| Abstract |
Global Positioning System (GPS) measurements were conducted in the Northern Negros Geothermal Project (NNGP) in West -Central Philippines to determine the feasibility of using it as a complement to a microgravity survey. If suitable, GPS had the potential to speed up the leveling component of a microgravity survey and significantly reduce the time required for its conduct. At present, first-order precise differential leveling measurements are conducted in tandem with microgravity measurements to determine the free-air correction factor to be applied when reducing gravity values. The accuracy standard is a function of the traverse loop length, defined by the formula: Allowable Closure Error (ACE) < 5mm * L, where L=total length of closed loop To compare GPS with differential leveling data, a loop was simulated from GPS data by doubling the vector length connecting GPS station pairs, taking into account the forward and backward runs of a traverse. Results satisfying the accuracy standard were obtained from the measurements. It was not consistent, however, because there were also measurements that failed to comply with the criterion (11mm closure error for 8mm ACE). It was also observed that the error magnitude was more significant over stations located in forested areas, probably due to obstructed line of sight between the satellites and GPS receivers, and multipath effects. Further tests with more measurement sets and possible improvements were recommended such as an upgrade in instrumentation from single frequency to dual frequency GPS receivers. |