The Greenland Gravitational Constant Experiment

An Airy‐type geophysical experiment was conducted in a 2‐km‐deep hole in the Greenland ice cap at depths between 213 m and 1673 m to test for possible violations of Newton's inverse square law. The experiment was done at Dye 3, the location of a Distant Early Warning Line radar dome and the site of the deepest of the Greenland Ice‐Sheet Program (GISP) drill holes. Gravity measurements were made at eight depths in 183‐m intervals with a LaCoste&Romberg borehole gravity meter. Prior to the experiment the borehole gravity meter was calibrated with an absolute gravity meter, and the wireline depth‐rinding system used in the borehole logging was calibrated in a vertical mine‐shaft against a laser geodimeter. The density of the ice in the region was calculated from measurements taken from ice cores obtained from earlier drilling observations. Ice penetrating radar was employed in order to correct the gravity data for the topography of the ice‐rock interface. Surface gravity observations were made to assess the extent to which density variations in the sub‐ice rock could affect the vertical gravity gradient. The locations of the gravity observation points were determined with a combination of GPS recording, first‐order leveling, and EDM surveying. An anomalous variation in gravity totaling 3.87 mGal (3.87×10−5 m/s2) in a depth interval of 1460 m was observed. This may be attributed either to a breakdown of Newtonian gravity or to unexpected density variations in the rock below the ice.