Support minimized nonlinear acoustic inversion with absolute phase error correction

The predominant factors which prohibit the inversion of acoustic scattering data for the purposes of flaw characterization are 1) limited angular access to the flaw, 2) limited temporal frequency signal bandwidth, and 3) lack of absolute phase information between individual measurements (zero of time problem). An additional complication which impedes the data inversion is the non-linear dependence of the scattering data on the scattering object. This problem must be handled by either linearizing the problem or by applying an iterative procedure which may have questionable convergence properties. An approach to data inversion is presented here which shows potential in overcoming the aforementioned difficulties. This approach compensates for the lack of data by constructing a solution which yields simulated scattering consistent with the measured data, while simultaneously minimizing a functional measure of the support (i.e. volume) of the flaw. Such an approach to limited data inversion has proven effective in limited view X-ray CT applications when reconstructing discontinuous boundary flaws such as cracks and inclusions [1, 2, 3]. The application presented here is by-and-large analogous to the X-ray CT application, except for the additional complication of the lack of absolute phase between measurements. This zero-of-time problem is handled here by treating the absolute phase of each measurement as a variable in the minimization of the flaw support

Effects of the passage of Comet C/2013 A1 (Siding Spring) observed by the Shallow Radar (SHARAD) on Mars reconnaissance orbiter

The close passage of Comet C/2013 A1 (Siding Spring) to Mars provided a unique opportunity to observe the interaction of cometary materials with the Martian ionosphere and atmosphere using the sounding radar SHARAD (SHAllow RADar) aboard Mars Reconnaissance Orbiter. In two nightside observations, acquired in the 10 h following the closest approach, the SHARAD data reveal a significant increase of the total electron content (TEC). The observed TEC values are typical for daylight hours just after dawn or before sunset but are unprecedented this deep into the night. Results support two predictions indicating that cometary pickup O+ ions, or ions generated from the ablation of cometary dust, are responsible for the creation of an additional ion layer

Lamb Wave Scattering from Rivets

For structures with large surface areas, a full integrity evaluation can be a time-consuming operation. Lamb wave techniques allow this evaluation to be performed with waves propagating along one dimension of the inspection area while the probing transducers are moved in the perpendicular dimension, giving information about the presence of flaws within the entire scanned area. For riveted structures the scattering of the Lamb waves from the rivets is often the dominant feature in the measured response, masking the more subtle effects of Lamb wave interactions with the flaws of interest [1]. In this paper we consider the scattering of lowest mode symmetric and antisymmetric Lamb waves from model rivets, and derive analytic expressions for the scattered fields. With solutions of this type the disruptive effects of the rivets can be “processed out” of measured data in order to expose the signals which are due to the flaws in the structure

Subsurface structure of Planum Boreum from Mars Reconnaissance Orbiter Shallow Radar soundings

We map the subsurface structure of Planum Boreum using sounding data from the Shallow Radar (SHARAD) instrument onboard the Mars Reconnaissance Orbiter. Radar coverage throughout the 1,000,000- km2 area reveals widespread reflections from basal and internal interfaces of the north polar layered deposits (NPLD). A dome-shaped zone of diffuse reflectivity up to 12 ls (1-km thick) underlies twothirds of the NPLD, predominantly in the main lobe but also extending into the Gemina Lingula lobe across Chasma Boreale. We equate this zone with a basal unit identified in image data as Amazonian sand-rich layered deposits [Byrne, S., Murray, B.C., 2002. J. Geophys. Res. 107, 5044, 12 pp. doi:10.1029/2001JE001615; Fishbaugh, K.E., Head, J.W., 2005. Icarus 174, 444–474; Tanaka, K.L., Rodriguez, J.A.P., Skinner, J.A., Bourke, M.C., Fortezzo, C.M., Herkenhoff, K.E., Kolb, E.J., Okubo, C.H., 2008. Icarus 196, 318–358]. Elsewhere, the NPLD base is remarkably flat-lying and co-planar with the exposed surface of the surrounding Vastitas Borealis materials. Within the NPLD, we delineate and map four units based on the radar-layer packets of Phillips et al. [Phillips, R.J., and 26 colleagues, 2008. Science 320, 1182– 1185] that extend throughout the deposits and a fifth unit confined to eastern Gemina Lingula. We estimate the volume of each internal unit and of the entire NPLD stack (821,000 km3), exclusive of the basal unit. Correlation of these units to models of insolation cycles and polar deposition [Laskar, J., Levrard, B., Mustard, J.F., 2002. Nature 419, 375–377; Levrard, B., Forget, F., Montmessin, F., Laskar, J., 2007. J. Geophys. Res. 112, E06012, 18 pp. doi:10.1029/2006JE002772] is consistent with the 4.2-Ma age of the oldest preserved NPLD obtained by Levrard et al. [Levrard, B., Forget, F., Montmessin, F., Laskar, J., 2007. J. Geophys. Res. 112, E06012, 18 pp. doi:10.1029/2006JE002772]. We suggest a dominant layering mechanism of dust–content variation during accumulation rather than one of lag production during periods of sublimation

Air-coupled, focused ultrasonic dispersion spectrum reconstruction in plates

This paper presents and demonstrates a noncontact method for measuring the Lamb wave dispersion spectrum of a plate. Noncontact air-coupled source and receive transducers are used with line-focus mirrors and 50–700 kHz broadband apparatus for simultaneous measurement over a broad spectrum of refractive angles and multiple guided modes. Broadband, wide-angle wave forms are measured as a function of position. The Fourier transform of these wave forms from the t – x domain to the v – k domain gives an approximate spectrum of the dispersion relation. We measure the dispersion spectra of Lucite™, aluminum, balsa wood, and a carbon fiber epoxy laminate, and show that the measured spectra agree well with the dispersion relation calculated from Lamb wave theory

Radar sounding evidence for buried glaciers in the southern mid-latitudes of Mars

Lobate features abutting massifs and escarpments in the middle latitudes of Mars have been recognized in images for decades, but their true nature has been controversial, with hypotheses of origin such as ice-lubricated debris flows or glaciers covered by a layer of surface debris. These models imply an ice content ranging from minor and interstitial to massive and relatively pure. Soundings of these deposits in the eastern Hellas region by the Shallow Radar on the Mars Reconnaissance Orbiter reveal radar properties entirely consistent with massive water ice, supporting the debris-covered glacier hypothesis. The results imply that these glaciers formed in a previous climate conducive to glaciation at middle latitudes. Such features may collectively represent the most extensive nonpolar ice yet recognized on Mars

Shallow radar (SHARAD) sounding observations of the Medusae Fossae Formation, Mars

The SHARAD (shallow radar) sounding radar on the Mars Reconnaissance Orbiter detects subsurface reflections in the eastern and western parts of the Medusae Fossae Formation (MFF). The radar waves penetrate up to 580 m of the MFF and detect clear subsurface interfaces in two locations: west MFF between 150 and 155◦ E and east MFF between 209 and 213◦ E. Analysis of SHARAD radargrams suggests that the real part of the permittivity is ∼3.0, which falls within the range of permittivity values inferred from MARSIS data for thicker parts of the MFF. The SHARAD data cannot uniquely determine the composition of the MFF material, but the low permittivity implies that the upper few hundred meters of the MFF material has a high porosity. One possibility is that the MFF is comprised of low-density welded or interlocked pyroclastic deposits that are capable of sustaining the steep-sided yardangs and ridges seen in imagery. The SHARAD surface echo power across the MFF is low relative to typical martian plains, and completely disappears in parts of the east MFF that correspond to the radar-dark Stealth region. These areas are extremely rough at centimeter to meter scales, and the lack of echo power is most likely due to a combination of surface roughness and a low near-surface permittivity that reduces the echo strength from any locally flat regions. There is also no radar evidence for internal layering in any of the SHARAD data for the MFF, despite the fact that tens-of-meters scale layering is apparent in infrared and visible wavelength images of nearby areas. These interfaces may not be detected in SHARAD data if their permittivity contrasts are low, or if the layers are discontinuous. The lack of closely spaced internal radar reflectors suggests that the MFF is not an equatorial analog to the current martian polar deposits, which show clear evidence of multiple internal layers in SHARAD dat

Nightside ionosphere of Mars studied with local electron densities: A general overview and electron density depressions

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95308/1/jgra21490.pd

Enhanced ionization of the Martian nightside ionosphere during solar energetic particle events

Electron densities in the Martian nightside ionosphere are more than 90% of time too low to be detected by the Mars Advanced Radar for Subsurface and Ionosphere Sounding radar sounder on board the Mars Express spacecraft. However, the relative number of ionograms with peak electron density high enough to be detected represents a good statistical proxy of the ionospheric density. We focus on solar energetic particle (SEP) events, and we analyze their effects on ionospheric formation. SEP time intervals were identified in situ using the background counts recorded by the ion sensor of the ASPERA-3 instrument on board Mars Express. We show that peak electron densities during the SEP events are large enough to be detected in more than 30% of measurements, and, moreover, the reflections of the sounding signal from the ground almost entirely disappear. Nightside electron densities during SEP events are thus substantially increased as compared to normal nightside conditions