Late Time Response Research Articles

Simple estimation of late-time response for radar target identification

This paper proposes a conceptual technique for the simple estimation of the late-time response for radar target identification without a priori knowledge of the target geometry or orientation. In the proposed technique, the cross correlation between the backscattering response and transmitted wave is performed. Peaks will occur in the cross-correlation output when the transmitted wave is aligned with the same features in the received backscattering response. The commencement of the late-time response corresponds with the peak resulting from a superimposed pattern between the transmitted wave and late-time response. The matrix pencil method was exploited in order to extract the poles from the received backscattering response. Several simulations were performed to evaluate the performance of the proposed estimation technique. The simulation results confirmed the superiority of the proposed approach. In the special case of the transmission with a monocycle pulse, the commencement of the late-time response can be automatically selected from the third peak of the resulting cross-correlation output.

New Integral Formulas for the Elements of the Transient Ground Resistance Matrix of Multiconductor Lines

This paper proposes new formulas for the calculation of the transient ground resistance matrix elements based on the analytical solution of the inverse Laplace transform of the Sunde's logarithmic equation. The new formulas solve the computational issues in the representation of the transition between early time and late time response of the line that arises when adopting the expressions already available in the literature. The formulas are adopted for the calculation of the per-unit length voltage drop due to the transient ground resistance in a multiconductor overhead line and the results match those provided by the numerical inverse Fourier transform of Sunde's logarithmic equation.

Poles Extraction of Radar Target Based on Narrowband Frequency Response Approximation

Pole is the most important feature of radar target in resonance region, and it is attitude insensitive. The poles extraction requires UWB (ultra-wideband) late-time response signal or frequency response data, which is difficult to achieve through general equipment. This paper proposed a method for extracting poles from narrowband data. At first, the frequency response equation of the resonance region was derived. Then, the equation parameters were solved by the interior point method through the narrowband data. At last, the zeros of the transfer function denominator were calculated. FEKO simulation data test verifies that the method has extrapolation ability.

Identification of massive sulphide targets using the galvanic source EM signal from a sub-audio magnetic survey at the Far South project, Western Australia

The Far South project is located 5 km south along strike from the Deep South mine, where gold mineralisation is commonly associated with semi-massive pyrrhotite and pyrite. Data from a sub-audio magnetic (SAM) survey, set up in galvanic configuration, were acquired over the project principally to map stratigraphy and structure using the on-time magnetometric conductivity (MMC) and total magnetic intensity (TMI) responses. The off-time galvanic source electromagnetic (GSEM) data were subsequently extracted from the raw data and four late-time anomalous responses were identified. Two of these responses are strong late-time anomalies up to 350 m in strike length, clearly visible in the 60 ms time gate, and the remaining two are weaker mid-time, subtler and less diagnostic responses. Modelling of the GSEM responses was attempted using conventional electromagnetic (EM) modelling techniques, where certain assumptions had to be made for the survey geometry and current path. The assumed current path has an effect on the coupling of the inducing current with the target, so in order to validate the assumptions and GSEM modelling results, follow-up moving-loop and fixed-loop electromagnetic (MLEM and FLEM) surveys were required. These surveys confirmed well-defined conductive responses over all four follow-up areas. Modelling of the GSEM data over the two strongest anomalies is in good agreement with modelling of the MLEM/FLEM data. Modelling at a third target showed some similarity to results obtained by modelling the follow-up EM survey data, and one target was not defined clearly enough in the GSEM data to allow modelling and had to be modelled solely on the follow-up FLEM data. The results of these efforts confirm the ability to identify and produce a reasonably useful model of strong conductive targets from SAM GSEM data, indicating that the rapid acquisition SAM system is an effective tool for quick reconnaissance in sulphide exploration. Subsequent drilling identified sulphide mineralisation at all four conductors.

Representation of Electromagnetic Responses in Time Domain Using State-Space System Identification Method

This paper presents a control system methodology based on system identification (SI) to derive state matrices and system dynamics for the extrapolation of simulated time-domain electromagnetic (EM) responses using both input and output data. The method is applied to model transient responses for a dielectric resonator and a wideband slot antenna computed by the finite-integration method using non-Gaussian excitation pulses typically employed in digital signals. Using state-space SI formulation, a compact representation of the output time-domain signal is derived, including the early-time transient responses, and its numerical implementation depicts no instability in late-time responses plotted well beyond the steady state. Furthermore, all the system poles are found to lie within the unit circle in the complex plane. Excellent model corroboration is achieved with independently computed frequency domain data or measured results using modest computer resources.

The superparamagnetic response of transient AEM data

SUMMARY Several lines of VTEM data flown at different system elevations across a known sulphide body and surface cover with elevated superparamagnetic (SPM) properties were analysed with MAXWELL, layered-earth inversions, LEROIAIR and LEROI. The SPM material was modelled with frequency-dependent magnetic susceptibilities at shallow depth. Due to their slow late-time decay, SPM responses can be confused with responses of deep conductors and vice versa. Depending on the parameter weighting used, 1D inversions model all late-time responses as deep conductive material or as surficial SPM material. However, the joint 1D inversion of data acquired at different system elevations manages to recover a deep conductor from the sulphide anomaly and elevated SPM values at the location of the SPM response. For the modelled parameters, the VTEM data sets from two elevations (at 70 and 80 m) require a vertical separation of about 10 m to allow for the discrimination between the SPM and sulphide responses. For lower system elevations, less sensor separation is necessary due to the strong gradient of the SPM response. We suggest that two vertically separated receivers could be used to measure the AEM gradient and depending on the flying height of the transmitter, the vertical offset of the receivers should be between 2 and 40 m.

Accurate Extraction of Early-/Late-Time Responses Using Short-Time Matrix Pencil Method for Transient Analysis of Scatterers

We present a new efficient technique for accurately distinguishing the early-time and late-time modes of the transient response from multiscatterer targets. In multiple resonant-based scatterers where the complex natural resonances (CNRs) are used as the ID of the target, the early-time response of the second scatterer might overlap with the late-time of the first, introducing some difficulties in the detection of the second scatterer and additionally, identification of the first one. In such circumstances, the separation of the late-time CNRs (ID of the scatterer) from the early-time poles (scattering centers) is challenging. The smaller the distance between two scatterers, the more difficult the detection process is. Here, by sliding a window along the time axis and applying short-time matrix pencil method (STMPM), the poles corresponding to the early-time and late-time responses are distinguished. It will be shown that when the early-time response is located at the center of the sliding window, its CNRs are located on the imaginary axis of the pole diagram. By monitoring the zero-crossing points in time damping-factor diagram, the locations of the early-time responses (or equivalently scattering centers) can be detected. Some scenarios are discussed, simulated, and confirmed by measurement results to show the effectiveness of the proposed technique in practical applications.

Modelling the superparamagnetic response of AEM data

Several lines of VTEM data flown at different system elevations across a known sulphide body and surface cover with elevated superparamagnetic (SPM) properties were analysed with MAXWELL, layered-earth inversions (LEI), LEROIAIR and LEROI. The SPM material was modelled with frequency-dependent magnetic susceptibilities at shallow depth.Due to their slow late-time decay, SPM responses can be confused with responses of deep conductors and vice versa. Depending on the parameter weighting used, 1D inversions model all late-time responses as deep conductive material or as surficial SPM material. However, the joint 1D inversion of data acquired at different system elevations manages to recover a deep conductor from the sulphide anomaly and elevated SPM values at the location of the SPM response. For the modelled parameters, the VTEM datasets from two elevations (at 70 and 80 m) require a vertical separation of ~10 m to allow for the discrimination between the SPM and sulphide responses. For lower system elevations, less sensor separation is necessary due to the strong gradient of the SPM response.Following the determination of SPM parameters from VTEM survey data, these values were used to hypothesise the SPM response for a range of system geometries, showing that larger transmitter loops and larger offsets between transmitter and receiver loops reduce SPM effects. We suggest that two vertically separated receivers could be used to measure the airborne electromagnetic (AEM) gradient and depending on the flying height of the transmitter, the vertical offset of the receivers should be between 2 and 40 m. If gradient data are not collected, then EM responses measured during the transmitter on-time and x-component data, if available, might offer some model discrimination.Whereas synthetic data of the examined helicopter TEM systems VTEM, AEROTEM and HELITEM indicate a fairly high sensitivity to SPM effects, fixed-wing MEGATEM data are much less affected, due to the higher transmitter elevation and large transmitter loop – receiver separation. SPM effects on data of frequency-domain systems such as the RESOLVE system are also small.

Lightning Electromagnetic Fields and Their Induced Currents on Buried Cables. Part II: The Effect of a Horizontally Stratified Ground

We use a finite element method (FEM) to evaluate the effect of a horizontally stratified two-layer ground on underground lightning electromagnetic fields and their induced currents on the shield of buried cables. It is shown that the azimuthal component of the magnetic field in the upper soil layer is affected by the soil stratification only when this layer is more conductive than the lower soil layer. On the other hand, inside the lower soil layer, this component is always affected by the soil stratification. The vertical electric field in the upper soil layer is mainly determined by the conductivity of the same layer in particular at close observation points. However, this component inside a more conductive lower soil layer is identical to that corresponding to a homogeneous soil with the same property of the lower soil layer. The horizontal electric field inside a stratified ground always takes values in between the electric fields corresponding to one-layer homogenous grounds. We also present a comparison with available experimental data on induced currents of a shielded buried cable and show that, in agreement with recent studies, taking the soil stratification into account allows to improve the late-time response of the induced currents. We also show that the horizontal stratification of soil may result, in some cases, in an enhancement of the induced currents with respect to the case of a homogeneous ground characterized by the electrical properties of either of the two layers.

A Space–Time–Frequency Anticollision Algorithm for Identifying Chipless RFID Tags

In this paper, the short-time matrix pencil method (STMPM) is efficiently employed as a time-frequency analysis methodology for the identification of multiple chipless radio-frequency identification (RFID) tags presented in the main beam of the reader antenna. Each tag represents a unique ID in the backscattered signal. Here, the data are embedded on the tag as the complex natural resonances (CNRs) of the structure. In the cases where more than one tag exist in the main beam of the reader antenna, the frequency-domain response is not sufficient to successfully distinguish the IDs of the tags. By applying the STMPM technique to the time-domain response, the poles and their residues are obtained at each snapshot of time. In order to obtain the exact values of the turn-on times of the poles, which are the roundtrip time of the tags to the antenna, high resolution in the time domain is needed which deteriorates the resolution in the frequency domain. Using mathematical descriptions, a simple formulation is derived which exhibits a duality between late-time response in the time domain and early-time response in the frequency domain. Therefore, a space-frequency representation can be simply obtained from the application of the short-frequency matrix pencil method (SFMPM) to the frequency-domain response of the tags. By possessing the exact values of the turn-on times and pole/residues of the tags, their backscattered responses can be reconstructed and associated IDs can be retrieved. With the proposed method in this paper, some scenarios are studied and discussed. As an experimental result, three 3-bit tags with different IDs (ID1:101, ID2:111, and ID3:011) are fabricated and placed away from each other. Exploiting the presented technique, the IDs of the tags and their locations are successfully distinguished. The simulation results are validated by the measured ones. The ranging error obtained by the proposed method is less than 1 cm.

Complex-Natural-Resonance-Based Design of Chipless RFID Tag for High-Density Data

Each scatterer responds to the incident wave in a unique manner. The backscattered signal from the scatterer is affected by two distinct phenomena: early-time response which emanates from the scattering centers of the scatterer and late-time response which is the summation of the complex natural resonances (CNRs) with some weighting residues. Based on the singularity expansion method (SEM), CNRs are aspect-independent parameters which include some structural information of the scattering target. Based on this fact, the data can be embedded on the RFID tag as CNRs associated to the structural parameters. In this paper, a low-profile chipless RFID tag as a suitable device for high-density data is introduced. By incorporating several slots on the chipless RFID tag, the data can be encoded as CNRs on the structure. The designed RFID tag operates in the frequency band of 3.1-10.6 GHz. The data are decoded in the reader by applying short-time matrix pencil method (STMPM) to the transient backscattered response of the tag. In addition, the turn-on times, damping factors, and resonant frequencies of the poles are distinguished. After studying the effects of various parameters of the tag, a 24-bit tag is designed on the area of 24 × 24 mm2, simulated and measured. The measurement is performed in the frequency domain and, after applying inverse fast Fourier transform (IFFT), STMPM is employed to extract the signature of the tag. The tag is successfully read remotely via its scattered fields.

On the Electromagnetic Susceptibility of Hot Wire-Based Electroexplosive Devices to RF Sources

We present an analysis of the thermal response of a hot-wire electroexplosive device (EED) excited with different transient signals. First-order and second-order analytical models to calculate the thermal response of an EED are assessed taking as reference numerical simulations obtained using ANSYS. For the early-time response, when the time is much smaller than the thermal constant of the EED, the best approach corresponds to a first-order differential model in which the thermal capacitance is calculated with short-pulse excitations. A linear simplification to calculate the maximum temperature due to short excitations is also shown to be adequate. On the other hand, the most appropriate model for the late-time response is a second-order model. The models are used to assess the electromagnetic susceptibility of a wired EED for different electromagnetic pulsed environments. Radiated signals produced by a mesoband radiator, two types of radars, and a hyperband radiator are considered. The radar signal proved to be the most disturbing source because of its highest duty cycle and its flat spectral response around a specific frequency. Even the temperature firing threshold can be exceeded with the radiated field produced by a radar of 200 kW of output power located at a distance of 5 m.

Vertical electric source in transient marine CSEM: Effect of 3D inhomogeneities on the late time response

The time-domain marine controlled-source electromagnetic method, based on injection of electric currents into the sea via a vertical cable and measurements of the transient vertical electric field, is characterized by high sensitivity to resistive reservoirs and robustness with respect to distorting effects of lateral heterogeneities. This is due to the fact that a vertical electric dipole induces in a stratified medium only a transverse magnetic (TM) field. In addition, the vertical electric field is not directly contributed by the transverse electric (TE) component of the field scattered by heterogeneities. Nevertheless, a closed-form solution shows that the first order effect of a lateral heterogeneity displays itself as a parallel shift of the late time response curves. The effect is also observed in 3D responses evaluated by numerical solutions of the integral equation of the modified iterative dissipative method. Moreover, in “favorable” conditions, scattering on heterogeneities may change the law of the field decay. The parallel shift of the late time curves is caused by vertical polarization of the scatterer, while its horizontal polarization leads to an abnormally fast decay of the vertical electric field. The latter effect, observed against the background of the general decay of the free electromagnetic field, can be associated with “energy channeling from the TM to TE field.” Neither of the effects necessarily deteriorates the method sensitivity. Unlike the vertical electric field, the horizontal electromagnetic field is contributed by the scattered TE field. As a result, the abnormally fast decay of the vertical electric field is accompanied by an abnormally slow decay of the horizontal components. The transient horizontal electric field may become almost insensitive to the reservoir resistivity. In addition to unrealistically harsh requirements to the transmitter tilt, this may render accurate measurements of the horizontal electromagnetic field of a vertical electric bipole not feasible.

First Application of Progressing Cavity Pumps for Appraisal Well Testing in the Ugandan Albertine Graben Basin

Summary Over 28 production well tests and two interference tests have been conducted in the Albertine Graben basin in Uganda since the first oil discovery in 2006. The Uganda crude is moderately viscous, and low-gas/oil ratio (GOR), and later discoveries were mostly in shallow lower-energy systems. In addition, Uganda crude has relatively high pour points and wax appearance temperatures (WATs), which result in flow-assurance challenges during production testing operations. Production challenges brought about by the low energy scenario are offset by high formation permeability (mostly in the multi-Darcy range). Field X was discovered in 2008 by Well X-1, and subsequently appraised by several wells. Zone 1 of Well X-1 was production tested under natural flow without a rig. Although hydrocarbons were brought to surface successfully during the testing operation, the maximum flow rate achieved was approximately 262 BOPD, and a stable flow rate could not be sustained. Fluid-segregation effects distorted the early-time data. In addition, flow-assurance challenges prevented acquisition of late-time response from the pressure buildup (PBU). Therefore, important dynamic information, particularly in regard to flow barriers, was not obtained. A change in completion philosophy was proposed after detailed analysis of the test results. Well X-1 was retested with a new completion design incorporating real-time surface-readout (SRO) pressure and temperature monitoring capability, a hydraulically controlled inflow control valve (ICV), a progressing cavity pump (PCP), and a surface-controlled electrical heat trace system. High-quality well-test data was acquired, and the real-time data was used extensively throughout operations to optimize flow and buildup periods and well operating envelopes. Several challenges were faced during the first completion run, leading to the use of the contingent insert PCP system to retest Zone 1. This paper highlights the first application of a PCP completion in Uganda and describes the challenges faced during the completion and well-testing operation. The experience from Well X-1 has demonstrated that high-quality exploration and appraisal well-test data can be acquired using a PCP completion. Similar well-completion designs will be used in Uganda for appraisal of other discoveries.

Identification of Multiple Objects Using Their Natural Resonant Frequencies

A methodology for identification of multiple objects using their natural resonant frequencies is presented. This methodology is then applied to generate a library of poles of various objects in the frequency domain and compare them to the computed natural poles of unknown objects using the typical transient temporal late time response in the time domain. The Cauchy method is applied directly in the frequency domain to extract the Singularity Expansion Method (SEM) poles, and thus a library of poles of various perfectly conducting objects (PEC) is generated. The Matrix Pencil (MP) method is then applied to the late time response to compute the SEM poles for identification in the time domain. Simulation examples are analyzed to illustrate the potential of this proposed methodology.

EFFECT OF THRESHOLD VALUE ON THE PERFORMANCE OF NATURAL FREQUENCY-BASED RADAR TARGET RECOGNITION

In this paper, the performance analysis of the natural frequency-based radar target recognition in the time domain is considered. We investigate the dependence of the probability of correct classiflcation on a speciflc threshold value, and determine the optimum threshold value for two targets, and the sub-optimal threshold for multiple targets to maximize the probability of correct classiflcation. Based on the probability density function (PDF) of some quantity consisting of the projections of the late time response onto the column spaces of the matrices constructed using the natural frequencies of the speciflc targets, we propose how to determine an optimum threshold in the sense that the probability of correct classiflcation of two targets is maximized. By extending the scheme for two targets, we show how to determine a threshold value close to the optimal threshold for multiple targets. The scheme is validated by comparing the performance using the analytic method with that using the Monte-Carlo simulation.

Extraction of a Target Response from GPR Data for Identification of Buried Objects

A method of calibration for GPR responses is introduced in order to extract a target response from GPR data. This calibration procedure eliminates undesirable waveform distortion that is caused by antenna characteristics and multiple scattering effects between the antennas and the ground surface. An application result to measured GPR data shows that undesirable late-time responses caused by the antenna characteristics and multiple scattering effects are removed, and that the target response is clearly reconstructed. This result demonstrates that the waveform calibration of GPR data is significant and essential for reliable target identification.

Performance of natural frequency-based target detection in frequency domain

We consider the performance analysis of the natural frequency-based radar target detection in the frequency domain. Two cases of the detection are considered: one is for known residues, called the likelihood ratio test (LRT), and the other is for unknown residues, called the generalized likelihood ratio test (GLRT). In time-domain detection scheme [Mooney JE, Ding Z, Riggs L. Performance analysis of a GLRT in late-time radar target detection. Prog. Electromagn. Res. 1999;24:77–96], the authors showed that the decision statistic for the LRT and the decision statistic for the GLRT are Gaussian distributed and chi-square distributed, respectively. Note that the formulation in [Mooney JE, Ding Z, Riggs L. Performance analysis of a GLRT in late-time radar target detection. Prog. Electromagn. Res. 1999;24:77–96] is for the time-domain late time response and that the formulation proposed in this paper is for the frequency response. In this paper, we consider the detection problem in the frequency domain. The scheme is validated by comparing the detection performance using the analytical method with that using the Monte-Carlo simulation.

Quasiparticle-scattering measurements of laminar and turbulent vortex flow in the spin-down of superfluid3He-B

The dynamics of quantized vortices is studied in superfluid ${}^{3}$He-B after a rapid stop of rotation. We use Andreev reflection of thermal excitations to monitor vortex motion with quartz tuning-fork oscillators in two different experimental setups at temperatures below $0.2{T}_{c}$. Deviations from ideal cylindrical symmetry in the flow environment cause the early decay to become turbulent. This is identified from a rapid initial overshoot in the vortex density above the value before the spin-down and its subsequent decay with a ${t}^{\ensuremath{-}3/2}$ time dependence. The high polarization of the vortices along the rotation axis significantly suppresses the effective turbulent kinematic viscosity ${\ensuremath{\nu}}^{\ensuremath{'}}$ below the values reported for more homogeneous turbulence and leads to a laminar late-time response. The laminar dissipation down to $T<0.15{T}_{c}$ is determined from the precession frequency of the polarized vortex configuration. In the limit of vanishing normal-component density, it is found to approach a temperature-independent value, whose origin is currently under discussion.

Compensation of the Instrumental Decay in Measured Lightning Electric Field Waveforms

The proper characterization of lightning electric and magnetic fields is very important for a number of reasons, such as the study and design of lightning protection and the validation of return stroke models. Also, an accurate late time response of the sensor used to record electric fields radiated by a lightning return stroke is very important to correctly detect, classify, and locate lightning discharges. Lightning fields are commonly obtained by integrating the output of antennas that produce replicas of the derivatives of the wanted quantities. The integration is carried out using either passive or active analog circuits that introduce a distortion in the form of a decay time constant that may affect the late-time response of the sensor. We derive the transfer function required for the reconstruction of signals integrated using a single-pole integrator commonly used in field and current waveform measurements. Making use of the convolution theorem, we propose an equation that changes a signal that resulted from integration with a given time constant, to the signal that would have been obtained if a different time constant had been used. Finally, we propose an equation to fully compensate a measured signal from an insufficient time constant of an analog integrator.