The gravity method: new challenges for delineation diapiric structures in north-east Spain

The gravity method was the first geophysical technique that demonstrated its usefulness for oil exploration one century ago with the delimitation of the anticlinal structure of Egbell in 1916 from detailed measurements conducted by Roland von Eötvös using a torsion balance. This experience was followed by the discovery of the oil field related to the salt diapir of Nash (Texas, USA). Despite the development of other geophysical techniques, gravity prospecting continues to offer significant and sometimes essential evidence in many exploration projects. In recent decades, the emergence of a new generation of digital gravimeters, which has increased the precision of the measurements. The support of the GPS technique for accurate location and measuring of the height of the station, as well as the availability of digital terrain models, have renewed interest in this technique. This paper illustrates some recent applications of this technique in the southern Pyrenees that is an interesting evaporitic province characterized by the existence of several diapiric bodies of Triassic and Oligocene evaporites, related to a thrust-and-fold system. Gravimetry began with Galileo (1561-1642), who discovered the laws of free fall and of pendular motion. For this reason the unit in terms of which geophysicists express their gravity measurements is the ‘gal’, so named after Galileo. To Newton (1642-1727) we owe the law of universal gravitation and Huyghens (1629-1695) first used pendulums in clocks. Newton and Huyghens pointed out independently, and almost simultaneously, that the observed effect must be owing to the departure of the Earth from the spherical form. Geodesy was the first application of gravity measurement. The object was to throw light on the shape of the Earth and Bouguer (1698-1759) was probably the first to make pendulum observations for this purpose. His name is perpetuated in the Bouguer corrections and Bouguer anomalies.