Geophysical Methods

GEOPHYSICAL METHODS The methods used in Geophysics aim to map different physical properties of existing structures in the terrestrial interior. Many of these methods can be used both on a local scale, to investigate features a few meters deep, and on a global scale, to investigate the deep structure of the planet. Some traditional methods used in geophysical research are: Gravimetry aims to detect density variations in the subsurface by measuring local variations of the elements of the Earth's gravity field. These variations have several causes: the rotation and irregular shape of the Earth, gravitational effects of the Moon and the Sun, variations in altitude between the measurement points, and what is usually most important for geophysical exploration: the mass distribution of the planet. Gravimetric surveys can be used for both shallow and deep structures detection, requiring different procedures and data processing. The careful analysis of small disturbances in the gravity field produced by masses of different densities, combined with information geological, allows to identify and evaluate the economic potential of structures that can storage oil, deposits of various ores, as well as search for impact craters hidden under sediments, caves, regions of intensive volcanism, and any bodies whose density different from their surroundings. The pattern of where earthquakes occur around the world shows clearly that most of the earthquake activity is not random, but is concentrated in narrow earthquake belts. For instance, there are many earthquakes recorded in the middle of the Atlantic Ocean and around the edge of the Pacific Ocean, but not spread all around the Pacific Ocean. The analysis of these earthquake belts provided an important clue in the development of the theory of plate tectonics, that explains most of the observed global geological phenomena. It states that the outer shell of the Earth, or lithosphere (continental and oceanic), is made up of a number of rigid segments called lithospheric or tectonic plates. These plates are continually moving at rates of usually a few centimetres per year, that is about as fast as our fingernails grow. They are driven by huge forces deep within the Earth. Below the tectonic plates lies the asthenosphere, a layer that behaves like a fluid over very long time scales (thousands or millions of years). It is not easy to explain what drives the movement of tectonic plates, and there are almost a dozen of force components envolved in this process. Most earthquakes occur at the boundaries between the plates, where they can move together, move apart or slide along one another, and tremendous stresses are build up. In this way, the global distribution of earthquakes clearly delineates the edges of the tectonic plates. There is a category of geophysical methods that uses electric and electromagnetic fields to investigate the properties of structures and rocks that are normally found near the surface. The most used are: - electrical resistivity: uses an artificial electrical current to generate a difference in the electrical potential that is measured in several positions, which allows determining the apparent resistivity of the subsurface; - electromagnetic methods: they use the propagation of low frequency electromagnetic fields in the rocks and structures of the subsurface, allowing to determine information about such features. In the electroresistivity method, values of electrical potentials are measured at different points, seeking to infer the shapes of the subsurface structures from the calculated electrical resistivities. The electrical potential on the surface or inside the earth depends on the intensity and location of the sources of artificial electrical currents used and the geoelectric structures present in the subsurface. The electrical resistivity of the rocks is directly related to the mechanisms of electrical conduction in the materials, and normally the electrolytic conduction in the liquid solutions contained in the pores of the rocks is the predominant factor, but the presence of metallic minerals and graphite in the rocks can also contribute significantly. This method uses a continuous or alternated artificial electric current of low frequency (below 10 Hz), injected into the ground by means of two electrodes located on the surface or inside it. Thus, the redistribution of the surface and volumetric electrical charges contributes to originate and sustain an electric field in a volume around the electrodes. The potential difference between points distributed in the region is measured through other electrodes, with various specific configurations. The analysis of this information allows the elaboration of a profile with the electrical properties in the subsoil, which can be interpreted according to the geological or geotechnical situation of the study region. The GPR (ground penetration radar) is a method that uses high frequency electromagnetic waves that propagate in the subsurface, interact with rocks and structures and return to the surface, where they are captured and analyzed. From this analysis it is possible to locate targets of interest for geotechnical, forensic or archaeological exploration or investigation, usually located in the depth of a few meters. As a non-destructive investigation method, GPR has direct application in archeology, normally being used to find targets located at low depths, less than 5 m; it can be used as the main method or as a complementary method to other techniques, such as magnetometry. Usually the GPR gives a good idea of the depth of the sources of magnetic anomalies, allowing a safe demarcation of sites for excavations. A distinctive feature of the GPR method is the reduction of the investigation time, as the regions of interest can be visualized preliminarily in real time, during the data collection. GPR data allows one to accurately characterize archaeological features such as buried urns and ducts, occupation soils, building bases and tunnels, separating their signal from the material deposited later. Among the signatures typically visible in GPR records, the lateral discontinuities in the records caused by the overturning of the subsoil during burials, and the features originated by the presence of urns, ducts and construction foundations stand out.