Abstract：Based on the instantaneous spectral analysis (ISA) method, low-frequency shadows (LFSs) have been used as an effective hydrocarbon indicator. But the cause of these shadows is unclear. We try here to find out the cause with numerical simulation method. With equivalent fluid-saturated media models, we perform the forward modeling by visco-elastic wave equation based on the Kelvin-Voigt model. Then we obtain corresponding ISA sections of different seismograms using time frequency continuous wavelet transform (TFCWT). On low-frequency (e.g. 15Hz) ISA sections, the spectral energy of the area under oil-bearing reservoirs is much stronger than that of the same area on high-frequency ISA sections (e.g.>55Hz). Our study reveals that LFS is more apparent if reservoir is thicker (>λ/2) and quality factors are lower (QP<90 and QS<70). It is concluded that LFSs will appear around the first interface with impedance difference under reservoirs.
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