Ray Paths Research Articles

An attenuating, isotropic and heterogeneous uppermost inner core: evidence from global PKiKP-PKIKP amplitude ratio tomography

SUMMARY The Earth's inner core is solidifying from the liquid outer core, where convection currents power the geodynamo. Constraining properties of the inner core is fundamentally important, yet seismic models do not reconcile body wave and normal mode attenuation properties. Here, we analyse high signal-to-noise ratio core-refracted (PKIKP) and core-reflected (PKiKP) waves generated by earthquakes recorded globally between 1987 and 2021. These phases are excellent indicators of uppermost inner core (UIC) properties due to their low angular separation at the core–mantle boundary and similar ray paths. We analyse their amplitude ratios (ARs) and time delays (DTs) relative to synthetic waveforms and estimate the required attenuation perturbations in a linearized attenuation tomography. The UIC is cylindrically isotropic and heterogeneous in both seismic velocity and attenuation. Contrary to the paradigm that the inner core is seismically hemispherical, we find more complex patterns of attenuation that correlate with seismic velocities, according to both Akaike criterion and Student's t-test. The UIC beneath NE Asia is weakly attenuating and seismically slower, in contrast to the expected properties associated with the quasi-Eastern Hemisphere. Beneath South America, where the core is thought to grow fastest, AR values are more diverse, possibly reflecting interdendritic melt inclusions, and DTs show an E–W gradient. The UIC is seismically slow and weakly attenuating beneath the Atlantic, but strong anomalies emerge near West Africa. Attenuation slightly changes with depth conceivably implying an increase in melt degree or a change in inner core growth rate with time. These 3-D heterogeneities are inconsistent with simple models of core translation or lopsided growth, indicating that more intricate processes are needed to explain inner core structure and evolution.

Active VLF Transmission Experiments Between the DSX and VPM Spacecraft

AbstractThis study presents results from magnetic field line conjunctions between the medium‐Earth orbiting Demonstration and Science Experiments (DSX) satellite and the low‐Earth orbiting (LEO) very low frequencies (VLF) Propagation Mapper (VPM) satellite. DSX transmitted at VLF toward VPM, which was equipped with a single‐axis dipole electric field antenna, when the two spacecraft passed near the same magnetic field line. VPM did not observe DSX signals in any of the 27 attempted conjunction experiments; the goal of this study, therefore, is to explain why DSX signals were not received. Explanations include (a) the predicted power at LEO from DSX transmissions was too low for VPM to observe; (b) VPM's trajectory missed the “spot” of highest intensity due to the focused ray paths reaching LEO; or (c) rays mirrored before reaching VPM. Different combinations of these explanations are found. We present ray‐tracing analysis for each conjunction event to predict the distribution of power and wave normal angles in the vicinity of VPM at LEO altitudes. We find that, for low‐frequency (below 4 kHz) transmissions, nearly all rays mirror before reaching LEO, resulting in low amplitudes at LEO. For mid‐ and high‐frequency transmissions (∼8 and 28 kHz respectively), the power at LEO is above the noise threshold of the VPM receiver (between 0.5 μV/m and 1 μV/m). We conclude that the antenna efficiency and plasmasphere model are critical in determining the predicted power at LEO, and are also the two most significant sources of uncertainty that could explain the apparent discrepancy between predicted amplitudes and VPM observations.

Acoustic emission source location on a cylindrical shell structure through grouped sensors based analytical solution and data field theory

Cylindrical shell structures are widely used in the fields of mining engineering, tunneling engineering and material structures, where acoustic emission (AE) source location plays an important role in monitoring engineering fractures. The commonly used AE source location method for cylindrical shell structures has several shortcomings. First, it must select the shortest ray path from multiple possible ray paths, making it hard to derive an analytical solution and is inconvenient for iterative solutions. Moreover, a local optimal solution may be obtained through a single time event location. To address these issues, we propose an AE source location method, which combines grouped sensors based analytical solution and data field theory. Synthetic tests of 18,360 generated events with Gaussian noises (mean value = 0us, standard deviation = 2.5us) included for P-wave travel time show an average location error within 2 cm. Furthermore, the average location error of 53 pencil-lead breaks is 2.43 cm. Moreover, the results verify that it is necessary to remove the P-wave arrival time system error between sensors prior to AE event location. In conclusion, the proposed analytical solution is helpful for rapid AE location on a cylindrical shell structure, and the application of data field can overcome the local optimal problem of a single time location.

A New Approach to Modeling Focused Infrared Heating Based on Quantum Mechanical Formulations

The focused infrared (IR) heating method is an energy-efficient heating technology for engineering applications. Numerical models of focused IR heating technology have been introduced based on the theory of ray optics. The ray optics-based IR models have provided good simulation results; however, they are mathematically complex because the ray optics models need to account for the complex paths of the IR rays and the geometrical information of the heating devices. This paper presents a new approach for modeling the focused IR heating method using quantum mechanical formulations. Even though the IR heating condition is not a pure quantum phenomenon, it is efficient to employ the concept of the superposition principle of wave functions in IR distribution modeling. The proposed model makes an abstraction by replacing the distributed IR rays with an energy particle with independent wave functions at different eigenstates, based on the Schrödinger equation. The new approach results in a simpler equation for modeling the focused IR heating method. An electrical-thermal simulation of the focused IR heating with the new model provides results in good agreement with the experimental data.

Internal shear layers in librating spherical shells: the case of periodic characteristic paths

Internal shear layers generated by the longitudinal libration of the inner core in a spherical shell rotating at a rate $\varOmega ^*$ are analysed asymptotically and numerically. The forcing frequency is chosen as $\sqrt {2}\varOmega ^*$ such that the layers issued from the inner core at the critical latitude in the form of concentrated conical beams draw a simple rectangular pattern in meridional cross-sections. The asymptotic structure of the internal shear layers is described by extending the self-similar solution known for open domains to closed domains where reflections on the boundaries occur. The periodic ray path ensures that the beams remain localised around it. Asymptotic solutions for both the main beam along the critical line and the weaker secondary beam perpendicular to it are obtained. The asymptotic predictions are compared with direct numerical results obtained for Ekman numbers as low as $E=10^{-10}$ . The agreement between the asymptotic predictions and numerical results improves as the Ekman number decreases. The asymptotic scalings in $E^{1/12}$ and $E^{1/4}$ for the amplitudes of the main and secondary beams, respectively, are recovered numerically. Since the self-similar solution is singular on the axis, a new local asymptotic solution is derived close to the axis and is also validated numerically. This study demonstrates that, in the limit of vanishing Ekman numbers and for particular frequencies, the main features of the flow generated by a librating inner core are obtained by propagating through the spherical shell the self-similar solution generated by the singularity at the critical latitude on the inner core.

A reappraisal of active tectonics along the Fethiye–Burdur trend, southwestern Turkey

SUMMARYWe investigate active tectonics in southwestern Turkey along the trend between Fethiye, near the eastern end of the Hellenic subduction zone, and Burdur, on the Anatolian plateau. Previously, regional GNSS velocities have been used to propose either (1) a NE-trending zone of strike-slip faulting coined the Fethiye–Burdur Fault Zone, or (2) a mix of uniaxial and radial extension accommodated by normal faults with diverse orientations. We test these models against the available earthquake data, updated in light of recent earthquakes at Arıcılar (24 November 2017, Mw 5.3), Acıpayam (20 March 2019, Mw 5.6) and Bozkurt (8 August 2019, Mw 5.9), the largest in this region in the last two decades. Using Sentinel-1 InSAR and seismic waveforms and arrival times, we show that the Arıcılar, Acıpayam and Bozkurt earthquakes were partially or fully buried ruptures on pure normal faults with subtle or indistinct topographic expressions. By exploiting ray paths shared with these well-recorded modern events, we relocate earlier instrumental seismicity throughout southwestern Turkey and incorporate these improved hypocentres in an updated focal mechanism compilation. The southwestern Fethiye–Burdur trend is dominated by ESE–WNW trending normal faulting, even though most faults evident in the topography strike NE–SW. This hints at a recent change in regional strain, perhaps related to eastward propagation of the Gökova graben into the area or to rapid subsidence of the Rhodes basin. The northeastern Fethiye–Burdur trend is characterized by orthogonal normal faulting, consistent with radial extension and likely responsible for the distinct physiography of Turkey’s Lake District. We find that the 1971 Mw 6.0 Burdur earthquake likely ruptured a NW-dipping normal fault in an area of indistinct geomorphology near Salda Lake, contradicting earlier studies that place it on well-expressed faults bounding the Burdur basin, and further highlighting how damaging earthquakes are possible on faults that would prove difficult to identify beforehand. Overall, our results support GNSS-derived kinematic models that depict a mix of uniaxial and radial extension throughout southwestern Turkey, with no evidence from focal mechanisms for major, active strike-slip faults anywhere along the Fethiye–Burdur trend. Normal faulting orientations are consistent with a stress field driven primarily by contrasts in gravitational potential energy between the elevated Anatolian plateau and the low-lying Rhodes and Antalya basins.

Michelson’s analysis of errors in his 1881/87 experiments of the two swimmers contest; Necessity of correcting college textbooks

The Paper highlights discovery from 2000 of two Errors (ERM81/87) by Michelson in analysis of his experiments ME81/87. ERM81/87 has also been addressed in series of previous articles by the authors. ERM81/87 consisted of assuming the transverse path of light ray 2 or of SW2 swimmer, to be an isosceles triangle (Error 1) inclined with angle β’ in referential frame (RF) ether/water (E/W), or a double orthogonal path (in RF device/bank (D/B)), thus calculating the wrong result, t2 < t1, or Δt = t1-t2 > 0, (Error 2), states SW2 wins the contest. In ERM81/87 it is proven firstly, by Demonstration 1, based on logic, starting from the non-observance in Michelson’s analysis, of rules of correctness and fair play, in development of the two swimmers contest (TSC). Rresulted in correct transversal path as a right triangle, with t2 = t1 (in RF, E/W), the correct contest result is a draw. Demonstration 2 consists, in correction of SW2 route, resulting under fair play conditions, for transversal path, a right triangle with hypotenuse inclined at angle 2β (in RF, E/W) and t2 = t1 or Δt = 0, with the contest result in a tie. Demonstration 3 is based on a new Generalised Case (GC) of the TSC where the orientation of the starting path of SW1 can be inclined at any angle α, always one obtains, t1 = t2, the correct result. Evidence provided shows Michelson designed his interferometer by analogy with TSC, Michelson considered SW2 the winner. Evidence provided in 1892 by Lorenz expressed doubts about the correctness of Michelson’s analysis of ME81/87, without finding errors. We show, an initial error (ERM81) was committed by Michelson in ME81analysis, erroneously superposing paths for SW2 (in RF, D/B), and in incorrect calculations of t2 and Δt obtaining t2 < t1, an error communicated to him by Potier, which also mentioned the correct result Δt = 0, without any demonstration. In analysis of ME87, Michelson acknowledged the error, modified the path of SW2 to isosceles triangle (in RF, E/W), and made new t2 calculation. But he reduced by only half, the Δt compared to Δt in ME81, meaning the basic error in Δt (ERM87) has persisted, until today. We emphasize that errors ERM81/87 in TSC are repeatedly printed, and appear in all college physics textbooks around the world, exemplifying by presenting four such excerpts from college textbooks from the US and Canada. Thus, a correction of ERM81/87 is required, starting with the correction of the TSC, by admitting the correct result of equality of SW1 and SW2, in all college physics textbooks. As result the ether can be reintroduced into physics with multiple consequences including re-evaluation of the Special Relativity Theory.

Slab control on the mega-sized North Pacific ultra-low velocity zone

Ultra-low velocity zones (ULVZs) are localized small-scale patches with extreme physical properties at the core-mantle boundary that often gather at the margins of Large Low Velocity Provinces (LLVPs). Recent studies have discovered several mega-sized ULVZs with a lateral dimension of ~900 km. However, the detailed structures and physical properties of these ULVZs and their relationship to LLVP edges are not well constrained and their formation mechanisms are poorly understood. Here, we break the degeneracy between the size and velocity perturbation of a ULVZ using two orthogonal seismic ray paths, and thereby discover a mega-sized ULVZ at the northern edge of the Pacific LLVP. The ULVZ is almost double the size of a previously imaged ULVZ in this region, but with half of the shear velocity reduction. This mega-sized ULVZ has accumulated due to stable mantle flow converging at the LLVP edge driven by slab-debris in the lower mantle. Such flow also develops the subvertical north-tilting edge of the Pacific LLVP.

Thermal improvement of linear Fresnel solar system utilizing Al2O3-water nanofluid and multi-way twisted tape

To enhance the thermal treatment of Linear Fresnel Reflector (LFR) unit, multi-way twisted tape (MWTT) was applied in current research. Twelve mirrors and one tube with trapezoidal cavity around it, generates new LFR unit. The operating fluid is Al2O3-water and for predicting the properties, temperature dependent formulas were employed. Finite volume simulations results in finding the distribution of velocity, temperature, entropy generation. Also, thermal performance was measured. The geometrical factors are number of wings (nw), revolution (TR) and width (ER) of tapes. To evaluate the modeling procedure, various examinations with numerical and experimental publications were verified. To extract the distribution of absorb flux, SolTrace and ANSYS FLUET have been utilized and they are in good agreement. The output of SolTrace demonstrates the path of rays and selecting higher number of rays results in higher accuracy. Highest absorb flux occurs in bottom of pipe. As mixing of fluid improves with rise of revolution and width of tapes, Nu increases and augmentation in interaction of fluid with wall enhances the friction factor. Increasing number of wings can enhance the secondary flow. So, stronger impingement with wall can augment the friction factor. Nu enhances with rise of nw about 4.384% when TR = 2, ER = 0.34,V˙ = 19. Augmenting TR makes Nu to increase about 7.135% when nw = 4, ER = 0.1, V˙ = 19. Besides, with selecting higher ER, friction factor augments about 20.019% at TR = 2, nw = 2, V˙ = 6.3. To evaluate the amount of available work of this solar unit, components of entropy generation (Sgen,f, Sgen,h) have been measured. Thermal irreversibility declines with increase of turbulent intensity while friction component increases. Thermal efficiency (ηth) augments with growth of TR, ER and nw while it decreases with enhance of inlet temperature. As Tin rises, ηthreduces about 31.51% when TR = 2, nw = 2, ER = 0.1, V˙ = 19. Inclusion of nano-powders can enhance the useful heat and ηthaugments about 0.153%.

Distinguishing Convective-Transition Moisture-Temperature Relationships with a Constellation of Polarimetric Radio Occultation Observations in and near Convection

Convective transition statistics serve as diagnostics for the parameterization of convection in climate and weather forecast models by characterizing the dependence of convection on the humidity-temperature environment. The observed strong pickup of precipitation as a function of layer-averaged water vapor and temperature is captured in models with varying accuracy. For independent observational verification, a low-Earth orbiting satellite constellation of Global Navigation Satellite System (GNSS) polarimetric radio occultation (PRO) measurements would be spaced such that adjacent RO would capture different profiles within and immediately adjacent to convection. Here, the number of profile observations needed to distinguish between convective transition relations by different tropospheric temperature ranges is determined, over different tropical oceanic basins. To obtain these, orbit simulations were performed by flying different satellite constellations over global precipitation from the Global Precipitation Measurement (GPM) mission, varying the numbers of satellites, orbit altitude, and inclination. A 45-degree orbit inclination was found to be a good tradeoff between maximizing the number of observations collected per day, and the desired 50–150-km spacing between individual RO ray paths. Assuming a set of reasonable assumptions for net data yield, three tropospheric temperatures can be distinguished by 1 K with a six-month on-orbit duration from a constellation of at least three satellites.

The Effects of Heavy Precipitation on Polarimetric Radio Occultation (PRO) Bending Angle Observations

Abstract Following the successful launch of the Spanish PAZ mission the proof of concept experiment “Radio Occultation and Heavy Precipitation with PAZ” (ROHP-PAZ) started operating in May 2018. The ROHP-PAZ observations demonstrated that precise measurements of the phase shift between horizontal and vertical polarizations from Global Navigation Satellite System (GNSS) L-band signals are sensitive to oriented hydrometeors along the ray paths. While this differential phase shift measurement as a function of time has proven very useful, the regular radio occultation (RO) intermediate products from different polarized channels, such as bending angle and phase retrievals on the domain of impact parameter, have never been exploited. In this research, we studied the characteristics of polarimetric phase and bending angle difference retrieved by the radio-holographic (RH) method to mitigate atmospheric multipath effect and to explore their use in data assimilation. To validate RH approach in polarimetric retrievals, we performed end-to-end simulations where the hydrometeors are modeled by the effective refractivity with different horizontal extents. The simulation results demonstrate that the strong precipitation (&gt;15 mm h−1) with 40-km horizontal extent can be detected with the retrieved bending angle shift. The calibration process on the impact parameter domain has also been developed to extract the differential phase and bending angle shift from the actual polarimetric RO data. Statistics from the PAZ data shows that the mean retrieved RH polarimetric phase shift with various horizontal extent is approximately proportional to the tangent point location rain rate at a ratio of 0.02 rad (mm h−1)−1.

The Lunar Geophysical Network Landing Sites Science Rationale

Abstract The Lunar Geophysical Network (LGN) mission is proposed to land on the Moon in 2030 and deploy packages at four locations to enable geophysical measurements for 6–10 yr. Returning to the lunar surface with a long-lived geophysical network is a key next step to advance lunar and planetary science. LGN will greatly expand our primarily Apollo-based knowledge of the deep lunar interior by identifying and characterizing mantle melt layers, as well as core size and state. To meet the mission objectives, the instrument suite provides complementary seismic, geodetic, heat flow, and electromagnetic observations. We discuss the network landing site requirements and provide example sites that meet these requirements. Landing site selection will continue to be optimized throughout the formulation of this mission. Possible sites include the P-5 region within the Procellarum KREEP Terrane (PKT; (lat: 15°; long: −35°), Schickard Basin (lat: −44.°3; long: −55.°1), Crisium Basin (lat: 18.°5; long: 61.°8), and the farside Korolev Basin (lat: −2.°4; long: −159.°3). Network optimization considers the best locations to observe seismic core phases, e.g., ScS and PKP. Ray path density and proximity to young fault scarps are also analyzed to provide increased opportunities for seismic observations. Geodetic constraints require the network to have at least three nearside stations at maximum limb distances. Heat flow and electromagnetic measurements should be obtained away from terrane boundaries and from magnetic anomalies at locations representative of global trends. An in-depth case study is provided for Crisium. In addition, we discuss the consequences for scientific return of less than optimal locations or number of stations.

Adjoint Waveform Tomography of South America

AbstractWe used 3D spectral‐element seismic wave simulations and data from 112 earthquakes and 1,311 seismic stations, totalizing 20,884 unique ray paths, to construct an adjoint waveform tomographic model of South America. We performed 23 conjugate‐gradient iterations using exponentiated phase (EP) measurements. Our final model (SAAM23, South American Adjoint Model—iteration 23) shows a 50% decrease in the EP misfit relative to its 3D starting model. We further assessed the phase misfit reduction by using cross‐correlation travel‐time measurements of 53 earthquakes not included in the inversion. We estimated SAAM23 resolution using point‐spread function tests and density coverage analysis. The Nazca Slab is well imaged and is shown to be continuous in the 300–500 km depth range. Beneath northern South America, the slab traverses the mantle transition zone and continues into the lower mantle. In the central and southern part of South America, the slab appears to flatten near the 650 km discontinuity before continuing into the lower mantle. In the stable Precambrian platform, both cratons (Amazonian and São Francisco), as well as covered cratonic blocks beneath the intracratonic Paraná and Parnaíba basins (Paranapanema and Parnaíba, respectively), show high velocities at lithospheric depths. The seismic Lithosphere/Asthenosphere boundary (LAB) agrees well with published values obtained by S‐wave receiver functions. In the Amazonian craton, the positive lithospheric S‐wave velocity anomalies and LAB depth increase with the average age of the geochronological provinces. No lithospheric high‐velocity anomalies were found beneath the Río de la Plata Craton.

Grouping design method dependence on an illumination system and large off-axis distance for an anamorphic extreme ultraviolet lithography objective

High-numerical-aperture (NA) anamorphic extreme ultraviolet lithography (EUVL) is the next-generation technology to achieve a 3 nm node and below. Current EUVL anamorphic projection objectives in publications are designed with small off-axis distance of the field of view (FOV). Further, the ray path of the illumination system will be obscured by the last mirror of the objective. In fact, as the NA increases, the diameter of the last mirror of the objective becomes larger. Thus, the off-axis distance of the FOV should be larger to ensure that the ray path of the illumination system is not obscured by the objective. It is well known that the design of an anamorphic EUVL objective with large off-axis distance is very challenging. In this paper, a high-NA anamorphic EUVL objective with large off-axis distance is designed. The value of the large off-axis distance that satisfies the unobscured constraint is first analyzed. Then an 8× demagnification isomorphic objective with large off-axis distance is designed by a grouping design process. Finally, the XY polynomial surface is adopted to achieve a 4× demagnification perpendicular to the scanning direction. This method is implemented to design a NA 0.55 anamorphic EUVL objective with large off-axis distance. The performance of the designed objective demonstrates that this design method is very effective and will provide a good structure of the objective for the illumination system design.

Blue-shifted deep ocean currents in the equatorial Indian Ocean

Spectra from two decades of zonal current data at $$\sim $$ 4000 m in the central and western equatorial Indian Ocean show a shift in the dominant frequencies from the west to the east. The 120–180-day period is stronger at 77 $$^\circ $$ E, the 60–120-day period at 83 $$^\circ $$ E, and the 30–90-day period at 93 $$^\circ $$ E. The weakening of lower frequencies near the eastern boundary can be explained using theoretical ray paths of Kelvin waves and reflected Rossby waves. The equatorial Kelvin wave forced by winds reflects from the eastern boundary as Rossby waves with different meridional modes. After reflection, the low (high) frequency Rossby beams travel a larger (shorter) distance before reaching the bottom, thereby creating a shadow zone, a region with low wave energy, between the ray path and the eastern boundary. The shift in frequency with longitude is not evident in the top 1000 m, where the current is dominated by the semi-annual cycle.

Radio Network Forensic with mmWave Using the Dominant Path Algorithm

For the last two decades, cybercrimes are growing on a daily basis. To track down cybercrimes and radio network crimes, digital forensic for radio networks provides foundations. The data transfer rate for the next-generation wireless networks would be much greater than today’s network in the coming years. The fifth-generation wireless systems are considering bands beyond 6 GHz. The network design of the next-generation wireless systems depends on propagation characteristics, frequency reuse, and bandwidth variation. This article declares the channel’s propagation characteristics of both line of sight (LoS) and non-LOS (NLoS) to construct and detect the path of rays coming from anomalies. The simulations were carried out to investigate the diffraction loss (DL) and frequency drop (FD). Indoor and outdoor measurements were taken with the omnidirectional circular dipole antenna with a transmitting frequency of 28 GHz and 60 GHz to compare the two bands of the 5th generation. Millimeter-wave communication comes with a higher constraint for implementing and deploying higher losses, low diffractions, and low signal penetrations for the mentioned two bands. For outdoor, a MATLAB built-in 3D ray tracing algorithm is used while for an indoor office environment, an in-house algorithmic simulator built using MATLAB is used to analyze the channel characteristics.

The B-Scan Image Simulation Method of a Ground-Penetrating Radar Mounted on a Drone Using a High-Frequency Technique

A B-scan image simulation method of a ground-penetrating radar mounted on a drone using a high-frequency technique is proposed. The proposed simulation method aims to predict drone mounted-GPR receiving signals in a complex three-dimensional (3D) scenario, including the reflected signal from various ground types, backscattered signals from considered underground objects, and backscattered volume clutter signals from underground random object clusters. The modeled scenario, including various 3D drone trajectories, target objects, underground random object clusters, ground types, antenna radiation patterns, and desired transmitting time-domain short pulse signals, are taken into account. A high-frequency technique consisting of the geometrical optic (GO) ray-tracing technique to find the GO ray paths, and the GO ray field approximation technique to find the electromagnetic (EM) fields of every GO ray path, are employed instead of conventional numerical methods. This is useful for EM problems with large computational domains. The efficiency in terms of computational time and computer resources is better than that of numerical techniques. Finally, the effect of drone positioning errors, ground types, underground random object clusters, and the antenna radiation patterns on A-scan and B-scan images of a drone-mounted GPR will be illustrated. It is found that the distortion of the hyperbolic signature of underground target objects in the B-scan image occurred by unwanted back-scattered signals or radar clutter from the ground surface, volume clutter from underground random object clusters, and drone positioning errors. Moreover, a high directivity antenna enhances the intensity of the hyperbolic signature. The proposed simulation method will be useful for predicting the drone mounted-GPR signals in various complex 3D scenarios, having various kinds of transmitting signals, and target object configurations and antenna types.

Quantitative mapping of thickness variations along a ray path using geometrical full waveform inversion and guided wave mode conversion

Quantitative guided wave thickness mapping in plate-like structures and pipelines is of significant importance for the petrochemical industry to accurately estimate the minimum remaining wall thickness in the presence of corrosion, as guided waves can inspect a large area without needing direct access. Although a number of inverse algorithms have been studied and implemented in guided wave reconstruction, a primary assumption is widely used: the three-dimensional guided wave inversion of thickness is simplified as a two-dimensional acoustic wave inversion of velocity, with the dispersive nature of the waves linking thickness to velocity. This assumption considerably simplifies the inversion procedure; however, it makes it impossible to account for mode conversion. In reality, mode conversion is quite common in guided wave scattering with asymmetric wall loss, and compared with non-converted guided wave modes, converted modes may provide greater access to valuable information about the thickness variation, which, if exploited, could lead to improved performance. Geometrical full waveform inversion (GFWI) is an ideal tool for this, since it can account for mode conversion. In this paper, quantitative thickness reconstruction based on GFWI is developed in a plate cross-section and applied to study the performance of thickness reconstruction using mode conversion.

3-D Electromagnetic Modeling Explains Apparent-Velocity Increase in Crosshole GPR Data-Borehole Fluid Effect Correction Method Enables to Incorporating High-Angle Traveltime Data

For high-resolution crosshole ground-penetrating radar (GPR) tomography, a wide-range of ray path angles are required, including transmitter-receiver pairs with high-angles. However, artifacts have been observed in the inverted GPR tomograms when high-angle data were incorporated in ray-based inversion (RBI) tomography due to not well-understood increasing apparent velocities for increasing ray-angles. To reduce these artifacts, it is common practice to limit the angular aperture to a threshold between 30° and 50°, which reduces the spatial resolution. We apply 3-D finite-difference time-domain GPR modeling including borehole fluid and resistive loaded finite-length antenna (FLA) models to study the increase of apparent velocity with increasing ray path angle. This study shows that the strong refraction of the electromagnetic waves at the borehole interface between water and subsurface is one of the reasons for these not well-understood phenomena. We introduce a novel borehole-fluid effect correction (BFEC) that relocates the transmitter and receiver positions to the location, where the refraction is occurring to remove any influence of the borehole such that the remaining traveltimes can be inverted using an RBI. BFEC improves the estimated apparent-velocity (relative permittivity) values and enables the incorporation of wide-angle ray paths resulting in more accurate tomograms. We verify the BFEC with a simple layered subsurface model and a realistic synthetic model. By applying curved-ray RBI without and with the BFEC, the subsurface structures are reconstructed with more details for the BFEC data and the average relative error model reduced from 13% to below 8% for the high-resolution inhomogeneous model.

Seismic source location using the shortest path method based on boundary discretisation scheme for microseismic monitoring in underground mines

In seismic monitoring at underground mines, the velocity models can be highly complex, particularly in presence of major geological structures . In practice, homogeneous velocity models have been widely used for the purposes of seismic event location in mine seismology . Although heterogeneous velocity models can be incorporated, ray tracing methods need be adopted in these velocity models to compute the travel times for accurately locating seismic events. These methods require each geological unit to be discretised to grids (corner-node or center-node discretisation scheme), which requires large computational capacity. In this research, the Shortest Path Method (SPM) (Moser, 1991) based on a novel Boundary Discretisation Scheme (BDS) is proposed to compute the travel times for seismic event location in heterogeneous velocity models. Only the boundaries of each geological unit are discretised to nodes. The graphical network is constructed by establishing ray paths between discretised nodes on the boundaries of geological units. Dijkstra’s algorithm (Dijkstra et al., 1959) using heap sorting method (Cao et al., 1993) is applied to search for the shortest ray paths with the first arrivals for seismic event location. The accuracy and efficiency of this method are validated using a 2D synthetic example. The seismic events recorded during the field monitoring of a dyke-roadway intersected area are located with the proposed BDS-SPM method. The results demonstrate that, compared with the conventional location method using homogeneous velocity models, the proposed method can provide more accurate seismic event locations for microseismic monitoring in underground mines.