Brent Minchew

Polarimetric Analysis of Backscatter From the Deepwater Horizon Oil Spill Using L-Band Synthetic Aperture Radar

We analyze the fully-polarimetric Uninhabited Aerial Vehicle Synthetic Aperture Radar (UAVSAR) data acquired on June 23, 2010, from two adjacent, overlapping flight tracks that imaged the main oil slick near the Deepwater Horizon (DWH) rig site in the Gulf of Mexico. Our results show that radar backscatter from both clean water and oil in the slick is predominantly from a single surface scatterer, consistent with the tilted Bragg scattering mechanism, across the range of incidence angles from 26° to 60°. We show that the change of backscatter over the main slick is due both to a damping of the ocean wave spectral components by the oil and an effective reduction of the dielectric constant resulting from a mixture of 65-90% oil with water in the surface layer. This shows that synthetic aperture radar can be used to measure the oil volumetric concentration in a thick slick. Using the H/A/α parameters, we show that surface scattering is dominant for oil and water whenever the data are above the noise floor and that the entropy (H) and α parameters for the DWH slick are comparable to those from the clean water. The anisotropy, A, parameter shows substantial variation across the oil slick and a significant range-dependent signal whenever the backscatter in all channels is above the instrument noise floor. For slick detection, we find the most reliable indicator to be the major eigenvalue of the coherency matrix, which is approximately equal to the total backscatter power for both oil in the slick and clean sea water.

A physical model for seismic noise generation from sediment transport in rivers

Measuring sediment flux in rivers remains a significant problem in studies of landscape evolution. Recent studies suggest that observations of seismic noise near rivers can help provide such measurements, but the lack of models linking observed seismic quantities to sediment flux has prevented the method from being used. Here, we develop a forward model to describe the seismic noise induced by the transport of sediment in rivers. The model provides an expression for the power spectral density (PSD) of the Rayleigh waves generated by impulsive impacts from saltating particles which scales linearly with the number of particles of a given size and the square of the linear momentum. After incorporating expressions for the impact velocity and rate of impacts for fluvially transported sediment, we observe that the seismic noise PSD is strongly dependent on the sediment size, such that good constraints on grain size distribution are needed for reliable estimates of sediment flux based on seismic noise observations. The model predictions for the PSD are consistent with recent measurements and, based on these data, a first attempt at inverting seismic noise for the sediment flux is provided.

Determining the mixing of oil and sea water using polarimetric synthetic aperture radar

Knowledge of the characteristics of spilled oil in the ocean is important for cleanup operations, predictions of the impact on wildlife, and studies of the nature of the ocean surface and currents. Herein I discuss a method for evaluating the characteristics of oil in a marine environment using synthetic aperture radar (SAR) and present a new, simple classification, called the oil/water mixing index (Mdex), to quickly assess the results. I link the Mdex results to the Bonn Agreement for Oil Appearance Codes (BAOAC) for aerial observers and demonstrate the Mdex on Uninhabited Aerial Vehicle SAR (UAVSAR) data collected June 23, 2010 over the former site of the Deepwater Horizon (DWH) drilling rig. The Mdex map shows a more heterogeneous oil swath than do radar backscatter images and features within the oil are consistent with features present in previously published, near-coincident optical imagery. The Mdex results indicate that most of the oil near the DWH was mixed with sea water to a minimum depth of a few millimeters, though some areas containing relatively thin films are observed.

Fault-zone controls on the spatial distribution of slow-moving landslides

Slow-moving landslides (earthflows) can dominate hillslope sediment flux and landscape erosion in hilly terrain with mechanically weak, fine-grained rock. Controls on the occurrence of slow-moving landslides are poorly constrained and need to be understood for landscape evolution models, sediment budgets, and infrastructure and hazards planning. Here, we use airborne interferometric synthetic aperture radar (InSAR) and aerial photographs to document 150 previously unidentified active earthflows along the central, creeping portion of the San Andreas fault, California. The earthflows move seasonally in response to winter rainfall, occur on hillslopes at ∼20%–40% gradients (less than typically associated with rapid, catastrophic landslides), and have similar morphological characteristics to earthflows in different climatic and tectonic settings. Although our data extend up to 10 km from the fault trace, ∼75% of detected landslides occur within 2 km of the active fault. Topographic, precipitation, and rock type metrics alone are not enough to explain the observed spatial distribution of earthflows. Instead, we hypothesize that earthflows cluster near the creeping San Andreas fault because of a fault-induced zone of reduced bulk-rock strength that increases hillslope susceptibility to failure. In addition, similar lithology, topography, and climate exist north of the creeping section of the fault, yet earthflows there are rare. This may be due to large-magnitude earthquakes episodically triggering coseismic rapid landslides, which preferentially remove weak rock from the fault damage zone. Our analysis suggests that the necessary conditions for earthflow formation in central California include some combination of reduced rock strength, fine-grained sedimentary rock, threshold precipitation and relief, and possibly the absence of large-magnitude earthquakes. These conditions likely hold for earthflow development in other areas, and our work suggests that local variations in rock strength and seismicity, such as those associated with fault zones, need to be taken into account in order to accurately predict earthflow occurrence.

On the Synergistic Use of SAR Constellations’ Data Exploitation for Earth Science and Natural Hazard Response

Several current and expected future SAR satellites missions (e.g., TanDEM-X (TDX)/PAZ, COSMO-SkyMed (CSK), and Sentinel-1A/B) are designed as constellations of SAR sensors. Relative to single satellite systems, such constellations can provide greater spatial coverage and temporal sampling, thereby enabling better control on interferometric decorrelation and lower latency data access. These improvements lead to more effective near real-time disaster monitoring, assessment and response, and a greater ability to constrain dynamically changing physical processes. Using observations from the CSK system, we highlight examples of the potential for such imaging capabilities to enable advances in Earth science and natural hazards response.

Tidally induced variations in vertical and horizontal motion on Rutford Ice Stream, West Antarctica, inferred from remotely sensed observations

To better understand the influence of stress changes over floating ice shelves on grounded ice streams, we develop a Bayesian method for inferring time-dependent 3-D surface velocity fields from synthetic aperture radar (SAR) and optical remote sensing data. Our specific goal is to observe ocean tide-induced variability in vertical ice shelf position and horizontal ice stream flow. Thus, we consider the special case where observed surface displacement at a given location can be defined by a 3-D secular velocity vector, a family of 3-D sinusoidal functions, and a correction to the digital elevation model used to process the SAR data. Using nearly 9 months of SAR data collected from multiple satellite viewing geometries with the COSMO-SkyMed 4-satellite constellation, we infer the spatiotemporal response of Rutford Ice Stream, West Antarctica, to ocean tidal forcing. Consistent with expected tidal uplift, inferred vertical motion over the ice shelf is dominated by semidiurnal and diurnal tidal constituents. Horizontal ice flow variability, on the other hand, occurs primarily at the fortnightly spring-neap tidal period (M_(sf)). We propose that periodic grounding of the ice shelf is the primary mechanism for translating vertical tidal motion into horizontal flow variability, causing ice flow to accelerate first and most strongly over the ice shelf. Flow variations then propagate through the grounded ice stream at a mean rate of ∼29 km/d and decay quasi-linearly with distance over ∼85 km upstream of the grounding zone.

On the Use of Simulated Airborne Compact Polarimetric SAR for Characterizing Oil–Water Mixing of the Deepwater Horizon Oil Spill

Compact polarimetry (CP) synthetic aperture radar (SAR) is a form of coherent dual-pol SAR that has been shown to have great potential for maritime surveillance applications such as ship and ice detection. In this paper, we demonstrate the potential of CP data for oil spill characterization. As the availability of CP data is limited at this time, we simulate CP image data from UAVSAR L-Band quad-polarized images. We reconstruct quad-pol SAR data (termed pseudo-quad) from these simulated CP SAR data, and calculate an oil-water mixing index, termed Mdex. We show that the differences between the pseudo-quad and quad-pol Mdex maps are negligible. This contributes to the case that CP SAR has great potential for multiple applications in maritime surveillance.

Polarimetric decomposition analysis of the Deepwater Horizon oil slick using L-band UAVSAR data

We report here an analysis of the polarization-dependence of L-band radar backscatter from the main slick of the Deepwater Horizon oil spill, with specific attention to the utility of polarimetric decomposition analysis for discrimination of oil from clean water and identification of variations in the oil characteristics. For this study we used data collected with the UAVSAR instrument from opposing look directions directly over the main oil slick. We find that both the Cloude-Pottier and Shannon entropy polarimetric decomposition methods offer promise for oil discrimination, with the Shannon entropy method yielding the same information as contained in the Cloude-Pottier entropy and averaged intensity parameters, but with significantly less computational complexity.

Multiple glacier surges observed with airborne and spaceborne interferometric synthetic aperture radar

Mechanical properties of glacier beds impose fundamental constraints on glacier flow across a wide range of timescales [1]. Despite their importance in governing glacier dynamics, basal mechanics are not well understood, particularly where glaciers are underlain by deformable till [2]. While some till samples have been retrieved from beneath several glaciers and tested in laboratories in order to ascertain till rheology [3, 4], limitations on clast sizes imposed by apparatus dimensions and the difficulty of understanding and reproducing subglacial environments in the lab necessitate observations of the mechanical properties of in situ tills [5]. Such observations are sparse, owing to the inherent difficulty in attaining them, and this observational paucity has helped foment persistent uncertainties concerning the proper rheology of subglacial till and the rheological dependence on mechanical, thermal, and hydrological forcing [1, 2].

Geodetic Imaging of Time-Dependent Three-Component Surface Deformation: Application to Tidal-Timescale Ice Flow of Rutford Ice Stream, West Antarctica

We present a method for inferring time-dependent three-component surface deformation fields given a set of geodetic images of displacements collected from multiple viewing geometries. Displacements are parameterized in time with a dictionary of displacement functions. The algorithm extends an earlier single-component (i.e., single line of sight) framework for time-series analysis to three spatial dimensions using combinations of multitemporal, multigeometry interferometic synthetic aperture radar (InSAR) and/or pixel offset (PO) maps. We demonstrate this method with a set of 101 pairs of azimuth and range PO maps generated for a portion of the Rutford Ice Stream, West Antarctica, derived from data collected by the COSMO-SkyMed satellite constellation. We compare our results with previously published InSAR mean velocity fields and selected GPS time series and show that our resulting three-component surface displacements resolve both secular motion and tidal variability.