Airborne ground penetrating radar is a suitable tool to perform cost-effective surveys of the underground of large possibly nonaccessible areas. To tackle the imaging problem underlying this technology, we propose a regularized inverse scattering procedure. The capabilities of the approach to achieve a stable solution and its imaging performance are first assessed against synthetic data. Then, results obtained by processing data collected by means of a gated stepped frequency system mounted on a helicopter are provided as examples of on-field test cases.

A Tomographic Approach for Helicopter-Borne Ground Penetrating Radar Imaging

This study calculates area, volume and elevation changes of two
glaciers on James Ross Island, Antarctica, during the period
1979–2006. Davies Dome is a small ice cap. Whisky Glacier is a
valley glacier. Ground-penetrating radar surveys indicate ice
thickness, which was used for calculations of the bed
topography and volume of both glaciers. Maximum measured ice
thicknesses of Davies Dome and Whisky Glacier are 83 +/- 2 and
157 +/- 2 m, respectively. Between 1979 and 2006, the area of
the ice cap decreased from 6.23 +/- 0.05 km2 to 4.94 +/- 0.01
km2 (–20.7%), while the area of the valley glacier reduced from
2.69 +/- 0.02 km 2 to 2.40 +/- 0.01 km2 (–10.6%). Over the same
period the volume of the ice cap and valley glacier reduced
from 0.23 +/- 0.03 km3 to 0.16 +/- 0.02 km3 (–30.4%) and from
0.27 +/- 0.02 km3 to 0.24 +/- 0.01 km3 (–10.6%), respectively.
The mean surface elevation decreased by 8.5 +/- 2.8 and 10.1
+/- 2.8 m. The average areal (0.048–0.011 km2 a–1) and
volumetric (0.003–0.001 km3 a–1) changes are higher than the
majority of other estimates from Antarctic Peninsula glaciers.

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Ice thickness, areal and volumetric changes of Davies Dome and Whisky Glacier (James Ross Island, Antarctic Peninsula) in 1979–2006

To forecast rock glacier movements, it is necessary to have dependable information on their internal structures and physical properties. A first attempt to expand our knowledge of a rock glacier in the Swiss Alps involved acquiring ground-based ground-penetrating radar (GPR) data along numerous profiles using different acquisition systems and antennae with different nominal frequencies. Images derived from these ground-based data were inconsistent and unreliable. For our second attempt, we recorded GPR data using a helicopter-mounted system. The helicopter GPR sections were surprisingly good, with consistent images along adjacent and intersecting profiles. Internal shear horizons, ice-rich and ice-poor regions and the bedrock interface were well delineated on the helicopter GPR images. Besides providing excellent-quality images, the helicopter GPR system allowed areas of the rock glacier to be surveyed that would have been difficult or impossible to access for a ground-based study. Because near-surface heterogeneity does not seem to have a major effect on helicopter GPR data acquired across a rugged rock glacier, we suggest that helicopter GPR surveying might be useful for investigating many terrains covered by heterogeneous loose material, including debris avalanches, scree slopes and rockfalls. Copyright © 2015 John Wiley & Sons, Ltd.

Evaluation of Ground-Based and Helicopter Ground-Penetrating Radar Data Acquired Across an Alpine Rock Glacier

Tete Rousse is a small polythermal glacier located in the Mont Blanc area (French Alps) at an altitude of 3100 to 3300 m In 1892, an outburst flood from this glacier released about 200 000m(3) of water mixed with ice, causing much damage A new accumulation of melt water in the glacier was not excluded The uncertainty related to such glacier conditions initiated an extensive geophysical study for evaluating the hazard Using three-dimensional surface nuclear magnetic resonance imaging (3-D-SNMR), we showed that the temperate part of the Tete Rousse glacier contains two separate water-filled caverns (central and upper caverns) In 2009, the central cavern contained about 55 000m(3) of water Since 2010, the cavern is drained every year We monitored the changes caused by this pumping in the water distribution within the glacier body Twice a year, we carried out magnetic resonance imaging of the entire glacier and estimated the volume of water accumulated in the central cavern Our results show changes in cavern geometry and recharge rate: in two years, the central cavern lost about 73% of its initial volume, but 65% was lost in one year after the first pumping We also observed that, after being drained, the cavern was recharged at an average rate of 20 to 25m(3) d(-1) during the winter months and 120 to 180m(3) d(-1) in summer These observations illustrate how ice, water and air may refill englacial volume being emptied by artificial draining Comparison of the 3-D-SNMR results with those obtained by drilling and pumping showed a very good correspondence, confirming the high reliability of 3-D-SNMR imaging

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Monitoring water accumulation in a glacier using magnetic resonance imaging

. We report on a successful application of the horizontal-to-vertical spectral ratio (H / V) method, generally used to investigate the subsurface velocity structures of the shallow crust, to estimate the Antarctic ice sheet thickness for the first time. Using three-component, five-day long, seismic ambient noise records gathered from more than 60 temporary seismic stations located on the Antarctic ice sheet, the ice thickness measured at each station has comparable accuracy to the Bedmap2 database. Preliminary analysis revealed that 60 out of 65 seismic stations on the ice sheet obtained clear peak frequencies (f0) related to the ice sheet thickness in the H / V spectrum. Thus, assuming that the isotropic ice layer lies atop a high velocity half-space bedrock, the ice sheet thickness can be calculated by a simple approximation formula. About half of the calculated ice sheet thicknesses were consistent with the Bedmap2 ice thickness values. To further improve the reliability of ice thickness measurements, two-type models were built to fit the observed H / V spectrum through non-linear inversion. The two-type models represent the isotropic structures of single- and two-layer ice sheets, and the latter depicts the non-uniform, layered characteristics of the ice sheet widely distributed in Antarctica. The inversion results suggest that the ice thicknesses derived from the two-layer ice models were in good concurrence with the Bedmap2 ice thickness database, and that ice thickness differences between the two were within 300 m at almost all stations. Our results support previous finding that the Antarctic ice sheet is stratified. Extensive data processing indicates that the time length of seismic ambient noise records can be shortened to two hours for reliable ice sheet thickness estimation using the H / V method. This study extends the application fields of the H / V method and provides an effective and independent way to measure ice sheet thickness in Antarctica.

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Antarctic ice sheet thickness estimation using the horizontal-to-vertical spectral ratio method with single-station seismic ambient noise

To determine subglacial topography and internal features of the Fourcade Glacier on King George Island in Antarctica, helicopter-borne and ground-towed ground-penetrating radar (GPR) data were recorded along four profiles in November 2006. Signature deconvolution, f-k migration velocity analysis, and finite-difference depth migration applied to the mixed-phase, single-channel, ground-towed data, were effective in increasing vertical resolution, obtaining the velocity function, and yielding clear depth images, respectively. For the helicopter-borne GPR, migration velocities were obtained as root-mean-squared velocities in a two-layer model of air and ice. The radar sections show rugged subglacial topography, englacial sliding surfaces, and localised scattering noise. The maximum depth to the basement is over 79 m in the subglacial valley adjacent to the south-eastern slope of the divide ridge between Fourcade and Moczydlowski Glaciers. In the ground-towed profile, we interpret a complicated conduit above possible basal water and other isolated cavities, which are a few metres wide. Near the terminus, the GPR profiles image sliding surfaces, fractures, and faults that will contribute to the tidewater calving mechanism forming icebergs in Potter Cove.

J. Lee

Papers

Helicopter-borne and ground-towed radar surveys of the Fourcade Glacier on King George Island, Antarctica