A regional gravity survey of Jameson Land, Eastern Greenland

Abstract

Some 60 gravity stations have been established within and adjoining the Cerro Prieto geothermal field for the purpose of monitoring the small temporal changes due to fluid withdrawal and subsidence. Each station consists of a permanent concrete slab whose initial elevation has been determined by the CFE's second-order levelling survey. It is intended that these stations will be reoccupied and relevelled at time intervals ranging from 6 to 12 months; so far, only the initial occupations have occurred, with 70% of the stations having been completed.Each station occupation consists of a minimum of four sets of four readings apiece, each set read on a different day. Four persons participated equally in making the gravity observations, utilizing two G-model LaCoste and Romberg gravity meters. Three base stations serve as reference points for the field work: (1) an external base in Mexicali, occupied at the beginning and end of every field session; (2) an internal base in the Cerro Prieto area, occupied every 4 or 5 hr to check instrumental drift and possible tares; and (3) a base on the Cerro Prieto volcano, occupied every 3–4 weeks to verify changes which might occur within a long-duaration occupation period. Adverse problems encountered in the field have included rough roads, windy periods, temperature extremes and locally high seismic activity due to both natural and cultural activity.Data reduction has so far included reduction of meter readings to observed gravity values, dedrifting (using Longman's 1959 tidal formulas), calculation of gravity differences between bases and stations, and initial statistical evaluation (analysis of variance). Precision is expected to be approx. 10 μgal. Future reduction will include calculation of Bouguer anomaly values (to supplement the values currently available) and complete statistical regression analyses to study various error sources; this latter task may result in a redesign of field and reduction techniques. Two full days of tidal monitoring and a second reoccupation of all stations will be done in November and December of 1978.