Study of Earth‐tides by gravitational measurements

Abstract

The existence of earth‐tides has been shown by (1) studies of long‐period ocean‐tides and (2) the tilt of land‐surfaces usually measured by horizontal pendulums, and (3) by observations on the variation of gravity at a given place. The method involving the study of ocean‐tides may be dismissed as having only historical interest since any significant measurement of earthtides by this method is quite impracticable. The second method has been used extensively since horizontal pendulums or equivalent devices of adequate sensitivity for measuring the tilt of landsurfaces have long been available. However, though the required measurements may be made, the problem of interpretation in terms of vertical displacement necessarily involves the integration of many simultaneous observations covering a large area, for normal variations in the effective rigidity of the Earth's crust due to variations in lithology or structural features result in spurious local deformations which must be eliminated from the broad‐scale picture if a true measure of the vertical tidal displacement is to be obtained. Clearly, observations at an excessively large number of stations would be required and the final interpretation would present an enormous problem. Indeed, the difficulties appear quite insurmountable if one is to judge from results thus far obtained and the reports of recent work (Travaux de la Association de Géodésie, Union Géodésique et Géophysique Internationale, Rapport sur les Marées de I' Écorce Terrestre, Paris, 1933).