Multiscale estimation of excess mass from gravity data
CASTALDO, R. 1, FEDI, M. 2 e FLORIO G.2,
1 Istituto per il Rilevamento Elettromagnetico dell’Ambiente, IREA-CNR, I-80147 Naples, Italy
2 Dipartimento di Scienze della Terra, dell’Ambiente e delle Risorse, Università degli Studi di Napoli Federico II, Largo San Marcellino 10, 80138 Napoli, Italy
Geophysical Journal International, 197 ( 3 ), pp. 1387 - 1398, anno 2014
We describe a multiscale method to estimate the excess mass of gravity anomaly sources, based on the theory of source moments. Using a multipole expansion of the potential field and considering only the data along the vertical direction, a system of linear equations is obtained. The choice of inverting data along a vertical profile can help us to reduce the interference effects due to nearby anomalies and will allow a local estimate of the source parameters. A criterion is established allowing the selection of the optimal highest altitude of the vertical profile data and truncation order of the series expansion. The inversion provides an estimate of the total anomalous mass and of the depth to the centre of mass. The method has several advantages with respect to classical methods, such as the Gauss' method: (i) we need just a 1-D inversion to obtain our estimates, being the inverted data sampled along a single vertical profile; (ii) the resolution may be straightforward enhanced by using vertical derivatives; (iii) the centre of mass is also estimated, besides the excess mass; (iv) the method is very robust versus noise; (v) the profile may be chosen in such a way to minimize the effects from interfering anomalies or from side effects due to the a limited area extension. The multiscale estimation of excess mass method can be successfully used in various fields of application. Here, we analyse the gravity anomaly generated by a sulphide body in the Skelleftea ore district, North Sweden, obtaining source mass and volume estimates in agreement with the known information. We show also that these estimates are substantially improved with respect to those obtained with the classical approach.
Key-words: Inverse theory; Gravity anomalies and Earth structure; Geopotential theory.
THE REAL CASE OF THE UDDEN MASSIVE SULPHIDE BODY (SWEDEN)
a) residual gravity map of the Udden sulphide body; b) data misfit error (top), excess mass estimation (middle) and center of mass estimation (bottom) as a function of the number of terms of the series expansion (kN). The selected solutions are relative to the minimum of the error (kN =8).