Passive and active electric methods: New frontiers of application

In the field of applied geophysics, the geoelectric methods have been important for about a century, particularly for shallow and near-surface investigations. Geoelectrical methods are used extensively to locate buried targets that are conductive and resistive in nature. The purpose of active geoelectric surveys is to determine the subsurface resistivity distribution by making measurements on the ground surface. The ground resistivity is related to various geologic parameters such as the mineral and fluid content, porosity, and degree of water saturation in the rock. Geoelectric surveying has greatly improved and has become an important and useful tool in archaeological and monumental heritage conservation state studies and in hydrogeology, in environmental and engineering applications. Schlumberger brothers introduce the resistivity method in the second decade of the twentieth century. In the origin of this technique, the center point of the electrode array remains fixed, but the spacing between the electrodes is increased to obtain more information about the deeper sections of the subsurface. The traditional horizontal layering techniques for interpreting geoelectric resistivity data are rapidly being replaced with two-dimensional (2D) and three-dimensional (3D) models of interpretations, especially in complex and heterogeneous subsurface media. Field techniques have advanced from the measurements made at separate and independent points to automated measuring systems with multielectrode array along the measurement profiles. In the case of self-potential (SP) methods, where no subsurface energization using external artificial source is done, differences in natural ground potentials are measured between any two points on the ground surface. SP is the naturally occurring electric potential of the earth resulting from geologic, geochemical and hydrologic interactions that cause electric potentials to exist in the earth in the vicinity of the measurement point. Since 1830, the SP method has been employed in the search for minerals, but it is very rarely used in archaeological prospection because related phenomena are not very well known. The self-potentials are measured in millivolts (mv) relative to a "survey base," where the potential is arbitrarily assigned to be 0V. Starting in the first years of the 1980s, the data acquisition was carried out manually, making labor-intensive and slow survey and often producing low-quality data. Successively, a range of automated multielectrode and multichannel data acquisition systems were developed. These methods allow fast and accurate data acquisition. Since the electric techniques are regularly used by a vast number of geoscientists, authors use this chapter to expose the new research development reached in the field of instrumentation, data processing, and interpretation.