Man-made electromagnetic noises causing difficulty in geomagnetic and geoelectric observations in city area

In city area, there are several types of electromagnetic noises originated in daily activities of human beings. The noises possibly cause difficulty to detect geomagnetic and geoelectric signatures. It is important for investigators who are studying geomagnetic effects for biology, medicine and so on to be aware that such noises are present in city area. Among these noises, the one due to electric railways driven by direct-current electric power supply might give the largest influence because this type of noise continues almost all the time and have complex wave forms. The characteristics of the noises are briefly introduced by showing a theoretical background and some experimental studies.