Surface Velocity Measurements Research Articles

Dynamic instability of marine-terminating glacier basins of Academy of Sciences Ice Cap, Russian High Arctic

AbstractIce sheets and smaller ice caps appear to behave in dynamically similar ways; both contain slow-moving ice that is probably frozen to the bed, interspersed with fast-flowing ice streams and outlet glaciers that terminate into the ocean. Academy of Sciences Ice Cap (Akademii Nauk ice cap; 5570 km2), Severnaya Zemlya, Russian High Arctic, provides a clear example of this varied flow regime. We have combined satellite measurements of elevation change and surface velocity to show that variable ice-stream dynamics dominate the mass balance of the ice cap. Since 1988, the ice cap has lost 58±16 Gt of ice, corresponding to ~3% of its mass or 0.16mm of sea-level rise. The climatic mass balance is estimated to be close to zero, and terminus positions have remained stable to within a few kilometers, implying that almost all mass loss has occurred through iceberg calving. The ice-cap calving rate increased from ~0.6 Gt a–1 in 1995 to ~3.0 Gt a–1 in 2000–02, but has recently decreased to ~1.4 Gt a–1 due to a likely slowdown of the largest ice stream. Such highly variable calving rates have not been reported before from High Arctic ice caps, suggesting that these ice masses may be less stable than previously thought.

0101 Tracking the flow state in the lee of a Backward-Facing Step in asimulated open channel using a POD-based Unscented Kalman Filter

Existing systems using field PIV or high frequency radar to measure surface velocity over a certain area shows apromising possibility to construct real-time monitoring systems of depth-scale flows based on surface velocity. In thisscenario, we propose Proper Orthogonal Decomposition-based Unscented Kalman Filtering, namely POD-UKF, asthe first step toward constructing such systems. Radial Basis Function is used as state-space models of POD-UKF.Measurement models are POD-based regression models using Kernel Ridge Regression. Data used in this work is1600 velocity snapshots of simulated flow over a Backward-Facing Step in an open channel. Estimation results areshown for tracking flow information at some subsurface regions based on "virtual sensors" of surface velocity.

Flow dynamics of an accumulation basin: a case study of upper Kahiltna Glacier, Mount McKinley, Alaska

AbstractWe interpreted flow dynamics of the Kahiltna Pass Basin accumulation zone on Mount McKinley, Alaska, USA, using 40,100 and 900 MHz ground-penetrating radar profiles and GPS surface velocity measurements. We found dipping, englacial surface-conformable strata that experienced vertical thickening as the glacier flowed westward from a steep, higher-velocity (60 m a-1) region into flat terrain associated with a 90° bend in the glacier and lower velocities (15 m a-1) to the south. Stratigraphy near the western side of the basin was surface-conformable to ˜170m depth and thinned as flow diverged southward, down-glacier. We found complex strata beneath the conformable stratigraphy and interpret these features as buried crevasses, avalanche debris and deformed ice caused by up-glacier events. We also suggest that basin dimensions, bed topography and the sharp bend each cause flow extension and compression, significantly contributing to conformable and complex strata thickness variations. Our findings show that surface-conformable stratigraphy continuous with depth and consistent strata thicknesses cannot be assumed in accumulation basins, because local and up- glacier terrain and flow dynamics can cause structural complexities to occur under and within surface- conformable layers.

Investigation of Heat Transfer Performance of Nanofluids Flow in a Microchannel

Abstract Miniaturization and increase in performance of electronic devices, resulted in an increase of energy density loads generated. In recent times, micro-scale cooling devices such as microchannel heat sinks have evolved as a plausible solution to the above heat transfer challenge. Nanofluids emerged as a good candidate in improving the cooling performance of micro cooling systems. Nanofluids are colloidal suspensions consisting of nano-sized particles (less than 100nm) dispersed in a base fluid. Nanofluids are considered ideal for micro-channel devices because they not only improve the heat transfer capabilities, due to increased thermal conductivity, but also minimize the clogging problem. Present work experimentally investigates the heat transfer performance of nanofluids through a microchannel with constant temperature wall boundary condition. Laminar flow of SiO2-water nanofluids inside a rectangular microchannel flow assembly is examined. The effect of flow rate on thermal performance of nanofluid is analyzed along with variation in thermo-physical properties. Interestingly, experimental results shows heat transfer enhancement at lower flow rates and heat transfer degradation at higher flow rates. Theoretical reasoning for this kind of opposing trend is given based on flow conditions and thermo-physical properties of nanofluids. Moreover, Novel near surface velocity measurement of Nanofluids compared with that of regular fluid at the same condition.

Synchronized vibrations measurements at multiple locations using a single continuously scanning laser doppler vibrometer. Applications to non-contact sensing of human body vibrations

Laser Doppler vibrometry (LDV) is a non-contact method to measure surface velocity. Typical LDV configurations uses one fixed laser (He-Ne) beam at a specific point and orientation on a surface under test. A continuously scanning laser Doppler vibrometry (CSLDV) technique using a single laser beam performing quickly a long scan was developed to measure surface velocity at each laser position during its scan. This fast CSLDV can then replace several fixed LDVs. This technique is especially advantageous for sensing human body natural vibrations (typically below 100 Hz) at multiple locations (e.g., along small muscles) as it does not require traditional skin-mounted sensors (e.g., accelerometers array), which eliminates mass artifacts and the set-up time to attach those sensors. Experimental measurements were conducted using a CSLDV with a 200 Hz linear scan rate, over scan lengths up to 5 cm, to measure low-frequency vibrations (f<100 Hz) on gel samples which mimic human soft tissues. Validations of the CSLDV measurements were done using an array of several fixed LDVs distributed along the same scan line. The effects of speckle noise on CSLDV measurements will be quantified. Applications of this CSLDV technique for active and passive elastography measurements will be presented.

Discharge estimation combining flow routing and occasional measurements of velocity

Abstract. A new procedure is proposed for estimating river discharge hydrographs during flood events, using only water level data at a single gauged site, as well as 1-D shallow water modelling and occasional maximum surface flow velocity measurements. One-dimensional diffusive hydraulic model is used for routing the recorded stage hydrograph in the channel reach considering zero-diffusion downstream boundary condition. Based on synthetic tests concerning a broad prismatic channel, the "suitable" reach length is chosen in order to minimize the effect of the approximated downstream boundary condition on the estimation of the upstream discharge hydrograph. The Manning's roughness coefficient is calibrated by using occasional instantaneous surface velocity measurements during the rising limb of flood that are used to estimate instantaneous discharges by adopting, in the flow area, a two-dimensional velocity distribution model. Several historical events recorded in three gauged sites along the upper Tiber River, wherein reliable rating curves are available, have been used for the validation. The outcomes of the analysis can be summarized as follows: (1) the criterion adopted for selecting the "suitable" channel length based on synthetic test studies has proved to be reliable for field applications to three gauged sites. Indeed, for each event a downstream reach length not more than 500 m is found to be sufficient, for a good performances of the hydraulic model, thereby enabling the drastic reduction of river cross-sections data; (2) the procedure for Manning's roughness coefficient calibration allowed for high performance in discharge estimation just considering the observed water levels and occasional measurements of maximum surface flow velocity during the rising limb of flood. Indeed, errors in the peak discharge magnitude, for the optimal calibration, were found not exceeding 5% for all events observed in the three investigated gauged sections, while the Nash-Sutcliffe efficiency was, on average, greater than 0.95. Therefore, the proposed procedure well lend itself to be applied for: (1) the extrapolation of rating curve over the field of velocity measurements (2) discharge estimations in different cross sections during the same flood event using occasional surface flow velocity measures carried out, for instance, by hand-held radar sensors.

Effects of grain size and boundary structure on the dynamic tensile response of copper

Plate impact experiments have been carried out to examine the influence of grain boundary characteristics on the dynamic tensile response of Cu samples with grain sizes of 30, 60, 100, and 200 μm. The peak compressive stress is ∼1.50 GPa for all experiments, low enough to cause an early stage of incipient spall damage that is correlated to the surrounding microstructure in metallographic analysis. The experimental configuration used in this work permits real-time measurements of the sample free surface velocity histories, soft-recovery, and postimpact examination of the damaged microstructure. The resulting tensile damage in the recovered samples is examined using optical and electron microscopy along with micro x-ray tomography. The free surface velocity measurements are used to calculate spall strength values and show no significant effect of the grain size. However, differences are observed in the free surface velocity behavior after the pull-back minima, when reacceleration occurs. The magnitude of the spall peak and its acceleration rate are dependent upon the grain size. The quantitative, postimpact, metallographic analyses of recovered samples show that for the materials with grain sizes larger than 30 μm, the void volume fraction and the average void size increase with increasing grain size. In the 30 and 200 μm samples, void coalescence is observed to dominate the void growth behavior, whereas in 60 and 100 μm samples, void growth is dominated by the growth of isolated voids. Electron backscatter diffraction (EBSD) observations show that voids preferentially nucleate and grow at grain boundaries with high angle misorientation. However, special boundaries corresponding to Σl (low angle, < 5 °) and Σ3 (∼60 ° <111> misorientation) types are more resistant to void formation. Finally, micro x-ray tomography results show three dimensional (3D) views of the damage fields consistent with the two dimensional (2D) surface observations. Based on these findings, mechanisms for the void growth and coalescence are proposed.

Comparison of modal analysis results of laser vibrometry and nearfield acoustical holography measurements of an aluminum plate.

Two methods for non-contact vibration measurements are compared using modal analysis. Laser vibrometry and nearfield acoustical holography are used to measure mode shapes of an aluminum plate excited by a mechanical shaker with random noise. Laser vibrometry provides a direct measurement of the panel surface velocity, while the nearfield acoustical holography measurements are used to calculate the surface velocity indirectly. Both data sets are compared in frequency, damping, and mode shapes. The results are also compared to finite element and theoretical analyzes. Results are presented from all four approaches.

Defining conditions for bulking and debulking in lahars

Research Article| July 01, 2011 Defining conditions for bulking and debulking in lahars E.E. Doyle; E.E. Doyle † 1Joint Centre for Disaster Research, Massey University, P.O. Box 756, Wellington 6140, New Zealand †E-mail: emmadoyle79@gmail.com Search for other works by this author on: GSW Google Scholar S.J. Cronin; S.J. Cronin 2Institute of Natural Resources, Massey University, Private Bag 11 222, Palmerston North 4442, New Zealand Search for other works by this author on: GSW Google Scholar J.-C. Thouret J.-C. Thouret 3Laboratoire Magmas et Volcans UMR 6524, Centre National de la Recherche Scientifique (CNRS), Université Blaise Pascal, Observatoire de Physique de Globe at Clermot-Ferrand (OPGC) et Institute of Research and Development (IRD), 5 rue Kessler, 63038 Clermont-Ferrand cedex, France Search for other works by this author on: GSW Google Scholar GSA Bulletin (2011) 123 (7-8): 1234–1246. https://doi.org/10.1130/B30227.1 Article history received: 17 Dec 2009 rev-recd: 12 Jul 2010 accepted: 09 Aug 2010 first online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation E.E. Doyle, S.J. Cronin, J.-C. Thouret; Defining conditions for bulking and debulking in lahars. GSA Bulletin 2011;; 123 (7-8): 1234–1246. doi: https://doi.org/10.1130/B30227.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Through measurements at Semeru Volcano, East Java, we define the conditions under which bulking (entrainment of sediment and pore water) and debulking (dilution and sedimentation) occur in rain-triggered volcanic floods (lahars). Two observation sites were installed 510 m apart, along the Curah Lengkong River, 11.5 km southeast of Semeru's summit. This 30-m-wide box valley, with a gravel and lava base, represents a real-world flume analogy. Pore-pressure sensors provided stage measurements, a broad-band seismograph gave insight into sediment content and frictional-collisional behavior, video cameras were used to measure surface velocities, and direct bucket samples were taken. Eight rainfall-induced lahars were recorded, lasting 1–3 h with heights of 0.5–2 m, peak velocities of 3–7 m/s, and discharges of 25–250 m3/s. Flows ranged from typical (<40 wt% sediment) to coarse and dense hyperconcentrated flows (50–60 wt% sediment). Multiple distinct flow “packets” occurred within the complex lahars, and were used to determine internal changes between sites. From the multiparameter data set at each site, volumetric bulking and wave shortening, due to portions of the lahar accelerating toward the flow front, are identified. Initial debulking of lahars between sites may reflect drainage into the dry substrate. Estimates of discharge and volume at each site lead to the quantification of bulking and debulking by these actively flowing lahars along the channel reach. From this, we observe that bulking can be localized to certain parts of lahars, resulting in intraevent increases in peak discharge that are greater than what would occur if bulking was evenly distributed throughout the flow. Such data are essential for the development of numerical descriptions and hazard models for mass flows. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Effect of window reflections on photonic Doppler velocimetry measurements

Photonic Doppler velocimetry (PDV) has rapidly become a standard diagnostic for measuring velocities in dynamic compression research. While free surface velocity measurements are fairly straightforward, complications occur when PDV is used to measure a dynamically loaded sample through a window. Fresnel reflections can severely affect the velocity and time resolution of PDV measurements, especially for low-velocity transients. Shock experiments of quartz compressed between two sapphire plates demonstrate how optical window reflections cause ringing in the extracted PDV velocity profile. Velocity ringing is significantly reduced by using either a wedge window or an antireflective coating.

Mechanical properties of tantalum with different types of microstructure under high-rate deformation

The paper presents experimental data on the mechanical behavior of coarse-grained (80 μm) tantalum and cold-forged fine-grained (1–2 μm) tantalum under static, dynamic and shock wave loading. The data includes σ−ɛ compression diagrams for a strain rate of 10−3–103 s−1, impact velocity dependences of the relative change in cylinder length in Taylor impact tests, and time dependences of the free surface velocity in the Ta specimens under shock wave loading. It is found that the 80-fold decrease in Ta grain size only slightly affects the strength properties of the material under static and dynamic loading. Measurements of the free surface velocity under shock wave loading at ∼ 17 GPa point to a decrease in elastic precursor in the fine-grained tantalum by ∼35% and to an increase in its critical fracture stress by 15–20% compared to those in the coarse-grained tantalum. A complex strain rate dependence of the tantalum strength properties was revealed. The microstructure of the coarse-grained Ta specimens loaded at a shock wave pressure of 20–130 GPa for different times was examined, and the dislocation density and the number of shear bands formed under shock wave loading were measured.

3D conformal MRI-controlled transurethral ultrasound prostate therapy: validation of numerical simulations and demonstration in tissue-mimicking gel phantoms

MRI-controlled transurethral ultrasound therapy uses a linear array of transducer elements and active temperature feedback to create volumes of thermal coagulation shaped to predefined prostate geometries in 3D. The specific aims of this work were to demonstrate the accuracy and repeatability of producing large volumes of thermal coagulation (>10 cc) that conform to 3D human prostate shapes in a tissue-mimicking gel phantom, and to evaluate quantitatively the accuracy with which numerical simulations predict these 3D heating volumes under carefully controlled conditions. Eleven conformal 3D experiments were performed in a tissue-mimicking phantom within a 1.5T MR imager to obtain non-invasive temperature measurements during heating. Temperature feedback was used to control the rotation rate and ultrasound power of transurethral devices with up to five 3.5 × 5 mm active transducer elements. Heating patterns shaped to human prostate geometries were generated using devices operating at 4.7 or 8.0 MHz with surface acoustic intensities of up to 10 W cm−2. Simulations were informed by transducer surface velocity measurements acquired with a scanning laser vibrometer enabling improved calculations of the acoustic pressure distribution in a gel phantom. Temperature dynamics were determined according to a FDTD solution to Pennes’ BHTE. The 3D heating patterns produced in vitro were shaped very accurately to the prostate target volumes, within the spatial resolution of the MRI thermometry images. The volume of the treatment difference falling outside ±1 mm of the target boundary was, on average, 0.21 cc or 1.5% of the prostate volume. The numerical simulations predicted the extent and shape of the coagulation boundary produced in gel to within (mean ± stdev [min, max]): 0.5 ± 0.4 [−1.0, 2.1] and −0.05 ± 0.4 [−1.2, 1.4] mm for the treatments at 4.7 and 8.0 MHz, respectively. The temperatures across all MRI thermometry images were predicted within −0.3 ± 1.6 °C and 0.1 ± 0.6 °C, inside and outside the prostate respectively, and the treatment time to within 6.8 min. The simulations also showed excellent agreement in regions of sharp temperature gradients near the transurethral and endorectal cooling devices. Conformal 3D volumes of thermal coagulation can be precisely matched to prostate shapes with transurethral ultrasound devices and active MRI temperature feedback. The accuracy of numerical simulations for MRI-controlled transurethral ultrasound prostate therapy was validated experimentally, reinforcing their utility as an effective treatment planning tool.

Performance of image-based velocimetry (LSPIV) applied to flash-flood discharge measurements in Mediterranean rivers

Summary Flash-floods that occur in Mediterranean regions result in significant casualties and economic impacts. Remote image-based techniques such as Large-Scale Particle Image Velocimetry (LSPIV) offer an opportunity to improve the accuracy of flow rate measurements during such events, by measuring the surface flow velocities. During recent floods of the Ardeche river, LSPIV performance tests were conducted at the Sauze–Saint Martin gauging station without adding tracers. The rating curve is well documented, with gauged discharge ranging from 4.8 m 3 s −1 to 2700 m 3 s −1 , i.e., mean velocity from 0.02 m s −1 to 2.9 m s −1 . Mobile LSPIV measurements were carried out using a telescopic mast with a remotely-controlled platform equipped with a video camera. Also, LSPIV measurements were performed using the images recorded by a fixed camera. A specific attention was paid to the hydraulic assumptions made for computing the river discharge from the LSPIV surface velocity measurements. Simple solutions for interpolating and extrapolating missing or poor-quality velocity measurements, especially in the image far-field, were applied. Theoretical considerations on the depth-average velocity to surface velocity ratio (or velocity coefficient) variability supported the analysis of velocity profiles established from available gauging datasets, from which a velocity coefficient value of 0.90 (standard deviation 0.05) was derived. For a discharge of 300 m 3 s −1 , LSPIV velocities throughout the river cross-section were found to be in good agreement (±10%) with concurrent measurements by Doppler profiler (ADCP). For discharges ranging from 300 to 2500 m 3 s −1 , LSPIV discharges usually were in acceptable agreement (

A grain-scale study of spall in brittle materials

The evolution of spall for a brittle material is investigated under variance of anisotropy, grain boundary fracture energy, and loading. Because spall occurs in the interior of the specimen, fundamental studies of crack nucleation and growth are needed to better understand surface velocity measurements. Within a cohesive approach to fracture, we illustrate that for anisotropic materials, increases in the fracture energy cause a transition in crack nucleation from triple-points to entire grain boundary facets. Analysis of idealized flaws reveals that while crack initiation and acceleration are strong functions of the fracture energy, flaws soon reach speeds on the order of the Rayleigh wave speed. Finally, simulated surface velocities of spalled configurations are correlated with microstructural evolution. These fundamental studies of nucleation, growth, and spall attempt to link atomic separation to the macroscopic spall strength and provide a computational framework to examine the evolution of spall and the impact on the simulated surface velocity field.

An efficient sound source determination process based on half-space boundary element method

In this study, sound source localization and identification of a refrigerator were accomplished by using the half-space boundary element method (BEM) based on surface velocity measurements. Surface velocities of the refrigerator were measured by a vibration test set-up; and two main dominant excitation frequencies were determined. The velocity data were fed to half-contact BE model of the refrigerator built in our “in-house” BEM computer code. The code calculated surface acoustic pressure distributions. The surface pressures exposed the location of sound sources of the refrigerator. Near-field sound radiation pattern of the localized sources were obtained by the surface pressure-input to the computer code.

Enhancing accuracy in reconstruction of vibro‐acoustic responses of a complex structure using Helmholtz equation least squares based nearfield acoustical holography.

In traditional nearfield acoustical holography, the acoustic pressures in the near field are measured, and the entire acoustic field including the normal component of the surface velocity are reconstructed. This type of approach may be appropriate for reconstructing the acoustic pressure field, but not for reconstructing normal surface velocity in practice because not enough nearfield information is collected to yield satisfactory reconstruction of the normal surface velocity. In this paper, we propose to supplement the acoustic pressure measurements with a few normal surface velocity measurements, which can be used as benchmarks in optimizing the reconstruction of normal surface velocity. These data are taken as input to the Helmholtz equation least squares method based NAH to reconstruct the acoustic pressure and normal velocity on the surface of a vibrating structure. The accuracy in reconstruction is examined experimentally on a baffled rectangular plate with clamped boundary conditions subject to random excitations. To validate the results, the reconstructed field acoustic pressures are compared with those measured by microphones, and the normal surface velocities were compared with the benchmark values collected by using a scanning laser vibrometer under the same condition.

Dynamic fracture and spall in aluminum with helium bubbles

Investigation of the dynamic properties of aluminum targets with helium bubbles is presented. The targets were obtained by melting pure aluminum with 0.15% wt.10B powder. The solid targets were neutron irradiated to get homogeneous helium atoms inside the aluminum boron 10 matrix according to the reaction 10B + n → 7Li+4He. Helium atoms further accumulated into bubbles by diffusion in the bulk aluminum. Shock wave experiments were performed by accelerating the aluminum impactor into different targets: (1) pure aluminum, (2) Al-10B, and (3)Al-10B with different radii and concentrations of helium bubbles. The spall strength was calculated and analyzed from the free surface velocity measurements. It was found that the addition of 10B in pure aluminum reduces the spall strength of the material by 25–32%. However, irradiated sample with helium bubbles was found to have higher spall strength compared to samples without bubbles. This finding was reconstructed by numerical simulations. The impacted targets were collected after the impact experiments and examined by TEM. These targets were compared to TEM pictures before the impact. The number of helium atoms in the bubbles was calculated from the electron energy loss spectrum (EELS). TEM comparison between the pre-impacted and the impacted targets shows bubbles coalescence and EELS measurements demonstrate a reduction of the helium atoms concentration in the bubbles from ~1028 m−3 before the impact to ~1027 m−3 after the impact.

Dynamics and mass balance of Taylor Glacier, Antarctica: 1. Geometry and surface velocities

Taylor Glacier, Antarctica, exemplifies a little‐studied type of outlet glacier, one that flows slowly through a region of rugged topography and dry climate. This glacier, in addition, connects the East Antarctic Ice Sheet with the McMurdo Dry Valleys, a region much studied for geomorphology, paleoclimate, and ecology. Here we report extensive new measurements of surface velocities, ice thicknesses, and surface elevations, acquired with InSAR, GPS, and GPR. The latter two were used to construct elevation models of the glacier's surface and bed. Ice velocities in 2002–2004 closely matched those in 2000 and the mid‐1970s, indicating negligible interannual variations of flow. Comparing velocities with bed elevations shows that, along much of the glacier, flow concentrates in a narrow axis of relatively fast flowing ice that overlies a bedrock trough. The flow of the glacier over major undulations in its bed can be regarded as a “cascade”; it speeds up over bedrock highs and through valley narrows and slows down over deep basins and in wide spots. This pattern is an expected consequence of mass conservation for a glacier near steady state. Neither theory nor data from this Taylor Glacier study support the alternative view, recently proposed, that an outlet glacier of this type trickles slowly over bedrock highs and flows fastest over deep basins.

Dynamics and mass balance of Taylor Glacier, Antarctica: 2. Force balance and longitudinal coupling

Taylor Glacier, Antarctica, exemplifies an ice sheet outlet that flows through a region of rugged topography and dry climate. In contrast to other well‐studied outlets, Taylor Glacier moves very slowly, despite a thickness of order 1 km and driving stresses averaging 1.5 bars. Here we analyze new measurements of glacier geometry and surface velocity to elucidate flow dynamics of Taylor Glacier. Force balance and basal temperatures are calculated at six locations along the glacier's length using an algorithm developed for this study. The effects of stress‐gradient coupling on longitudinal flow variations are also examined; we ask whether Kamb and Echelmeyer's (1986) linearized theory adequately describes the observed response of flow to large‐amplitude variations in driving stress. The force balance calculations indicate that no basal motion is needed to explain the observed flow of Taylor Glacier. Inferred basal temperatures are within a few degrees of the melting point in regions of kilometer‐thick ice and well below the melting point elsewhere; deformation of subfreezing ice largely controls the flow of Taylor Glacier. Basal drags are mostly in the range 0.9 to 1.2 bars, and lateral drags are in the range 0.2 to 0.5 bar. Stress‐gradient coupling strongly reduces the variability of velocities along the glacier. The velocity variations can be described as the convolution of a forcing function with a spatial filter, as Kamb and Echelmeyer suggested, but the form of the forcing function differs from the theoretical relation derived for small‐amplitude perturbations (the power on driving stress is one, not three).

Estimation of the Surface Velocity Field of the Aletsch Glacier Using Multibaseline Airborne SAR Interferometry

This paper presents a methodology to process airborne interferometric synthetic aperture radar (SAR) data to measure surface velocity fields (SVFs) of temperate glaciers, and applies it to data acquired over the Aletsch glacier. The first part of this paper deals with the main limitation in airborne interferometric SAR to retrieve reliable interferometric products, namely, the existence of the so-called residual motion errors - inaccuracies on the order of a few centimeters in the navigation system. An extended multisquint approach is proposed for their estimation in the case of nonstationary scenes. The second part of this paper expounds an efficient methodology to derive SVFs with airborne systems, where the line-of-sight displacement is estimated using differential interferometry and the along-track component by estimating the azimuth coregistration offsets. The necessary steps to finally obtain the 3-D SVF are also presented, as well as the possibility of combining different acquisition geometries. Airborne interferometric SAR data acquired by the Experimental SAR system of the German aerospace center over the Aletsch glacier, located in the Swiss Alps, are used to evaluate the performance of the proposed approach. The motion of the corner reflectors deployed in the scene is retrieved with an accuracy between 1 and 5 cm/day using L-band data.