NOVELTY - The gravimeter (10) has a chamber to contain a gas, and having a longitudinal axis. A cavity ring-down spectrometer (CRDS) measures a density of the gas at a location along the longitudinal axis using an optical cavity having an optical axis and configured to resonate light rays that are absorbed by the gas in the optical cavity. The optical axis has a vector component perpendicular to the longitudinal axis. A processor (8) receives a density measurement from the cavity ring-down spectrometer and estimates the gravitational acceleration using the density measurement.    USE - Gravimeter used e.g. for hydrocarbon production, geothermal production, and carbon dioxide sequestration, for estimating gravitational acceleration under a surface of the earth.    ADVANTAGE - Increases sensitivity and, thus, the accuracy of the gravity measurements, since CRDSs are located as far apart from each other as practical within the constraints of the gravimeter that are necessary for being conveyed within the confines of the borehole, the CRDSs being highly sensitive spectrometer that provides higher accuracy than other spectrometers, which is necessary for making gravitational acceleration measurements where differences in gas density measurements may be very small and can measure molar fractions down to the parts per trillion level. The length of the chamber and, thus, the distance between CRDSs are constrained due to the necessity of being able to be conveyed through the borehole. The smaller distance between CRDSs further leads to smaller differences in gas density measurements requiring highly sensitive gas density measurements.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for a gravitational acceleration estimation method.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of a gravimeter disposed in a borehole penetrating the earth.    Earth formation (4)    Armored wireline (6)    Downhole electronics (7)    Processor (8)    Gravimeter (10)