Description:
The gravity meter senses the acceleration of gravity to one part in one billion. Careful controls must be exercised in the field and in data reduction to realize the full potential of the instrument. The device senses the total gravity field of the earth, but local mass excesses or deficiencies can be detected as minor changes in the pull of gravity. For the purposes of most geologic investigations, it is not necessary to measure the absolute gravity field, rather the relative changes from one point to another. This simplifies the procedure considerably.
In the field, the instrument is leveled and a reading is taken by nulling the apparatus with a dial that moves a system of delicate springs and levers attached to a mass. When nulled, the reading is taken off the dial. In the office, this reading is changed to relative gravity by multiplying by the calibrated dial constant. Also, corrections are made for areal change in elevation because, at the earth's surface, an error of about 17 feet equates to one milligal error. A further correction for the earth's mass between sea level and instrument is carried out using the best value of mass. A drift correction is made by use of repeat readings at a base station; most of the drift is tidal effect. Finally, a correction is made for the local irregularities in the terrain; this is a difficult and time consuming correction.
