Active seismic studies in valley glacier settings: strategies and limitations

Abstract

Subglacial tills play an important role in glacier dynamics but are difficult to characterize in situ. Amplitude Variation with Angle (AVA) analysis of seismic reflection data can distinguish between stiff tills and deformable tills. However, AVA analysis in mountain glacier environments can be problematic: reflections can be obscured by Rayleigh wave energy scattered from crevasses, and complex basal topography can impede the location of reflection points in 2-D acquisitions. We use a forward model to produce challenging synthetic seismic records in order to test the efficacy of AVA in crevassed and geometrically complex environments. We find that we can distinguish subglacial till types in moderately crevassed environments, where ‘moderate’ depends on crevasse spacing and orientation. The forward model serves as a planning tool, as it can predict AVA success or failure based on characteristics of the study glacier. Applying lessons from the forward model, we perform AVA on a seismic dataset collected from Taku Glacier in Southeast Alaska in March 2016. Taku Glacier is a valley glacier thought to overlay thick sediment deposits. A near-offset polarity reversal confirms that the tills are deformable.

Grounding zone subglacial properties from calibrated active source seismic methods

“…Upstream of the grounding zone two retrieved estimates exhibit properties similar to those of water (kilometer 0-0.5); however, neither are unambiguous. values typical of those observed in dewatered tills ( Figure 6-7, Table 4) (Zechmann et al, 2018). Our estimates of V p and ρ for all lines are close to those estimated by Luthra et al (2016) in their active source seismic study of a major sticky spot beneath Whillans Ice Plain.…”

“…Source size (A 0 ) is often estimated using the ratio of the primary bed return amplitude (A i ) and the long path multiple amplitude (A m,i ) (e.g. Röthlisberger, 1972;Smith, 1997;Peters et al, 2008;Brisbourne et al, 2017;Zechmann et al, 2018), as this approach can remove the need for an independent estimate of attenuation. However, low impedance contrast at the bed, low signal to noise ratios, or closely spaced subglacial reflectors, can all complicate the amplitude ratio method of determining source amplitude.…”

“…To investigate the basal properties beneath Whillans Ice Stream's grounding zone, and the distance over which the transition to the freely slipping ice shelf occurs, we use active source seismic techniques that are commonly applied in studies addressing the basal boundary of glaciers and ice sheets (e.g. Anandakrishnan, 2003;Smith, 2007;Brisbourne et al, 2017;Zechmann et al, 2018;Muto et al, 2019). These methods require the source amplitude and path effects to be estimated, which is often challenging due to variability in source and receiver coupling, and strong vertical gradients in density and seismic velocity in the firn.…”

Detecting and characterising an englacial conduit network within a temperate Swiss glacier using active seismic, ground penetrating radar and borehole analysis

“…Shot or geophone coupling corrections were not performed as previous studies found this correction to be insensitive to the calculated reflectivity (Zechmann and others, 2018). Additionally, we anticipated this variation to be relatively small in comparison to the large range of first order multiple amplitude values and p-wave attenuation parameters used in Eqn (1).…”

Grounding zone subglacial properties from calibrated active-source seismic methods

“…To further investigate the basal properties beneath Whillans Ice Stream's grounding zone, we here revisit the active-source seismic data reported by Horgan et al (2013b) and apply and extend amplitude analysis methods previously used in studies addressing the basal boundary of glaciers and ice sheets (e.g. Anandakrishnan, 2003b;Smith, 2007;Holland and Anandakrishnan, 2009;Brisbourne et al, 2017;Zechmann et al, 2018;Muto et al, 2019). These methods require source amplitude and path effects to be estimated, which is often challenging due to variability in source and receiver coupling and strong vertical gradients in density and seismic velocity in the firn.…”