Specific features of variations in the characteristics of VLF signals when the lunar shadow propagated along the path during the solar eclipse of March 29, 2006

The effects of the solar eclipse of March 29, 2006, in the signals of ULF radio stations, in the intensity of regular radio noise at frequencies of 0.3–10 kHz, and in the number of atmospherics received in Yakutsk mostly from the west have been considered. The observations were performed using a multichannel parallel analyzer-recorder (11 channels in the frequency band 0.47–8.7 kHz), one-point lightning direction and range finder (0.3–100 kHz), narrow-sector radio noise direction-finder (0.3–10 kHz), recorder of signals from VLF radio stations, and broadband radio noise recorder (0.3–100 kHz). A GPS clock was used to synchronize a recorder of signals from VLF radio stations. The effect was observed in radio signals, radio noise, and number of atmospherics from the direction 270° ± 20° counted off clockwise from the north during the last stage of the eclipse (～ 1100–1200 UT), when the lunar shadow approached the line of the nighttime terminator and obscured part of the signal propagation path. The effect was observed as an enhancement of the received signals by a factor of ～1.2, a factor of ～1.4 increase in the number of atmospherics, and a change in the radio station phase values.     

Airborne VLF Measurements and Variations of Ground Conductivity: A Tutorial

Airborne very low frequency (VLF) data are routinely collected by national agencies and commercial companies together with other passive geophysical measurements of the static magnetic field and radiometric data. The purpose of this paper is to demonstrate that both standard three-component VLF and tensor VLF (TVLF) data contain a lot of useful quantitative and qualitative information about the electrical conductivity distribution in the upper few hundred meters of the crystalline basement. We first give a new derivation of the fundamental transfer functions (the tipper) used in the TVLF technique. We then show that the tipper can be estimated from simultaneous measurements of the wave magnetic fields from at least two transmitters with somewhat different frequencies, and present a simple model by which the maximum error introduced by the difference in frequencies can be found. Single transmitter scalar VLF maps emphasise those conductive structures that have dominant strikes in the direction of the transmitter. Multiple transmitter transfer functions are dependent only upon the underlying conductivity structure. Two dimensional structures can be quantitatively modelled by modern inversion methods developed originally for deep electromagnetic magnetotelluric (MT) soundings. In such cases three-component VLF measurements can be modelled easily upon appropriate rotation of the co-ordinate system to “strike” co-ordinates. Single frequency transfer functions (tippers) have real and imaginary parts that carry information on not only lateral contrasts in conductivity, as usually stated in text books, but, taken together, they provide a robust tool for determining the background conductivity level away from distinct conductors, and they can also be used to discriminate between deep and shallow conductors. Based upon simulations using multi-frequency data, it can be concluded that such a new development would dramatically increase the resolving power of airborne VLF measurements.     

分布式电磁法仪器系统设计及实现

本文介绍一种采用大功率稳流发射、低噪声测量、宽频带接收以及分布式同步等技术,自主研制的分布式多功能电磁法仪器系统.系统包括大功率电磁法发射机、分布式电磁法接收机、磁场传感器、整流源等设备.采用ARM芯片和FPGA芯片进行发射机的整机控制和信号整形发射,采用PC104工控机和FPGA芯片进行接收机的整机控制和信号处理.在人工场源模式下实现了可控源音频大地电磁法（CSAMT）、谱激电法（SIP）和时域激电法（TDIP）等测量功能;在天然场源模式下实现了大地电磁法（MT）、音频大地电磁法（AMT）的测量功能.发射机在满功率发射的情况下连续可靠运行时间大于12 h,接收机的动态范围大于120 dB,接收机可接收信号频率范围是0.001 Hz~32 kHz.通过典型矿区的野外实验和应用,表明本系统的性能总体上达到了国际先进水平.     

SPEAR-induced field-aligned irregularities observed from bi-static HF radio scattering in the polar ionosphere

Experimental results from SPEAR HF heating experiments in the polar ionosphere are examined. Bi-static scatter measurements of HF diagnostic signals were carried out on the Pori (Finland)–SPEAR–St. Petersburg path at operational frequencies of 11,755 and 15,400 kHz and the London–SPEAR–St. Petersburg path at frequencies of 12,095 and 17,700 kHz, using a Doppler spectral method. The SPEAR HF heating facility generates heater-induced artificial field-aligned small-scale irregularities (AFAIs), which can be detected by HF diagnostic bi-static radio scatter techniques at St. Petersburg at a distance of about 2000 km. In accordance with the Bragg condition, HF bi-static backscatters were sensitive to small-scale irregularities having spatial sizes of the order of 9–13 m across the geomagnetic field line. The properties and behaviour of AFAIs have been considered in the winter and summer seasons under quiet magnetic conditions and under various status of the polar ionosphere (the presence of “thick” and “thin” sporadic Es layers, different structures of the F2 layer). The experimental results obtained have shown that AFAIs can be excited in the F as well as in the E regions of the polar ionosphere. The excitation of a very intense wide-band spectral component with an abrupt increase in the spectral width up to 16–20 Hz has been found in the signals scattered from striations. Along with a wide-band component, a narrow-band spectral component can be also seen in the Doppler sonograms and in the average spectra of the signals scattered from the SPEAR-induced striations. AFAIs were excited even when the HF heater frequency was up to 0.5 MHz larger than the critical frequency. A simulation of the ray geometry for the diagnostic HF radio waves scattered from AFAIs in the polar ionosphere has been made for the geophysical conditions prevailing during experiments carried out in both the winter and summer seasons.     

An application of waveform denoising for microseismic data using polarization–linearity and time–frequency thresholding

Noise suppression or signal‐to‐noise ratio enhancement is often desired for better processing results from a microseismic dataset. In this paper, a polarization–linearity and time–frequency‐thresholding‐based approach is used for denoising waveforms. A polarization–linearity filter is initially applied to preserve the signal intervals and suppress the noise amplitudes. This is followed by time–frequency thresholding for further signal‐to‐noise ratio enhancement in the S transform domain. The parameterisation for both polarization filter and time–frequency thresholding is also discussed. Finally, real microseismic data examples are shown to demonstrate the improvements in processing results when denoised waveforms are considered in the workflow. The results indicate that current denoising approach effectively suppresses the background noise and preserves the vector fidelity of signal waveform. Consequently, the quality of event detection, arrival‐time picking, and hypocenter location improves.     

基于LWPC和IRI模型的NWC台站信号传播幅度建模分析

频率为3~30 kHz的甚低频（VLF,Very Low Frequency）电磁波具有波长长、传播距离远的特点,能够沿地面-低电离层波导进行传播,在通信、导航等许多领域都被广泛应用.基于波导模理论的长波传播模型（LWPC,Long-Wavelength Propagation Capability）能够用于计算甚低频波的传播路径及幅度,进而研究耀斑、磁暴、地震等事件对电离层的扰动.本文利用国际电离层参考模型（IRI,International Reference Ionosphere）对LWPC中电子密度和碰撞频率进行改进,并将模拟结果与武汉大学VLF接收机实际观测到的NWC （North West Cape）台站信号幅度进行比较分析,结果表明改进后LWPC模型得到的幅度及变化趋势与实际值更加接近.LWPC模型给出的电子密度与IRI模型得到的电子密度在日间基本一致,但是在夜间存在差异,造成夜间部分区域NWC台站信号幅度的差异性,验证了电离层电子密度对于VLF信号传播具有的重要影响.传播路径上的晨昏变化也可以引起VLF信号幅度分布的突变,在日出和日落时间段内存在明显的过渡区域.基于IRI模型的LWPC,改善了VLF电波传播过程的预测分析效果,提供了一种长波导航通信质量的评估方法.     

The LF radio anomaly observed before the Mw = 6.5 earthquake in Crete on October 12, 2013

On October 12, 2013, an earthquake with M_w = 6.5 occurred in the southern Hellenic Arc, approximately 20 km off the west coast of Crete. The main shock, the focal depth of which is on the order of 40 km, was followed by aftershocks felt in the nearby cities and villages, although the aftershock sequence was poor. The epicentre was located at approximately 60 km from a radio receiver in Crete (CRE), which belongs to the European VLF/LF Radio Network. Several days before the earthquake, a clear disturbance occurred in one of the ten radio signals that the CRE receiver sampled. The disturbance, which can be considered an anomaly, appeared in the 216 kHz radio signal radiated by the Radio Monte Carlo (MCO) transmitter. The radio path MCO-CRE crossed directly over the epicentre area of the aforementioned earthquake. In this work, we present a detailed analysis of the MCO signal anomaly using spectral tools. We also investigate the behaviour of other radio signals sampled by the CRE receiver and consider other possible causes of disturbances on the MCO radio signal. We conclude that the disturbance in the MCO radio signal is a convincingly possible precursor of the earthquake in Crete. Emission of electromagnetic waves with a frequency band that includes 216 kHz from the focal zone of the earthquake can provide a satisfactory explanation of the radio anomaly.     

Low‐frequency passive seismic experiments in Abu Dhabi,United Arab Emirates: implications for hydrocarbon detection

Low‐frequency passive seismic experiments utilizing arrays of 3‐component broadband seismometers were conducted over two sites in the emirate of Abu Dhabi in the United Arab Emirates. The experiments were conducted in the vicinity of a producing oilfield and around a dry exploration well to better understand the characteristics and origins of microtremor signals (1–6 Hz), which had been reported as occurring exclusively above several hydrocarbon reservoirs in the region. The results of the experiments revealed that a strong correlation exists between the recorded ambient noise and observed meteorological and anthropogenic noises. In the frequency range of 0.15–0.4 Hz, the dominant feature is a double‐frequency microseism peak generated by the non‐linear interactions of storm induced surface waves in the Arabian Sea. We observed that the double‐frequency microseism displays a high variability in spectral amplitude, with the strongest amplitude occurring when Cyclone Gonu was battering the eastern coast of Oman; this noise was present at both sites and so is not a hydrocarbon indicator. Moreover, this study found that very strong microtremor signals in the frequency range of 2–3 Hz were present in all of the locations surveyed, both within and outside of the reservoir boundary and surrounding the dry exploration well. This microtremor signal has no clear correlation with the microseism signals but significant variations in the characteristics of the signals were observed between daytime and nighttime recording periods that clearly correlate with human activity. High‐resolution frequency‐wavenumber (f‐k) spectral analyses were performed on the recorded data to determine apparent velocities and azimuths of the wavefronts for the microseism and microtremor events. The f‐k analyses confirmed that the double‐frequency microseism originates from wave activity in the Arabian Sea, while the microtremor events have an azimuth pointing towards the nearest motorways, indicating that they are probably being excited by traffic noise. Results drawn from particle motion studies confirm these observations. The vertical‐to‐horizontal spectral ratios of the data acquired in both experiments show peaks around 2.5–3 Hz with no dependence on the presence or absence of subsurface hydrocarbons. Therefore, this method should not be used as a direct hydrocarbon indicator in these environments. Furthermore, the analyses provide no direct evidence to indicate that earthquakes are capable of stimulating the hydrocarbon reservoir in a way that could modify the spectral amplitude of the microtremor signal.     

Surface wave attenuation based polarization attributes in time-frequency domain for multicomponent seismic data

In the paper, we propose a surface wave suppression method in time-frequency domain based on the wavelet transform, considering the characteristic difference of polarization attributes, amplitude energy and apparent velocity between the effective signals and strong surface waves. First, we use the proposed method to obtain time–frequency spectra of seismic signals by using the wavelet transform and calculate the instantaneous polarizability at each point based on instantaneous polarization analysis. Then, we separate the surface wave area from the signal area based on the surface-wave apparent velocity and the average energy of the signal. Finally, we combine the polarizability, energy, and frequency characteristic to identify and suppress the signal noise. Model and field data are used to test the proposed filtering method.     

多重自相关在微弱低频电磁信号检测中的应用

低频电磁信号在地下电性结构探测中具有重要作用,经过长距离传播,信号衰减,可能被淹没于噪声中.利用多重自相关检测方法,对微弱低频电磁信号进行检测,并与自适应滤波法和离散小波变换法进行对比.利用Matlab对3种算法进行仿真研究.结果表明,多重自相关法能更好抑制噪声,有效检测微弱信号,检测性能优于小波变换法和自适应滤波法.     

Seismic directional random noise suppression by radial‐trace time–frequency peak filtering using the Hurst exponent statistic

Radial‐trace time–frequency peak filtering filters a seismic record along the radial‐trace direction rather than the conventional channel direction. It takes the spatial correlation of the reflected events between adjacent channels into account. Thus, radial‐trace time–frequency peak filtering performs well in denoising and enhancing the continuity of reflected events. However, in the seismic record there is often random noise whose energy is concentrated in certain directions; the noise in these directions is correlative. We refer to this kind of random noise (that is distributed randomly in time but correlative in the space) as directional random noise. Under radial‐trace time–frequency peak filtering, the directional random noise will be treated as signal and enhanced when this noise has same direction as the signal. Therefore, we need to identify the directional random noise before the filtering. In this paper, we test the linearity of signal and directional random noise in time using the Hurst exponent. The time series of signals with high linearity lead to large Hurst exponent value; however, directional random noise is a random series in time without a fixed waveform and thus its linearity is low; therefore, we can differentiate the signal and directional random noise by the Hurst exponent values. The directional random noise can then be suppressed by using a long filtering window length during the radial‐trace time–frequency peak filtering. Synthetic and real data examples show that the proposed method can remove most directional random noise and can effectively recover the reflected events.     

Is a type of low frequency ULF magnetic disturbance a kind of earthquake precursors?

A type of relatively low frequency ULF magnetic disturbance has been generally observed by the magnetic induction-meter since it’s operation at Baijiatuan observatory in Beijing. As the amplitudes and frequencies of such disturbance signals change with time and are different from the disturbances produced by some man-made magnetic sources, therefore they are usually taken as a kind of earthquake precursors for ULF electro-magnetic emission. However, a comparative analysis with magnetic storms on the magnetogram obtained by variometerat the same station indicates that such signals always occur in the period of magnetic storm occurrence and so far we have not found the cases where such signals occur with no magnetic storms, therefore this kind of ULF magnetic disturbances should not be taken as a ULF earthquake precursors of the electromagnetic emission at present. The main features of such signals are as follows: the signals occur discontinuously during storms and the duration of each time section with such signals and the occurrence rates of the sections and the disturbance amplitudes are usually related to the types and intensities of storms. The wave form characteristics of the ULF disturbance are also related to the types of storms. Generally, the amplitudes and durations for SC storms are stronger and longer respectively than those for GC stroms, and if a storm is with largerK index, then relatively large amplitudes and higher rates and longer durations as well as variable frequencies will be observed andvice versa. Most of the start time and the time section with strongest disturbance recorded by the ULF unit and by the variometer are not consistent with each other, and the same one is only about 44%. The periods and intensities of such disturbance signals are in the ranges of few to several seconds and 0.04–8 nT respectively. The predominant frequency is about 0.06 Hz and certain energies are also distributed on the harmonic frequencies. Contribution No. 95A0063, Institute of Geophysics, SSB, China.     

The structure of extremely low frequency field reflected by the ionosphere

The amplitudes of variations in the magnetic and electric fields at extremely low frequencies were studied on the Kola Peninsula during a five-day-long experiment under different geophysical conditions. These studies demonstrated that the influence of the ionosphere is distinguishable at frequencies of <10 Hz and the structure of field variations reflected by the ionosphere is similar to that of the transverse wave. It is established that the ratio between semimajor axes of polarization ellipses of the electric and magnetic fields is independent of the state of the ionosphere and is likely determined by the deep structure of the crust beneath the site of the experiment.     

Effect of the ionosphere on electromagnetic waves from ground-based emitter in the frequency band 1–10 Hz

The results of the first experimental reception of electromagnetic signals, emitted by the ground antenna at frequencies of 1–10 Hz in the transition zone in the case when the distance from the transmitter is comparable with the equivalent waveguide height, are presented. The works were performed episodically in 2006, in different seasons and at different time of day, under quiet geomagnetic conditions. A pronounced effect of the state of the ionosphere on the received signal value was found out at distances about 80 km from the transmitter; in this case the seasonal manifestations were more substantial than the daily ones. The obtained results indicate that it is necessary to take into account the effect of the ionosphere on the wave amplitude in the transition zone, when the electromagnetic sounding of the Earth’s structure is performed at frequencies of 1–10 Hz, and that it is reasonable to use artificial waves in this band to perform quasi-vertical sounding of the ionosphere.     

Using GPS–GLONASS–GALILEO data and IRI modeling for ionospheric calibration of radio telescopes and radio interferometers

VHF and HF radio signals are widely used to observe the Sun and pulsars. Nowadays, large low-frequency radio astronomical arrays (LOFAR, 30–240 MHz; MIRA, 80–300 MHz) are being constructed to record radiation of pulsars at the maximum distance. registration of the solar radio emission intensity at fixed frequencies and in the spectral VHF band is very important along with other methods of monitoring of coronal mass ejections. Interpreting radio astronomical data is known to be necessary to take into account possible distortions of these signals in the Earth ionosphere. However, in contrast to modern navigation systems (Global Position System (GPS), GLObal NAvigation Satellite System (GLONASS), GALILEO), in which a very accurate reconstruction of ionosphere parameters is a built-in function, in present-day radio astronomy a retrieve of ionosphere transfer characteristics has not been appropriately worked out. This collides with increasing requirements to accuracy of the analysis of a radio emission amplitude profile and to the angular and polarizing resolution of radio telescopes of new generation (LOFAR, SKA, etc.). We have developed a method and software to calculate the ionosphere rotation measure (RM) and dispersion measure (DM). We used the ionosphere model IRI-2001, magnetic field model IGRF-10, and the ionosphere total electron content values obtained from GPS measurements. The obtained values of DM and RM were recalculated into characteristics of the phase delay, Faraday amplitude modulation, and polarization changes. We calculated ones for different levels of geomagnetic activity as well as different angular positions of radio sources. Our main idea is to use a signal of navigation satellites (GPS, GLONASS, GALILEO) as a testing signal from a “reference” source located at minimal angle distance from a source studied. Our project allows development of methods and systems of ADAPTIVE RADIO ASTRONOMY, adaptive to the non-uniform and non-stationary ionosphere, by analogy with known systems of adaptive optics intended to adapt optical telescopes to varying conditions of the optically non-uniform and non-stationary troposphere.     

Quasi-periodic very low frequency emissions, very low frequency chorus, and geomagnetic Pc4 pulsations (Event on April 3, 2011)

The results of an analysis of ground-based observations of very low frequency (VLF) emissions in Scandinavia (L ∼ 5) in April 2011 are discussed. A detailed study is conducted of an non-typical event (April 3, 2011) of simultaneous generation of VLF chorus at frequencies below 3 kHz and quasi-periodic VLF emissions (QP) in the band of 4–6 kHz, which were not discrete emissions but consisted of separate short (about 20 s) bursts of hiss. It is shown that these emissions were mainly characterized by right-hand polarization, which indicates the location of the exit point of waves from the ionosphere near the point of ground observations. Based on an analysis of the spectral characteristics of emissions, it is concluded that the generation regions of chorus and QP emissions were located at different L shells. The appearance of QP emissions coincided with the excitation of resonance geomagnetic pulsations of the Pc4 range in the magnetosphere with a period that was close to the quasi-period of repetition of spectral forms in QP emissions. However, based on the available data, it is not possible to conclude that these geomagnetic pulsations caused the quasi-periodic generation of bursts of VLF hiss. The time shift between the peaks of QP and geomagnetic pulsations was inconsistent and varied from one burst of hiss to another. It is suggested that the discussed QP emissions were a result of the development of self-oscillations in the Earth’s radiation belts.     

Daytime VLF emissions at the Sodankyla Observatory (L ∼ 5.3) at the front of high-speed solar wind streams

We discuss the results of an analysis of digital high-sensitivity ground-based observations of very low frequency (VLF) emissions, carried out in Northern Finland (L = 5.3) in May–June 2012. During this period of time, we found that three high-speed solar wind streams approached the Earth’s magnetosphere and at the front of these fluxes long-lasting intense daytime bursts of VLF emissions were generated in two frequency bands: above and below ～2.5 kHz. At frequencies above ～2.5–3.0 kHz, there were VLF hiss waves, the temporal structure of which consisted of a quasi-periodic sequence of separate stronger spots of noise signals. The low-frequency band was represented by chorus waves, superimposed on intense hiss emissions at frequencies below ～1.5 kHz. The high-frequency (f > 2.5 kHz) waves were elliptic and, predominately, left-hand polarized and the low-frequency waves were right-hand polarized. It was supposed that high-frequency VLF hiss waves were generated at L < 5 and VLF chorus waves were generated at L > 5. We discuss a possible scenario of the generation and propagation of the VLF emissions observed.     

Airborne EM skin depths

Skin depth is an electromagnetic (EM) scale length that provides a measure of the degree of attenuation experienced by a particular frequency of an EM system. As has been discussed in the literature, skin depth is not a complete measure of the depth of investigation, but the two may be related. Frequency‐domain airborne EM systems employ pairs of transmitter and receiver coils that use a frequency range from several hundred hertz to over 100 kHz. For elevated dipoles, both geometrical and frequency‐dependent attenuation of the induced fields must be considered. For airborne EM systems it is possible to define a skin depth based only on the electric field induced by the transmitter. A vertical decay scale length, here defined from the at‐surface position of maximum electric field, enables the same skin‐depth estimate to be obtained for both cases of vertical and horizontal dipolar excitation. Such dipolar skin depths associated with towed‐bird and fixed‐wing airborne systems are studied in relation to frequency, conductivity and sensor elevation. Dipolar skin depths are found to be much smaller than their plane‐wave counterparts except at high frequency (>50 kHz) and in combination with high conductivity. For the majority of airborne systems the influence of altitude on skin depth is highly significant. Dipolar skin depths increase with increasing sensor elevation. Low frequencies display the greatest sensitivity. At low elevation (<40 m), geometrical attenuation dominates the behaviour of the skin depth. The study indicates that typical low‐altitude airborne surveys provide vertically compact assessments of subsurface conductivity, well suited to near‐surface, environmental applications.     

Frequency dependent polarization analysis of ambient seismic noise recorded at a broadband seismometer in the central United States

We present a new approach to polarization analysis of seismic noise recorded by three-component seismometers.It is based on statistical analysis of frequency-dependent particle motion properties determined from a large number of time windows via eigenanalysis of the 3-by-3,Hermitian,spectral covariance matrix.We applied the algorithm to continuous data recorded in 2009 by the seismic station SLM,located in central North America.A rich variety of noise sources was observed.At low frequencies (0.05 Hz) we observed a tilt-related signal that showed some elliptical motion in the horizontal plane.In the microseism band of 0.05-0.25 Hz,we observed Rayleigh energy arriving from the northeast,but with three distinct peaks instead of the classic single and double frequency peaks.At intermediate frequencies of 0.5-2.0 Hz,the noise was dominated by non-fundamental-mode Rayleigh energy,most likely P and Lg waves.At the highest frequencies (3 Hz),Rayleigh-type energy was again dominant in the form of Rg waves created by nearby cultural activities.Analysis of the time dependence of noise power shows that a frequency range of at least 0.02-1.0 Hz (much larger than the microseism band) is sensitive to annual,meteorologically induced sources of noise.     

Modelling and straight‐ray tomographic imaging studies of cross‐hole radio‐frequency electromagnetic data for mineral exploration

Radio‐frequency electromagnetic tomography (or radio imaging method) employs radio‐frequency (typically 0.1–10 MHz) electromagnetic wave propagation to delineate the distribution of electric properties between two boreholes. Currently, the straight‐ray imaging method is the primary imaging method for the radio imaging method data acquired for mineral exploration. We carried out synthetic studies using three‐dimensional finite‐element modelling implemented in COMSOL Multiphysics to study the electromagnetic field characteristics and to assess the capability of the straight‐ray imaging method using amplitude and phase data separately. We studied four sets of experiments with models of interest in the mining setting. In the first two experiments, we studied models with perfect conductors in homogeneous backgrounds, which show that the characteristics of the electromagnetic fields depend mainly on the wavelength. When the borehole separations are less than one wavelength, induction effects occur; conductors with simple geometries can be recovered acceptably with amplitude data but are incorrectly imaged on the phase tomogram. When the borehole separations are longer than two wavelengths, radiation effects play a major role. In this case, phase tomography provides images with acceptable quality, whereas amplitude tomography does not provide satisfactory results. The third experiment shows that imaging with both original and reciprocal datasets is somewhat helpful in improving the imaging quality by reducing the impact of noise. In the last experiment, we studied models with conductive zones extended into the borehole plane with different lengths, which were not accurately recovered with amplitude tomography. The experiment implies that it is difficult to determine the extent of a mineralised zone that has been intersected by one of the boreholes. Due to the large variation of the wavelength in the radio‐frequency range, we suggest investigating the local electric properties to select an operating frequency prior to a survey. We conclude that straight‐ray tomography with either amplitude or phase data cannot provide high‐quality imaging results. We suggest using more general methods based on full electromagnetic modelling to interpret the data. In circumstances when computational time is critical, we suggest saving time by using either induction methods for borehole separations less than one wavelength or wave‐based methods (only radiation fields are considered) for borehole separation larger than two wavelengths.