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A commercial finite element approach to modelling Glacial Isostatic Adjustment on spherical self-gravitating compressible earth models
Full text linkSeismicity of Ireland, and why it is so low: How the thickness of the lithosphere controls intraplate seismicity
Full text linkIreland and neighbouring Britain share much of their tectonic history and are both far from
active plate boundaries at present. Their seismicity shows surprising lateral variations, with
very few earthquakes in Ireland but many low-to-moderate ones in the adjacent western Britain.
Understanding the cause of these variations is important for our understanding of the basic
mechanisms of the intraplate seismicity distributions and for regional hazard assessment. The
distribution of microseismicity within Ireland and its underlying causes have been uncertain
due to the sparsity of the data sampling of the island, until recently. Here, we use the data from
numerous recently deployed seismic stations in Ireland and map its seismicity in greater detail
than previously. The majority of detectable seismic events are quarry and mine blasts. These
can be discriminated from tectonic events using a combination of the waveform data, event
origin times, and the epicentres’ proximity to quarries and mines, catalogued or identified
from the satellite imagery. Our new map of natural seismicity shows many more events
than known previously but confirms that the earthquakes are concentrated primarily in the
northernmost part of the island, with fewer events along its southern coast and very few deeper
inland. Comparing the seismicity with the recently published surface wave tomography of
Ireland and Britain, we observe a strong correspondence between seismicity and the phase
velocities at periods sampling the lithospheric thickness. Ireland has relatively thick, cold
and, by inference, mechanically strong lithosphere and has very few earthquakes. Most Irish
earthquakes are in the north of the island, the one place where its lithosphere is thinner,
warmer and, thus, weaker. Western Britain also has relatively thin lithosphere and numerous
earthquakes. By contrast, southeastern England and, probably, eastern Scotland have thicker
lithosphere and, also, few earthquakes. The distribution of earthquakes in Ireland and Britain
is, thus, controlled primarily by the thickness and mechanical strength of the lithosphere. The
thicker, colder, stronger lithosphere undergoes less deformation and features fewer earthquakes
than thinner, weaker lithosphere that deforms more easily. Ireland and Britain are tectonically
stable and the variations in the lithospheric thickness variations across them are estimated to
be in a 75–110 km range. Our results thus indicate that moderate variations in the lithospheric
thickness within stable continental interiors can exert substantial control on the distributions
of seismicity and seismic hazard—in Ireland, Britain and elsewhere around the worl
Determining subsurface temperature & lithospheric structure from joint geophysical-petrological inversion: A case study from Ireland
Full text linkHigh quality maps of the geothermal gradient and temperature are essential when assessing geothermal potential for a region. However, determining geothermal potential is a challenge as direct measurements of in situ temperature are sparse and individual geophysical methods are sensitive to a range of parameters, not solely temperature. Here, we develop a novel approach to determine the geothermal gradient using a new joint geophysical-petrological inversion where seismic velocities and density in the mantle are related to temperature and bulk composition within a thermodynamic framework. Large datasets of phase velocities of seismic surface-waves are now incorporated into the inversion, and provide essential constraints on the lithospheric thickness and temperature, which shape the crustal geotherms to a significant extent. We also include all available measurements of the surface heat flow, radiogenic heat production (RHP) and thermal conductivity within the crust, to further constrain the temperature and geothermal gradient, in particular in the top few kilometres of the crust. We use Ireland as a case study and show how our new methodology can reproduce the results of previous work but also improve on them, thanks to the complementary sensitivities of the full range of data. Lithospheric and crustal thicknesses have primary controls on the temperature gradient, with areas of thinner lithosphere showing higher geothermal gradients. In some locations, anomalously warm geotherms result from high RHP within crustal granitic rocks, mudstone and shales. RHP is above continental averages across all Ireland, likely due to a crust with mostly felsic lithology. The new methodology provides a robust workflow for determining the geothermal potential in areas with limited direct temperature measurements, facilitating knowledge creation for the transition to sustainable energy sources and energy self-sufficiency
Einstein's General Theory of Relativity: A Concise Introduction
No full textEinstein's general theory of relativity can be a notoriously difficult subject for students approaching it for the first time, with arcane mathematical concepts such as connection coefficients and tensors adorned with a forest of indices. This book is an elementary introduction to Einstein's theory and the physics of curved space-times that avoids these complications as much as possible. Its first half describes the physics of black holes, gravitational waves and the expanding Universe, without using tensors. Only in the second half are Einstein's field equations derived and used to explain the dynamical evolution of the early Universe and the creation of the first elements. Each chapter concludes with problem sets and technical mathematical details are given in the appendices. This short text is intended for undergraduate physics students who have taken courses in special relativity and advanced mechanics
Improved Type III solar radio burst detection using congruent deep learning models
Full text linkSolar flares are energetic events in the solar atmosphere that are often linked with solar radio bursts (SRBs). SRBs are observed at metric to decametric wavelengths and are classified into five spectral classes (Type I–V) based on their signature in dynamic spectra. The automatic detection and classification of SRBs is a challenge due to their heterogeneous form. Near-real time detection and classification of SRBs has become a necessity in recent years due to large data rates generated by advanced radio telescopes such as the LOw Frequency ARray (LOFAR). For this study, we implemented congruent deep learning models to automatically detect and classify Type III SRBs. We generated simulated Type III SRBs, which were comparable to Type IIIs seen in real observations, using a deep learning method known as the generative adversarial network (GAN). This simulated data were combined with observations from LOFAR to produce a training set that was used to train an object detection model known as you only look once (YOLOv2). Using this congruent deep learning model system, we can accurately detect Type III SRBs at a mean Average Precision (mAP) value of 77.71%
An Estimate of the Effect of 3D Heterogeneous Density Distribution on Coseismic Deformation Using a Spectral Finite-Element Approach
No full textUNDERSTANDING THE CAPABILITIES OF THE TELLUS FREQUENCY- AND TIME-DOMAIN AIRBORNE ELECTROMAGNETIC DATA
Full text linkAirborne Electromagnetic (AEM) geophysics has been increasingly applied to investigate large areas of the
subsurface due to its time and cost efficiency in producing spatially extensive datasets and its ability to
provide high-resolution information about electrical resistivity properties of the shallow subsurface. As part
of the geophysics component of the Tellus programme, time-domain electromagnetic (TDEM) data were
measured in one survey block in 2014 along with frequency-domain electromagnetic (FDEM) data acquired
in all other data blocks since 2011. A number of questions regarding the resolution capabilities of the two
Tellus EM systems used, and the resolution of the resistivity models remain. This restricts the confidence
with which resultant models might be interpreted and limits the geological value that might be derived
from them. This study will compare the capabilities of the FDEM and TDEM systems using synthetic and co-
located Tellus AEM datasets. Additionally, with the aim of investigating how the resistivity structure of the
subsurface along the Tellus AEM profiles compares with ground data, an Electrical Resistivity Tomography
(ERT) survey was carried out in September 2022 along a 470 m portion of a co-located Tellus AEM profile
Nowcasting geoelectric fields in Ireland using magnetotelluric transfer functions
Full text linkGeomagnetically induced currents (GIC) driven by geoelectric fields pose a hazard to ground-based infrastructure, such as power grids and pipelines. Here, a new method is presented for modelling geoelectric fields in near real time, to provide valuable information to help mitigate the impact of GIC. The method uses magnetic field measurements from the Magnetometer Network of Ireland (MagIE; https://www.magie.ie), interpolates the geomagnetic field variations between magnetometers using spherical elementary current systems (SECS), and estimates the local electric field using a high-density (< 40 km) network of magnetotelluric transfer functions (MT-TF) encompassing the island. The model was optimised to work in near real time, with a correction curve applied to the geoelectric field time series. This approach was successfully validated with measured electric fields at four sites for a number of geomagnetic storms, providing accurate electric fields up to a 1-minute delay from real time, with high coherence (0.70 – 0.85) and signal-to-noise ratio (SNR; 3.2 – 6.5) relative to measured electric field validation time series. This was comparable to a standard non-real-time geoelectric field model (coherence = 0.80 − 0.89 and SNR = 4.0 − 7.0). The impact of galvanic distortion on the model was also briefly evaluated, with a galvanic distortion correction leading to a more homogeneous representation of the direction of the electric field, at a regional scale
Interferometric imaging of the type IIIb and U radio bursts observed with LOFAR on 22 August 2017
Full text linkContext. The Sun is the source of different types of radio bursts that are associated with solar flares, for example. Among the most frequently observed phenomena are type III solar bursts. Their radio images at low frequencies (below 100 MHz) are relatively poorly studied due to the limitations of legacy radio telescopes.
Aims. We study the general characteristics of types IIIb and U with stria structure solar radio bursts in the frequency range of 20–80 MHz, in particular the source size and evolution in different altitudes, as well as the velocity and energy of electron beams responsible for their generation.
Methods. In this work types IIIb and U with stria structure radio bursts are analyzed using data from the LOFAR telescope including dynamic spectra and imaging observations, as well as data taken in the X-ray range (GOES and RHESSI satellites) and in the extreme ultraviolet (SDO satellite).
Results. In this study we determined the source size limited by the actual shape of the contour at particular frequencies of type IIIb and U solar bursts in a relatively wide frequency band from 20 to 80 MHz. Two of the bursts seem to appear at roughly the same place in the studied active region and their source sizes are similar. It is different in the case of another burst, which seems to be related to another part of the magnetic field structure in this active region. The velocities of the electron beams responsible for the generation of the three bursts studied here were also found to be different
Seismic ambient noise imaging above an underground mine: An example from Boliden Tara Mines, Ireland
No full textWe use a passive seismic data set recorded from June 2017 to July 2018 by twenty-one stations deployed in a five
by five km grid to the south-west of the Boliden Tara Mines orebody (Eastern Ireland) to image the shallower part
of the Carboniferous rock units. We cross-correlate one year of ambient noise between all station pairs to
generate cross-correlation functions that are rich in surface waves. Then we investigate the locations of the noise
sources by applying f-k cross-correlation beamforming. We find two persistent noise sources associated with
oceanic microseisms and a cluster of anthropogenic activities, respectively. We undertake dispersion analysis to
extract group velocity measurements in the 1–9 Hz band that we invert using the fast marching method to create
Rayleigh wave velocity maps. This is followed by a wavelet-based sparsity constrained tomographic method to
produce a 3-D shear wave velocity model down to 800 metres. The results highlight low and high-velocity
anomalies related to the structural and stratigraphic heterogeneity within the Upper Dark limestone formation.
In addition, the velocity gradient of our model agrees with the sonic log velocity profile available adjacent
to the study area, suggesting that this sparse network can be used to obtain reliable broad-scale information and
could also guide the design phase of more expensive and involved experiments, such as active seismic reflection
imaging