Accumulation Layer PickingUsing FMCW CReSIS Radar and North-Central Greenland Ice Core Data

Renee’ Butler, David Braaten, Sam Buchanan, Kyle PurdonCenter for Remote Sensing of Ice Sheets (CReSIS), University of Kansas, Lawrence KS 66045, Haskell Indian Nations University, Lawrence KS 66046

AbstractTwo picking tools were used; one for training and one for picking the layers. The one for training is the surface/bed picking tool which specifically picks the surface of the ice and the bedrock. The new tool is used for picking layers in between the surface and the bed; but usually not all the way down to the bed. The data being used was collected along the ice divide in North-Central Greenland in 2007 with frequency-modulated continuous wave (FMCW) radar. The layers being picked are from a 70km segment and is deeper than 30 m to a max depth of 100 m and date back more than 400 yrs. The layers are used in a similar manner as tree rings to understand whether or not annual layers are missing and if the vertical distance between layers (slab thickness) includes more than one year of snowfall. The slab thickness is related to the annual accumulation of snowfall; thicker slabs mean more snow and thinner slabs mean less snow during the year. Nearby ice core data will be used to understand the chronology of the layers. The ice core data also provides ice density profile measurements; which will be used to more accurately calculate the radar signal propagation speed through the ice. This will provide increased accuracy of determining the internal layer depths.

Surface/Bed Picking ToolThis tool is MATLAB software. The GeoTIFF is selected as the background image to the echogram (the visible data being picked). The Standard source type was good for most of the examples, but some were more challenging. For a surface that is difficult to see QLook or IMG_01 help it to become more visible. For the bed MVDR or IMG_02 help it to become more visible. The source data was usable Raw but Averaged smoothed it out and helped the bed become more visible. The tools Max point and Snake were used to pick the surface and bed layers. The tool Enter point was used to insert a manual point to make corrections for the snake tool.

Layer Picking ToolThis tool is MATLAB software that is still under development so some things mentioned are subject to change. This picker is used to pick the internal layers of the ice sheet in North-Central Greenland. The tool enter point was used to enter the points for the layer; these points were interpolated to create the layer data that was used. The data included an average of one thousand points plotted but only about three to four hundred points were plotted. The layers nearer to the surface appeared thicker than the layers towards the bottom of the frame. Sometimes the layers seem to disappear and then reappear. The areas where the layer disappeared were unpickable. The first layer picked took longer to pick, but once the tool becomes more familiar the third and fourth layers were quicker to pick. The results were recorded in two-way travel time; which is the time it takes the radar to travel into the ice and bounce back; and needed to be converted into depth.

Picking

Figure 1: Layers already picked Figure 2: Picking (red layer is selected)

When first starting to pick expect to go slow but then speed up as the tool becomes more familiar. It took an average of six hours to pick the first layer, but then it took about three to four hours for the third and fourth layer. It was harder to pick the deeper the layers went. Deeper layers became harder to see and closer together.

Results

Figure 3: Depth of Layers Figure 4: Depth Between Layers

The layers picked are between 59-62m deep. The slab thickness varies between thick and thin. Some layers have more accumulation (such as the dip in Figure 4).

ConclusionsSome double checking is needed to make sure the thicker areas of layers is true and not just a line jump in the picking. Also, the layer thickness is converted from two-way time into meters for depth. More conversions are need to compare the density to the ice core samples. Also, the density will be used to find the water content to better understand how much accumulated.