Introduction and application of a 3-D gravity method
Chen-Fong Yu
Abstract
In order to solve the subsurface geological problem, we use a gravity inversion method for determining the volumes of bodies with pre-established density contrasts. The method works step-by-step on a prismatic partition of the subsurface volume, expanding the anomalous bodies to fit the observed gravity values in a systematic exploration of model possibilities. The process is treated in a 3-D context; at the same time, it can determine a simple regional trend. Moreover, positive and negative density contrasts are simultaneously accepted. The solution is obtained by a double condition: (1) the L2-fitness to the observed gravity data (model fitness) and (2) the minimization of the total (weighted) anomalous mass (model smoothness). A positive parameter is used to balance the two minimization terms. The method is applied to a simulated example and also to a real example: the volcanic island of Gran Canaria and Lanzarote(Canary Islands,Spain).
At Lanzarote,a main intrusive body is located under the central-eastern area and could correspond to a dilated volcanic activity of shield formation.Moreover, the SW and NE extreme areas of the island show smaller and shallower positive bodies,interpreted as less-developed magmatic intrusions. Conversely, several density lows offer interesting shallow alignments,which could be associated with a fracture system corresponding to structural stress, and also correlate with historic eruptions, such as, for instance, the Timanfaya eruption. The monitoring of several geophysical parameters at two underground geodynamic stations,in the NE zone of the island and Timanfaya, shows characteristic differences between the two zones which confirm crustal anomalies in the second station.
References
Camacho,A.G.,Montesinos,F.G.,and Vieira,R.,2000:Gravity inversion by means of growing bodies.Geophysics,65,95-101
(Abstract) (Full text)
Camacho,A.G.,Montesinos,F.G.,Vieira,R.,and Arnoso,J.,2001:Modelling of crustal anomalies of Lanzarote (Canary Islands) in light of gravity data.Geophys.J.Int.,147,403-414