EFFECTS OF URBAN NOISE ON ELECTROMAGNETIC METHODS
BAI Deng-hai 1) WANG Li-feng 1) SUN Jie 1) ZHU Jin-fang 2) HUANG Zong-lin 2) HUANG Dan-qing 3) HE Zhao-hai 1) ZU Jin-hua 1) LIAN Yu-fang 1) Quentin Yarie 4) Volker Schaepe 5) 1) Institute of Geology, China Seismological Bureau, Beijing 100029, China 2) Seismological Bureau of Fujian Province, Fuzhou 350003, China 3) Institute of Geophysical Exploration, China Coal Geological Bureau, Zhuo Zhou 072750, Hebei Province, China 4) GEONICS LIMITED, 8-1745, Meyerside Drive, Mississauga, Ontario L5T 1C6, Canada 5) DMT Mines & More Division, Deutsche Montan Technologie GmbH, Am Technologiepark 1, 45307 Essen, Germany$$$$
Of the geoelectromagnetic techniques, multi-electrode Direct Current (DC) and Transient Electromagnetic (TEM) methods are the most powerful tools for shallow soundings. For city active fault detection, urban noise is a key problem in the use of electrical or electromagnetic methods. Effects of some major noises on DC and TEM are discussed on the basis of the experiments carried out in Fuzhou City in 2001. The experiments show that underground noises (pipes, cables, etc.) are most harmful to DC soundings, while for TEM, in addition to the underground noises, the aerial noises (power lines, metal sheds, etc.) will also lead to serious effects. Even so, effective soundings can be obtained providing that the noises are not too strong and some proper countermeasures are taken. In the experiments, specific measurement environments, including aerial and underground power lines and cables, water supply pipelines, roads, metal sheds, waste disposal sites, etc., were chosen as the urban noise sources. A set of RESECS instruments from DMT, Germany, were used for DC test, and EM-47/EM-67 by Geonics, Canada, for TEM test. The results of the experiments show that for DC soundings if the underground noise is not too strong, an effective record generally can be obtained, and especially the results would be much improved if the sampling time window of the instrument could be adjusted according to the noise distribution. We strongly recommend, therefore, that an instrument with real-time display of injection current and measurement potential, having adjustable time window be used for the active fault detection in urban areas. The configuration of electrodes is also important in some cases. For TEM soundings, it is better to set the measurement traverse at least 50m away from power lines, roads, cables, and big pipes. If the pipes are not big and not densely distributed within the transmitter loop, good data can be obtained at the sites several meters away from the pipes. In an area with electric current channeling, different configurations of the transmitter loops of different dimensions should be tested before the measurement.