ポーラリメトリック・ボアホールレーダによる地下き裂の性状評価と水理性推定への応用

Tohoku University佐藤源之課

Radar Polarimetry Analysis Applied to Single-Hole Fully Polarimetric Borehole Radar

科研費報告書収録論文(課題番号:14102024/研究代表者:佐藤源之/ポーラリメトリック・インターフェロメトリックレーダによる地雷検知に関する研究

Evaluating GPR polarization effects for imaging fracture channeling and estimating fracture properties

This study investigates the polarization properties of GPR signals for imaging flow channeling in a discrete fracture. In particular this study examines if cross-polarized components could be used to image channels in a horizontal fracture. To understand how the polarization of radar waves affects imaging of channelized flow in a horizontal fracture, i) a series of numerical forward models was created with varying fracture aperture, channel orientation, and varying fracture water electrical conductivity, and ii) mulitpolarization field data were used to monitor dipole flow saline tracer tests in a subhorizontal fracture. Numerical modeling demonstrated that the cross-polarized data held useful information about channels but only when the channel is oriented oblique to the E-W wavefield orientation. When the channel is oriented oblique to survey line, summation of the cross-polarized and co-polarized components results in an accurate representation of the total scattered energy from the channel. When the channel is oriented parallel or orthogonal to survey line summation the co-polarized components represent the total scattered energy. In addition to numerical modeling multipolarization, time lapse GPR field data was acquired at the Altona Flat Rock test site in New York State. These surveys were conducted under varying artificial hydraulic gradients, to investigate channeled transport of different concentrations of saline tracer through the fracture and to highlight flow channels between wells. Amplitude analysis of the cross-polarized components reveals flow channeling in an E-W orientation which suggests good well connectivity in that direction. N-S amplitude trends suggest poor hydraulic connectivity. In conclusion, this investigation reveals that cross-polarized components of GPR signals contain useful information for imaging channeled flow in fractured media

Quantifying the Relationship Among Ground Penetrating Radar Reflection Amplitudes, Horizontal Sub-Wavelength Bedrock Fracture Geometries, and Fluid Conductivities

Accurate characterization of subsurface fractures is indispensible for contaminant transport and fresh water resource modeling because discharge is cubically related to the fracture aperture; thus, minor errors in aperture estimates may yield major errors in a modeled hydrologic response. Ground penetrating radar (GPR) has been successfully used to noninvasively estimate fracture aperture for sub-horizontal fractures at outcrop scale, but limits on vertical and horizontal resolution are a concern. Theoretical formulations and field tests have demonstrated increased GPR amplitude response with the addition of a saline tracer in a sub-millimeter fracture; however, robust verification of existing theoretical equations without an accurate measure of aperture variation across a fracture surface is difficult. The work presented here is directed at better verification of theoretical predictions of GPR amplitude and phase response. For sub-vertical resolution features, the response of a 1000 MHz PulseEKKO Pro transducer to a fluid-filled bedrock fracture analog composed of two plastic (UHMW-PE) blocks was measured, where fracture aperture ranged from 0-40 ± 0.3 mm and fluid conductivity from 0-5700 ± 5 mS/m. The GPR profiles were acquired down the centerline of the block, horizontally stacked to reduce errors, normalized to the control response at zero aperture, used to calculate reflection coefficient by dividing by the magnitude of the direct wave, and used to calculate the instantaneous phase. For sub-horizontal resolution features, lateral fracture extent ranged from 0-20 cm and fluid conductivity from 20-5700 ± 5 mS/m. GPR profiles were acquired parallel and perpendicular to the fracture. Comparison of the measured GPR response to analytical and numerical modeling suggests that numerical modeling best predicts both amplitude and phase variations due to changes in fracture aperture and conductivity. The Widess equation combined with an empirically derived scaling factor also predicts GPR amplitude response but not phase. Future applications to inversions of field data to map subsurface fracture networks will rely on easily invertible models, and numerical modeling using GPRMax2D can help develop a theoretical model for computationally effective and accurate inversion

Radar Sub-surface Sensing for Mapping the Extent of Hydraulic Fractures and for Monitoring Lake Ice and Design of Some Novel Antennas.

Hydraulic fracturing, which is a fast-developing well-stimulation technique, has greatly expanded oil and natural gas production in the United States. As the use of hydraulic fracturing has grown, concerns about its environmental impacts have also increased. A sub-surface imaging radar that can detect the extent of hydraulic fractures is highly demanded, but existing radar designs cannot meet the requirement of penetration range on the order of kilometers due to the exorbitant propagation loss in the ground. In the thesis, a medium frequency (MF) band sub-surface radar sensing system is proposed to extend the detectable range to kilometers in rock layers. Algorithms for cross-hole and single-hole configurations are developed based on simulations using point targets and realistic fractured rock models. A super-miniaturized borehole antenna and its feeding network are also designed for this radar system. Also application of imaging radars for sub-surface sensing frozen lakes at Arctic regions is investigated. The scattering mechanism is the key point to understand the radar data and to extract useful information. To explore this topic, a full-wave simulation model to analyze lake ice scattering phenomenology that includes columnar air bubbles is presented. Based on this model, the scattering mechanism from the rough ice/water interface and columnar air bubbles in the ice at C band is addressed and concludes that the roughness at the interface between ice and water is the dominate contributor to backscatter and once the lake is completely frozen the backscatter diminishes significantly. Radar remote sensing systems often require high-performance antennas with special specifications. Besides the borehole antenna for MF band subsurface imaging system, several other antennas are also designed for potential radar systems. Surface-to-borehole setup is an alternative configuration for subsurface imaging system, which requires a miniaturized planar antenna placed on the surface. Such antenna is developed with using artificial electromagnetic materials for size reduction. Furthermore, circularly polarized (CP) waveform can be used for imaging system and omnidirectional CP antenna is needed. Thus, a low-profile planar azimuthal omnidirectional CP antenna with gain of 1dB and bandwidth of 40MHz is designed at 2.4GHz by combining a novel slot antenna and a PIFA antenna.PhDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/120674/1/wujf_1.pd

レーダを利用した空隙を持つ物質中の液体移動モニタリング

Tohoku University佐藤源

Parametric Inversion Technique for Location of Cylindrical Structures by Cross-Hole Measurements

科研費報告書収録論文(課題番号:14102024/研究代表者:佐藤源之/ポーラリメトリック・インターフェロメトリックレーダによる地雷検知に関する研究

The WISDOM Radar: Unveiling the Subsurface Beneath the ExoMars Rover and Identifying the Best Locations for Drilling

The search for evidence of past or present life on Mars is the principal objective of the 2020 ESA-Roscosmos ExoMars Rover mission. If such evidence is to be found anywhere, it will most likely be in the subsurface, where organic molecules are shielded from the destructive effects of ionizing radiation and atmospheric oxidants. For this reason, the ExoMars Rover mission has been optimized to investigate the subsurface to identify, understand, and sample those locations where conditions for the preservation of evidence of past life are most likely to be found. The Water Ice Subsurface Deposit Observation on Mars (WISDOM) ground-penetrating radar has been designed to provide information about the nature of the shallow subsurface over depth ranging from 3 to 10 m (with a vertical resolution of up to 3 cm), depending on the dielectric properties of the regolith. This depth range is critical to understanding the geologic evolution stratigraphy and distribution and state of subsurface H2O, which provide important clues in the search for life and the identification of optimal drilling sites for investigation and sampling by the Rover's 2-m drill. WISDOM will help ensure the safety and success of drilling operations by identification of potential hazards that might interfere with retrieval of subsurface samples

An Effective Method for Borehole Imaging of Buried Tunnels

Detection and imaging of buried tunnels is a challenging problem which is relevant to both geophysical surveys and security monitoring. To comply with the need of exploring large portions of the underground, electromagnetic measurements carried out under a borehole configuration are usually exploited. Since this requires to drill holes in the soil wherein the transmitting and receiving antennas have to be positioned, low complexity of the involved apparatus is important. On the other hand, to effectively image the surveyed area, there is the need for adopting efficient and reliable imaging methods. To address these issues, in this paper we investigate the feasibility of the linear sampling method (LSM), as this inverse scattering method is capable to provide almost real-time results even when 3D images of very large domains are built, while not requiring approximations of the underlying physics. In particular, the results of the reported numerical analysis show that the LSM is capable of performing the required imaging task while using a quite simple measurement configuration consisting of two boreholes and a few number of multiview-multistatic acquisitions

Which fractures are imaged with Ground Penetrating Radar? Results from an experiment in the Äspö Hardrock Laboratory, Sweden

Identifying fractures in the subsurface is crucial for many geomechanical and hydrogeological applications. Here, we assess the ability of the Ground Penetrating Radar (GPR) method to image open fractures with sub-mm apertures in the context of future deep disposal of radioactive waste. GPR experiments were conducted in a tunnel located 410 m below sea level within the Äspö Hard Rock Laboratory (Sweden) using 3-D surface-based acquisitions (3.4 m × 19 m) with 160 MHz, 450 MHz and 750 MHz antennas. The nature of 17 identified GPR reflections was analyzed by means of three new boreholes (BH1-BH3; 9–9.5 m deep). Out of 21 injection and outflow tests in packed-off 1-m sections, only five provided responses above the detection threshold with the maximum transmissivity reaching 7.0 × 10−10 m2/s. Most GPR reflections are situated in these permeable regions and their characteristics agree well with core and Optical Televiewer data. A 3-D statistical fracture model deduced from fracture traces on neighboring tunnel walls show that the GPR data mainly identify fractures with dips between 0 and 25°. Since the GPR data are mostly sensitive to open fractures, we deduce that the surface GPR method can identify 80% of open sub-horizontal fractures. We also find that the scaling of GPR fractures in the range of 1–10 m2 agrees well with the statistical model distribution indicating that fracture lengths are preserved by the GPR imaging (no measurement bias). Our results suggests that surface-GPR carries the resolution needed to identify the most permeable sub-horizontal fractures even in very low-permeability formations, thereby, suggesting that surface-GPR could play an important role in geotechnical workflows, for instance, for industrial-scale siting of waste canisters below tunnel floors in nuclear waste repositories