GRAVITY STUDIES - ESTABLISHMENT OF REFERENCE BASE STATIONS IN JOHNSTOWN, PENNSYLVANIA

The Pan-American Center for Earth and Environmental Studies maintains archives of reference gravity base stations in North America. These reference locations provide base stations to support regional geologic studies. We found no reference gravity stations to support our work near Johnstown, PA and have established three new stations using the Altoona, PA reference station. Our work was accomplished on 9/20/2008 using a LaCoste-Romberg gravimeter (Model G). The data loop between Altoona and Johnstown was obtained during 3:04-5:54 EDT, a time of virtually no tidal change. We visited the former reference gravity stations located at the Bedford, PA airport, and at the campus of Indiana University of PA. These base stations are no longer usable because the benchmarks are gone. Our three new base stations are at or near the University of Pittsburgh at Johnstown and are publically accessible. The first station is in the Krebs Building, in the stairwell on the basement level at the NE corner of the building. A brass plate on the basement wall documents an observed gravity value on the adjacent floor of 980012.27 mGal. We estimate the decimal latitude for this station at N 40.2754 degrees. The second base station is located at a benchmark on Berwick Road, 1.5 km southeast of the UPJ campus. This benchmark appears at the southern edge of the Geistown 7.5' quadrangle map, with an elevation of 1808 ft. The benchmark is on the south side of a bridge structure and has the following characteristics: elevation 551.548 m, decimal latitude N 40.2574 deg., and observed gravity 980039.00 mGal. The calculated Bouguer anomaly value for this benchmark location is -45.62 mGal. The third reference gravity station is located on the UPJ campus at a field used for training in field mapping and geophysics. In 2009 we will build a permanent monument for this base station, which has the following characteristics: elevation 666.017 m, decimal latitude N 40.2643 deg., longitude -78.8262, observed gravity 980017.64 mGal, and a Bouguer anomaly value of -45.09 mGal.