Since physical processes within a volcano generate seismological signals on its surface, seismolo... more Since physical processes within a volcano generate seismological signals on its surface, seismology plays an important role in determining the internal volcano dynamics. Long period (LP) events (f=0.2 - 2 Hz), often occurring in swarms, are of particular interest to achieve this task as their repetitive nature provides a good basis for monitoring the temporal changes of the upper part of a volcano on different time scales (from days to years). The similarity of the waveforms within a swarm suggests that they are generated by non-destructive repetitive or slowly changing source process. Although the proposed LP source models relate this type of events to resonance of fluid-filled cracks, dykes and conduits, their exact origin is still under debate. Consequently, in an effort to better understand the exact origin of LP seismicity, inverting seismic waveform for LP source mechanisms is becoming increasingly common. The most critical aspect in such modelling procedures lies in the accurate calculation of the Green's functions. It was shown in the recent literature that a fine scale shallow structure (hundred metres) which lies below the resolution limits commonly exhibited by 3D velocity models of volcanic interiors can lead to apparently stable, but erroneous source models. In addition, the simultaneous search for a source location may also converge in a considerably wrong solution (~ 1 km). Such unreliable (and possibly misleading) solutions are particularly pronounced when a dataset is limited to the records obtained from a sparse network placed in the far-field of the source (~ 5-10 km). Our recent results suggested that this issue can be overcome by using a dense broadband network, placed directly above the source. Therefore, in this study we use 30 broadband stations placed within 2 km from the Etna summit. The LP activity recorded by this network in June 2008 was located using the relative time delays between all station pairs, obtained by the cross-correlation of similar waveforms across the network. This lead to an unprecedented high-resolution short-term spatio-temporal image of the LP source zone. In this study we perform a simultaneous inversion for source mechanism and locations, using the same high resolution near-field dataset. The aims of this exercise are (i) to test our ability to reconstruct a consistent image of the source zone by using different methods (source inversion vs. cross-correlation location technique), and (ii) to constrain the source mechanisms of the recorded LP sequence. This may lead to an improved spatio-temporal image of the shallow internal dynamic of Etna volcano.
Long Period seismic events are viewed with particular interest on volcanoes as they are thought t... more Long Period seismic events are viewed with particular interest on volcanoes as they are thought to be a strong indication of a fluid-filled shallow plumbing system. The accepted model for the generation of these events is that of a fluid-filled resonating conduit, although the details of the resonance triggering mechanism are still hotly debated. Motivated by results from large scale numerical simulations we have recently made very near-field recordings of LP events at several volcanoes, and in particular conducted a very high resolution experiment at Mt Etna. Results clearly demonstrate the existence of very short duration pulse-like LP events, which are not consistent with the conduit resonance model. 3D full wavefield numerical simulations in heterogeneous models with DEM topography show that apparent source related resonance is caused by strong path effects, unless the data are recorded within a few hundred metres of the source. An analysis of these pulse-like sources indicates ...
ABSTRACT A passive seismic experiment was conducted in April/May 2010 in the Albertine Graben reg... more ABSTRACT A passive seismic experiment was conducted in April/May 2010 in the Albertine Graben region in Uganda to record low-frequency seismic signals and explore the possibility of their exploitation in this area as a direct hydrocarbon indicator (DHI). Recordings were made at locations directly overlying both hydrocarbon and water-bearing strata within the sedimentary basin as well as reference sites external to the basin, directly on the basement. Contrary to findings published in some literature to date, we found that spatial variations in the analysed wavefield parameters correlate with the underlying geology rather than the presence or absence of hydrocarbons. Inversion of the surface-wave (fundamental mode) dispersion curve as well as the observed horizontal-to-vertical spectral ratio of both surface and body waves provide evidence that the observed spectral variations can be explained solely by a simple layered/gradient velocity model, without the presence of any kind of anomaly that could be attributed exclusively to a hydrocarbon reservoir. Consequently, it is recommended that knowledge of the geological and velocity structure is sought when analysing passive low-frequency seismic data sets. This is a fundamental prerequisite in order to guard against misinterpretation of the spatial variation of seismic derived attributes as DHIs.
ABSTRACT A passive seismic experiment was conducted in April/May 2010 in the Albertine Graben reg... more ABSTRACT A passive seismic experiment was conducted in April/May 2010 in the Albertine Graben region in Uganda to record low-frequency seismic signals and explore the possibility of their exploitation in this area as a direct hydrocarbon indicator (DHI). Recordings were made at locations directly overlying both hydrocarbon and water-bearing strata within the sedimentary basin as well as reference sites external to the basin, directly on the basement. Contrary to findings published in some literature to date, we found that spatial variations in the analysed wavefield parameters correlate with the underlying geology rather than the presence or absence of hydrocarbons. Inversion of the surface-wave (fundamental mode) dispersion curve as well as the observed horizontal-to-vertical spectral ratio of both surface and body waves provide evidence that the observed spectral variations can be explained solely by a simple layered/gradient velocity model, without the presence of any kind of anomaly that could be attributed exclusively to a hydrocarbon reservoir. Consequently, it is recommended that knowledge of the geological and velocity structure is sought when analysing passive low-frequency seismic data sets. This is a fundamental prerequisite in order to guard against misinterpretation of the spatial variation of seismic derived attributes as DHIs.
The heterogeneities of elastic properties in the Earth's interior exist over all scales. Seis... more The heterogeneities of elastic properties in the Earth's interior exist over all scales. Seismic waves propagating through heterogeneous media are scattered thus the waveforms are distorted. Consequently, our ability to image the source is compromised. Time reversal technique can utilize the reciprocity of linear elastodynamics and can be applied effectively in heterogeneous media. In this work, the refocusing properties of time reversal wave fields in heterogeneous solid half-spaces are investigated numerically. Here the 3D numerical simulations are performed with a spectral-element method by using open-source software package SPECFEM3D. The simulation results indicate that in the regime where the heterogeneous scale is of the order of the dominant seismic wavelength, the refocusing fields become statistical stable that they are independent of the particular realizations of random fields. Furthermore, the super-resolution phenomenon is also observed as the width of focal spot c...
ABSTRACT Long-period (LP) volcano seismic events often precede volcanic eruptions and are viewed ... more ABSTRACT Long-period (LP) volcano seismic events often precede volcanic eruptions and are viewed with considerable interest in hazard assessment. They are usually thought to be associated with resonating fluid-filled conduits although alternative models involving material failure have recently been proposed. Through recent field experiments, we uncovered a step-like displacement component associated with some LP events, outside the spectral range of the typically narrow-band analysis for this kind of event. Bespoke laboratory experiments with step tables show that steps of the order of a few micrometers can be extracted from seismograms, where long-period noise is estimated and removed with moving median filters. Using these constraints, we observe step-like ground deformation in LP recordings near the summits of Turrialba and Etna Volcanoes. This represents a previously unobserved static component in the source time history of LP events, with implications for the underlying source process.
Ocean gravity waves are driven by atmospheric pressure systems. Their interactions with one anoth... more Ocean gravity waves are driven by atmospheric pressure systems. Their interactions with one another and the production of standing waves in the water column generate pressure changes at the sea floor. These pressure fluctuations are the cause of continuous background seismic noise known as microseisms. The levels of microseism activity vary as a function of the sea state and increase during periods of intensive ocean wave activity. In 2011 a seismic network was deployed along the west coast of Ireland to continuously record microseisms generated in the Atlantic Ocean. This project aims to determine the characteristics of the causative ocean gravity waves through calibration of the microseism data with ocean buoy data. In initial tests we are using a Backpropagation Feed-forward Artificial Neural Network (BP ANN) to establish the underlying relationships between microseisms and ocean waves. If successful these tools could then be used to estimate ocean wave heights and wave periods u...
Currently there is strong interest in monitoring temporal changes in seismic wave velocity in var... more Currently there is strong interest in monitoring temporal changes in seismic wave velocity in various geological settings. These settings can range from volcano monitoring to reservoir monitoring amongst others. Green's functions are often used to monitor temporal variations in seismic velocity as their arrival times contain information about velocity changes. The velocities can be measured through the cross correlation of Green's functions for a given pair of stations. Correlation of ambient noise is typically used for Green's function retrieval. The great advantage of using noise is that noise is continuous in time and there are no natural explosive or repeatable sources required. However temporal and spatial variations of non-uniformly distributed noise sources may lead to apparent changes in Green's functions which are related to the source not the path. This could lead to a misinterpretation of temporal changes in wave velocity. Ireland is a good location in whi...
Long Period (LP) volcano seismic events have been observed at many volcanoes around the world. Sw... more Long Period (LP) volcano seismic events have been observed at many volcanoes around the world. Swarms of LP events are often recorded prior to or during volcanic eruptions, and can be used to provide an insight into a volcano’s internal dynamics. Within these swarms numerous recorded LP events can have almost identical waveforms, thus it can be assumed that these events have both similar source locations and source mechanisms; these events can be grouped together into families. An accurate source location is crucial in order to further understand the source processes involved in producing LP events, however locating these events can be difficult due to their emergent onset and poorly differentiated P- and S-waves. In this study we present a location method, an extension to the double-difference location technique, which can simultaneously determine the absolute location of a family of LP events and the relative source locations of each event within the family in order to image struc...
Long period (LP) seismic events occur at volcanoes across the world, but the source processes gen... more Long period (LP) seismic events occur at volcanoes across the world, but the source processes generating these events are still relatively poorly understood. LPs often occur in increasing numbers before volcanic eruptions, so a better understanding of the source is a major aim of volcano seismology. Full-waveform moment tensor inversion has been carried out at many volcanoes in order to attempt to constrain the mechanism of LPs. An experiment was carried out in 2011 at Turrialba volcano in Costa Rica, where 25 temporary seismic stations were deployed in addition to the three permanent stations. The aim of this experiment was to perform 3D full-waveform moment tensor inversion as accurately as possible, by using a dense seismic network with stations especially concentrated across the summit of the volcano, which has been shown by previous studies to reduce path effects and therefore improve the solution. Source locations are obtained by implementing a grid search while carrying out t...
Since physical processes within a volcano generate seismological signals on its surface, seismolo... more Since physical processes within a volcano generate seismological signals on its surface, seismology plays an important role in determining the internal volcano dynamics. Long period (LP) events (f=0.2 - 2 Hz), often occurring in swarms, are of particular interest to achieve this task as their repetitive nature provides a good basis for monitoring the temporal changes of the upper part of a volcano on different time scales (from days to years). The similarity of the waveforms within a swarm suggests that they are generated by non-destructive repetitive or slowly changing source process. Although the proposed LP source models relate this type of events to resonance of fluid-filled cracks, dykes and conduits, their exact origin is still under debate. Consequently, in an effort to better understand the exact origin of LP seismicity, inverting seismic waveform for LP source mechanisms is becoming increasingly common. The most critical aspect in such modelling procedures lies in the accurate calculation of the Green's functions. It was shown in the recent literature that a fine scale shallow structure (hundred metres) which lies below the resolution limits commonly exhibited by 3D velocity models of volcanic interiors can lead to apparently stable, but erroneous source models. In addition, the simultaneous search for a source location may also converge in a considerably wrong solution (~ 1 km). Such unreliable (and possibly misleading) solutions are particularly pronounced when a dataset is limited to the records obtained from a sparse network placed in the far-field of the source (~ 5-10 km). Our recent results suggested that this issue can be overcome by using a dense broadband network, placed directly above the source. Therefore, in this study we use 30 broadband stations placed within 2 km from the Etna summit. The LP activity recorded by this network in June 2008 was located using the relative time delays between all station pairs, obtained by the cross-correlation of similar waveforms across the network. This lead to an unprecedented high-resolution short-term spatio-temporal image of the LP source zone. In this study we perform a simultaneous inversion for source mechanism and locations, using the same high resolution near-field dataset. The aims of this exercise are (i) to test our ability to reconstruct a consistent image of the source zone by using different methods (source inversion vs. cross-correlation location technique), and (ii) to constrain the source mechanisms of the recorded LP sequence. This may lead to an improved spatio-temporal image of the shallow internal dynamic of Etna volcano.
Long Period seismic events are viewed with particular interest on volcanoes as they are thought t... more Long Period seismic events are viewed with particular interest on volcanoes as they are thought to be a strong indication of a fluid-filled shallow plumbing system. The accepted model for the generation of these events is that of a fluid-filled resonating conduit, although the details of the resonance triggering mechanism are still hotly debated. Motivated by results from large scale numerical simulations we have recently made very near-field recordings of LP events at several volcanoes, and in particular conducted a very high resolution experiment at Mt Etna. Results clearly demonstrate the existence of very short duration pulse-like LP events, which are not consistent with the conduit resonance model. 3D full wavefield numerical simulations in heterogeneous models with DEM topography show that apparent source related resonance is caused by strong path effects, unless the data are recorded within a few hundred metres of the source. An analysis of these pulse-like sources indicates ...
ABSTRACT A passive seismic experiment was conducted in April/May 2010 in the Albertine Graben reg... more ABSTRACT A passive seismic experiment was conducted in April/May 2010 in the Albertine Graben region in Uganda to record low-frequency seismic signals and explore the possibility of their exploitation in this area as a direct hydrocarbon indicator (DHI). Recordings were made at locations directly overlying both hydrocarbon and water-bearing strata within the sedimentary basin as well as reference sites external to the basin, directly on the basement. Contrary to findings published in some literature to date, we found that spatial variations in the analysed wavefield parameters correlate with the underlying geology rather than the presence or absence of hydrocarbons. Inversion of the surface-wave (fundamental mode) dispersion curve as well as the observed horizontal-to-vertical spectral ratio of both surface and body waves provide evidence that the observed spectral variations can be explained solely by a simple layered/gradient velocity model, without the presence of any kind of anomaly that could be attributed exclusively to a hydrocarbon reservoir. Consequently, it is recommended that knowledge of the geological and velocity structure is sought when analysing passive low-frequency seismic data sets. This is a fundamental prerequisite in order to guard against misinterpretation of the spatial variation of seismic derived attributes as DHIs.
ABSTRACT A passive seismic experiment was conducted in April/May 2010 in the Albertine Graben reg... more ABSTRACT A passive seismic experiment was conducted in April/May 2010 in the Albertine Graben region in Uganda to record low-frequency seismic signals and explore the possibility of their exploitation in this area as a direct hydrocarbon indicator (DHI). Recordings were made at locations directly overlying both hydrocarbon and water-bearing strata within the sedimentary basin as well as reference sites external to the basin, directly on the basement. Contrary to findings published in some literature to date, we found that spatial variations in the analysed wavefield parameters correlate with the underlying geology rather than the presence or absence of hydrocarbons. Inversion of the surface-wave (fundamental mode) dispersion curve as well as the observed horizontal-to-vertical spectral ratio of both surface and body waves provide evidence that the observed spectral variations can be explained solely by a simple layered/gradient velocity model, without the presence of any kind of anomaly that could be attributed exclusively to a hydrocarbon reservoir. Consequently, it is recommended that knowledge of the geological and velocity structure is sought when analysing passive low-frequency seismic data sets. This is a fundamental prerequisite in order to guard against misinterpretation of the spatial variation of seismic derived attributes as DHIs.
The heterogeneities of elastic properties in the Earth's interior exist over all scales. Seis... more The heterogeneities of elastic properties in the Earth's interior exist over all scales. Seismic waves propagating through heterogeneous media are scattered thus the waveforms are distorted. Consequently, our ability to image the source is compromised. Time reversal technique can utilize the reciprocity of linear elastodynamics and can be applied effectively in heterogeneous media. In this work, the refocusing properties of time reversal wave fields in heterogeneous solid half-spaces are investigated numerically. Here the 3D numerical simulations are performed with a spectral-element method by using open-source software package SPECFEM3D. The simulation results indicate that in the regime where the heterogeneous scale is of the order of the dominant seismic wavelength, the refocusing fields become statistical stable that they are independent of the particular realizations of random fields. Furthermore, the super-resolution phenomenon is also observed as the width of focal spot c...
ABSTRACT Long-period (LP) volcano seismic events often precede volcanic eruptions and are viewed ... more ABSTRACT Long-period (LP) volcano seismic events often precede volcanic eruptions and are viewed with considerable interest in hazard assessment. They are usually thought to be associated with resonating fluid-filled conduits although alternative models involving material failure have recently been proposed. Through recent field experiments, we uncovered a step-like displacement component associated with some LP events, outside the spectral range of the typically narrow-band analysis for this kind of event. Bespoke laboratory experiments with step tables show that steps of the order of a few micrometers can be extracted from seismograms, where long-period noise is estimated and removed with moving median filters. Using these constraints, we observe step-like ground deformation in LP recordings near the summits of Turrialba and Etna Volcanoes. This represents a previously unobserved static component in the source time history of LP events, with implications for the underlying source process.
Ocean gravity waves are driven by atmospheric pressure systems. Their interactions with one anoth... more Ocean gravity waves are driven by atmospheric pressure systems. Their interactions with one another and the production of standing waves in the water column generate pressure changes at the sea floor. These pressure fluctuations are the cause of continuous background seismic noise known as microseisms. The levels of microseism activity vary as a function of the sea state and increase during periods of intensive ocean wave activity. In 2011 a seismic network was deployed along the west coast of Ireland to continuously record microseisms generated in the Atlantic Ocean. This project aims to determine the characteristics of the causative ocean gravity waves through calibration of the microseism data with ocean buoy data. In initial tests we are using a Backpropagation Feed-forward Artificial Neural Network (BP ANN) to establish the underlying relationships between microseisms and ocean waves. If successful these tools could then be used to estimate ocean wave heights and wave periods u...
Currently there is strong interest in monitoring temporal changes in seismic wave velocity in var... more Currently there is strong interest in monitoring temporal changes in seismic wave velocity in various geological settings. These settings can range from volcano monitoring to reservoir monitoring amongst others. Green's functions are often used to monitor temporal variations in seismic velocity as their arrival times contain information about velocity changes. The velocities can be measured through the cross correlation of Green's functions for a given pair of stations. Correlation of ambient noise is typically used for Green's function retrieval. The great advantage of using noise is that noise is continuous in time and there are no natural explosive or repeatable sources required. However temporal and spatial variations of non-uniformly distributed noise sources may lead to apparent changes in Green's functions which are related to the source not the path. This could lead to a misinterpretation of temporal changes in wave velocity. Ireland is a good location in whi...
Long Period (LP) volcano seismic events have been observed at many volcanoes around the world. Sw... more Long Period (LP) volcano seismic events have been observed at many volcanoes around the world. Swarms of LP events are often recorded prior to or during volcanic eruptions, and can be used to provide an insight into a volcano’s internal dynamics. Within these swarms numerous recorded LP events can have almost identical waveforms, thus it can be assumed that these events have both similar source locations and source mechanisms; these events can be grouped together into families. An accurate source location is crucial in order to further understand the source processes involved in producing LP events, however locating these events can be difficult due to their emergent onset and poorly differentiated P- and S-waves. In this study we present a location method, an extension to the double-difference location technique, which can simultaneously determine the absolute location of a family of LP events and the relative source locations of each event within the family in order to image struc...
Long period (LP) seismic events occur at volcanoes across the world, but the source processes gen... more Long period (LP) seismic events occur at volcanoes across the world, but the source processes generating these events are still relatively poorly understood. LPs often occur in increasing numbers before volcanic eruptions, so a better understanding of the source is a major aim of volcano seismology. Full-waveform moment tensor inversion has been carried out at many volcanoes in order to attempt to constrain the mechanism of LPs. An experiment was carried out in 2011 at Turrialba volcano in Costa Rica, where 25 temporary seismic stations were deployed in addition to the three permanent stations. The aim of this experiment was to perform 3D full-waveform moment tensor inversion as accurately as possible, by using a dense seismic network with stations especially concentrated across the summit of the volcano, which has been shown by previous studies to reduce path effects and therefore improve the solution. Source locations are obtained by implementing a grid search while carrying out t...
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Papers by Ivan Lokmer