[en] A rotary-wing, reconnaissance, high sensitivity, radiometric and magnetic survey was performed in the Big Bend area of Texas. Four 1:250,000 scale NTMS quadrangles (Marfa, Ft. Stockton, Presidio, and Emory Peak) were surveyed. A total of 7,529 line miles (12,115 kilometers) of data were collected utilizing a Sikorsky S58T helicopter. Traverse lines were flown in an east-west direction at 3.0 mile (5 kilometer) spacing, with tie lines flown in a north-south direction at 12.5 mile (20 kilometer) spacing. The data were digitally recorded at 1.0 second intervals. The NaI terrestrial detectors used in this survey had a total volume of 2,154 cubic inches. The magnetometer employed was a modified ASQ-10 fluxgate system. The radiometric data was normalized to 400 feet terrain clearance and is presented in the form of computer listings on microfiche and as stacked profile plots. Profile plots are contained in Volume II of this report. A geologic interpretation of the radiometric and magnetic data is included as part of this report