In seismic data processing removal of very low frequency is the conventional approach but the preservation of low frequency become important in highly absorptive media, where we are interested in extracting the geological information associated with low frequencies. The reflection coefficient is the function of frequency in fluid saturated media and anomalously high amplitude at low frequency can indicate presence of hydrocarbon which is normally not observed in conventional seismic data. Continuous wavelet transform (CWT) is used for extraction of low frequency from the seismic data, which is a spectral decomposition method. The application of spectral decomposition includes layer thickness determination, stratigraphic visualization and direct hydrocarbon detection (DHI), which have a great significance in hydrocarbon exploration.
The concept of reflectivity of low frequency band of seismic energy related to presence of hydrocarbons is established theoretically for quite some time. Now efforts are made to establish this in the industry for detection of hydrocarbons. The present study is the case study from Western offshore basin, India where data is processed with preservation of low frequencies and taking utmost care while selecting input parameters like band pass filter, wavelet matching and amplitude matching. Broadly, the processing steps included standard signal processing techniques without affecting the relative amplitudes and preserving low end of frequency bandwidth.
To examine low frequency, input data has been analyzed for the maximum frequency present in the study area. The input data for low frequency analysis was taken as Pre Stack Time Migration (PSTM) volume. Using in-house developed program for complex continuous wavelet transform (CCWT) of seismic traces low frequency volumes from input PSTM data were extracted. We have analyzed where the peak amplitude has occurred with respect to scale in low frequency volume. This peak amplitude value with respect to low frequency extracted from the volume may signify the indication of hydrocarbon bearing zones. In a properly processed low frequency section/volume the amplitude brightening may be used as direct hydrocarbon indicator. The frequency dependent reflectivity which is supposed to be higher at lower frequency range for a fluid saturated interface should produce higher amplitude on a low frequency section compared to the surrounding (with no fluid saturation). From the seismic data suitable low frequency volume is generated using continuous wavelet transform (CWT) method.
The present study reveals that how low frequency based seismic data processing can help to identify the high amplitude at low frequency in fluid saturated zones, which is not observed in conventional processed data. This methodology is significantly useful for generation of locales for exploration/exploitation in complex lithological environment where other approaches fail.