Directional Borehole Radar Research Articles

3-D Directional Borehole Radar Imaging Based on Echo Separation

3-D Directional Borehole Radar Imaging Based on Echo Separation

A 3-D Directional Borehole Radar Imaging Method for Rough Fractures: Numerical Simulation and Analysis

A 3-D Directional Borehole Radar Imaging Method for Rough Fractures: Numerical Simulation and Analysis

3D Imaging Algorithm of Directional Borehole Radar Based on Root-MUSIC

A directional borehole radar consists of one transmitting antenna in the borehole and four receiving antennas distributed at equal angles in a ring. The receiving antennas can determine the depth and orientation of targets beside the borehole. However, the problem of target orientation determination and 3D imaging algorithms remains a technological challenge. The MUSIC (multiple signal classification) algorithm requires a peak search, so the accuracy of the operation is limited by the angle interval. Based on the MUSIC algorithm, the Root-MUSIC algorithm is proposed and implemented. By replacing the spectral peak search with calculating the roots of the polynomials greatly improves the orientation recognition accuracy. Finally, the results obtained using the above algorithm are verified with synthetic data and compared with the results of the MUSIC algorithm. The results show that both the MUSIC algorithm and the Root-MUSIC algorithm can achieve very good orientation determination and 3D imaging results. In terms of accuracy, the Root-MUSIC algorithm has an obvious improvement compared with the MUSIC algorithm.

Analysis of the Borehole Effect in Borehole Radar Detection.

The directional borehole radar can accurately locate and image the geological target around the borehole, which overcomes the shortcomings that the conventional borehole radar can only detect the depth of the target and the distance from the borehole. The directional borehole radar under consideration consists of a transmitting antenna and four receiving antennas equally distributed on the ring in the borehole. The nonuniformity caused by the borehole and sonde, as well as the mutual coupling among the four receiving antennas, will have a serious impact on the received signal and then cause interference to the azimuth recognition for the targets. In this paper, Finite difference time domain (FDTD), including the subgrid, is applied to study these effects and interferences, and the influence of borehole, sonde, and mutual coupling among the receiving antennas is found. The results show that, without considering the sonde and the fluid in the borehole, the one transmitting and one receiving borehole radar system does not have resonance, but the wave pattern of the reflected wave will have obvious distortion. For the four receiving antennas of the borehole radar system, there is obvious resonance, which is caused by the multiple reflections between the receiving antennas. However, when the fluid in the borehole is water and the relative permittivity of the sonde is low to a certain extent, the resonance disappears; that is, the generation of resonance requires a large relative permittivity material between the receiving antennas. When the influence of the sonde is considered, the resonance disappears because the relative permittivity of the sonde is low, which makes the propagation speed of the electromagnetic wave between the antennas accelerate and lose the conditions for resonance. In addition, the diameters of the sonde and the circular array of the receiving antennas can affect the received signal: the higher the diameter of the sonde and the higher the diameter of the circular array are, the better the differentiation of the received signal. The development of the research provides scientific guidance for the design and application of borehole radar in the future.

Consideration of Oblique Incidence in 3-D Imaging of a Planar Interface With a Circular Dipole Array in an Air-Filled Borehole

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.

Linear Prediction-Based DOA Estimation for Directional Borehole Radar 3-D Imaging

Directional borehole radar (BR) 3-D imaging is a challenging problem due to its limited observation space within a borehole. We present a linear prediction (LP)-based direction-of-arrival (DOA) estimation and a migration-based 3-D imaging algorithm for the directional BR, which is composed of a transmitting antenna and a uniform circular array including four small receiving antennas. Only the azimuth is estimated, while the elevation and the distance are included in the imaging process. LP is a useful tool for DOA estimation in the radar domain, but a single linear array (LA) has azimuth ambiguity in DOA estimation. The four receiving antennas are treated as two orthogonal uniform LAs. The combination of two arrays removes the ambiguity in DOA estimation. In addition, the moving window and the threshold-based estimation increase the calculation efficiency. According to the estimated azimuth angles, the measured signal at each depth is decomposed to the corresponding directions. Therefore, there is a time-series signal in each direction at each depth. After processing the measured signals at all depths, we obtain a 3-D data set that can be used for horizontal- and radial vertical-section displays. Then, the radial vertical-section data are processed by migration which transforms the time domain signal into a wavefield-based 3-D image. Simulated data from a fracture model are used to validate the algorithm, and the reconstructed image can reveal the real model well. It is concluded that the novel method we propose is an effective one with real-time processing capability.

Probe rotation effects on direction of arrival estimation in array‐type directional borehole radar

ABSTRACTThis paper discusses the relationship between feeding line delay compensation and direction of arrival estimation in an array‐type directional borehole radar. In this radar, since the space available for an array antenna is limited by the borehole diameter, direction of arrival estimation is based on small travel time differences among array elements, and an accurate compensation of feeding line delays is important. Computer simulation confirmed that the radar probe rotation in a borehole leads to errors in the direction of arrival estimation of greater than 15° if the delays associated with the feeding array antenna elements are not compensated to within 0.1 ns. This may be caused by a failure to measure the time delays of electrical circuits around the feeding points of the antenna elements. In this case, we suggest that the lengths of system components other than the coaxial cables should be kept to less than 3 cm. Based on these investigations, we developed an array‐type directional borehole radar for a geotechnical project to locate foundation piles. In a field experiment, we confirmed that direction of arrival estimation errors were below about 15°, although the radar probe rotated through more than 180° during the measurement, thanks to correct compensation of the cables. With the correct compensation, we demonstrated three‐dimensional location of a buried cylindrical conducting object, which was located at 2 m from the radar in wet soil. We were able to estimate the reflection point position with an accuracy of 34 cm, which is the averaged error of the three‐dimensional location, while allowing the radar probe to rotate.

Improved Discrimination of Subsurface Targets Using a Polarization-Sensitive Directional Borehole Radar

We propose a polarization-sensitive directional borehole radar with a dipole array and loop antenna in a borehole (DALAB), along with an algorithm for radar signal processing. This radar enables us to discriminate between a vertical conducting cylinder and inhomogeneity in soil in 3-D space. The antenna configuration difference, antenna coupling, and borehole effects were calibrated using the method of moments (MoM) in the proposed algorithm. According to computer simulation, the magnitude of the polarization ratio (MPR) of the incident wave may be used for discrimination between signals from targets and background reflections. A prototype of the borehole radar with the DALAB was constructed and tested in laboratory and field experiments. In the laboratory, we confirmed that the DALAB can estimate the polarization state of an incident wave despite the presence of a water layer. In field testing, we confirmed that the MPR was nearly 0 dB, as theoretically estimated when the DALAB received electromagnetic waves radiated by a tilted dipole antenna. We also carried out single-hole measurements with the DALAB and found that the MPR of a reflected wave from a vertical conducting cylinder was more than 20 dB, whereas that from soil inhomogeneity was less than about 20 dB. This implies that we may discriminate between a conducting cylinder and soil inhomogeneity and determine target locations in three dimensions using the DALAB and the MPR.

Interference Criterion for Coaxial-Fed Circular Dipole Array Antenna in a Borehole

In this paper, we propose a coaxial-fed circular dipole array in a borehole (CFCAB) for directional borehole radar and present a design method for the CFCAB. In the CFCAB, a circular dipole array is arranged around a conducting cylinder. When a wave is incident on the antenna, we may divide the signals received at the antenna into two components. One is caused by the waves arriving at the antenna directly, and the other is due to the waves scattered at the conducting cylinder. We proposed a criterion to quantify the influence of the conducting cylinder on the radar measurement. We show results of experiments in air and in granite to examine the validity of the criterion. Making use of the criterion, we designed and built the directional borehole radar system with the CFCAB working at around 200 MHz. Conducting the field experiments in granite, we confirmed that the CFCAB can estimate the directions of arrival waves on the time-frequency plane, as we expected with the criterion.

Estimating the 3D position and inclination of a planar interface with a directional borehole radar

ABSTRACTIn this paper, we present a signal processing method suitable for estimating the position of a planar interface and its inclination using a directional borehole radar. The receiving directive antenna in a radar system is a coaxial‐fed circular dipole array antenna in a borehole (CFCAB), which we have proposed previously. The method combines least‐square fitting with a model of plane‐wave incidence and uses an inverse boundary scattering transform. This approach makes it possible to estimate parameters from measurements collected at only two narrowly separated depths of a radar sonde. We give a numerical example to verify the proposed method. In this simulation, the array data were generated with the Method of Moments modified for a borehole radar. We conducted field experiments in the Nakatatsu mine in Japan, where there is a fault in the skarn. We constructed a 3D image of the fault with a directional borehole radar and the signal processing method. The image agrees well with the information obtained from boring core samples and observations in the gallery.

A Novel Direction-Finding Algorithm for Directional Borehole Radar

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> A directional borehole radar system has been developed for the purpose of 3-D imaging of subsurface targets in a single-hole measurement. The radar system is equipped with a uniform circular array consisting of four dipole antennas as a receiver in order to realize azimuth bearing sensitivity. We propose a new direction-finding (DF) algorithm that is suitable for directional borehole radar measurement, and we apply this algorithm to actual field measurement data. This algorithm is based on the Adcock DF antenna principle where the complex time series (analytic signal) expression, the optimization, and the filtering procedure are incorporated to provide more accurate estimation. The algorithm was first verified in a transmission measurement in boreholes with a cross-hole configuration (15 m apart from each other) by estimating a direction of the incident wave from a transmitter to the receiver. Finally, the algorithm was applied to single-hole measurement data to demonstrate the ability to detect the 3-D location of a subsurface tunnel which was located 5.5 m from the borehole. The result showed fairly good agreement with the actual location of the tunnel, i.e., to an azimuth estimation error of within 10 <formula formulatype="inline"><tex>$^{\circ}$</tex></formula>. </para>

A Novel Directional Borehole Radar System Using Optical Electric Field Sensors

A directional borehole radar system using an array antenna connected to passive optical electric field sensors was developed and evaluated by laboratory and field experiments. This system uses a single dipole antenna for a transmitter and a four-dipole element circular array for a receiver. The received signals are transmitted through optical fibers from optical electric field sensors. The receiver array measures the phase differences between four dipole antennas and estimates the azimuth direction of the incoming reflection wave. The accuracy of the phase measurement was first evaluated in a laboratory test and then tested in a field experiment. To compensate for the sensitivity differences among the sensors, we propose an <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">in situ</i> calibration technique. The dominant operating frequency of the system is 70 MHz. Subsequently, we used borehole radar for subsurface measurements in a vertical borehole in granite. Estimation of the azimuth orientations of subsurface fractures was successfully demonstrated.

Resonance analysis of a circular dipole array antenna in cylindrically layered media for directional borehole radar

In this paper, we discuss the influence of a resonance on estimating direction of arrival (DOA) with a circular dipole array in a borehole (CAB). The resonance is caused by the phase-sequence of currents on the dipole antennas. Making use of method of moments (MoM) analysis, we predict resonant frequencies of the CAB and describe a mechanism for the resonance theoretically, making use of the fact that a plane wave can be broken down into cylindrical harmonics. In order to examine the agreement between the MoM and experimental data, we propose a signal processing method to extract phase-sequence components from the actual received array signal. This method is useful for confirming excitation of the phase-sequence resonance in experimental data, as predicted by the MoM. Using the MoM, we examine the influence of the resonances on DOA estimation, and we conclude that the resonances give some spurious solutions. In order to confirm these results, we conduct experiments at a field test site. We arrange the CAB, which consisted of seven receiving dipole antennas. In this experiment, a plane wave is incident on the CAB from a transmitter in another borehole. We apply the proposed signal method, and the measured results demonstrate the existence of the phase-sequence resonance in a physically real CAB. The spurious solutions found to occur in the DOA estimation at the resonant frequencies are also found experimentally.

A Directional Borehole Radar System

In this paper we present the simulation and design of a directional borehole radar. In addition we discuss an imaging method for the radar system. The antenna system contains an electric dipole which is in one direction shielded by a cylindrical perfectly conducting reflector. The radiation pattern of the reflected wavefield is computed by first solving the integral equation. This equation combines the unknown electric surface current density on the reflector and the known incident field from the electric dipole. Once the electric surface current density is known, the radiation pattern of the system is computed using the integral representation over the reflector and the dipole. The radiation patterns for various configurations have been computed in order to find an optimal configuration. A prototype of the antenna system based on an optimal configuration has been built, and the directional radiation pattern has been measured in the plane perpendicular to the antenna system. The measurements were in good agreement with the computations. Subsequently a three dimensional imaging method for the borehole radar is presented. Here a deconvolution is carried out in the angular direction, making use of the computed radiation pattern. Some imaging results will be shown.

Super-resolution of coherent targets by a directional borehole radar

An analytical method based on the multiple signal classification (MUSIC) algorithm is applied to three-dimensional (3D) estimation of target positions using a directional borehole radar. A cylindrical conformal array on a conducting cylinder in a borehole was used for experimental measurements estimating the position of targets. It is also shown that the algorithm provides much better resolution than the Fourier-based algorithm in both a computer simulation and real tests. Soil at the experimental site is not necessarily uniform, and there can be many clutter sources such as small stones in the subsurface. For this situation, it is difficult for the super-resolution technique to work properly. Results of the experiments show that the super-resolution technique for locating targets, despite being seriously limited by both the space available for the array and the bandwidth, is promising for directional borehole radar. In the developed algorithm, it is assumed that the distribution of electromagnetic constants inside a borehole is known before estimation of target positions.

Analysis of a guided wave along a conducting structure in a borehole

A guided wave along a borehole is often observed in borehole radar measurements. These guided waves deform the antenna pattern and can cause artefacts in radar measurements. A water‐filled borehole or a conducting logging cable can function as a waveguide for electromagnetic waves under some conditions. We describe the theoretical characteristics of such a guided wave in a borehole and compare them with our experiments. The measured signal discussed was obtained with a directional borehole radar. This radar uses a cylindrical conformal array antenna as receiver and is a model of a conducting structure in a borehole. The induced field around the borehole was compared with the theory. The most fundamental symmetrical and asymmetrical modes were TM01 and HE11, and they were identified in the measured signals using time–frequency distribution analysis and by observation of the azimuthal field distribution of the magnetic field.