Existing achievements about Baotou Fault demonstrate it as a buried eastern boundary of the Baiyanhua Basin in Hetao active fault subsidence zone,striking NE.More data is needed to assess its activity.Located in the relay ramp between Wulashan Fault and Daqingshan Fault,Baotou Fault's activity is of great importance to discuss the linkage mode and the response to the earthquake of the adjacent fault.Also it is necessary to the knowledge of the characteristic of the seismic tectonic in local area.Recently it is prevalent to combine shallow seismic profile and composite drilling section to study the activity of the buried fault.Shallow seismic profile indicates that Baotou Fault is a normal fault,inclining to NW.The displacement of the Tg at 75m underground is 25m.Composite drilling section indicates that it is a growth fault,the up-break point of which is 45.6m underground and ends in brownish red clay strata of early Pleistocene.In comparison,the upper Late Pleistocene strata are out of the influences of the tectonic subsidence zone.Baotou Fault's activity is limited to the early Pleistocene.

Mohr circle analysis is a qualitative method for interpretation of magnetotelluric (MT) data. It uses various rotational invariants of impedance tensors and their composite as its basic parameters, so as to visually and clearly displays the features, such as dimensions, 2D-deviation degree, anisotropy, strike orientation, and impedance of the main axis of electrical structures. The conspicuous feature of the method is that it can be used for determining the dimensions of structures. For 1D structure, the Mohr circle is degraded into a point on the abscissa. For 2D structure, the center of Mohr circle is on the abscissa, where the longer the radius of circle, the sharper the electrical anisotropy is. And for 3D structure, the center of Mohr circle deviates from the abscissa, and the farther the deviation of the center from the abscissa, the sharper the 3D feature is. The Mohr circle analysis method was used in analysis of 76 magnetotelluric sounding data from the whole sounded area in west Sichuan-east Tibet. The result obtained shows that 2D regional structures are the main features of the region, and the lateral anisotropy is less intense. The strike of regional structures is approximately sub-longitudinal and the maximum deviation does not exceed ?30?. The analysis result of data only from four observation sites is presented in detail here due to the limited space of the paper.

To test the effectiveness of DC and TEM soundings for urban active faults, experiments were carried out in Fuzhou City, Fujian Province in 2001. This paper is focusing on the test results of the Bayishuiku-Shanggan Fault, which is the major fault crossing the downtown of Fuzhou City. Two DC(DC-Ⅰ-1 and DC-Ⅰ-23)and three TEM(TEM-Ⅰ-1, TEM-Ⅰ-2 and TEM-Ⅰ-3)traverses were deployed in Bayishuiku area. DC experiment used a RESECS equipment. An array of 96 electrodes(192m long)was employed for the measurement, while a 16 electrode string(32m long)is prepared for rolling ahead at the end of the array for a long traverse. Electrode spacing is 2m. TEM used EM-47 and EM-67 systems. For EM-47 the measurements were made at the center of the transmitter loop with a dimension of 40m×40m. A large fixed loop with side length of 100～200m was employed for EM-67. Two adjacent loops overlap half of the side length. Site spacing is 10m for both EM-47 and EM-67. DC models show the electric structures at a depth range of 1 to 30m, while TEM models indicate those of 10 to 300m depths. DC model coincides excellently with the TEM model in the same traverse. All the DC and TEM models represent a layered structure with a strong vertical low resistivity anomaly in the middle part of each profile, which is inferred to be the main strand of the Bayishuiku-Shanggan Fault in the area. The fault appears to be several ten meters wide dipping NNE into the basement. The experiments suggest that the results will be much improved by using the combined DC and TEM method.

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.

An electromagnetic array profiling(EMAP) and magnetotelluric sounding (MT) have been conducted in the Sanhe-Pinggu M8 earthquake area, China. The EMAP profile is 31.8km in length, while the total length of two MT profiles is 150.05km.The profiles reveal the principal features of the electrical structures of the studied area. The relation of all these features to the development and occurrence of strong earthquakes is discussed based on the analysis of the principal characteristics of shallow and deep electrical structures. The electrical structures of Sanhe-Pinggu M8 earthquake area are characterized by its significant layering, and the area as a whole can be classified into 7 electrical layers. A high conductivity layer can be recognized at a certain depth of the electrical structures of the Sanhe-Pinggu area. The depth of the central part of the high conductivity layer is about 20km,and the average thickness of the layer is about 11.5km. Several gradient zones of abrupt variation of resistivity were observed within the high conductivity layer, and it is believed that they might bear a certain relation to the nucleation and occurrence of strong earthquakes. Anomalous bodies of various scales with very low resistivity have been found along both the NE-trending and NW-trending observation traverses in the studied area. The resistivity of these bodies is one magnitude less than that of the high conductivity layer. They appear as isolated blocks, instead of layers. There is no direct link between the deep-seated and surficial faults inferred from the variation of the electrical structures. This may indicate the differences and discontinuity of the shallow and deep-seated structures. The lithosphere thickness is thicker in mountainous region and thinner in plain region. The depth of the lower boundary of lithosphere in this area varies from 70km to 94 km.

The affection of a two dimension inclination terrain mainly causes the distortion of the TM mode data,that decreases apparent resistivity curves of the TM mode. The affection could be ignored if the inclination angle is less than 10 degrees. The affection becomes more severe,as the inclination angle increases. The affection are also related with periods,the longer the period,the more severe the distortion causes,and remarkably affected periods become shorter as the inclination angle increases. In order to reduce these affections,two kinds of models which are homogenous half space with a flat surface and with a slope surface are respectively considered,magnetotelluric (MT) response functions have been computed and correction coefficients have been obtained.

The Combination of Electromagnetic Array Profiling (EMAP) with Magnetotelluric Method (MT) is that MT data can be simultaneously oblained while EMAP measurement is carried out. This technique has the following advantages: 1. The information is gereatly expanded and the field measurement is more efficient. 2. It could be a remedy for the possible weakness of the insufficient deep seated data in case of EMAP measurement only. 3. It provides an opportunity of synthetical study for both EMAP and MT data.

Because of the limitations of the tradional analysis methods for magnetotelluric impedance,the universal canonical decomposition has been proposed. By this method the eight scalar parameters provided by the main coordinate system describe the transfer features,polarization state of the field source in the Earth system as well as their relations. Therefore these are a parameter system with definite physical implications and to be easy to explain. This paper studies the characters of the canonical decomposition method under some tectonic conditions and the polarization problem of the field source. A simple example of application of this method is also given.

Effect of topography is one of the main problems for the MTS work in mountain areas. To this problem we have made theoretical analyses and numerical modeling by using a two dimensional model in homogeneous halfspace. Our results indicate that effect of topography depends on the type of MTS data, mode of polarization, electromagnetic wave period of field source, survey site, as well as the form and scale of topography. In addition, we also give the estimates on the extent of topography effect in terms of calculations by a cliff model.

Mathematical formulas of three-dimensional(3D) Magnetotelluric(MT) responses affected by shear,twist and anisotropy are derived based on the principle of two-dimensional(2D) MT impedance tensor decomposition,which are applicable not only to complicated 3D structure,but also to 2D or 1D situations.Mohr circle analysis are also made for 3D MT responses and the properties in the cases of both weak and strong distortions.

Mohr circles and their application to Magnetotelluric(MT)impedance tensor analysis aredescribed in this paper,It iS illustrated that the rotational invariants of MT impedance tensors,having the advantages of providing variety of MT in a formation and stability,are frequently usedin MT data interpretation,Mohr circle,connecting the different forms of invariant impedancetensors,becomes an important graphic dignostic tool in studying MT impedance tensor properties.With the help of this powerful tool,some MT parameters,such as structural dimensional-ty,skewness,anisotropy,can be expressed visually and simply in the circle diagrams which giveimportant information for studying both vertical and lateral variation of the conductivity structures.

The features of deep conductivity structure along Xiangshui-Mandal Geoscience Transect are described in this paper basing on newly obtained magnetotelluric data. Large resistivity contrasts can be found on both borders of North China platform. The depth of the crustal high conductivity layer changes abruptly from 21km to 34km beneath Nei Mongol fold system on the northwestern end of the transect. While it is absent beneath Subei-Jiaonan terrane on the southeastern end of the transect,where two high conductivity layers are detected in the upper mantle. The crustal high conductivity layer is well developed in the Cenozoic basins within North China Platform with a relatively shallow depth. The depth of the upper mantle high conductivity layer along the transect ranges from 60 to 127kin,with a general tendancy of decreasing in depth from northwest to southeast,forming local uplifts beneath Ho-Bao basin,Central Hebei depression and Tan-Lu fault zone. It reflects the characteristics of the regional geology and the deep tectonic movements.

Based on the data conducted from 18 MT sites in west Yunnan, the deep conductivity characterics along the longitudial and transverse directions of the geological structure are studied, The results indicate that the deep eletrical structure in this area is a multiple-layer structure with 4-5 electrical layers. There is great lateral inhomogeneity in the deep eletrical structure which are apparently controlled by the regional geological structure. There exists a widespread highly conductive layer in the crust, and two upper mantle highly conductive layer uplift zones are found, one is situated in Jianchuan-Heqing with the principal axis of NNW direction and another in Tengchong-luxi with approximately NS direction.The relationship between the electrical structure of the crust and upper mantle and the geotectonics, the formation of northwest Yunnan rift basin and the relationship between seismic activity and the deep geological structure are also discussed in this paper.