have been determined to be very important in the hydrocarbon distribution. Two key fault systems have been identified to control the hydrocarbon entrapment in the study area: major, deep, E–W normal faulting that affected the northern area, which caused extensive structural deformation, and a much shallower ENE–WSW fault system to south. The shale type and distribution and the basic reservoir properties, were enhanced through analysing the logging data using the standard clastic models. The reservoir is composed of three clastic compartments (S1, S2 and S3) which are separated by thin shale beds. No hydrocarbons were found in the S1 top unit which had considerable shale volume of a dispersed shale type. Considerable to good hydrocarbon content up to 57% and 61%, associated with a structural uplift, was identified in front of the S2 and S3 units, respectively. A dispersed/laminated shale model was assigned for these two units with a laminated style priority. The hydrocarbon entrapment in the study area was caused by the sealing characteristics of the overlying layers as well as the juxtaposition of the reservoir against other seal rocks. The distribution maps show an increase in hydrocarbon saturation toward the west, north and northeast which coincided with areas of low seismic-derived properties of velocity, acoustic impedance and negative reflection coefficient. An elongated structural horst of good hydrocarbon potentiality was identified in the upthrow of the major fault. This prospective structural high is a key to explore hydrocarbons in nearby areas and other basins of similar geologic characteristics.

Computer modeling in mechanics of circulation

Some cardiovascular pathology connected with variations in geometry, wall properties and microcirculatory disorders can be studied by computer simulations. The mathematical model that allows to calculate the parameters of blood circulation – velocities of blood and pressure, displacements of the artery walls - for a complex vascular tree in real time is proposed.The geometrical model is based on the detailed postmortem measurements on the systemic arterial trees (more than 1000 arterial segments). The aortic model consists of 32 aortic segments and 59 side branches of the aorta including the larger and medium vessels. Mathematical model of blood flow in the system is described by Womersley model of the pulsatile viscous flow in the viscoelastic tube using the pressure and volumetric rate continuity conditions at the bifurcations of arteries. The Windkessel and structured tree outflow boundary conditions at the outlets of the branches have been used. The solution has been found as superposition of the forward and backward running waves. Based on the model, blood circulation parameters were calculated in the aortic model (91 tubes). The calculation results correspond to in vivo measurements. It was shown most of the branches have zero wave reflection coefficients but the large branches like celiac, renal and iliac arteries could produce noticeable wave reflections. The smaller branches possess negative wave reflection coefficient and, thus, contribute to the blood suction effect and lower aortic resistance to the blood flow. It is shown, the individual geometry plays an essential role in the location of the positive and negative wave reflection sites along the aorta and, thus, in the pressure and flow patterns and blood distribution into the branches. The influence of occlusion of the iliac arteries, low/high wall rigidity, and total length of aorta are studied on different individual geometries. The model can be used for determination of the individual parameters for patient-specific cardiovascular models and further modeling of the outcomes of the surgical and therapeutic procedures.

Elastic parameters from constrained AVO inversion: Application to a BSR in the Mahanadi offshore, India

We have estimated P-wave velocities (VP) and thicknesses from traveltimes data, and S-wave velocities (VS) and densities (ρ) from amplitudes versus offset (AVO) data by constraining the VP for a set of seismic reflection data including a bottom simulating reflector or BSR (marker for gas hydrates) in the Mahanadi offshore. The elastic parameters (VP, VS and ρ) of the whole-layered model have been derived simultaneously using the genetic algorithm global optimization technique that converges into a global minimum. The result shows a low velocity (1506 m/s) free-gas zone below the BSR that lies at 241 m below the sea floor. The presence of underlying free-gas is further corroborated by the observation of increasing negative reflection coefficients with angles. The VP and VS for gas hydrates bearing sediments above the BSR are estimated to be 1655 and 468 m/s respectively, and match reasonably with the sonic log at site NGHP-01-08A passing through the seismic line. Gas-hydrates and free-gas have been qualitatively estimated as 7.0% and 0.9% using three-phase weighted equation of Lee and Gassmann equation respectively. The combined usage of VP and VS corresponding to a Poisson ratio of 0.4565 provides a better estimation of gas hydrates, which shows comparable result that has been estimated using the Simandoux equation to the resistivity log data at site NGHP-01-08A.

Possible link between weak bottom simulating reflections and gas hydrate systems in fractures and macropores of fine-grained sediments: Results from the Hikurangi Margin, New Zealand

Small amounts of free gas in interstitial sediment pores are known to significantly lower compressional (P-) wave velocity (Vp). This effect, combined with moderately elevated Vp from the presence of gas hydrates, is usually thought to be the cause for the often observed strong negative reflection coefficients of bottom simulating reflections (BSRs) at the base of gas hydrate stability (BGHS). At several locations however, weak BSRs have been observed, which are difficult to reconcile with a presence of gas in sediment pores. We here present a rock physics model for weak BSRs on the Hikurangi Margin east of New Zealand. Thin sections of a fine-grained mudstone sample from a submarine outcrop in the vicinity of a weak BSR show macroscopic porosity in the form of fractures and intrafossil macropores. We apply the Kuster-Toksöz theory to predict seismic velocities for a rock with water-saturated interstitial micropores and gas or hydrates in macroscopic pore space simulating fractures or compliant macropores. We match field observations of a weak BSR with a reflection coefficient of −0.016 with two end-member models; (1) rocks with gas hydrate-filled voids with a concentration of <4% of bulk sediment overlying water-filled voids, or (2) fully gas-saturated voids at a concentration of <2% beneath water-filled voids. A natural system is likely to consist of a combination of these end-members and of macroporosity filled with a mixture of water and gas or hydrate. Our results suggest weak BSRs may be caused by gas hydrate systems in fractures and macropores of fine-grained sediments with fully water-saturated interstitial pore space. Gas may be supplied into the macroscopic pore space by diffusion-driven short-range migration of methane generated within the gas hydrate stability field or, our favoured model based on additional geologic considerations, long-range advective migration from deeper sources along fractures.

Density profiles in open superdiffusive systems

We numerically solve a discretized model of Lévy random walks on a finite one-dimensional domain with a reflection coefficient r and in the presence of sources. At the domain boundaries, the steady-state density profile is nonanalytic. The meniscus exponent μ, introduced to characterize this singular behavior, uniquely identifies the whole profile. Numerical data suggest that μ = α/2 + r(α/2 - 1), where α is the Lévy exponent of the step-length distribution. As an application, we show that this model reproduces the temperature profiles obtained for a chain of oscillators displaying anomalous heat conduction. Remarkably, the case of free-boundary conditions in the chain corresponds to a Lévy walk with negative reflection coefficient.

Oxidative gating of water channels (aquaporins) in Chara by hydroxyl radicals

ABSTRACTHydroxyl radicals (*OH) as produced in the Fenton reaction (Fe2+ + H2O2 = Fe3+ + OH– + *OH) have been used to reversibly inhibit aquaporins in the plasma membrane of internodes of Chara corallina. Compared to conventional agents such as HgCl2, *OH proved to be more effective in blocking water channels and was less toxic to the cell. When internodes were treated for 30 min, cell hydraulic conductivity (Lp) decreased by 90% or even more. This effect was reversed within a few minutes after removing the radicals from the medium. In contrast to HgCl2, radical treatment reduced membrane permeability of small lipophilic organic solutes (ethanol, acetone, 1‐propanol, and 2‐propanol) by only 24 to 52%, indicating some continued limited movement of these solutes across aquaporins. The biggest effect of *OH treatment on solute permeability was found for isotopic water (HDO), which largely used water channels to cross the membrane. Inhibition of aquaporins reduced the diffusional water permeability (Pd) by about 70%. For the organic test solutes, which mainly use the bilayer to cross the membrane, channel closure caused anomalous (negative) osmosis; that is, cells had negative reflection coefficients (σs) and were transiently swelling in a hypertonic medium. From the ratio of bulk (Lp or osmotic permeability coefficient, Pf) to diffusional (Pd) permeability of water, the number (N) of water molecules that align in water channels was estimated to be N = Pf/Pd = 46 (on average). Radical treatment decreased N from 46 to 11, a value still larger than unity, which would be expected for a membrane lacking pores. The gating of aquaporins by *OH radicals is discussed in terms of a direct action of the radicals when passing the pores or by an indirect action via the bilayer. The rapid recovery of inhibited channels may indicate an easy access of cytoplasmic antioxidants to closed water channels. As hydrogen peroxide is a major signalling substance during different biotic and abiotic stresses, the reversible closure of water channels by *OH (as produced from H2O2 in the apoplast in the presence of transition metals such as Fe2+ or Cu+) may be downstream of the H2O2 signalling. This may provide appropriate adjustments in water relations (hydraulic conductivity), and a common response to different kinds of stresses.

Bandwidth Enhancement of Traveling-Wave Photodetector by Impedance Mismatch

In this paper, we report bandwidth enhancement phenomena of traveling-wave photodetectors (TWPDs) by output impedance mismatch with an open input facet. The impulse response obtained by the extended finite difference time domain (FDTD) method shows that internal reflection of microwaves with a negative reflection coefficient at the load enlarges the bandwidth of TWPDs.

AVO analysis by offset-limited prestack migrations of crustal seismic data

The physical causes of reflections from the crystalline crust are still a matter of debate. The polarity of the wavelet and the amplitude-versus-offset (AVO) dependence of seismic reflection data can help to reveal the nature of a reflector. In contrast to seismic data from sedimentary environments, however, it is very difficult to get polarity and AVO information from crystalline basement data. Reflections in prestack data are typically weak, incoherent and often obscured by numerous diffractions. There is, however, a better chance of obtaining reliable AVO information from prestack-migrated data after true-amplitude processing, as the migration process unravels the jumble of reflections and diffractions and improves the signal/noise ratio. Important steps in this processing sequence are careful editing followed by surface-consistent amplitude corrections for source and receiver coupling to the ground. Migration-velocity models can be obtained from refraction seismics and/or from migration-velocity analyses. The migration itself is of Kirchhoff-type and applied to COF gathers (COF = Common OFfset) to get one migrated section for each COF. These COF-migrated images can then be used to get AVO functions for specific reflections. Examples from the DEKORP near-vertical reflection line KTB8502 show some strong reflections, from beneath a granite body, at about 9 to 11 km depth. Both the positive and negative reflection coefficients of the reflectors are consistent with an interval velocity model obtained from migration-velocity analyses. AVO functions from these reflections include offsets from zero to 12 km and reflection angles from zero to about 30°. Absolute reflection coefficients are surprisingly small with maximum values of 0.015. Two-layer modelling has been done to match the AVO dependence of these reflections.

Reconciling physical properties with surface seismic data from a layered mafic intrusion

Compositional layering of intrusive rocks is often cited as a source of seismic reflectivity in the crystalline crust. Direct evidence for this is based primarily on synthetic seismograms calculated from laboratory measurements of rock velocity and density, or estimates of aggregate rock properties from measurements of mineral velocities and densities. Little is known about how in situ effects such as fracturing, chemical alteration, or anisotropy, may change the reflectivity character of the surface seismic data. An ideal dataset for examining the hazards of using physical properties data for estimating crustal reflectivity occurs at the Nellie intrusion, a middle Proterozoic-age layered mafic intrusion that lies beneath the Permian Basin of west Texas. Due to a well that penetrated 4.5 km of the intrusion, we have a dataset that spans the scale from compositional data on the intrusion derived from well cuttings, to log data, to surface reflection data. Sonic and density log data from the well measure velocities and densities 10 to 15% below the values calculated from modal mineralogy. We attribute this difference primarily to pervasive post-emplacement alteration and fracturing of the rock mass in situ. Thus, this unique dataset provides a quantitative estimate of the magnitude of difference between observed and theoretical velocities that can be expected for crystalline rocks in the upper crust and its cause. Despite the difference, analysis of synthetic seismograms shows that primary compositional variation of the intrusion is still responsible for sub-horizontal layered reflectivity in both the log-based and petrology-based synthetic seismograms. Boundaries of intrusive cycles are characterized by large negative reflection coefficients due to the juxtaposition of mafic-rich basal layers against plagioclase-rich layer tops. In detail, only the log-based synthetic provides a good match to the surface seismic reflection data. This suggests that physical properties from modal mineralogy are useful in obtaining a generalized model for reflectivity, but that differences in bulk physical properties of the rock mass preclude the possiblity of obtaining a one-to-one match with surface seismic data.

Irreversible thermodynamics of transport across charged membranes. Part VI. Frictional interactions and coupling effects in transport of acid through anion exchange membranes

Having performed experiments described in [1–3] the friction coefficients, f ik, coupling coefficients, q ik, and diffusion indexes for ions and water, D i , have been computed in order to discuss the interactions on transport of acid (HCl) through anion exchange membranes. Two membranes were examined — diffusion dialysis Neosepta AFN-7 and standard grade Neosepta AM-1 (Tokuyama Corp.). The frictional interactions and coupling effects in AFN-7 membrane, the dual mode sorption, high diffusion coefficient, D HCl , and negative reflection coefficient, σ, can be explained postulating the dual mechanism of transport: a simple diffusion of acid dissolved in internal water combined with a kind of facilitated transport of the “absorbed” acid.

Diffusion dialysis Transport phenomena by irreversible thermodynamics

The transport phenomena in the permeation of acid and salt across anion-exchange membranes used to separate electrolytes by diffusion dialysis (DD) were studied. The experiments and computations were performed as imposed by irreversible thermodynamics. The phenomena are discussed in terms of the phenomenological ( L p, L D and L pD), reflection (σ) and permeability (υ) coefficients. The negative reflection coefficient indicated facilitated permeation of acid across the anion-exchange membranes. The negative osmotic flow was one of the phenomena increasing the permeation rate of acid. The data suggested that efficient separation by DD resulted from two effects: facilitated transport of one component (an acid) and restrictions in the sorption and permeation of the other one (a salt) by Donnan exclusion.

Modeling of nonlinear active regions in TLM (distributed circuits)

A transmission line matrix (TLM) method for modeling distributed circuits containing nonlinear active elements and structures is demonstrated. The scattering properties of the nonlinear element are expressed in terms of differential equations that are solved by stepwise integration with the time step of the TLM algorithm. A cavity oscillator with an active diode with a finite cutoff frequency has been modeled. Spurious oscillations of the TLM mesh that regularly occur when including negative conductivity or pulse reflection coefficients larger than unity were effectively suppressed. >

Equivalent circuit model for positive/negative reflection—type SAW reflectors

AbstractAn analysis method is proposed herein which uses an equivalent circuit model as a means to evaluate positive/negative reflection‐type (PNR) SAW reflectors. The PNR reflector consists of reflecting elements which have 180° phase difference (positive and negative reflection coefficients). Each element is spaced with a period of one‐quarter wavelength. Compared with the conventional reflectors, these new reflectors have larger reflection‐efficiencies. In this paper, an equivalent circuit model for the PNR reflectors is presented. Circuit parameters can be determined theoretically by applying the finite‐element method to an infinite array. Examples are computed for two kinds of PNR reflectors on 128° Y‐X LiNbO3 or ST‐X quartz substrate. For each of them, we determined the circuit parameters and analyzed the reflection characteristics. We obtained good agreement between theoretical and experimental results.

High-throughput GaAs PIN electrooptic modulator with a 3-dB bandwidth of 96 GHz at 13 μm

GaAs PIN traveling-wave modulators with terminating resistors of various resistances operated at 1.3 microm have been fabricated from material grown by organometallic vapor phase epitaxy (OMVPE) on n(+) (100) GaAs substrates. It was found that modulators with positive reflection coefficients at the load exhibted initial rolloff in the frequency response. However, modulators with negative reflection coefficients at the load exhibited some peaking behavior at low frequencies at the expense of lower detected microwave powers. Based on this observation, a compromised PIN traveling-wave modulator was designed and measured to have a 3-dB bandwidth of 9.6 GHz.

New Types of SAW Reflectors and Resonators Consisting of Reflecting Elements with Positive and Negative Reflection Coefficients

New Types of SAW Reflectors and Resonators Consisting of Reflecting Elements with Positive and Negative Reflection Coefficients

New Types of SAW Reflector and Resonator Consisting of Reflecting Elements with Alternative (Positive and Negative) Reflection Coefficients

New types of SAW grating reflector and resonator configurations are proposed which consist of reflecting elements with the opposite sign of the reflection coefficients, and which have the advantages of large reflectivity, wide stopband and small mode conversion into radiative bulk waves. The principles of operation and the possible configurations of the reflector are described. It is confirmed by experiments on metal strip gratings that the proposed configurations provide very efficient reflection characteristics and a small size resonator with very small capacitance ratio.

Mass transport through charge-micromosaic membranes

Electrolyte and water fluxes were measured through a charge-micromosaic membrane of which the length of a repeating cation- and anion-exchange unit was 50 to 100 nm. The overall transport number of K + ions in the membrane was about 0.5 at KCl solutions of 10 ±6 to 10 −3 mol-cm −3. The stagnant layers on both sides of the membrane played an important role in electrolyte transport. Negative osmosis or negative reflection coefficients were observed through the membrane for KCl, NaCl, and LiCl solutions of 10 −6 to 3 x 10 −4 mol-cm −3.