Description: A great obstacle for seismic surveys on firn-covered ice masses is the ability of firn to strongly attenuate seismic energy and divert downward ray paths away from the vertical because of the velocity gradient. The standard way to overcome these limitations is the drilling of shotholes about 10-30 m deep. However, drilling of shotholes is a time and energy consuming task. Another possibility is to use vibroseismic sources at the surface and increase the signal-to-noise ratio by repeated stacking. However, compared to explosive charges, vibroseismic signals are bandlimited per se. As a third variant, we investigate the usage of ordered patterns of surface charges consisting of detonation cord. Previous applications of detonation cord only explored their general comparison to bulk explosives when deployed in a linear fashion, i.e. a single line. Our approach extends these results to other geometries, like fan- or comb-shaped patterns. These have two advantages: first, over the pattern area a locally plane wave is generated, limiting the spherical and velocity-gradient induced spreading of energy during propagation; second, the ratio between seismic wave speed of the firn and the detonation cord of typically about 1:5 causes the wave to propagate in an angle downward. When using large offsets like a snow streamer, it is possible to direct the refected energy towards the streamer, depending on offset range and reflector depth. We compare the different source types for several surveys conducted in Antarctica in terms of frequency spectra. Our results show that ordered patterns of detonation cord serve as suitable seismic surface charges, avoiding the need to drill shotholes. Moreover, an example of a short profile with patterned surface charges is presented. The technique can be of advantage for surveys in remote areas, which can only be accessed by aircrafts.

Description: Explosive seismic reflection data from Halvfarryggen, a 910m thick local ice dome of the Antarctic ice sheet, show numerous laterally continuous reflections within the ice between 300 and 870m depth.We compare the quality of data obtained with explosive sources with that obtained using a vibroseis source for detecting englacial reflections with a snowstreamer, and investigate the origin of englacial reflections. We find vibroseis in combination with a snowstreamer is ten times more productive than explosive seismics. However, englacial reflections are more clearly visible with explosives, which have a broader bandwidth signature, than the vibroseis, which is band-limited at the high-frequency end to 100 Hz. Only the strongest and deepest englacial reflection is detected with vibroseis. We interpret the majority of englacial reflections to originate from changes in the crystal orientation fabric in closely spaced layers, less than the vibro-seismic tuning thickness of 13.5 m. Phase analysis of the lowermost englacial reflector, 40m above the bed, indicates a sharp increase in seismic wave speed. We interpret this reflector as a transition to a vertical single-maximum fabric. Our findings support current results from anisotropic ice-flow models, that crystal fabric is highly anisotropic at ice domes, both laterally and vertically.

Description: We carried out a combined vibro-explosive seismic survey on Halvfarryggen, a local ice dome south of Neumayer III. The Vibroseis survey was grid shaped to provide spatial information about the glaciological and geological substructure. The center survey line was also surveyed with explosive reflection and refraction seismic set-ups. The ice bed contact we interpret as a frozen till layer overlaying bedrock. Velocity analysis from refractions seen in far offset data, we obtain a pressure-wave velocity 〉5000 m/s. We therefore interpret the bedrock as igneous.

Description: We carried out a combined vibro-explosive seismic survey on Halvfarryggen, a local ice dome south of the German research station Neumayer III. The Vibroseis survey was grid shaped to provide spatial information about the glaciological and geological substructure. The center survey line was also surveyed with explosive reflection and refraction seismic set-ups. The ice bed contact we interpret as a frozen till layer overlaying bedrock. From velocity analysis of refractions seen in far offset data, we obtain a pressure-wave velocity 〉5000 m/s. We therefore interpret the bedrock as igneous, which supplements previous magnetic and gravimetric data sets.