Variable density Bouguer processing of gravity data from Herault, France

Abstract

The Herault permit for oil is located in the south of France in very hilly terrain between Mount Aigoual (1500 m) and the sea at Sete. In the preliminary stage of exploration, it was decided that new Bouguer maps, including a variable density one, and a surface density map should be produced. France is covered by a national gravity map published by BRGM on a scale of 1:80000. It was decided that the original data should be recovered and new data acquired to infill the blanks in order to get stations on an average grid of 1.2 by 1.2 km. The existing data had been acquired along roads with a station spacing of 800 to 1200 m, but the road lines were 3 to 6 km apart. Very detailed mining gravity data were also available from the area around Lodeve. After appropriate sampling of the latter data set, 2041 old stations were retained and 619 new stations were planned (Fig. 1). In most of the permit, the new stations were located along tracks or footpaths over the hills, whereas the old grid was located mainly in the valleys. With 2660 stations in total, the number of stations per 100 km2 varies between 60 and 70, and the largest remaining blanks in very inaccessible areas are about 3 by 3 km in size. The data from these stations were processed using a generalised Nettleton method. After isostatic corrections, structures show up much better on the variabie density Bouguer anomaly map, facilitating interpretation. The surface density map shows the low density of the granites, and the high density of the Permian and Jurassic dolomites.