scholarly journals A low-frequency ice-penetrating radar system adapted for use from an airplane: test results from Bering and Malaspina Glaciers, Alaska, USA

2009 ◽  
Vol 50 (51) ◽  
pp. 93-97 ◽  
Author(s):  
Howard Conway ◽  
Ben Smith ◽  
Pavan Vaswani ◽  
Kenichi Matsuoka ◽  
Eric Rignot ◽  
...  
Keyword(s):  
Low Frequency ◽  
Radar System ◽  
Ice Thickness ◽  
Radio Waves ◽  
Low Frequencies ◽  
Throat Region ◽  
High Attenuation ◽  
Glacier System ◽  
Dielectric Absorption ◽  
Thickness Measurements

AbstractIce-thickness measurements are needed to calculate fluxes through fast-flowing outlet glaciers in Greenland, Alaska, Patagonia and Antarctica. However, relatively high attenuation of radio waves by dielectric absorption and volume scattering from englacial water hampers detection of the bed through warm deep ice. In the past we have had success measuring ice thickness of temperate glaciers using a ground-based monopulse radar system operating at low frequencies (2 MHz). Here we adapt the same system to operate from an airplane. Test flights over Bering Glacier, Alaska, USA, detected the bed through ice up to 1250m thick. Flights across the Seward–Malaspina Glacier system, Alaska, resolved the ice thickness of Malaspina Glacier, but strong hyperbolic-shaped returns obscured the bed echo through the Seward throat. It is likely that this clutter in the signal was caused by off-nadir returns from chaotic surface crevasses that are ubiquitous in the throat region.

scholarly journals Bioinspired Low-Frequency Material Characterisation

2012 ◽  
Vol 2012 ◽  
pp. 1-12 ◽  
Author(s):  
C. Hopper ◽  
S. Assous ◽  
P. B. Wilkinson ◽  
D. A. Gunn ◽  
P. D. Jackson ◽  
...  
Keyword(s):  
Sound Velocity ◽  
Medical Physics ◽  
Low Frequency ◽  
High Sensitivity ◽  
Prediction Errors ◽  
Low Frequencies ◽  
Longitudinal Sound Velocity ◽  
Velocity Prediction ◽  
Thickness Measurements

New-coded signals, transmitted by high-sensitivity broadband transducers in the 40–200 kHz range, allow subwavelength material discrimination and thickness determination of polypropylene, polyvinylchloride, and brass samples. Frequency domain spectra enable simultaneous measurement of material properties including longitudinal sound velocity and the attenuation constant as well as thickness measurements. Laboratory test measurements agree well with model results, with sound velocity prediction errors of less than 1%, and thickness discrimination of at least wavelength/15. The resolution of these measurements has only been matched in the past through methods that utilise higher frequencies. The ability to obtain the same resolution using low frequencies has many advantages, particularly when dealing with highly attenuating materials. This approach differs significantly from past biomimetic approaches where actual or simulated animal signals have been used and consequently has the potential for application in a range of fields where both improved penetration and high resolution are required, such as nondestructive testing and evaluation, geophysics, and medical physics.

scholarly journals Low-frequency radar sounding of ice in East Antarctica and southern Greenland

2014 ◽  
Vol 55 (67) ◽  
pp. 138-146 ◽  
Author(s):  
J. Mouginot ◽  
E. Rignot ◽  
Y. Gim ◽  
D. Kirchner ◽  
E. Le Meur
Keyword(s):  
Low Frequency ◽  
Radar Data ◽  
East Antarctica ◽  
Radar Signal ◽  
Airborne Radar ◽  
Ice Thickness ◽  
Decameter Wavelength ◽  
Radar Sounder ◽  
Thickness Measurements ◽  
Southwest Greenland

AbstractWe discuss a decameter-wavelength airborne radar sounder, the Warm Ice Sounding Explorer (WISE), that provides ice thickness in areas where radar signal penetration at higher frequencies is expected to be limited. Here we report results for three campaigns conducted in Greenland (2008, 2009, 2010) and two in Antarctica (2009, 2010). Comparisons with higher-frequency radar data indicate an accuracy of ±55 m for ice-thickness measurements in Greenland and ±25 m in Antarctica. We also estimate ice thickness of the Qassimiut lobe in southwest Greenland, where few ice-thickness measurements have been made, demonstrating that WISE penetrates in strongly scattering environments.

Graeme Reade Anthony (‘Bill’) Ellis 1921–2011

2013 ◽  
Vol 24 (1) ◽  
pp. 96
Author(s):  
Robert Delbourgo ◽  
Peter M. McCulloch
Keyword(s):  
Galactic Disk ◽  
Geomagnetic Latitude ◽  
Low Frequency ◽  
The Sun ◽  
Radio Waves ◽  
Low Frequencies ◽  
Radio Emissions ◽  
Radio Receivers ◽  
Practical Research ◽  
The University

Graeme Reade Anthony Ellis (universally known as ?Bill') was a pioneer in the area of low-frequency radio observations. By exploiting Hobart's geomagnetic latitude and the lack of background radio noise there, he was able to make major discoveries at these low frequencies (principally in the frequency range 1?10�MHz).Among the questions he pursuedwere the propagation/dispersion/reflection of radio waves in the ionosphere and the detection of radio emissions from the Sun, the galactic disk and Jupiter. He built innovative radio receivers and de-dispersers to gain information about the radio sources, for example about the Sun via aurorae and about the influence of Io on the Jovian emissions. It is thanks to Ellis' practical research investigations and clever experimental methods that radio astronomy at the University of Tasmania is today firmly established and internationally recognized.

scholarly journals Ice thickness, internal structure and subglacial topography of Bowles Plateau ice cap and the main ice divides of Livingston Island, Antarctica, by ground-based radio-echo sounding

2009 ◽  
Vol 50 (51) ◽  
pp. 49-56 ◽  
Author(s):  
Yu.Ya. Macheret ◽  
J. Otero ◽  
F.J. Navarro ◽  
E.V. Vasilenko ◽  
M.I. Corcuera ◽  
...  
Keyword(s):  
Internal Structure ◽  
Sea Level ◽  
Low Frequency ◽  
South Shetland Islands ◽  
Ice Thickness ◽  
Radio Waves ◽  
Ice Cap ◽  
Livingston Island ◽  
Radio Echo Sounding ◽  
Echo Sounding

AbstractWe present the results of low-frequency (20 MHz) radio-echo sounding (RES) carried out in December 2000 and December 2006 on the main ice divides of Livingston Island, South Shetland Islands (SSI), Antarctica, and Bowles Plateau, Antarctica, respectively, as well as high-frequency (200 MHz) RES on the latter, aimed at determining the ice thickness, internal structure and subglacial relief. Typical ice thickness along the main ice divides is ~150 m, reaching maxima of ~200 m. On Bowles Plateau the ice is much thicker, with an average of 265 m and maxima of ~500 m. The bed below the main ice divides is above sea level, while part of the outlet glaciers from Bowles Plateau lies significantly below sea level, down to –120 m. The strong scattering of the radio waves in the areas under study constitutes further evidence that the ice in the accumulation area of the ice masses of the SSI is temperate. Typical thickness of the firn layer in Bowles Plateau is 20–35 m, similar to that found in King George ice cap. A strong internal reflector within the firn layer, interpreted as a tephra layer from the 1970 eruption at Deception Island, has allowed a rough estimate of the specific mass balances for Bowles Plateau within 0.20–0.40ma–1w.e., as average values for the period 1970–2006.

Radio and Electrical Measurements on Glacial Streams

1987 ◽  
Vol 33 (114) ◽  
pp. 239-242
Author(s):  
M. E. R. Walford
Keyword(s):  
Electrical Conductivity ◽  
Water Channel ◽  
Water System ◽  
Low Frequency ◽  
Water Channels ◽  
Radiative Loss ◽  
Radio Waves ◽  
Electrical Measurements ◽  
Water Conductivity ◽  
Glacier Surface

AbstractWe discuss the suggestion that small underwater transmitters might be used to illuminate the interior of major englacial water channels with radio waves. Once launched, the radio waves would naturally tend to be guided along the channels until attenuated by absorption and by radiative loss. Receivers placed within the channels or at the glacier surface could be used to detect the signals. They would provide valuable information about the connectivity of the water system. The electrical conductivity of the water is of crucial importance. A surface stream on Storglaciären, in Sweden, was found, using a low-frequency technique, to have a conductivity of approximately 4 × 10−4 S m−1. Although this is several hundred times higher than the conductivity of the surrounding glacier ice, the contrast is not sufficient to permit us simply to use electrical conductivity measurements to establish the connectivity of englacial water channels. However, the water conductivity is sufficiently small that, under favourable circumstances, radio signals should be detectable after travelling as much as a few hundred metres along an englacial water channel. In a preliminary field experiment, we demonstrated semi quantitatively that radio waves do indeed propagate as expected, at least in surface streams. We conclude that under-water radio transmitters could be of real practical value in the study of the englacial water system, provided that sufficiently robust devices can be constructed. In a subglacial channel, however, we expect the radio range would be much smaller, the environment much harsher, and the technique of less practical value.

scholarly journals Earless toads sense low frequencies but miss the high notes

2017 ◽  
Vol 284 (1864) ◽  
pp. 20171670 ◽  
Author(s):  
Molly C. Womack ◽  
Jakob Christensen-Dalsgaard ◽  
Luis A. Coloma ◽  
Juan C. Chaparro ◽  
Kim L. Hoke
Keyword(s):  
High Frequency ◽  
Species Differences ◽  
Low Frequency ◽  
Auditory Brainstem ◽  
Low Frequencies ◽  
Hearing Sensitivity ◽  
Sensory Pathways ◽  
Airborne Sound ◽  
Vibrational Sensitivity ◽  
Species Specific

Sensory losses or reductions are frequently attributed to relaxed selection. However, anuran species have lost tympanic middle ears many times, despite anurans' use of acoustic communication and the benefit of middle ears for hearing airborne sound. Here we determine whether pre-existing alternative sensory pathways enable anurans lacking tympanic middle ears (termed earless anurans) to hear airborne sound as well as eared species or to better sense vibrations in the environment. We used auditory brainstem recordings to compare hearing and vibrational sensitivity among 10 species (six eared, four earless) within the Neotropical true toad family (Bufonidae). We found that species lacking middle ears are less sensitive to high-frequency sounds, however, low-frequency hearing and vibrational sensitivity are equivalent between eared and earless species. Furthermore, extratympanic hearing sensitivity varies among earless species, highlighting potential species differences in extratympanic hearing mechanisms. We argue that ancestral bufonids may have sufficient extratympanic hearing and vibrational sensitivity such that earless lineages tolerated the loss of high frequency hearing sensitivity by adopting species-specific behavioural strategies to detect conspecifics, predators and prey.

scholarly journals Coherent radar ice thickness measurements over the Greenland ice sheet

2001 ◽  
Vol 106 (D24) ◽  
pp. 33761-33772 ◽  
Author(s):  
S. Gogineni ◽  
D. Tammana ◽  
D. Braaten ◽  
C. Leuschen ◽  
T. Akins ◽  
...  
Keyword(s):  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Ice Thickness ◽  
Coherent Radar ◽  
Thickness Measurements

scholarly journals Measuring and inferring the ice thickness distribution of four glaciers in the Tien Shan, Kyrgyzstan

2020 ◽  
pp. 1-18
Author(s):  
Lander Van Tricht ◽  
Philippe Huybrechts ◽  
Jonas Van Breedam ◽  
Johannes J. Fürst ◽  
Oleg Rybak ◽  
...  
Keyword(s):  
Cross Validation ◽  
Thickness Distribution ◽  
Tien Shan ◽  
Ice Thickness ◽  
Ice Volume ◽  
In Situ Data ◽  
Radio Echo Sounding ◽  
Thickness Measurements ◽  
Tien Shan Mountains

Abstract Glaciers in the Tien Shan mountains contribute considerably to the fresh water used for irrigation, households and energy supply in the dry lowland areas of Kyrgyzstan and its neighbouring countries. To date, reconstructions of the current ice volume and ice thickness distribution remain scarce, and accurate data are largely lacking at the local scale. Here, we present a detailed ice thickness distribution of Ashu-Tor, Bordu, Golubin and Kara-Batkak glaciers derived from radio-echo sounding measurements and modelling. All the ice thickness measurements are used to calibrate three individual models to estimate the ice thickness in inaccessible areas. A cross-validation between modelled and measured ice thickness for a subset of the data is performed to attribute a weight to every model and to assemble a final composite ice thickness distribution for every glacier. Results reveal the thickest ice on Ashu-Tor glacier with values up to 201 ± 12 m. The ice thickness measurements and distributions are also compared with estimates composed without the use of in situ data. These estimates approach the total ice volume well, but local ice thicknesses vary substantially.

A waveform inversion technique for measuring elastic wave attenuation in cylindrical bars

Geophysics ◽  
1992 ◽  
Vol 57 (6) ◽  
pp. 854-859 ◽  
Author(s):  
Xiao Ming Tang
Keyword(s):  
Elastic Wave ◽  
Wave Attenuation ◽  
Waveform Inversion ◽  
Finite Size ◽  
Low Frequency ◽  
Frequency Range ◽  
Multiple Reflections ◽  
Low Frequencies ◽  
The Time Domain ◽  
Attenuation Values

A new technique for measuring elastic wave attenuation in the frequency range of 10–150 kHz consists of measuring low‐frequency waveforms using two cylindrical bars of the same material but of different lengths. The attenuation is obtained through two steps. In the first, the waveform measured within the shorter bar is propagated to the length of the longer bar, and the distortion of the waveform due to the dispersion effect of the cylindrical waveguide is compensated. The second step is the inversion for the attenuation or Q of the bar material by minimizing the difference between the waveform propagated from the shorter bar and the waveform measured within the longer bar. The waveform inversion is performed in the time domain, and the waveforms can be appropriately truncated to avoid multiple reflections due to the finite size of the (shorter) sample, allowing attenuation to be measured at long wavelengths or low frequencies. The frequency range in which this technique operates fills the gap between the resonant bar measurement (∼10 kHz) and ultrasonic measurement (∼100–1000 kHz). By using the technique, attenuation values in a PVC (a highly attenuative) material and in Sierra White granite were measured in the frequency range of 40–140 kHz. The obtained attenuation values for the two materials are found to be reliable and consistent.

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