Depth of principal mass anomalies contributing to the earth's geoidal undulations and gravity anomalies
Title:
Depth of principal mass anomalies contributing to the earth's geoidal undulations and gravity anomalies
Author:
Bowin, Carl
Appeared in:
Marine geodesy
Paging:
Volume 7 (1983) nr. 1-4 pages 61-100
Year:
1983
Contents:
To help decipher the distribution of the principal mass anomalies within the earth, an analysis is made of the earth's potential field by simultaneously considering both gravity and geoid anomalies. Ancillary knowledge about the earth from astronomical, angular momentum, seismological, and plate tectonic studies aids in deciding between deep or broad shallow-mass anomalies as the causative source of certain potential field anomalies. The geoid (or gravity) anomaly for an individual gravity potential field feature as a function of increasing spatial resolution (decreasing wavelength) is obtained by accumulating stepwise the contributions of spherical harmonic coefficients for each degree and order for any selected location. This is equivalent to displaying the spectral content of an anomaly. The ratio of gravity to geoid (g/N) for each individual harmonic degree is independent of the harmonic coefficient values, being determined only by the degree and values for normal gravity and the earth's radius. This g/N ratio value has an equivalent point mass depth. Thus, the gravity and geoid contributions from each individual degree have a fixed ratio value in proportion to that of point masses at a depth of some fraction of the earth's radius, and this depth places a greater constraint on the depth of the source mass anomaly than can be obtained from wavelength considerations. Curves are presented that quantify the g/N relationships for two simple geometric degrees 2 and 3 geoid and gravity anomalies. A preliminary decomposition of the earth's gravity field is made using these new considerations. This decomposition comprises four main sources for mass anomalies: (1) mass anomalies at the core-mantle boundary region expressed principally in coefficients of harmonic degrees 2* and 3*; (2) mantle anomalies, and the mass excess of the deeper portions of plate convergent zones, expressed principally in coefficients of harmonic degrees 4* through 10*; (3) upper mantle mass anomalies in the outer 600 km of the earth revealed by residual geoid anomalies from radar altimeter observations; and (4) crustal anomalies revealed by residual free-air gravity anomalies. The observational data cited do not distinguish perfectly between these four sources of mass anomalies, as well as between other possible lesser contributors. However, the evidence is sufficiently reasonable to give credence to the decomposition. The degree 10 geoid thus provides an estimate of the combined contributions from mass anomalies at the core-mantle boundary region and from the deep mantle and deeper parts of plate convergent zones. Its subtraction from radar altimeter data indicates mass anomalies in the outer 600 km, which in turn provides new information regarding the nature of convection within the upper mantle.
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