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Measuring and analyzing earth's gravitational field at the surface is one of the main tools to study the structure and composition of the Earth. The gravity method helps Geology in describing subsoil lithological variation based on density anomalies.
Seismic reflection method is based on the propagation of seismic waves inside the earth. This method analyzes the reflected waves which are generated on or near the surface by a seismic source, travel down into the ground, reflect at a discontinuity (density and velocity contrast) and travel back up to the ground surface. Digital processing of seismic reflection data offers the opportunity to create cross-sectional images of the subsurface (seismic reflection section) showing the amplitude of the reflected wave at the correct horizontal and depth coordinate of the reflection point.
Seismic refraction method is based on analyzing the travel times of waves refracted under the critical angle on subsoil interfaces (velocity contrasts). Seismic energy is radiated downwards into the ground from a seismic source on or near the ground surface (e.g. weight drop, explosives,etc.), critically refracted at the interface travelling along it and radiated back to the surface. Observed refracted waves travel-times versus distance are converted into seismic velocity cross-sections of the subsurface.
Borehole geophysical logging techniques are based on measuring and recording continuously the physical properties of interest along a borehole using different probes. Analysis of the correlation between the different records (logs) allows to characterize lithological section and formation fluids.
The Magnetotelluric method (MT) uses natural electromagnetic fields (electrical storms, ionosferic currents) as a source to study subsoil structure. Simultaneous recording of the horizontal electric and magnetic field variations on earth surface allows to determine the electrical resistivity variation as a function of the frequency (depth). Frequency band for MT data ranges from 0.001 Hz to 100 Hz that makes it possible to study depths of up to hundreds of km. For shallow studies (around 1 km investigation depth), control source audiomagnetotellurics method (CSAMT) or audiomagnetotellurics methods (AMT) are required. Frequency band in this type of studies ranges from 1 Hz to 100 KHz.
Vertical Electrical Sounding (VES) is a electrical resistivity method that involves injecting current into the earth and measuring the potential difference between a pair of points. In this way, apparent resistivity of the subsoil as a function of depth can be obtained. VES technique can be applied to a wide range of studies with a maximum investigation depth of about 200 m depending on the acquisition system.
This method puts emphasis on the measurement of anomalies of the Earth's magnetic field, caused by the presence of reservoirs or structures on the subsurface. This technique allows to assess changes of nT in the Earth's magnetic field caused by the presence of some minerals. Those minerals are characterized by high magnetic susceptibility such as the magnetite.
Information regarding an avalanche observed recently. Mapped based on direct observation of the event or observation following it. The information is obtained based on the nivometeorological observation network, rescue groups, countryside wardens, mountain guides, and ski and mountain resort personnel. In the event of important episodes and accidents the IGC technicians also gather information. The elements mapped mainly include the extent to which avalanches affect populated areas, roads, ski resorts and people who carry out mountain activities. Data began to be gathered in 1986. It is shown in blue.
Information regarding an area exposed to an avalanche. Mapped based on analysis of the terrain and of signs that avalanches leave in their path, mainly, in forests. In Avalanche Zones, avalanches occur frequently and are of different sizes. Avalanche Zones have been systematically mapped throughout the region. They are shown in orange. There are two types of Avalanche Zones: zones where avalanches usually follow the same path, shown in orange, and zones where avalanches usually follow different paths, shown in pale orange.
The Preliminary studies of geological hazards are executed by specific order of the General Direcció d'Urbanisme. The aim of this study is to carry out a preliminary evaluation of the natural geologic hazard to determine if in the studied scope, indications of geologic processes exist that could cause risk situations. The studies are focused on the hazards of natural origin related to: - Movements of the land (slope movements and collapses). - Floodings. - Cones of deyección and erosive processes associated with rock falls and landslides - Vulnerable water-bearing of special interest for the urban consumption. - Avalanches. The estimation of the natural hazard is based on the magnitude and the degree of activity of the associated geomorphological phenomena to the slope dynamics, fluvial and torrential. From these parameters, the following degrees of natural danger are determined: - Very low hazard: Zones where a exhibition to active phenomena has been detected (without defined hazard). - Low hazard: Set out zones to phenomena of low activity and from medium-sized to small magnitude. - Average hazard: Zones with phenomena of medium-sized frequency and magnitude. - High hazard: Zones with phenomena of high fequency and medium-high magnitude. From this evaluation of the geologic hazard of the territory two types of situations are distinguished: - Areas in which the accomplishment of additional studies of geologic hazard is recommended. Usually, they correspond to scopes from Very Low to Low hazard, generally, is not necessary to realise other studies that pertinent the geotechnical information for the construction of the different structures. - Areas where the accomplishment of additional studies is recommended. Normally, they correspond to areas which contain scopes from medium to high hazards. In these areas, like general criterion, before realising any city-planning action, is recommended to carry out detailed studies, previous studies to the definition of the uses of the ground that can be compatible, that in detail evaluates certain aspects of the geologic hazard and its possible effects on the planned action.