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.     

The origins of Antarctic rock glaciers: periglacial or glacial features?

In extensively glaciarized permafrost areas such as Northern Victoria Land, rock glaciers are quite common and are considered postglacial cryotic landforms. This paper reveals that two rock glaciers in Northern Victoria Land (at Adélie Cove and Strandline) that are located close to the Italian Antarctic Station (Mario Zucchelli Station) should have the same origin, although they were previously mapped as Holocene periglacial landforms and subsequently considered ice‐cored and ice‐cemented rock glaciers, respectively. In fact, by integrating different geophysical investigations and borehole stratigraphy, we show that both landforms have similar internal structures and cores of buried glacier ice. Therefore, this kind of rock glacier is possibly related to the long‐term creep of buried ice rather than to permafrost creep alone. This interpretation can be extended to the larger part of the features mapped as rock glaciers in Antarctica. In addition, a high‐reflective horizon sub‐parallel to the topographic surface was detected in Ground Probing Radar (GPR) data over a large part of the study area. Combining all the available information, we conclude that it cannot be straightforwardly interpreted as the base of the active layer but rather represents the top of a cryo‐lithological unit characterized by ice lenses within sediments that could be interpreted as the transition zone between the active layer and the long‐term permafrost table. More generally, knowledge of the subsurface ice content and, in particular, the occurrence of massive ice and its depth is crucial to make realistic and affordable forecasts regarding thermokarst development and related feedbacks involving GHG emissions, especially in the case of cryosoils rich in carbon content. Copyright © 2017 John Wiley & Sons, Ltd.     

Resolution of GPR bowtie antennas: An experimental approach

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.     

Cylindrical-wave approach for the electromagnetic scattering problem by buried two-dimensional objects

A spectral-domain method, for the solution of the two-dimensional electromagnetic plane-wave scattering by a finite set of perfectly-conducting or dielectric cylinders buried in a dielectric half-space, has been developed. The scattered field is represented in terms of a superposition of cylindrical waves, and use is made of the plane-wave spectrum to take into account the reflection and transmission of such waves by the interface. The problem is solved for both the near- and the far-field regions, for TM and TE polarizations. In this work we briefly resume the theoretical basis of our approach. For configurations in which more obstacles are buried in the ground, and they are near to one another, we give details about the convergence rate of our method, and about the properties of our algorithms for the integration of cylindrical functions. With our technique it is possible to simulate two-dimensional buried obstacles of general shape, by means of a suitable set of circular-section cylinders: in this paper we show preliminary results of simulations carried out using arrays of same-radius circular cylinders, and of different-radius circular cylinders.     

Implementation of ADI-FDTD subgrids in ground penetrating radar FDTD models

Realistic numerical modeling of ground penetrating radar (GPR) using the finite-difference time-domain (FDTD) method could greatly benefit from the implementation of subgrids – supporting finer spatial resolution – into the conventional FDTD mesh. This is particularly important, when parts of the computational domain need to be modeled in detail or when there are features or regions in the overall computational mesh with values of high dielectric constant supporting propagation of waves at very short wavelengths. A scheme that simplifies the process of implementing these subgrids into the traditional FDTD method is presented. This scheme is based on the combination of the standard FDTD method and the unconditionally stable alternating-direction implicit (ADI) FDTD technique. Because ADI-FDTD is unconditionally stable its time-step can be set to any value that facilitates the accurate calculation of the fields. By doing so, the two grids can efficiently communicate information across their boundary without requiring to use a costly time-interpolation scheme. This paper discusses the performance of ADI-FDTD subgrids when implemented into the traditional FDTD method, using different communication schemes for the information exchange at the boundary of the two grids. The developed algorithm, can handle cases where the subgrid crosses dielectrically inhomogeneous media. In addition, results from the comparison between the proposed scheme and a commonly employed purely FDTD subgridding technique are presented.     

自适应Chirplet变换及其在探地雷达目标探测中的应用(英文)

探地雷达不仅能够探测金属目标体,而且能够探测非金属目标体,而成为UX0和地雷探测的一种重要的浅部地球物理方法。但是在地雷和UX0探测中,目标体埋藏深度浅,在探地雷达数据信噪比较低情况下,地表和土壤层的反射严重干扰对目标体的拾取。本文采用自适用Chirplet变换来消除地表层和土壤层变化的干扰,并在Radon—Wigner分布的基础上,采用自适用Chirplet变换来拾取目标体的信号。通过对实际探测实验数据应用证明,本方法处理结果比传统的偏移方法具有较高的信噪比,并能清晰地提取目标体信号。     

Railway track inspection using GPR

Swiss Federal Railways (SBB) inspect their railway tracks at regular intervals. The first step of track renewal planning is a geotechnical study. Inspection is focused on the thickness of the ballast, on subsoil material penetrating upwards into the ballast and on geotechnical properties of subgrade and subsoil materials. Up to now, the inspection has been done mainly by digging trenches at evenly spaced intervals and in locations of special interest. In order to evaluate the benefits and limits of GPR railway track inspections, three GPR surveys were carried out on three different railway lines. Data were acquired using a mobile system travelling at 10 km/h. Subsequent to radar data acquisition, trenches were dug. The positioning of some of the trench locations was based on preliminary GPR results in order to support the interpretation of GPR data. Only those trenches were available during interpretation of radar data. In addition, SBB performed their usual investigation programme. This provided an opportunity for checking the radar results in great detail.     

地质雷达在乌拉山山前断裂探测中的应用

地质雷达是利用电磁波对地下不同电性介质进行探测的地球物理仪器，其探测速率快、分辨率高，可弥补探槽和其他地球物理方法存在探测盲区的缺陷，正在越来越多地应用于活动断层探测领域。本文以乌拉山山前断裂为例开展地质雷达探测工作，使用无人机正射影像技术对测线进行地形校正，获得断层浅部地质雷达图像。研究结果表明，本文研究方法能有效反映探槽揭露的地层单元和断层分布。本次探测中，雷达波形图像特征为:浅地表的土壤层反射波总体较弱；粗粒沉积为主的砾石层反射波总体较强，同相轴连续性好；细粒沉积为主的砂层反射波弱于砾石层，波形以中、高频为主，同相轴具有弱连续性；对于洪冲积地区，地质雷达能分辨具有一定特征的地层单元，这为剖面图像的断层识别提供了标志；通过无人机正射影像技术对地质雷达测线进行地形校正，有利于获得更为准确的探测结果。     

铁路路基雷达探测工作参数的设计

介绍了使用路检车装载探地雷达(Ground Penetrating Radar)对铁路路基检测的原理,结合野外探测过程中对测量深度、精度和行车速度的要求,给出了GPR现场工作测线布置的原则,理论上给出天线中心频率、时窗、采样率、测量点距的计算公式.讨论了连续测量和离散测量的关系,考虑到连续测量时水平精度的要求,给出行车速度必须满足的两个条件,从而确定行车速度的上限.为保证野外检测工作的顺利进行,结合实际情况给出了GPR工作参数的设计原则和流程.