H. A. Zebker

Stanford University, Palo Alto, California, United States

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Publications (334)701.57 Total impact

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    Laura E Erban, Steven M Gorelick, Howard A Zebker
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    ABSTRACT: Groundwater exploitation is a major cause of land subsidence, which in coastal areas poses a flood inundation hazard that is compounded by the threat of sea-level rise (SLR). In the lower Mekong Delta, most of which lies <2 m above sea level, over-exploitation is inducing widespread hydraulic head (i.e., groundwater level) declines. The average rate of head decline is ~0.3 m yr−1, based on time-series data from 79 nested monitoring wells at 18 locations. The consequent compaction of sedimentary layers at these locations is calculated to be causing land subsidence at an average rate of 1.6 cm yr−1. We further measure recent subsidence rates (annual average, 2006–10) throughout the Delta, by analysis of interferometric synthetic aperture radar (InSAR), using 78 ALOS PALSAR interferograms. InSAR-based subsidence rates are 1) consistent with compaction-based rates calculated at monitoring wells, and 2) ~1–4 cm yr−1 over large (1000s of km2) regions. Ours are the first mapped estimates of Delta-wide land subsidence due to groundwater pumping. If pumping continues at present rates, ~0.88 m (0.35–1.4 m) of land subsidence is expected by 2050. Anticipated SLR of ~0.10 m (0.07–0.14 m) by 2050 will compound flood inundation potential. Our results suggest that by mid-century portions of the Mekong Delta will likely experience ~1 m (0.42–1.54 m) of additional inundation hazard.
    Environmental Research Letters 08/2014; 9(8):084010. · 4.09 Impact Factor
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    ABSTRACT: Gravity measurements and elevation data from the Cassini mission have been used to create shape, global topography and gravity anomaly models of Titan that enable an improved understanding of its outer ice I shell structure. We provide constraints on the averaged ice shell thickness and its long-wavelength lateral variations, as well as the density of the subsurface ocean using gravity anomalies, the tidal Love number k2 measurement and long-wavelength topography. We found that Titan’s surface topography is consistent with an approximate isostatically compensated ice shell of variable thickness, likely in a thermally conductive or in a subcritical convective state, overlying a relatively dense subsurface ocean.
    Icarus 07/2014; 236:169–177. · 2.84 Impact Factor
  • Jingyi Chen, Howard A. Zebker, Paul Segall, Asta Miklius
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    ABSTRACT: We present here an Small BAseline Subset (SBAS) algorithm to extract both transient and secular ground deformations on the order of millimeters in the presence of tropospheric noise on the order of centimeters, when the transient is of short duration and known time, and the background deformation is smooth in time. We applied this algorithm to study the 2010 slow slip event as well as the secular motion of Kīlauea's south flank using 49 TerraSAR-X images. We also estimate the tropospheric delay variation relative to a given reference pixel using an InSAR SBAS approach. We compare the InSAR SBAS solution for both ground deformation and tropospheric delays with existing GPS measurements and confirm that the ground deformation signal andtropospheric noise in InSAR data are successfully separated. We observe that the coastal region on the south side of the Hilina Pali moves at a higher background rate than the region north side of the Pali. We also conclude that the 2010 SSE displacement is mainly horizontal and the maximum magnitude of the 2010 SSE vertical component is less than 5 mm.
    Journal of Geophysical Research: Solid Earth 07/2014; 119(8). · 3.44 Impact Factor
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    ABSTRACT: Interferometric synthetic aperture radar (InSAR) is a remote sensing method that maps relative ground surface deformation. In previous work, we investigated the relationship between deformation and hydraulic head change in the San Luis Valley, CO, USA, and determined that we must quantify the spatially variable uncertainty in the InSAR deformation measurement in order for these data to be used to predict hydraulic head. In this study, we modify a commonly applied multitemporal technique, Small Baseline Subset (SBAS) analysis, to process InSAR data in an area where pumping for crop irrigation creates seasonally variable deformation. We propagate the uncertainty due to decorrelation through the InSAR processing chain and calculate the uncertainty in the deformation for all selected pixels. The standard deviation of the uncertainty in the deformation ranges from 1 to 5 mm. Finally, we investigate how the InSAR coherence affects the standard deviation of the estimated deformation. Through a synthetic study, we show that given the mean coherence and standard deviation of coherence, we can determine the mean standard deviation of the final deformation estimates. This allows us to optimize InSAR processing to identify which pixels can provide the uncertainty desired in the final deformation time series.
    IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing 07/2014; 7(7):2992-3001. · 2.83 Impact Factor
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    ABSTRACT: Wildfire is a major disturbance in the Arctic tundra and boreal forests, having a significant impact on soil hydrology, carbon cycling, and permafrost dynamics. This study explores the use of the microwave Interferometric Synthetic Aperture Radar (InSAR) technique to map and quantify ground surface subsidence caused by the Anaktuvuk River fire on the North Slope of Alaska. We detected an increase of up to 8 cm of thaw-season ground subsidence after the fire, which is due to a combination of thickened active layer and permafrost thaw subsidence. Our results illustrate the effectiveness and potential of using InSAR to quantify fire impacts on the Arctic tundra, especially in regions underlain by ice-rich permafrost. Our study also suggests that surface subsidence is a more comprehensive indicator of fire impacts on ice-rich permafrost terrain than changes in active layer thickness alone.
    Geophysical Research Letters 05/2014; 41:3906-3913. · 4.46 Impact Factor
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    ABSTRACT: The sustainability of the confined aquifer system in the San Luis Valley, Colorado is of utmost importance to the valley's agricultural economy. There is a dearth of hydraulic head measurements in the confined aquifer to which the current groundwater flow model can be calibrated. Here we investigate the extent to which spatially and temporally dense measurements of deformation from Interferometric Synthetic Aperture Radar (InSAR) data can be used to interpolate and extrapolate temporal and spatial gaps in the head dataset by calibrating with InSAR at the monitoring well locations. We conduct this calibration at 11 wells where we expect sufficient deformation for reliable InSAR measurement, given the accepted level of uncertainty (˜ 1 cm). In the San Luis Valley crop growth degrades the quality of the InSAR signal, which means that the high quality deformation data may not be collocated with the wells. We use kriging to estimate the deformation directly at the well locations. We find that the calibration is valid at three well locations where the seasonal magnitude of the deformation is much larger than the uncertainty of the InSAR measurement. At these well locations we predict head prior to and within the temporal sampling window of the head measurements. We find that 59% of the InSAR-predicted hydraulic head values agree with the measured values, within the uncertainty of the data. Given our success in extending the hydraulic head data temporally, the next step in our research is to use InSAR data to interpolate spatially between head measurements.
    Water Resources Research. 05/2014; 50(5).
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    ABSTRACT: [1] We construct the depth profile—the bathymetry—of Titan's large sea Ligeia Mare, from Cassini RADAR data collected during the 23 May 2013 (T91) nadir-looking altimetry fly-by. We find the greatest depth to be about 160 m and a seabed slope that is gentler towards the northern shore, consistent with previously imaged shoreline morphologies. Low radio signal attenuation through the sea demonstrates that the liquid, for which we determine a loss tangent of 3 ± 1*10-5, is remarkably transparent, requiring a nearly pure methane-ethane composition, and further that microwave absorbing hydrocarbons, nitriles, and suspended particles be limited to less than the order of 0.1% of the liquid volume. Presence of nitrogen in the ethane-methane sea, expected based on its solubility and dominance in the atmosphere, is consistent with the low attenuation, but that of substantial dissolved polar species or suspended scatterers is not.
    Geophysical Research Letters. 02/2014;
  • A.C. Chen, H.A. Zebker
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    ABSTRACT: Interferometric synthetic aperture radar (InSAR) is a valuable tool for the study of geophysical phenomena such as crustal deformation, ice motion and structure, and vegetation canopy depths, but it is adversely affected by uncharacterized inhomogeneities in ionospheric propagation delay. Ionospheric disturbances distort both InSAR phase and correlation maps. Here, we present a method to compensate ionospheric propagation variations using accurate image coregistration. This significantly improves both the interferometric coherence and phase accuracy. An azimuth gradient in the total electron content (TEC) from a spatially variable ionosphere results in a range-dependent azimuth phase gradient being added to the phase histories of the pixels being imaged. These phase gradients are equivalent to Doppler shifts, and thus they cause azimuth offsets between the actual and imaged positions of the pixels. Measuring these offsets accurately permits estimation of the gradient and correction of the interferograms for much of the phase distortion, resulting in more accurate estimates of coherence. We show an example over Greenland where the TEC variation causes the correlation to drop from about 0.7 to about 0.2 in one region if spatially varying offsets are not accounted for; it also adds an estimated 4.4 radians of interferometric phase over an 80 km InSAR scene. After applying our algorithm, we find that the correlation in regions affected by the ionospheric inhomogeneity becomes comparable to correlation in the rest of the image. In a more challenging example over Iceland, we show that our method improves the correlation from 0.15 to 0.25 in some areas.
    IEEE Transactions on Geoscience and Remote Sensing 01/2014; 52(1):60-70. · 2.93 Impact Factor
  • Nature Geoscience 01/2014; 7:493-496. · 11.67 Impact Factor
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    ABSTRACT: radar observations of the surface of Ligeia Mare collected during the 23 May 2013 (T91) Cassini flyby show that it is extremely smooth, likely to be mostly methane in composition, and exhibits no surface wave activity. The radar parameters were tuned for nadir-looking geometry of liquid surfaces, using experience from Cassini's only comparable observation, of Ontario Lacus on 21 December 2008 (T49), and also include coincident radiometric observations. Radar echoes from both passes show very strong specular radar reflections and limit surface height variations to 1 mm rms. The surface physical temperature at 80°N is 92 +/- 0.5 K if the sea is liquid hydrocarbon and the land is solid hydrocarbon, essentially the same as Cassini CIRS measurements. Furthermore, radiometry measurements over the surrounding terrain suggest dielectric constants from 2.2 to 2.4, arguing against significant surface water ice unless it is extremely porous.
    12/2013; 41(2).
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    ABSTRACT: Drained thermokarst lake basins (DTLBs) are ubiquitous landforms on arctic tundra lowlands, but their present-day dynamic states are seldom investigated. Here we report results based on high-resolution Interferometric Synthetic Aperture Radar (InSAR) measurements using space-borne data for a study area located near Prudhoe Bay, Alaska where we focus on the seasonal thaw settlement within DTLBs, averaged between 2006 and 2010. The majority (14) of the 18 DTLBs in the study area analyzed exhibited seasonal thaw settlement of 3-4 cm. However, four of the DTLBs analyzed exceeded 4 cm of thaw settlement, with one basin experiencing up to 12 cm. Combining the InSAR observations with the in situ active layer thickness measured using ground penetrating radar and mechanical probing, we calculated thaw strain, an index of thaw settlement strength along a transect across the basin that underwent large thaw settlement. We found thaw strains of 10-35% at the basin center, suggesting the seasonal melting of ground ice as a possible mechanism for the large settlement. These findings emphasize the dynamic nature of permafrost landforms, demonstrate the capability of the InSAR technique to remotely monitor surface deformation of individual DTLBs, and illustrate the combination of ground-based and remote sensing observations to estimate thaw strain. Our study highlights the need for better description of the spatial heterogeneity of landscape-scale processes for regional assessment of surface dynamics on arctic coastal lowlands.
    The Cryosphere Discussions 12/2013; 7(6):5793-5822.
  • Howard A. Zebker
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    ABSTRACT: Phased arrays can be formed by separating elements in time as well as space. In this configuration, surface or target velocities may be observed, rather than the radar backscatter at each position in an image or area. We have used time series data acquisitions using imaging radars to map and measure mm-scale velocities at m-scale resolutions over areas tens of km in size. Our applications are mainly for geophysical use, where our main results to date are in mapping natural hazards such as earthquakes and volcanos, groundwater management systems, and ice motion in the polar regions. In all of these the fundamental observable is velocity of the surface, at a precision of mm to cm/yr. The data we use are acquired by spaceborne imaging radar systems, typically by a single satellite operating in a repeat orbit. Repeat times for the systems range from 11 to 46 days, so in many cases we analyze data acquired over months to years, an appropriate time scale for most of the processes we study.
    2013 IEEE International Symposium on Phased Array Systems and Technology (ARRAY 2013); 10/2013
  • D Hemingway, F Nimmo, H Zebker, L Iess
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    ABSTRACT: Several lines of evidence suggest that Saturn's largest moon, Titan, has a global subsurface ocean beneath an outer ice shell 50 to 200 kilometres thick. If convection is occurring, the rigid portion of the shell is expected to be thin; similarly, a weak, isostatically compensated shell has been proposed to explain the observed topography. Here we report a strong inverse correlation between gravity and topography at long wavelengths that are not dominated by tides and rotation. We argue that negative gravity anomalies (mass deficits) produced by crustal thickening at the base of the ice shell overwhelm positive gravity anomalies (mass excesses) produced by the small surface topography, giving rise to this inverse correlation. We show that this situation requires a substantially rigid ice shell with an elastic thickness exceeding 40 kilometres, and hundreds of metres of surface erosion and deposition, consistent with recent estimates from local features. Our results are therefore not compatible with a geologically active, low-rigidity ice shell. After extrapolating to wavelengths that are controlled by tides and rotation, we suggest that Titan's moment of inertia may be even higher (that is, Titan may be even less centrally condensed) than is currently thought.
    Nature 08/2013; 500(7464):550-2. · 42.35 Impact Factor
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    ABSTRACT: Deep aquifers in South and Southeast Asia are increasingly exploited as presumed sources of pathogen- and arsenic-free water, although little is known of the processes that may compromise their long-term viability. We analyze a large area (>1,000 km(2)) of the Mekong Delta, Vietnam, in which arsenic is found pervasively in deep, Pliocene-Miocene-age aquifers, where nearly 900 wells at depths of 200-500 m are contaminated. There, intensive groundwater extraction is causing land subsidence of up to 3 cm/y as measured using satellite-based radar images from 2007 to 2010 and consistent with transient 3D aquifer simulations showing similar subsidence rates and total subsidence of up to 27 cm since 1988. We propose a previously unrecognized mechanism in which deep groundwater extraction is causing interbedded clays to compact and expel water containing dissolved arsenic or arsenic-mobilizing solutes (e.g., dissolved organic carbon and competing ions) to deep aquifers over decades. The implication for the broader Mekong Delta region, and potentially others like it across Asia, is that deep, untreated groundwater will not necessarily remain a safe source of drinking water.
    Proceedings of the National Academy of Sciences 08/2013; · 9.81 Impact Factor
  • Jaime Lien, Howard Zebker
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    ABSTRACT: Interferometric synthetic aperture radar (InSAR) is an effective tool for measuring temporal changes in the Earth's surface and producing high accuracy, wide coverage images of crustal deformation fields. Decorrelation due to spatial and temporal baseline is a major limiting factor in estimating the deformation signal, but may be ameliorated by using persistent scatterer (PS) techniques. Phase unwrapping and subsequent deformation estimation on the spatially sparse PS network depend largely on the accurate selection of PS pixels and the density of the network. Many additional pixels can be added to the PS list if we are able to identify those in which a dominant scatterer exhibits partial, rather than complete, correlation across all radar scenes. In this work, we discuss and compare statistical methods to model, characterize, and select partially correlated PS pixels.
    IGARSS 2013 - 2013 IEEE International Geoscience and Remote Sensing Symposium; 07/2013
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    ABSTRACT: Cassini RADAR SARtopo and altimetry data are used to construct a global gridded 1 × 1° elevation map, for use in Global Circulation Models, hydrological models and correlative studies. The data are sparse, and so most of the map domain (∼90%) is populated with interpolated values using a spline algorithm. The highest (∼+520 m) gridded point observed is at 48°S, 12°W. The lowest point observed (∼1700 m below a 2575 km sphere) is at 59°S, 317°W: this may be a basin where liquids presently in the north could have resided in the past. If the deepest point were once a sea with the areal extent of present-day Ligeia Mare, it would be ∼1000 m deep. We find four prominent topographic rises, each ∼200 km wide, radar-bright and heavily dissected, distributed over a ∼3000 km arc in the southeastern quadrant of Titan (∼40–60°S, 15–150°W).
    Icarus 07/2013; 225(1):367–377. · 2.84 Impact Factor
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    ABSTRACT: Cassini-derived gravity and topography data suggest that Titan’s ice shell is largely rigid and that its surface has undergone extensive erosion.
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    ABSTRACT: Despite the abundance of rock glaciers in the Sierra Nevada of California, USA, few efforts have been made to measure their surface flow. Here we use the interferometric synthetic aperture radar (InSAR) technique to compile a benchmark inventory describing the kinematic state of 59 active rock glaciers in this region. In the late summer of 2007, these rock glaciers moved at speeds that range from 14 cmyr(-1) to 87 cmyr(-1), with a regional mean value of 53 cmyr(-1). Our inventory reveals a spatial difference: rock glaciers in the southern Sierra Nevada moved faster than the ones in the central Sierra Nevada. In addition to the regional mapping, we also conduct a case study to measure the surface flow of the Mount Gibbs rock glacier in fine spatial and temporal detail. The InSAR measurements over this target reveal (1) that the spatial pattern of flow is correlated with surface geomorphic features and (2) a significant seasonal variation of flow speed whose peak value was 48 cmyr(-1) in the fall of 2007, more than twice the minimum value observed in the spring of 2008. The seasonal variation lagged air temperatures by three months. Our finding on the seasonal variation of surface speed reinforces the importance of a long time series with high temporal sampling rates to detect possible long-term changes of rock glacier kinematics in a warming climate.
    The Cryosphere 01/2013; 7(4):1109-1119. · 4.37 Impact Factor
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    ABSTRACT: Land subsidence is a common problem in vulnerable deltas. The Nile Delta is no exception. The impacts of land subsidence are heightened by the economic, social and historical importance of the delta to Egypt. A major debate has evolved in the past two decades concerning whether the land surface of the Nile Delta is subsiding. The debate is certainly problematic in light of the fact that current measures of subsidence across the delta are rough estimates at best. To date, knowledge of subsidence rates in the delta is limited to long-term geologic averages that assume spatial uniformity and temporal consistency. In this study, we apply persistent scatterer interferometry (PSI) to measure the magnitude and monitor the spatial and temporal variations of land subsidence in the Nile Delta, during 1993–2000, using synthetic aperture radar interferometric data of 5.66 cm wavelength. The average measured rates of local subsidence in two major cities in the delta, namely Mansura and Greater Mahala, are –9 and –5 mm year–1, respectively. The observed deformation features imply that subsidence in both cities is controlled mainly by local groundwater processes. Our PSI measurements indicate that no regional subsidence has occurred in either city between 1993 and 2000. The slight regional subsidence that is expected to occur over time due to the natural compaction of deltaic sediments most likely has been masked by surface displacements caused by seasonal oscillations in the groundwater level.
    Remote Sensing Letters 12/2012; 3(7):621-630. · 1.43 Impact Factor

Publication Stats

10k Citations
701.57 Total Impact Points


  • 1992–2014
    • Stanford University
      • • Department of Electrical Engineering
      • • Department of Geophysics
      Palo Alto, California, United States
  • 2011
    • NORUT Northern Research Institute
      Tromsø, Troms, Norway
  • 2010
    • Johns Hopkins University
      • Applied Physics Laboratory
      Baltimore, Maryland, United States
  • 1988–2007
    • California Institute of Technology
      • Jet Propulsion Laboratory
      Pasadena, CA, United States