Howard A. Zebker

Stanford University, Palo Alto, California, United States

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Publications (345)779.01 Total impact

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    ABSTRACT: We use repeat synthetic aperture radar (SAR) observations and complementary altimetry passes acquired by the Cassini spacecraft to study the scattering properties of Titan's empty lake basins. The best-fit coefficients from fitting SAR data to a quasi-specular plus diffuse backscatter model suggest that the bright basin floors have a higher dielectric constant, but similar facet-scale rms surface facet slopes, to surrounding terrain. Waveform analysis of altimetry returns reveals that nadir backscatter returns from basin floors are greater than nadir backscatter returns from basin surroundings and have narrower pulse widths. This suggests that floor deposits are structurally distinct from their surroundings, consistent with the interpretation that some of these basins may be filled with evaporitic and/or sedimentary deposits. Basin floor deposits also express a larger diffuse component to their backscatter, which is likely due to variations in subsurface structure or an increase in roughness at the wavelength scale (Hayes, A.G. et al. [2008]. Geophys. Res. Lett. 35, 9). We generate a high-resolution altimetry radargram of the T30 altimetry pass over an empty lake basin, with which we place geometric constraints on the basin's slopes, rim heights, and depth. Finally, the importance of these backscatter observations and geometric measurements for basin formation mechanisms is briefly discussed.
    Icarus 09/2015; DOI:10.1016/j.icarus.2015.09.043 · 3.04 Impact Factor
  • Chen · A. Parsekian · K. Schaefer · E. Jafarov · S.K. Panda · L. Liu · T. Zhang · H.A. Zebker ·
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    ABSTRACT: This data set includes estimates of permafrost Active Layer Thickness (ALT; cm), and calculated uncertainties, derived using a ground-penetrating radar (GPR) system in the field in August 2014 near Toolik Lake and Happy Valley on the North Slope of Alaska. GPR measurements were taken along 10 transects of varying length (approx. 1 to 7 km). Traditional ALT estimates from mechanical probing every 100 to 500 m along each transect are also included. These data are suitable for future studies of how ALT varies over relatively large geological features, such as hills and valleys, wetland areas, and drained lake basins.
  • L. Liu · K. M. Schaefer · A. C. Chen · A. Gusmeroli · H. A. Zebker · T. Zhang ·
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    ABSTRACT: Thawing of ice-rich permafrost followed by surface subsidence results in irregular, depressed landforms known as thermokarst. Many remote sensing studies have identified thermokarst landforms and mapped their changes. However, the intrinsic dynamic thermokarst process of surface subsidence remains a challenge to quantify and is seldom examined using remote sensing methods. In this study we used spaceborne interferometric synthetic aperture radar (InSAR) data to map surface subsidence trends at a thermokarst landform located near Deadhorse on the North Slope of Alaska. A pipeline access road constructed in the 1970s triggered the thawing of the permafrost, causing subsequent expansion of the thermokarst landform. Using Phased Array type L band Synthetic Aperture Radar images acquired by the Advanced Land Observing Satellite-1, our InSAR analysis reveals localized thermokarst subsidence of 2–8 cm/yr between 2006 and 2010, equivalent to an ice volume loss of about 1.2 × 107 m3/yr. Comparisons between InSAR subsidence trends and lidar microtopography suggest a characteristic time of 8 years of thermokarst development. We also quantitatively explain the difficulty, uncertainties, and possible biases in separating thermokarst-induced, irreversible subsidence from cyclic seasonal deformation. Our study illustrates that InSAR is an effective tool for mapping and studying active thermokarst processes and quantifying ice loss.
    Journal of Geophysical Research: Earth Surface 09/2015; 120(9):n/a-n/a. DOI:10.1002/2015JF003599 · 3.44 Impact Factor
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    ABSTRACT: Active layer thickness (ALT) is a critical parameter for monitoring the status of permafrost that is typically measured at specific locations using probing, in situ temperature sensors, or other ground-based observations. Here we evaluated the Remotely Sensed Active Layer Thickness (ReSALT) product that uses the Interferometric Synthetic Aperture Radar technique to measure seasonal surface subsidence and infer ALT around Barrow, Alaska. We compared ReSALT with ground-based ALT obtained using probing and calibrated, 500 MHz Ground Penetrating Radar at multiple sites around Barrow. ReSALT accurately reproduced observed ALT within uncertainty of the GPR and probing data OPEN ACCESS Remote Sens. 2015, 7 3736 in ~76% of the study area. However, ReSALT was less than observed ALT in ~22% of the study area with well-drained soils and in ~1% of the area where soils contained gravel. ReSALT was greater than observed ALT in some drained thermokarst lake basins representing ~1% of the area. These results indicate remote sensing techniques based on InSAR could be an effective way to measure and monitor ALT over large areas on the Arctic coastal plain.
    Remote Sensing 03/2015; 7(4):3735-3759. DOI:10.3390/rs70403735 · 3.18 Impact Factor

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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. DOI:10.1088/1748-9326/9/8/084010 · 3.91 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 08/2014; 119(8). DOI:10.1002/2014JB011156 · 3.44 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. DOI:10.1016/j.icarus.2014.03.018 · 3.04 Impact Factor
  • J.A. Reeves · Rosemary 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 07/2014; 7(7):2992-3001. DOI:10.1109/JSTARS.2014.2322775 · 3.03 Impact Factor
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    ABSTRACT: Titan's surface-atmosphere system bears remarkable similarities to Earth's, the most striking being an active, global methane cycle akin to Earth's water cycle. Like the hydrological cycle of Earth, Titan's seasonal methane cycle is driven by changes in the distribution of solar energy. The Cassini spacecraft, which arrived at Saturn in 2004 in the midst of northern winter and southern summer, has observed surface changes, including shoreline recession, at Titan's south pole and equator. However, active surface processes have yet to be confirmed in the lakes and seas in Titan's north polar region. As the 2017 northern summer solstice approaches, the onset of dynamic phenomena in this region is expected. Here we present the discovery of bright features in recent Cassini RADAR data that appeared in Titan's northern sea, Ligeia Mare, in July 2013 and disappeared in subsequent observations. We suggest that these bright features are best explained by the occurrence of ephemeral phenomena such as surface waves, rising bubbles, and suspended or floating solids. We suggest that our observations are an initial glimpse of dynamic processes that are commencing in the northern lakes and seas as summer nears in the northern hemisphere.
    Nature Geoscience 06/2014; 7(7):493-496. DOI:10.1038/NGEO2190 · 11.74 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 06/2014; 41:3906-3913. DOI:10.1002/2014GL060533 · 4.20 Impact Factor
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    Laura E Erban · Steven M Gorelick · Howard A Zebker · Scott Fendorf ·

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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.
    05/2014; 50(5). DOI:10.1002/2013WR014938

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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.
    03/2014; 41(5). DOI:10.1002/2013GL058618
  • Howard Zebker · Alex Hayes · Mike Janssen · Alice Le Gall · Ralph Lorenz · Lauren Wye ·
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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.
    01/2014; 41(2). DOI:10.1002/2013GL058877
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    Albert C. Chen · Howard 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. DOI:10.1109/TGRS.2012.2236098 · 3.51 Impact Factor

Publication Stats

12k Citations
779.01 Total Impact Points


  • 1992-2014
    • Stanford University
      • • Department of Electrical Engineering
      • • Department of Geophysics
      Palo Alto, California, United States
  • 1988-2006
    • California Institute of Technology
      • Jet Propulsion Laboratory
      Pasadena, California, United States