Geophysics And The Environment

GEOPHYSICS AND THE ENVIRONMENT The knowledge of the history of the planet, since its origin, and the mechanisms that promote changes in the environment, are also the object of Geophysics, together with areas such as Geology, Oceanography and Atmospheric Sciences, for example. Mapping the contamination of aquifers and urban regions is important for the proper management of waste disposal; investigating the location and possible contamination of aquifers is important for urban planning and mitigation of environmental disasters; mapping and monitoring waste disposal zones is important to prevent damage to the environment and populations. Geophysical methods are very efficient in helping with these tasks. Landfills are one of the cheapest and most used forms of waste disposal in several countries around the world. However, the disposal of household, industrial and health care solid waste in landfills can have serious environmental consequences, including risks of contamination of surface and groundwater due to the toxic material produced, the leachate. In the event of cracks in the structures of these landfills, the problems associated with the operation and subsequent phases can be aggravated. This shows the need for detailed knowledge of the subsurface structure of the area in which a landfill is to be implanted. The depth of the rocky substrate, the depth of the water level in relation to the base of the landfill and the direction of the underground flow are fundamental in the characterization of an area that will be used for waste disposal. Usually, the direct determination of these characteristics is limited for technical and economic reasons. Geophysical methods, however, allow obtaining information with a good level of precision on the geological and hydrological conditions of the subsurface quickly and at reduced costs, both in the design phase and in the monitoring of the operations. Among the most applied geophysical methods in this scenario are the electrical and electromagnetic methods, which allow estimating important parameters in the characterization of waste disposal areas, such as the rock depth sound, the depth of the static level and the direction of the groundwater flow. The non-invasive nature of the geophysical methods, which normally do not affect and do not destroy natural or artificial sealing layers, and the relative low cost, speed and ease of application of data collection, make them very suitable for environmental studies. The use of geophysical methods in the characterization of an area affected by polluting substances consist of the detection and mapping of the affected area, searching for the depth of the saturated zone, the determination of the direction of the underground flow and the characterization of the rocky substrate depth. The study of these characteristics allows the assessment of the pollution level of the site and can help to guide measures for the monitoring and the remediation of the affected area. The geophysical techniques used for the characterization of the rocky substrate for the purpose of geotechnical mapping normally present a very satisfactory behavior, allowing the determination of the spatial variations of some important attributes. The joint application of vertical electric sounding (VES) and seismic refraction is a good tool in obtaining the physical parameters of the soil-rock transition interface and in determining the depth of the rocky substrate, as well as the depth of the water level. A limitation of this joint methodology may be of an economic nature, as the seismic refraction technique in regions of tropical climate, where soil layers can be quite thick, requires the use of powerful seismic sources, which may involve explosive charges. In addition to the problems in handling these loads, the cost involved increases considerably, and is not feasible for preliminary large-scale studies. However, this methodology can become interesting in the case of detailed works for the implementation of major engineering works, where the cost/benefit ratio reaches more favorable values.