Transmitter-receiver Geometry Research Articles

Lake Experimentation of in-Band Full-Duplex Underwater Acoustic Communications With a Receiving Array

In-band full-duplex (IBFD) communications is a promising technique to achieve spectral-efficient communication in the underwater environment. To achieve IBFD underwater communications, suppression of self-interference (SI) is a prerequisite. In this paper, SI suppression using physical separation and digital SI cancellation is investigated under different transmitter-receiver geometries. An iterative IBFD receiver combining joint channel estimation, SI cancellation, and multichannel decision feedback equalizer is developed. Due to the potential deterioration of the channel estimation accuracy, a digital SI cancellation control mechanism is used to regulate the suppression process. The feasibility of coherent IBFD underwater acoustic communications was tested in a lake environment. The two SI suppression methods, the SI cancellation control mechanism, and the receiver performance were evaluated using the experimental measurements. A bit error rate (BER) in the order of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10^{-2}$ </tex-math></inline-formula> was achieved at the range of 80 m for IBFD communications.

Lake experimentation of in-band full-duplex underwater acoustic communications

In-band full-duplex (IBFD) communications is a promising technique to achieve spectral-efficient communication in the underwater environment. Characteristics of self-interference and methods to suppress the self-interference are crucial to attaining IBFD acoustic communications. In this effort, we analyze the characteristic of the self-interference for the underwater acoustic channel through experimental measurements. Self-interference reduction using physical separation and digital cancellation is investigated under different transmitter-receiver geometries. An iterative IBFD receiver combining joint channel estimation, SI cancellation, and multichannel decision feedback equalizer is developed. The feasibility of coherent IBFD underwater acoustic communications was tested in a lake environment. The self-interference suppression methods and the receiver performance were evaluated using the experimental measurements. IBFD communications was demonstrated at 80 m. With 10 dB local self-interference source level reduction, IBFD communications was extended to the 170 m range. [Work supported by the National Science Foundation.]

A semi-automated procedure for the emitter–receiver geometry characterization of motor-controlled lidars

Abstract. To correctly understand and interpret lidar-acquired signals and to provide high-quality data, the characterization of the lidar transmitter–receiver geometry is required. For example, being fundamental to correctly align lidar systems, this characterization is useful to improve the efficiency of the alignment procedure. In addition, some applications (e.g. air quality monitoring) need to quantitatively interpret the observations even in the range where the overlap between the telescope field of view and the laser beam is incomplete. This is generally accomplished by correcting for the overlap function. Within the frame of lidar-based networks (e.g. ACTRIS/EARLINET, the Aerosol, Clouds and Trace Gases Research Infrastructure/European Aerosol Research Lidar Network), there is a need to define standardized approaches to deal with lidar geometry issues. The multi-wavelength multi-telescope Rayleigh–Mie–Raman “9-eyes” system in Rome Tor Vergata, part of ACTRIS/EARLINET, has the capability, through computer-controlled servomotors, to change the orientation of the laser beams and the 3D position of the diaphragm of the receiving optical system around the focal point of the telescopes. Taking advantage of these instrumental design characteristics an original approach to characterize the dependency of the acquired signal from the system relative transmitter–receiver geometry (the mapping procedure) was developed. The procedure consists in a set of programs controlling both the signal acquisition as well as the motor movements. The approach includes solutions to account for atmospheric and laser power variability likely to occur during the mapping sessions. The paper describes in detail the developed procedure and applications such as the optimization of the telescope/beam alignment and the estimation of the overlap function. The results of the mapping applied to a single combination of telescope-laser beam are shown and discussed. The effectiveness of the mapping-based alignment was successfully verified by comparing the whole signal profile and the outcome of the telecover test, adopted in EARLINET, for a manual and a mapping-based alignment. A significant signal increase and lowering of the full overlap height (from 1500 m to less than 1000 m) was found. The overlap function was estimated down to 200 m and compared against the one obtained from a geometric model. The developed procedure also allowed estimating the absolute and relative tilt of the laser beam. The mapping approach, even in simplified versions, can be adapted to other lidars to characterize and align systems with non-motorized receiving geometry.

1D electromagnetic response modeling with arbitrary source-receiver geometry based on vector potential and its implementation in MATLAB

The calculation of 1D electromagnetic (EM) responses of dipole sources is considered as a fundamental computation kernel in several geophysical applications, ranging from EM sensitivity analysis to high-dimensional EM forward modeling and inversion. To solve the EM responses caused by electric and magnetic dipoles with different orientations, we have developed an open-source MATLAB code for a complete treatment, which allows for an arbitrary transmitter-receiver geometry in the multilayered earth. A complete derivation of the EM field expressions with simplified exponent terms for all traditional dipole sources is accomplished based on the vector potential differential equations. The quadrature-with-extrapolation algorithm is adapted for calculating the integration of Bessel functions to produce EM responses with a desired accuracy. The MATLAB digit functions are used to alleviate the instability problem caused by the exponent terms. The accuracy of our code is verified by comparing the results with the published data and self-checks on an infinite thin sheet. A layered model is designed to test the capability of our code for all traditional dipole sources by placing the receiver at different depths. In the end, we apply our code to marine and airborne controlled-source EM explorations. The results indicate that our code is accurate and can potentially be used to calculate 1D EM responses for general EM problems.

Algorithm 1001

IPscatt is a free, open-source MATLAB toolbox facilitating the solution for time-independent scattering (also known as time-harmonic scattering) in two- and three-dimensional settings. The toolbox has three main application cases: simulation of the scattered field for a given transmitter-receiver geometry; the generation of simulated data as well as the handling of the real-world data from Institute Fresnel; and the reconstruction of the contrast from several measured, scattered fields. In each case, a variety of options tailored to the needs of practitioners is provided. For example, the toolbox allows the simulation of the scattered near field as well as of the far field. Also, it provides methods for the modeling of the incident field as point sources as well as plane waves. Finally, many common geometries of transmitters and receivers are included out of the box. Regarding the reconstruction, the provided functions implement the regularization scheme that relies on a primal-dual algorithm and was introduced by F. Bürgel, K. S. Kazimierski, and A. Lechleiter [ Journal of Computational Physics 339 (2017), 1–30]. This article provides a survey of the mathematical concepts in scattering, connects them with the provided implementation, gives an overview of the software framework as well as its application areas, and compares it with existing software packages solving the same problem.

Disrupted layered lithology can jeopardise airborne electromagnetic interpretation

The majority of airborne electromagnetic (AEM) data is processed using stitched consecutive 1D approximations, from which conductivity depth or CDI sections can be produced. Obtaining calculated variable layer depths from AEM is one of the appealing assets of the method. The current induced by an AEM system in the nearsurface circulates preferentially at a radial distance from the horizontal transmitter (commonly called footprint), the section plotted below the receiver is actually generated from currents induced in the general vicinity and not directly below the reviver-transmitter system. Detection of palaeochannels, faults and other laterally varying structures are common geophysical mapping targets. Caution should be taken when interpreting these horizontally-anisotropic targets; especially if smaller that the system 'Annulus of resolution' or close to edges of discontinuous layering. All AEM systems have different transmitter-receiver geometries, moments, wave forms and others specifics. Forward modelling and conductivity-transforms provide a good method to understand CDI sections and for system comparison. When 2D/3D effects of discontinuities are present, 1D modelling incorrectly predicts weak conductors at depth. Within a conductive layer where conductivity changes with facies or geometrical thickness, 1D approximations work well. 2D processing of 2D data is clearly needed, but existing inversion methods are too slow. 2D/3D structures with abrupt conductivity boundaries resolved with an isotropic layered assumption must always be queried.

An airborne electromagnetic system with a three-component transmitter and three-component receiver capable of detecting extremely conductive bodies

Extremely conductive bodies, such as those containing valuable nickel sulfides, have a secondary response that is dominated by an in-phase component, so this secondary response is very difficult to distinguish from the primary field emanating from the transmitter (because by definition they are identical in temporal shape and phase). Hence, an airborne electromagnetic (AEM) system able to identify the response from the extremely conductive bodies in the ground must be able to predict the primary field to identify and measure the secondary response of the extremely conductive body. This is normally done by having a rigid system and bucking out the predicted primary (which will not change significantly due to the rigidity). Unfortunately, these rigid systems must be small and are not capable of detecting extremely conductive bodies buried deeper than approximately 100 m. Another approach is to measure the transmitter current and geometry and subtract the primary mathematically, but these measurements must be extremely accurate and this is difficult or expensive, so it has not been done successfully for an AEM system. I exploit the geometric relationship of the primary fields from a three-component (3C) dipole transmitter. If the transmitter is mathematically rotated so that one axis points to the receiver, then linear combinations of the fields measured by a 3C receiver can be combined in such a way that the primary fields from the transmitter sum to zero and cancel. Alternatively, the measured transmitter current and response could be used to estimate the transmitter-receiver geometry and then to predict and remove the primary field. Any residual must be the secondary coming from a conductive body in the ground. Hence, extremely conductive bodies containing valuable minerals can be found. An AEM system with a 3C transmitter and a 3C receiver should not be too difficult to build.

Diffraction Corrections for Second Harmonic Beam Fields and Effects on the Nonlinearity Parameter Evaluation

The nonlinearity parameter is frequently measured as a sensitive indicator in damaged material characterization or tissue harmonic imaging. Several previous studies have employed the plane wave solution, and ignored the effects of beam diffraction when measuring the non-linearity parameter β. This paper presents a multi-Gaussian beam approach to explicitly derive diffraction corrections for fundamental and second harmonics under quasilinear and paraxial approximation. Their effects on the nonlinearity parameter estimation demonstrate complicated dependence of β on the transmitter-receiver geometries, frequency, and propagation distance. The diffraction effects on the non-linearity parameter estimation are important even in the nearfield region. Experiments are performed to show that improved β values can be obtained by considering the diffraction effects.

An Efficient and Accurate Method for Calculating Nonlinear Diffraction Beam Fields

This study develops an efficient and accurate method for calculating nonlinear diffraction beam fields propagating in fluids or solids. The Westervelt equation and quasilinear theory, from which the integral solutions for the fundamental and second harmonics can be obtained, are first considered. A computationally efficient method is then developed using a multi-Gaussian beam (MGB) model that easily separates the diffraction effects from the plane wave solution. The MGB models provide accurate beam fields when compared with the integral solutions for a number of transmitter-receiver geometries. These models can also serve as fast, powerful modeling tools for many nonlinear acoustics applications, especially in making diffraction corrections for the nonlinearity parameter determination, because of their computational efficiency and accuracy.

Development of attenuation and diffraction corrections for linear and nonlinear Rayleigh surface waves radiating from a uniform line source

In recent studies with nonlinear Rayleigh surface waves, harmonic generation measurements have been successfully employed to characterize material damage and microstructural changes, and found to be sensitive to early stages of damage process. A nonlinearity parameter of Rayleigh surface waves was derived and frequently measured to quantify the level of damage. The accurate measurement of the nonlinearity parameter generally requires making corrections for beam diffraction and medium attenuation. These effects are not generally known for nonlinear Rayleigh waves, and therefore not properly considered in most of previous studies. In this paper, the nonlinearity parameter for a Rayleigh surface wave is defined from the plane wave displacement solutions. We explicitly define the attenuation and diffraction corrections for fundamental and second harmonic Rayleigh wave beams radiated from a uniform line source. Attenuation corrections are obtained from the quasilinear theory of plane Rayleigh wave equations. To obtain closed-form expressions for diffraction corrections, multi-Gaussian beam (MGB) models are employed to represent the integral solutions derived from the quasilinear theory of the full two-dimensional wave equation without parabolic approximation. Diffraction corrections are presented for a couple of transmitter-receiver geometries, and the effects of making attenuation and diffraction corrections are examined through the simulation of nonlinearity parameter determination in a solid sample.

Full-Polarization Modeling of Monostatic and Bistatic Radar Scattering From a Rough Sea Surface

Fully polarimetric radars have advantages compared to more conventional single-polarization radars when measuring ocean wave characteristics. However, the theoretical analysis of full-polarization radar scattering frequently presents a challenge. For example, the classical composite model fails to correctly predict the cross-polarization components of the normalized radar cross section (NRCS). The new version of the numerical implementation of the small slope approximation of the second order, the SSA2, is presented in this paper. The SSA2 correctly predicts the behavior of out-of-plane bistatic scattering for an arbitrary set of polarization indices. The SSA2, as contrasted with the composite model, accounts for Bragg scattering of the second order which is essential for modeling cross-polarization scattering. We present the results of numerical simulations of both monostatic and bistatic cross sections and scattering amplitude correlators for various ocean conditions and transmitter-receiver geometries using the SSA2. We compare them with available experimental data and other modeling results.

Dynamic orientation of receiver arrays using particle swarm optimisation

Considering a time difference of arrival (TDOA) localisation system, it is well known that the accuracy depends on the transmitter–receiver geometry, time differences estimation error and the selection of the localisation algorithm. Although many studies related to TDOA estimation and emitter location finding can be found in the literature, the effect of transmitter–receiver geometry on positioning accuracy has not been investigated sufficiently. Most of the studies focused on figuring out the optimum receiver arrays by minimising the Cramer-Rao lower bound (CRLB) or maximising the Fisher information matrix. In this reported work, depending on the target position, the optimum receiver arrays are dynamically oriented. By using particle swarm optimisation (PSO) the target is localised and the offset angle which makes the CRLB minimum is determined. After rotating the receiver array by the offset angle, the target is localised again. At the end of the process, a significant decrement in the positioning error is obtained. To show the effectiveness of the proposed method, a uniform angular array and a cross array are selected as optimum two-dimensional geometries.

A Deterministic Satellite-to-Indoor Entry Loss Model

Advances in satellite and receiver technology make former denied areas like indoors accessible for satellite based radio services. As a consequence, suitable penetration loss models are of significant importance for service availability planning. Standard penetration loss models ignore the building layout in relation to the transmitter-receiver geometry. Within this paper, we introduce the term entry loss as a measure for the additional loss of a wideband impulse received at the geometrical line-of-sight path delay caused by the building materials. Knowing the received power caused by indoor multipath propagation, the penetration loss and the entry loss are deterministically related. As a contribution, we propose an entry loss model which is able to include the geometrical relation of walls, receiver and transmitter positions. Within the model, we treat walls, windows and doors as apertures and, therefore, as sources for signal penetration and diffraction. A verification of the model using wideband channel sounder measurement data is performed. By simulations, we show the comparability between existing models and the proposed one. Moreover, a short description on the usage of the proposed entry loss model for wideband channel simulations is provided.

Review of three airborne EM systems

Time Domain Airborne Electromagnetic (TDEM) systems are defined by a set of technical specifications, which include dipole moment, bandwidth, transmitter waveform and transmitter-receiver geometry. Comprehensive analysis of these specifications is fundamental in understanding how they define the target response. For example, a system optimised for mapping deep, discrete ore bodies is not necessarily the ideal solution for mapping regolith where good vertical resolution may be required. Data acquired by three TDEM systems developed by Fugro Airborne Surveys are used to demonstrate the effects that different system specifications have on the response of an exploration target.

Idealized Bistatic RADAR GMTI Detection with Space-Time Processing

In a recent paper by the author a novel approach was utilized for determining upper-bound ground moving target indication (GMTI) radar detection performance with two-dimensional space-time processing. That model utilized an idealized clutter model and other simplifying assumptions to clarify the method. The method is extended here to the general bistatic case. It is shown that the elements of the clutter covariance matrix can be obtained from cross-correlations of the convolutions of (appropriately transformed) transmit and receive aperture functions at different times and locations. A simulation based on this theory was developed that accommodates arbitrary transmitter-receiver geometry. When this general bistatic model is specialized to the monostatic case, it is shown to agree both theoretically and computationally with the earlier results.

The application of airborne geophysics in the exploration for iron ore in the Zouerate area of Mauritania

An airborne time domain electromagnetic and magnetic survey (Geotech VTEM system; VTEM is a helicopter-borne Time Domain Electromagnetic (TDEM) system with concentric loop transmitter receiver geometry.) was flown over the Kedia mountain and Mhaoudat Intrusive in Mauritania. Hematite has been mined in both these areas for many years. The objectives of this survey were to determine if additional ore bodies existed below old mined areas and to find and delineate possible new ore bodies. A detailed structural interpretation, using the magnetic and EM data, was done with the assumption that major faults could be a controlling factor on the occurrence of iron ore. Decay curve analysis was done of the VTEM data as a fast, first pass delineation of conductive areas. Good correlation with existing mines was found. CDI?s were consequently calculated for all flight lines. These were used to calculate conductivity contour maps at various depth intervals below the surface. Conductive areas were correlated with the magnetic data as well as the known geology. In several areas known iron ore was associated with the edges of observed anomalies. These anomalies continued at depth and distance, indicating possible extensions of the ore. Further anomalies occurred in a sand covered area south of Mhaoudat, leading to the possibility of new iron ore deposits.

Layered earth inversions of AEM bathymetry data incorporating aircraft attitude and bird offset – a case study of Torres Strait

The potential of obtaining water depths derived from AEM data that are accurate to ~0.5 m cannot be fully realised if the variable geometry between transmitter and receiver is neglected. The rigorous approach of monitoring the bird offset in three dimensions is currently too costly. However, neglecting the bird offset variations from the assumed fixed nominal offset is problematic because the measured decay shape is sensitive to the transmitter–receiver geometry. An alternative procedure is to invert both layered earth parameters and geometric parameters using a non-linear least-squares method. This inversion procedure has been tested on 25 Hz GEOTEM data from a survey flown in the Torres Strait. The accuracy of the inverted water depths was appraised using data from laser airborne depth sounding surveys. Good agreement, to within ~1 m was obtained between the different methods.

2D marine controlled-source electromagnetic modeling: Part 2 — The effect of bathymetry

Marine controlled-source electromagnetic (CSEM) data are strongly affected by bathymetry because of the conductivity contrast between seawater and the crust below the seafloor. We simulate the marine CSEM response to 2D bathymetry using our new finite element (FE) code, and our numerical modeling shows that all electric and magnetic components are influenced by bathymery, but to different extents. Bathymetry effects depend upon transmission frequency, seabed conductivity, seawater depth, transmitter-receiver geometry, and roughness of the seafloor topography. Bathymetry effects clearly have to be take into account to avoid the misinterpretation of marine CSEM data sets.

Quantifying the Effects That Changes in Transmitter-Receiver Geometry Have on the Capability of an Airborne Electromagnetic Survey System to Detect Good Conductors

Time-domain electromagnetic systems are most sensitive to highly conductive targets during the transmitter on-time. Data collected during the on-time is highly influenced by the geometric relationship between the transmitter and the receiver. Unless corrections can be applied for these geometric variations, the ability to correctly detect and identify highly conductive targets can not be fully realized. Variables affecting the relative position between the transmitter and receiver include changes in the position of the receiver with respect to the transmitter, variations in transmitter attitude, and transmitter loop deformation. The level of accuracy in the determination of the relative geometry required to permit complete removal of the effects due to geometric variations is different for each of the variables and is dependent on the depth of the target. The most influential geometric variation involves changes in the relative position of the receiver with respect to the transmitter along the direction of flight; this variable, therefore, requires the highest degree of accuracy. The accuracy required for receiver motions along the vertical direction is roughly one order of magnitude less, and in the lateral direction, several orders of magnitude less. The objective of this paper is to relate the sensitivity of the response to transmitter-receiver geometry to the ability of an airborne, time-domain electromagnetic system to resolve a highly conductive vertical target. This is achieved by a detailed analysis of the effect of the individual x, y, and z distances between the transmitter and the receiver, the transmitter attitude (roll, pitch, and yaw), and transmitter deformation. © 2006 Canadian Institute of Mining, Metallurgy and Petroleum. All rights reserved.

A case study of AEM bathymetry in Geographe Bay and over Cape Naturaliste, Western Australia, Part 2: 25 and 12.5 Hz GEOTEM

We interpret airborne EM response data recorded by the GEOTEM 12.5 and 25 Hz systems flown over shallow seawater, using conductivity-depth imaging (CDI) to estimate sea depths to 65 m. We observed non-monotonic decay in the vertical component of the 25 Hz data recorded at survey altitude. Nonmonotonic decay had an adverse effect on the CDI results when the processing was run with positivity constraints. The removal of positivity constraints resulted in significant improvements in the quality of CDIs for interpreting water depth. Layered-earth modelling further showed that this non-monotonic decay in the 25 Hz data was due to variations in the transmitter-receiver geometry over the highly conductive seawater. Currently, 12.5 Hz airborne EM is unsuitable for bathymetric mapping unless latetime system noise can be reduced. CDIs from dBx/dt component data at 25 Hz provide the most accurate interpreted sea depths. For this dataset, the use of B-field responses computed from dB/dt observations does not offer any clear advantage over dB/dt data for interpreting sea depths from CDIs.