Microgravimetry and the theory, measurement and application of gravity gradients

Abstract

The theory, measurement, and application of gravity gradients is explored. Exphasis is on (a) the development of analytical and interpretive techniques using gravity-gradient data, and (b) the development of practical field microgravimetric techniques for gravity-gradient measurement or determination. Three field test cases evaluate the techniques and assess their practicality. The analytical and interpretive techniques are also applied to data from a published high-resolution gravity survey. Four key questions, posed in the first chapter, are addressed: (a) Do the measured gradients exhibit greater resolving power and improved detectability thresholds than gravity? The resolution advantage of gravity gradients is shown to hold regardless of depth of the anomalous structure, while the detection advantage of gravity gradients over gravity is limited to structures < 10 to 15 m depth. (b) Do relations which exist theoretically between vertical and horizontal gradients exist between measured gradients? For the cases examined in this paper, namely (1) the case where both vertical and horizontal gradients are measured, and (2) the case where the horizontal gradient is measured and the vertical gradient is obtained as its Hilbert transform, all of the theoretical relations between gradients can be recognized between the measured gradients. (c) How does discrete sampling along a profile line affect the ability to recognize diagnostic features in the gradient profiles and other representations of the data? Two primary effects are identified: (1) filtering effects, and (2) amplitude attenuation and spatial spectra broadening. (d) Is it preferable to measure the horizontal gradient profile and calculate the vertical gradient profile from it as suggested by some authors, is the reverse procedure preferable, or must both gradients be measured for the methods to be usable? On the basis of the present work, the preferable procedure is to measure horizontal gradient profiles and then compute vertical gradient profiles using the Hilbert transform; reasons presented for this conclusion differ from those of other authors.