Probe measurements of ICRF surface waves in the TORTUS tokamak

Accurate and reliable measurement of surface waves is important to Comprehensive Nuclear-Test-Ban Treaty (CTBT) monitoring because the M s :m b , discriminant and its regional variants can in many cases unambiguously identify events as earthquakes or explosions. Surface wave processing at the International Data Center (IDC) is designed to be completely automated and is performed using the program Maxsurf. Maxsurf searches for surface wave characteristics in the expected surface wave arrival time window for all continuous long-period and broadband data in the IDC processing stream. The Prototype IDC GSETT3 Reviewed Event Bulletin (REB) now contains a very large and growing data set of surface wave measurements. Users of this data set need to be aware of processing changes and calibration errors in the GSETT3 experimental bulletin. The prototype International Monitoring System (IMS) surface wave detection threshold is approximately one magnitude unit lower than the detection threshold of other global networks that use visual identification of surface waves. Surface wave identification and measurement can be improved through development of regionalized earth models, phase-matched filtering and the use of path corrected spectral magnitudes in place of M s . Regionalized earth models are developed through tomographic inversion of a very large data set of phase and group velocity dispersion measurements. Discrimination capability can be improved through the use of maximum likelihood magnitudes and maximum likelihood upper bounds.

An overview of surface wave methods and a reliability study of a simplified inversion technique

Measurements are presented of the edge wave magnetic fields generated by fast wave antennas in the TORTUS Tokamak. All measurements were made in a deuterium plasma and at frequencies in the range 2< omega / omega ci<4, where omega ci is the ion cyclotron frequency. High Q cavity modes were observed only for the m=+1 mode, although many other cavity modes were expected on the basis of cold plasma calculations. High m modes were also observed, not as cavity modes, but in the form of an evanescent beam guided along steady magnetic field lines in the plasma edge. The three-dimensional structure of the beam was determined using toroidal and poloidal arrays of magnetic probes. The existence of the beam is attributed to Alfven resonance conditions at the plasma edge.

Techniques for extracting surface wave characteristics from radar backscatter have been investigated and improved over the last several decades. Much of this research has focused on the use of backscatter intensity from navigational radars for characterization of wave period and direction, and has clearly demonstrated accurate measurement of these wave characteristics. However, the precise determination of significant wave height has been more problematic due to the required application of a modulation transfer function. Furthermore, low sea states generally do not provide enough backscatter intensity for evaluation of wave characteristics, and thus, navigational radar measurements are restricted to relatively high sea states. More recently, techniques using Doppler velocities as a measurement of surface waves have been an area of increasing focus and development. An advantage of this approach is that no modulation transfer function is required, and only phase information is used from the backscattered radar signal. Recent research suggests that the relationship between Doppler velocities and wave height may be more consistent than that between radar backscatter intensity and wave height. In July 2010, surface waves were measured during an experiment at the Scripps Institution of Oceanography pier. Radar measurements were performed using a high-resolution pulse-Doppler instrumentation radar at low grazing angle (∼1 deg) with a pulse repetition frequency of 800 Hz and spatial resolutions of 10–30 cm. Radar data for X- and Ku-bands using both VV and HH polarizations were collected. Concurrent buoy measurements were also performed, along with the collection of wind speed and direction data. Measured seaways consisted of small significant wave heights (glassy conditions to &lt;1 m), and contained combinations of wind sea and swell. Doppler processing of the radar data provided estimates of surface wave orbital velocity spectra in wavenumber and frequency domains. The velocity spectra were converted to sea surface elevation spectra. Using these spectra, peak periods were computed as well as RMS wave heights, thus providing approximate significant wave heights. The methods for extracting wave spectra, peak periods, and significant wave heights are discussed, and results are compared with buoy measurements. When sufficient capillary waves existed on the sea surface, the radar and buoy measured wave spectra were in agreement, and analysis indicates that the instrumentation radar was able to detect and spectrally distinguish between wind seas and swell.

From 2002-2007, the Southeast Coastal Ocean Observing System (SEACOOS) deployed high frequency (HF) radars to overlook several venues stretching from the West Florida Shelf to the North Carolina Shelf. Based on extensive deliberations within SEACOOS, we decided to assess the two differing types of coastal ocean current radars within the southeast that were on the commercial market. The long-range SeaSondes (SS) were deployed to sense surface currents at hourly intervals and a 6 km resolution along the West Florida Shelf and the North Carolina Shelf. The medium and long-range Wellen Radars (WERA) were deployed along the Florida Straits and along the South Atlantic Bight with spatial resolutions of 1.2 to 3 km sampling at time scales of minutes. A common theme in these deployments was to sense the Loop Current, Florida Current and the Gulf Stream, which transport heat poleward as part of the gyre circulation.Several lessons were learned as part of these deployments, such as the need to protect against lightening strikes and the challenge of providing robust communication links between the remote sites and a central hub to make the data available in near real-time. Since states in the southeast and surrounding the Gulf of Mexico are prone to the passage of hurricanes, surface current and wave measurements during hurricanes are invaluable for improving storm surge and inundation models that are now being coupled to surface waves. In addition, significant wave heights (and directional surface wave spectra) are critical in the model assessment. Data quality and accuracy of the surface current and wave fields remain a central issue to search and rescue and safe maritime operations and to understanding the limitations of these radar systems. As more phased array systems (i.e., WERAs) are deployed for surface current and wave measurements, more attention needs to be placed on the interoperability between the two types of systems to insure the highest quality data possible is available to meet applied and operational goals. To insure the highest quality data possible, a full-time technician and a half-time IT specialist are needed for each installation as well as access to spares to keep these systems running consistently and to make quality observations available in near real-time.

This paper addresses two issues with regard to nonlinear ocean waves. (1) The first issue relates to the often-confused differences between the coordinates used for the measurement and characterization of ocean surface waves: The surface elevation and the complex modulation of a wave field. (2) The second issue relates to the very different kinds of physical wave behavior that occur in shallow and deep water. Both issues come from the known, very different behaviors of deep and shallow water waves. In shallow water one often uses the Korteweg-deVries that describes the wave surface elevation in terms of cnoidal waves and solitons. In deep water one uses the nonlinear Schrödinger equation whose solutions correspond to the complex envelope of a wave field that has Stokes wave and breather solutions. Here I make clear the relationships between the two ways of characterizing surface waves. Furthermore, and more importantly, I address the issues of matching the two types of wave behavior as the wave motion passes from deep to shallow water, or vice versa. For wave measurements we normally obtain the surface elevation with a wave staff, resistance gauge or pressure recorder for getting time series. Remote sensing applications relate to the use of lidar, radar or synthetic aperture radar for obtaining space series. The two types of wave behavior can therefore crucially depend on where the instrument is placed on the “ground track” or “field” over which the lidar or radar measurements are made. Thus the matching problem from deep to shallow water is not only important for wave measurements, but also for wave modeling. Modern wave models [Osborne, 2010, 2018, 2019a, 2019b] that maintain the coherent structures of wave dynamics (solitons, Stokes waves, breathers, superbreathers, vortices, etc.) must naturally pass from deep to shallow water where the nature of the nonlinear physics, and the form of the coherent structures, change. I address these issues and more herein. This paper is directed towards the development of methods for the real time measurement of waves by shipboard radar and for wave measurements by airplane and helicopter using lidar and synthetic aperture radar. Wave modeling efforts are also underway.

Novel experimental techniques to measure ultrasonic velocity and attenuation of surface waves on fluid‐filled porous natural rocks are presented. Our experimental results are consistent with the theoretical predictions of Feng and Johnson (1983). Depending on the interface conditions, i.e., whether the surface pores are open or closed, pseudo‐Rayleigh, pseudo‐Stoneley, and/or true Stoneley surface waves may exist on fluid‐saturated rocks. We have demonstrated the existence of the “slow” surface wave (true Stoneley mode) on fluid‐filled porous rocks with closed surface pores. The velocity and attenuation of the “slow” surface mode may be used to assess the dynamic permeability of porous formations.

Conventional active surface wave measurements performed using a transient or continuous source are often limited in the maximum depth of penetration due to the difficulty of generating lowfrequency energy with reasonably portable sources. This limitation may inhibit accurate seismic site response calculations because of the inability to define deeper structure. By combining measurements of surface waves generated by passive sources including microtremors and cultural noise, it is possible to overcome this problem and develop soil profiles to much larger depths. Passive surface wave measurements are performed using a two-dimensional array of receivers placed in a circular configuration. The frequency range measured during passive testing is often on the order of 1 to 10 Hz. The resulting dispersion curve is combined with the dispersion curve from an active test at the same location using an irregular, linear array. Generally, the passive and active measurements overlap in the frequency range of approximately 4 to 10 Hz and it is necessary to resolve differences between them. Near-field effects on array-based active surface wave methods were studied using numerical simulations for two typical soil conditions. Plots of normalized phase velocity vs. normalized array center capture the effects. The plots of the normalized parameters from the numerical simulation results agree well with the plots from experimental dispersion curves at two sites. Therefore, it may be concluded that the differences between the active and passive dispersion curves at low frequencies result from errors caused by near-field effects. It is recommended that a composite dispersion curve be obtained by using only passive dispersion data at low frequencies. The composite dispersion curve is then inverted to obtain the shear wave velocity profile. The proposed procedure is illustrated at two sites in Memphis, Tennessee and San Jose, California.

Surface waves generally do not penetrate into the interior of a solid and are useful for studying near surface anomalies. For homogeneous material, surface waves travel at a speed in all direction which is related to the speed of compressional and shear body waves. For anisotropic media, the general theory (Synge, 1957) shows that the velocity of propagation of surface waves varied with direction and for some direction may not exist. Confirmation of the theoretical result has not been established. This paper describes experimental measurements of surface and body waves for a homogeneous, isotropic material (Plexiglas ) and a homogeneous anisotropic material (Phenolite). Phenolite (Uren, 1989) has approximately transverse symmetry, similar to a finely layered composite material.

In order to investigate the gelation process of tungstic acid, surface wave measurements at several frequencies (50–200 Hz) were carried out with a deflected-laser-light detection system using surface waves excited with a PZT bimorph vibrator. During the gelation process, marked changes were observed in the surface wave velocity and amplitude. From the relationship between the frequency and wave number of the surface wave in the sol and gel states, it was found that the surface wave in the sol state was mainly the surface tension wave, and that in the gel state, the Rayleigh wave. We also estimated surface tension in the sol state and shear elastic modulus in the gel state from the present experimental results. The results of the present investigations demonstrate the usefulness of the surface wave measurement for studying the gelation process beyond the transition point.

Full waveform tomography for near-surface applications has garnered increased attention in recent years due to increasing computational capabilities and rapid developments in the continued search for hydrocarbon sources. Full waveform tomography attempts to solve an inversion problem whereby the entirety of the seismic waveforms measured at a site are matched with waveforms acquired from numerical simulations of wave propagation in a subsurface model. One of the strengths of the method is the ability to incorporate all types of waves into the inversion. With careful placement of measurement locations, combined recordings of body waves and surface waves can be exploited to improve the inversion results. One near-surface engineering application where a combined body and surface wave full waveform inversion (FWI) technique can potentially improve resolution capabilities is for evaluation of liquefaction triggering. Liquefaction of granular soils refers to the loss of shear strength caused by rapid dynamic loading as encountered in earthquake events. The evaluation of liquefaction triggering typically involves site characterization at pre-determined locations with standard penetration tests (SPT) and cone penetration tests (CPT) to estimate the resistance of the subsurface soils against liquefaction. Geophysical measurements from either downhole, crosshole, or surface-wave testing methods can be used to augment this SPT/CPT information. Since boreholes or CPT soundings are often already present at a site being investigated for liquefaction hazards, a receiver array could be simultaneously deployed at the surface and within the subsurface to capture both surface and body waves generated by a single source. This study uses numerical simulations to examine how the accuracy and resolution of FWI can be improved with combined body and surface wave measurements within the context of characterizing liquefaction triggering. The results demonstrated that the spatial extent of liquefaction is better estimated using the combined full waveform approach when compared to full waveform of only surface measurements and when compared to interpolation between localized in-situ measurements.

Previous studies of the first author and others have focused on low audible frequency (<1 kHz) shear and surface wave motion in and on a viscoelastic material comprised of or representative of soft biological tissue. A specific case considered has been surface (Rayleigh) wave motion caused by a circular disk located on the surface and oscillating normal to it. Different approaches to identifying the type and coefficients of a viscoelastic model of the material based on these measurements have been proposed. One approach has been to optimize coefficients in an assumed viscoelastic model type to match measurements of the frequency-dependent Rayleigh wave speed. Another approach has been to optimize coefficients in an assumed viscoelastic model type to match the complex-valued frequency response function (FRF) between the excitation location and points at known radial distances from it. In the present article, the relative merits of these approaches are explored theoretically, computationally, and experimentally. It is concluded that matching the complex-valued FRF may provide a better estimate of the viscoelastic model type and parameter values; though, as the studies herein show, there are inherent limitations to identifying viscoelastic properties based on surface wave measurements.

Discrete Alfven Waves (DAWs) have been observed as antenna resistance peaks and as enhanced edge fields in the TORTUS tokamak during Alfven wave heating experiments. A kinetic theory code has been used to calculate the antenna loading and the structure of the DAW fields for a range of plasma current and density profiles. There is fair agreement between the measured and predicted amplitude of the DAW fields in the plasma edge when both are normalized to the same antenna power.

[1] We have collected time series data of short oceanic waves as a part of the International Polar Year (IPY) 2007–2008. Using a shipboard laser wave slope (LAWAS) system operating at 900 nm, we have obtained wave slopes measurements up to 60 rad m−1 wave number. We have compared our in situ wave slopes with collocated and concurrent high-resolution upwind Normalized Radar Cross Sections (NRCS) collected by QuikSCAT. The LAWAS measured wave slope spectra were consistent with local wind speeds and QuikSCAT measured NRCS. Our measured short wave mean slopes indicate their enhancement by long waves (0–1 rad m−1) at small values of long-wave slope. Concurrent with wave slope measurements, the strength of the reflected LAWAS light beam was analyzed in terms of the light attenuation coefficient at 900 nm. We have observed a correlation between surface elevation and light attenuation. The mechanism of wave modulated beam attenuation was found to be related to the instantaneous long wave skewness.

Although in recent years surface wave methods have undergone significant development that has greatly enhanced their capabilities, little effort has been spent to determine the uncertainty associated with surface wave measurements. The objective of this study is to determine how the uncertainty of the experimental data is mapped into the uncertainty of the shear wave velocity profile via the inversion algorithm. The methodology developed in this study for estimating the uncertainty of the shear wave velocity profile from surface wave measurements is based on the assumption that the experimental data are normally distributed. The validity of this hypothesis was experimentally verified using data gathered at two sites in Italy where surface wave tests were performed using linear arrays of multiple receivers. The experimental dispersion curve measured at the site was subsequently inverted to obtain the expected shear wave velocity profile together with an estimate of the associated standard deviation. The final results show that uncorrelated noise has a very little influence on multistation surface wave tests, confirming their robustness for applications in noisy environments.