GEON Databases, Tools, and Application Development 76 Gravity Reduction Spreadsheet to Calculate the Bouguer Anomaly

Objective: Develop a portable and easy to use gravity reduction program using standardized methods and constants to calculate the Bouguer gravity anomaly. Current standards for reduc-tion of observed gravity to a modeled Bouguer anomaly are unregulated and vary among geo-physical textbooks, commercial software programs, and academic research spreadsheets avail-able for download from the Internet. Using new standards established by the U.S. Geological Survey and the North American Gravity Database Committee, we developed a spreadsheet for reduction of raw data to the Bouguer anomaly and, with the use of terrain correction, the Com-plete Bouguer anomaly. We view the spreadsheet as particularly useful for field data reduction and modeling where internet access is limited or unavailable. Description: The spreadsheet is based on Microsoft Excel, which is a common software applica-tion used by government agencies, research institutions, and private companies. The equations used in the spreadsheet are derived from the FORTRAN code written by Mike Webring of the U.S. Geological Survey and are the same as those used by the GeoNet Server accessible at the Pan-American Center for Earth and Environmental Studies. With the use of the Global Positioning System (GPS) for surveying station locations and alti-tudes, availability of digital terrain models, and enhanced computational capability, gravity modeling is a cost-effective tool in subsurface analysis ranging from basin to continental scale studies. Existing gravity data for North America are archived and readily accessible via the in-ternet at the Pan-American Center for Earth and Environmental Studies Web sit