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Vibroseismics on ice sheets and shelves

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Citation:
Eisen, O. , Kristoffersen, Y. , Hofstede, C. , Lambrecht, A. , Mayer, C. , Blenkner, R. and Kindermann, L. (2010): Vibroseismics on ice sheets and shelves , 24. Internationale Polartagung der Deutschen Gesellschaft für Polarforschung Obergurgl/Österreich. 10. September 2010. .
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Abstract:

We present first-time-ever results of active seismic measurements on an ice shelf with a vibroseismic source. The overall goal of this pilot study was to investigate the feasibility of vibroseismic operations on a porous firn layer to image ice and sedimentary structures and stratigraphies. Conventional explosive seismic surveys require the time- and energy-intensive drilling of 10-20 m deep boreholes to reach denser firn to avoid strong attenuation and ground roll. In contrast, vibroseismics can be operated directly from the surface. Vibro- and explosive seismic measurements were conducted in the 2009/10 field season in Dronning Maud Land, Antarctica, within the LIMPICS project. A Failing Y-1100 vibrator mounted on a truck on skis with a total mass of 16 t was used on the Ekströmisen ice shelf near Neumayer III, where ice is about 100-200 m thick and overlies about the same amount of water. A 60 channel snowstreamer was used for data acquisition. Measurement geometry yields 120 channels with 6.25 m group spacing. Moreover, calibrated hydrophones deployed in the water column underneath the ice shelf at PALAOA were used for quantifying propagating energy and wave characteristics. This enables us to determine absolute amplitudes of the two sources, reflection and absorption coefficients. Within several days, a total of 28.3 km of vibroseismic and 5.6 km of explosive seismic profiles were acquired. Our results show that the vibroseismic source is at least equal to the explosive source in terms of stratigraphic imaging capabilities. The explosive charge (300 g) yields higher frequencies than the employed vibroseismic 10-100 Hz, 10 s linear sweep but generates stronger ground roll. In combination with a snow streamer the vibroseismic operation with its known source characteristics enables us to easily obtain multi-fold coverage for sophisticated data processing along seismic over-ice traverses in comparably short time periods. We believe that this technique has the potential to considerably advance the utilization of seismic methods for glaciological and geological studies alike. Applications would include the determination of intra-ice properties, conditions at the basal interface and sub-ice geology, e.g. for pre-site surveys for sediment coring such as ANDRILL or the characterisation of subglacial lake environments.

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