城市活断层探测中电磁噪音和环境干扰对浅层电磁方法的影响

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     

Theoretical investigation of the ocean-coast effect at a passive continental margin

The idea that oceanic lithosphere is thinner than continental lithosphere is widely accepted even though one would like to see clearer evidence to support it. In fact, the very concept of lithosphere is still a matter of some debate. If there is indeed a variation in the thickness of the lithosphere at continental margins and if this change is associated with a lateral variation in electrical conductivity one may envisage detecting it with electromagnetic soundings methods. A model of a passive continental margin has therefore been investigated to test whether this would be feasible. It has been found that the well-known but strong ocean-coast effect masks the minor lithospheric effect in magnetotelluric soundings performed on the shore. Inductive soundings, on the other hand, are highly sensitive to lateral variations in electrical conductivity. An analysis in terms of the induction arrow has shown that such soundings carried out on land would be perfectly suitable to reveal a changing lithospheric thickness, if the continents merely extended to the oceanic coast. However, the presence of only a narrow continental shelf of 100 km width under 250 m of sea-water produces an overriding coast effect ahead of the margin, and thus renders electromagnetic methods unsuited to reveal a changing lithospheric thickness.     

Artificial neural networks for removal of couplings in airborne transient electromagnetic data

Modern airborne transient electromagnetic surveys typically produce datasets of thousands of line kilometres, requiring careful data processing in order to extract as much and as reliable information as possible. When surveys are flown in populated areas, data processing becomes particularly time consuming since the acquired data are contaminated by couplings to man‐made conductors (power lines, fences, pipes, etc.). Coupled soundings must be removed from the dataset prior to inversion, and this is a process that is difficult to automate. The signature of couplings can be both subtle and difficult to describe in mathematical terms, rendering removal of couplings mostly an expensive manual task for an experienced geophysicist. Here, we try to automate the process of removing couplings by means of an artificial neural network. We train an artificial neural network to recognize coupled soundings in manually processed reference data, and we use this network to identify couplings in other data. The approach provides a significant reduction in the time required for data processing since one can directly apply the network to the raw data. We describe the neural network put to use and present the inputs and normalizations required for maximizing its effectiveness. We further demonstrate and assess the training state and performance of the network before finally comparing inversions based on unprocessed data, manually processed data, and artificial neural network automatically processed data. The results show that a well‐trained network can produce high‐quality processing of airborne transient electromagnetic data, which is either ready for inversion or in need of minimal manual processing. We conclude that the use of artificial neural network scan significantly reduce the processing time and its costs by as much as 50%.     

THE ASSOCIATION OF RESISTIVITY SOUNDINGS*

A simple measure, the association parameter, is proposed for directly comparing the results of two electrical soundings. The use of this measure to classify field results and to gain some insight into geological structure before extensive depth interpretation is discussed. In particular it is shown that when used with soundings conducted using the tripotential technique the combined use of association parameter arid lateral inhomogeneity index can allow structural patterns to be discerned where otherwise they might be obscured. Possible extension of the technique is considered.     

The Assimilation of GPS Radio Occultation Data and its Impact on Rainfall Prediction along the West Coast of India during Monsoon 2002

In this study, the Weather Research and Forecasting (WRF-2.0.3.1) model with three-dimensional variational data assimilation (3DVAR) was utilized to study a heavy rainfall event along the west coast of India with and without the assimilation of GPS occultation refractivity soundings in the monsoon period of 2002. The WRF model is a next-generation mesoscale numerical weather prediction system designed to serve both operational forecasting and atmospheric research communities. The Global Positioning System (GPS) radio occultation (RO) refractivity data, processed by UCAR, were obtained from the CHAMP and SAC-C missions. This study investigates the impact of thirteen GPS occultation refractivity soundings only, as assimilated into the WRF model with 3DVAR, on the rainfall prediction over the western coastal mountain of India. The model simulation, with the finest resolution of 10 km, was in good agreement with rainfall observations, up to 72-h forecast. There are some subtle but important differences in predicted rainfalls between the control run CN (without the assimilation of refractivity soundings) and G13 (with the assimilation of thirteen GPS RO soundings). In general, the assimilation run G13 gives a better prediction in terms of both rainfall locations and amounts at later times. The moisture increments were analyzed at the initial and forecast times to assess the impact of GPS RO data assimilation. The results indicate that remote soundings in the forcing region could have significant impacts on distant downstream regions. It is anticipated, based on this study, that considerably occultation soundings available from the six-satellite constellation of FORMOSAT-3/COSMIC would have even more significant impacts on weather prediction in this region.     

Deep sounding with industrial power lines in conjunction with MTS measurements

This study is devoted to the development of a method for deep tensor electromagnetic inductive sounding of the Baltic Shield lithosphere with the use of mutually orthogonal industrial power lines alternately connected to a generator of up to 100 kW in power. The first results of the sounding are obtained at separations of up to 510 km from the source. The electrical conductivity of the Baltic Shield lithosphere in the crust-mantle transition zone (depths of 20–50 km) is numerically estimated from soundings with natural and controlled sources. The perspectives of future investigations are associated with synchronous observations on a 2-D network of measuring stations. This will enable the study of the relation of deep geoelectric parameters of the lithosphere to the geological structure of the crust in terms of 3-D models.     

Effect of non-uniform station coverage on the inversion for earthquake rupture history for a Haskell-type source model

In order to determine how reliably one can invert accelerograms to determine the rupture process details, when the station configuration is less than optimal, we use the vertical component of synthetic accelerograms for a Haskell-type earthquake rupture model, at stations in the vicinity of a dip-slip fault and solve the inverse problem. Of the various station configurations used, one is a uniform distribution and the others are very non-uniform. Faults of two different aspect ratios are considered. We mainly use much larger spatial and temporal cell sizes in the inversion than we use to construct the artificial data. The fault mechanism and the fault area are taken as known in the inversions. To solve the inverse problem, we use the method of linear programming and stabilize the solution by the use of physical constraints. The constraints of positivity of the slip rates on the fault is used in all cases in this study. In some cases, additional physical constraints such as preassigning the final moment, the rupture speed, and so on, are also used. We find that using a cell size almost double the wavelength of interest, we are able to reproduce the solution of the problem, even when we add a small amount of random noise to the artificial data, provided the source medium structure is known. We show that the best station configuration is when the stations are on the hanging wall, due to the fact that they provide the best illumination of the fault surface. This provides an incentive to install permanent ocean bottom strong ground motion stations in subduction zones. We also analyzed the effect of the rupture propagation direction on the results of the inversion showing that even four stations are sufficient to retrieve the rupture process if they are in the forward direction of the rupture propagation; the results for this case are better than when the four stations are placed in the backward direction, even when their positions are such that they illuminate the fault in exactly the same way as the four stations in the forward direction. Thus azimuthal distribution and the resulting illumination of the fault as well as the relation of the position of the stations to the direction of rupture propagation are more important than simply the number of stations. Finally, we find that proper knowledge of source medium structure is essential to recover the source process details reliably and that poor knowledge of crustal structure cannot be compensated by adding stations or by additional constraints.     

A MODIFIED GEOELECTRICAL PROCEDURE USING POLARDIPOLE ARRAYS—AN EXAMPLE OF APPLICATION TO DEEP EXPLORATION *

Dipole soundings are more sensitive to noise caused by lateral and superficial inhomogeneities than Schlumberger soundings. However, the former are preferable for deep explorations in view of the relatively short cables required. The simple solution of carrying out the field work by means of dipole spreads, and to transform the dipole resistivity diagrams into Schlumberger ones by means of proper formulae would be valid only for smooth and regular curves; but often, owing to the presence of lateral noises, the dipole data show a considerable scatter. For such cases a “continuous dipole sounding” method is proposed for which all successive dipoles are contiguous, so that all parts of the profiles are covered and interpolation is not necessary. Obviously the moving dipoles have lengths proportional to their distances, so that they appear equal in the usual bilogarithmic scale. It follows that only polar-dipole arrays may be used. The transition from a dipole to the corresponding Schlumberger apparent resistivity diagram requires an integration constant which is not unequivocally determined. Therefore, the solution is not unique, but all possible derived Schlumberger diagrams have a common part. Similarly, they have some common interpretative results, which may be referred to the original dipole diagram obtained in the field. A special measurement technique is required since the dipole-dipole voltages to be determined are noticeably smaller than the Schlumberger ones. This is true also because dipole soundings are used for great depths and for long distances between the two dipoles.     

Seismic Wavefields in the Deep Seafloor Area from a Submarine Landslide Source

We use the finite difference method to simulate seismic wavefields at broadband land and seafloor stations for a given terrestrial landslide source, where the seafloor stations are located at water depths of 1,900–4,300 m. Our simulation results for the landslide source explain observations well at the seafloor stations for a frequency range of 0.05–0.1 Hz. Assuming the epicenter to be located in the vicinity of a large submarine slump, we also model wavefields at the stations for a submarine landslide source. We detect propagation of the Airy phase with an apparent velocity of 0.7 km/s in association with the seawater layer and an accretionary prism for the vertical component of waveforms at the seafloor stations. This later phase is not detected when the structural model does not consider seawater. For the model incorporating the seawater, the amplitude of the vertical component at seafloor stations can be up to four times that for the model that excludes seawater; we attribute this to the effects of the seawater layer on the wavefields. We also find that the amplification of the waveform depends not only on the presence of the seawater layer but also on the thickness of the accretionary prism, indicating low amplitudes at the land stations and at seafloor stations located near the trough but high amplitudes at other stations, particularly those located above the thick prism off the trough. Ignoring these characteristic structures in the oceanic area and simply calculating the wavefields using the same structural model used for land areas would result in erroneous estimates of the size of the submarine landslide and the mechanisms underlying its generation. Our results highlight the importance of adopting a structural model that incorporates the 3D accretionary prism and seawater layer into the simulation in order to precisely evaluate seismic wavefields in seafloor areas.     

Magnetovariational information to improve distortion diagnostics in deep magnetotelluric soundings

An interpretative experience from nine magnetotelluric soundings was accomplished in the central region of Argentina (32°S?C34°S; 63°W?C69°W), from the Andean region in the west to the platform zone in the east. To do this, magnetovariational information was used to improve the distortion diagnostics in magnetotelluric curves. Using Pilar Geomagnetic Observatory as a reference site, horizontal magnetic transfer functions were estimated, which were compared with the integrate conductivity at each location in field. As a result, a rather simple methodology is proposed to better approach the accurate positions of normal curves. Results suggest that, in this way, better formal interpretations of soundings may be reached. In addition, a more clear and comprehensible knowledge about the nature of lateral in-homogeneities is obtained; e.g., discovering 3-D effects no suspected from tectonic maps. This methodology seems to be particularly useful when-as in the present case-magnetotelluric soundings are far away each others; i.e., when effective volumes of soundings are not interpenetrated. Horizontal magnetovariational information suggests two elongate conductivity anomalies (about N25°?C30°E), possibly associated with deep seated faults belonging to the South American regmatic network. These anomalies would be produced by partial melting in lower crust and possibly in the asthenospheric zone next to Andean Range. Another elongate anomaly (possibly of graphitic nature) is shown in the study region. It seems to be a marginal fault following the border between The Sierras Pampeanas dynamic zone and the South American craton. Magnetotelluric results indicate the study region can be considered as divided in a dynamic belt next to Andean Range and a cratonic zone eastward. The dynamic zone presents a well developed lower crust, with conductances ranging 300?C4300 Siemens and depths of about 20?C30 km. An asthenosphere close to the Andes with 1000 Siemens of conductance at 74 km depth is also observable. Heat flows of 63?C70 mW/m² are estimated next to Andes and 48 mW/m², eastward, close to South American platform. The cratonic zone presents a first conductive layer with a conductance of 270 Siemens underneath BUL sounding site, but it does not seem to present lateral development. An intermediate conductive layer is also present in this region, but it does not have so much development. Therefore, this layer would not have asthenospheric character; so, the lithosphere would be tied to upper mantle. Heat flows ranging 40?C35 mW/m² were estimated for this cratonic region.     

On the 3-D inversion of vertical electrical soundings: Application to the South Ismailia area—Cairo desert road, Cairo, Egypt

A code for 3-D resistivity modelling and inversion of vertical electrical soundings has been developed based on the finite-element technique and regularisation method. Synthetic data were used to test the effectiveness of the code and to examine the resolving capability of the Schlumberger array in investigating 3-D resistivity distributions. The code was applied to experimental data set constituted by 35 Schlumberger soundings collected near the Cairo city in order to study the subsurface resistivity distribution. The results have shown that valuable imaging of the subsurface resistivity distribution can be constructed even when the vertical electrical soundings are acquired in a sparse field data set.     

Analysis of electrojet-distorted magnetotelluric sounding curves

South America presents several unique geomagnetic features, one of which is the Equatorial Electrojet (EEJ), a current system which extends itself east to west in Northeastern Brazil for almost 3500 km. Considering the fact that the influence of this phenomenon at low magnetic latitudes can be detected at great distances from its central axis, it is important to simulate its effect in magnetotelluric (MT) exploration. To accomplish this and by the use of an integral equation scheme, we have modeled the MT response of confined three-dimensional (3D) bodies (dykes in an homogeneous half-space) and deep 3D structures (horst and graben located at Marajo Basin in Northern Brazil). As the inductive source, we replace the classical plane wave source by a common line of current, besides gaussian and undulatory electrojets. The results of the modelling indicate that the studied effect is prominent in both one-dimensional (1D) and 3D media for periods (T) greater than 10 s. It decreases with distance, but it is detectable as far as 3000 km from the center of the EEJ. It is also observed that for T greater than 10 s, the computed soundings can be strongly distorted, mainly by EEJ effects in the host medium which, in turn, cause changes in the final branch of the 3D soundings. For structures in the Marajo Basin, our results reveal that the 3D galvanic effect prevails in the interval 0.1<T<10 s, while source influence occurs mainly for T>10 s. On the other hand, automatic inversion of these 3D data shows that, in equatorial regions having complex geology, some errors arise in conventional 1D interpretation of the MT soundings. This is due to the superposition of the host medium response, the galvanic effect of 3D structures, and the EEJ influence.     

NUMERICAL MODELLING OF STANDARD AND CONTINUOUS VERTICAL ELECTRICAL SOUNDINGS1

Analytical solutions of vertical electrical soundings (VES) have mostly been applied to groundwater exploration and monitoring groundwater quality on terrains of fairly simple geology and geomorphology on which the electrode arrays are symmetrical (e.g. Schlumberger or Wenner configurations). The sounding interpretation assumes flat topography and horizontally stratified layers. Any deviations from these simple situations may be impossible to interpret analytically. The recently developed GEA-58 geoelectrical instrument can make continuous soundings along a profile with any colinear electrode configuration. This paper describes the use of finite-difference and finite-element methods to model complex earth resistivity distributions in 2D, in order to calculate apparent resistivity responses to any colinear current electrode distribution in terrains in which the earth resistivities do not vary along the strike. The numerical model results for simple situations are compared with the analytical solutions. In addition, a pseudo-depth section of apparent resistivities measured in the field with the GEA-58 is compared with the numerical solution of a real complex resistivity distribution along a cross-section. The model results show excellent agreement with the corresponding analytical and experimental data.     

DC resistivity methods for determining resistivity in the earth's crust

The use of Schlumberger and dipole arrays for crustal-scale resistivity soundings is considered. Advantages and disadvantages of the two methods are described. The depth to which resistivity may be determined from field measurements is discussed as well as the determination from the sounding curves of various parameters associated with layered structure. The interpretation of experimental data using reference curves as well as two approaches used in computer assisted interpretation are discussed.     

THE INFLUENCE OF LATERAL INHOMOGENEITIES ON THE DIPOLE METHODS*

The theory of electrical dipole soundings proved that this method can produce resistivity measurements, which are comparable with those obtained by electrical soundings of the Wenner or Schlumberger type. Their main advantage is the use of short cable lengths, which is important if the depth of penetration should be large. A considerable disadvantage of the dipole method is the great sensitivity to lateral discontinuities. Though these have an influence on the Schlumberger arrangement as well, they can disturb a dipole sounding to such an extent than an interpretation based on a horizontal layer case is no more possible. There are six different dipole arrays, which differ from each other with respect to the angle enclosed by the two dipole orientations-the current dipole AB and the measuring dipole MN. The theoretical comparison of the dipole arrays with the Schlumberger array concerning their sensitivity to lateral discontinuities is a useful basis for the choice of the most suitable configuration. Considering geological subsurface conditions the right choice of a dipole array can give an optimal result, i.e. a dipole sounding for which the sensitivity to lateral discontinuities is as small as possible under the given circumstances.     

Dynasonde observations of electron concentration gradients above Tromsø

The Dynasonde is a digital ionosonde which measures the amplitude and phase of radio echoes reflected from the ionosphere. In addition to the frequency and virtual range of each echo, one parameter of particular interest is the direction of arrival which allows horizontal structures in the ionosphere to be studied. Under suitable ionospheric conditions, namely where structures or strong gradients are present in the electron concentration, echoes may be observed out to horizontal distances of several hundred kilometres. We describe an automated method for latitudinally mapping these gradients in F-region peak electron concentration using the Dynasonde. Good agreement is found when the results are compared with a co-located incoherent scatter radar lending credibility to observations of electron concentration gradients by a Dynasonde alone. This is significant for sites where no supporting instruments exist. Although Dynasondes account for only a small fraction of digital ionosondes in use today, the techniques described in this paper could be extended to other digital ionosondes with similar directional capabilities. The combination of routine soundings, and the worldwide distribution of such instruments, would provide a powerful means to monitor ionospheric structures on a regular basis.     

风光发电系统在无人值守地震台站的设计

风光互补发电技术在边防、通信、公路等已得到广泛应用,能否将这一绿色、清洁、可再生能源应用于地震无人值守台站,山东省在龙口地震台已做先期试点,通过一年试运行,风光互补发电系统能够满足地震无人值守台站的工作要求。     

Research Note: The mean sensitivity depth of the electrical resistivity method

Until now, a simple formula to estimate the depth of investigation of the electrical resistivity method that takes into account the positions of all of the electrodes for a general four‐electrode array has not been available. While the depth sensitivity function of the method for a homogeneous infinite half‐space is well known, previous attempts to use it to characterize the depth of investigation have involved calculating its peak and median, both of which must be determined numerically for a general four‐electrode array. I will show that the mean of the sensitivity function, which has not been considered previously, does admit a very simple mathematical formula. I compare the mean depth with the median and peak sensitivity depths for some common arrays. The mean is always greater than or equal to the median that is always greater than the peak. All three measures give reasonable estimates to the depths of actual structures for most circumstances. I will further show that, for 1D soundings, the use of the mean sensitivity depth as the pseudo‐depth assigns an apparent resistivity to a given pseudo‐depth that is consistent between different arrays. One consequence of this is that smoother depth soundings are obtained as “clutches,” caused by a change in the depth sensitivity due to moving the potential electrodes, are effectively removed. I expect that the mean depth formula will be a useful “rule of thumb” for estimating the depth of investigation before the resistivity structure of the ground is known.     

洪涝异常与云南楚雄地区强震的相关分析

利用云南省123个气象台站1960年以来的降雨量资料,使用年降雨量距平百分率方法,分析楚雄强震与降雨量的关系,结果发现：年降雨量距平百分率≥0.25的台站数大于8,次年云南地区皆有Ms≥6.0地震,也是楚雄地区发生6级强震背景条件;楚雄地区强震前表现涝异常,即该区域强震具有涝—震特征.