Advancing Active Source Seismic Methods for Exploration of the Cryosphere

Active source seismic methods have been providing critical information to the study of the cryosphere by probing kilometers below surface to the ice-bed interface and imaging geologic formations beneath ice sheets. Considering the large expanses of rapidly changing ice masses around the globe, there is a need for improving the efficiency of active source seismic methods. Researchers at the Center for Remote Sensing of Ice Sheets (CReSIS) developed a new seismic streamer for reflection imaging through polar ice and utilized active source seismic surface wave methods to determine polar firn properties. These methods were tested on the Jakobshavn Glacier, Greenland, in summer 2007. Seismic streamers deployed in polar environments have been plagued by poor coupling to the snow surface and by wind noise contamination compared to manually buried geophones below the surface. A 24-channel seismic streamer prototype was constructed at The University of Kansas consisting of geophones mounted on metallic plates and towed behind a sled. The streamer was deployed on Jakobshavn Glacier alongside manually buried "control" geophones recording simultaneously the same explosive source signals. The streamer imaged seismic reflections from the bed at approximately 1.7 km depth and internal ice layers. In wind conditions up to 5 knots, streamer data were identical to control seismic data exhibiting bed reflections with frequency content in excess of 200 Hz. In 5-10 knot wind conditions, bed reflections and internal ice layers were clearly imaged by the streamer, although some wind noise was present compared to control data. In excess of 10 knot winds, streamer and control data showed increased noise content. Bed reflections were clearly recorded by the streamer, but internal layers were not discernible in single trace recordings. Multi-fold processing of streamer data enhanced signal-to-noise and improved imaging in windy conditions. It is estimated that in field conditions encountered at the Jakobshavn Glacier, streamer technology can yield a five- to ten-fold increase in seismic surveying efficiency without considerable loss of data quality. Seismic wavetrains contain surface waves propagating within one wavelength from surface. Their dispersive characteristics can be exploited to construct shear wave velocity profiles of the near-surface. We analyzed the phase velocity of Rayleigh waves by employing the Multichannel Analysis of Surface Waves (MASW) method and obtained shear wave velocity profiles of polar firn and glacial ice to approximately 78 meters depth. Shear wave velocities progressively increase from 900 m/s at the surface to 1800 m/s at 45 m depth. Between 45 and 78 m depth, shear wave velocity is predominantly 1800 m/s, which indicates that the firn-ice transition is at 45 m below surface. Surface wave methods do not require the generation and recording of shear waves and can map velocity inversions that cannot be detected by seismic refraction methods. Surface wave methods can provide continuous shear wave velocity mapping of polar firn which can help understand better firn mechanical properties and mechanisms of crevasse formation.