Cần đặt vấn đề tìm, thăm dò và xây dựng các giếng khoan khai thác nước ngầm nguồn gốc nội sinh. Lâu nay, do quan niệm nước ngầm do trên mặt ngấm xuống thành các tầng chứa nước nên người ta tìm rất tốn tiền mà không ra. | 324 Gerald Gabriel applying formulas for simple source bodies . Telford et al. 1990 . For most investigations in the frame of groundwater geophysics point spacing between 50 m and 100 m is sufficient. The surveyed area should exceed the dimension of the geological target in order to map the corresponding gravity anomaly completely and to distinguish between regional and residual gravity anomalies. In areas where less information is available a survey should start with a coarse net of points in areas of interest additional measurements can be performed later. -3-2-10123 relative position of level Fig. . Inclination-insensitive points of gravimeter G-662 on 02nd July 2002 1 Volt 1 mGal During the survey gravity stations have to be selected properly. Readings can be affected by microseismics natural and artificial and wind causing strong disturbances by trees . Gravity stations near steep slopes buildings frequented roads and ditches should be avoided. Ideally each gravity station within the surveyed area is recorded twice in order to avoid outliers. The reading procedure itself should be the same at each point of the survey. The dial has to be approached from the same side each time otherwise its backlash will cause errors that can amount to some 10 pGal. The reading can be taken as soon as the gravimeter output is stable best controlled by a feed-back system. The total time required for the observation of one single station is between 5 and 10 minutes. Therefore about 30 to 50 stations can be surveyed during a day provided that the time for transport between the stations is negligible. Unfavourable environmental conditions like bad weather or microseismics can cause delays. Local gravity investigations in the frame of groundwater geophysics will often be performed by densifying an existing data base by comple 11 Microgravimetry 325 menting measurements. In order to assure the compatibility of the gravity values resulting from different campaigns relative .
