Magnetotelluric data in the middle Rio Grande basin, Albuquerque volcanoes, New Mexico

The population in the Albuquerque-Santa Fe region of New Mexico is rapidly growing. The Santa Fe Group aquifer in the Middle Rio Grande Basin is the main source of municipal water for the greater Albuquerque metropolitan area. The capacity of this aquifer is more limited than previously thought. The Middle Rio Grande Basin, as defined hydrologically and used here, is the area within the Rio Grande Valley extending from Cochiti Dam downstream to the community of San Acacia. Because approximately 600,000 people (40 percent of the population of New Mexico live in the study area (Bartolino, 1999), water shortfalls could have serious consequences. Future growth and land management in the region depends on accurate assessment and protection of the region's groundwater resources. An important issue in defining the ground water resources is a better understanding of the hydrogeology of the Santa Fe Group and the other sedimentary deposits that fill the Rio Grande rift. The U.S. Geological Survey is currently conducting a series of studies of the Middle Rio Grande Basin in north-central New Mexico to address this issue. One objective of these studies is to improve the hydrogeologic models of the Middle Rio Grande Basin to help land managers plan for the development and use of its water resources. These studies involve a multi-disciplinary approach to better understand the critical aquifers in what is considered to be an intracontinental rift environment. Detailed geologic mapping, high-resolution airborne magnetic surveys, lithologic and geophysical logging of wells, surface-based electrical and magnetic surveys, enhanced satellite imagery, as well as hydrologic and hydrogeochemical data are being used to refine our understanding of the aquifer systems in the Middle Rio Grande Basin (Cole et al., 1999). An airborne time-domain electromagnetic (TEM) survey was flown for the USGS as part of this work. The primary purpose of the TEM survey was to map changes in electrical resistivity with depth that are related to lithologic variations important to the critical aquifers. However, typical conversion of recorded signal to resistivities obtained from TEM surveys are not unique and depend on proprietary algorithms used by the various companies that collect airborne TEM data. Even though the airborne TEM method has been used for many years as a geophysical prospecting tool, its use in hydrogeological mapping is recent. Therefore, before making maps of subsurface resistivity, an evaluation of what depths the TEM inversions were reliable to was needed. This was accomplished by correlating the TEM inversions with well logs and ground electromagnetic surveys (magnetotellurics). This report presents a description of the magnetotelluric (MT) soundings that were used to help calibrate the results of the airborne TEM survey. The purpose of this report is to release the magnetotelluric sounding data; no interpretation of the data is included.