A Radar and DC Resistivity Survey of Arid Ephemeral Stream Systems Near Yuma, Arizona: bedrock morphology, channel stratigraphy, and soil moisture content
Presented at the 2011 American Geophysical Union Fall Meeting
Andrew Genco, Dennis L. Harry, Colorado State University, Dept. of Geosciences, Fort Collins, Colorado, U.S.A.
Ground Penetrating Radar (GPR), Direct Current (DC) Electrical Resistivity, and laboratory resistivity measurements were collected to determine the depth to bedrock, subsurface stratigraphy, and spatial variations in soil moisture in fluvial channels in the arid Sonoran Desert region near Yuma, Arizona. Six sites were surveyed, three each at Yuma and Mohave Washes, which are two minimally disturbed ephemeral channel systems. The purpose of the survey is to determine whether large vegetation present in the larger channels is sustained primarily by soil moisture, or by rooting below the water table within the fractured bedrock. An abrupt change in the GPR character marks the contact between bedrock and alluvium, from abundant coherent reflections in the alluvium to minimal weak reflections and occasional diffractions in the bedrock.
Bedrock depths range from approximately 1 to 5 m at Mohave Wash, with the greater depths corresponding with the larger channels in the wash. At Yuma Wash the contact between bedrock and alluvium ranges from approximately 1 to 4 m depth, also with the greater depths being associated with the larger channels. Resistivity in the subsurface at Mohave Wash ranges from 130-1100 Ω-m in the upper 2 m and from 15-130 Ω-m below 2-5 m depth. At Yuma Wash, the resistivity ranges from 46-500 Ω-m in the upper 1 m, and from 4-46 Ω-m below approximately 1-4 m depth. Preliminary interpretation indicates that the potential root zone within the soil is limited to relatively shallow depths, less than 5 m thick in the places surveyed, with the soil likely being unsaturated above 1-2 m.