Papers by Sven Peter Näsholm
This study uses low-frequency, inaudible acoustic waves (infrasound) to probe wind and temperatur... more This study uses low-frequency, inaudible acoustic waves (infrasound) to probe wind and temperature fluctuations associated with breaking gravity waves in the middle atmosphere. Building on an approach introduced by Chunchuzov et al., infrasound recordings are used to retrieve effective sound-speed fluctuations in an inhomogeneous atmospheric layer that causes infrasound backscattering. The infrasound was generated by controlled blasts at Hukkakero, Finland and recorded at the IS37 infrasound station, Norway in the late summers 2014 - 2017. Our findings indicate that the analyzed infrasound scattering occurs at mesospheric altitudes of 50 - 75 km, a region where gravity waves interact under non-linearity, forming thin layers of strong wind shear. The retrieved fluctuations were analyzed in terms of vertical wave number spectra, resulting in approximate kz-3 power law that corresponds to the “universal“ saturated spectrum of atmospheric gravity waves. The kz-3 power law wavenumber ran...
Infrasound waves generated by phenomena at the Earth’s surface can travel to these levels before ... more Infrasound waves generated by phenomena at the Earth’s surface can travel to these levels before returning to the surface and being detected. Observations like travel time, change in backazimuth angle, and trace velocity contain integrated information of all the levels the wave travelled through. These often include stratospheric and mesospheric levels which are otherwise poorly observed.In this work we take a data assimilation technique, the Modulated Ensemble Transform Kalman Filter, which is commonly used in satellite data assimilation, and illustrate how it can be readily used for infrasound data assimilation. We highlight the similarities between the two problems, and the particular challenges in extracting information from summarised quantities. To our knowledge, this is the first work doing data assimilation with a full ray-tracing model as forward operator.
This study uses ground-based recordings of low-frequency, inaudible acoustic waves (infrasound) t... more This study uses ground-based recordings of low-frequency, inaudible acoustic waves (infrasound) to probe the wind and temperature fluctuations associated with internal gravity waves breaking in the middle atmosphere. Building on the approach introduced by Chunchuzov et al., the recorded waveforms are used to retrieve the effective sound speed fluctuations in an inhomogeneous atmospheric layer of infrasound backscattering. The retrieval procedure was applied to infrasound from controlled blasts related to the disposal of military explosives in Hukkakero, Finland and recorded at the IS37 station in Norway over a four-year period from 2014 to 2017. Our findings indicate that infrasound scattering occurs in the lower mesosphere between 50 and 75 km in altitude in a region where gravity waves interact due to strong nonlinear effects and form thin layers with strong wind shears. The retrieved effective sound speed fluctuations were then analysed in terms of the vertical wave number spectr...
Frontiers in Astronomy and Space Sciences, Dec 7, 2022
Modelling the spatial distribution of infrasound attenuation (or transmission loss, TL) is key to... more Modelling the spatial distribution of infrasound attenuation (or transmission loss, TL) is key to understanding and interpreting microbarometer data and observations. Such predictions enable the reliable assessment of infrasound source characteristics such as ground pressure levels associated with earthquakes, man-made or volcanic explosion properties, and ocean-generated microbarom wavefields. However, the computational cost inherent in full-waveform modelling tools, such as Parabolic Equation (PE) codes, often prevents the exploration of a large parameter space, i.e., variations in wind models, source frequency, and source location, when deriving reliable estimates of source or atmospheric properties-in particular for real-time and near-real-time applications. Therefore, many studies rely on analytical regression-based heuristic TL equations that neglect complex vertical wind variations and the range-dependent variation in the atmospheric properties. This introduces significant uncertainties in the predicted TL. In the current contribution, we propose a deep learning approach trained on a large set of simulated wavefields generated using PE simulations and realistic atmospheric winds to predict infrasound ground-level amplitudes up to 1000 km from a ground-based source. Realistic range dependent atmospheric winds are constructed by combining ERA5, NRLMSISE-00, and HWM-14 atmospheric models, and small-scale gravity-wave perturbations computed using the Gardner model. Given a set of wind profiles as input, our new modelling framework provides a fast (0.05 s runtime) and reliable (∼ 5 dB error on average, compared to PE simulations) estimate of the infrasound TL.
Mechanics of Time-Dependent Materials, 2020
This paper proposes some fractional nonlocal viscoelastic models which are under the framework of... more This paper proposes some fractional nonlocal viscoelastic models which are under the framework of both Eringen's nonlocal theory and gradient elasticity theory. Introducing different combinations of new mechanical elements derived from the spatial nonlocal theory, a set of time-space-fractional constitutive models for nonlocal material, such as the Kelvin-Voigt model and the Maxwell model, and their corresponding wave equations are presented. In addition, by applying the wave equations to describe the scattering attenuation from a general energy loss standpoint, the undetermined parameters of the presented constitutive models are obtained. As a discussion of the results, the scattering attenuation curve of the presented model is investigated and is found to be in good agreement with the Blair scattering model. Moreover, the nonlocal fractional Kelvin-Voigt model is applied to describe the creep of sand-bearing soft soil and then compared to existing models as well as experimental data.
Mathematical Geosciences, 2022
This study suggests a stochastic model for time series of daily zonal (circumpolar) mean stratosp... more This study suggests a stochastic model for time series of daily zonal (circumpolar) mean stratospheric temperature at a given pressure level. It can be seen as an extension of previous studies which have developed stochastic models for surface temperatures. The proposed model is a combination of a deterministic seasonality function and a Lévy-driven multidimensional Ornstein–Uhlenbeck process, which is a mean-reverting stochastic process. More specifically, the deseasonalized temperature model is an order 4 continuous-time autoregressive model, meaning that the stratospheric temperature is modeled to be directly dependent on the temperature over four preceding days, while the model’s longer-range memory stems from its recursive nature. This study is based on temperature data from the European Centre for Medium-Range Weather Forecasts ERA-Interim reanalysis model product. The residuals of the autoregressive model are well represented by normal inverse Gaussian-distributed random vari...
<p>An earthquake happened in 18 May 2020 early morning in the Kiruna underground iron ore m... more <p>An earthquake happened in 18 May 2020 early morning in the Kiruna underground iron ore mine, Northern Sweden having a magnitude Mw 4.9. Following the earthquake, the mine was immediately evacuated because of the risk of aftershocks. This event is the largest mining-induced earthquake that has ever taken place in Scandinavia and it produced signals recorded by three infrasound arrays at distances of 7 km (KRIS, Sweden), 155 km (IS37, Norway) and 286 km (ARCI, Norway). We explore seismo-acoustic features of this event recorded in near and far-field. This procedure allows us to track how the signal propagated in the solid earth until the seismometers located at various distances or transmitted to the atmosphere and propagated further to the infrasound stations. Our study also provides a detailed comparison between observed and predicted wave front characteristics at the arrays. We conduct a comparison of amplitude corrected for propagation effect versus magnitude and ground shaking amplitude. These results show that this mine-quake having &#8220;unconventional&#8221; source mechanism generated infrasound recorded up to ~300 km and provided ground shaking information as well as local amplification caused by topographic and geological features.</p>
&amp;amp;lt;p&amp;amp;gt;Like seismic waves traveling through the solid earth, infrasound... more &amp;amp;lt;p&amp;amp;gt;Like seismic waves traveling through the solid earth, infrasound waves traveling through the atmosphere are also sensitive to the medium properties &amp;amp;amp;#8211; in particular to temperature and wind. The exploitation of this information is particularly interesting in regions and altitude ranges where other&amp;amp;lt;br /&amp;amp;gt;measurements are sparse. In this work, we look at the climatology from first-arrival travel-times using a dataset of infrasound observations from northern Scandinavia, this is, in the context of stratospheric temperatures.&amp;amp;lt;/p&amp;amp;gt; &amp;amp;lt;p&amp;amp;gt;The same dataset has recently been exploited to estimate tropospheric and stratospheric cross-winds. This dataset spans 30 years and corresponds to explosions that are due to the destruction of ammunition at a military site in Finland conducted over the months of August and September; hence, it&amp;amp;lt;br /&amp;amp;gt;covers the period of transition from summer to winter stratosphere. The transition between summer and winter stratosphere is clear in the data. However, a significant travel-time variation between years produces inconclusive results when inferring stratospheric temperature trends over the 30 years analyzed. Still, when comparing the travel-times against regional stratospheric temperatures represented in atmospheric re-analysis models, there is a correspondence between models and infrasound data.&amp;amp;lt;/p&amp;amp;gt;
Geophysical Journal International, 2017
Seismic arrays enhance signal detection and parameter estimation by exploiting the time-delays be... more Seismic arrays enhance signal detection and parameter estimation by exploiting the time-delays between arriving signals on sensors at nearby locations. Parameter estimates can suffer due to both signal incoherence, with diminished waveform similarity between sensors, and aberration, with time-delays between coherent waveforms poorly represented by the wave-front model. Sensor-to-sensor correlation approaches to parameter estimation have an advantage over direct beamforming approaches in that individual sensor-pairs can be omitted without necessarily omitting entirely the data from each of the sensors involved. Specifically, we can omit correlations between sensors for which signal coherence in an optimal frequency band is anticipated to be poor or for which anomalous time-delays are anticipated. In practice, this usually means omitting correlations between more distant sensors. We present examples from International Monitoring System seismic arrays with poor parameter estimates resulting when classical f-k analysis is performed over the full array aperture. We demonstrate improved estimates and slowness grid displays using correlation beamforming restricted to correlations between sufficiently closely spaced sensors. This limited sensor-pair correlation (LSPC) approach has lower slowness resolution than would ideally be obtained by considering all sensor-pairs. However, this ideal estimate may be unattainable due to incoherence and/or aberration and the LSPC estimate can often exploit all channels, with the associated noise-suppression, while mitigating the complications arising from correlations between very distant sensors. The greatest need for the method is for short-period signals on large aperture arrays although we also demonstrate significant improvement for secondary regional phases on a small aperture array. LSPC can also provide a robust and flexible approach to parameter estimation on three-component seismic arrays.
Geophysical Research Letters, 2015
This study demonstrates probabilistic infrasound propagation modeling using realistic perturbatio... more This study demonstrates probabilistic infrasound propagation modeling using realistic perturbations. The ensembles of perturbed analyses, provided by the European Centre for Medium-Range Weather Forecasts (ECMWF), include error variances of both model and assimilated observations. Ensemble spread profiles indicate a yearly mean effective sound speed variation of up to 8 ms −1 in the stratosphere, exceeding occasionally 25 ms −1 for a single ensemble set. It is shown that errors in point estimates of effective sound speed are dominated by variations in wind strength and direction. One year of large mining explosions in the Aitik mine, northern Sweden, observed at infrasound array IS37 in northern Norway are simulated using 3-D ray tracing. Probabilistic propagation modeling using the ensembles demonstrates that small-scale fluctuations are not always necessary to improve the match between predictions and observations.
The sensitivity of correlation detectors increases greatly when the outputs from multiple seismic... more The sensitivity of correlation detectors increases greatly when the outputs from multiple seismic traces are considered. For single-array monitoring, a zero-offset stack of individual correlation traces will provide significant noise suppression and enhanced sensitivity for a source region surrounding the hypocenter of the master event. The extent of this region is limited only by the decrease in waveform similarity with increasing hypocenter separation. When a regional or global network of arrays and/or 3-component stations is employed, the zero-offset approach is only optimal when the master and detected events are co-located exactly. In many monitoring situations, including nuclear test sites and geothermal fields, events may be separated by up to many hundreds of meters while still retaining sufficient waveform similarity for correlation detection on single channels. However, the traveltime differences resulting from the hypocenter separation may result in significant beam loss ...
The IMS infrasound array IS37, near Bardufoss in northern Norway, started providing data in Octob... more The IMS infrasound array IS37, near Bardufoss in northern Norway, started providing data in October 2013. In the boreal summer, the stratospheric waveguide promotes the observation at IS37 of infrasound from numerous military and industrial sources of repeating explosions to the east and southeast at distances between 200 and 800 km. In August and September 2014, IS37 recorded for the first time infrasound signals from each of 15 ammunition destruction explosions at Hukkakero, a military site in northern Finland at a distance of 320 km. The first 12 of the explosions were large blasts with yields of approximately 20 tons and, for each of these events, an extensive wavetrain is observed. Approximately 18 minutes after the explosion (celerity 296 m/s) a long duration signal, rich in high frequencies, is observed that extends for approximately two minutes. Between 3 and 4 minutes after the arrival of this signal (celerity around 240 m/s) signals of far shorter duration and lower freque...
The Journal of the Acoustical Society of America, 2013
Frequency-dependent acoustical loss due to a multitude of physical mechanisms is commonly modeled... more Frequency-dependent acoustical loss due to a multitude of physical mechanisms is commonly modeled by multiple relaxations. For discrete relaxation distributions, such models correspond with causal wave equations of integer-order temporal derivatives. It has also been shown that certain continuous distributions may give causal wave equations with fractional-order temporal derivatives. This paper demonstrates analytically that if the wave-frequency ω satisfies ΩL≪ω ≪ΩH, a continuous relaxation distribution populating only Ω∈[ΩL,ΩH] gives the same effective wave equation as for a fully populated distribution. This insight sparks the main contribution: the elaboration of a method to determine discrete relaxation parameters intended for mimicking a desired attenuation behavior for band-limited waves. In particular, power-law attenuation is discussed as motivated by its prevalence in complex media, e.g., biological tissue. A Mittag–Leffler function related distribution of relaxation mech...
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Papers by Sven Peter Näsholm