High-resolution seismic imaging of gas accumulations and seepage in the sediments of the Ria de Aveiro barrier lagoon (Portugal) |
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Authors: | Henrique Duarte Luis Menezes Pinheiro Francisco Curado Teixeira José Hipólito Monteiro |
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Institution: | 1. Departamento de Geociências and CESAM, University of Aveiro, Campus de Santiago, 3810-193, Aveiro, Portugal 2. Marine Geology Department, INETInova??o, Estrada do Zambujal, Apartado 7586, 2720, Alfragide, Portugal 3. Instituto de Sistemas e Robótica, Instituto Superior Técnico, Torre Norte, 7° piso, Av. Rovisco Pais 1, 1049-001, Lisboa, Portugal
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Abstract: | Methane is a powerful greenhouse gas and an important energy source. The global significance and impact in coastal zones of
methane gas accumulation and seepage in sediments from coastal lagoon environments are still largely unknown. This paper presents
results from four high-resolution seismic surveys carried out in the Ria de Aveiro barrier lagoon (Portugal) in 1999, 2002
and 2003. These comprise three chirp surveys (RIAV99, RIAV02, RIAV02A) and one boomer survey (RIAV03). Evidence of extensive
gas accumulation and seepage in tidal channel sediments from the Ria de Aveiro barrier lagoon is presented here for the first
time. This evidence includes: acoustic turbidity, enhanced reflections, acoustic blanking, domes, and acoustic plumes in the
water column (flares). The stratigraphy and structural framework control the distribution and extent of gas accumulations
and seepage in the study area. In these shallow systems, however, tidal altitude variations have a significant impact on gas
detection using acoustic methods, by changing the raw amplitude of the enhanced seismic reflections, acoustic turbidity, and
acoustic blanking in gas-prone areas. Direct evidence of gas escape from drill holes in the surrounding area has shown that
the gas present in the Ria de Aveiro consists of biogenic methane. Most of the gas in the study area was probably generated
mainly in Holocene lagoon sediments. Evidence of faults affecting the Mesozoic limestones and clays underlying some of the
shallow gas occurrences, and the presence of high-amplitude reflections in these deeper units raise the possibility that some
of this gas could have been generated in deeper sedimentary layers, and then migrated upward through the fractured Mesozoic
strata. |
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