An interpretation of the 1997 airborne electromagnetic (AEM) survey, Fort Huachuca vicinity, Cochise County, Arizona

Abstract

Executive Summary -- In March of 1997, an airborne electromagnetic (AEM) survey of the Fort Huachuca Military Reservation and immediate surrounds (location map, http://geopubs.wr.usgs.gov/open-file/of99-007-b/index.jpg) was conducted. This survey was sponsored by the U.S. Army and contracted through the Geologic Division of the U.S. Geological Survey (USGS). Data were gathered by Geoterrex-Dighem Ltd. of Ottawa, Canada. The survey aircraft is surrounded by a coil through which a large current pulse is passed. This pulse induces currents in the Earth which are recorded by a set of three mutually perpendicular coils towed in a 'bird' about 100 m behind and below the aircraft. The bird also records the Earth's magnetic field. The system samples the Earth response to the electromagnetic pulse about every 16 m along the aircraft flight path. For this survey, the bulk of the flightpaths were spaced about 400 m apart and oriented in a northeast-southwest direction extending from bedrock over the Huachuca Mountains to bedrock over the Tombstone Hills. A preliminary report on the unprocessed data collected in the field was delivered to the U.S. Army by USGS in July 1997 (USGS Open-File Report 97?457). The final data were delivered in March, 1998 by the contractor to USGS and thence to the U.S. Army. The present report represents the final interpretive report from USGS. The objectives of the survey were to: 1) define the structure of the San Pedro basin in the Sierra Vista-Fort Huachuca-Huachuca City area, including the depth and shape of the basin, and to delineate large faults that may be active within the basin fill and therefore important in the hydrologic regime; 2) define near surface and subsurface areas that contain a large volume fraction of silt and clay in the basin fill and which both reduce the volume of available storage for water and reduce the permeability of the aquifer; and 3) to evaluate the use of the time domain electromagnetic method in the southwest desert setting as a means of mapping depth to water. Chapter one, written by M.E. Gettings, reports the results of the analysis of the aeromagnetic anomaly data. Depths to magnetic rocks computed from these data are in good agreement with depths from gravity anomaly models (Gettings and Houser, in prep.) and confirm and refine the location of the bedrock highs which reach to within 200 m of the surface in several parts of the study area. The highly faulted and generally shallow character of the basin within the study area deduced from the gravity studies is also evident in the aeromagnetic data. The caldera ring fault delimiting the buried structural southwest edge of the Tombstone caldera is expressed in the magnetic data and deeper intrusives extending outside the caldera to the southwest are inferred. Several magnetic bodies occur at shallow depths within the Precambrian granite of the Huachuca Mountains along the eastern foothills of the mountains. These are inferred to be Tertiary intrusives but remain to be confirmed by field work if any of their uppermost dikes or apotheses are exposed. Faults delineating the east-west trending bedrock high beneath the city of Sierra Vista appear to be shallow and should be investigated for surface expressions. Chapter two, written by Jeff Wynn, analyzes and interprets the conductivity depth transforms (CDTs) and provides a general evaluation of the data quality. He concludes that there is a good general correlation between the uppermost conductor seen in the CDTs and water table depth in many cases. Detailed comparisons between the ground-based vertical electric soundings (VES) and the CDTs are reported in this chapter. The two sets of data compare well in general for most sounding sites where the VES data are not noisy.