APPLICATION OF ISO-FREQUENCY MAPS TO TIME-LAPSE DATA

ABSTRACT. Time-frequency transforms are widely used in seismic exploration. These transforms enable analysis of the energy density of a non-stationary signal as functions of amplitude, time and frequency. The representation of energy density is not unique, and each transform has its advantages and disadvantages. The choice of which transform should be used depends on the application. In this paper, we propose a new way to analyze time-lapse anomalies using iso-frequency panels obtained by time-frequency transforms. We compared the iso-frequency panels of the Morlet Wavelet Transform and Choi-Williams Distribution. These panels revealed different characteristics and can provide additional information for the interpretation of time-lapse anomalies. We used seismic data from the Marimba field of the Campos Basin, Brazil, for which base and monitor acquisitions were held in 1984 and 1999, respectively. We also used a special filtering approach to enhance seismic resolution and remove noise, whereby we applied the curvelet transform to remove noise, and employed a tool to correct the residual moveout and inverse Q filtering for attenuation correction. Then we analysed the time-lapse anomalies using iso-frequency panels. The main time-lapse anomalies appeared in the form of clouds in the iso-frequency panels obtained by the Morlet Wavelet Transform approach. Iso-frequency panels obtained by Choi-Williams Distribution showed a higher sensitivity and resolution for analyzing the anomalies. Our results show the great potential of these transforms for visualization of time-lapse anomalies. Keywords : time-lapse anomalies, spectrogram, Morlet Wavelet Transform, Choi-Williams Distribution. RESUMO. Transformadas tempo-frequencia sao amplamente utilizadas na exploracao sismica. Estas transformadas permitem a analise da densidade de energia de um sinal nao-estacionario como funcoes de amplitude, tempo e frequencia. A representacao da densidade de energia de um sinal nao e unica, e cada transformada tem suas vantagens e desvantagens. A escolha da transformada que deve ser usada depende da aplicacao. Neste artigo, propomos uma nova abordagem para analisar anomalias de dados time-lapse usando paineis iso-frequencia obtidos atraves de transformadas tempo-frequencia. Comparamos os paineis iso-frequencia obtidos com a Transformada Wavelet de Morlet e a Distribuicao de Choi-Williams. Estes paineis revelaram diferentes caracteristicas que podem fornecer informacoes adicionais para a interpretacao de anomalias time-lapse . Os dados sismicos utilizados foram do Campo de Marimba da Bacia de Campos, Brasil, os quais as aquisicoes base e monitor foram realizadas em 1984 e 1999, respectivamente. Antes da analise dos paineis iso-frequencia, usamos um workflow para melhorar a resolucao sismica e a razao sinal-ruido. Neste workflow , aplicamos a Transformada Curvelet para remover ruidos aleatorios e coerentes, uma ferramenta para corrigir o moveout residual e um Q-filter para correcao dos efeitos da atenuacao. Apos este workflow , as principais anomalias time-lapse apareceram na forma de nuvens nos paineis iso-frequencia da Transformada Wavelet de Morlet. Ja os paineis iso-frequencia da Distribuicao de Choi-Williams apresentaram uma maior sensibilidade e resolucao para analise dessas anomalias. Os resultados mostraram o grande potencial dessas transformadas para a visualizacao e interpretacao de anomalias time-lapse . Palavras-chave : anomalias time-lapse , espectrograma, Transformada Wavelet de Morlet, Distribuicao Choi-Williams.