Electrical Resistivity Tomography Data along the Little Colorado River near Leupp, AZ 2019

The Little Colorado River alluvial aquifer near Leupp, Arizona was investigated as a possible source of irrigation water for the Leupp and Birdsprings Chapters of the Navajo Nation. The physical, chemical, and hydraulic characteristics of the alluvial aquifer were studied using geophysical surveys, installation of observation wells, water-level measurements, chemical analyses, groundwater pumping simulations, and review of previous investigations. Geophysical surveys and well borings were used to measure the thickness of the aquifer. Geophysical surveys included Electrical Resistivity Tomography (ERT) to better define the alluvial channel of the Little Colorado River near Leupp. ERT is a commonly used electrical geophysical technique for investigating the subsurface of the Earth. In this study it was tested in two separate areas to determine how effective the technique would be for determining the thickness of the alluvial sediments along the LCR. An Advanced Geosciences Inc. Supersting R8TM multi-channel resistivity/IP meter with a SwitchBox56® was used for the DC resistivity survey and all DC resistivity survey data was modeled using Advanced Geosciences Inc. Earthimager 2D software. For each survey line, 56 electrodes were placed on the ground with a spacing of 5 meters (except Line MB4 had a spacing of 6 meters) for the initial survey segment and when the segment was completed then the first 28 electrodes of the survey were moved to the end of the survey line in a roll-along technique. The survey line can be extended indefinitely in a leap-frog fashion even though each segment is only 56 electrodes at a fixed spacing of 5 meters. Resistivity measurements were made in different arrangements or arrays of electrodes, and for this survey dipole-dipole and strong-gradient arrays were used. For both array types, an electrical current is transmitted into the ground, and the resulting potential differences are measured at the surface (Sharma, 1997). Layers within the Earth that are electrically conductive or resistive will deflect or distort the normal potentials.