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Site surveys for the deployment of infrasound mobile stations in the Azores Islands
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Abstract(s)
Ondas sonoras de baixa frequência (< 20 Hz), conhecidas como infrassons, são geradas por fontes naturais e artificiais que induzem pequenas variações na pressão atmosférica, tais como erupções vulcânicas, sismos, explosões em pedreiras, explosões nucleares, entre outras. Através de canais na atmosfera, os infrassons podem propagar-se por longas distâncias sofrendo pouca atenuação. Desde o início do século XXI, os infrassons têm vindo a ser aplicados na monitorização de diversos tipos de eventos acústicos, tanto para aplicações militares como civis e científicas.
A história geológica das ilhas dos Açores tem registado importantes eventos sísmicos e vulcânicos, por isso, futuros eventos eruptivos dos seus vulcões ativos são expectáveis. Localizado no meio do Atlântico Norte, o arquipélago é uma excelente plataforma para desenvolver uma rede regional de monitorização por infrassons. A instalação de arrays de infrassons em caso de qualquer emergência, como a crise sismovulcânica de 2022 do Sistema Vulcânico Fissural de Manadas na Ilha de São Jorge, foi uma mais-valia para testar as redes de monitorização existentes do CIVISA/IVAR, particularmente para identificar sinais precursores da atividade vulcânica através da deteção de eventos sismo-acústicos, bem como uma ferramenta indispensável para monitorizar uma possível atividade eruptiva. De igual modo, o IVAR é responsável pela operação e manutenção da estação de infrassons IS42 do IMS, localizada na Ilha da Graciosa, e que desempenha um papel importante na deteção diversas fontes de infrassons no arquipélago dos Açores e globalmente. Assim sendo, a sua colaboração com outros arrays de infrassons torna-se fundamental em casos como o da crise de São Jorge relativamente ao array SJ1.
No entanto, antes instalação de qualquer array portátil de infrassons, devem ser avaliados e selecionados os locais com as melhores condições gerais, sobretudo com uma boa razão entre sinal e ruído. Portanto, este trabalho de investigação adaptou as metodologias conhecidas para os chamados site surveys direcionados para a instalação de arrays portáteis de infrassons nos Açores, hipotecando para isso cenários eruptivos nos vulcões centrais ativos nos Açores como fonte de infrassons de referência. Todas as etapas dos site surveys foram aqui detalhadas, nomeadamente (1) a seleção preliminar, (2) a avaliação in situ dos sítios pré-selecionados, que só foi possível na Ilha de São Miguel, e (3) o teste de sinal e consequente seleção final. Esta última etapa foi realizada na Ilha de São Jorge, dada a oportunidade que surgiu com a crise sismovulcânica. Assim, essa mesma metodologia foi adaptada para o Sistema Vulcânico Fissural de Manadas, tendo-se instalado o array SJ1 de modo a colaborar com as outras redes de monitorização, especialmente a sísmica, na deteção de eventos sismo-acústicos e, na eventualidade do início de atividade eruptiva.
ABSTRACT: Low-frequency sound waves (< 20 Hz), known as infrasound, are generated by natural and man-made sources, such as volcanic eruptions, seismic activity, quarry activities, nuclear explosions, among others, which induce micro pressure variations in the atmosphere. Thus, travelling through waveguides in atmosphere, infrasound can propagate over long distance with low attenuation. Therefore, since the beginning of the XXI century, infrasound techniques have been employed to monitor and analyse a myriad of acoustic events, from military to civil and scientific applications. The geological history of the Azores Islands has been recording important seismic and volcanic activity, which lead to expect future eruptive events from their active volcanoes. Therefore, located in the middle of the North Atlantic, the archipelago is an excellent platform to develop a regional infrasound monitoring network. The deployment of infrasound arrays in case of any emergency, as the 2022 volcanic unrest of Manadas fissure volcanic system on São Jorge Island, would be a valuable input to the existing monitoring networks of CIVISA/IVAR, particularly to identify precursor signals of volcanic activity through the detection of seismoacoustic events, and an indispensable tool for monitoring possible eruptive activity. Additionally, IVAR is responsible for the operation and maintenance of the IMS infrasound station IS42, located on Graciosa Island, which has a major contribution for the Azorean region in the detection of all type of infrasound sources from all the world, and would display a significant role in association with other infrasound arrays, as the situation in São Jorge Island with SJ1 array. Nevertheless, before the deployment of any infrasound array, site surveys must be conducted to select the sites with the best overall conditions, particularly with a good signal-to-noise ratio. Thus, this research work has adapted known methodologies for site surveys to deploy infrasound arrays in the Azores Islands, considering its central volcanoes as the infrasound source reference, through hypothetically eruptive scenarios. All steps of the survey were herein detailed, namely (1) the preliminary selection, (2) the assessment in situ of the pre-selected sites, which was only possible on São Miguel Island, and (3) field survey and final selection. This last step was conducted in São Jorge Island, given the opportunity that came up with the seismovolcanic crisis. Therefore, the same site survey methodology was applied for a volcanic fissure system and an infrasound array (SJ1) was deployed to support the other monitoring networks, specially the seismic one, in the detection of seismoacoustic events. Hence, SJ1 was set up to detect volcanic infrasound in case of an eruption start on Manadas fissure volcanic system.
ABSTRACT: Low-frequency sound waves (< 20 Hz), known as infrasound, are generated by natural and man-made sources, such as volcanic eruptions, seismic activity, quarry activities, nuclear explosions, among others, which induce micro pressure variations in the atmosphere. Thus, travelling through waveguides in atmosphere, infrasound can propagate over long distance with low attenuation. Therefore, since the beginning of the XXI century, infrasound techniques have been employed to monitor and analyse a myriad of acoustic events, from military to civil and scientific applications. The geological history of the Azores Islands has been recording important seismic and volcanic activity, which lead to expect future eruptive events from their active volcanoes. Therefore, located in the middle of the North Atlantic, the archipelago is an excellent platform to develop a regional infrasound monitoring network. The deployment of infrasound arrays in case of any emergency, as the 2022 volcanic unrest of Manadas fissure volcanic system on São Jorge Island, would be a valuable input to the existing monitoring networks of CIVISA/IVAR, particularly to identify precursor signals of volcanic activity through the detection of seismoacoustic events, and an indispensable tool for monitoring possible eruptive activity. Additionally, IVAR is responsible for the operation and maintenance of the IMS infrasound station IS42, located on Graciosa Island, which has a major contribution for the Azorean region in the detection of all type of infrasound sources from all the world, and would display a significant role in association with other infrasound arrays, as the situation in São Jorge Island with SJ1 array. Nevertheless, before the deployment of any infrasound array, site surveys must be conducted to select the sites with the best overall conditions, particularly with a good signal-to-noise ratio. Thus, this research work has adapted known methodologies for site surveys to deploy infrasound arrays in the Azores Islands, considering its central volcanoes as the infrasound source reference, through hypothetically eruptive scenarios. All steps of the survey were herein detailed, namely (1) the preliminary selection, (2) the assessment in situ of the pre-selected sites, which was only possible on São Miguel Island, and (3) field survey and final selection. This last step was conducted in São Jorge Island, given the opportunity that came up with the seismovolcanic crisis. Therefore, the same site survey methodology was applied for a volcanic fissure system and an infrasound array (SJ1) was deployed to support the other monitoring networks, specially the seismic one, in the detection of seismoacoustic events. Hence, SJ1 was set up to detect volcanic infrasound in case of an eruption start on Manadas fissure volcanic system.
Description
Dissertação de Mestrado, Vulcanologia e Riscos Geológicos, 25 de maio de 2023, Universidade dos Açores.
Keywords
Infrasound Volcanogy São Jorge Island Noise
Pedagogical Context
Citation
Jesus, Maria do Céu Neto de. (2022). "Site surveys for the deployment of infrasound mobile stations in the Azores Islands". 129 p. (Dissertação de Mestrado em Vulcanologia e Riscos Geológicos). Ponta Delgada: Universidade dos Açores, 2022. Disponível em http://hdl.handle.net/10400.3/6751