Penentuan Anomali Gayaberat Regional dan Residual Menggunakan Filter Gaussian Daerah Mamuju Sulawesi Barat

Gravity method is a geophysical method that has been frequently used in prospecting mineral resources. The parameter of searched object is based on variations of gravity acceleration measurements on the surface due to variations in sub-surface geological changes. Research area is located in Mamuju Area of West Sulawesi Province where tectonically a complex geological region, which is at a meeting of three large plates, the Pacific plate, the Indo-Australian plate and the Eurasian plate and the smaller Philippine plate. In addition, Mamuju is an area with a high radioactivity dose rate that has potency to radioactive minerals resources. The purpose of the research is to obtain gravity anomalies by using qualitative separation and interpretation of regional and residual gravity anomalies. Complete Bouguer Anomaly (CBA) value of the research area obtained from the measurements was 46.0 – 115.7 mGal. Based on the CBA map, the separation process of regional gravity anomalies and residual using Gaussian filtering technique conducted. This filtering technique works based on spectral analysis of gravity amplitude changes in spatial where the result is a cutoff wave number of 1.1736 x 10-3/meter and a wavelength of 5373.45 m. The regional and residual gravity anomalies range from 51.8 to 102 mGal and -10.4 to 14.8 mGal respectively. The depth of influence of each anomaly is calculated based on their spectral wavelengths, resulting 970.97 m and 100.21 m for regional and residual anomalies respectively. There are five zones based on the residual anomaly map, which are zones A, B, C, D and E. The heaviest positive gravity anomaly is found in zone A and B, which is predicted to be influenced by Adang lava with relative north – south distribution

Spatial distribution of hotspot material added to the lithosphere under La Réunion, from wide-angle seismic data

Wide-angle seismic lines recorded by ocean bottom and land seismometers provide a pseudo three dimensional investigation of the crust and upper mantle structure around the volcanically active hotspot island of La Réunion. The submarine part of the edifice has fairly low seismic velocities, without evidence for intrusives. An upper unit with a velocity-depth gradient is interpreted as made of material erupted subaerially then transported and compacted downslope. Between this unit and the top of the oceanic plate, imaged by normal incidence seismic reflection, a more homogeneous unit indicated by shadow zones on several wide-angle sections may correspond to lavas of a different nature, extruded underwater in the earlier phase of volcanism. Coincident wide angle and normal incidence reflections document that the oceanic plate is not generally downwarping toward the island but doming instead toward its southeastern part, with limited evidence for some intracrustal intrusion. Deeper in the lithosphere, the presence of a layer of intermediate velocity between the crust and the mantle is firmly established. It is interpreted as resulting from the advection of hotspot magmatic products. Possibly partially molten, and of a composition for which the crust is a density barrier. The extensive wide-angle coverage constrains the extent of this body. It does not show the elongated shape expected from plate drift above a steady hotspot supply. Alternative propositions can hence be considered, for example, that La Réunion is caused by a solitary wave of hotspot material or by a young hotspot. The size of the underplate, 140 km wide and up to 3 km thick, corresponds to less than half the volume of the edifice on top of the plate. (Résumé d'auteur