Gravimetry is a geophysical technique that allows the Earth's subsoil to be characterized by variations in the density of the materials present. The variation in the value of gravity along the surface is produced by different causes such as rotation, shape and the heterogeneous distribution of densities of the planet. Thus, the final objective of the method will be to determine the contribution of this heterogeneous distribution of densities in the crust with respect to the total value of gravity at a given point on the surface and thus support geological models.

Methodology

The heterogeneous distribution of subsoil densities leads to differences between the value of the observed gravity (gobs) and the theoretical gravity (gT) at a given point on the Earth's surface. This difference is called the gravimetric or Bouguer anomaly (AB) and to determine its value it is necessary to apply a series of corrections: height, surrounding topography, tides and atmosphere. This results in a Bouguer Anomaly (AB) which is due almost exclusively to variations in the densities of the materials that make up the subsoil. This is expressed with the formula of Telford et al. 2004, as follows:

In reality, the gravitational field is composed of a regional part, usually caused by deep structures, and a local part, usually caused by surface structures. In addition, the intensity of the gravitational field decreases with the square of the distance. Consequently, given a body of a given density, the gravimetric effect on the surface will be intense and focused when the body is shallow, and will be weaker and wider if it is at greater depth. Before interpreting the gravimetric data in the study area, it will be necessary to separate the regional field and the local field. In this way, local anomalies, hidden by regional anomalies, which will be associated with the geological structures under study, can be analyzed more easily.

Esquema simple sobre la separació del camp residual respecte del camp total mesurat. Δgres= Δg- Δgreg. (https://www.gravity.uni-kiel.de)

Gravimetric prospecting is carried out with specific instruments called relative gravimeters and the field configuration consists of the uniform distribution of measurements throughout a study area. The result is a distribution of Bouguer anomaly measurements on a surface, which can be interpreted through a series of subsurface structures with different characteristics of density, shape and depth.

Field data observation. Left) Distribution of measurement points on a 1:250000 orthophoto (ICGC, 2019) of the study area. The limit of the study area is marked in blue and the location of the Absolute Gravimetric Base used in this campaign is marked with a red circle. As can be seen in the image, the density of measurement points is greater on the North-West side of the study area. Right) Relative gravimeter CG-6 (Scintrex) and differential GPS (Trimble R8s) working at the same point.

Results of a gravimetric modeling. The upper graph corresponds to the fit between the observed gravimetric data (circles) and the calculated ones (black line), where the error (0.445; red line) is also shown. Each color of the geological section (lower graph) represents a formation (Fm.) with a differentiated density (D).

Applications

• Study of sedimentary basins with the determination of geological structures, location of faults and fractures, etc.
• Study of natural resource areas such as mineral deposits (diapirs, etc.).
• Detection of cavities (karst terrain).
• Geodesy.
• Regional geological maps and modeling of geological sections.
• Civil engineering – Geotechnics.
• Archaeology.

References

Telford, W.; Geldart, L.; Sheriff, R. (2004). Gravity Methods. Applied Geophysics (Second ed.). Cambridge University Press. New York, USA.