Verification of the virtual bandwidth SAR (VB-SAR) scheme for centimetric resolution subsurface imaging from space

This work presents the first experimental demonstration of the virtual bandwidth synthetic aperture radar (VB-SAR) imaging scheme. VB-SAR is a newly-developed subsurface imaging technique which, in stark contrast to traditional close-proximity ground penetrating radar (GPR) schemes, promises imaging from remote standoff platforms such as aircraft and satellites. It specifically exploits the differential interferometric synthetic aperture radar (DInSAR) phase history of a radar wave within a drying soil volume to generate high- resolution vertical maps of the scattering through the soil volume. For this study, a stack of C-band VV polarisation DInSAR images of a sandy soil containing a buried target was collected in the laboratory whilst the soil moisture was varied - firstly during controlled water addition, and then during subsequent drying. The wetting image set established the moisture-phase relationship for the soil, which was then applied to the drying DInSAR image set using the VB-SAR scheme. This allowed retrieval of high resolution VB-SAR imagery with a vertical discrimination of 0.04m from a stack of 1m vertical resolution DInSAR images. This work unequivocally shows that the basic principles of the VB-SAR technique are valid and opens the door to further investigation of this promising technique

Remarks on the Classical Size of D-Branes

We discuss different criteria for `classical size' of extremal Dirichlet p-branes in type-II supergravity. Using strong-weak coupling duality, we find that the size of the strong-coupling region at the core of the (p<3)-branes, is always given by the asymptotic string scale, if measured in the weakly coupled dual string metric. We also point out how the eleven-dimensional Planck scale arises in the classical 0-brane solution, as well as the ten-dimensional Planck scale in the D-instanton solution.Comment: 8 pp, harvma

Born-Infeld Theory and Stringy Causality

Fluctuations around a non-trivial solution of Born-Infeld theory have a limiting speed given not by the Einstein metric but the Boillat metric. The Boillat metric is S-duality invariant and conformal to the open string metric. It also governs the propagation of scalars and spinors in Born-Infeld theory. We discuss the potential clash between causality determined by the closed string and open string light cones and find that the latter never lie outside the former. Both cones touch along the principal null directions of the background Born-Infeld field. We consider black hole solutions in situations in which the distinction between bulk and brane is not sharp such as space filling branes and find that the location of the event horizon and the thermodynamic properties do not depend on whether one uses the closed or open string metric. Analogous statements hold in the more general context of non-linear electrodynamics or effective quantum-corrected metrics. We show how Born-Infeld action to second order might be obtained from higher-curvature gravity in Kaluza-Klein theory. Finally we point out some intriguing analogies with Einstein-Schr\"odinger theory.Comment: 31 pages, 4 figures, LaTex; Some comments and references adde

Sub-seasonal thaw slump mass wasting is not consistently energy limited at the landscape scale

Predicting future thaw slump activity requires a sound understanding of the atmospheric drivers and geomorphic controls on mass wasting across a range of timescales. On sub-seasonal timescales, sparse measurements indicate that mass wasting at active slumps is often limited by the energy available for melting ground ice, but other factors such as rainfall or the formation of an insulating veneer may also be relevant. To study the sub-seasonal drivers, we derive topographic changes from single-pass radar interferometric data acquired by the TanDEM-X satellites. The estimated elevation changes at 12 m resolution complement the commonly observed planimetric retreat rates by providing information on volume losses. Their high vertical precision (around 30 cm), frequent observations (11 days) and large coverage (5000 km2) allow us to track mass wasting as drivers such as the available energy change during the summer of 2015 in two study regions. We find that thaw slumps in the Tuktoyaktuk coastlands, Canada, are not energy limited in June, as they undergo limited mass wasting (height loss of around 0 cm day−1) despite the ample available energy, suggesting the widespread presence of early season insulating snow or debris veneer. Later in summer, height losses generally increase (around 3 cm day−1), but they do so in distinct ways. For many slumps, mass wasting tracks the available energy, a temporal pattern that is also observed at coastal yedoma cliffs on the Bykovsky Peninsula, Russia. However, the other two common temporal trajectories are asynchronous with the available energy, as they track strong precipitation events or show a sudden speed-up in late August respectively. The observed temporal patterns are poorly related to slump characteristics like the headwall height. The contrasting temporal behaviour of nearby thaw slumps highlights the importance of complex local and temporally varying controls on mass wasting

Imaging subsurface soil moisture dynamics using tomopgraphic profiling: observations and modelling

Depth-resolved radar imaging at L- to X-band has barely been applied to soils owing to limitations imposed by wave absorption within the soil and the resolutions attainable from air- or spaceborne platforms. Rather, soils are commonly studied using radar systems that cannot resolve the depth component. In this study, we adapt tomographic profiling to image the wetting and drying of sandy soil using a ground-based radar with a depth resolution of about 10 cm. The depth-resolving capabilities are achieved using synthetic aperture processing of the measurements obtained with downward pointing antennas operating at C-band with 2 GHz bandwidth. The observed subsurface backscatter appears to be governed by the local soil moisture content and the soil moisture content above (absorption). When the soil moisture content changes, the observed differential interferometric phase and coherence are consistent with the notion that the total depth-averaged interferometric return is governed by volume scattering and wave propagation within the soil. However, existing models of the depth-averaged interferometric coherence do not include variations in the volume scattering power induced by soil moisture changes, which the backscatter observations indicate exist. Besides improving our understanding of the radar backscatter from heterogeneous soils, depth-resolved imaging may in future also provide direct information about the spatial variability of soil properties and soil moisture dynamics