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This paper presents the spatial distribution of shallow gas structures and classifies them on the basis of two different data sets of CHIRP seismic records, one from the Conceição Lagoon (CL) and the other from North Bay (NB), both on Santa Catarina Island, Southern Brazil. Side scan sonar data from the CL were used to facilitate the understanding. The sub bottom (SB) seismic data were processed and interpreted by means of the SeisPrho software, the side scan sonar (SSS) data by SonarWiz5 software and the spatial extension being measured with the help of GIS. The shallow gas structures were defined in accordance with their shapes in the seismic recordings (echo-character). At the CL, shallow gas accumulations were found in the form of seepages and features presenting shallow gas structures between the surface and 8.20 ms (around 12.3 m). Accumulations of gas were found in the form of Acoustic Blanking with Acoustic Plume, and also Black Shadows. Pockmarks were found on the lagoon floor and associated with gas seepages (average size diameter 0.97 ± 0.19 m and density from 54 to 242 units per 50 m2). In the NB three types of shallow gas features were found in the seismic profile, namely Acoustic Blanking, Turbidity Pinnacles and Intra-sedimentary plumes. The depth varied from the surface to 12.10 ms (around 18.15 m). In both environments, the gas is escaping from the sediment into the water column. The Pockmarks in the CL and the Acoustic Plume features and sediment rich in total sulfur in the NB validate these findings.

Demarco et al.: Seismic attributes in CHIRP sub-bottom profiles 332Marine substrate response from the analysis of seismic attributes in CHIRP sub-bottom profiles This paper presents an evaluation of the response of seismic reflection attributes in different types of marine substrate (rock, shallow gas, sediments) using seafloor samples for ground-truth statistical comparisons. The data analyzed include seismic reflection profiles collected using two CHIRP subbottom profilers (Edgetech Model 3100 SB-216S), with frequency ranging between 2 and 16 kHz, and a number (38) of sediment samples collected from the seafloor. The statistical method used to discriminate between different substratum responses was the non-parametric KruskalWallis analysis, carried out in two steps: 1) comparison of Seismic Attributes between different marine substrates (unconsolidated sediments, rock and shallow gas); 2) comparison of Seismic Attributes between different sediment classes in seafloors characterized by unconsolidated sediments (subdivided according to sorting). These analyses suggest that amplituderelated attributes were effective in discriminating between sediment and gassy/rocky substratum, but did not differentiate between rocks and shallow gas. On the other hand, the Instantaneous Frequency attribute was effective in differentiating sediments, rocks and shallow gas, with sediment showing higher frequency range, rock an intermediate range, and shallow gas the lowest response. Regarding grain-size classes and sorting, statistical analysis discriminated between two distinct groups of samples, the SVFS (silt and very fine sand) and the SFMC (fine, medium and coarse sand) groups. Using a Spearman coefficient, it was found that the Instantaneous Amplitude was more efficient in distinguishing between the two groups. None of the attributes was able to distinguish between the closest grain size classes such as those of silt and very fine sand. O presente trabalho tem por objetivo apresentar uma avaliação da resposta dos atributos sísmicos (Amplitude Instantânea, Amplitude RMS, Energia e Frequência Instantânea) em diferentes tipos de substratos marinhos, correlacionando-os com características sedimentológicas das amostras coletadas. Foram analisados perfis sísmicos obtidos com um perfilador de subsuperfície com sinal do tipo CHIRP modelo SB-216S da marca EdgeTech, com frequência de trabalho de 2 e 16 kHz. O método se deu a partir da análise estatística não-paramétrica de Kruskal-Wallis foi aplicada para comparar o comportamento dos atributos com as diferentes classes dos grãos das amostras (subagrupadas segundo o grau de seleção) e com diferentes feições. Com base na análise dos resultados, foi possível distinguir dois grupos distintos nas amostras, o grupo SAMF (silte e areia muito fina) e o grupo AFMG (areia fina, areia média e areia grossa). Como conclusão, pode-se dizer que os atributos não foram capazes de distinguir entre as classes mais próximas dos grãos. Utilizando o coeficiente de Spearman foi verificado que o atributo "Amp...

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This paper presents the identification and depth of gas features detected in the Conceição Lagoon and in the North Bay, located on the Island of Santa Catarina, Brazil. The lines were obtained by seismic acoustic profiler of high resolution, using the equipment CHIRP (frequency between 1 and 16 KHz), then the data were spatially processed and interpreted with the use of free software. The high resolution seismic data showed the presence of shallow gas classified as Acoustic blanket (acoustic blanking), Turbidity pinnacles, Intra-sedimentary plumes, Black shadows and gas escape as Acoustic plumes.