We propose a preliminary 3D velocity model of sw Iberia and its adjacent Atlantic area, and inves... more We propose a preliminary 3D velocity model of sw Iberia and its adjacent Atlantic area, and investigate the possibilities of such a model. The velocity model is based on previous studies of the crust in SW Iberia published by various authors; it is integrated in afinite-difference wave propagation code. First, wefocus on three recent earthquakes, and compare observed and synthetic seismograms. Then, we concentrate on the largest recorded earthquake in the region, and compare macroseismic observations with synthetic seismic intensity. Finally, using the same method, we test three hypothetical modelsfor the 1755 earthquake source, and determine which source produces bestfit between observed and synthetic intensitie
<p>GPR data sometimes present subsampling problems that prevent an effectiv... more <p>GPR data sometimes present subsampling problems that prevent an effective study about the existence of buried structures in an archaeological site. This is a frequent problem, related with the profile spacing used in the survey, when this is performed in parallel profiles to construct a 3D dataset. This difficulty can be lessened by decreasing the profile spacing, but that increases the survey time, or can be experimented in the data processing. INT-FFT algorithm is GPR data densification approach, complementary to the other standard operations, that allows to reconstruct missing data from the combined use of mathematical transforms and predictive filters. To calculate the missing signal, two requirements must be checked: (1) the data in the frequency domain must be limited in a range of values; and (2) must be able to be represented by a distribution of Fourier coefficients. Both conditions are verified in GPR data. Based on seismic trace interpolation, INT-FFT algorithm uses an open access routine (Suinterp, from Seismic Unix package) to interpolate the GPR profiles, that makes use of automatic event identification routines, through the calculation of spatial derivatives, to identify discontinuities in space from the detection very subtle changes in the signal, thus allowing for more efficient interpolation without artifacts or signal deterioration. The approach was successfully tested using GPR datasets from the archaeological site of Roman Villa of Horta da Torre (Fronteira, Portugal). The results show that there was an increase of the geometric sharpness of the GPR planimetry and has not produced any numerical artefacts. The tests performed to apply the methodology to GPR-3D data allowed to assess the interpolation efficiency, the level of recovery of missing data and the level of information lost when one chooses to increase the distance between profiles in the acquisition stage of the data.</p><p> </p><p>Acknowledgment: The work was supported by the Portuguese Foundation for Science and Technology (FCT) project UIDB/04683/2020 - ICT (Institute of Earth Sciences) and by the INTERREG 2014-2020 Program, through the "Innovación abierta e inteligente en la EUROACE" Project, with the reference 0049_INNOACE_4_E.</p>
Trabajo presentado al III Congresso Luso-Extremadurense “Ciência e Tecnologias”, celebrado en la ... more Trabajo presentado al III Congresso Luso-Extremadurense “Ciência e Tecnologias”, celebrado en la Universidad de Évora del 25 al 26 de noviembre de 2019
ABSTRACT The first broadband (BB) seismic stations were deployed in Portugal in the 1990s, and ev... more ABSTRACT The first broadband (BB) seismic stations were deployed in Portugal in the 1990s, and ever since their number had steadily increased. Portugal is currently covered by a network of 35 broadband stations in mainland Portugal, which is complemented by stations in the islands of Madeira and Azores, as well as stations in Morocco. In the period 2010 - 2012, project WILAS - "West Iberia Lithosphere and Astenosphere Structure" (PTDC/CTE-GIX/097946/2008), deployed 30 additional temporary seismic BB stations in mainland Portugal. The WILAS stations, in addition to the permanent and TOPOIBERIA stations, provided a full and dense coverage of the Iberian Peninsula. In this presentation we will characterize the ambient seismic noise recorded at BB stations deployed in Portugal (mainland, Azores and Madeira) and Morocco. We analyse all time periods of data available since the instruments were installed. The noise is characterized by means of probability density functions (PDFs) of power spectral density (PSDs) of continuous, overlapping, 1-hour segments of data. Time-series of noise levels at different frequencies and spectrograms are computed to visualize the variations of ambient noise over different time periods and frequency bands. We observe the expected diurnal periodicity at high frequencies and seasonal variation at long periods. There is a clear increase of the noise amplitude in the microseismic band during the Winter, when more storms occur in the adjacent Northern Atlantic. We correlate sea level, storm activity, and other atmospheric parameters with the variations in ambient noise level. The analysis performed gives clues concerning data quality (poor quality data is clearly identified), Earth structure (a correlation is visible between sedimentary basins and amplification of seismic noise), and sources of ambient noise at different frequency bands.
Temporary seismic stations were quickly deployed after the mainshock (first station was installed... more Temporary seismic stations were quickly deployed after the mainshock (first station was installed 1h30 after the mainshock) , as a collaborative effort between ICT (Instituto de Ciências da Terra, Évora) and IDL (Instituto Dom Luiz). The full deployment encompassed 14 broadband stations (CMG 6TD, 30 s) and 21 short-period stations (CDJ, 2.0 Hz). The 35 stations, deployed around the epicentral area, together with the backbone permanent seismic network, allow a unique study of microseismic activity during this crisis.
Waveform modeling is an important and helpful instrument of modern seismology that may provide va... more Waveform modeling is an important and helpful instrument of modern seismology that may provide valuable information. However, synthesizing seismograms requires to define many parameters, which differently affect the final result. Such parameters may be: the design of the grid, the structure model, the source time functions, the source mechanism, the rupture velocity. Variations in parameters may produce significantly different seismograms.
Obtain three-dimensional models of the physical properties of buried structures in the subsurface... more Obtain three-dimensional models of the physical properties of buried structures in the subsurface by inversion of GPR data is an appeal to Archaeology and a challenge to Geophysics. Along the research of solutions to resolve this issue stand out two major problems that need to be solved: 1) Establishment the basis of the computation that allows assign numerically in the synthetic radargrams, the physical conditions at which the GPR wave were generated; and 2) automatic comparison of the computed synthetic radargrams with the correspondent observed ones. The influence of the pulse shape in GPR data processing was a studied topic. The pulse outline emitted by GPR antennas was experimentally acquired and this information has been used in the deconvolution operation, carried out by iterative process, similarly the approach used in seismology to obtain the receiver functions. In order to establish the comparison between real and synthetic radargrams, were tested automatic image adjustmen...
We propose a preliminary 3D velocity model of sw Iberia and its adjacent Atlantic area, and inves... more We propose a preliminary 3D velocity model of sw Iberia and its adjacent Atlantic area, and investigate the possibilities of such a model. The velocity model is based on previous studies of the crust in SW Iberia published by various authors; it is integrated in afinite-difference wave propagation code. First, wefocus on three recent earthquakes, and compare observed and synthetic seismograms. Then, we concentrate on the largest recorded earthquake in the region, and compare macroseismic observations with synthetic seismic intensity. Finally, using the same method, we test three hypothetical modelsfor the 1755 earthquake source, and determine which source produces bestfit between observed and synthetic intensitie
<p>GPR data sometimes present subsampling problems that prevent an effectiv... more <p>GPR data sometimes present subsampling problems that prevent an effective study about the existence of buried structures in an archaeological site. This is a frequent problem, related with the profile spacing used in the survey, when this is performed in parallel profiles to construct a 3D dataset. This difficulty can be lessened by decreasing the profile spacing, but that increases the survey time, or can be experimented in the data processing. INT-FFT algorithm is GPR data densification approach, complementary to the other standard operations, that allows to reconstruct missing data from the combined use of mathematical transforms and predictive filters. To calculate the missing signal, two requirements must be checked: (1) the data in the frequency domain must be limited in a range of values; and (2) must be able to be represented by a distribution of Fourier coefficients. Both conditions are verified in GPR data. Based on seismic trace interpolation, INT-FFT algorithm uses an open access routine (Suinterp, from Seismic Unix package) to interpolate the GPR profiles, that makes use of automatic event identification routines, through the calculation of spatial derivatives, to identify discontinuities in space from the detection very subtle changes in the signal, thus allowing for more efficient interpolation without artifacts or signal deterioration. The approach was successfully tested using GPR datasets from the archaeological site of Roman Villa of Horta da Torre (Fronteira, Portugal). The results show that there was an increase of the geometric sharpness of the GPR planimetry and has not produced any numerical artefacts. The tests performed to apply the methodology to GPR-3D data allowed to assess the interpolation efficiency, the level of recovery of missing data and the level of information lost when one chooses to increase the distance between profiles in the acquisition stage of the data.</p><p> </p><p>Acknowledgment: The work was supported by the Portuguese Foundation for Science and Technology (FCT) project UIDB/04683/2020 - ICT (Institute of Earth Sciences) and by the INTERREG 2014-2020 Program, through the "Innovación abierta e inteligente en la EUROACE" Project, with the reference 0049_INNOACE_4_E.</p>
Trabajo presentado al III Congresso Luso-Extremadurense “Ciência e Tecnologias”, celebrado en la ... more Trabajo presentado al III Congresso Luso-Extremadurense “Ciência e Tecnologias”, celebrado en la Universidad de Évora del 25 al 26 de noviembre de 2019
ABSTRACT The first broadband (BB) seismic stations were deployed in Portugal in the 1990s, and ev... more ABSTRACT The first broadband (BB) seismic stations were deployed in Portugal in the 1990s, and ever since their number had steadily increased. Portugal is currently covered by a network of 35 broadband stations in mainland Portugal, which is complemented by stations in the islands of Madeira and Azores, as well as stations in Morocco. In the period 2010 - 2012, project WILAS - "West Iberia Lithosphere and Astenosphere Structure" (PTDC/CTE-GIX/097946/2008), deployed 30 additional temporary seismic BB stations in mainland Portugal. The WILAS stations, in addition to the permanent and TOPOIBERIA stations, provided a full and dense coverage of the Iberian Peninsula. In this presentation we will characterize the ambient seismic noise recorded at BB stations deployed in Portugal (mainland, Azores and Madeira) and Morocco. We analyse all time periods of data available since the instruments were installed. The noise is characterized by means of probability density functions (PDFs) of power spectral density (PSDs) of continuous, overlapping, 1-hour segments of data. Time-series of noise levels at different frequencies and spectrograms are computed to visualize the variations of ambient noise over different time periods and frequency bands. We observe the expected diurnal periodicity at high frequencies and seasonal variation at long periods. There is a clear increase of the noise amplitude in the microseismic band during the Winter, when more storms occur in the adjacent Northern Atlantic. We correlate sea level, storm activity, and other atmospheric parameters with the variations in ambient noise level. The analysis performed gives clues concerning data quality (poor quality data is clearly identified), Earth structure (a correlation is visible between sedimentary basins and amplification of seismic noise), and sources of ambient noise at different frequency bands.
Temporary seismic stations were quickly deployed after the mainshock (first station was installed... more Temporary seismic stations were quickly deployed after the mainshock (first station was installed 1h30 after the mainshock) , as a collaborative effort between ICT (Instituto de Ciências da Terra, Évora) and IDL (Instituto Dom Luiz). The full deployment encompassed 14 broadband stations (CMG 6TD, 30 s) and 21 short-period stations (CDJ, 2.0 Hz). The 35 stations, deployed around the epicentral area, together with the backbone permanent seismic network, allow a unique study of microseismic activity during this crisis.
Waveform modeling is an important and helpful instrument of modern seismology that may provide va... more Waveform modeling is an important and helpful instrument of modern seismology that may provide valuable information. However, synthesizing seismograms requires to define many parameters, which differently affect the final result. Such parameters may be: the design of the grid, the structure model, the source time functions, the source mechanism, the rupture velocity. Variations in parameters may produce significantly different seismograms.
Obtain three-dimensional models of the physical properties of buried structures in the subsurface... more Obtain three-dimensional models of the physical properties of buried structures in the subsurface by inversion of GPR data is an appeal to Archaeology and a challenge to Geophysics. Along the research of solutions to resolve this issue stand out two major problems that need to be solved: 1) Establishment the basis of the computation that allows assign numerically in the synthetic radargrams, the physical conditions at which the GPR wave were generated; and 2) automatic comparison of the computed synthetic radargrams with the correspondent observed ones. The influence of the pulse shape in GPR data processing was a studied topic. The pulse outline emitted by GPR antennas was experimentally acquired and this information has been used in the deconvolution operation, carried out by iterative process, similarly the approach used in seismology to obtain the receiver functions. In order to establish the comparison between real and synthetic radargrams, were tested automatic image adjustmen...
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