Nicholas Holschuh

Power loss in dipping internal reflectors, imaged using ice-penetrating radar

Abstract The geometry of ice-sheet internal layers is frequently interpreted as an indicator of present and past ice-sheet flow dynamics. One of the primary goals of radio-echo sounding is to accurately reproduce that layer geometry. Internal layers show a loss in reflection amplitude as a function of increasing dip angle. We posit that this energy loss occurs via several mechanisms: destructive interference in trace stacking, energy dispersion through synthetic aperture radar (SAR) processing and off-nadir ray path losses. Adjacent traces collected over a dipping horizon contain reflection arrivals which are not in phase. Stacking these traces results in destructive interference. When the phase shift between adjacent traces exceeds one-half wavelength, SAR processing, which otherwise coherently combines data from dipping reflectors, disperses the energy, reducing image quality further. Along with amplitude loss from destructive stacking and SAR dispersion, imaging reflectors from off-nadir angles results in additional travel time and thus additional englacial attenuation relative to horizontal reflectors at similar depths. When selecting radar frequency, spatial sample rate and stacking interval for a given survey, the geometry of the imaging target must be considered. Based on our analysis, we make survey design recommendations for these parameters.

Decoding ice sheet behavior using englacial layer slopes.

The complex flow fields of the Antarctic and Greenland ice sheets deform layers deposited as snow at the ice-sheet surface, leaving a record of the regional flow history and/or local transitions in basal boundary conditions within the geometry of ice-sheet layers. Ice-penetrating radar reveals these layers, but interpretations are limited by the challenges of quantitatively and reproducibly comparing observations with model output. We present a conceptual framework that relates along-track reflector slope to gradients in the steady-state velocity field of ice sheets. This method makes effective use of englacial reflectors in regions where it is challenging to image continuous layers, and avoids the error propagation inherent to tracer-transport methods, developing the potential for formal radar data assimilation in future modeling studies. We apply our method to radar data collected at the grounding line of Whillans Ice Stream, where enhanced bed-friction produces characteristic reflector slopes reproducible using a higher-order ice flow model.

Characteristics of the sticky spot of Kamb Ice Stream, West Antarctica

Amplitude analysis of reflection seismic data reveals the presence of highly variable bed conditions under the main sticky spot and adjacent regions of the Kamb Ice Stream (KIS—formerly ice stream C). The sticky spot, which is a zone of bed that imparts high basal resistance to ice flow, is situated on a local topographic high composed of consolidated sediments or sedimentary rock. Any meltwater draining from upglacier along the base of the ice is routed around the sticky spot. The ice over the sticky spot includes, in at least some places, a seismically detectable basal layer containing a low concentration of debris, which locally thickens to 40 m over a topographic low in the bed. The ice-contact basal material ranges from dilated and highly porous to more-compacted and stiff, and perhaps locally frozen. The softer material is preferentially in topographic lows, but there is not a one-to-one correspondence between basal character and basal topography. We speculate that the 40-m-thick frozen-on debris layer formed by glaciohydraulic supercooling of lake-drainage events along a basal channel during the former, active phase of the ice stream. We also speculate that loss of lubricating water, perhaps from piracy upstream, contributed to the slowdown of the ice stream, with drag from the sticky spot playing an important role, and with the basal heterogeneity greatly increasing after the slowdown of the ice stream.

Persistent Tracers of Historic Ice Flow in Glacial Stratigraphy near Kamb Ice Stream, West Antarctica

This archive includes a pdf containing a data description, a location map, and quick-look images of the data, as well as 11 .mat (MATLAB) binary files containing the radar data.

Evaluating the Duration and Continuity of Potential Climate Records From the Allan Hills Blue Ice Area, East Antarctica

The current ice core record extends back 800,000 years. Geologic and glaciological evidence suggests that the Allan Hills Blue Ice Area, East Antarctica, may preserve a continuous record that extends further back in time. In this study, we use ice-penetrating radar and existing age constraints to map the internal stratigraphy and age structure of the Allan Hills Main Ice Field. The dated isochrones provide constraints for an ice flow model to estimate the age of ice near the bed. Previous drilling in the region recovered stratigraphically disturbed sections of ice up to 2.7 million years old. Our study identifies a site ~5 km upstream, which likely preserves a continuous record through Marine Isotope Stage 11 with the possibility that the record extends back 1 million years. Such records would provide new insight into the past climate and glacial history of the Ross Sea Sector. Plain Language Summary Ice cores currently provide detailed, continuous records of Earth’s climate and atmosphere over the past 800,000 years. Discrete ice samples with ages up to 2.7 million years have been recovered from the Allan Hills Blue Ice Area, East Antarctica, indicating that the region may preserve a continuous record that extends beyond 800,000 years. In this study, we use ice-penetrating radar and an ice flow model to identify an optimal site for a continuous ice core record from the Allan Hills, which may extend over the last 1 million years. Such a long ice core record would help us better understand the fundamental drivers of Earth’s climate.

Constraining attenuation uncertainty in common midpoint radar surveys of ice sheets

For common-offset radar data, there is no clear way to disentangle path effects from reflector characteristics, so efforts to determine the physical properties at the bed using reflection amplitude are inherently limited by the constraints on englacial attenuation. In this study, we identify the theoretical considerations required for interpreting bistatic radar surveys, and use data collected on the Northeast Greenland Ice Stream (NEGIS) and Kamb Ice Stream to compute local attenuation profiles. We found that failing to correct for angle-dependent controls on return power (including antenna directivity, the reflection coefficient, and refractive focusing) can bias the computed attenuation rates as much as 30 dB/km for reflectors at 1 km depth. Because the radiation characteristics are the dominant source of uncertainty in our data, we recommend either a simplified survey design for the future (where the antennae are decoupled from the ice surface), or additional data collection to constrain the near-field permittivity and its effect on the radiation pattern. Depth-averaged attenuation rates computed using CMP methods for deep reflectors yield values >10 dB/km higher than attenuation rates computed using common-offset techniques with the same data. We attribute these anomolously high attenuation rates to additional wavenumber (and therefore, angle) dependent interferences between sub-wavelength reflectors.