A geophysical investigation can provide valuable geologic information needed to characterize sand and gravel deposits and can be an attractive complement to more common characterization methods like drilling. Although such investigations have already been conducted, a comparison of the different geophysical methods used in these investigations apparently has never been done. For this reason, a study was initiated by the Mineral Resources Program of the U. S. Geological Survey. The goal is to determine the advantages and the limitations of different geophysical methods when used to evaluate alluvial sand and gravel deposits. The study is focused on those geophysical methods that are commonly available because these are most likely to be used by industry. Furthermore, the study is focused on surface geophysical methods.

    Heretofore, four different methods have been evaluated. Three of the four methods — time-domain electromagnetic soundings, frequency-domain electromagnetic profiling, DC resistivity soundings — are similar in that they all measure the electrical resistivity of the ground with depth. Ground penetrating radar, however, maps changes in the dielectric permittivity and/or the electrical resistivity with depth. For all four methods, the objective is to relate the measured physical quantity to the stratigraphy of the alluvial sediments.

    The study was conducted at two sites in the South Platte River valley, northeast of Denver, Colorado. One site was adjacent to an active sand and gravel pit, where the sediments and the underlying bedrock are well exposed. These alluvial sediments are roughly 7 m thick and consist of gravel, sand, and some clay. The bedrock beneath these sediments is mudstone. The water table is within the alluvial sediments — there are roughly 6 m of unsaturated sediments and 1 m of saturated sediments. The thickness of the unsaturated sediments was accurately determined with the time-domain electromagnetic soundings (Figure 1) and the DC resistivity soundings (Figure 2); the thickness was determined moderately accurately with the frequency-domain electromagnetic profiling. The thickness of the saturated sediments could not be determined with any of the tested methods because its electrical conductivity is practically identical to that of bedrock. Sedimentary structures, such as foreset beds, were detected with ground penetrating radar; the bedrock surface was possibly detected when a low-frequency, high-power radar antenna was used.

    The other site was a transect across the entire South Platte River valley, and the subsurface geology here had been determined from 12 test holes. The alluvial sediments consisted of gravel, sand, and some clay, and the sediments are covered with soil that is 1 to 2 m thick. In the center of the transect, the sediments and the soil are between 9 and 16 m thick; on the western and the eastern sides, they are between 15 and 25 m thick. The underlying bedrock is mostly shale. Across the entire transect, the water table is believed to be 2 or 3 m below the ground surface. In the center of the transect, the combined thickness of the sediments and the soil was accurately determined with the time-domain electromagnetic soundings and the DC resistivity soundings. However, on the western and the eastern sides of the transect, the thickness could not be determined with either method; rather, layers in the alluvial sediments that are not present in the center of the transect were detected. Ground penetrating radar could not detect any sediments beneath the soil because the soil is clay-rich.

Fig. 1. TEM soundings


Fig. 2. DC resistivity soundings