Influence of dispersive soil electromagnetic properties on hand‐held time domain electromagnetic sensors

Although metal detectors remain the workhorses of humanitarian demining, it is well established that the performance of both continuous wave (frequency domain) and pulsed induction (time domain) detectors can be severely compromised by so‐called ‘soil‐effects’. Generally, problem soils reduce the signal‐to‐noise ratio and increase the false‐detection rate. In certain locations, the soil‐effect is so severe as to render the detector practically inoperable. The current study is part of an ongoing international effort to establish and quantify the influence of soil electromagnetic properties on the operation of metal detectors and related sensor technologies. In particular, we examine the relative influence of soil electrical conductivity, magnetic susceptibility and associated frequency dependence on the time domain electromagnetic (TDEM) response of pulsed induction metal detectors and related small‐scale TDEM sensors.     

Acoustic properties of sediments saturated with gas hydrate, free gas and water

We obtain the wave velocities and quality factors of gas‐hydrate‐bearing sediments as a function of pore pressure, temperature, frequency and partial saturation. The model is based on a Biot‐type three‐phase theory that considers the existence of two solids (grains and gas hydrate) and a fluid mixture. Attenuation is described with the constant‐Q model and viscodynamic functions to model the high‐frequency behaviour. We apply a uniform gas/water mixing law that satisfies Wood's and Voigt's averages at low and high frequencies, respectively. The acoustic model is calibrated to agree with the patchy‐saturation theory at high frequencies (White's model). Pressure effects are accounted by using an effective stress law for the dry‐rock moduli and permeabilities. The dry‐rock moduli of the sediment are calibrated with data from the Cascadia margin. Moreover, we calculate the depth of the bottom simulating reflector (BSR) below the sea floor as a function of sea‐floor depth, geothermal gradient below the sea floor, and temperature at the sea floor.     

地震前兆电磁波监测仪的研制

为了提高地方地震队伍监测能力,需要有一些操作简单、容易安装、性能好、工作稳定、不须人工经常管理而又能记录到有价值的地震前兆信息的监测仪器。最近研制出的数字记录的超低频电磁波监测仪可自动采集数据,数据可以在仪器内存贮长达一个月,平时不须管理,随时可以通过手工查阅、连接电脑或远程遥控等３种方式获取数据,适合地方地震监测点使用。介绍了仪器研制的科学依据、研制意义和仪器的工作原理,详细了仪器实现自动化、数     

声学探测技术在天津隐伏断层探测中的应用

利用单道地震等水上声学探测技术,对天津海河断裂的浅部构造和地层错断信息进行了探测。探测结果表明,在水深较深,相对宽阔的河道内该方法可以获得较好的结果。在天津塘沽地区,海河断裂的上断点深度位于河底面以下30m左右,对应的最新活动时代为Q_p~3—Q_h~1,这与前期的钻孔勘探结果基本吻合。在渤海近海海域,海河断裂呈一组NWW-NEE向分布的断裂带,其上断点距离海底的深度小于30m,断层断错和影响的地层年代为全新世早中期,断层活动特征与塘沽地区的基本一致。     

煤样变形破坏声电效应的演化规律及机理研究

通过实验研究不同煤样变形破坏全过程的声发射和电磁辐射效应的演化规律,分析了煤样变形破坏过程中电磁辐射和声发射信号的相关性和煤样变形破坏的微观机理,提出了煤样变形破坏过程中电磁辐射和声发射信号分别是在煤样内的裂纹面滑移和裂纹扩展时产生的观点,较好地解释了煤样变形破坏过程中声电效应的变化规律和特征,为进一步提高电磁辐射及声发射、电磁辐射综合预测煤岩动力灾害的可靠性和准确性提供了理论基础.     

Soil properties and performance of landmine detection by metal detector and ground-penetrating radar — Soil characterisation and its verification by a field test

Metal detectors have commonly been used for landmine detection, and ground-penetrating radar (GPR) is about to be deployed for this purpose. These devices are influenced by the magnetic and electric properties of soil, since both employ electromagnetic techniques. Various soil properties and their spatial distributions were measured and determined with geophysical methods in four soil types where a test of metal detectors and GPR systems took place. By analysing the soil properties, these four soils were classified based on the expected influence of each detection technique and predicted soil difficulty. This classification was compared to the detection performance of the detectors and a clear correlation between the predicted soil difficulty and performance was observed. The detection performance of the metal detector and target identification performance of the GPR systems degraded in soils that were expected to be problematic. Therefore, this study demonstrated that the metal detector and GPR performance for landmine detection can be assessed qualitatively by geophysical analyses.     

Application of Acoustic Technique to Surveying a Buried Fault in Tianjin

We carried out surveying on the shallow structure and faulted-stratum of the Haihe fault in Tianjin using acoustic surveying methods such as the single-channel seismic technique. The result shows that the method can obtain satisfactory results in wide and deep river courses. It also shows that in the Tanggu area of Tianjin, the upper fault point of Haihe fault is about 30m beneath the river bed and the corresponding latest active period is Qp^3- Qh^1 , which is consistent with the former borehole survey result. In the offshore area of the Bohai Sea, Haihe fault shows as a NWW-NEE strike dense fault zone and its upper fault point is less than 30m beneath the seabed. It shows that the active characteristics of Haihe fault in the Bohai Sea correspond to the Tanggu area.     

Acoustic wave propagation in saturated porous media: reformulation of the Biot/Squirt flow theory

A model of wave propagation in fluid-saturated porous media is developed where the principal fluid/solid interaction mode affecting the propagation of the acoustic wave results from the conjunction of the Biot and the Squirt flow mechanism. The difference between the original Biot/Squirt (BISQ) flow theory and the new theory, which we call the reformulated BISQ, is that the average fluid pressure term appearing in the dynamic equation for a two component solid/fluid continuum is independent of squirt flow length. P-velocity and attenuation relate to measurable rock physical parameters: the Biot's poroelastic constants, porosity, permeability, pore fluid compressibility and viscosity. Modelling shows that velocity and attenuation dispersion obtained using the reformulated BISQ theory are of the same order of magnitude as those obtained using the original BISQ theory. Investigation on permeability effect on velocity and attenuation dispersion indicate that the transition zone in velocity and attenuation peak, occurring both at the relaxation frequency, shifts toward high frequency when permeability decreases. This behaviour agrees with Biot's theory prediction.     

声波方法在城市活断层探查与评价中的应用--人工地震、雷达和声波方法的比较与组合

断层上断点深度是确定平原隐伏断层活动年代的关键数据。对同一断层用不同物探方法获得的数据相差可达几十到几百米，这一差值可能导致对断层活动性的判定结论完全相反，由于缺乏宽频带、中心频率高的人工震源，只能采用多种方法联合探测。人工地震波、电磁波和声波的频率越高，分辨率越高，但衰减快，可探测深度也降低。相反，频率低，分辨率下降，但探测深度增大。人工地震波频率为数十赫兹，探测深度大，分辨能力低。雷达电磁波为10^6Hz量级，分辨能力强，但探测深度很小。声波为数千赫兹，分辨能力优于人工地震，探测深度大于雷达，性状介于二者之间，适合厚层第四纪沉积层的超浅层探查。声波高分辨率超浅层探测的初步结果表明．以往华北平原东部某些断层活动性的论断可能要做很大的修正。     

基于弯曲元技术的含水合物松散沉积物声学特性研究

含水合物松散沉积物的声学特性对海上天然气水合物地球物理勘探和资源评价具有重要意义.研制了适用于高压条件下含水合物沉积物声学特性探测的纵横波一体化新型弯曲元换能器,提出利用频谱分析(FFT)和小波分析(WT)相结合的方法获取纵横波速度,并进行了多个轮次的水合物声学特性模拟实验研究.结果表明,新型弯曲元技术可以灵敏探测松散沉积物中水合物的生成和分解,随着水合物饱和度(Sh)的增大,纵横波速度呈规律性增长:当ShSh>60%时声速随着水合物饱和度增加又快速增长,表明水合物可能重新胶结沉积物颗粒生成.     

Mapping the spatial variation of soil water content at the field scale with different ground penetrating radar techniques

Two ground penetrating radar (GPR) techniques were used to estimate the shallow soil water content at the field scale. The first technique is based on the ground wave velocity measured with a bistatic impulse radar connected to 450 MHz ground-coupled antennas. The second technique is based on inverse modeling of an off-ground monostatic TEM horn antenna in the 0.8–1.6 GHz frequency range. Data were collected on a 8 by 9 m partially irrigated intensive research plot and along four 148.5 m transects. Time domain reflectometry, capacitance sensors, and volumetric soil samples were used as reference measurements. The aim of the study was to test the applicability of the ground wave method and the off-ground inverse modeling approach at the field scale for a soil with a silt loam texture. The results for the ground wave technique were difficult to interpret due to the strong attenuation of the GPR signal, which is related to the silt loam texture at the test site. The root mean square error of the ground wave technique was 0.076 m³ m⁻³ when compared to the TDR measurements and 0.102 m³ m⁻³ when compared with the volumetric soil samples. The off-ground monostatic GPR measured less within-field soil water content variability than the reference measurements, resulting in a root mean square error of 0.053 m³ m⁻³ when compared with the TDR measurements and an error of 0.051 m³ m⁻³ when compared with the volumetric soil samples. The variability between the two GPR measurements was even larger with a RSME of 0.115 m³ m⁻³. In summary, both GPR methods did not provide adequate spatial information on soil water content variation at the field scale. The main reason for the deviating results of the ground wave method was the poor data quality due to high silt and clay content at the test site. Additional reasons were shallow reflections and the dry upper soil layer that cannot be detected by the ground wave method. In the case of off-ground GPR, the high sensitivity to the dry surface layer is the most likely reason for the observed deviations. The off-ground GPR results might be improved by using a different antenna that allows data acquisition in a lower frequency range.     

Two-dimensional development of the dynamic coupled consolidation scaled boundary finite-element method for fully saturated soils

The scaled boundary finite-element method is a powerful tool used to analyse far-field boundary soil–structure interaction problems. In this paper, the method is extended to include Biot's coupled consolidation in order to deal with fully saturated soil as a two-phase medium. The advantages of this method are explained in this paper. The detailed formulation considers the general two-dimensional (2D) analysis case, accounting for body forces and surface tractions in both bounded and unbounded media.     

Field test of sub‐basalt hydrocarbon exploration with marine controlled source electromagnetic and magnetotelluric data

The recent use of marine electromagnetic technology for exploration geophysics has primarily focused on applying the controlled source electromagnetic method for hydrocarbon mapping. However, this technology also has potential for structural mapping applications, particularly when the relative higher frequency controlled source electromagnetic data are combined with the lower frequencies of naturally occurring magnetotelluric data. This paper reports on an extensive test using data from 84 marine controlled source electromagnetic and magnetotelluric stations for imaging volcanic sections and underlying sediments on a 128‐km‐long profile. The profile extends across the trough between the Faroe and Shetland Islands in the North Sea. Here, we focus on how 2.5D inversion can best recover the volcanic and sedimentary sections. A synthetic test carried out with 3D anisotropic model responses shows that vertically transverse isotropy 2.5D inversion using controlled source electromagnetic and magnetotelluric data provides the most accurate prediction of the resistivity in both volcanic and sedimentary sections. We find the 2.5D inversion works well despite moderate 3D structure in the synthetic model. Triaxial inversion using the combination of controlled source electromagnetic and magnetotelluric data provided a constant resistivity contour that most closely matched the true base of the volcanic flows. For the field survey data, triaxial inversion of controlled source electromagnetic and magnetotelluric data provides the best overall tie to well logs with vertically transverse isotropy inversion of controlled source electromagnetic and magnetotelluric data a close second. Vertical transverse isotropy inversion of controlled source electromagnetic and magnetotelluric data provided the best interpreted base of the volcanic horizon when compared with our best seismic interpretation. The structural boundaries estimated by the 20‐Ω·m contour of the vertical resistivity obtained by vertical transverse isotropy inversion of controlled source electromagnetic and magnetotelluric data gives a maximum geometric location error of 11% with a mean error of 1.2% compared with the interpreted base of the volcanic horizon. Both the model study and field data interpretation indicate that marine electromagnetic technology has the potential to discriminate between low‐resistivity prospective siliciclastic sediments and higher resistivity non‐prospective volcaniclastic sediments beneath the volcanic section.     

The use of gypsum spheres for identifying water flow routes in soils

Gypsum (plaster of paris) has been cast into spheres and placed in soils; weight loss has been used to identify relative water flow routes. Theoretical considerations and laboratory experimentation show that solutional weight loss of the material used increases with increasing water flow, but is independent of pH above pH 4. Results for gypsum sphere weight loss are presented for soils where moisture conditions have been measured independently using tensiometers. The data suggest that the weight loss method provides a viable time-integrated demonstration of relative water flow routes.     

Transport and fate of nonaqueous phase liquid (NAPL) in variably saturated porous media with evolving scales of heterogeneity

 A stochastic simulation is performed to study multiphase flow and contaminant transport in fractal porous media with evolving scales of heterogeneity. Numerical simulations of residual NAPL mass transfer and subsequent transport of dissolved and/or volatilized NAPL mass in variably saturated media are carried out in conjunction with Monte Carlo techniques. The impact of fractal dimension, plume scale and anisotropy (stratification) of fractal media on relative dispersivities is investigated and discussed. The results indicate the significance of evolving scale of porous media heterogeneity to the NAPL transport in the subsurface. In general, the fractal porous media enhance the dispersivities of NAPL mass plume transport in both the water phase and the gas phase while the influence on the water phase is more significant. The porous media with larger fractal dimension have larger relative dispersivities. The aqueous horizontal dispersivity exhibits a most significant increase against the plume scale.     

Seasonal changes in surface level of a salt marsh creek

Nineteen surveys were carried out over a two-year period to determine the surface height of a salt marsh creek located on the north side of the River Esk, Cumbria, England. An AGA 112 electromagnetic distance measurer mounted on a Wild TI theodolite was used in conjunction with acrylic reflectors to follow the form of the ground. The results showed that there were no significant net trends in surface level, although seasonal variations of the order of 2 cm occurred. Vegetated areas responded in similar fashion to bare surfaces. It is thought that the elevational changes were attributable to the swelling of clay particles during the winter months rather than the effects of erosion and accretion.     

Characterizing incipient motion of low fines content soils with varying compositions,water contents,and relative densities

Laboratory flume experiments were conducted to quantify the effects of the soil characteristics on the critical shear stress of low fines content soil samples collected from the Montauk shores in New York. The collected soils were reconstituted at five different fines contents, ranging between 0 and 20%. These soil mixtures were composed of two initial water contents, dry of optimum and optimum moistures, and two relative densities, one moderate dense and the other dense. The strength indices of...     

Sensitivity and inversion of marine electromagnetic data in a vertically anisotropic stratified earth

The controlled‐source electromagnetic (CSEM) and magnetotelluric method (MT) are two techniques that can be jointly used to explore the resistivity structure of the earth. Such methods have, in recent years, been applied in marine environments to the exploration and appraisal of hydrocarbons. In many situations the electric properties of the earth are anisotropic, with differences between resistivity in the vertical direction typically much higher than those in the horizontal direction. In cases such as this, the two modes of the time‐harmonic electromagnetic field are altered in different ways, implying that the sensitivity to the earth resistivity may vary significantly from one particular resistivity component (scalar, horizontal or vertical) to another, depending on the measurement configuration (range, azimuth, frequency or water depth). In this paper, we examine the sensitivity of the electromagnetic field to a vertically anisotropic earth for a typical set of configurations, compare inversion results of synthetic data characterizing a vertically anisotropic earth obtained using the isotropic and anisotropic assumptions and show that correctly accounting for anisotropy can prevent artefacts in inversion results.     

Inversion of inline and broadside marine controlled‐source electromagnetic data with constraints derived from seismic data

We present a structural smoothing regularization scheme in the context of inversion of marine controlled‐source electromagnetic data. The regularizing hypothesis is that the electrical parameters have a structure similar to that of the elastic parameters observed from seismic data. The regularization is split into three steps. First, we ensure that our inversion grid conforms with the geometry derived from seismic. Second, we use a seismic stratigraphic attribute to define a spatially varying regularization strength. Third, we use an indexing strategy on the inversion grid to define smoothing along the seismic geometry. Enforcing such regularization in the inversion will encourage an inversion result that is more intuitive for the interpreter to deal with. However, the interpreter should also be aware of the bias introduced by using seismic data for regularization. We illustrate the method using one synthetic example and one field data example. The results show how the regularization works and that it clearly enforces the structure derived from seismic data. From the field data example we find that the inversion result improves when the structural smoothing regularization is employed. Including the broadside data improves the inversion results even more, due to a better balancing between the sensitivities for the horizontal and vertical resistivities.     

The Application of the Acoustic Method to the Exploration of Urban Active Faults and a Comparison with the Artificial Earthquake, and Radar Methods

The depth of upper fault point is the key data for ascertaining the active age of a buried fault on a plain. The difference of depth obtained from same fault may be dozens to several hundred meters when using different geophysical methods. It can result in the absolutely opposite conclusions when judging fault activity. Because of a lack of an artificial earthquake source with wide band and high central-frequency, many kinds of methods have to be used together. The higher the frequency of the artificial earthquake wave, electromagnetic wave and sonic wave, the higher the resolution. However the attenuation is also very fast and the exploration depth is very shallow. The reverse is also true. The frequency of artificial seismic waves is in the tens of Hz. Its exploration depth is big and the resolution is poor. The frequency of radar electromagnetic waves is about a million Hz, indicating that the resolving power is better, but the exploration depth is very shallow. However, the acoustic frequency is thousands of Hz, its resolving power is better than that of the artificial earthquake method and the exploration depth is larger than that of the radar method. So it is suitable for extra-shallow exploration in the thick deposit strata of the Quaternary. The preliminary results detected using the high frequency acoustic method in extra-shallow layers indicates that previous inferences about some fault activity in the eastern part of the North China plain may need to be greatly corrected.