Problems

Other GPR work on coastal dunes (Wenell and others, 1998) shows that the water table remains flat under coastal dunes. However, in the Cape Henry profiles, the water table reflection arches up into dunes and hills as shown in the Picnic Area line (below).

The velocity of EM energy traveling through dry sand was measured at Cape Henry as 0.14 m/ns. In saturated sand, velocity is approximately half of the dry velocity (0.07 m/ns). This causes dry units to appear compressed, resulting in a water table with an apparent arch. Since pulseEKKO software assigns only one velocity value for each profile, profiles with low hills that are close to the water table, will always have partially incorrect depth values. For example, the velocity applied to the Picnic Area profile is for dry sand. The resulting depth values are true for the area above the water table, but twice the correct value for the area below the water table.

In most Cape Henry profiles, there was no reflection marking the boundary between dunes or ridges and the land surface that they formed upon. This confirms that ridge formation occurred at the same time as beach progradation, and suggests that parabolic dune migration also occurred soon after ridge formation, leaving no time for soil development.