The technique is based on the passive measurement of variations in gravitational acceleration caused by changes in the density of the subsoil. Each type of rock has its own characteristic density and can be assessed by means of gravimetric data. Using highly-sensitive instruments called gravimeters – which make zero impact on the environment – measurements are taken (usually from an airplane or ship) to produce a survey. This shows the different types of rocks and their geometry down to a depth of several kilometres. Such a methodology has the great advantage of being able to obtain information in remote areas, quickly and at less than a twentieth of the cost of seismic prospecting.
In certain contexts it is also possible to use low-resolution satellite data to identify regions favourable to exploration. The technology has evolved enormously since the late 1990s,with the entry of a sub-discipline of gravimetry known as gravity gradiometry (GG) which has shown some success in exploration research. Compared to the simple measurement of the vertical component of the gravity field, in GG it is possible to measure increasingly small changes in gravity in all directions to obtain what is referred to as the gravity tensor. It is made up of nine elements, each relating to density distribution in the subsoil in a particular direction. Of these, it is the component relating to vertical variation – known as Tzz – that is most useful for the identification of specific types of rock within the subject area, the location of reservoirs and structural highs, the mapping of faults in the subsoil and reconstruction of the geological-structural setting.For each component, the tensor makes it possible to distinguish increasingly small local variations in density, greatly improving the interpretation of the first 4-5km of depth in particular. Thus one can obtain valuable information about the type of rock in the area and its geometry within the subsoil. It can, for example, distinguish not only between sedimentary and volcanic rocks, but also make out carbonate rocks, clays and/or sands, which in many cases represent potential hydrocarbon reservoirs.
The use of data
Eni pioneered this process in the early 2000s. Data acquired by GG was interpreted using proprietary algorithms – the result of research projects conducted in collaboration with the Federico II University of Naples – to allow calculation of the gravity tensor. It can be used in a variety of exploration contexts where it is necessary to obtain information in a short time, and where other types of data such as seismic, geological or well types have limitations. This research has allowed Eni to maintain high technological standards in competition with other major oil companies.