Airborne gravity measurements over mountainous areas by using a LaCoste & Romberg air-sea gravity meter

This paper introduces a new approach to airborne gravity data reduction well-suited for surveys flown at high altitude with respect to gravity sources (mountainous areas). Classical technique is reviewed and illustrated in taking advantage of airborne gravity measurements performed over the western French Alps by using a LaCoste & Romberg air-sea gravity meter. The part of nongravitational vertical accelerations correlated with gravity meter measurements arc investigated with the help of coherence spectra. Beam velocity has proved to be strikingly correlated with vertical acceleration of the aircraft. This finding is theoretically argued by solving the equation of the gravimetric system (gravity meter and stabilized platform). The transfer function of the system is derived, and a new formulation of airborne gravity data reduction, which takes care of the sensitive response of spring tension to observable gravity field wavelengths, is given. The resulting gravity signal exhibits a residual noise caused by electronic devices and short-wavelength Eotvos effects. The use of dedicated exponential filters gives us a way to eliminate these high-frequency effects. Examples of the resulting free-air anomaly at 5100-m altitude along one particular profile are given and compared with free-air anomaly deduced from the classical method for processing airborne gravity data, and with upward-continued ground gravity data. The well-known trade-off between accuracy and resolution is discussed in the context of a mountainous area.