We propose an outer pipe to reduce a direct wave in a thin single-hole borehole radar probe in a thick water-filled borehole. The outer pipe replaces the medium, such as water inside the borehole, with low-permittivity materials, such as air and plastics. According to numerical calculations, the cylindrical water layer makes the direct wave from the transmitting loop antenna to the receiving one have significant power and narrow frequency bandwidth. This is caused by the low attenuation of the TE01 surface wave when there is a cylindrical water layer. The MoM analysis showed that wearing the outer pipe on the radar probe decreased the direct wave’s power more than the reflected wave from the subsurface objects, improving the detection of that reflected wave. We realized the radar system with the outer pipe by attaching the two acrylic pipes with different diameters. With this outer pie, we conducted field experiments to estimate the position of metal ore near the borehole in skarn with the loop antenna array type borehole radar. The direct wave having oscillation prevented the detection of the reflected wave from the sphalerite vein in the time domain without the outer pipe. However, attaching the outer pipe highlighted that reflected wave.

A horizontal loop array antenna (HLA) in a vertical borehole is utilized to estimate the 3-D position of a horizontal conducting cylinder. According to the computer simulation, the horizontal loop antenna can detect reflected waves from a thin conducting cylinder with a diameter of 1 cm in the soil. However, a vertical dipole antenna hardly does. Another computer simulation demonstrated that the singlehole borehole radar probe with the HLA could detect a reflected wave from a horizontal conducting cylinder just after a direct wave in the time domain and estimate the 3-D positions of the reflection point on the cylinder. The prototype borehole radar with the HLA was applied to estimate the 3-D positions of the horizontal cylinder. This radar measured a conducting cylinder in wet soil and estimated the 3-D reflection points on the conducting cylinder.

We present the experimental results of the fractures in the andesite rock in Ishikoshi town, Miyagi, Japan, measured by the polarization-sensitive directional borehole radar. This radar has a dipole array and loop antenna, which are sensitive to the wave polarization and direction of the arrival wave. In the test site, many fractures spatially separated in the rock, and the borehole radar detected one of the fractures. We found that this fracture's reflected wave does not have much cross-polarization component and estimated the reflection point position reasonably in the three-dimensional space. This result is similar to numerical simulation results, which simulated the fracture with a thin dielectric water slab in this simulation.

This paper shows that loading ferrite beads on feeding lines causes significant errors in direction estimation in a loop array antenna in a borehole radar. This contradicts the conventional wisdom that ferrite bead loading avoids undesirable scattering. The numerical simulation of a plane wave incidence on the loop array found that the ferrite loading worsened the DOA estimation by about 30°. We conducted a field experiment of cross-hole measurement in soil. The azimuth angle estimation error of the loop array without the ferrite loading was less than 25 °, while that with the ferrite loading ranged between 30° and 120°.

In this article, we claim that a vertical dipole array antenna in a vertical borehole may sense a horizontally polarized plane wave incoming from outside the borehole, which worsens the antenna’s directivity. This is caused by cylindrical inhomogeneity around the antenna. According to the problem formulation and numerical simulation, the spatial spectrum of the electromagnetic field from an incident plane wave depends on the wave polarization because of scattering excitation in a medium inside a borehole. Unique field experiments in which a dipole antenna is moved on a plane perpendicular to the axis of a borehole confirmed this phenomenon. The numerical simulation predicted that the estimation of an inclined conducting cylinder direction has an error due to the horizontally polarized wave’s scattering. Single-hole measurements were performed to estimate the direction of an inclined conducting cylinder in soil to a dipole array. We found that the estimated direction of the reflected wave had an error of several degrees in the azimuth angle, as numerically expected.

We present full polarimetric borehole radar and field experiment results with this radar. This radar consists of four dipole and two loop array antenna elements for each transmitter and receiver. We conducted field experiments to measure a conducting cylinder in soil with the radar. The experimental data showed the co-polarization components are larger than the cross-polarization ones in magnitudes of reflection from a vertical conducting cylinder. It also showed that the cross-polarization components in an inclined conducting cylinder reflection got larger than that in the vertical conducting cylinder. These results agree with the polarimetric property in conducting cylinder scattering. Also, we confirmed the experimental data was in line with the reciprocity theorem.

We investigate the influence of a horizontally polarized wave's oblique incidence on the direction of arrival (DOA) estimation with a dipole array antenna in a borehole. According to a computer simulation, the horizontally polarized wave component of the reflected wave from the conducting cylinder influences the wave's azimuth angle estimation. We conducted a field experiment in soil with a similar situation to the computer simulation. The experimental results confirmed the phenomena predicted by the computer simulation.

We propose a directive antenna for borehole radar, using a horizontally polarized wave in a vertical borehole. This antenna is an array of several horizontal loop antennas, arranged vertically. When a plane wave is incident on the antenna, the differences in arrival times at the array elements enables us to estimate the direction of arrival (DOA). We present a simple model representing the arrival time difference and propose an algorithm for DOA estimation. Further, we synthesized the array signal by analyzing the electromagnetic field using the method of moments (MoM) and applied the proposed algorithm to the signal. We found that mutual coupling between antenna elements affects DOA estimation, and that a space of about 3 cm between antenna elements prevents mutual coupling. Using the MoM analysis, we simulated a cross-hole measurement to demonstrate the antenna’s ability. We carried out field experiments in wet soil to examine the antenna’s ability. The proposed loop array antenna in a water-filled borehole received a direct wave from a source in another borehole. After applying the proposed algorithm to the measured data, we found that we could estimate the DOAs to the source with an error of less than 15° in the azimuth angle and 10° in the elevation angle.

Objectives There is no report on antibody titers after vaccination against SARS-CoV-2 in Japanese dialysis patients. As dialysis is different between Japan and other countries, changes in antibody titers were examined. Methods Baseline characteristics and anti-spike protein antibody titers (Roche) over 90 days after administration of the BNT162b2 messenger RNA vaccine were investigated in dialysis patients. Results The maximum anti-spike protein antibody titer after the second dose was 738 (327 to 1143) U/mL and was reached at 19 (17 to 24) days after the second dose. Antibody titers decreased over time, with titers of 770 (316 to 1089) U/mL at 15 days, 385 (203 to 690) U/mL at 30 days, 254 (138 to 423) U/mL at 60 days, and 208 (107 to 375) U/mL at 90 days after the second dose. When an antibody titer of 137 U/mL was assumed to be a measure related to breakthrough infection, the proportion of subjects with antibody titers exceeding this level was 90.1% at 15 days, 85.3% at 30 days, 75.0% at 60 days, and 65.4% at 90 days after the second dose. When a decrease in antibody titers below the assumed breakthrough level was defined as an event, subjects with a pre-dialysis albumin ≥ 3.5 g/dL were significantly less likely to experience an event than subjects with a pre-dialysis albumin < 3.5 g/dL. Conclusions The presence of anti-spike protein levels ≥ 313 U/mL at 30 days after the second vaccine dose might be a factor in maintaining enough antibody titers at 90 days after. Whether an additional vaccine dose is needed should be determined based on indicators serving as factors in maintaining antibody titers as well as the status of the spread of infection.

PreviousNext No AccessProceedings of the 14th SEGJ International Symposium, Online, 18–21 October 2021Estimation of the direction of an object by directional borehole radar with dipole array for a transmitter and a receiverAuthors: Yuki TsujikawaSatoshi EbiharaKoki KoyamaTomoya KakoHisaya YamamotoYuki TsujikawaOsaka Electro-Communication UniversitySearch for more papers by this author, Satoshi EbiharaOsaka Electro-Communication UniversitySearch for more papers by this author, Koki KoyamaOsaka Electro-Communication UniversitySearch for more papers by this author, Tomoya KakoOsaka Electro-Communication UniversitySearch for more papers by this author, and Hisaya YamamotoOsaka Electro-Communication UniversitySearch for more papers by this authorhttps://doi.org/10.1190/segj2021-085.1 SectionsAboutPDF/ePub ToolsAdd to favoritesDownload CitationsTrack CitationsPermissions ShareFacebookTwitterLinked InRedditEmail Abstract We present a directional borehole radar system with dipole array antennas used as a transmitter and a receiver in this paper. This radar can estimate the azimuth direction of a radar target position at both the transmitter and receiver. Thanks to this capability, the radar may precisely estimate the azimuth angle even if the media around the borehole are inhomogeneous. Dipole antenna elements are arranged in a circle at both the transmitter and the receiver. We use the difference in arrival times measured at the dipole antenna elements to estimate an object’s direction. In our prototype borehole radar systems, there are four dipole antenna elements for the transmitter and the same number of elements for the receiver. These antenna elements are packed in an FRP vessel within the length of about one meter. We demonstrated a field experiment with the radar in wet soil. A radar target is an inclined cylindrical conducting cylinder, positioned about one meter from the borehole. The error in the estimated azimuth angles of the conducting cylinder ranged from a few degrees to about 30 degrees. Averaging the calculated values, we evaluated the azimuth direction within an error of 15 degrees. We found the multiple antennas at both the transmitter and receiver improved the estimated azimuth angles in the field experiment data. Keywords: borehole radar, direction finding, directional antenna, dipole array antennaPermalink: https://doi.org/10.1190/segj2021-085.1FiguresReferencesRelatedDetails Proceedings of the 14th SEGJ International Symposium, Online, 18–21 October 2021ISSN (online):2159-6832Copyright: 2021 Pages: 349 publication data© 2021 Published in electronic format with permission by the Society of Exploration Geophysicists of JapanPublisher:Society of Exploration GeophysicistsSociety of Exploration Geophysicists of Japan HistoryPublished Online: 29 Nov 2021 CITATION INFORMATION Yuki Tsujikawa, Satoshi Ebihara, Koki Koyama, Tomoya Kako, and Hisaya Yamamoto, (2021), "Estimation of the direction of an object by directional borehole radar with dipole array for a transmitter and a receiver," SEG Global Meeting Abstracts : 323-326. https://doi.org/10.1190/segj2021-085.1 Plain-Language Summary Keywordsborehole radardirection findingdirectional antennadipole array antennaPDF DownloadLoading ...

PreviousNext No AccessProceedings of the 14th SEGJ International Symposium, Online, 18–21 October 2021Measurement of wave polarization scattered by a cylindrical conductor with a dipole and loop array borehole radarAuthors: Satoshi EbiharaKoki KoyamaHisaya YamamotoYuki TsujikawaTomoya KakoSatoshi EbiharaOsaka Electro-Communication UniversitySearch for more papers by this author, Koki KoyamaOsaka Electro-Communication UniversitySearch for more papers by this author, Hisaya YamamotoOsaka Electro-Communication UniversitySearch for more papers by this author, Yuki TsujikawaOsaka Electro-Communication UniversitySearch for more papers by this author, and Tomoya KakoOsaka Electro-Communication UniversitySearch for more papers by this authorhttps://doi.org/10.1190/segj2021-084.1 SectionsAboutPDF/ePub ToolsAdd to favoritesDownload CitationsTrack CitationsPermissions ShareFacebookTwitterLinked InRedditEmail Abstract We present a borehole radar system that can radiate and receive horizontally and vertically polarized waves. This capability can help the characterization of a radar target. In this study we designed the radar with two loops and four dipole antennas for both a transmitter and a receiver. The two kinds of antennas can radiate or receive two perpendicular polarizations of waves independently. We made a prototype of the radar system and conducted field experiments in wet soil. The radar was in a vertical borehole, while the radar target was a vertical or inclined conducting cylinder underground. Both the targets are one meter apart from the borehole. We defined the antenna voltage ratio as the loop antenna signal voltage divided by the dipole one. We found that the amplitude ratio in the inclined cylinder’s case is twenty times larger than that of the vertical one when the vertical dipole is a transmitter. This result may lead to improved characterization of the radar target. Keywords: borehole radar, polarization, dipole antenna, loop antennaPermalink: https://doi.org/10.1190/segj2021-084.1FiguresReferencesRelatedDetails Proceedings of the 14th SEGJ International Symposium, Online, 18–21 October 2021ISSN (online):2159-6832Copyright: 2021 Pages: 349 publication data© 2021 Published in electronic format with permission by the Society of Exploration Geophysicists of JapanPublisher:Society of Exploration GeophysicistsSociety of Exploration Geophysicists of Japan HistoryPublished Online: 29 Nov 2021 CITATION INFORMATION Satoshi Ebihara, Koki Koyama, Hisaya Yamamoto, Yuki Tsujikawa, and Tomoya Kako, (2021), "Measurement of wave polarization scattered by a cylindrical conductor with a dipole and loop array borehole radar," SEG Global Meeting Abstracts : 319-322. https://doi.org/10.1190/segj2021-084.1 Plain-Language Summary Keywordsborehole radarpolarizationdipole antennaloop antennaPDF DownloadLoading ...

In this article, we investigated the arrival times of plane waves incident on a circular dipole array in a borehole (CAB). Our theoretical and experimental findings suggest that the elevation angle of the received wave strongly affects plane wave arrival times in a borehole installation. If a plane wave is incident at the critical elevation angle (CEA), there is little arrival time difference among the signals at the CAB. On the other hand, when the plane wave is incident at an elevation angle steeper than the CEA, the wave arrival order at the CAB becomes opposite to the normal incidence case. In order to clarify the reason why these phenomena occur, we decompose a transmission matrix, which represents the effect of the borehole, into three components. We found that the azimuthal component of the electric field contributes to the observed phenomena significantly, and two of the three decomposed components cancel each other out at the CEA. For verification of the phenomena in a cross-hole borehole radar measurement, we performed a numerical simulation and carried out experimental verification at a field test site. Both data sets confirmed that the arrival time differences among the signals at the CAB were minimized at the elevation angle predicted to be the CEA. In addition, we found the reverse of the incident wave's arrival order at the CAB.

PreviousNext No Access18th International Conference on Ground Penetrating Radar, Golden, Colorado, 14–19 June 2020S11 measurement of dipole antenna in a boreholeAuthors: Satoshi EbiharaKouhei OkamotoSyouhei KobayashiSatoshi EbiharaOsaka Electro-Communication University, JapanSearch for more papers by this author, Kouhei OkamotoOsaka Electro-Communication University, JapanSearch for more papers by this author, and Syouhei KobayashiOsaka Electro-Communication University, JapanSearch for more papers by this authorhttps://doi.org/10.1190/gpr2020-091.1 SectionsAboutPDF/ePub ToolsAdd to favoritesDownload CitationsTrack CitationsPermissions ShareFacebookTwitterLinked InRedditEmail Abstract In this paper, we propose a radar system for S11 measurement of a dipole antenna in a borehole. The total length and diameter of the dipole antenna in the system are 1.8 m and 7 cm, respectively. The operating frequencies are between 1 MHz and 800 MHz. A small vector network analyzer (VNA), stick personal computer, fiber media converter, and battery are inside the dipole antenna. A cable between the dipole antenna and ground surface equipment is made of only optical fiber. This means that there is no conductor, excluding the dipole antenna itself, and unnecessary electromagnetic scattering is thus avoided. We conducted field experiments in sandy soil and granite at a test site. After applying the inverse Fourier transform to the acquired S11 data, we obtained time domain signals. From these, we could identify the reflected waves from subsurface objects such as a geological interface. We also detected reflected waves from both ends of the dipole antenna. The arrival times of those waves corresponded to the dielectric property of the medium around the borehole. Keywords: borehole measurements, dielectric measurements, Fourier, fractures, geologyPermalink: https://doi.org/10.1190/gpr2020-091.1FiguresReferencesRelatedDetails 18th International Conference on Ground Penetrating Radar, Golden, Colorado, 14–19 June 2020ISSN (online):2159-6832Copyright: 2020 Pages: 455 publication data© 2020 Published in electronic format with permission by the Society of Exploration GeophysicistsPublisher:Society of Exploration Geophysicists HistoryPublished Online: 11 Nov 2020 CITATION INFORMATION Satoshi Ebihara, Kouhei Okamoto, and Syouhei Kobayashi, (2020), "S11 measurement of dipole antenna in a borehole," SEG Global Meeting Abstracts : 348-351. https://doi.org/10.1190/gpr2020-091.1 Plain-Language Summary Keywordsborehole measurementsdielectric measurementsFourierfracturesgeologyPDF DownloadLoading ...

PreviousNext No Access18th International Conference on Ground Penetrating Radar, Golden, Colorado, 14–19 June 2020Borehole effects on direction finding of a planar interface with an array-type directional borehole radarAuthors: Satoshi EbiharaShyuhei KotaniKengo FujiwaraYuta KimuraTakaaki ShimomuraRyota UchimuraSatoshi EbiharaOsaka Electro-Communication University, JapanSearch for more papers by this author, Shyuhei KotaniOsaka Electro-Communication University, JapanSearch for more papers by this author, Kengo FujiwaraOsaka Electro-Communication University, JapanSearch for more papers by this author, Yuta KimuraOsaka Electro-Communication University, JapanSearch for more papers by this author, Takaaki ShimomuraOsaka Electro-Communication University, JapanSearch for more papers by this author, and Ryota UchimuraOsaka Electro-Communication University, JapanSearch for more papers by this authorhttps://doi.org/10.1190/gpr2020-090.1 SectionsAboutPDF/ePub ToolsAdd to favoritesDownload CitationsTrack CitationsPermissions ShareFacebookTwitterLinked InRedditEmail Abstract We introduce the possible locations of the critical receiver position (CRP) to estimate the direction of arrival (DOA) with an array-type directional borehole radar. The circular dipole array antenna at the CRP in a borehole cannot estimate the DOA. Our method would be useful when we estimate the direction of a planar interface, such as a fault in the rock. We derive an analytical equation representing the possible CRP locations, considering the positions of the antennas and the planar interface geometrically. We output the numerical values of the possible CRP locations after estimating the dip angle of the planar interface in a radargram. We applied the proposed method to analyze reflected waves from a real-world fault in the rock. We confirmed that the actual CRP was inside the possible CRP locations. Making use of the CRP, we correctly estimated the direction of arrival of the reflected wave from a fault. Keywords: faults, estimation, electromagnetic, processing, numericalPermalink: https://doi.org/10.1190/gpr2020-090.1FiguresReferencesRelatedDetails 18th International Conference on Ground Penetrating Radar, Golden, Colorado, 14–19 June 2020ISSN (online):2159-6832Copyright: 2020 Pages: 455 publication data© 2020 Published in electronic format with permission by the Society of Exploration GeophysicistsPublisher:Society of Exploration Geophysicists HistoryPublished Online: 11 Nov 2020 CITATION INFORMATION Satoshi Ebihara, Shyuhei Kotani, Kengo Fujiwara, Yuta Kimura, Takaaki Shimomura, and Ryota Uchimura, (2020), "Borehole effects on direction finding of a planar interface with an array-type directional borehole radar," SEG Global Meeting Abstracts : 344-347. https://doi.org/10.1190/gpr2020-090.1 Plain-Language Summary KeywordsfaultsestimationelectromagneticprocessingnumericalPDF DownloadLoading ...

We consider the effects of oblique incidence of a wave on the creation of a 3-D image of a planar interface with an array-type directional borehole radar. In this study, we focus on the situation where the radar probe is close to the planar interface. In such circumstances, the reflected wave from the planar interface may be incident on the receiving array antenna at very steep elevation angles. As a result, borehole effects cause differences in the arrival times of the wave at the array elements, and consequently some errors emerge in imaging the planar interface. Observing the arrival time differences, we present an algorithm to compensate those errors in creating a 3-D image of a planar interface. Computer simulations predict that the errors may occur when the circular dipole array antenna is in an air-filled borehole in rock. Numerical simulations show that our proposed algorithm generates a 3-D image of an interface around an exact position, whereas conventional methods produce some spurious images opposite to the correct position (i.e., out of position by 180°). We then applied the proposed method to analyze reflected waves from a real-world fault in rock. A 3-D image of the fault could be successfully created, which was not possible using the conventional method.

PreviousNext No AccessProceedings of the 13th SEGJ International Symposium, Tokyo, Japan, 12–14 November 2018Consideration of Oblique Incidence in Planar Interface Measurement with a Dipole Array Antenna of Directional Borehole RadarAuthors: Satoshi EbiharaShyuhei KotaniKengo FujiwaraSatoshi EbiharaOsaka Electro-Communication Univ.Search for more papers by this author, Shyuhei KotaniOsaka Electro-Communication Univ.Search for more papers by this author, and Kengo FujiwaraOsaka Electro-Communication Univ.Search for more papers by this authorhttps://doi.org/10.1190/SEGJ2018-038.1 SectionsAboutPDF/ePub ToolsAdd to favoritesDownload CitationsTrack CitationsPermissions ShareFacebookTwitterLinked InRedditEmail Abstract We discuss how signals of a directional borehole radar can be processed when a plane wave is obliquely incident on the receiving dipole array antenna of the radar. This is done by measuring the planar interface intersecting the borehole with the directional borehole radar. We show that the receiving antenna position where the antenna fails in the direction of arrival (DOA) estimation exists because of the reflected wave's incidence with the critical elevation angle. We propose choice of a DOA estimation method from two methods before and after this position during scanning of the radar probe. This is because we need to consider the electromagnetic field abnormality caused by the oblique incidence. We applied the proposed method to analyze reflected waves from a real fault in a rock. In this application, we estimated the DOA successfully even if the incident elevation angle of the arriving wave is steeply oblique. Keywords: directional borehole radar, dipole array antenna, fault, single-hole measurementPermalink: https://doi.org/10.1190/SEGJ2018-038.1FiguresReferencesRelatedDetails Proceedings of the 13th SEGJ International Symposium, Tokyo, Japan, 12–14 November 2018ISSN (online):2159-6832Copyright: 2019 Pages: 588 publication data© 2018 Published in electronic format with permission by the Society of Exploration Geophysicists of JapanPublisher:Society of Exploration GeophysicistsSociety of Exploration Geophysicists of Japan HistoryPublished Online: 29 Apr 2019 CITATION INFORMATION Satoshi Ebihara, Shyuhei Kotani, and Kengo Fujiwara, (2019), "Consideration of Oblique Incidence in Planar Interface Measurement with a Dipole Array Antenna of Directional Borehole Radar," SEG Global Meeting Abstracts : 146-149. https://doi.org/10.1190/SEGJ2018-038.1 Plain-Language Summary Keywordsdirectional borehole radardipole array antennafaultsingle-hole measurementPDF DownloadLoading ...

Summary We investigated cross-polarization characteristics of a dipole array antenna in a water-filled borehole. We theoretically determined the electromagnetic fields when a plane wave is incident to multiple cylindrical layers. According to our analysis, the dipole array antenna may be characterized by a spatial frequency spectrum of the dipole array signals. If a cross-polarized wave is incident on the antenna, the absolute value of the spectrum is maximized to the first order. Furthermore, there is a phase difference of 90 deg. between the co-polarization and cross-polarization incidence angles at the first order spatial frequency. Numerical simulations confirmed that the cross-polarization characteristics in the spatial frequency spectrum may be observed as suggested in the theoretical analysis. Field experiments in soil were carried out at a test site. Data equivalent to signals of a dipole array in a borehole were obtained using a transmitting loop in another borehole. In the experimental data, cross-polarization characteristics were observed similar to those deduced from the theoretical and numerical analyses.

We propose an algorithm for creating a 3-D image of a planar interface with an array-type directional borehole radar. In this paper, we focus on the situation when the radar probe is close to the planar interface. In such circumstances, the reflected wave from the planar interface may be incident on the receiving array antenna at very steep elevation angles. This causes unusual arrival times of the wave at the array elements, and consequently some errors in imaging the planar interface. In our numerical simulation, the proposed algorithm generated a 3-D image of an interface around an exact position whereas a conventional method produced some spurious images opposite the correct position (i.e., off by 180 deg.). We then applied the proposed method to analyze reflected waves from a real-world fault in a rock. Using the proposed algorithm, we could successfully create a 3-D image of the fault, although this was not possible with the conventional method.

A new design for a pulse directional borehole radar with a dipole array antenna is proposed and tested. In this radar, a pulse generator excites a pulse, with peak voltage of 110 V and low jitter at a feeding point of the transmitter. The receiver measured the difference of arrival times at several receiving dipole elements to estimate the direction of the arrival wave. We compensated for time delays due to the cables which connect the dipole elements. Field experiments were carried out with the pulse directional borehole radar in tuff; the system received a reflected wave from a metal cylinder located several meters from the radar antenna. After the cable delay compensation, we successfully estimated the 3-D location of the metal cylinder. The results offield measurements using a stepped-frequency directional borehole radar are also included for comparison.

In this study we investigated the influence of a borehole on the estimation of wave arrival direction using a dipole array antenna in a directional borehole radar system. According to computer simulation, if a plane wave is incident at certain elevation angles, there is little arrival time difference among the dipole array signals. In this case, the estimation of arrival direction fails. When the elevation angle of wave incidence is very steep, the estimated direction of arrival becomes opposite to the true direction (i.e., off by 180 deg.). We conducted a cross-hole measurement to verify these phenomena. We observed little arrival time difference among the dipole array signals at a certain elevation angle we deemed the critical angle. For a wave incident on the dipole array at very steep angles, field experiments found that the estimated direction of arrival was almost opposite to the true direction like the computer simulation.