Source-receiver Separation Research Articles

Green's functions of and emission into discrete anisotropic and hyperbolic baths

In this paper, we study wave propagation in generic Hermitian local periodic baths and investigate the effects of anisotropy and quasibreaking of periodicity on resonant emission into the band of the bath. We asymptotically decompose the Green's function into long-range traveling waves composed of all wave vectors (near) resonant at the emitter frequency and rapidly decaying evanescent waves. Our approximation then converges exponentially with increasing source-receiver separation $\mathbit{\ensuremath{\rho}}$ when resonant wave packets with group velocity parallel to $\mathbit{\ensuremath{\rho}}$ exist. In hyperbolic media this condition may not be satisfied, and we find that the exponential decay length of oscillating evanescent waves locally around caustics generally depends as a power law with exponent 3/2 on the angle made between $\mathbit{\ensuremath{\rho}}$ and the caustic. For $\mathbit{\ensuremath{\rho}}$ beyond the caustic we observe that the Green's function can become almost imaginary, which results in exclusively incoherent emitter-emitter interactions and allows the simulation of purely dissipative systems with short-range interactions. Here the interaction length is tunable via the separation vector of the emitters. We finally probe the hyperbolic dispersion beyond the previous regimes by applying an artificial gauge field on the lattice. We find that emission resonant with the corresponding open orbits in the Brillouin zone is quasi one dimensional, in contrast to an isotropic environment. The quasi-one-dimensional emission is further topologically protected against local and global lattice perturbations and periodically refocusing, offering a robust bidirectional transport of excitations in higher-dimensional media.

Evaluating an Unconventional Shale Completion with Repeat Crosswell Tomography

Evaluation of hydraulic fracturing relies on indirect inference from measurements that are distorted by distance and noise of various kinds, so reservoir monitoring technology that places the measurement as close to the completion as possible is important. Crosswell tomography is just such a technology, placing source and sensor in the plane of the feature to be analysed, and is an excellent technology for making in situ estimates of subsurface properties in simple geologic settings when source-receiver separation is small and when noise levels are low. Since these conditions are not typical in-field scale surveys of unconventional shales, this study overcomes these deficits by incorporating vertically transverse isotropy in raytracing and applying a strong source effort to increase signal-to-noise. The survey used Schlumberger’s Ztrac source and multicomponent geophones to both generate and record compressional and shear wave energy, matching baseline and repeat surveys within a few days of hydraulic fracturing to detect subtle timelapse changes prior to flowback and fluid diffusion. The result is a unique characterization of fracture creation due to hydraulic fracturing, highlighting an out of zone response and subsequent impact on the bounding limestone layers.

The Horizontal Loop Electromagnetic (HLEM) Response of Ifewara Transcurrent Fault,Southwestern Nigeria: A Computational Results

The need to accurately interpret geological models that approximate mineralized zones in a Basement Complex terrain necessitate the development of horizon loop electromagnetic method (HLEM) forward modeling solutions for such scenarios. The focus of the present work is on finding rapid forward modeling solutions for synthetic HLEM data as an aid in exploration for moderate to deep conductive mineral exploration targets.The main thrust is obtaining idealized HLEM models that are required for geological interpretation of the subsurface in such environment. The original HLEM equations developed by Wesley were extended to represent a horizontally stratified earth with a conductive approximated by shear zone. From these equations a computer program was written to calculate the HLEM responses for optimal conductor model with known values of coil separations (L), depth of burial (z) and angle of dip of the target.The thin conductive model was used because it is simple and suitable for different geological scenarios. The accuracy of the approximate forward solution has been confirmed for HLEM systems with various geometric ranges, frequencies and conductivities. Three models having varying overburden thickness, dip angle of target and source-receiver separation were used in the forward modeling. The effect of varying the dip angle,overburden thickness and coil separation was studied in all the three models used. The result obtained from the forward modeling showed that variation of the dip angle gave rise to changes in the amplitudes of the anomalies generated, while that of overburden and coil separation gave rise to changes in anomaly shape. Also, the geometry and position of the causative body were precisely delineated.

Early-photon reflectance fluorescence molecular tomography for small animal imaging: Mathematical model and numerical experiment.

The paper presents an original approach to time-domain reflectance fluorescence molecular tomography (FMT) of small animals. It is based on the use of early arriving photons and state-of-the-art compressed-sensing-like reconstruction algorithms and aims to improve the spatial resolution of fluorescent images. We deduce the fundamental equation that models the imaging operator and derive analytical representations for the sensitivity functions which are responsible for the reconstruction of the fluorophore absorption coefficient. The idea of fluorescence lifetime tomography with our approach is also discussed. We conduct a numerical experiment on 3D reconstruction of box phantoms with spherical fluorescent inclusions of small diameters. For modeling measurement data and constructing the sensitivity matrix we assume a virtual fluorescence tomograph with a scanning fiber probe that illuminates and collects light in reflectance geometry. It provides for large source-receiver separations which correspond to the macroscopic regime. Two compressed-sensing-like reconstruction algorithms are used to solve the inverse problem. These are the algebraic reconstruction technique with total variation regularization and our modification of the fast iterative shrinkage-thresholding algorithm. Results of our numerical experiment show that our approach is capable of achieving as good spatial resolution as 0.2 mm and even better at depths to 9 mm inclusive.

Sediment sound speeds from a monostatic (multibeam) sonar

Time-averaged sediment layer sound speeds, or interval velocities are generally obtained from source-receiver separations at various offsets. In marine environments, the traditional measurement technique employs a towed broadband source and a long towed receive array, often kilometers in length. A method is described for estimating interval velocities using a simpler, monostatic configuration. The method is first tested using simulated data from various layered seabed structures with roughness at each interface; the resulting estimated interval velocity is within less than 1% of the true value. Monostatic measured data from the Gulf of Lion are also presented which exhibit many characteristics similar to the simulated data. The method applied to the measured data yield an interval velocity of 1569 m/s in an 18 m sediment layer. This accords with nearby independent data from cores and wide-angle reflection analysis. [Work supported by ONR Littoral Geosciences and Optics program.]

Perturbing sound speed field estimates to study quality of acoustic ranging

Acoustic ranging is clearly dependent on the sound speed field c(x,y,z), where the points (x,y,z) are constrained to the two-dimensional region defined by a submerged source at a certain point in the ocean and a submerged receiver at a different but unknown point in the ocean. An in-situ estimate of c(x,y,z) based solely on a sound speed profile taken at the source of known location and one taken at the receiver at a unknown location has been used to obtain reasonable ranging estimates over large source-receiver separation [Sarma etal., IEEE UASP (2019)]. We discuss an approach to perturb this estimate of c(x,y,z) in a physically sound manner consistent with internal wave and other ocean processes in order to study the effect on the accuracy of ranging methods. The approach can indeed be applied to any estimate of c(x,y,z). The ultimate motivation for this work is in-situ determination of the inherent variability expected for a specific range estimate from any range estimation technique.

Sediment interval velocities from a monostatic multibeam sonar.

Interval velocities for marine sediments are generally obtained from source-receiver separations at various offsets. A method is described for estimating interval velocities using a monostatic configuration. Testing is performed using simulated data from rough layered seabeds and interval velocity is estimated within less than 1% of the true value. Monostatic multibeam data from the Gulf of Lion are also presented which exhibit many characteristics similar to the simulated data. The method applied to the measured data yield an interval velocity of 1569 m/s in an 18 m sediment layer. This accords with nearby independent data from cores and wide-angle reflection analysis.

Shallow water propagation of mid-frequency cross-shelf acoustics on the Chukchi Shelf

The shallow water Canada Basin Acoustic Propagation Experiment (SW CANAPE) was conducted to study the effects of oceanographic variability on broadband acoustic fields in the Arctic. The physics of the acoustic waveguide on the northeastern edge of the Chukchi Shelf are influenced by dynamic boundary conditions and spatio-temporal fluctuations in the water column temperature and salinity profiles. Several oceanographic and acoustic receiving arrays were deployed across the Chukchi Shelf out to the shelf break region. Linear frequency modulated (LFM) signals were transmitted by two sources on the shelf for a long period of time. The influence of small scale, short-term water column variability, and dynamic upper boundary conditions including open water, marginal, and solid ice zones on shallow water propagation is shown for a 10 km source-receiver separation with well-defined water column properties measured at the source, receiver, and a mid-point along the cross-shelf acoustic path. [Work supported by ONR 321OA.]

A spectral approach to geoacoustic inversion from sparse impulsive sources

A common approach to geoacoustic inversion is to match the temporal structure displayed by modeled and measured acoustic data produced at a small number of frequencies by range sampling source emissions over a broad interval between a source and a receiver. An alternative approach is to match spectral structure in modeled and measured acoustic data for a small number of source-receiver separations from a fine frequency sample over a broad bandwidth. The latter approach is demonstrated with Shallow Water '06 experiment data in which the acoustic sources were imploding light bulbs. Geoacoustic parameter values obtained by this approach are compared to values obtained from previous inversions, and the limits of validity of the approach are discussed.

The effect of sound speed profile on shallow water shipping sound maps.

Sound mapping over large areas can be computationally expensive because of the large number of sources and large source-receiver separations involved. In order to facilitate computation, a simplifying assumption sometimes made is to neglect the sound speed gradient in shallow water. The accuracy of this assumption is investigated for ship generated sound in the Dutch North Sea, for realistic ship and wind distributions. Sound maps are generated for zero, negative and positive gradients for selected frequency bands (56 Hz to 3.6 kHz). The effect of sound speed profile for the decidecade centred at 125 Hz is less than 1.7 dB.

Green’s Function Using Schelkunoff Integrals for Horizontal Electric Dipoles Over an Imperfect Ground Plane

Recently, Schelkunoff integrals have been used to formulate a Green’s function for analysis of radiation from a vertical electric dipole over an imperfect ground plane. Schelkunoff integrals were proved to be more suitable for numerical computation for large radial distances than the Sommerfeld integrals which are used conventionally to deal with antennas over an imperfect ground. This is because Schelkunoff integrals have no convergence problem on the tail of the contour of integration, especially when the fields are calculated near the boundary separating the media and for large source–receiver separations. In this paper, the Schelkunoff integrals are utilized to derive a Green’s function for the case of a horizontal electric dipole radiating over an imperfect ground plane (a two-media problem where the lower medium is lossy). A detailed comparison between the presented expressions and the conventional ones based on Sommerfeld integrals is illustrated both numerically and analytically.

Calling site choice and its impact on call degradation and call attractiveness in the gray treefrog, Hyla versicolor

To evaluate the effects of calling site on call degradation, we broadcast synthetic advertisement calls of male gray treefrogs through forest, over open terrain, and across pond water. Calls were recorded at distances of 1, 2, 4, 8, 16, and 32 m. We varied speaker and microphone heights for a total of five elevation combinations ranging from surface level to a height of 1.5 m. We quantified structural degradation in recorded calls using “∆V,” a measure of relative sound energy in call pulses and interpulse intervals. A subset of recorded calls was used in two-speaker discrimination tests with females. Finally, we examined male selection of perch height by recording locations of calling males on ladder-like trellises positioned around the periphery of a breeding pond. We found the greatest degradation for calls broadcast through forest followed by calls transmitted across open terrain and then pond water. At relatively small source-receiver separations, elevation had only small effects on degradation. However, for separations greater than 4 m (especially through forest), elevation had a significant impact on ∆V—with calls broadcast and recorded near the substrate particularly vulnerable to degradation. Choice tests demonstrated that such levels of degradation could significantly reduce a male’s attractiveness. This may, in part, explain why males only seldom called from low rungs of trellises.

The effect of signal bandwidth on partially saturated broadband ocean acoustic transmission scintillation in a shallow water waveguide

The acoustic field in a shallow water waveguide at small source-receiver separations <3 km are partially saturated. Here, we examine the scintillation statistics of partially saturated broadband ocean acoustic transmissions in the low to mid frequency range collected using a high-resolution towed horizontal coherent hydrophone array system. The standard deviation of intensity fluctuations and scintillation index are quantified as a function of signal bandwidth and source-receiver separation. The amplitude and intensity distributions are compared with corresponding distribution models for the partially saturated field to determine the number of independent statistical fluctuations present in the broadband data.

A follow up low frequency propagation experiment in Currituck Sound

An initial low frequency propagation experiment was conducted in September 2013 in which a Combustive Sound Source (CSS) generated transient acoustic signals in water depths of approximately 2.5 m [Costley et al. J. Acoust. Soc. Am. 136, 2178 (2014)]. High amplitude signals were generated mid-water column and recorded with bottom mounted hydrophones at distances up to 200 m. A time harmonic, two-dimensional axisymmetric finite element model under-predicted the magnitudes of the recorded pressure signals. A follow up experiment was conducted in October 2014. In addition to using the CSS and the bottom mounted hydrophones, a 4-element vertical line array was deployed. Measurements were made at source-receiver separations from approximately 10 m to 1000 m in 100 m increments. Additionally, acoustic properties of the sediment were obtained through in-situ measurements [McNeese et al. J. Acoust. Soc. Am. 136, 2252 (2014)]. The results of the experiment, along with results of the finite element model using updated sediment properties, will be presented. [Work supported by the U.S. Army Engineer Research and Development Center. Permission to publish was granted by Director, Geotechnical &amp; Structures Laboratory.]

Seabed Characterization Using Acoustic Communication Signals on an Autonomous Underwater Vehicle With a Thin-Line Towed Array

Sediment classification was demonstrated using the self-noise of an autonomous underwater vehicle (AUV) received on a short towed array. The adopted approach was to separate the direct path and the surface- and bottom-reflected signals. Electrical interference from the source was used to verify source receiver separation. The amplitude ratio of the bottom reflected to the direct path signal levels, after compensating for the differences in absorption, spreading losses, and beam patterns, yields the bottom-reflection loss, at the applicable grazing angle. The latter is calculated from the travel time difference between the direct path and bottom-reflected signals. The method is self-calibrating, requiring absolute calibration of neither sound source nor receivers. The definitive isolation of the reflected and direct path signals and the self-calibrating property make this approach robust. The reflection loss may be compared to known seabed models to estimate sediment type.

Step-off, vertical electromagnetic responses of a deep resistivity layer buried in marine sediments

A frequency-domain, marine controlled-source electromagnetic (CSEM) method has been applied successfully in deep water areas for detecting hydrocarbon (HC) reservoirs. However, a typical technique with horizontal transmitters and receivers requires large source–receiver separations with respect to the target depth. A time-domain EM system with vertical transmitters and receivers can be an alternative because vertical electric fields are sensitive to deep resistive layers. In this paper, a time-domain modelling code, with multiple source and receiver dipoles that are finite in length, has been written to investigate transient EM problems. With the use of this code, we calculate step-off responses for one-dimensional HC reservoir models. Although the vertical electric field has much smaller amplitude of signal than the horizontal field, vertical currents resulting from a vertical transmitter are sensitive to resistive layers. The modelling shows a significant difference between step-off responses of HC- and water-filled reservoirs, and the contrast can be recognized at late times at relatively short offsets. A maximum contrast occurs at more than 4 s, being delayed with the depth of the HC layer.

Pitfalls of 1D inversion of small-loop electromagnetic data for detecting man-made objects

In order to interpret field data from small-loop electromagnetic (EM) instruments with fixed source–receiver separation, 1D inversion method is commonly used due to its efficiency with regard to computation costs. This application of 1D inversion is based on the assumption that small-offset broadband EM signals are insensitive to lateral resistivity variation. However, this assumption can be false when isolated conductive bodies such as man-made objects are embedded in the earth. Thus, we need to clarify the applicability of the 1D inversion method for small-loop EM data. In order to systematically analyze this conventional inversion approach, we developed a 2D EM inversion algorithm and verified this algorithm with a synthetic EM data set. 1D and 2D inversions were applied to synthetic and field EM data sets. The comparison of these inversion results shows that the resistivity distribution of the subsurface constructed by the 1D inversion approach can be distorted when the earth contains man-made objects, because they induce drastic variation of the resistivity distribution. By analyzing the integrated sensitivity of the small-loop EM method , we found that this pitfall of 1D inversion may be caused by the considerable sensitivity of the small-loop EM responses to lateral resistivity variation. However, the application of our 2D inversion algorithm to synthetic and field EM data sets demonstrate that the pitfall of 1D inversion due to man-made objects can be successfully alleviated. Thus, 2D EM inversion is strongly recommended for detecting conductive isolated bodies, such as man-made objects, whereas this approach may not always be essential for interpreting the EM field data. ► The resistivity of the subsurface constructed by the 1D inversion can be distorted. ► Small-loop EM response has considerable sensitivity to lateral resistivity variation. ► The application of 2D EM inversion can alleviate the pitfall of 1D EM inversion. ► 2D EM inversion is strongly recommended for detecting man-made objects.

Interferometric determination of the high-intensity laser-pulse-material interaction site

We developed a method that accurately determines an unknown position of the high-intensity laser-pulse-material interaction site on the front side of a plate. It is based on interferometric measurements of a normal displacement at known positions on the plate’s rear side. The displacement is caused by reflections of various pulsed-laser-induced mechanical waves. We have superseded the long-established time-of-flight approach with the improved, triple-echo method. To accurately locate the origin of the laser-induced ultrasound on the plate with a known thickness, we only need to detect the arrivals of the first three consecutive mode unconverted waves. Our method works without knowing the propagation velocities of various ultrasonic waves and additionally solves some time-related drawbacks of the conventional time-of-flight approach. The relative uncertainty of the measured source-receiver separations obtained with the presented method is less than 0.01.

The relation between the waveguide invariant, multipath impulse response, and ray cycles

The waveguide invariant, β, that manifests itself as interference fringes or "striations" in a plot of frequency vs source-receiver separation, is usually thought of as a modal phenomenon. This paper shows that striations can be explained simply through the variation of the eigenray arrival times with range, in short, the variation of the multipath impulse response. It is possible to calculate β for a number of sound speed profiles analytically and to find what β depends on, why it switches from one value to another, how it depends on source-receiver depth, how it depends on variable bathymetry, and how smooth the sound speed profile needs to be for clear fringes. The analytical findings are confirmed by calculating striation patterns numerically starting from eigenray travel times in various stratified environments. Most importantly the approach throws some light on what can be deduced from β alone and the likelihood and utility of striations in reverberation.

Estimation of modal group velocities with a single receiver for geoacoustic inversion in shallow water

Due to the expense associated with at-sea sensor deployments, a challenge in underwater acoustics has been to develop methods requiring a minimal number of sensors. This paper introduces an adaptive time-frequency signal processing method designed for application to a single source-receiver sensor pair. The method involves the application of conjugate time-frequency warping transforms to improve the SNR and resolution of the time-frequency distribution (TFD) of the measured field. Such refined knowledge of the TFD facilitates efforts to extract tomographic information about the propagation medium. Here the method is applied to the case of modal propagation in a shallow ocean range independent environment to extract a refined TFD. Given knowledge of the source-receiver separation, the refined TFD is used to extract the frequency dependent group velocities of the individual modal components. The extracted group velocities are then incorporated into a computationally light tomographic inversion method. Simulated and experimental results are discussed.