Constraining attenuation uncertainty in common midpoint radar surveys of ice sheets

Abstract

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 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 common midpoint 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 anomalously high attenuation rates to additional wavenumber‐dependent (and therefore, angle‐dependent) interferences between subwavelength reflectors.

A framework for attenuation method selection evaluated with ice-penetrating radar data at South Pole Lake

“…This result is encouraging given that prior attempts to resolve attenuation measurements with depth have shown the opposite result (i.e. decreasing attenuation with depth) (Matsuoka and others, 2010; Holschuh and others, 2016). On the other hand, results from Method 2 are significantly lower than expected (even negative) for most of the reflectors close to the surface, and there is high variance between results for all layers throughout the ice column.…”

“…The value from each reflector represents an interval attenuation rate, so taken together the result is a depth-attenuation profile. This method is analogous to that used for common-midpoint surveys (Holschuh and others, 2016). Unlike the bed, internal reflectors are typically specular, and are less likely to have spatially variable electrical properties over short wavelengths, so the |∂[ R ]/∂ z | ≪ |∂[ P c ]/∂ z | assumption (also required for Method 1) is more likely valid.…”

“…Even carefully designed experiments, such as common-midpoint surveys, which eliminate the effects of spatial variability in ice temperature, impurity concentration and basal reflectivity by isolating differences in reflected power as a function of the travel path in the ice, often suffer from geometric (angle-dependent) effects. Common-midpoint surveys can therefore result in erroneously high attenuation rates (Holschuh and others, 2016).…”

Five decades of radioglaciology

“…While these multi-channel sounders do achieve some diversity in viewing angle and englacial propagation, true bistatic observations and tomographic inversions can be exploited to provide much richer constraints on subsurface properties including, for example, using commercial pulsed ground-penetrating radar (GPR) systems in common mid-pointor borehole configurations to achieve wider (though coherence-limited) offsets (e.g. Kofman and others, 2015; Holschuhand others, 2016; Patterson and others, 2017; Church and others, 2019).…”

“…Hills and others, 2020; Jeofry and others, 2020). Additionally, investigating attenuation with variable offset can constrain englacial, attenuation, though there is a limit on the maximum offset achievable with commercial GPR systems (Holschuhand others, 2016). These empirical attenuation values can either be used to correct losses to enable reflectivity interpretation or interpreted themselves as a proxy for englacial temperature.…”