Understanding the causes of low frequency shadow below gas hydrates

Summary

Gas hydrates consider as one of the prospective natural resource that can supply energy. The seismic method has successfully been applied to locate gas hydrates. Their presence is revealed by bottom simulating reflectors (BSR) which represent the seismic signature of the base of gas-hydrate saturated sediments, with a layer partially saturated with free gas below the BSR. It is observed that the seismic response of the BSR is characterized by low frequency, which it is called the low frequency shadow (LFS). The possible causes of LFS are the presence of free gas or normal moveout (NMO) stretching. In order to have a deep understanding of the low frequency shadow causes, 1D and 2D synthetic seismograms and spectrograms are computed. The 1D and 2D simulations of seismic signal are based on rock physics and numerical modelling, considered the effect of the seismic attenuation and NMO stretch. The results of numerical seismograms and spectrograms show that attenuation affects the lower interface with minimum amplitudes for lower values. The quantification of the maximum frequency is obtained as the shift of the centroid spectrum. Moreover, the non-stretch NMO corrections improve the resolution to detect the BSR layer, and a stacked trace can be achieved without loss of frequencies. As a result, using an appropriate rock physics method is required to obtain valuable knowledge about the effect of the different parameters on the wave properties.

• bottom simulating reflectors (BSR)
• low frequency shadow (LFS)
• seismic attenuation and normal moveout (NMO)