TOPSAR Data Research Articles

Geological Mapping in Western Tasmania Using Radar and Random Forests

Mineral exploration and geological mapping of highly prospective areas in western Tasmania, southern Australia, is challenging due to steep topography, dense vegetation, and limited outcrop. Synthetic aperture radar (SAR) can potentially penetrate vegetation canopies and assist geological mapping in this environment. This study applies manual and automated lithological classification methods to airborne polarimetric TopSAR and geophysical data in the Heazlewood region, western Tasmania. Major discrepancies between classification results and the existing geological map generated fieldwork targets that led to the discovery of previously unmapped rock units. Manual analysis of radar image texture was essential for the identification of lithological boundaries. Automated pixel-based classification of radar data using Random Forests achieved poor results despite the inclusion of textural information derived from gray level co-occurrence matrices. This is because the majority of manually identified features within the radar imagery result from geobotanical and geomorphological relationships, rather than direct imaging of surficial lithological variations. Inconsistent relationships between geology and vegetation or geology and topography limit the reliability of TopSAR interpretations for geological mapping in this environment. However, Random Forest classifications, based on geophysical data and validated against manual interpretations, were accurate (∼90%) even when using limited training data (∼0.15% of total data). These classifications identified a previously unmapped region of mafic–ultramafic rocks, the presence of which was verified through fieldwork. This study validates the application of machine learning for geological mapping in remote and inaccessible localities but also highlights the limitations of SAR data in thickly vegetated terrain.

Three dimensional reconstruction of coastal bathymetry using TOPSAR c-band

This paper reports on a study carried out for the generation of three-dimensional (3D) ocean bathymetry using airborne TOPSAR data. The main objective of this study is to utilize fuzzy arithmetic for constructing ocean bathymetry from polarized remote sensing data such as a TOPSAR image. In doing so, two three-dimensional surface models, the Volterra model and a fuzzy B-spline model which construct a global topological structure between the data points were used to support an approximation of real surface. The best reconstruction of coastal bathymetry of the test site in Kuala Terengganu, Malaysia, was obtained with polarised C bands TOPSAR acquired with VV polarization. With 10 m spatial resolution of TOPSAR data, an accuracy of (root mean square) ± 0.03 m was found with Cvv band. Key words: Topographic synthetic aperture radar data, fuzzy B-spline model, Volterra model, three-dimensional ocean bathymetry.

An Imaging Approach for TOPSAR Data Based on Azimuth Pre-processing and Post-processing

Burst工作模式和方位波束主动扫描使得TOPSAR(Terrain Observation by Progressive scans SAR)回波信号同时具备ScanSAR和聚束模式回波信号特点，即方位输出时间混叠和多普勒频谱混叠。虽然时域和频域同时升采样操作能够解决混叠问题，但同时带来成像运算量明显提高和内存消耗显著增大的问题。针对TOPSAR回波信号特殊的方位时频关系，该文提出一种结合方位向预处理和后处理的TOPSAR成像方法。该方法利用复制拼接和去斜的方法来解决混叠问题，利用信号截取操作剔除干扰信号，同时还可以减少采样点数，提高算法运算效率。点目标和分布目标的仿真结果验证了这种成像方法的有效性。

TOPSAR data focusing based on azimuth scaling preprocessing

Both Doppler spectral aliasing and azimuth output time folding simultaneously exist in TOPSAR (Terrain Observation by Progressive Scans) raw data. Resampling in both Doppler frequency and azimuth time domain can resolve the azimuth aliasing problem but with the seriously increased computational complexity and memory consumption. According to the special characteristics of TOPSAR raw data support in the slow time/frequency domain (TFD), the presented azimuth scaling preprocessing step is introduced to not only resolve the Doppler spectral aliasing problem but also reduce the increased azimuth samples. Furthermore, the correction of sawtoothed azimuth antenna pattern (AAP) becomes easy to be implemented. The following conventional stripmap processor can be adopted to focus the residual TOPSAR raw data but with the result of azimuth aliased TOPSAR image. The mosaic approach, which has been presented to unfold azimuth aliased ScanSAR image, is exploited to resolve the problem of azimuth output folding in TOPSAR mode. Simulation results and pulse response parameters are given to validate the presented imaging approach.

3-D visualizations of coastal bathymetry by utilization of airborne TOPSAR polarized data

Multi-frequency C and L bands in the TOPSAR data have been utilized to reconstruct three-dimensional (3-D) bathymetry pattern. The main objective of this study is to utilize fuzzy arithmetic to reduce the errors arising from speckle in synthetic aperture radar (SAR) data when constructing ocean bathymetry from polarized SAR data. In doing so, two 3-D surface models, the Volterra algorithm and a fuzzy B-spline (FBS) algorithm, which construct a global topological structure between the data points, were used to support an approximation to the real surface. Volterra algorithm was used to express the non-linearity of TOPSAR data intensity gradient based on the action balance equation (ABC). In this context, a first-order kernel of Volterra algorithm was used to express ABC equation. The inverse of Volterra algorithm then performed to simulate 2-D current velocities from CVV and LHH band. Furthermore, the 2-D continuity equation then used to estimate the water depth. In order to reconstruct 3-D bathymetry pattern, the FBS has been performed to water depth information which was estimated from 2-D continuity equation. The best reconstruction of coastal bathymetry of the test site in Kuala Terengganu, Malaysia, was obtained with polarized L and C bands SAR acquired with HH and VV polarizations, respectively. With 10 m spatial resolution of TOPSAR data, bias of –0.004 m, the standard error mean of 0.023 m, r 2 value of 0.95, and 90% confidence intervals in depth determination was obtained with LHH band.

Model-Based Estimation of Forest Canopy Height in Red and Austrian Pine Stands Using Shuttle Radar Topography Mission and Ancillary Data: A Proof-of-Concept Study

In this paper, accurate tree stand height retrieval is demonstrated using C-band Shuttle Radar Topography Mission (SRTM) height and ancillary data. The tree height retrieval algorithm is based on modeling uniform tree stands with a single layer of randomly oriented vegetation particles. For such scattering media, the scattering phase center height, as measured by SRTM, is a function of tree height, incidence angle, and the extinction coefficient of the medium. The extinction coefficient for uniform tree stands is calculated as a function of tree height and density using allometric equations and a fractal tree model. The accuracy of the proposed algorithm is demonstrated using SRTM and TOPSAR data for 15 red pine and Austrian pine stands (TOPSAR is an airborne interferometric synthetic aperture radar). The algorithm yields root-mean-square (rms) errors of 2.5-3.6 m, which is a substantial improvement over the 6.8-8.3-m rms errors from the raw SRTM minus National Elevation Dataset Heights.

Quality of TOPSAR topographic data for volcanology studies at Kilauea Volcano, Hawaii: An assessment using airborne lidar data

We use airborne lidar data for the summit area of Kilauea Caldera, Hawaii, to explore the utility of topographic data collected by the TOPSAR airborne interferometric radar for volcanology studies. The lidar data are processed to a spatial resolution of 1 m/pixel, compared to TOPSAR with a spatial resolution of 5 m. Over a variety of fresh volcanic surfaces (pahoehoe and aa lava flows, ash falls and fluvial fans), TOPSAR data are shown to have a typical vertical offset compared to the lidar data of no more than ∼2–3 m. Larger differences between the two data sets and TOPSAR data drop-outs are found to be concentrated around steep scarps such as the walls of pit craters and ground cracks associated with the Southwest Rift Zone. A comparison of these two data sets is used to explore the utility of TOPSAR to interpret the topography of volcanic features close to the spatial resolution of TOPSAR, such as spatter ramparts, fractures, a perched lava flow, and eroded ash deposits. Comparison of the TOPSAR elevation and the lidar first-return minus the return from the ground surface (the so-called “bald Earth” data) for vegetated areas reveals TOPSAR penetration into the tree canopy is typically at least 10% and no more than ∼50%, although a wide range of penetration values from 0% to 90% has been identified. Our results are significant because they show that TOPSAR data for volcanoes can reliably be used to measure regional slopes and the thickness of lava flows, and have value for the validation of coarser spatial resolution digital elevation data (such as SRTM) in areas where lidar data have not been collected.

The effect of savanna fires on SAR backscatter in northern Australia

This paper investigates the potential for utilizing multi-frequency, quad-polarized Synthetic Aperture Radar (SAR) for delineation of fire scars in the tropical savannas of northern Australia. Fire regimes and regional land management in the study area are contentious issues for the multiple stakeholders. Thus, the ability to accurately map fire scars and assess impacts on the mosaic of vegetation community types is crucial for effective landscape management. In this study identification of a fire scar resulting from a late dry season fire was assessed using TopSAR data. The assessment was achieved through comparison with optical datasets acquired both pre-burn and post-burn. By examining five representative vegetation communities, it was concluded that only C-band SAR data were affected sufficiently by fire in this environment to detect and map fire scars. Despite the intensity of the fire event, the resistance of vegetation communities to fire damage resulted in insignificant changes to the L- and P-band SAR data. Further investigation is required to determine if this behaviour can be exploited to improve the above-ground woody biomass mapping currently reliant on optical data.

AIRSAR/TOPSAR Image Processing and Interpretation: An Application Example in Far West Texas, U.S.A.

JPL/NASA's AIRSAR/TOPSAR is a multipolarimetric, multifrequency, and interferometric airborne synthetic aperture radar capable of imaging in C‐, L‐, and P‐ bands (5.7, 24.5, and 68 cm) with 10 m spatial resolution. This paper introduces and explains 5 conversion processes that users need to apply to data products received from JPL's integrated AIRSAR processor. Numerical algorithms and approaches to remove both speckle and banding noises that are common in AIRSAR data have been investigated and evaluated. Among statistical adaptive speckle removal algorithms, G‐MAP (Gamma Maximum A Posteriori) had the best performance. For banding removal, a new method that we call combined principal components analysis (CPCA) has been developed. This method is proved very effective with our data. Radar signature differences were studied and compared at different wavelengths and between radar and optical (ETM+) images. Some subsurface objects (like water pipelines) were clearly visible in the radar images, especially from the P‐ band. Data fusion based on the color transform technique was employed to integrate Landsat 7 (30 m ETM+ data fused with the accompanying 15 m panchromatic data) and TOPSAR data after speckle and banding removal. The resulting fused image (10 m) brought out new features that were not evident in the original images and helped identify many features whose origin was not clear in the original images.

Volumetric characteristics of lava flows from interferometric radar and multispectral satellite data: the 1995 Fernandina and 1998 Cerro Azul eruptions in the western Gal�pagos

We have used a suite of remotely sensed data, numerical lava flow modeling, and field observations to determine quantitative characteristics of the 1995 Fernandina and 1998 Cerro Azul eruptions in the western Galapagos Islands. Flank lava flow areas, volumes, instantaneous effusion rates, and average effusion rates were all determined for these two eruptions, for which only limited syn-eruptive field observations are available. Using data from SPOT, TOPSAR, ERS-1, and ERS-2, we determined that the 1995 Fernandina flow covers a subaerial area of 6.5×106 m2 and has a subaerial dense rock equivalent (DRE) volume of 42×106 m3. Field observations, ATSR satellite data, and the FLOWGO numerical model allow us to determine that the effusion rate declined exponentially from a high of ~60–200 m3 s-1 during the first few hours to <5 m3 s-1 prior to ceasing after 73 days, with a mean effusion rate of 4–16 m3 s-1. Integrating the ATSR-derived, exponentially declining effusion rate over the eruption duration produces a total (subaerial + submarine) DRE volume of between 27 and 100×106 m3, the range in values being due to differing assumptions about heat loss characteristics; only values in the higher part of this range are consistent with the independently derived subaerial volume. Using SPOT, TOPSAR, ERS-1, and ERS-2 data, we determine that the 1998 Cerro Azul flow is 16 km long, covers 16 km2, and has a DRE volume of 54×106 m3. FLOWGO produces at-vent velocity and effusion rate values of 11 m s-1 and ~600 m3 s-1, respectively. The velocity value agrees well with the 12 m s-1 estimated in the field. The mean effusion rate (total DRE volume/duration) was 7–47 m3 s-1. Dike dimensions, fissure lengths, and pressure gradients along the conduit based on magma chamber depth estimates of 3–5 km produce mean effusion rates for the two eruptions that range over nearly four orders of magnitude, the range being due to uncertainty in the magma viscosity, dike dimensions, and pressure gradient between magma chamber and vent. Although somewhat consistent with mean effusion rates from other techniques, their wide range makes them less useful. The exponentially declining effusion rates during both eruptions are consistent with release of elastic strain being the driving mechanism of the eruptions. Our results provide independent input parameters for previously published theoretical relationships between magma chamber pressurization and eruption rates that constrain chamber volumes and increases in volume prior to eruption, as well as time constants of exponential decay during the eruption. The results and theoretical relationships combine to indicate that at both volcanoes probably 25–30% of the volumetric increase in the magma chamber erupted as lava onto the surface. In both eruptions the lava flow volumes are less than 1% of the magma chamber volume.

Fusion of hyperspectral and radar data using the IHS transformation to enhance urban surface features

The Intensity–Hue–Saturation (IHS) transformation is used to integrate the high spectral resolution, provided by hyperspectral data (Airborne Visible Infrared Imaging Spectrometer, AVIRIS), and the surface texture information, derived from radar data (Topographic Synthetic Aperture Radar, TOPSAR), into a single image of an urban area. This transformed image is superimposed on the Digital Elevation Model (DEM) data derived from TOPSAR data to create a 3D perspective view. The ambiguity of several urban land cover types is resolved to a larger degree using the higher spectral and spatial resolutions and the synergistic visual content provided by the fused image in a 3D perspective. For urban areas that are at risk from geological hazards (e.g., avalanches, mudflows, and debris flows), the fused image provides a cost-effective product, rich in the information necessary for assessment and mitigation of these hazards.

Application of high-resolution, interferometric DEMs to geomorphic studies of fault scarps, Fish Lake Valley, Nevada–California, USA

Recent developments in interferometric radar remote sensing provide a method for deriving detailed topographic and slope information for fault-scarp detection. In October 1996, the airborne TOPSAR instrument [IEEE Trans. Geosci. Remote Sens., 30 (1992) 933] was flown over southwest Nevada and east-central California. Topography calculated from TOPSAR data are in the form of a high-resolution (5-m spatial grid) digital elevation model (DEM). In this study, we focus upon the large, steep fault scarps that cut alluvium and alluvial fans in Fish Lake Valley, east of the White Mountains of Nevada–California. A series of topographic profiles extracted from the DEM reveal that the larger fault scarps are greater than 40 m in height and that the average midsection slope angle for all measured scarps is approximately 23°. These large scarps are the product of multiple offsets rather than a single event. Other relevant geomorphic features present in the digital topography include splays and benches along the main fault, levees, cutbanks, gullies incising fault scarp slopes, shutterridges, offset drainage, and small normal faults with scarp heights of only 4–6 m. Field work corroborated general geomorphologic landforms, confirmed fault-scarp morphometry, and aided the evaluation of the accuracy of the DEM. We are also able to assess fault segmentation models that divide the Fish Lake Valley fault zone (FLVFZ) into discrete segments based upon surface-rupture characteristics.

TOPSAR wave spectra model and coastal erosion detection

This paper presents work done utilizing TOPSAR data to detect shoreline change along the Terengganu coast (Malaysia). TOPSAR data were used to extract information on wave spectra. This wave spectra information was then used to model shoreline changes by investigating the wave refraction patterns. From these patterns, the volume transport at several locations was estimated. The shoreline change model developed was designed to cover a 20 km stretch of shoreline of Kuala Terengganu. The model utilized data from aerial photographs, TOPSAR data and ground truth data. The location of sedimentation and erosion along the shoreline of Kuala Terengganu was estimated. The wave spectra extracted from TOPSAR data showed wavelengths ranging from 20 m to 175 m. The main direction of the waves given by the spectra was from the northeast. The wave refraction patterns varied, showing both convergence and divergence, indicating erosion and sedimentation locations, respectively. A comparison between the TOPSAR shoreline change model and aerial photographs and ground truth data showed a significant relationship. Finally, the regression model showed that erosion occurred particularly at Sultan Mahmed Airport, at a rate of −1.5 m/year. The maximum rate of sedimentation along the 20 km stretch was 1 m/year.

Fusion of airborne polarimetric and interferometric SAR for classification of coastal environments

AIRSAR and TOPSAR data were acquired over the wetlands of Bolivar Peninsula along the Gulf coast of Texas for mapping land cover types and topographic features such as beach ridges, dunes, and relict storm features. Classification of land cover over this wetlands and uplands environment is difficult because of the similarity of spectral signatures of the vegetation types. In addition, because the distribution of vegetation communities in coastal marshes is strongly related to salinity, which in turn is largely dictated by frequency and duration of inundation, surface topography is critical to determination of the vegetation characteristics at any location. The potential advantages of multisensor classification, including, in particular, topographic information from a TOPSAR DEM are investigated. An approach which employs a class dependent feature selection procedure in conjunction with pairwise Bayesian classifiers is proposed and applied to the polarimetric and interferometric SAR data.

Comparison of Greenland Ice Sheet topography measured by TOPSAR and airborne laser altimetry

A digital elevation model (DEM) produced from airborne interferometric synthetic aperture radar (TOPSAR) data of a 5 km/spl times/65 km region of the Greenland Ice Sheet is compared with multiple airborne laser altimeter profiles of the same surface. TOPSAR-derived elevation values were calculated on a 10 m/spl times/10 m grid. Surface topography at wavelengths greater than one kilometer is accurately reproduced. A planar adjustment of -8 m with along-track and cross-track tilts of 0.17 and 2.4 m/km, respectively, is required to match the two elevation data sets. Residual noise of the TOPSAR data at subkilometer wavelengths is less than two meters. This study underscores the need for independent elevation data for controlling TOPSAR elevation surveys.

Slopes, lava flow volumes, and vent distributions on Volcán Fernandina, Galápagos Islands

Digital elevation data from TOPSAR, an airborne synthetic aperture radar system that uses interferometry to derive topography, are used to determine slope distributions and lava flow thicknesses on Fernandina Volcano, Galápagos Islands. Four extracaldera slope regions are defined (from the coast inland): A coastal plain (average slope ∼2°), an apron (6°–12°), steep slopes 250–600 m high (20°–43°), and a 0.5‐ to 1‐km‐wide summit platform (∼10°). Lava flows and vents are mapped using Shuttle Imaging Radar‐C (SIR‐C), SPOT, Landsat Thematic Mapper (TM), and air photos. Over 500 flows are identified and categorized as young, intermediate, or old based on albedo, vegetation cover, and margin preservation. By area, young flows constitute 55% of the island, 34% are intermediate, and 7% are old. The aa:pahoehoe ratio of the young flows is 85:15, whereas for the intermediate flows it is 58:42. Of the 423 vents that were classified, 236 are radial and 80% of these are in the apron, 143 are arcuate and 94% of these are in the summit platform, and 46 have transitional orientations and are about equally divided between the steep slopes and summit platform; 95% of all vents are within 13 km of the caldera center. TOPSAR data allow flow volumes to be estimated, and young flows range from &lt;0.01 to 0.12 km3, with a total volume of 2.3 km3. By volume, 91% of the young lava erupted from radial vents below the steep slopes, many of which are concentrated within the SE apron about 5–6 km from the caldera. Similar concentrations to the NE, NW, and SW consist of young and intermediate flows. Different proportions of lava flows and vents form the different slope regions; the coastal plain averages 0.1 vents/km2 and the slightly steeper apron averages 0.6 to 0.9 vents/km2, increasing inland. The summit platform averages 4.7 vents/km2, and this concentration supports previously proposed mechanisms for producing higher elevations and steeper slopes in the central part of the volcano. Temporal changes in the plumbing system and/or magma supply, rate are suggested by the change in aa:pahoehoe ratio; it appears that in the past, low effusion rate eruptions were more common (perhaps from a filled caldera), whereas more recently, high effusion rate eruptions have dominated.

Digital topography of volcanoes from radar interferometry: an example from Mt Vesuvius, Italy

A new airborne radar technique can generate digital topographic data for volcanoes at a scale of 10 m spatial and 1–5 m vertical, with a swath width of ∼6.4 km. Called TOPSAR, the intrument is an interferometric radar flown on the NASA DC-8 aircraft. TOPSAR data permit the quantification of volcano slopes, volumes, and heights, and as such will be valuable for the analysis of lava flows, domes, and lahar channels. This instrument will be flown over several volcanoes in the near future, providing volcanologists with valuable data sets for the analysis of high-resolution topography. We briefly illustrate the potential use of TOPSAR data through examples from Mt Somma and Vesuvius, Italy.