Geophysical survey of Rhoads Pond, Bryn Mawr College campus, during lab for GEOL 205 (Sedimentary Materials and Environments), Fall 2003.

The map and subsequent images below show the location and results of a preliminary geophysical survey of Rhoads Pond using ground-penetrating radar.

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The survey was made from an inflatable rubber boat, moved by lines attached fore and aft held taut by field assistants on shore. Unfortunately the boat contained aluminum supports in the flooring (see photos) and these caused some interference with the GPR antennas. This interference was particularly problematic in the middle of the main basin line. We repositioned the antennas and got cleaner data in the 2nd and 3rd FOREBAY lines.

The first several plots illustrate a) unprocessed GPR data, b) the effects of subsequent processing, and c) the interpreted GPR record. Conductivity of the pond was measured shortly before the survey. Comparison of the relatively low conductivity values with published data allowed a reasonably confident assignment of the standard freshwater signal velocity = 0.033 meters per nanosecond; this velocity is used to assign depth (m) to the righthand side of the plots. The bottom of the pond was clearly imaged in most cases, but the interpretation of reflections below the pond floor was more ambiguous. Some of these may have been "ringing" associated with the sediment/water interface. The actual PulseEKKO data and header files can be downloaded here.

The GPR transect lines and start/end points were not surveyed, although we attempted to maintain shot spacings of 25 cm using a non-metallic tape measure.

We anticipate acquiring cleaner and more precisely positioned data in future surveys by using a fiberglass boat to minimize metal interference, and by simultaneously tracking the boat position with a fixed onshore total station.

The field photos below show our rubber boat, the alignment of the antennae in the boat, and various stages of survey in main bay and forebay. Click on any of these images for a page with larger photos and captions.

Figure 1. Map of tracklines.

Figure 2. Field data and survey parameters.

Figure 3. GPR data with gain applied.

Figure 4. GPR data processed to emphasize pond floor reflections. Processing parameters listed at bottom of figure.

 

Figure 6. Muted wiggle-trace plot of processed GPR data, with interpreted reflections overlaid.

Figure 7. Processed data from FOREBAY line 2; this line is parallel to previous line, in opposite direction (see map). There is a hint of two separate reflections at the bottom, possibly indicative of original (landscaped) pond floor, with new sediment on top. On the left side of plot, strong subbottom reflections are metamorphic bedrock, which crops out along the forebay inlet.

Figure 8. Processed data for FOREBAY line 3; this shorter line is perpendicular to previous two lines (see map).

Department of Geology