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1.
Reduction of reflections from above surface objects in GPR data 总被引：2，自引：0，他引：2
During a ground-penetrating radar (GPR) survey, special attention must be paid to objects located above the earth's surface. Due to the low-loss character of electromagnetic propagation in air and high velocity, above-surface reflections or diffractions can overwhelm subsurface events, making the interpretation a difficult task. The relative sensitivity of reflections and diffractions originating from above-surface objects is a function of the antenna radiation characteristics, the lateral and vertical dimensions of the objects and their position with respect to the antennas. The largest amplitude reflections and diffractions are expected when the polarization of the electric field is parallel to the long-axis of the object. Near the surface in the E-plane, the electric field is vertically polarized and has a larger amplitude than the horizontally polarized electric field in the H-plane. Numerical modeling of reflections from three above surface objects (a vertical plane and elongated horizontal and vertical objects) demonstrate that the largest amplitude difference occurs when an elongated vertical object is present in the E- or H-plane. The calculated reflection from the elongated vertical object present in the E-plane was 21 times larger than when it was present in the H-plane. In 60-m long field data sets, reflections from interfering trees present in the E-plane were at several positions >15 times larger and on average 6 times larger than when the trees were present in the H-plane. These large amplitude differences indicate that appropriate orientation of the antennas can be used to minimize the effects of above-surface reflections and diffractions. 相似文献
2.
Analyses of travel times and amplitudes of crosshole georadar data provide estimates of the electromagnetic velocity and attenuation of the probed media. Whereas inversions of travel times are well established and robust, ray-based inversions of amplitudes depend critically on the complex directive properties of the georadar antennae. We investigate the variations of radiation patterns in the presence of water-filled boreholes and/or changes of electrical material properties in the vicinity of the transmitters or receivers. To assess the implications of such complicating factors for ray-based georadar amplitude tomography, we generate crosshole georadar data for a suite of canonical models using a finite difference time domain (FDTD) solution of Maxwell's equations in cylindrical coordinates. The emitting dipole-type antenna is approximated by an infinitesimal vertical electric dipole, whereas a corresponding receiving antenna is emulated by recording the vertical component of the transmitted electric field. Inversions of the amplitudes of these synthetic data demonstrate that the presence of water-filled boreholes as well as changes in the material properties along the boreholes may cause substantial artifacts in the estimated attenuation structure. Furthermore, our results indicate that ray-based amplitude tomography of crosshole georadar data is unable to constrain absolute values of attenuation. Despite these inherent limitations, the method is surprisingly robust at detecting and constraining relative changes in attenuation. In particular, we find the method to be highly effective for locating conductivity contrasts that are not associated with corresponding changes in dielectric permittivity, and hence, cannot be located by travel time tomography alone. 相似文献
3.
In order to improve the reliability of the ground penetrating radar (GPR) method in identifying subsurface sinkholes and karst cavities, laboratory investigations have been performed. The main objective of this work was to examine the relationship between horizontal/vertical voids dimensions and wavelengths of various antennas, and the corresponding GPR responses. Emphasis was given to the investigation of the factors that cause the appearance of reverberation phenomena in the signal pattern.The tests were conducted in 5 m × 10 m area by 2-m-deep trench filled with homogenous, dry sand. The voids models (empty fiberglass cylinders in diameters of 0.6 m, 1.0 m, 1.5 m and 2.4 m, with various heights) were buried vertically with their tops at depths of between 0.7 and 1.5 m. Investigations were performed for the various model conditions by towing 500, 300 and 100 MHz antennas along a pre-established grid, for the various model conditions.The GPR data collected using the 500 MHz bistatic antenna above the 1.0-m- and the 1.5-m-diameter cylinders, and using the 300 MHz bistatic antenna above the 1.5-m-diameter cylinder, confirmed the presence of a reverberation phenomenon, i.e. a strong convex signal pattern, containing a series of high amplitude extending oscillations with reduced frequency.Based on past practical GPR experience of void detection and presently obtained experimental data, two rules of thumbs may be adopted for the prediction of the appearance of resonant radar pictures:
- 1. The void diameter larger than the wavelength in air of the antenna used.
- 2. The vertical size of the empty void not significantly smaller than its horizontal dimension.
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X. Drobert C. Fauchard Cte E. Le Brusq E. Guillanton J. Y. Dauvignac Pichot 《Journal of Applied Geophysics》2001,47(3-4)
In the field of road construction and maintenance, the need for information on the thickness of very thin road layers is not satisfied by means of commercial pulse GPR, due to the inability of such devices to operate over ranges of several gigahertz. As a result, research has focused on the design of a step-frequency radar technique, able to work with very high-frequency synthetic pulses.An ultrawide band antenna, belonging to the family of Vivaldi antennas, has been developed for road applications. It has been created using stripline technology and yields a band width greater than one decade. During an initial step, this antenna was tested on various bituminous concrete samples with a network analyzer. Different parameters were studied, including band width, offset between antennas, and height and shape of the frequency-dependent pulse.A second step involved GPR dynamic measurements. A customized software program enabled recording data from the network analyzer. Several radar profiles were developed from selected road construction and maintenance test sites (e.g. the Circular Pavement Fatigue Test Track, composed of a number of known structures). Results show improved resolution when compared to a commercial impulse GPR system. 相似文献
6.
Antenna field 2 of the Tromsø Heating facility consists of six rows of six horizontal crossed full wave dipoles aligned with the rows running geographic east-west. In previous experiments on ELF/VLF wave generation it has been the practice to feed the rows in parallel, with HF radiation amplitude modulated at the ELF/VLF frequency it was desired to radiate (AM configuration). Here we describe how the antenna array was also configured so that it could be fed with a continuous wave (CW) input power but still carry information at an ELF/VLF frequency. To effect this the three southern most rows of the antenna array were driven with a CW signal at 4.04 MHz and the three northernmost rows with a CW signal at frequency greater than 4.04 MHz by the ELF/VLF frequency it was desired to radiate (CW configuration). Experiments were performed with modulation/difference frequencies of 565 and 2005 Hz and the signals were received at the Lycksele Geophysical Observatory, 500 km south of the heating facility. The signals were typically 11 dB greater in the AM than the CW configuration, despite the fact that the average power delivered to the ionosphere in the CW configuration was four times that in the AM configuration. Significant harmonic radiation (both odd and even) was produced in the AM configuration but no harmonic radiation was detected in the CW configuration. A simple theory has been developed to model the fields produced by HF heating using the two techniques (AM,CW). A good agreement has been obtained between the experimental observations and model computations. The model has been extended to show how the relative efficiency of generation, AM/CW, varies with the frequency of the ELF/VLF radiation and HF antenna element spacing. Radiation patterns for the ‘ionospheric ELF/VLF antenna’ have also been derived for the two generation techniques. 相似文献
7.
Definitive superparamagnetic source identification through spatial,temporal, and amplitude analysis of airborne electromagnetic data 
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下载免费PDF全文 James Macnae 《Geophysical Prospecting》2017,65(4):1071-1084
The aim of this paper is to add confidence to existing methods using decay shape analysis to detect superparamagnetic responses in airborne electromagnetic data. While expensive to acquire, vertical spatial gradient measurements of the electromagnetic signals can discriminate near‐surface superparamagnetic sources. This research investigated the use of horizontal spatial gradients and amplitude information as further indicators of superparamagnetic. High horizontal gradients were shown both theoretically and in field data to help discriminate superparamagnetic from deep mineral targets. Further, superparamagnetic responses have characteristically small amplitudes inconsistent with realistic mineral exploration targets at shallow depths. 相似文献
8.
Jorge Luís Porsani Evert Slob Robson S. Lima David Nakamura Leite 《Journal of Applied Geophysics》2010,70(1):1-8
GPR (Ground Penetrating Radar) results are shown for perpendicular broadside and parallel broadside antenna orientations. Performance in detection and localization of concrete tubes and steel tanks is compared as a function of acquisition configuration. The comparison is done using 100 MHz and 200 MHz center frequency antennas. All tubes and tanks are buried at the geophysical test site of IAG/USP in São Paulo city, Brazil. The results show that the long steel pipe with a 38-mm diameter was well detected with the perpendicular broadside configuration. The concrete tubes were better detected with the parallel broadside configuration, clearly showing hyperbolic diffraction events from all targets up to 2-m depth. Steel tanks were detected with the two configurations. However, the parallel broadside configuration was generated to a much lesser extent an apparent hyperbolic reflection corresponding to constructive interference of diffraction hyperbolas of adjacent targets placed at the same depth. Vertical concrete tubes and steel tanks were better contained with parallel broadside antennas, where the apexes of the diffraction hyperbolas better corresponded to the horizontal location of the buried target disposition. The two configurations provide details about buried targets emphasizing how GPR multi-component configurations have the potential to improve the subsurface image quality as well as to discriminate different buried targets. It is judged that they hold some applicability in geotechnical and geoscientific studies. 相似文献
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机载探地雷达可以用于人类无法到达的危险地区、植被严重覆盖的地下目标体探测,然而由于机载探地雷达的特殊性,影响机载探地雷达探测效果的因素包括天线的极化方向、天线的飞行高度以及地表粗糙度等.为了研究这些影响因素与探测效果之间的关系,用三维时间域有限差分模拟电磁波的传播过程,以沙漠地区地下空洞掩体的机载探地雷达探测为实例,分别模拟了不同天线极化方向、天线高度及地表粗糙度情况下的机载探地雷达剖面,分析了各因素对机载探地雷达探测地下空洞目标体的影响.天线极化方向与目标体走向垂直更有利于地下目标体探测;天线距离地表越近,可以获得更高分辨率的雷达剖面;沙漠地表起伏越大,雷达剖面中的散射杂波能量越强,浅部地下目标体信号容易被掩盖.为了消除起伏地形造成的散射杂波,提出用逆时偏移成像技术对共炮集机载探地雷达数据进行偏移成像,成像结果优于基尔霍夫偏移成像结果. 相似文献
10.
Junliang Gao Xiaojun Zhou Li Zhou Jun Zang Qiang Chen Haoyu Ding 《Ocean Dynamics》2018,68(12):1663-1681
Oscillations within an enclosed rectangular harbor and a set of partially opened rectangular harbors with various widths and locations of entrance induced by cubic water surface disturbances with different initial heights and locations are simulated using FUNWAVE 2.0 model. The height and the location of the cubic water surface disturbance refer to its thickness and its relative horizontal position inside the harbor, respectively. The water depth inside and outside all harbors is set to be constant. The aim of this paper is to investigate how different heights and locations of the water surface disturbance and various widths and locations of harbor entrance affect the oscillations inside the harbor. Results show that for the given harbors and the range of the initial height of water surface disturbance studied in this paper, all the response amplitudes of various eigenfrequencies increase linearly with the initial height of water surface disturbance. The variations of the initial location of water surface disturbance along the backwall and sidewall of the harbor can significantly change the transverse and longitudinal oscillation patterns of various modes, respectively. The effects of the variations of the width and location of the harbor entrance on the response amplitudes of various resonant modes both depend on the relative positions of their node lines and antinode lines to the harbor entrance. 相似文献
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At Delft Geotechnics the technique of ground-penetrating radar is in use for the detection of buried objects such as pipes. In order to give our ‘measurements in the field’ a more quantitative interpretation, a series of experiments has been started under well-defined conditions. A cylindrical vessel contains water, simulating wet soil. A pulsed transmitting antenna (TA) is mounted above the water-surface irradiating horizontal underwater pipes. The reflected pulses are detected by a horizontal receiving dipole (AP, i.e. ‘air-probe’) in the air. The reflecting objects used in the experiments are steel pipes, PVC rods and hollow PVC pipes filled with air or water. The depth of the pipes varies from 0.25 to 2 m. The strength of the reflected pulse depends on the type of pipe, its diameter, its depth, the electromagnetic properties of the water and also on the strength and polarization of the incident E-field. The latter is (mainly) parallel to the axis of the pipe in the present experiments. The experimental results have been compared with calculated results using the theory of a continuous plane wave, incident on an infinitely long cylinder in a homogeneous dielectric medium with the same dielectric constant as water. In a previous paper an experiment was described in which a movable receiving dipole in the water measured the transverse, mutually orthogonal Eø- and Eθ-components of the pulses emitted by the TA. The amplitudes of Eθ versus depth, measured in that paper, are used in the calculations as amplitudes of the incident field. The attenuation of the scattered field is accounted for by assuming exponential damping. The relative amplitudes of the scattered field for different pipes lying at the same depth are reproduced very well by this simple theory. Also the relative values of the scattered amplitudes for a given pipe lying at different depths are described neatly, provided the refraction of the scattered field at the water-air interface is accounted for. 相似文献
13.
为使物理模拟实验效果与实际探测情形更为接近,在室外自然条件下建立中尺度土壤石油污染实验模型,油污染区扩展深度超过1 m,采用实地探测中常用的500 MHz雷达天线进行长期定时探测.通过实测雷达图像特征、土壤含水量含油量分析,并对比前人开展的小尺度室内模拟试验结果,综合评价探地雷达对油污染区的探测效果.研究表明探地雷达探测图像异常特征与污染区扩散阶段密切相关:包气带内油污染区会引起振幅增强;毛细带的油污染区则表现为水位面反射轴附近清晰可辨的高幅异常区,且水位面反射轴呈下凹状;随扩散过程持续进行,异常区下移与水位面反射轴相交,并产生水平扩张.当污染土含油饱和度大于20%时,可通过雷达图像异常区圈定污染范围;当污染土含油饱和度大于15%时,可通过频谱图出现低频响应的位置圈定污染区水平范围.中尺度实验结果与室内小尺度模拟结果具有一致性,可作为油污染区雷达图像异常的解译依据. 相似文献
14.
At Delft Geotechnics the technique of ground-penetrating radar is in use for the detection of buried objects such as pipes. To enable us to give our ‘measurements in the field’ a more quantitative interpretation than can be deduced from these alone, a series of experiments has been started under well-defined conditions. A cylindrical vessel containing water simulates wet soil. Mounted horizontally above the water surface is a pulsed triangular half-wave dipole which is used as a transmitting antenna (TA). It has a carrier-frequency of about 160 MHz and a pulse repetition-frequency of about 50 kHz. A movable receiving dipole (‘probe’) in the water measures the transverse, mutually orthogonal Eφ,- and Eθ-components of the pulses as a function of probe-position (r, θ, φ) and of the height h of the TA above the water surface. When these pulses are Fourier-transformed, the transverse electric fields Eφ and Eθ at 200 MHz are obtained. The resulting field patterns are compared with computational results on the basis of the theory of the continuous wave, infinitesimal electric dipole (‘point dipole’). It can be concluded that:
- 1 Far-field conditions have not fully developed at a depth of about 2.50 m, the largest value of the radius r at which field patterns were measured, although it represents a distance of about 15 wavelengths.
- 2 The attenuation constant of the tapwater used, as deduced from E-field measurements for θ= 0, 2.50 m < r < 2.75 m, is slightly less than the value measured using a network analyser and air line combination, in agreement with (1).
- 3 E φ field patterns calculated using the value of the conductivity σ corresponding to the former value of the attenuation constant agree reasonably well with the measured patterns for r≤ 2.50 m and for θ < 20° at all antenna heights considered. Calculated Eφ patterns do not agree so well with the measured patterns when h is close to zero. With increasing height the agreement inproves.
- 4 In accordance with the theory of the point-dipole, the angular distribution of the radiation patterns of the TA becomes wider as the frequency decreases.
- 5 The normalized underwater pulse-spectra shift to lower frequencies with increasing r. This can be explained since the attenuation constant of the water rises with rising frequency.
15.
Fernando I. Rial Manuel Pereira Henrique Lorenzo Pedro Arias Alexandre Novo 《Journal of Applied Geophysics》2009,67(4):367
Since target reflections directly depend on the emitted pulse characteristics, a key factor for carrying out a successful GPR survey is to know as much as possible about the transmission features of the antennas used. This information is very important in order to choose the right antennas and set the appropriate configuration parameters for a specific survey. With this in mind this paper deals with the development of a set of laboratory experiments on the resolution capabilities of three bowtie antennas at frequencies of 500, 800 and 1000 MHz. Results from these measurements give a first estimation of the resolution of the antennas under test, showing the advantage of performing experiments rather than relying only on theoretical assumptions. The results are also expressed in terms of the central wavelength for each antenna and compared with some theoretical estimations proposed in the specialized bibliography. 相似文献
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均匀弹性半空间表面或内部震源产生的地震波场的解析解属于Lamb问题,采用Cagniard-deHoop方法,对与水平面呈任意夹角的表面线源,求解了其作用于弹性半空间时的拉普拉斯-傅里叶双积分变换解.以δ-脉冲函数为例,给出了任意方向作用力下波场的构成,并定量解出了P波、S波、首波和Rayleigh波等各波的位移表达式,分析了不同作用方向下各波位移的相对大小.建立数值模型并进行数值模拟,模拟结果验证了理论研究的正确性.研究成果为近地表地震波场的研究提供了理论依据. 相似文献
17.
The identification of buried cables, pipes, conduits, and other cylindrical utilities is a very important task in civil engineering. In the last years, several methods have been proposed in the literature for tackling this problem. Most commonly employed approaches are based on the use of Ground Penetrating Radars, i.e., they extract the needed information about the unknown scenario starting from the electromagnetic field collected by a set of antennas. In the present paper, a statistical method, based on the use of smart antenna techniques, is used for the localization of a single buried object. In particular, two efficient algorithms for the estimation of the directions of arrival of the electromagnetic waves scattered by the targets, namely the MUltiple SIgnal Classification and the Support Vector Regression, are considered and their performances are compared. 相似文献
18.
分析了现有闪电电场变化测量仪存在的不足之处,提出了采用一台测量仪同时获得闪电电场快、慢变化特性的技术方案,通过对测量仪天线、电路、传输方式的优化,大大降低了系统的复杂程度,提高了可靠性;采用全介质光缆传输信号,可避免在信号传输过程中因电磁耦合带来的干扰;采用系统函数逆变换反演被测电场时域特性,既可省去一套专测电场慢变化的仪器,且弥补了以往"电场慢变化测量仪"性能的不足.测量结果表明,快变化测量系统的测量带宽为80 Hz~87.5 MHz.采用研制成的电场变化测量仪对自然闪电进行多次测量后,将反演运算得到的波形与实测波形对比,验证了所提方案的可行性和技术上的先进性. 相似文献
19.
In the framework of the program for setting the Callisto spectrometer network into operation, the spectral measurements were
carried out at the sites of spectrometer locations in India and Russia in winter 2006. The results achieved at Badary, the
site where the Siberian Solar Radio Telescope (SSRT) is located, are presented. The measurements were performed using a broadband
log-periodic antenna connected to the Callisto spectrometer developed at the Institute of Astronomy (Zurich). The results
of these measurements should explain whether spectral studies at frequencies below 1 GHz can be performed using such antennas
or new antennas should be developed. The presented results are compared with the similar results obtained in Switzerland in
the frequency intervals of interest for radio astronomy. Concerning electromagnetic noise, Badary is a better site for observing
the Sun in the 50–800 MHz frequency range as compared to observatories in Switzerland. 相似文献
20.
The seismic wave field is considerably influenced by local structures close to the source and to the receiver. This applies to sources and receivers situated close to localized inhomogeneities, to structural interfaces, to the earth's surface, etc. In this paper we concentrate our attention mainly to the ray-theoretical radiation patterns of point sources situated close to the structural interfaces and to the earth's surface. In numerical modeling of high-frequency seismic wave fields by the ray method, the interaction of the source with the earth's surface has not usually been taken into account.The proposed procedure of the computation of the radiation patterns of point sources situated directly on structural interfaces and on the earth's surface is based on the zero-order approximation of the ray method, assuming that the length of the ray between the source and the receiver is long. The derived equations are extended to point sources located close to structural interface, to the earth's surface and to thin transition layers using the hybrid ray-reflectivity method, seeervený (1989). The thin layer need not be homogeneous; it may include an arbitrary inner layering (transition layers, laminas, etc.) The only requirement is for the layer to be thin. Roughly speaking, we require its thickness to be less than one quarter of the prevailing wavelength. The hybrid ray-reflectivity method describes well even certain non-ray effects (tunneling.S
* waves, etc.). Explicit analytical expressions for radiation patterns for all above listed point sources are found. These expression have a local character and may be easily implemented into computer codes designed for the routine computation of ray amplitudes and synthetic ray seismograms in 2-D and 3-D, laterally varying isotropic layered and block structures by the ray method.Numerical examples of radiation patterns ofP andS waves of point sources situated close to the earth's surface and to a thin low-velocity surface layer are presented and discussed. The explosive point source (center of dilatation) and the vertical and horizontal single force point sources are considered. It has been ascertained that the radiation patterns of point sources depend drastically on the depth of the source below the surface even if the depths vary within one quarter of the prevailing wavelength. 相似文献