Options
Bean, Christopher J.
Preferred name
Bean, Christopher J.
Official Name
Bean, Christopher J.
Research Output
Now showing 1 - 10 of 29
- PublicationDiffraction imaging of sedimentary basins: An example from the Porcupine Basin(Copernicus, 2021-04-30)
; ; ; ; iffraction imaging is the technique of separating diffraction energy from the source wavefield and processing it independently. As diffractions are formed from objects and discontinuities, or diffractors, which are small in comparison to the wavelength, if the diffraction energy is imaged, so too are the diffractors. These diffractors take many forms such as faults, fractures, and pinch-out points, and are therefore geologically significant. Diffraction imaging has been applied here to the Porcupine Basin; a hyperextended basin located 200km to the southwest of Ireland with a rich geological history. The basin has seen interest both academically and industrially as a study on hyperextension and a potential source of hydrocarbons. The data is characterised by two distinct, basin-wide, fractured carbonates nestled between faulted sandstones and mudstones. Additionally, there are both mass-transport deposits and fans present throughout the data, which pose a further challenge for diffraction imaging. Here, we propose the usage of diffraction imaging to better image structures both within the carbonate, such as fractures, and below.453 - PublicationCoseismic velocity variations caused by static stress changes associated with the 2001 Mw=4.3 Agios Ionis earthquake in the Gulf of Corinth, Greece(American Geophysical Union, 2010-07)
; ; ; The analysis of temporal variations in the seismic velocity across faults can be used to estimate in situ stress changes. Seismic velocity of propagation depends on the fault stiffness, which is a function of stress. The coda wave interferometry technique is applied to seven families of repeating earthquakes (multiplets) recorded on the southern shore of the Gulf of Corinth, Greece, to estimate high precision velocity changes in the Earth ’ s crust associated with the M w = 4.3 Agios Ioanis earthquake. Results show that the Agios Ioanis event causes a perturbation in elastic properties at seismogenic depth, resulting in a reduction of 0.2% in the seismic velocity. The results are not consistent with either damage induced by dynamic stresses nor a fluid transient origin. In contrast, both the spatial distribution and magnitude of the velocity perturbation correlate well with modeled static stress variations. This suggests that the measured changes in the mechanical properties of the seismogenic crust can be attributed to a change in static stress field associated with the M w = 4.3 Agios Ioanis earthquake. The velocity changes indicate an unclamping of the Pyrgaki fault at depth, which has local hazard implications267 - PublicationWave height quantification using land based seismic data with grammatical evolutionAccurate, real time, continuous ocean wave height measurements are required for the initialisation of ocean wave forecast models, model hindcasting, and climate studies. These measurements are usually obtained using in situ ocean buoys or by satellite altimetry, but are sometimes incomplete due to instrument failure or routine network upgrades. In such situations, a reliable gap filling technique is desirable to provide a continuous and accurate ocean wave field record. Recorded on a land based seismic network are continuous seismic signals known as microseisms. These microseisms are generated by the interactions of ocean waves and will be used in the estimation of ocean wave heights. Grammatical Evolution is applied in this study to generate symbolic models that best estimate ocean wave height from terrestrial seismic data, and the best model is validated against an Artificial Neural Network. Both models are tested over a five month period of 2013, and an analysis of the results obtained indicates that the approach is robust and that it is possible to estimate ocean wave heights from land based seismic data.
419 - Publication2D Synthetic dataset of numerical simulations of long-period seismicity in a volcanic edifice and related sensitivity kernelsThis work describes the data used in the EPSL research article “Quantifying strong seismic propagation effects in the upper volcanic edifice using sensitivity kernels”. The dataset is generated in order to investigate to what extent the seismic signals recorded on volcanoes are affected by near surface velocity structure. Data were calculated using the computational spectral elements scheme SPECFEM2D, where the wave propagation beneath Mount Etna volcano, Italy, was simulated in both homogeneous and heterogeneous models. The heterogeneous model comprises a low-velocity superficial structure (top several hundred meters) based on the previously published studies. Several different source mechanisms and locations were used in the simulations. The seismic wavefield was “recorded” by 15 surface receivers distributed along the surface of the volcano. The associated sensitivity kernels were also computed. These kernels highlight the region of the velocity model that affects the recorded seismogram within a desired time window. The text files describing the velocity models used in the simulations are also provided. The data may be of interest to volcano seismologists, as well as earthquake seismologists studying path effects and wave propagation through complex media.
160Scopus© Citations 1 - PublicationPropagation of microseisms from the deep ocean to landOcean-generated microseisms are faint Earth vibrations that result from pressure fluctuations at the sea floor generated by the interaction between ocean surface gravity waves, and are continuously recorded as low frequency seismic noise. Here we investigate microseism propagation away from deep-ocean source regions using the spectral element method for an oceanic model that contains realistic northeast Atlantic Ocean irregular-layered structure composed of water, sediment, and upper crust. It also includes structural heterogeneities and continental slope and shelf bathymetry. Numerical simulations of coupled acoustic and elastic wave propagation in both simplified models and the full realistic model show that most microseism energy is confined to sediment and water column waveguides. We also show that a significant portion of microseism energy is reflected back to the deep ocean by the continental slope, while only a small fraction of deep-ocean-generated microseism energy reaches land. We conclude that terrestrially observed microseisms are largely generated in shallow water on continental shelves.
Scopus© Citations 27 398 - PublicationStatistical selection of the 'best' seismic source mechanisms from inversions of synthetic volcanic long-period eventsInversions for the source mechanism of long‐period (LP) seismicity recorded on volcanoes have become increasingly common and are used to interpret fluid‐driven processes. The source mechanism considered for LP inversions usually consists of a symmetric moment tensor with and without single forces. Also, constrained inversions have been performed where one presumes a specific source geometry that reduces the degrees of freedom in the inversion. To select the correct solution from the different possible mechanisms, the Akaike information criterion (AIC) has been used. However, since AIC performs well only if the inverted model is close to the true model, we tested its ability to select the correct model in LP inversions. Using synthetic data sets generated on Mt. Etna, Italy, with a tomography velocity model and the Green’s functions calculated for a simplified, homogeneous velocity model, we have investigated (1) if any of the inversion source models can recover the true mechanism and (2) the ability of the Akaike and Bayesian information criteria (BIC) to select the correct model. Results show that in some cases it is possible to recover the mechanism but never the source magnitude and that the BIC is a better measure than the AIC in selecting the true source model, although in numerous cases both criteria fail to select the correct solution. Therefore, the BIC should be used as opposed to the AIC if it is necessary to select an appropriate source. Caution should be used when using the statistical measure in any seismic inversion application.
276Scopus© Citations 14 - PublicationValidation of elastic wave measurements of rock fracture compliance using numerical discrete particle simulationsWe test various methods of quantifying the compliance of single and multiple rock fractures from synthetic ultrasonic data. The data are generated with a 2D discrete particle scheme which has previously been shown to treat fractures in agreement with linear-slip theory. Studying single fractures, we find that delays derived from peak amplitudes do not correspond to group delays, as might be expected. This is due to waveform distortion caused by the frequency-dependent transmission across the fracture. Instead the delays correspond to an expression for phase delays, which we derive from linear-slip theory. Phase delays are a unique function of compliance, whereas group delays are non-uniquely related to compliance. We believe that this property of group delays has hindered the wider application of deriving fracture compliances from traveltimes. We further show that transmission coefficients derived from waveform spectra yield more accurate fracture compliances than those obtained from ratios of signal peak amplitudes. We also investigate the compliance of a set of parallel fractures. Fracture compliance can only be determined from transmission coefficients if the fracture spacing is so large that the first arriving pulse is not contaminated by reverberations. In the case of contamination the direct measurement of group or phase delays is not practical. However, we demonstrate that in such cases of strong waveform distortion the coda wave interferometry method is very effective for determining relative fracture compliance. First break delays in the fracture set data are related to those observed in single fracture simulations. This means that fracture set compliance can be estimated from first break data if used together with numerical simulations.
Scopus© Citations 20 364 - PublicationMoment tensor inversion for the source location and mechanism of long period (LP) seismic events from 2009 at Turrialba volcano, Costa Rica(Elsevier, 2013-05-15)
; ; ; ; ; ; Long-period (LP) seismic events were recorded during the temporary installation of a broadband seismic network of 13 stations from March to September 2009 on Turrialba volcano, Costa Rica. Over 6000 LPs were extracted using a modified STA/LTA method and a family consisting of 435 similar LP events has been identified. For the first time at Turrialba volcano, full-waveform moment tensor inversion is performed to jointly determine the location and source mechanism of the events. The LPs in the family are likely to be caused by crack mechanisms dipping towards the southwest at angles of approximately 10 to 20°, located at shallow depths (< 800 m) below the active Southwest and Central craters. As the locations are so shallow, the most probable causes of crack mechanisms are hydrothermal fluids resonating within or 'pulsing' through a crack. The waveforms observed at the summit stations suggest a 'pulsing' mechanism, but source resonance with a high degree of damping is also possible.504Scopus© Citations 15 - PublicationTremor-rich shallow dyke formation followed by silent magma flow at Bárðarbunga in Iceland(Nature, 2017-04)
; ; ; ; ; ; ; The Bárðarbunga eruption in Iceland in 2014 and 2015 produced about 1.6 km3 of lava. Magma propagated away from Bárðarbunga to a distance of 48 km in the subsurface beneath Vatnajökull glacier, emerging a few kilometres beyond the glacier's northern rim. A puzzling observation is the lack of shallow (<3 km deep), high-frequency earthquakes associated Q.1 with shallow dyke formation near the subaerial and subglacial eruptive sites, suggesting that near-surface dyke formation is seismically quiet. However, seismic array observations and seismic full wavefield simulations reveal the presence and nature of shallow, pre-eruptive, long-duration seismic tremor activity. Here we use analyses of seismic data to constrain therelationships between seismicity, tremor, dyke propagation and magma flow during the Bárðarbunga eruption. We show that although tremor is usually associated with magma flow in volcanic settings, pre-eruptive tremor at Bárðarbunga was probably caused by swarms of microseismic events during dyke formation, and hence is directly associated with fracturing of the upper 2-3 km of the crust. Subsequent magma flow in the newly formed shallow dyke was seismically silent, with almost a complete absence of seismicity or tremor. Hence, we suggest that the transition from temporarily isolated, large, deep earthquakes to many smaller, shallower, temporally overlapping earthquakes (< magnitude 2) that appear as continuous tremor announces the arrival of a dyke opening in the shallow crust, forming a pathway for silent magma flow to the Earth's surface.778Scopus© Citations 36 - PublicationHelicopter vs. volcanic tremor: Characteristic features of seismic harmonic tremor on volcanoesWe recorded high-frequency (> 10 Hz) harmonic tremor with spectral gliding at Hekla Volcano in Iceland. Particle motion plots indicated a shallow tremor source. We observed up to two overtones beneath our Nyquist frequency of 50 Hz and could resolve a source of closely spaced pulses of very short duration (0.03-0.1 s) on zoomed seismograms. Volcanic tremor with fundamental frequencies above 5 Hz, frequency gliding and/or repetitive sources similar to our observations were observed on different volcanoes around the world. However, this frequency content, duration and occurrence of volcano-related tremor was not observed in the last 35 years of seismic observations at Hekla. Detailed analysis reveals that this tremor was related to helicopters passing the volcano. This study relates the GPS track of a helicopter with seismic recordings of the helicopter at various distances. We show the effect the distance, number of rotor blades and velocity of the helicopter has on the observed up and down glidings at up to 40 km distance. We highlight similarities and differences between volcano-related and helicopter tremor in order to help avoid misinterpretations.
Scopus© Citations 29 505
- «
- 1 (current)
- 2
- 3
- »