Borehole Radar Data Research Articles

Extracting mud invasion information using borehole radar — A numerical study

In hydrocarbon drilling, mud filtrate penetrates permeable formations and alters the pore fluid characteristics in the immediate vicinity of the borehole. Typically, the prevailing in situ pore fluids are displaced by the invading mud filtrate, which leads to gradually changing distributions of the fluid and electrical properties. Understanding this invasion process is crucial for the interpretation of logging data and associated reservoir evaluations. Conventional logging methods tend to be inadequate for this purpose as their resolution is too low. We find that invasion depth can be determined from borehole radar data using an optimized antenna configuration and time-lapse measurements. A series of parametric sensitivity analyses provide information about the effects of variations of the rock and fluid properties on the identification and extraction of borehole radar signals reflected from the invasion front. Our results suggest that by embedding the radar antennas in cavities filled with an absorbing dielectric material, it is possible to minimize the interference arising from the metal components of the logging tool. In the simulated reservoir scenario, a time-lapse measurement mode with a time interval of at least 6 h can reliably extract the radar signals reflected from the invasion front, and the proposed borehole radar has a lateral detection range from 0.15 to 1 m. A comprehensive range of parametric sensitivity analyses indicates that the signals reflected from the invasion front are principally influenced by oil viscosity, porosity, and mud and formation water salinity, as well as by molecular diffusion coefficient and cementation exponent. These properties and parameters should be carefully explored and assessed when applying borehole radar to evaluate mud invasion information in a reservoir environment.

An Application of Reflected Guided Borehole Radar Waves to Detection of Subhorizontal Structures

Borehole radar (BHR) waves will be severely attenuated when a borehole is filled with high-conductivity (HC) media. As a result, BHR side-reflections are significantly weakened, i.e., the capability to detect the targets far from the borehole is restricted. However, V-shaped reflected guided waves (RGWs) are commonly observed in BHR data from such HC boreholes. They are often regarded as unwanted contamination to conventional BHR surveys and should be avoided or suppressed. However, we advocate that the RGWs, as a type of reflected wave traveling along the borehole, contain geological information about the surroundings of the borehole and can be used to locate and recognize geological boundaries, such as lithological interfaces and fractures that intersect the borehole. Unlike conventional BHR surveys mainly imaging the structures near-parallel to the borehole, the RGWs can effectively sense near-normal ones. In addition, we can use RGWs to distinguish between echoes from fractures and those from bedding planes. This is achieved by recognizing different phase characteristics of RGWs for these structures: the RGWs from both sides of a fracture have the same phase, whereas the ones from a stratigraphic interface have opposite phases. Synthetic and real data are used to demonstrate the feasibility of detecting subhorizontal structures by RWGs. RGWs extend BHR measurements and allow a better understanding of subsurface structures. They help to predict and reduce mining risks and monitor potential geological hazards.

Characteristics of a fracture network surrounding a hydrothermally altered shear zone from geophysical borehole logs

Abstract. Hydrothermally active and altered fault/shear zones in crystalline rocks are of practical importance because of their potential similarities with petrothermal reservoirs and exploitable natural hydrothermal systems. The petrophysical and hydraulic characterization of such structures is therefore of significant interest. Here, we report the results of corresponding investigations on a prominent shear zone of this type located in the crystalline Aar massif of the central Swiss Alps. A shallow borehole was drilled, which acutely intersects the core of the shear zone and is entirely situated in its surrounding damage zone. The focus of this study is a detailed characterization of this damage zone based on geophysical borehole measurements. For this purpose, a comprehensive suite of borehole logs, comprising passive and active nuclear, full-waveform sonic, resistivity, self-potential, optical televiewer, and borehole radar data, was collected. The migrated images of the borehole radar reflection data together with the optical televiewer data reveal a complicated network of intersecting fractures in the damage zone. Consequently, the associated petrophysical properties, notably the sonic velocities and porosities, are distinctly different from intact granitic formations. Cluster analyses of the borehole logs in combination with the structural interpretations of the optical televiewer data illustrate that the variations in the petrophysical properties are predominantly governed by the intense brittle deformation. The imaged fracture network and the high-porosity zones associated with brittle deformation represent the main flow pathways. This interpretation is consistent with the available geophysical measurements as well as the analyses of the retrieved core material. Furthermore, the interpretation of the self-potential and fluid resistivity log data suggests a compartmentalized hydraulic behavior, as evidenced by inflows of water into the borehole from different sources, which is likely to be governed by the steeply dipping structures.

Application of Borehole Radar Data Processing Based on Empirical Mode Decomposition

Borehole Radar (BHR) uses ultra-wideband electromagnetic (EM) waves to image discontinuities in formations. It has been a major bottleneck to extend BHR applications to obtain a clear and high-resolution radar profile in a complex and noisy environment, which increases ambiguity in the geology interpretation. To avoid this increased ambiguity in the geology interpretation, we proposed a scheme based on the empirical mode decomposition (EMD) and complex signal analysis theory to process the BHR data with low signal to noise ratio (SNR). The scheme includes four steps. First, the original radar profile is pre-processed to avoid mode confusion and noise interference to the radar echo. Next, the EMD method is used to process a single-channel radar dataset and to analyze the frequency components of the radar signal. Various intrinsic modes of the pre-processing radar profile are also obtained by using EMD. Finally, we reconstruct the intrinsic mode profile, which contains information about the formation, calculate the complex signals of the reconstructed radar profile using the Hilbert transform, extract the three instantaneous attributes (instantaneous amplitude, instantaneous phase, and instantaneous frequency), and draw the separate instantaneous attributes profiles. This processing scheme provides both the conventional time-distance profile also in addition to the three instantaneous attributes. The additional attributes reduce ambiguity when evaluating the original radar profile and avoid the deviation relying solely on a conventional time-distance profile. An actual radar profile, which was obtained by a BHR system in a limestone fracture zone, is used to verify the effectiveness of instantaneous attributes for improving interpretation accuracy. The results demonstrate that the EMD method is superior in processing the BHR signal under a low SNR and has the capability to separate the high-low components of the radar echo effectively.

Energy Flow Domain Reverse-Time Migration for Borehole Radar

A modified 2-D reverse-time migration algorithm in the energy flow domain, called EF-RTM, is proposed for short impulse borehole radar (BHR) imaging. The key of the approach is based on Poynting's theorem, which allows decomposition of the energy flux density (EFD) derived from the source and receiver wave fields in different wave-propagation directions. Then, imaging conditions, for example, zero-lag cross correlation (prestack migration) and zero-time imaging principle (poststack migration), are applied to the decomposed EFD-field components to obtain the migrated sections. The characteristics of the resulting images can be optionally combined or separately used for better BHR data imaging, interpretation, and analysis. In this paper, the EF-RTM algorithm is validated by numerical modeling and real field data and compared with the conventional RTM algorithm. All the results show that this new EF-RTM method is superior to the conventional RTM method: it inherits the high precision of RTM with additional imaging advantages, such as natural wave-field decomposition in different directions, improved migrated cross-range resolution, a better focus of the target's shape, and migration noise reduction.

Suppressing Reflected Guided Waves From Contaminated Borehole Radar Data

In borehole radar (BHR) surveys, if the radar is either suspended by a conductive communication cable or run in a conductive water, guided waves may be induced. When they encounter impedance changes, for example, at geological interfaces, the guided waves will be reflected. These reflected guided waves (RGWs) often are relatively strong. They present themselves as obliquely striped patterns in the B-scan image, and, severely contaminate the reflections of interest. In this letter, we compare three signal processing methods to suppress RGWs: 1) running a moving average error filter along a specified dip—as in $\tau $ -p filtering; 2) pie-slice frequency–wavenumber (F-K) filtering; and 3) F-K migration-based filtering method. The third migration-based method is novel. All three methods are based on the characteristics and generating mechanism of RGWs. Real BHR data collected with a monostatic radar system illustrate their relative effectiveness.

Virtual multi-offset reflection profiling with interferometric imaging for borehole radar

Interferometry is one of the most advanced signal-processing and imaging techniques. It is widely used in geophysical detection and remote sensing. Application of interferometric processing significantly improves the resolution in geophysical imaging. This paper reports the application of an interferometric imaging approach to cross-hole multi-offset transmission (CHMOT) borehole radar data to generate a virtual single-hole, multi-offset reflection (SHMOR) profile with the validation of a real SHMOR data set. In subsurface material property imaging, SHMOR is an effective technique for interface and fracture detection. However, borehole radar survey in SHMOR fashion is not practical for most cases. Transforming cross-hole transmission mode radar data to virtual single-hole, multi-offset reflection data using a wave interferometric virtual source (WIVS) approach was proposed previously but not fully validated with real SHMOR data. In this study, we compare WIVS-derived virtual SHMOR to real SHMOR profiles using data sets acquired in two boreholes (SIMA1 and SIMA2) drilled into crystalline igneous bedrocks. As the calibration and validation, the reflection from borehole SIMA2 (as the known object) is clearly imaged by both the real and WIVS-derived virtual SHMOR profiles in SIMA1. The diffractions and amplitude decay from sharp stratigraphic change also registered at both the real and WIVS-derived virtual SHMOR profiles. The results of this study demonstrate the potential of the WIVS approach to improve structural imaging in bedrocks for hydrogeological, geothermal, and petroleum reservoir development applications.

Adaptive Filtering Processing for Target Signature Enhancement in Monostatic Borehole Radar Data

In B-scan data measured by a pulse-type monostatic borehole radar, target signatures are seriously obscured by two clutters that differ in orientation and intensity. The primary clutter appears as a nearly constant time delay, which is caused by internal ringing between antenna and transceiver in the radar system. The secondary clutter occurs as an oblique time delay due to the guided borehole wave along the logging cable of the radar antenna. This issue led us to perform adaptive filtering processing for orientation-based clutter removal. This letter describes adaptive filtering processing consisting of a combination of edge detection, data rotation, and eigenimage filtering. We show that the hyperbolic signatures of a dormant air-filled tunnel target can be more distinctly enhanced by applying the proposed approach to the B-scan data, which are measured in a well-suited test site for underground tunnel detection.

Shaft sinking risk analysis through the integration of borehole radar and acoustic televiewer data in deep geotechnical boreholes

The 2.06 Ga Bushveld Igneous Complex of South Africa is the world?s largest layered igneous intrusion. Situated to the north of the Johannesburg, it extends ~450 km from east to west and ~300 km north to south (Simmat et al., 2006), and has a maximum thickness of ~8 km. It is estimated to contain over half the world?s Platinum Group Element (PGE) resources (Cawthorn, 1999). The two PGE-bearing chromitite layers in the Bushveld Complex, the Merensky Reef and the UG2, occur within pyroxenites of the Critical Zone, show high lateral continuity and commonly dip at relatively shallow angles (2°-20°) towards the centre of the complex. These are overlain by a succession of norite and gabbronorite formations of the Main Zone, representing a more mafic phase of the intrusion. The project area is the location of a planned new Anglo Platinum mine on the western limb of the Bushveld, south of the Pilanesberg Alkaline Complex, a later volcanic event. Major capital investment is required to sink the twin shafts, accessing the underground platinum reefs from ~700 m depth. Due to the proximity to the Pilanesberg, dykes and faults are pervasive throughout the deposit. Several dyke varieties occur and these differ in composition, texture and orientation, and consequently represent different geotechnical hazards. Dolerite dykes tend to be hard and competent with similar rock strength properties to that of the surrounding Bushveld rocks, and are not expected to represent significant geotechnical problems. Lamprophyre dykes, however, are often soft, weather easily when exposed and demonstrate significant contrast in rock strength properties to the Bushveld rocks. Shaft geotechnical boreholes were drilled along the two proposed shaft axes and were logged with a suite of geophysical tools to detect potentially hazardous structures that may influence shaft sinking operations. This paper examines the integration of borehole radar (BHR) and acoustic televiewer (ATV) data down the proposed main- and ventilation shaft positions.

Full-waveform inversion algorithm for interpreting crosshole radar data: a theoretical approach

Ground-penetrating radar is a useful tool for civil and environmental engineering fields because of its high resolving power and non-destructive measurements. This paper presents a method of full-waveform inversion of borehole radar data for imaging permittivity structures. The inversion algorithm is based on a conjugate gradient search for the minimum of an error functional relating to the difference between measured and predicted data. A small model perturbation in the functional can be efficiently calculated by propagating the data error back into the model in reverse time and correlating the field generated by the back-propagation with the corresponding incident field at each point. A finite difference time domain (FDTD) method is used for solving Maxwell’s equations to obtain incident electromagnetic wavefields. Back-propagated wavefields satisfy adjoint Maxwell’s equations, which are stable in reverse time and can be solved by the same FDTD scheme. The imaging scheme is applied to crosshole radar configuration, thereby demonstrating its capability to reconstruct permittivity structures. Tests on a two-dimensional synthetic model produce good images of target scatterers and show stable convergence.

Wave Interferometry Applied to Borehole Radar: Virtual Multioffset Reflection Profiling

Based on wave-interferometry principles, we describe a procedure to synthesize monostatic and multioffset borehole radar reflection data from cross-hole radar tomography data. The procedure is equivalent to placing multiple transmitting sources in the receiving hole and conducting wide-angle reflection and refraction surveys. The procedure is illustrated using transmission and reflection data generated by numerical simulation of electromagnetic waves for a simple fracture model. The numerically simulated reflection data compare favorably with the reflection data synthesized using the wave-interferometry method. An experimental test of this procedure is also applied to a piece of field data from a pair of boreholes in crystalline igneous bedrock. The results demonstrate the potential of practical use of wave-interferometry methods for extracting reflection information from cross-hole radar tomography data.

MoM Analysis of Dipole Antennas in Crosshole Borehole Radar and Field Experiments

In this paper, we propose a method-of-moments (MoM) analysis that includes the borehole effects on crosshole borehole radar, and we verify that the MoM represents the crosshole borehole radar data correctly. We derive the far-field approximation of an electric field radiated by an electric point source. In this derivation, we assume that the point source is in a cylindrically layered medium, while the observed electric field is in another cylindrically layered medium. The equations that are derived are used as a transfer function to connect the two impedance matrices - one for the transmitting antenna and the other one for the receiving one. In these matrices, the influence of the scattered field by the borehole is included completely. We conducted field experiments in granite with a controlled borehole medium in order to investigate the influence of the borehole. In this paper, we estimated the relative amplitude and the group delay of the received signals between the water-filled borehole case and the air-filled borehole case. This paper showed that the estimated amplitude and delay depend on the frequency and that the proposed MoM could predict these frequency dependencies.

Reconstruction From Antenna-Transformed Radar Data Using a Time-Domain Reconstruction Method

This paper discusses the reconstruction of the subsurface from radar data using a time-domain reconstruction method. Before the reconstruction is conducted, data preprocessing is carried out. The preprocessing transforms measured data transmitted and received by 3-D dipole antennas into equivalent data excited and received by 3-D ideal electric dipoles and, then, into equivalent data excited and received by 2-D ideal electric dipoles. Distributions of permittivity, conductivity, and permeability are reconstructed from the preprocessed data by a 2-D forward-backward time-stepping waveform inverse scattering technique. Two 2-D examples of permittivity and conductivity reconstruction from synthetic noisy and field borehole radar data are demonstrated to show the effectiveness of the proposed method

Borehole radar investigations for locating ice ring formed by cryogenic condition in an underground cavern

A small underground pilot plant was constructed and operated at Korea Institute of Geoscience and Mineral Resources for investigating the feasibility of a new concept of storing liquefied natural gas (LNG) in a lined hard rock cavern. In the real operation of the pilot plant, liquid nitrogen (LN2) was stored instead of LNG for safety purpose. A drainage system controls the development of an impervious ring of ice formed due to storage of LN2 in extremely low temperature. The ice ring formed around the storage cavern is very important because this can act as the final barrier to secure the LNG storage. We carried out borehole radar surveys to study the applicability of the borehole radar reflection method to locating the ice ring formation. Prior to conducting the fieldwork, we performed numerical experiments of borehole radar reflection survey; through the analysis of the numerical modeled data we obtained the most appropriate interpretation strategies for locating the ice rings. Before and after storing the LN2 in the underground cavern, we conducted borehole radar reflection and crosshole level scanning surveys. The data obtained at the two stages of the surveys were compared in order to identify changes occurring in basement rock after storing the LN2. The interpretation of the borehole radar data clearly showed that the ice rings only developed under and in front of the LNG storage cavern of the pilot plant, but not over it. Through the numerical modeling experiments and field monitoring exercises, we were able to know that borehole radar reflection technique is an effective method for locating ice rings formed in basement rock for storing material of extremely low temperature in an underground cavern.

Characterization of an unstable rock mass based on borehole logs and diverse borehole radar data

Unstable rocky slopes are major hazards to the growing number of people that live and travel though mountainous regions. To construct effective barriers to falling rock, it is necessary to know the positions, dimensions and shapes of structures along which failure may occur. To investigate an unstable mountain slope distinguished by numerous open fracture zones, we have taken advantage of three moderately deep (51.0–120.8 m) boreholes to acquire geophysical logs and record single-hole radar, vertical radar profiling (VRP) and crosshole radar data. We observed spallation zones, displacements and borehole radar velocity and amplitude anomalies at 16 of the 46 discontinuities identified in the borehole optical televiewer images. The results of the VRP and crosshole experiments were disappointing at our study site; the source of only one VRP reflection was determined and the crosshole velocity and amplitude tomograms were remarkably featureless. In contrast, much useful structural information was provided by the single-hole radar experiments. Radar reflections were recorded from many surface and borehole fracture zones, demonstrating that the strong electrical property contrasts of these features extended some distance into the adjacent rock mass. The single-hole radar data suggested possible connections between 6 surface and 4 borehole fractures and led to the discovery of 5 additional near-surface fracture zones. Of particular importance, they supplied key details on the subsurface geometries and minimum subsurface lengths of 8 of the 10 previously known surface fracture zones and all of the newly discovered ones. The vast majority of surface fracture zones extended at least 40–60 m into the subsurface, demonstrating that their depth and surface dimensions are comparable.

Fracture Characterization Using Borehole Radar: Numerical Modeling

Forward modeling of borehole radar data for a series of synthetic discrete-fracture network (DFN) models provides a conceptual framework for interpreting experimental field data at fractured rock sites. A finite-difference time-domain (FDTD) radar wave propagation model was developed for this purpose. Synthetic examples demonstrate the utility of single-hole reflection-mode and cross-hole transmission-mode borehole radar for (1) identification of fracture location and orientation, and (2) identification of fracture pore-fluid properties, which might change as a result of tracer tests or flooding exercises in support of resource development or site remediation. A two-dimensional, synthetic DFN was generated statistically based on hypothetical distributions of fracture length, orientation, aperture, permeability, and inter-connectivity. The DFN includes a zone of permeable fractures embedded within a network of lower-permeability fractures and a low-permeability rock matrix. We modeled the unconnected and non-permeable fractures as being filled with freshwater. To simulate tracer experiments, contaminant releases, or engineered-remediation processes, we considered alternately the inter-connected, permeable fractures to be filled with freshwater air, or saline water (tracer). Synthetic radar data sets for both single-hole reflection and cross-hole transmission modes were generated. The features in synthetic radargrams were then examined and compared to the DFN model to evaluate the likelihood of identifying fracture location, orientation, and pore fluid in field situations. This comparison demonstrates that (1) the replacement of freshwater with saline water in permeable fractures generally increases the amplitude of reflections from permeable, connected fractures; and (2) in general, radar reflection-mode data contains more information about fracture properties than transmission-mode data.

Borehole Radar Delineation of the Ventersdorp Contact Reef in Three Dimensions

A large portion of South Africa‘s gold reserve comes from the Ventersdorp Contact Reef (VCR). The VCR is a thin, tabular ore body, generally less than 1.2 m thick. The geometry of the VCR is mainly controlled by slopes, terraces, and faults. The gold is concentrated in palaeo-river channels situated on river terraces and not on the confining terrace risers or slopes, so there is a good correlation between topography and grade. The geophysical problem is to determine the reef geometry before mining begins, in order to plan optimal ore extraction.Borehole radar has already been shown to be a suitable technique for mapping topography on the VCR because of the favourable electrical property contrast between the footwall rocks and the overlying lavas. Here, a forward model is used to resolve the directional ambiguity inherent in low-frequency borehole radar data and to accurately position the target reflection in space. The data from five boreholes within a 200 m square block of ground are combined in a single model to estimate the topography of the ore body within the block. The illumination lines from the radar forward model are positioned in 3D. They are incorporated with other ore-body elevation data from mapping and drilling to improve the geological model of the ore body within the block.The spatial resolution of the geological model is substantially improved with borehole radar. Small faults and other changes in topography are accurately positioned, giving warning of oncoming changes in mining conditions. Terraces and slopes are mapped, facilitating planning of the optimal extraction of the gold within the block. In areas where exploration drilling is primarily undertaken to determine the structure of the ore body rather than its grade, borehole radar provides an effective alternative to conventional drilling.

Sparse Borehole Radar Array Imaging of Orebodies in Three Dimensions

Broadband very high frequency (VHF) borehole radars (BHR) can be used as tactical tools to map orebodies, faults and marker horizons; to identify hazards well in advance of mining and to optimize mine development. In principle, radar profiles and cross-hole scans from fanned arrays of boreholes can be used to synthesize three-dimensional images interferometrically if: borehole trajectories are accurately known; signal-to-noise ratios are adequate; the spatial sampling rate is sufficient; target echoes are well correlated, and if the target space is sufficiently uncluttered to limit the formation of mirages during reconstruction. However, automatic methods of projecting data into three-dimensional image space make stringent demands upon rock homogeneity, translucence and the accuracy of borehole trajectories. These demands can be relaxed by reconstructing objects from parts of geologically plausible 3D primitives such as cylinders, planes and spheres. In this paper we show that a number of useful primitives derive from a common element—a hoop (with semi-circular shell cross-section) of finite radius, centered on the borehole. The parameters defining the hoop’s plane, radius and thickness can be recovered sequentially from borehole radar data to determine the shape of curved 3D reflectors. We show how kinematic mapping can be applied in practice by using data acquired in a Western Australian mine. This survey was shot using [Formula: see text] borehole radars from a fanned array of [Formula: see text] long boreholes. The holes were drilled from a single station at relative angles of [Formula: see text]. Both cross-hole transmission and single-hole reflection surveys were performed. Reflection ranges of [Formula: see text] were achieved in the rocks hosting the deposit. Echo patterns correlated by using kinematic primitives were used to establish the geometry of an arched interface that was located approximately [Formula: see text] above the uppermost edge of the borehole array.

Three-dimensional kinematic imaging of borehole radar data

Broadband, 10-100 MHz, VHF borehole radar (BHR) can be used to map ore bodies, faults and marker horizons, and to identify hazards in advance of mining. Mine boreholes are often drilled in fans. BHR data acquired from these boreholes can be used to reconstruct targets in 3D. Significant progress has been made in using synthetic aperture radar interferometry (InSAR) to reconstruct 3D images from sparse arrays. However, automatic methods of projecting data into 3D image space, such as migration and InSAR, make stringent demands upon rock homogeneity, translucence, and the accuracy of borehole trajectories. These demands can be relaxed by kinematic mapping using geologically plausible 3D primitives such as cylinders, planes, and spheres. In this paper we show how interactive kinematic mapping can be applied in practice to BHR data acquired in a Western Australian mine. BHR data were shot from a fan of ~200 m long boreholes. The holes were drilled from a single station at relative angles of ~5°. Both cross-hole transmission and single-hole reflection surveys were performed. Reflection ranges of 50-60 m were achieved in the rocks hosting the deposit. Echo patterns correlated using kinematic primitives were used to infer the geometry of an interface that was interpreted to be approximately 20 m above the uppermost borehole in the array.

Avaliação da influência da interface solo-ar sobre os dados colhidos em poços com o radar polarimétrico

The main purpose of this paper is to determine the minimum depth value beyond which it is possible to state that borehole-radar data are not significantly affected by the lateral wave and the wave reflected at the ground-air interface. This depth value is of paramount importance for cross-borehole electromagnetic probing. Three methods of solving the problem in question are presented: a numerical method, a graphical method and an analytical method. Five practical examples illustrate the use of the methods. Thus the advantages of the proposed methods over the classical way of solving the same problem are stressed. Thanks to the new methods, it is possible to avoid the unfruitful tasks of collecting and processing data that will be discarded later, when it is discovered that they are significantly degraded by the ground-air interface. The costs of the unproductive tasks may be high in geotomographic electromagnetic surveys owing to the large amount of data they typically involve. Practical examples show that the proposed methods make easier the planning of such surveys and the interpretation of geotomographs. The cause of artifacts in a geotomograph cannot be ascribed to the ground-air interface when the methods indicate that its influence over the borehole-radar data is negligible. The plane of incidence contains both boreholes. An electric dipole, placed in a borehole, can emit transverse magnetic (TM) waves. An axial slot on a conducting cylinder can emit transverse electric (TE) waves from a borehole. The proposed methods apply to both these cases.