Integration of GPS and levelling for subsidence monitoring studies at Costa Bolivar oil fields, Venezuela
Monitoring of ground subsidence has been traditionally performed by means of geodetic levelling techniques. Geodetic levelling is slow and costly, requiring long connection lines to stable areas, and higher densification in critical areas to properly depict the deformation behaviour. The Global Positioning System (GPS) has been envisioned as an attractive alternative in the domain of deformation monitoring, bringing about potential savings without signification deterioration in accuracy. The Cost Bolivar oil fields in Venezuela have been subject to subsidence since 1926 at a rate of 20 cm/year. The monitoring scheme has been based on geodetic levelling and an already obsolete computational methodology. A full evaluation of the whole scheme has revealed a total uncertainty of 20 to 30 mm at the 95% confidence level for the subsidence determination and of 15 to 20 mm at the 95% confidence level for the absolute elevations. A methodology in integrate GPS with levelling in order to modernize and optimize the present monitoring scheme has been designed. The results of pilot tests to evaluate the real accuracy of GPS in the area using WM101 receivers, show an accuracy of 29 mm independent of baseline length. Accuracy standards developed for the optional integration reveal, however, that relative GPS accuracies in the order of 10 to 15 mm are needed for compatible results with levelling. The results of an economic analysis on the designed integration networks show savings in the order of 26% in the cost of one campaign which is an indication of the feasibility of GPS when used in combination with levelling for subsidence monitoring studies.