Errors in Radar CMP Velocity Estimates Due to Survey Geometry, and Their Implication for Ice Water Content Estimation

Barrett, Brian; Murray, Tavi; Clark, Roger A.

doi:10.2113/jeeg12.1.101

Cited by 42 publications

(17 citation statements)

References 16 publications

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“…We then used the synthetic traveltimes for calculating the reflector depths and the depth-averaged velocities (averaged from the surface to the reflector depths) subject to the NMO equations. Differences in depth-averaged velocities were smaller than 0.5 %, and differences in reflector depths were smaller than 0.5 m. Similar to the findings of Barrett et al (2007), this confirms that in our case the NMO approximation essentially holds, even for comparatively large horizontal offsets and a continuously changing depth-velocity function. This must not always be the case and ray tracing easily allows the NMO approximation to be checked for each specific setting.…”

Section: Benefits Of Traveltime Inversion Using Ray Tracingsupporting

confidence: 90%

Constraining variable density of ice shelves using wide-angle radar measurements

Drews

Brown²,

Matsuoka

et al. 2016

The Cryosphere

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Abstract. The thickness of ice shelves, a basic parameter for mass balance estimates, is typically inferred using hydrostatic equilibrium, for which knowledge of the depthaveraged density is essential. The densification from snow to ice depends on a number of local factors (e.g., temperature and surface mass balance) causing spatial and temporal variations in density-depth profiles. However, direct measurements of firn density are sparse, requiring substantial logistical effort. Here, we infer density from radio-wave propagation speed using ground-based wide-angle radar data sets (10 MHz) collected at five sites on Roi Baudouin Ice Shelf (RBIS), Dronning Maud Land, Antarctica. We reconstruct depth to internal reflectors, local ice thickness, and firn-air content using a novel algorithm that includes traveltime inversion and ray tracing with a prescribed shape of the depthdensity relationship. For the particular case of an ice-shelf channel, where ice thickness and surface slope change substantially over a few kilometers, the radar data suggest that firn inside the channel is about 5 % denser than outside the channel. Although this density difference is at the detection limit of the radar, it is consistent with a similar density anomaly reconstructed from optical televiewing, which reveals that the firn inside the channel is 4.7 % denser than that outside the channel. Hydrostatic ice thickness calculations used for determining basal melt rates should account for the denser firn in ice-shelf channels. The radar method presented here is robust and can easily be adapted to different radar frequencies and data-acquisition geometries.

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Section: Benefits Of Traveltime Inversion Using Ray Tracingsupporting

confidence: 90%

Constraining variable density of ice shelves using wide-angle radar measurements

Drews

Brown²,

Matsuoka

et al. 2016

The Cryosphere

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“…6) was used to constrain the lateral extent of the intrusions. Blakely et al (2016) have also developed a method to retrieve the edge of a body and its depth (Fig. 5) by using the reciprocal of the horizontal gradient at the zero contour of the tilt derivative grid (Fairhead et al, 2008;Salem et al, 2007).…”

Section: -D Forward Modelsmentioning

confidence: 99%

Revisiting Austfonna, Svalbard, with potential field methods – a new characterization of the bed topography and its physical properties

Dumais

Brönner

2020

The Cryosphere

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Abstract. With hundreds of metres of ice, the bedrock underlying Austfonna, the largest icecap on Svalbard, is hard to characterize in terms of topography and physical properties. Ground-penetrating radar (GPR) measurements supply ice thickness estimation, but the data quality is temperature dependent, leading to uncertainties. To remedy this, we include airborne gravity measurements. With a significant density contrast between ice and bedrock, subglacial bed topography is effectively derived from gravity modelling. While the ice thickness model relies primarily on the gravity data, integrating airborne magnetic data provides an extra insight into the basement distribution. This contributes to refining the range of density expected under the ice and improving the subice model. For this study, a prominent magmatic north–south-oriented intrusion and the presence of carbonates are assessed. The results reveal the complexity of the subsurface lithology, characterized by different basement affinities. With the geophysical parameters of the bedrock determined, a new bed topography is extracted and adjusted for the potential field interpretation, i.e. magnetic- and gravity-data analysis and modelling. When the results are compared to bed elevation maps previously produced by radio-echo sounding (RES) and GPR data, the discrepancies are pronounced where the RES and GPR data are scarce. Hence, areas with limited coverage are addressed with the potential field interpretation, increasing the accuracy of the overall bed topography. In addition, the methodology improves understanding of the geology; assigns physical properties to the basements; and reveals the presence of softer bed, carbonates and magmatic intrusions under Austfonna, which influence the basal-sliding rates and surges.

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“…Several studies have successfully performed this type of analysis [Murray et al, 2000, with colocated ice core measurements showing only small errors in EM wave speed (ranging from 1.4% to 5.7% depending on frequency) [Eisen et al, 2002]. Small uncertainties in EM wave speeds are especially important to inversions for water content in ice, as Barrett et al [2007] found that an 8% deviation in the EM wave speed results in an 80% error in inferred water content.…”

Section: Introductionmentioning

confidence: 99%

Constraining attenuation uncertainty in common midpoint radar surveys of ice sheets

et al. 2016

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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.

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Errors in Radar CMP Velocity Estimates Due to Survey Geometry, and Their Implication for Ice Water Content Estimation

Cited by 42 publications

References 16 publications

Constraining variable density of ice shelves using wide-angle radar measurements

Constraining variable density of ice shelves using wide-angle radar measurements

Revisiting Austfonna, Svalbard, with potential field methods – a new characterization of the bed topography and its physical properties

Constraining attenuation uncertainty in common midpoint radar surveys of ice sheets

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