Geophysical Surveys, Levee Certification Geophysical Investigations, DC Resistivity

Abstract

The Yuma Area Levee system, running along the northern boundary of the city of Yuma (Arizona), trends parallel to sections of the Gila and Colorado Rivers. These Levees constructed from the surrounding earth, are composed of an assortment of sands, silts, clays, and quarried rock (which armors the river side of each levee structure). Sections of the levee system, and portions of the levee system near Walker’s Ranch have been chosen for investigation to determine structural integrity and areas susceptible to slope failure or internal erosion in the event of flooding. By using direct current resistivity (DCR), an assessment of the subsurface levee structure can be made, identifying material properties within the levee itself that indicate the presence of sands, clays, or anomalous void spaces if present. This data is used to help determine locations for levee remediation. This report discusses results from the DCR surveys performed at various locations along the levees. In general, areas of high electrical resistivity can represent locations of clean sands and gravels which may be prone to internal erosion at flood stage river levels. Areas of localized very low resistivity can indicate the presence of steel or other metallic materials such as culverts. This report provides a table of all direct current resistivity soundings and their quality of processed data. A few of the soundings revealed very high root mean square (RMS) error. These soundings will be disregarded. Direct current resistivity soundings with lower RMS error more accurately reveal areas of high resistivity sands and gravels within the levee structure. Only soundings classified as Good Data and Very Good Data will be used in evaluating the subsurface conditions when considering possible levee remediation locations. Direct Current Resistivity Summary Using a multi-electrode direct current (DC) resistivity system, DC resistivity Wenner surveys were performed at 14 locations along the Levees north of Yuma, AZ. These surveys were interpreted through multiple inversions using the Earth Imager 2D software package (Advanced Geosciences, Inc.). Background Areas of high resistivity may indicate areas containing sands, gravels, other openwork coarse grained materials, or void space within the structure (although large void spaces are unlikely). Structures including culverts and steel pipes may appear as low resistivity (high conductivity) areas. However, because these structures have limited diameters, DCR measurements may not be sensitive to influence of their material properties. Areas of lower resistivity usually indicate higher clay content or a relative increase of water saturation.