1 Application of Geophysical Methods to Waste Disposal Studies Cristina Pomposiello, Cristina Dapeña, Alicia Favetto and Pamela Boujon Instituto de Geocronología y Geología Isotópica (INGEIS, CONICET-UBA) Argentina 1. Introduction Geophysical methods provide information on the distribution of certain physical parameters in the sub-surface, which can be linked to the direct observations. Thus it is called an indirect observation method and it does not provide a "photo" of the sub-surface but it suggests a model of the underground derived from interpreting the distribution of these physical parameters. Clay and granite, for example, have different densities, acoustic velocities, elastic parameters, electrical conductivities, magnetic susceptibilities, and dielectric constants. So, geophysical methods are designed to exploit some of the physical properties of a target feature that is in contrast with its host environment, e.g., the low density nature of a void is in contrast to the high density nature of surrounding bedrock, etc. Geophysics should never be a stand-alone tool, but complementary to direct observations, which provide geological/hydrogeological background information (such as some of those methods seen in Table 1). There are two general types of geophysical methods: 1) active, which measure the sub- surface response to electromagnetic, electrical, and seismic energy generated by artificial sources; and 2) passive, which measure the earth's ambient magnetic, electrical, and gravitational fields. Geophysical instruments are designed to map spatial variations in the physical properties of the Earth. A gravimeter, for example, is designed to measure spatial variations in the strength of Earth’s gravitational field. Sanitary landfill is the most common way to eliminate solid urban wastes. They have a heterogeneous structure due to random origin of the disposed waste. Geophysical methods are particularly valuable because they are non-destructive and non-invasive. An important problem associated with this practice is leachate production and the related groundwater contamination. Leachate electrical conductivity is often much higher than that of natural groundwater and it is this large contrast that enables contamination plumes to be detected using geophysical methods. Ground Penetrating Radar (GPR) and Electrical methods such as Electrical Tomography (ET) and Vertical Electrical Sounding (VES) have been found to be especially useful for these kinds of environmental studies, due to the conductive nature of most contaminants, and they can be important tools for the detection and mapping of landfills, trenches, buried wastes and drums, or other underground structures.