## 3 Results
### Digital Elevation Data  
In the terrain data, the Rimaal structure is gently undulating with low topographic relief. The radar defined crater periphery is coincident with a semicircular-like basin in the DEM (Figure 6).  
A topographic profile derived from these data shows the structure as a bowl-like shallow depression that is nearly 45 m deep with a general tilt from west to east (Figure 7). The formation does not display a clear raised rim, particularly at its eastern side, and the wall is not as steep. The perpendicular alignment of the eastern side of the rim to the direction of the prevailing northeastern trade wind could be the reason for the rapid erosion of the eastern side in comparison to the western side of the structure. Although the terrain profile shows the interior of the structure to be flat with barely any sign of topographic expressions, PALSAR reveals a distinct surface roughness to the structure's interior (Figure 7). In PALSAR, the inner ring and central peak appears to be made up of significantly coarser textured material than that of the structure annular basin which is characterized by fine-textured material.  
The shallow depth of the Rimaal structure could be the result of filling by deposits, including impact breccia, as well as Aeolian and fluvial sediments. According to [7] for a complex impact structure, the true depth (the depth to the crater true floor from the top of the outer rim), and the apparent depth (the depth to the top of the breccia lens from the top of the outer rim), can follow the following empirical relationship:  
\[d_{a}=0.13\;D^{1.06}\;and\;d_{t}=0.28\;d^{1.02} \tag{1}\]  
where \(D\) is the final rim diameter in km, and \(d_{a}\) and \(d_{t}\) are the apparent and true depths in meters, respectively. Applying the formula in [7] to the Rimaal structure of a 10.5 km in diameter, yields an apparent depth of 243 m and an actual depth of 412 m. Based on the topographic data, the current depth of the structure is only 45 m, which means that about 80% of the crater's depression is filled by debris and sediments. The deep erosion to the outer rim would have also contributed to the shallowness of the Rimaal structure. As apparent from the DEM topographic data, the structure betrays considerable modifications by extensive long-term fluvial erosion. The SRTM-delineated drainage network in the area illustrates the presence of river channels of southern flow directions (Figure 8), which drain toward the ancient mega-lake of Northern Dafur [29].  
Figure 6: The topography of the Rimaal structure. (**a**) A terrain oblique 3D perspective view of the structure. (**b**) An oblique, 3D perspective view produced by superimposing the PALSAR L-HH image over the SRTM data.  
Figure 8: _Cont._  
Figure 7: NW-SE topographic and roughness profiles of the Rimal Structure. (**a**) Roughness profile generated from PALSAR L-band radar image. (**b**) Topographic profile extracted from SRTM elevation data. It shows the structure as a bowl-like depression with a depth of approximately 45 m deep and a general tilt from west to east. Although the terrain profile shows the interior of the structure to be relatively flat, PALSAR reveals a distinct surface roughness expression to the structure’s interior with a clear central peak which appears to be made up of significantly coarse textured material than that of the structure’s annular trough.