The Electrical Resistivity (ER) methods are used for locating and mapping groundwater sources, natural groundwater flow paths, groundwater contamination, and archeological remnants. These methods measure voltages associated with electric currents flowing in the ground. These currents may be currents introduced into the earth through electrodes or they may be natural currents due to earth processes. This type of testing is useful in detecting changes in apparent resistivity or apparent conductivity both laterally and vertically. When the goal is to map an area for lateral changes in resistivity, then the profiling technique is used. When the goal is to create a pseudo-depth section in the earth, then the sounding technique is used. These two techniques are often used in conjunction to fully understand the problem.