Table of Content

05 June 2001, Volume 23 Issue 2

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Brief Report

DEVELOPMENT OF MARINE MAGNETOTELLURIC PROSPECTING TECHNIQUE IN CHINA

Wei Wenbo, Deng Ming, Tan Handong, Jin Sheng

2001, 23(2):  131-137. 

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Because of the difficulties to obtain the conductivity of the rock on the seafloor, no marine electromagnetic prospecting has been done in the field of geophysical prospecting in China till now. But the electrical parameter is the better one in reflecting the lithology and physical state of rock than some other parameters. The development of marine electromagnetic method is strongly needed. The marine magnetotellurics, which studies the distribution of conductivity of medium in different depths through collecting magnetotelluric data on the seafloor, is proved to be an effective method. After two years of hard work, we have accomplished the study of marine magnetotelluric prospecting. Due to the rapid development of society, the need for mineral and energy resources is presently increasing fast, which leads to the decrease of the deposit of important mineral, oil and natural gas, and the lack of the targets for prospecting and exploiting on the land. Relatively, the abundant sea resource is rather attractive. Therefore it is imperative to develop the methods for prospecting the resources on the seafloor in China Sea, which will provide the broad applied prospects for the marine electromagnetic methods.

A BRIEF DESCRIPTION OF DISTRIBUTIVE PASSIVE ELECTROMAGNETIC SYSTEM AND ITS APPLICATION

Lin Pinrong, Zhao Ziyan

2001, 23(2):  138-142. 

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The Distributive Passive Electromagnetic system is briefly called DPEM which utilizes passive-source-natural or cultural electromagnetic fields as the sources of the system to measure the electromagnetic responses of the earth and to determine the underground distribution of electric properties and to detect geological structures. It is flexible for complex geographic and geological environments. Its major features include synchronous array remote surveys for different geographic coordinates points with high signal to noise ratio, high resolution and good effectiveness.The system is unclosed and can be fitted with the subsystems(coordinate points)by requiring for synchronous area-covering survey. Its frequency range is 0～16kHz. Depth sounding for different geological targets at depths from near surface to 1 km and even up to upper mantle can be realized. Parameters of apparent resistivity, impedance phase, and anomaly phase can be acquired. Three field work examples are given in this paper.

MEASUEMENT OF NETWORK-MT IN TWO AREAS OF NE CHINA FOR STUDY OF UPPER MANTLE CONDUCTIVITY STRUCTURE OF THE BACK-ARC REGION

Zhao Guoze, Tang Ji, H. Utada, M. Uyeshima, M. Ichiki, Ma Mingzhi, Huang Zhiwei, Wand Hongqi

2001, 23(2):  143-152. 

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The upper mantle electrical conductivity structure of the Pacific back-arc beneath northeastern China was investigated by the long-period magnetotelluric (Network-MT) at Changchun city and Sanchahe site. Time series data of the long baseline electric field were collected by using land telephone line cables and three component magneic field variations were recorded at Changchun observatory by digital flux magnetometer. Network-MT responeses have been obtained up to 10⁵～10⁶s, which were inverted to one-dimensional model down to depths of about 1 000km. Resulted models show that: (1) The discontinuity were detected at 100～200km, and in mantle transition zone for around 300 km and 500～600 km depth, respectively. (2) The discontinuity appears at 850km for the lines in Changchun. (3) The conductivity almost monotonically increases with depth in the smooth conductivity models. (4) The conductivity reaches 1 S/m at 600 km depth in Sanchahe sites and at 850 km in Changchun sites, respectively. The above results indicate the discontinuity and the heterogeneity of the mantle and can be compared with the results obtained by petrology studies.

ELECTRICAL CONDUCTIVITY AT CRUSTAL AND UPPER MANTLE DEPTHS UNDER CENTRAL EUROPE

V. Červ, S. Kováčiková, J. Pek, J. Pěčová, O. Praus

2001, 23(2):  153-165. 

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The model of the electrical resistivity distribution within the Bohemian Massif (BM) to crustal and uppermost-mantle depths was derived from a series of magnetotelluric (MT) stations in the Czech and Slovak Republics. Estimates of electric resistivity to crustal and mid-mantle depths were obtained especially from MT data for periods ranging from pulsations to long-period variations, and further from the magnetic continuum spectrum method using the data of the Budkov and Prhonice permanent observatories (Geophysical Institute, Acad. Sci, Czech Republic, Prague). Zones of prominent geoelectrical inhomogeneities were mapped from distribution of the geomagnetic induction vectors across the European area. The ranges of apparent resistivities in the Bohemian Massif were compared with those obtained from induction scale lengths (ISL) and W₁-responses at relevant central European observatories. The conductivity ranges were estimated from the resistivity/depth graph corresponding to the substitute perfect conductor inversion.

LONG PERIOD MAGNETOTELLURIC STUDIES IN THE WESTERN PART OF THE BOHEMIAN MASSIF

V. Červ, J. Pek, O. Praus

2001, 23(2):  153-165. 

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A long period magnetotelluric profile in SW Bohemia, in the western part of the Bohemian Massif, close to the KTB deep borehole in Oberpfalz, Germany, is studied. The data roughly fit a composite model of a 2D regional structure, striking E-W, overlaid by a layer of highly distorting near-surface inhomogeneities. The distorting layer displays large anisotropy with preferred conductivity in NW-SE to NNW-SSE direction. Several variants of 2D models for the deep regional structure are discussed, including an anisotropic model which can qualitatively explain the discrepancy of the principal directions indicated by the MT and GDS data. The relation of the results obtained along the particular profile to the regional geoelectrical background is studied.

ELECTRICAL STRUCTURES OF THE CRUST AND UPPER MANTLE IN SANHE-PINGGU M8 EARTHQUAKE AREA, CHINA

Deng Qainhui, Wang Jijun, Tang Ji, Zhan Yan, Sun Jie, Li Guoshen, Xiao Sidong, Hong Fei, Ye Qing

2001, 23(2):  178-185. 

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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.

PRELIMINARY STUDY ON ELECTRIC STRUCTURE IN THE JINGBOHU VOLCANO AREA OF THE HEILONGJIANG PROVINCE

Zhu Renxue, Fu Weizhou, Meng Lingshun, Chen Honzhou, Zhao Yi

2001, 23(2):  186-190. 

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In the Jingbohu volcano area of Heilongjiang province, MT measurement of 30 sites has been done. The results are as follows. The explored zone is non 1D structure. The central part is 2D structure. Its strike direction is north-east. In the vertical orientation there are three resistivity layers. Upside resistivity is middling value. Middle is high value. Bottom is low value. The surrounding resistivity of the explored area is larger. In the middle part of the explored area there is low resistivity (relative to periphery) vertical columniation of which the section is not regular, and its strike direction is north-east, and its resistivity has a “middle-high-low” style from the top to the bottom.

ELECTRIC CONDUCTIVITY AND MAGMA CHAMBER AT THE TIANCHI VOLCANO AREA IN CHANGBAISHAN MOUNTAIN

Tang Ji, Deng Qianhui, Zhao Guoze, Bai Denghai, Jin Guangwen, Li wenjun, Xuan Fei, Zhan Yan, Liang Jinge, Pu Xinhua, Wang Jijun, Li Guoshen, Hong Fei, Ma Mingzhi, Chen Fengxue

2001, 23(2):  191-200. 

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Observations of magnetotellurics have been carried out at 46 sites along five profiles in shape of umbrella and along a northwest direction profile in the Tianchi volcano area and its vicinity in Changbaishan Mountain northeast China. There are 61 sites including 15 sites which were done in early time. The data processing has been made by using the Robust technique. High S/N ratio resistivity and phase have been obtained. The Bahr’s impedance tensor decomposition was applied for eliminating distortion. The induction arrow vector was calculated. The two-dimensional inversion method (RRI) is used for data interpretation. The results show that (1)There is low resistivity correspondence Tianchi water pool near the surface. There are four low resistivity zones corresponding to Jingjiang hot spring, Julongquan hot spring, gate of mountain and its vicinity and Shuangmu peak at the depth of above 5 km.(2)Low resistivity body at the depth about 12km is located the carter and its north and east area. The body is 20 to 30km thick and 15～25km wide. It is inferred that this anomaly body may be a magma chamber. (3)The crustal electric conductivity structure has obvious difference in direction of SN in this area. There is a high resistivity zone in the south part and low resistivity zone in the north part.

A TRIAL STUDY OF DATA ACQUISITION TECHNIQUES FOR CEMP IN MOUNTAINOUS AREAS OF WESTERN CHINA

He Zhanxiang, Jiang wenbo, Luo Weifneg, Song Qunhui, Li Weili

2001, 23(2):  201-206. 

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The data acquisition techniques for CEMP in mountainous areas of western China have specific problems comparing with other regions. For instance, the routine methods are usually difficult to be used and/or the quality of acquired data can not be ensured. We make comparison analyses to some problems on the basis of experiments porformed in recent years. These are concerning with how to choose electrode spacing in a flexible manner, how to control record time, what is the effect of measarement sites on data quality, the density of megnetic stations and effect of remote and near references. We summarize the characteristics of implication for CEMP in mountain areas of western China and emphasize the importance of study on the data acquisition techniques for CEMP in these areas.

APPLICATION OF CEMP TO EXPLORATION FOR PALEO-BURIED-MOUNTAIN STRUCTURES

Sun Weibin, Zhang Changjiang, Yuan Zhineng

2001, 23(2):  207-211. 

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With the development of MT techniques, their application has been transferred from traditional mineral survey to the target exploration of anomaly bodies beneath the ground. The exploration accuracy has been greatly improved. In this paper we present the high density CEMP and its application to exploration for paleo-buried-mountain structures.

REMOTE REFERENCE MT TECHNIQUE AND THE APPLICATION TO THE FIELD EXPERIMENT

Huang Zhe

2001, 23(2):  212-216. 

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Results of more than thousand filed measurements show that the remote reference technique is a powerful tool for improving the MT data quality, especially for the noisy environment. Experiments indicate that a reasonable spacing between the local station and the reference station is important for different frequency bands. For high frequency band 10～20km is better. For middle frequency band 50～100km is reasonable. For low frequency band the spacing should be greater than 100km.

THE INVESTIGATION ON PLANE WAVE CHARACTERISTICS OF GEOMAGNETIC FIELD BY OBSERVED MT DATA

Chen Musen, Wang Yousheng, Ye Baiyuan

2001, 23(2):  217-221. 

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The remote reference MT technigue was used in the MT measurement experiment. The electric fields were recorded at site 113-214 and the magnetic fields were recorded at 4 sites including site 113-214. The apparent resistivity and impedance phase curves have been obtaind for sites 113-214 by using electric fields at this site and magnetic fields at all 4 sites, respectively. The agreement is reached among 4 groups of apparent resistivity and phase curves. The experiment indicates that the hypothesis on plan wave of electromagnetic source can be satisfied.

WAVELET-BASED DENOISING OF MT TIME SERIES

He Lanfang, Wang Xuben, He Zhanxiang, Li Chengfang

2001, 23(2):  222-226. 

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Acting as a major method of geophysical prospecting, Magnetotelluric Sounding (MT) is effective in oil and gas prospecting, geothermal survey and deeper earth prospecting. But the undeveloped data processing leads that MT has low resolution, which is the major factor that retards MT from being widely used. There is no method being able to obtain unbiased estimates of the transfer function when there is stonger correlated noise in both the electric and the magnetic time series. Wavelet analysis could decompose the composite signal, which consists of several components of different frequencies, into a series of signal block. So it is an effective method for dissociating the noise from signal. In this paper, we use wavelet analysis to denoise MT data by decomposition and reconstruction. The result is more acceptable than the conventional denoised result.

ONE-DIMENSIONAL MAGNETOTELLURIC CURVE COMPARISON INVERSION WITH PHASE INFORMATION

Zhang Dahai, Xu Shizhe

2001, 23(2):  227-231. 

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On the basis of the magnetotelluric curve comparison method inversion, we introduce phase information into the inversion procedure. It means inserting phase correction parameter into apparent resistivity correction element during the inversion, which makes the program more rapid and the procedure more robust. Comparing with other inversion methods of model research, the result of using curve comparision method with phase information can represent the electrical property distribution of the model correctly, and can be used as the initial model of multi-dimension inversion.

A FEASIBILITY STUDY OF RAPID TWO-DIMENSIONAL MT CURVE COMPARISON INVERSION

Zhang Dahai, Xu Shizhe

2001, 23(2):  232-237. 

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We have developed a new two-dimensional inversion method for magnetotelluric data inversion. This method is based on one-dimensional magnetotelluric curve comparison method with phase information, using resistivity and impedance phase data set from 1D inversion as the initial model, and then modeling with 2D finite element method(FEM). In successive iteration, model resitivity is modified by the 1D inversion procedure at depth and by 2D FEM in survey line. We can obtain a data set approximately approached real resitivity distribution after inversion process. The result shows that inversion process is feasible, and has the advantages of simplity, rapidity and avoiding the caculation of partial derivative matrix.

ADVANCES OF MT MIGRATION IMAGING TECHNIQUE AND ITS CAPABILITY OF SOLVING COMPLICATED GEOLOGICAL PROBLEMS

Yu Peng, Wang Jialin, Wu Jiansheng, Chen Bing

2001, 23(2):  238-244. 

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We summarize the achievements and defects on the study of EM imaging techniques. We put forward the migration imaging technique of magnetotelluric data using the improved finite difference method, which increases greatly the accuracy of the difference equation and the imaging resolution. Our aim is to raise the research level to a processing and interpretation stage for real observed data and complicated geo-electric models better than the simple models test. Based on the calculation results of the observed MT data in the Songliao Basin, the approach we provide has been testified as a more accurate and effective measurement for MT imaging, especially for the interpretation of complicated geological problems.

THE EFFECT OF TWO TYPES OF TURN-OFF CURRENT ON TEM RESPONSES AND THE CORRECTION TECHNIQUES

Bai Denghai, Maxwell Meju

2001, 23(2):  245-251. 

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The effect of turn-off current is a knotty problem in TEM method, especially for shallow soundings. Turn-off time is also a major parameter for evaluating the performance of instruments. Raiche (1984) discussed this problem and suggested a correction method using a ramp (linear) turn-off current expression. In this paper two types of turn-off current are discussed for the central loop configuration and a new correction strategy is suggested. According to this technique the accurate all-time apparent resistivity can be obtained in the case that the secondary and primary fields are recorded simultaneously.

TIME-DOMAIN BEHAVIOR OF TRANSIENT ELECTROMAGNETIC FIELD FOR 2D MODEL

Chen Mingsheng, Yan Shu, Shi Xianxin, Xie Haijun

2001, 23(2):  252-256. 

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In this paper we analyze the transient responses directly in frequency-domain using time-domain difference (FDTD). Since two-dimensional structure is convenient to be shown by figures, we use line source 2D geo-electrical models. By computing “smoke ring” diffusion in the earth and vertical electromotive force on the earth about uniform two-layer and uniform two-layer with a conductor indicates that transient fields have delay phenomenon. It means that once an anomalous body causes transient responses, this response might delay a long time. In engineering explorations, we have used later time delay to prospecting sallower bodies, such as abandoned coal miners located at 20m～150m depth range and have had good results. The numerical resolutions of this study support the previous practices theoretically.

TECHNICAL PROBLEMS OF MT STATIC-SHIFT USING TEM SOUNDING CORRECTION

Yao Zhilong, Wang Qingyi, Hu Yuping, Liu Junchang

2001, 23(2):  257-263. 

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The static-shift caused by shallow inhomogeneouties is one of the major factors which lead to severe problems in the interpretation of magnetotelluric (MT) data. It has been shown that this problem can be solved by using time-domain Transient ElectroMagnetic (TEM) sounding. In this paper, the characteristic of MT static shift and the principle of correction by using TEM method are presented. Besides, three technical key points, the features of TEM survey system (TEMS-3S), the data processing software and the correcting techniques, are discussed, and a case history is given as well to illustrate the effectiveness of the correction method for MT static-shift.

A BRIEF DESCRIPTION OF THE PROTEM-37 TRANSIENT ELECTRIMAGNETIC SOUNDING SYSTEM AND ITS APPLICATION

Zhan Yan, Zhao Guoze, Liang Jingge, Tang Ji, Deng Qianhui, Li Wenjun, Zhao Junmeng, Fan Hongrui, Zhao Yonggui, Li Qun

2001, 23(2):  264-270. 

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PROTEM-37 instrument possesses good qualities such as high resolution, large dynamic range, strong stability, and large sounding depth. In this article, the sounding principle, the work equipment, out-field work steps and data processing and interpretation of PROTEM-37 are described briefly. More over, the application of PROTEM-37 in the underground cave detecting, metal mineral prospecting and investigation in the hydrological aspect of a coal field is presented. All of these examples show that the PROTEM-37 instrument is effective in practice, and the TEM sounding method is an EM sounding method with extensive application, high resolution and large sounding depth.

TRIAL WITH LOTEM TO INVESTIGATE DETAILED GEOLOGIAL STRUCTURE IN THE AREA COVERED WITH CARBONATITE

Yan Liangjun, Hu Wenbao, Chen Qingli, Hu Jiahua

2001, 23(2):  271-276. 

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The paper presents the trial method and results with Long-offset & window Transient Electromagnetic Method (LoTEM) to investigate detailed geological structure in the area covered with carbonatite. We chose a well known local structure in Sangzhi Shimen synclinorium, Siwang mountain structure whose area is about 200 km²,to test the LoTEM. The rugged topography area is covered with carbonatite, and the surface geological condition gives priority to the complicated structure. The resistivity of the surface carbonatite is greater than 500Ω·m, and the target layer is Silurian marlite with resistivity value about 10Ω·m, and its buried depth is one to two kilometers. These give the better geophysical preconditions to the working area. Two sources were setup in the field. We finished eight lines with total 212 geophysical sites. The grid was arranged with equality (2km×0.5km).Althought there is strong cultural noise in the area, LoTEM features good anti disturbance. The rate of higher quality data is more than 80%. The EMF curve is credible before 1s and it guaranteed the data quality for interpretation. Through data processing and inversion, resistivity versus depth profiles of eight lines were obtained, and they have good coherence with the seismic profile. The results show that LoTEM can be used in prospecting for detailed structure in complicated areas.

THE APPLICATION OF ELECTRICAL MEASUREMENTS TO EARTHQUAKE PREDICTION IN CHINA

Zhao Yulin, Lu Jun, Zhang Hongkui, Qian Wei, Qian Fuye

2001, 23(2):  277-285. 

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Resistivity measurement with an active source has been practised for thirty years at an enormous number of station in China. Resistivity decreases of several percent, which began 2～3 years prior to the Tangshan earthquake and other events and gradual increases afterward were observed. These anomalous variations were larger than their background fluctuations and hence statistically significant. Resistivity decreases were generally seen first and larger at stations close to epicenters and later and smaller at more distant stations. This implies that in the preseismic intermediate term stage, the fault was closed and the accumulated strain started first in the epicentral area and then migrated outwards from the epicentre. Anomalies in self-potential of the specific pattern of rapid onset and slow decay recorded at the station XZ are almost simultaneous with oil blowouts from the borehole W11. These short term or impending earthquake precursors migrated towards the epicentre, i.e., strain energy releasing occurred first at stations more distant to the epicentre, later at closer stations and at last the main shock took place. For short periods the industrial interference magnetotellurics (IIMT) determined that the Tangshan earthquake took place in the resistive crust. TM mode (H polarization) data at long periods of the MT measurements determined that electrical properties of resistive zone and the existence of a deep fault zone (fluid leakage paths) was found in resistive crust of the Tangshan earthquake area. Electrical measurements are the geophysical measurements most closely related to fluid volumes, connectivities and pressures, and applicable to earthquake prediction.

A MANTLE TEMPERATURE MODEL DERIVED FROM AN INVESTIGATION OF THE THERMAL HISTORY OF THE EARTH

Takesi Yukutake

2001, 23(2):  286-291. 

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A mantle temperature profile is presented, which was obtained by solving the global energy balance equations. A sharp temperature increase across the mid-mantle transition zone inhibits a simple downward-extrapolation from the near surface data. However, this problem is solved in the context of studying the cooling process of the Earth. As the present-day value, the temperature at the base of the mantle was obtained to be 2 940K. At the mid-mantle transition zone, the temperature became 2 040 K with a sharp change of 310 K across the mid-boundary layer. At the base of the surface boundary layer it was obtained as 1 610 K.

MONITORING TECHNIQUE BASED ON THE ELECTROMETRIC METHOD INTERWELL

Zhang Jincheng

2001, 23(2):  292-300. 

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The basic theory and the test way of the interwell motitoring technique based on the electrometric method are presented. By using the application cases in the Daqing oil field, the Dagang oilfield, and a Shanxi coal mine, it is demonstrated that this technique can play an important role in oil field development, the evaluation for technologic measures and exploration of coal bed gas. Its usefulness and popularization in the future are also described.

A FINITE ELEMENT CODE FOR FULL 3D ELECTROMAGNETIC LOGGING RESPONSE COMPUTATION

Yang Wei

2001, 23(2):  301-306. 

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Considering the influence of conductivity and displacement current at the meantime, we derive a new functional formulation of A-Φ and develop a 3D Finite Element code to simulate electromagnetic well logging responses in full 3D and a wide frequency range. We compute the field distribution of a horizontal and a vertical magnetic field coil. We also give the response of induction well logging in deviated wells. Finally we analyze the influence of coil radius and anisotropy.

A PILOT COMPREHENSIVE GEOPHYSICAL STUDY OF THE SONGLIAO BASIN

Xu Xuejun, Zhang Lien, Yan Hongrui, Jiang Jilian, Tian Xiaodong

2001, 23(2):  307-313. 

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Physical properties of rocks, velocity, density, magnetic susceptibility and resistivity are analyzed statistically to look into their inherent relationships. First, various useful information is used to establish a comprehensive physical geological model. Then the model is modified according to the inversions of gravity, magnetism, MT and seismic data. Finally, a modified comprehensive model is obtained by the integrated inversion of all the available geophysical data.

IMPROVED RECONSTRUCTION ALGORITHMS FOR RESISTIVITY TOMOGRAPHY

Liu Guoqiang, Tao Guo, Ke Shizhen, Yang Huizhu, Jiang Jiya

2001, 23(2):  314-320. 

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We have developed an improved reconstruction algorithm for resistivity tomography. In this algorithm the perturbation approach is employed for calculating the Fr閏het derivatives. Comparing with other four methods the perturbation approach is more efficient as it needs only one time of forward calculation for each iteration. Theoretical analysis and numerical experiments show that this method has higher stability and computing efficiency. An example of field data processing demonstrates that this method is valid. This method can also be generalized to suit for seismic velocity tomography and electromagnetic wave tomography techniques since it is derived for nonhomogenious Helmholtz equations.

STUDY ON SOME PROBLEMS FOR 3D RESISTIVITY INVERSION USING CONJUGATE GRADIENT

Wu Xiaoping, Wang Tongtong

2001, 23(2):  321-327. 

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The 3D resistivity inversion using the conjugate gradient(CG) method has been developed in recent years and has many problems that remain to be studied.This paper studies how the Lagrange factor λ and CG iterative times influence the results of 3D inversion.The main purpose is to yield good results of 3D inversion.