In Oct 2005, a helicopter-borne transient electromagnetic (TEM) survey was flown over the Tooele Army Depot (TEAD) using the SkyTEM dual transmitter system. Data were acquired over an 8kmx8 km area with a line spacing of 200m. Laterally constrained one-dimensional (1-D) inversion was used to produce resistivity interval maps that were used to refine the location of hydrogeological units defined by faults and delineate an uplifted basement block – both significant hydraulically controlling features.
TEAD covers over 25,000 acres 35 miles southwest of Salt Lake City, UT at the south end of Tooele Valley. Mountains border the valley to the east, west and south, with the Great Salt Lake forming the northern boundary. The basin fill material in the vicinity of TEAD consists primarily of coarse-grained alluvial fan sediments. The most notable geologic feature is a shallow but largely buried bedrock block within the eastern portion of TEAD composed of Paleozoic interbedded quartzite, sandstone, and limestone. Drilling and geophysical data suggest that the northwest margin of the block is fault bounded, the southeast margin may be defined by a fault, and the bedrock contact plunges to the south and the northeast beneath alluvial cover. TEAD served as a site for the service, maintenance and storage of military equipment and vehicles. From 1942 to 1988 various hazardous materials were disposed of through wastewater flowing into unlined ditches, spreading areas and lagoons resulting in groundwater contamination. After monitor wells were used to characterize a trichloroethylene (TCE) plume, a pump and treat system was put in place to isolate and remediate the plume. In 2004 the system was shut down and an extensive monitoring program was implemented.
SkyTEM is a time-domain, electromagnetic helicopter system designed for hydrogeophysical and environmental investigations (Sørensen and Auken, 2004). The SkyTEM system is designed for mapping geological structures for groundwater and environmental investigations, and was developed as a rapid alternative to ground-based TEM surveying. Independent of the helicopter, the entire system is carried as an external sling load suspended from a helicopter. The SkyTEM system is unique in that a dual transmitter is used for high vertical resolution of the near surface in addition to deep penetration of the subsurface. The receiver is located approximately one meter above a rigid transmitter frame. Laser altimeters, GPS and inclinometers are positioned on the frame. Transients are recorded from 10 µsec to 10 msec.
The electronics, a power generator and an antenna for transmission of data to ground personnel are secured on the cable roughly half way between the transmitter and the helicopter. Only the pilot is required on board the helicopter. Very accurate data are acquired because of the very low internal noise and no drift, both of which are important for resolution of low resistivity constraints common in hydrogeophysical investigations.
