48 research outputs found
Entropy-Bayesian Inversion of Time-Lapse Tomographic GPR data for Monitoring Dielectric Permittivity and Soil Moisture Variations
No full textIn this study, we evaluate the possibility of monitoring soil moisture variation using tomographic ground penetrating radar travel time data through Bayesian inversion, which is integrated with entropy memory function and pilot point concepts, as well as efficient sampling approaches. It is critical to accurately estimate soil moisture content and variations in vadose zone studies. Many studies have illustrated the promise and value of GPR tomographic data for estimating soil moisture and associated changes, however, challenges still exist in the inversion of GPR tomographic data in a manner that quantifies input and predictive uncertainty, incorporates multiple data types, handles non-uniqueness and nonlinearity, and honors time-lapse tomograms collected in a series. To address these challenges, we develop a minimum relative entropy (MRE)-Bayesian based inverse modeling framework that non-subjectively defines prior probabilities, incorporates information from multiple sources, and quantifies uncertainty. The framework enables us to estimate dielectric permittivity at pilot point locations distributed within the tomogram, as well as the spatial correlation range. In the inversion framework, MRE is first used to derive prior probability distribution functions (pdfs) of dielectric permittivity based on prior information obtained from a straight-ray GPR inversion. The probability distributions are then sampled using a Quasi-Monte Carlo (QMC) approach, and the sample sets provide inputs to a sequential Gaussian simulation (SGSim) algorithm that constructs a highly resolved permittivity/velocity field for evaluation with a curved-ray GPR forward model. The likelihood functions are computed as a function of misfits, and posterior pdfs are constructed using a Gaussian kernel. Inversion of subsequent time-lapse datasets combines the Bayesian estimates from the previous inversion (as a memory function) with new data. The memory function and pilot point design takes advantage of the spatial-temporal correlation of the state variables. We first apply the inversion framework to a static synthetic example and then to a time-lapse GPR tomographic dataset collected during a dynamic experiment conducted at the Hanford Site in Richland, WA. We demonstrate that the MRE-Bayesian inversion enables us to merge various data types, quantify uncertainty, evaluate nonlinear models, and produce more detailed and better resolved estimates than straight-ray based inversion; therefore, it has the potential to improve estimates of inter-wellbore dielectric permittivity and soil moisture content and to monitor their temporal dynamics more accurately
An Efficient Algorithm for the Synthesis of Laser Altimetry Waveforms
No full textAirborne and spaceborne laser ranging is an emerging technology for capturing data on physical surfaces. An ever increasing number of applications takes advantage of the dense sampling, the high accuracy, and the direct way to obtain 3-D surface points that characterizes laser ranging methods. Most laser altimeters estimate the range to the terrain surface by measuring the round-trip time of flight of a short laser pulse. Laser systems are suitable sensors for rapid surface reconstruction since surface points can easily be computed from the measured range, and from the position and altitude measurements. The small illuminated footprint size and the high precision time measurements ensure reliable ranging and sub-dm vertical range resolution even from orbital altitudes. The heart of the laser system is the laser ranging. The accuracy of the timing (or ranging) performance is governed by the characteristics of the laser system and terrain surface, as well as the waveform processing. Closed form expression describing the shape of the return waveform have been derived only for simple cases, such as flat or uniformly sloping terrain (Gardner, 1982). Laser altimetry simulators are developed to explore the relationship between the altimeter's design, performance and terrain characteristics for more complex surface topography. The Laser altimetry Simulator developed at NASA Goddard Space Flight Center (Abshire et al., 1984) provides a mean for computing the receiver waveform for user defined, arbitrary two-dimensional (height versus along track distance) terrain geometry and for a few, simple 3-D surface types. To investigate the performance of laser systems over complex topography we developed a 3D space-time laser altimetry simulator, which we call Byrd Polar Research Center (BPRC) simulator. This report describes the BPRC simulator. First a brief description of laser altimeter systems is presented followed by the discussion of the characteristics of the returned laser pulse shape for simple surfaces and ranging schemes. The algorithm implemented in the BPRC simulator for waveform computation is summarized in the third chapter. Returned waveforms are presented and compared with results obtained by the NASA/GSFC simulator in the last, fourth chapter
An Algorithm And Application For Visualization And Analysis Of Scanning Laser Altimeter Data
No full textIce sheet elevation changes have been measured by repeat airborne laser altimetery in Greenland since 1991. The Airborne Topographic Mapper (ATM) system, which has been mounted in a NASA P-3 aircraft, includes a scanning laser altimeter, INS and differential GPS. During the post-processing the measured data are converted into measurements of ice sheet elevation relative to the Earth ellipsoid (Krabill et aI., 1995). Since laser scanner data sets are composed of a huge amount of points, obtaining the swath contour and locating overlapping areas between different swaths in an efficient way is not a simple task. To make this task more feasible a thinned data set called ICESS has been created from the laser data collected over the Greenland ice sheet (Martin, 1997). Since the surface is reasonably smooth, the laser swath was modeled as a series of planes, each characterized by a center elevation, a north-south slope, and an east-west slope. However, for many applications such as mapping the rough surface of outlet glaciers or surveying urban areas, the original dense laser points are needed. Our major goal is to develop software to access, display, and manipulate the original binary data files. By using a single application the user should be able to determine the overlap between several surveys, compute elevation changes, and create OEMs and contour maps. So far we were mainly focusing on the first part of the job, namely, providing tools for locating the laser swaths and their overlapping areas.
In this report we present the algorithm that we recommend for an efficient contour extraction from laser swaths data and the application that was developed for this purpose. The report is organized as follows. First the major objectives of the development of the algorithm are presented and then the algorithm itself is described. The fourth chapter describes the application functionality, and it is followed by a documentation of the software modules and the installation guide
Investigating Long-term Behavior of Outlet Glaciers in Greenland
No full textRepeat surveys by airborne laser altimetry in the 1990s have revealed significant thinning of outlet glaciers draining the interior of the Greenland Ice Sheet, with thinning rates up to several meters per year. To fully appreciate the significance of these recent glacier changes, the magnitude of retreat and surface lowering must be placed within the broader context of the retreat since the Last Glacial Maximum and, more significantly, of the retreat following the temporary glacier advance during the Little Ice Age (LIA). The LIA maximum stand is marked by trimlines, sharp boundaries between recently deglacifated unvegetated rocks, and vegetated surfaces at higher elevations. The objective of this project was to demonstrate the use of remote sensing data to map these trimlines and other glacial geomorphologic features
Response of a marine-terminating Greenland outlet glacier to abrupt cooling 8200 and 9300 years ago
No full textLong-term records of Greenland outlet-glacier change extending beyond the satellite era can inform future predictions of Greenland Ice Sheet behavior. Of particular relevance is elucidating the Greenland Ice Sheet's response to decadal- and centennial-scale climate change. Here, we reconstruct the early Holocene history of Jakobshavn Isbræ, Greenland's largest outlet glacier, using 10Be surface exposure ages and 14C-dated lake sediments. Our chronology of ice-margin change demonstrates that Jakobshavn Isbræ advanced to deposit moraines in response to abrupt cooling recorded in central Greenland ice cores ca. 8,200 and 9,300 years ago. While the rapid, dynamically aided retreat of many Greenland outlet glaciers in response to warming is well documented, these results indicate that marine-terminating outlet glaciers are also able to respond quickly to cooling. We suggest that short lag times of high ice flux margins enable a greater magnitude response of marine-terminating outlets to abrupt climate change compared to their land-terminating counterparts
