Field, map, and aerial photoreconnaissance in the Lake Alvord basin has focused on identifying late Pleistocene depositional shoreline features (e.g., tombolos, spits, barriers). Features in different areas of the basin are well defined, and their spatial extents are easily mapped; however, absolute—or even relative—ages of shoreline features are not clear. Ground penetrating radar (GPR) was used to distinguish between intermediate and highstand stage shorelines during what is thought to have been the latest Pleistocene, threshold-controlled lake cycle. Radar transects of 280 and 600 m imaged a spit and a baymouth barrier at sites in the northeastern quadrant of the basin where transects were aligned normal to the strike of each depositional geomorphic feature. Signal penetration with 100 MHz antennas was shallow (∼4 m), but resolution was sufficient to locate and identify gross morphostratigraphic features. Flooding surfaces are shown to correspond to intermediate stage lake surface elevations, and the absence of a flooding surface at the elevation of the highest shoreline indicates this to be the maximum lake surface elevation during this cycle. Elevations of intermediate lake stage elevations and highstand stage elevations were consistent at the two sites, with the highstand elevations corresponding closely to the basin threshold at Big Sand Gap. These data provide a first-order approximation of lake stage sequence and the degree of postdepositional neotectonic activity and illustrate the utility of GPR when used in context with field measurements in distinguishing transgressive and highstand features.

We used ground penetrating radar (GPR) data to help determine the spatial distribution and the subsurface geometry of clastic dikes at the Hanford Site. This information will help to improve the understanding of the hydrological role of these ubiquitous clastic dikes at the Hanford Site. We collected 100 MHz ground penetrating radar (GPR) 3-D surface reflection data at two sites, the 216-S-16 (S-16) Pond and the Army Loop Road sites, and 2-D reflection data along a 6.9 km linear transect (Traverse site) near the Army Loop Road site. The dikes are distinguished in the GPR data by a strongly attenuated zone, disruptions in the continuity of reflections, and diffractions where reflections are disrupted. In general, the data quality is better at the Army Loop Road and Traverse sites than at the S-16 Pond site, probably due to the presence of cobbles at the S-16 Pond site. A high-moisture, fine-grained unit probably causes the strong reflections at the Army Loop Road site and the Traverse survey site. The signal penetration varies between 5 and 12 m below the land surface.