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Diez A.,Alfred Wegener Institute for Polar and Marine Research | Diez A.,Karlsruhe Institute of Technology | Diez A.,University of California at San Diego | Eisen O.,Alfred Wegener Institute for Polar and Marine Research | And 10 more authors.
Cryosphere | Year: 2015

We investigate the propagation of seismic waves in anisotropic ice. Two effects are important: (i) sudden changes in crystal orientation fabric (COF) lead to englacial reflections; (ii) the anisotropic fabric induces an angle dependency on the seismic velocities and, thus, recorded travel times. Velocities calculated from the polycrystal elasticity tensor derived for the anisotropic fabric from measured COF eigenvalues of the EDML ice core, Antarctica, show good agreement with the velocity trend determined from vertical seismic profiling. The agreement of the absolute velocity values, however, depends on the choice of the monocrystal elasticity tensor used for the calculation of the polycrystal properties. We make use of abrupt changes in COF as a common reflection mechanism for seismic and radar data below the firn-ice transition to determine COF-induced reflections in either data set by joint comparison with ice-core data. Our results highlight the possibility to complement regional radar surveys with local, surface-based seismic experiments to separate isochrones in radar data from other mechanisms. This is important for the reconnaissance of future ice-core drill sites, where accurate isochrone (i.e. non-COF) layer integrity allows for synchronization with other cores, as well as studies of ice dynamics considering non-homogeneous ice viscosity from preferred crystal orientations. © 2015 Author(s).

Eisen O.,Alfred Wegener Institute for Polar and Marine Research | Hofstede C.,Alfred Wegener Institute for Polar and Marine Research | Diez A.,Alfred Wegener Institute for Polar and Marine Research | Kristoffersen Y.,University of Bergen | And 4 more authors.
Polar Science | Year: 2015

We present implementations of vibroseis system configurations with a snowstreamer for over-ice long-distance seismic traverses (>100km). The configurations have been evaluated in Antarctica on ice sheet and ice shelf areas in the period 2010-2014. We discuss results of two different vibroseis sources: Failing Y-1100 on skis with a peak force of 120kN in the frequency range 10-110Hz; IVI EnviroVibe with a nominal peak force of 66kN in the nominal frequency range 10-300Hz. All measurements used a well-established 60 channel 1.5km snowstreamer for the recording. Employed forces during sweeps were limited to less than 80% of the peak force. Maximum sweep frequencies, with a typical duration of 10s, were 100 and 250Hz for the Failing and EnviroVibe, respectively. Three different concepts for source movement were employed: the Failing vibrator was mounted with wheels on skis and pulled by a Pistenbully snow tractor. The EnviroVibe was operated self-propelled on Mattracks on the Antarctic plateau. This lead to difficulties in soft snow. For later implementations the EnviroVibe with tracks was put on a polyethylene (PE) sled. The sled had a hole in the center to lower the vibrator baseplate directly onto the snow surface. With the latter setup, data production varied between 20km/day for 6-fold and 40km/day for single fold for 9h/day of measurements. The combination of tracks with the PE-sled was especially advantageous on hard and rough surfaces because of the flexibility of each component and the relatively lose mounting. The systems presented here are suitable to obtain data of subglacial and sub-seabed sediment layers and englacial layering in comparable quality as obtained from marine geophysics and land-based explosive surveys. The large offset aperture of the streamer overcomes limitations of radar systems for imaging of steep along-track subglacial topography. With joint international scientific and logistic efforts, large-scale mapping of Antarctica's and Greenland's subglacial geology, ice-shelf cavity geometries and sea-bed strata, as well as englacial structures can be achieved. © 2014 Elsevier B.V. and NIPR.

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