Julius Fricke - Academia.edu (original) (raw)
Papers by Julius Fricke
Infrasound Technology Workshop, 2010, Tunis, Tunesia, 2010
In this thesis, the possibilities of infrasonic interferometry for probing the troposphere and st... more In this thesis, the possibilities of infrasonic interferometry for probing the troposphere and stratosphere with microbaroms are explored. Infrasonic interferometry determines the delay time between two sensors by cross correlating their infrasound recordings. The obtained delay time can be used to estimate the Green’s function, which is the impulse response of the medium. Until now this approach was applied in other research fields, e.g., oceanography, seismology and ultrasonics, but it was used only occasionally for infrasound studies. In this thesis, the new research field of infrasonic interferometry is explored from the theoretical basics, via numerical experiments with synthetic data, up to the application to tropospheric and stratospheric microbaroms.Applied Geophysics and Petrophysic
Journal of Geophysical Research: Atmospheres, 2014
• Infrasonic interferometry can be applied to microbaroms • The infrasonic wave of microbaroms is... more • Infrasonic interferometry can be applied to microbaroms • The infrasonic wave of microbaroms is coherent up to a distance of 40 km • The estimation of surface wind speed by infrasonic interferometry is possible
A sound intensity probe is an excellent means in many fields of acoustics to track acoustical ener... more A sound intensity probe is an excellent means in many fields of acoustics to track acoustical energy flow. In room acoustics this method is rarely applied in practise but is none the less appealing, e.g., to examine the timing and localization of scattered and diffracted reflections. Unfavourable are common single point measurements, which tend to lack a correct qualitative assessment in the ”early”, i.e., anisotropic sound field. The reason is attributed to the interference pattern in the scope of early reflections. Our work uses a scalable spherical array by which sound pressure is recorded on the surface of a sphere, using efficient spatial sampling. Based on near-field acoustical holography the sound-pressure is reconstructed on an equally spaced square grid. By calculating the pressure-gradient for each point of the lattice, the intensity vector is reconstructed inside a large volume. In a preliminary approach we simulated and measured the sound field of different rooms using the propo...
In this paper a localization method is analyzed, which uses Acoustic Vector Sensors (AVS). An AVS... more In this paper a localization method is analyzed, which uses Acoustic Vector Sensors (AVS). An AVS measures the directed velocity and the pressure in an area approaching a single point. In order to localize sound sources, the Multiple Signal Classification (MUSIC) was applied, which determines the signal and the noise components. MUSIC only uses the noise components (Noise Subspace) to estimate the direction of arrival. To estimate the quality of this method, the accuracy and the resolution of the localization were compared with an established pressure-based method. The accuracy and resolution of the AVS-based method are higher. MUSIC was more robust against calibration errors of phase and frequency. Our measurements showed that the combination of AVS, and MUSIC provides an efficient localization system.
ABSTRACT The travel time of infrasound through the stratosphere depends on the temperature profil... more ABSTRACT The travel time of infrasound through the stratosphere depends on the temperature profile and the wind speed. These atmospheric conditions can be estimated by determining the travel times between different receivers (microbarometers). Therefore the determination of the travel time of infrasound between different receivers becomes more and more important. An approach to determine the travel time is infrasound interferometry. In this work, the infrasound interferometry is applied to synthetic data of active and passive sources refracted by the stratosphere is tested. The synthetic data were generated with a raytracing model. The inputs of the raytracing model are the atmospheric conditions and a source wavelet. As source wavelet we used blast waves and microbaroms. With the atmospheric conditions and the source wavelet the raytracing model calculates the raypath and the travel time of the infrasound. In order to simulate the measurement of a receiver the rays which reach the receiver need to be found. The rays which propagate from a source to the receiver are called eigen rays. The simulation of the receiver measurements takes into account the travel time along the eigen rays, the attenuation of the different atmospheric layers, the spreading of the rays and the influence of caustics. The simulated measurements of the different receivers are combined to synthetic barograms. Two synthetic experiments were performed with the described model. In the first experiment the interferometry was applied to barograms of active sources like blast waves. The second experiment with microbaroms tests the applicability of interferometry to barograms of passive sources. In the next step infrasound interferometry will be applied to measured barograms. These barograms are measured with the 'Large Aperture Infrasound Array' (LAIA). LAIA is being installed by the Royal Netherlands Meteorological Institute (KNMI) in the framework of the radio-astronomical 'Low Frequency Array' (LOFAR) initiative. LAIA will consist of thirty microbarometers with an aperture of around 100 km. The in-house developed microbarometers are able to measure infrasound up to a period of 1000 seconds, which is in the acoustic-gravity wave regime. The results will also be directly applicable to the verification of the 'Comprehensive Nuclear-Test-Ban Treaty' (CTBT), where uncertainties in the atmospheric propagation of infrasound play a dominant role. This research is made possible by the support of the 'Netherlands Organisation for Scientific Research' (NWO).
ABSTRACT Ambient noise interferometry attempts to reconstruct the impulse response of a linear sy... more ABSTRACT Ambient noise interferometry attempts to reconstruct the impulse response of a linear system undergoing excitations by a random forcing. The method has proven to be quite general, finding applications in diverse fields such as ultrasonics, helioseismology, regional and global seismology, ocean acoustics, and exploration seismology. Due to the pervasive random forcing of microbaroms, the study of atmospheric infrasound can benefit from approaches developed in the field of interferometry as well. Moreover, continuous noise sources, such as microbaroms, provide a means to quantify the strong time-dependent changes inherent in the structure of the atmosphere. For pairs of infrasonic sensors separated by distances between 1 and 15 km, direct arrivals have been observed in long-time correlations of ambient noise from stations within 3 different networks: the network operated by the Alaska Volcano Observatory, the I53US array, and the Utah 07 experiment. As in seismic ambient noise studies, these interferometric arrivals show dispersive properties characteristic of guided wave propagation. As a result, infrasonic ambient noise interferometry can provide information on the gross structure of the atmospheric boundary layer. At Fourpeaked Volcano, Alaska, a surface-based temperature inversion produced a strong waveguide between two infrasound sensors. The propagation time of the interferometric arrivals shifted over the course of several days in close correspondence with regional temperature trends at nearby meteorological stations. Independent ECMWF data confirms the existence of a temperature inversion during this time period. Analysis of 4 infrasound sensors near Park City, Utah, further demonstrates the ability of infrasonic ambient noise interferometry to retrieve guided waves between stations separated by 5 to 10 km. Among the 4 stations, we observe strong interferometric arrivals on different station pairs at different times, attesting to variable atmospheric conditions over the array. Correlations of ambient noise on the different elements of the I53US array enable the comparison of interferometry with traditional beamforming. Multiple small aperture arrays of infrasound sensors within the Utah 07 experiment open up the possibility of cross-correlating ambient noise on pairs of phased arrays. Proper phasing of arrays prior to cross-correlation should enhance the ability to detect stratospheric and thermospheric returns. These arrivals are expected to hold important information on upper atmospheric processes, such as Sudden Stratospheric Warming events.
ABSTRACT Long range infrasound propagation from explosions often concerns stratospheric paths bet... more ABSTRACT Long range infrasound propagation from explosions often concerns stratospheric paths between source and receiver. Such paths highly depend on the wind and temperature structure around the stratopause, which is between 40 and 50 km in the northern hemisphere winter. In summer, the stratosphere is much more stable than in winter as energy can not propagate through the tropopause from the troposphere into the stratosphere. Furthermore, dramatic events like a Sudden Stratospheric Warming (SSW) happen every winter on the northern hemisphere and disturb, or even reverse, the circumpolar vortex flow. SSWs can occur in a couple of days given rise to temperature changes of tens of degrees Celsius in the stratosphere. Understanding infrasound propagation under these strongly varying upper atmospheric conditions is of importance for source identification. But also, inversely, when infrasound is used a passive atmospheric probe for the upper atmosphere. In this presentation, a climatology of stratospheric infrasound detections from explosions is presented from seismo-acoustic studies. The high variability in winter time observations (northern hemisphere) versus summer time is partly explained by the assumed propagation through atmospheric models. Explanations are sought in terms of unresolved stratospheric fine scale structure.
ABSTRACT The estimation of the traveltime of infrasound through the atmosphere is interesting for... more ABSTRACT The estimation of the traveltime of infrasound through the atmosphere is interesting for several applications. For example, it could be used to determine temperature and wind of the atmosphere, since the traveltime depends on these atmospheric conditions (Haney, 2009). In this work the traveltime is estimated with infrasonic interferometry. In other words, we calculate the crosscorrelations of data of spatially distributed receivers. With this method the traveltime between two receivers is determined without the need for ground truth events. In a first step, we crosscorrelate synthetic data, which are generated by a raytracing model. This model takes into account the traveltime along the rays, the attenuation of the different atmospheric layers, the spreading of the rays and the influence of caustics. In these numerical experiments we show that it is possible to determine the traveltime through infrasonic interferometry. We present the results of infrasonic interferometry applied to measured data. Microbaroms are used in the crosscorrelation approach. Microbaroms are caused by ocean waves and are measured by the 'Large Aperture Infrasound Array' (LAIA). LAIA is being installed by the Royal Netherlands Meteorological Institute (KNMI) in the framework of the radio-astronomical 'Low Frequency Array' (LOFAR) initiative. LAIA consists currently of around twenty receivers (microbarometers) with an aperture of around 100 km, allowing for several inter-station distances. Here, we show the results of crosscorrelations as a function of receivers distance, to assess the signal coherency. This research is made possible by the support of the 'Netherlands Organization for Scientific Research' (NWO). Haney, M., 2009. Infrasonic ambient noise interferometry from correlations of microbaroms, Geophysical Research Letters, 36, L19808
The traveltime of infrasound through the atmosphere depends on the temperature and the wind. The ... more The traveltime of infrasound through the atmosphere depends on the temperature and the wind. The basic idea of this project is to use the traveltime to estimate these atmospheric conditions. In order to measure the traveltime we can calculate the crosscorrelation between infrasound receivers (microbarometers). This method is called interferometry. On this poster we present the tests of the Interferometry with synthetic data (barograms). In future we will use the measured ambient noise of the 'Large Aperture Infrasound Array ' (LAIA) instead of synthetic data. influence of caustics. Eigen rays are the rays which connect source and receiver (figure 1 and 2). 3. Results Figure 3 shows the results of a crosscorrelation between the synthetic data of receiver 1 and receiver 2. The highest correlation is reached at traveltimes between 753s and 756s. 1. Background Infrasound propagates over wide ranges and up to thermosopheric altitudes through the atmosphere. The traveltime depends on the temperature and wind structure. We are going to to use the traveltime to estimate these atmospheric conditions. 2. Methods In order to measure the traveltime we can calculate the crosscorrelation between the data of two infrasound receivers (microbarometers). This Figure 2. Eigen rays to receiver 2 of 241 Infrasound source distributed from-60km till 60km 4. Conclusion By calculating the crosscorrelation of the synthetic data, it was possible to determine the traveltime between two receivers. The next step will be the inversion of the model to estimate the atmospheric conditions by the traveltime. In future we will use the method is called Interferometry. Here we present the tests of the inter-measured ambient ferometry with synthetic data (barograms). To simulate the synthetic data, we implemented a raytracing model. It takes into account the Figure 3. Normalized synthetic barograms of receiver 2. noise of the 'Large
The Journal of the Acoustical Society of America, 2013
ABSTRACT It has been theoretically shown by Wapenaar (2006) that the non-reciprocal Green'... more ABSTRACT It has been theoretically shown by Wapenaar (2006) that the non-reciprocal Green's function can be retrieved with cross-correlation. Thus, interferometric techniques can be applied to a moving medium such as the atmosphere. Numerical experiments have shown that the delay times of stratospherically refracted infrasound can be obtained from cross-correlation between pairs of microbarometers. Doing so, information about the wind and temperature around the stratopause can be passively gathered from the stationary phase with, for example, the continuous noise of microbaroms. Actual measurements have been used from the Large Aperture Infrasound Array (LAIA) in the Netherlands. LAIA consists of several microbarometers with inter-station distances ranging from a few kilometers to tens of kilometers and is ideally suited to assess the results from theoretical and numerical experiments in practice. Results will be shown on the correlation length of infrasound from microbaroms and the effect of wind and temperature on the delay times retrieved from cross-correlations.
The Journal of the Acoustical Society of America, 2013
The atmospheric wind and temperature can be estimated through the traveltimes of infrasound betwe... more The atmospheric wind and temperature can be estimated through the traveltimes of infrasound between pairs of receivers. The traveltimes can be obtained by infrasonic interferometry. In this study, the theory of infrasonic interferometry is verified and applied to modeled stratospherically refracted waves. Synthetic barograms are generated using a raytracing model and taking into account atmospheric attenuation, geometrical spreading, and phase shifts due to caustics. Two types of source wavelets are implemented for the experiments: blast waves and microbaroms. In both numerical experiments, the traveltimes between the receivers are accurately retrieved by applying interferometry to the synthetic barograms. It is shown that microbaroms can be used in practice to obtain the traveltimes of infrasound through the stratosphere, which forms the basis for retrieving the wind and temperature profiles. V
Journal of Geophysical Research: Atmospheres, 2012
Long-range infrasound propagation strongly depends on the state of the stratosphere. Infrasound c... more Long-range infrasound propagation strongly depends on the state of the stratosphere. Infrasound can be efficiently ducted between the Earth's surface and the stratopause under a favorable wind and temperature structure between 40 and 50 km altitude. Understanding infrasound propagation under variable stratospheric conditions is of importance for a successful verification of the Comprehensive Nuclear-Test Ban Treaty, in which infrasound is used as a verification technique. Inversely, infrasound observations can be used in acoustic remote sensing of the upper atmosphere. In previous studies, attention has been paid to the strength and direction of the circumpolar vortex wind. In this study, an analysis is made of the temperature effect in the stratosphere on infrasound propagation. A case study is presented from an explosion during a sudden stratospheric warming. During such conditions, the size of the classical stratospheric shadow zone ($200 km) appeared to be reduced by a factor of 2. The occurrence of such conditions is quantified by evaluating 10 years of atmospheric specifications. It unexpectedly appeared that the size of the shadow zone can become smaller than 100 km, which is confirmed by evaluating infrasound detections from mining blasts in southwestern Siberia, Russia. These results are valid over a latitudinal range of 20 N to 60 N, which is determined by the stratospheric surf zone.
ABSTRACT The travel time of infrasound through the atmosphere depends on the temperature and the ... more ABSTRACT The travel time of infrasound through the atmosphere depends on the temperature and the wind. These atmospheric conditions could be estimated by measuring the travel times between different receivers (microbarometers). For such an estimation an inverse model of the propagation of infrasound through the atmosphere is essential. In the first step it is useful to build a forward model. The inputs of our raytracing model are the atmospheric conditions and the positions of source and receiver. The model consists of three elements the source, the channel and the receiver. The source is a blast wave or microbaroms. The channel is the atmosphere and it takes into account the travel time along the eigen ray, the attenuation of the different atmospheric layers, the spreading of the rays and the influence of caustics. Each receiver is reached by different rays (eigen rays). To determine the eigen rays is part of the receiver element. As output the model generates synthetic barograms. The synthetic barograms can be used to explain measured barograms. Furthermore the synthetic barograms can also be used to evaluate the determination of the travel time. The accurate travel time is for the inverse model as input essential. Since small changes of the travel time lead to big changes of the output (temperature and wind). The travel time between two receivers is determined by crosscorrelating the barograms of these two receivers. This technique was already successfully applied in the troposphere (Haney, 2009). We show that the same can be achieved with more complicated stratospheric phases. Now we compare the crosscorrelation of synthetic barograms with the crosscorrelation of measured barograms. These barograms are measured with the 'Large Aperture Infrasound Array' (LAIA). LAIA is being installed by the Royal Netherlands Meteorological Institute (KNMI) in the framework of the radio-astronomical 'Low Frequency Array' (LOFAR) initiative. LAIA will consist of thirty microbarometers with an aperture of around 100 km. The in-house developed microbarometers are able to measure infrasound up to a period of 1000 seconds, which is in the acoustic-gravity wave regime. The results will also be directly applicable to the verification of the 'Comprehensive Nuclear-Test-Ban Treaty' (CTBT), where uncertainties in the atmospheric propagation of infrasound play a dominant role. This research is made possible by the support of the 'Netherlands Organisation for Scientific Research' (NWO). Haney, M., 2009. Infrasonic ambient noise interferometry from correlations of microbaroms, Geophysical Research Letters, 36, L19808, doi:10.1029/2009GL040179
Infrasound Technology Workshop, 2010, Tunis, Tunesia, 2010
In this thesis, the possibilities of infrasonic interferometry for probing the troposphere and st... more In this thesis, the possibilities of infrasonic interferometry for probing the troposphere and stratosphere with microbaroms are explored. Infrasonic interferometry determines the delay time between two sensors by cross correlating their infrasound recordings. The obtained delay time can be used to estimate the Green’s function, which is the impulse response of the medium. Until now this approach was applied in other research fields, e.g., oceanography, seismology and ultrasonics, but it was used only occasionally for infrasound studies. In this thesis, the new research field of infrasonic interferometry is explored from the theoretical basics, via numerical experiments with synthetic data, up to the application to tropospheric and stratospheric microbaroms.Applied Geophysics and Petrophysic
Journal of Geophysical Research: Atmospheres, 2014
• Infrasonic interferometry can be applied to microbaroms • The infrasonic wave of microbaroms is... more • Infrasonic interferometry can be applied to microbaroms • The infrasonic wave of microbaroms is coherent up to a distance of 40 km • The estimation of surface wind speed by infrasonic interferometry is possible
A sound intensity probe is an excellent means in many fields of acoustics to track acoustical ener... more A sound intensity probe is an excellent means in many fields of acoustics to track acoustical energy flow. In room acoustics this method is rarely applied in practise but is none the less appealing, e.g., to examine the timing and localization of scattered and diffracted reflections. Unfavourable are common single point measurements, which tend to lack a correct qualitative assessment in the ”early”, i.e., anisotropic sound field. The reason is attributed to the interference pattern in the scope of early reflections. Our work uses a scalable spherical array by which sound pressure is recorded on the surface of a sphere, using efficient spatial sampling. Based on near-field acoustical holography the sound-pressure is reconstructed on an equally spaced square grid. By calculating the pressure-gradient for each point of the lattice, the intensity vector is reconstructed inside a large volume. In a preliminary approach we simulated and measured the sound field of different rooms using the propo...
In this paper a localization method is analyzed, which uses Acoustic Vector Sensors (AVS). An AVS... more In this paper a localization method is analyzed, which uses Acoustic Vector Sensors (AVS). An AVS measures the directed velocity and the pressure in an area approaching a single point. In order to localize sound sources, the Multiple Signal Classification (MUSIC) was applied, which determines the signal and the noise components. MUSIC only uses the noise components (Noise Subspace) to estimate the direction of arrival. To estimate the quality of this method, the accuracy and the resolution of the localization were compared with an established pressure-based method. The accuracy and resolution of the AVS-based method are higher. MUSIC was more robust against calibration errors of phase and frequency. Our measurements showed that the combination of AVS, and MUSIC provides an efficient localization system.
ABSTRACT The travel time of infrasound through the stratosphere depends on the temperature profil... more ABSTRACT The travel time of infrasound through the stratosphere depends on the temperature profile and the wind speed. These atmospheric conditions can be estimated by determining the travel times between different receivers (microbarometers). Therefore the determination of the travel time of infrasound between different receivers becomes more and more important. An approach to determine the travel time is infrasound interferometry. In this work, the infrasound interferometry is applied to synthetic data of active and passive sources refracted by the stratosphere is tested. The synthetic data were generated with a raytracing model. The inputs of the raytracing model are the atmospheric conditions and a source wavelet. As source wavelet we used blast waves and microbaroms. With the atmospheric conditions and the source wavelet the raytracing model calculates the raypath and the travel time of the infrasound. In order to simulate the measurement of a receiver the rays which reach the receiver need to be found. The rays which propagate from a source to the receiver are called eigen rays. The simulation of the receiver measurements takes into account the travel time along the eigen rays, the attenuation of the different atmospheric layers, the spreading of the rays and the influence of caustics. The simulated measurements of the different receivers are combined to synthetic barograms. Two synthetic experiments were performed with the described model. In the first experiment the interferometry was applied to barograms of active sources like blast waves. The second experiment with microbaroms tests the applicability of interferometry to barograms of passive sources. In the next step infrasound interferometry will be applied to measured barograms. These barograms are measured with the 'Large Aperture Infrasound Array' (LAIA). LAIA is being installed by the Royal Netherlands Meteorological Institute (KNMI) in the framework of the radio-astronomical 'Low Frequency Array' (LOFAR) initiative. LAIA will consist of thirty microbarometers with an aperture of around 100 km. The in-house developed microbarometers are able to measure infrasound up to a period of 1000 seconds, which is in the acoustic-gravity wave regime. The results will also be directly applicable to the verification of the 'Comprehensive Nuclear-Test-Ban Treaty' (CTBT), where uncertainties in the atmospheric propagation of infrasound play a dominant role. This research is made possible by the support of the 'Netherlands Organisation for Scientific Research' (NWO).
ABSTRACT Ambient noise interferometry attempts to reconstruct the impulse response of a linear sy... more ABSTRACT Ambient noise interferometry attempts to reconstruct the impulse response of a linear system undergoing excitations by a random forcing. The method has proven to be quite general, finding applications in diverse fields such as ultrasonics, helioseismology, regional and global seismology, ocean acoustics, and exploration seismology. Due to the pervasive random forcing of microbaroms, the study of atmospheric infrasound can benefit from approaches developed in the field of interferometry as well. Moreover, continuous noise sources, such as microbaroms, provide a means to quantify the strong time-dependent changes inherent in the structure of the atmosphere. For pairs of infrasonic sensors separated by distances between 1 and 15 km, direct arrivals have been observed in long-time correlations of ambient noise from stations within 3 different networks: the network operated by the Alaska Volcano Observatory, the I53US array, and the Utah 07 experiment. As in seismic ambient noise studies, these interferometric arrivals show dispersive properties characteristic of guided wave propagation. As a result, infrasonic ambient noise interferometry can provide information on the gross structure of the atmospheric boundary layer. At Fourpeaked Volcano, Alaska, a surface-based temperature inversion produced a strong waveguide between two infrasound sensors. The propagation time of the interferometric arrivals shifted over the course of several days in close correspondence with regional temperature trends at nearby meteorological stations. Independent ECMWF data confirms the existence of a temperature inversion during this time period. Analysis of 4 infrasound sensors near Park City, Utah, further demonstrates the ability of infrasonic ambient noise interferometry to retrieve guided waves between stations separated by 5 to 10 km. Among the 4 stations, we observe strong interferometric arrivals on different station pairs at different times, attesting to variable atmospheric conditions over the array. Correlations of ambient noise on the different elements of the I53US array enable the comparison of interferometry with traditional beamforming. Multiple small aperture arrays of infrasound sensors within the Utah 07 experiment open up the possibility of cross-correlating ambient noise on pairs of phased arrays. Proper phasing of arrays prior to cross-correlation should enhance the ability to detect stratospheric and thermospheric returns. These arrivals are expected to hold important information on upper atmospheric processes, such as Sudden Stratospheric Warming events.
ABSTRACT Long range infrasound propagation from explosions often concerns stratospheric paths bet... more ABSTRACT Long range infrasound propagation from explosions often concerns stratospheric paths between source and receiver. Such paths highly depend on the wind and temperature structure around the stratopause, which is between 40 and 50 km in the northern hemisphere winter. In summer, the stratosphere is much more stable than in winter as energy can not propagate through the tropopause from the troposphere into the stratosphere. Furthermore, dramatic events like a Sudden Stratospheric Warming (SSW) happen every winter on the northern hemisphere and disturb, or even reverse, the circumpolar vortex flow. SSWs can occur in a couple of days given rise to temperature changes of tens of degrees Celsius in the stratosphere. Understanding infrasound propagation under these strongly varying upper atmospheric conditions is of importance for source identification. But also, inversely, when infrasound is used a passive atmospheric probe for the upper atmosphere. In this presentation, a climatology of stratospheric infrasound detections from explosions is presented from seismo-acoustic studies. The high variability in winter time observations (northern hemisphere) versus summer time is partly explained by the assumed propagation through atmospheric models. Explanations are sought in terms of unresolved stratospheric fine scale structure.
ABSTRACT The estimation of the traveltime of infrasound through the atmosphere is interesting for... more ABSTRACT The estimation of the traveltime of infrasound through the atmosphere is interesting for several applications. For example, it could be used to determine temperature and wind of the atmosphere, since the traveltime depends on these atmospheric conditions (Haney, 2009). In this work the traveltime is estimated with infrasonic interferometry. In other words, we calculate the crosscorrelations of data of spatially distributed receivers. With this method the traveltime between two receivers is determined without the need for ground truth events. In a first step, we crosscorrelate synthetic data, which are generated by a raytracing model. This model takes into account the traveltime along the rays, the attenuation of the different atmospheric layers, the spreading of the rays and the influence of caustics. In these numerical experiments we show that it is possible to determine the traveltime through infrasonic interferometry. We present the results of infrasonic interferometry applied to measured data. Microbaroms are used in the crosscorrelation approach. Microbaroms are caused by ocean waves and are measured by the 'Large Aperture Infrasound Array' (LAIA). LAIA is being installed by the Royal Netherlands Meteorological Institute (KNMI) in the framework of the radio-astronomical 'Low Frequency Array' (LOFAR) initiative. LAIA consists currently of around twenty receivers (microbarometers) with an aperture of around 100 km, allowing for several inter-station distances. Here, we show the results of crosscorrelations as a function of receivers distance, to assess the signal coherency. This research is made possible by the support of the 'Netherlands Organization for Scientific Research' (NWO). Haney, M., 2009. Infrasonic ambient noise interferometry from correlations of microbaroms, Geophysical Research Letters, 36, L19808
The traveltime of infrasound through the atmosphere depends on the temperature and the wind. The ... more The traveltime of infrasound through the atmosphere depends on the temperature and the wind. The basic idea of this project is to use the traveltime to estimate these atmospheric conditions. In order to measure the traveltime we can calculate the crosscorrelation between infrasound receivers (microbarometers). This method is called interferometry. On this poster we present the tests of the Interferometry with synthetic data (barograms). In future we will use the measured ambient noise of the 'Large Aperture Infrasound Array ' (LAIA) instead of synthetic data. influence of caustics. Eigen rays are the rays which connect source and receiver (figure 1 and 2). 3. Results Figure 3 shows the results of a crosscorrelation between the synthetic data of receiver 1 and receiver 2. The highest correlation is reached at traveltimes between 753s and 756s. 1. Background Infrasound propagates over wide ranges and up to thermosopheric altitudes through the atmosphere. The traveltime depends on the temperature and wind structure. We are going to to use the traveltime to estimate these atmospheric conditions. 2. Methods In order to measure the traveltime we can calculate the crosscorrelation between the data of two infrasound receivers (microbarometers). This Figure 2. Eigen rays to receiver 2 of 241 Infrasound source distributed from-60km till 60km 4. Conclusion By calculating the crosscorrelation of the synthetic data, it was possible to determine the traveltime between two receivers. The next step will be the inversion of the model to estimate the atmospheric conditions by the traveltime. In future we will use the method is called Interferometry. Here we present the tests of the inter-measured ambient ferometry with synthetic data (barograms). To simulate the synthetic data, we implemented a raytracing model. It takes into account the Figure 3. Normalized synthetic barograms of receiver 2. noise of the 'Large
The Journal of the Acoustical Society of America, 2013
ABSTRACT It has been theoretically shown by Wapenaar (2006) that the non-reciprocal Green'... more ABSTRACT It has been theoretically shown by Wapenaar (2006) that the non-reciprocal Green's function can be retrieved with cross-correlation. Thus, interferometric techniques can be applied to a moving medium such as the atmosphere. Numerical experiments have shown that the delay times of stratospherically refracted infrasound can be obtained from cross-correlation between pairs of microbarometers. Doing so, information about the wind and temperature around the stratopause can be passively gathered from the stationary phase with, for example, the continuous noise of microbaroms. Actual measurements have been used from the Large Aperture Infrasound Array (LAIA) in the Netherlands. LAIA consists of several microbarometers with inter-station distances ranging from a few kilometers to tens of kilometers and is ideally suited to assess the results from theoretical and numerical experiments in practice. Results will be shown on the correlation length of infrasound from microbaroms and the effect of wind and temperature on the delay times retrieved from cross-correlations.
The Journal of the Acoustical Society of America, 2013
The atmospheric wind and temperature can be estimated through the traveltimes of infrasound betwe... more The atmospheric wind and temperature can be estimated through the traveltimes of infrasound between pairs of receivers. The traveltimes can be obtained by infrasonic interferometry. In this study, the theory of infrasonic interferometry is verified and applied to modeled stratospherically refracted waves. Synthetic barograms are generated using a raytracing model and taking into account atmospheric attenuation, geometrical spreading, and phase shifts due to caustics. Two types of source wavelets are implemented for the experiments: blast waves and microbaroms. In both numerical experiments, the traveltimes between the receivers are accurately retrieved by applying interferometry to the synthetic barograms. It is shown that microbaroms can be used in practice to obtain the traveltimes of infrasound through the stratosphere, which forms the basis for retrieving the wind and temperature profiles. V
Journal of Geophysical Research: Atmospheres, 2012
Long-range infrasound propagation strongly depends on the state of the stratosphere. Infrasound c... more Long-range infrasound propagation strongly depends on the state of the stratosphere. Infrasound can be efficiently ducted between the Earth's surface and the stratopause under a favorable wind and temperature structure between 40 and 50 km altitude. Understanding infrasound propagation under variable stratospheric conditions is of importance for a successful verification of the Comprehensive Nuclear-Test Ban Treaty, in which infrasound is used as a verification technique. Inversely, infrasound observations can be used in acoustic remote sensing of the upper atmosphere. In previous studies, attention has been paid to the strength and direction of the circumpolar vortex wind. In this study, an analysis is made of the temperature effect in the stratosphere on infrasound propagation. A case study is presented from an explosion during a sudden stratospheric warming. During such conditions, the size of the classical stratospheric shadow zone ($200 km) appeared to be reduced by a factor of 2. The occurrence of such conditions is quantified by evaluating 10 years of atmospheric specifications. It unexpectedly appeared that the size of the shadow zone can become smaller than 100 km, which is confirmed by evaluating infrasound detections from mining blasts in southwestern Siberia, Russia. These results are valid over a latitudinal range of 20 N to 60 N, which is determined by the stratospheric surf zone.
ABSTRACT The travel time of infrasound through the atmosphere depends on the temperature and the ... more ABSTRACT The travel time of infrasound through the atmosphere depends on the temperature and the wind. These atmospheric conditions could be estimated by measuring the travel times between different receivers (microbarometers). For such an estimation an inverse model of the propagation of infrasound through the atmosphere is essential. In the first step it is useful to build a forward model. The inputs of our raytracing model are the atmospheric conditions and the positions of source and receiver. The model consists of three elements the source, the channel and the receiver. The source is a blast wave or microbaroms. The channel is the atmosphere and it takes into account the travel time along the eigen ray, the attenuation of the different atmospheric layers, the spreading of the rays and the influence of caustics. Each receiver is reached by different rays (eigen rays). To determine the eigen rays is part of the receiver element. As output the model generates synthetic barograms. The synthetic barograms can be used to explain measured barograms. Furthermore the synthetic barograms can also be used to evaluate the determination of the travel time. The accurate travel time is for the inverse model as input essential. Since small changes of the travel time lead to big changes of the output (temperature and wind). The travel time between two receivers is determined by crosscorrelating the barograms of these two receivers. This technique was already successfully applied in the troposphere (Haney, 2009). We show that the same can be achieved with more complicated stratospheric phases. Now we compare the crosscorrelation of synthetic barograms with the crosscorrelation of measured barograms. These barograms are measured with the 'Large Aperture Infrasound Array' (LAIA). LAIA is being installed by the Royal Netherlands Meteorological Institute (KNMI) in the framework of the radio-astronomical 'Low Frequency Array' (LOFAR) initiative. LAIA will consist of thirty microbarometers with an aperture of around 100 km. The in-house developed microbarometers are able to measure infrasound up to a period of 1000 seconds, which is in the acoustic-gravity wave regime. The results will also be directly applicable to the verification of the 'Comprehensive Nuclear-Test-Ban Treaty' (CTBT), where uncertainties in the atmospheric propagation of infrasound play a dominant role. This research is made possible by the support of the 'Netherlands Organisation for Scientific Research' (NWO). Haney, M., 2009. Infrasonic ambient noise interferometry from correlations of microbaroms, Geophysical Research Letters, 36, L19808, doi:10.1029/2009GL040179