Herndon Bay GPR

GPR data are acquired in time, not depth. The data are recorded and processed relative to the ground surface. In order to present the GPR transect as something resembling a geologic cross section, it has to be surface normalized or topographically corrected.

Topographic Correction of GPR Profiles Based on Laser Data
Abstract.

Data obtained by GPR (Ground Penetrating Radar) are displayed as a continuous cross-sectional profile. Surface, generally, is not flat. As a result, the image becomes distorted and the depth calculated from the surface no longer represents the true and exact position of electrically distinctive layers and objects in materials. In order to get real geologic cross section, GPR data must be corrected. This is paper discusses a new method using the color point cloud data obtained by a Vehicle-borne laser scanning system to compensate for elevation fluctuate. Elevation profile can be extracted from topographic data of survey site acquired using laser scanner, which can then be used to offset the error of GPR data. Through the discrete points in the survey line, each trace of the profile has its own elevation value showing a vertical difference from the reference profile with maximum elevation, then time shifts value of traces vertical offset versus the reference trace of profile can be obtained. At last, the results of topographic correction for radargrams that look extremely like the real geologic cross section are presented, which allows us to get a better profile interpretation and position of the objects and layers in the subsurface.

Di Zhang et al 2014 IOP Conf. Ser.: Earth Environ. Sci. 17 012251

Zhang.png

The GPR transect in Moore et al., was also corrected for terrain (surface normalization) using LiDAR.
Moore_GPR1.png

The data were acquired with a 300 ns recording window. This is approximately 5-9 m. The depths on the GPR transect are gross approximations due to the variability of the velocity field. While a surface normalized GRP transect looks like a geologic cross section, it is not. It is a geophysical approximation of a geologic cross section.

Moore_GPR2.png

On depth sections, the top of the Black Creek Group mud facies is essentially flat from the extant basin to second oldest rim. The mud facies under the oldest rim is about 1 m higher than the rest of the rims and basin.

Geoprobe core data reveal wave ravinement into the underlying Cretaceous muds, with muddy sand incorporated throughout the oldest sand rims during the initial period of high-energy lacustrine processes (Figure 4). Coring of sand rims demonstrates the scoured nature of the underlying mud facies, with an elevation drop between the older remnant basin surface to the east and the more recent basin due to scour associated with the initial period of migration and sand rim construction (Figures 3c and 4).

pg 155

[…]

The fact that Carolina bays can migrate, yet maintain their characteristic oval shape, orientation, and rim sequences demonstrates that these landforms are oriented lakes shaped by lacustrine and eolian processes. Clear evidence of basin scour into the underlying Cretaceous sandy mud, reveals that Carolina bays are capable of migrating while backfilling remnant basins with a regressive sequence of paleoshoreline deposits as the position of the basin margin changes through time.

pg 167

moorexsect.png

 

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