H. A. Zebker

Stanford University, Palo Alto, California, United States

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Publications (337)653.22 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. · 3.58 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. · 3.16 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; · 3.44 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;
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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;
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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;
  • J.A. Reeves, R. Knight, H.A. Zebker
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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 01/2014; 7(7):2992-3001. · 2.87 Impact Factor
  • 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. · 3.47 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.
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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
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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. · 3.16 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. · 1.62 Impact Factor
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    ABSTRACT: On November 6, 2011, Cassini RADAR obtained a unique data set during a flyby of Enceladus. We will discuss the observation design and processing and present the data in preliminary form.
  • Jingyi Chen, Howard A. Zebker
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    ABSTRACT: Phase artifacts in interferometric synthetic aperture radar (InSAR) images frequently degrade the interpretability of the phase and correlation signatures of terrain. Often, these distortions are attributed to spatially variable ionospheric propagation delays at two different SAR acquisition times. We present here L-band InSAR data from Iceland, California, and Hawaii. The California and Hawaii interferograms show no significant ionospheric artifacts, while the Iceland interferogram shows a maximum misregistration of three pixels in the azimuth direction, which leads to severe phase decorrelation artifacts in the InSAR image. We relate the misregistration of complex pixels seen in the interferograms to the gradient of the ionospheric total electron content (TEC) observed by global positioning system (GPS) data and confirm that indeed the phase artifacts in the Iceland interferogram are due to dispersive ionospheric propagation rather than other decorrelation factors such as neutral atmospheric delays. We develop a method to measure the spatial TEC variation at synthetic aperture length scales using dual-frequency GPS carrier phase data. We solve for the GPS data ambiguities using a low-resolution ionosphere reference derived from either available ionospheric observations or the GPS carrier phase data themselves. GPS observations show directly the level of ionospheric variability, and the spatial TEC gradient as observed by GPS predicts the misregistration of complex pixels in interferograms in all three areas. This confirmation of the cause of the image artifacts suggests that they can be routinely corrected from the InSAR data alone, provided that the sensor measures the change in TEC along the radar swath.
    IEEE Transactions on Geoscience and Remote Sensing 01/2012; 50(4):1227-1239. · 3.47 Impact Factor
  • AGU Fall Meeting Abstracts. 01/2012; 1:01.
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    ABSTRACT: The San Luis Valley (SLV) is an 8000 km2 region in southern Colorado that is home to a thriving agricultural economy. The valley is currently in a period of extreme drought, with county and state regulators facing the challenge of developing appropriate management policies for both surface water and ground water supplies. Legislation passed in 2004 requires that hydraulic head levels within the confined aquifer system stay within the range experienced in the years 1978 - 2000. While some measurements of hydraulic head exist, greater spatial and temporal sampling would be very valuable in understanding the behavior of the confined aquifer system. Interferometric synthetic aperture radar (InSAR) data provide spatially dense maps of deformation of Earth's surface, with one pixel representing the deformation of a 50 m by 50 m area on the ground. This deformation can be related to hydraulic head change in the confined aquifer system. The ability to map these changes, over time, in the SLV will provide critical information about the groundwater system. In this study we used data from the European Space Agency's ERS-1 and ERS-2 satellites, which have 31 acquisitions archived from 1992 - 2001. We applied small baseline subset (SBAS) analysis to create a time series of deformation for all pixels with high data quality. We find that the seasonal deformation measured by InSAR mimics hydraulic head measurements made in the confined aquifer system. We also find that the deformation occurring in the confined aquifer system is primarily elastic and recoverable in nature. At many well locations there are gaps in the hydraulic head record during the period relevant for the 2004 legislation. We find that high quality InSAR data exist during those time periods and can be used to fill historical gaps in hydraulic head data. We have processed the deformation time series for 2500 km2 of area on the ground at a spatial resolution of ~ 50 m. We find it useful to visualize the deformation over such a large area throughout time using an animation depicting the time series deformation patterns. InSAR can be used in this way, as a qualitative tool to see how the extent of groundwater pumping and/or the compressibility of sediments vary throughout the valley. The animation also allows us to identify InSAR acquisitions that show strong atmospheric signals, which were not removed during SBAS analysis.
    AGU Fall Meeting Abstracts. 12/2011;
  • H. A. Zebker, A. G. Hayes, L. Wye
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    ABSTRACT: The scatterometry mode of the Cassini RADAR is the premier dataset with which to investigate the scattering properties of Titan's surface. The scatterometry mode observes a wider range of incidence angles, has acquired near-global coverage, includes more robust radiometric calibration, and can discern features at lower signal levels than is possible with the fine-resolution synthetic aperture radar (SAR) mode. The downside to scatterometry analysis is that the real aperture surface footprints are much coarser than the SAR resolution. Here we present high-resolution backscatter maps derived from Cassini scatterometer observations at ~5-20 km resolution, coarser than the SAR observations (0.3-1km resolution) but finer than the 100 km resolution offered by real aperture scatterometer data reduction. These new products are made possible by analyzing the range delay and phase of the scatterometry measurements, rather than using the total beam-integrated power computation approach in real-aperture reduction. Cassini scatterometer data are acquired using a low-bandwidth chirp modulation on the transmitted signal, and each observation consists of a burst of about 8 transmit pulses. Using a coherent back projection algorithm, we process the data to improved resolution by about a factor of 10 in each dimension over real aperture values, although not all pass geometries have range/Doppler surface contours to support this resolution. Nonetheless, the finer resolution offered on well-contoured passes implies that we can estimate the backscatter curve for features much smaller than has been possible to date. The existence of multiple observations of each of these finer features means that we can better constrain surface roughness and dielectric constant properties than is possible from the SAR data alone, where limited observations exist of any single feature. Here we present initial reductions of the scatterometry data set and show that we can predict resolution performance by examining the range and Doppler contour diagrams from each pass. These images display moderate resolution Titan backscatter maps of areas not before imaged at fine resolution. The contour analysis in addition provides a way to schedule future Cassini observations of un-investigated areas in order to make the best use of spacecraft resources.
    AGU Fall Meeting Abstracts. 12/2011;
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    ABSTRACT: In the San Luis Valley (SLV), Colorado legislation passed in 2004 requires that hydraulic head levels in the confined aquifer system stay within the range experienced in the years 1978-2000. While some measurements of hydraulic head exist, greater spatial and temporal sampling would be very valuable in understanding the behavior of the system. Interferometric synthetic aperture radar (InSAR) data provide fine spatial resolution measurements of Earth surface deformation, which can be related to hydraulic head change in the confined aquifer system. However, change in cm-scale crop structure with time leads to signal decorrelation, resulting in low quality data. Here we apply small baseline subset (SBAS) analysis to InSAR data collected from 1992 to 2001. We are able to show high levels of correlation, denoting high quality data, in areas between the center pivot irrigation circles, where the lack of water results in little surface vegetation. At three well locations we see a seasonal variation in the InSAR data that mimics the hydraulic head data. We use measured values of the elastic skeletal storage coefficient to estimate hydraulic head from the InSAR data. In general the magnitude of estimated and measured head agree to within the calculated error. However, the errors are unacceptably large due to both errors in the InSAR data and uncertainty in the measured value of the elastic skeletal storage coefficient. We conclude that InSAR is capturing the seasonal head variation, but that further research is required to obtain accurate hydraulic head estimates from the InSAR deformation measurements.
    Water Resources Research 12/2011; 47(12):12510-. · 3.15 Impact Factor

Publication Stats

8k Citations
653.22 Total Impact Points


  • 1984–2014
    • Stanford University
      • • Department of Electrical Engineering
      • • Department of Geophysics
      Palo Alto, California, United States
  • 2011
    • NORUT Northern Research Institute
      Tromsø, Troms, Norway
  • 1988–2007
    • California Institute of Technology
      • Jet Propulsion Laboratory
      Pasadena, CA, United States
  • 2006
    • The University of Arizona
      • Department of Planetary Sciences
      Tucson, Arizona, United States