Knowledge Bank

The general principles of the use of known density anomalies for gravity field modelling are reviewed with special emphasis on local applications and utilization of high degree and order spherical harmonic reference fields. The natural extension to include also unknown density anomalies will be studied within the framework of geophysical inversion methods, and the prospects for " hybrid" gravity field modelling/inversion methods will be outlined. A very simple case of such methods is the determination of representative topographic densities through collocation with parameters. The topography, being the dominant density anomaly, together with the isostatic compensation, may be taken into account by various types of terrain reductions. Special attention is given to residual terrain reductions, i.e. using spherical harmonic expansions of the topography as a reference. The auxillary quantity "the terrain correction'' is investigated in detail, and possibilities for approximations (so called ''linear approximation") in terrain effect computations are evaluated through models and actual data. Frequency domain methods using the Fast Fourier Transform are studied on a theoretical base, and used for error studies to investigate the resolution of topographic data versus the attainable accuracies of computed terrain effects. Actual topography and gravity field data is analyzed by FFT for many different areas of USA and two areas of the Pacific, yielding power spectra, degree-variances and covariance functions for the topography and the gravity field. Results show topographic covariance functions to be of exponential type, and that use of terrain reductions as expected produce a gravity field of less variance, longer correlation length and higher degree of isotropy.