VARIATIONS OF VERTICAL GRAVITY GRADIENT IN NEW YORK CITY AND ALPINE, NEW JERSEY

Abstract

Careful gravity measurements with La Coste‐Romberg geodetic gravimeters were carried out in tall buildings on a floor‐to‐floor basis in New York City and on the Armstrong Tower, Alpine, New Jersey. Corrections for the instrumental drift and tidal gravity variation and for the Bouguer effect, topography, mass of the buildings, and subway and basement excavations have been applied to the gravity data, which are tied to the absolute gravity value of the National Gravity Base Station of Washington, D. C. The observed gravity versus elevation curves are nonlinear, particularly near the surface of the ground; the slope of the observed gravity anomaly versus elevation curves reverses sign at an elevation of about 170 ft for the campus buildings and about 350 ft for the downtown buildings, and is nearly linear without a reversal for the Armstrong Tower. The vertical gradients vary substantially even within short distances. Comparisons of the corrected observed gradients with the theoretical gradients of gravity are made. The anomalous gradient anomalies are positive and are correlated with the positive isostatic surface gravity anomalies. Calibration of gravimeters against the observed vertical gradient of gravity to an accuracy of ±2 μgal is definitely feasible provided the gradient is predetermined to a comparable accuracy by a standard instrument.