Abstract: 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.

Key words: urban noise, multi-electrode direct current method, transient electromagnetic method

摘要： 城市噪音主要指由于城市设施和人类活动(如电网、管线、通讯设备、建筑物、交通等)对电磁信号的干扰.克服城市噪音、提高信噪比是保证城市活断层的电磁探测能否成功的关键.因而,在福州市活断层的电磁试验探测中,噪音观测与分析被列为试验的一项重要内容^①.文中将主要讨论不同类型的噪音对高密度电法(DC)和瞬变电磁法(TEM)的影响,并提出一些实用的解决办法.试验表明,影响高密度电法观测质量的主要因素是地下噪音源,如地下管道、地下电缆等,地面以上噪音的影响相对较小.对于瞬变电磁法,不管地面以下还是地面以上的噪音源都将产生影响.但是,只要仪器的性能好,对不同噪音源采取相应的措施,可以获得满意的结果.

关键词: 城市噪音, 高密度电法, 瞬变电磁法