Effect of the mesoscale eddies on seismic wave propagation

In offshore exploration the water column is still taken as a homogeneous media. This assuming should be proper only for shallow water. In deep sea, the mesoscale structures, such as eddies, fronts and currents, have recently been discovered in seismic reflection data. It is found that if only the temperature steps are large enough, such as a few hundredths of a degree over only a few meters, the reflecting features can be seen in seismic data. The fine structure of eddies can be mapped with a high lateral resolution using standard seismic reflection techniques. Thus it is necessary to study the effect of eddies on seismic wave propagation in deep sea. Based on fluid-dynamical equations, we derive an appropriate partial differential equation for the pressure field in water with eddies, including current effects. Moreover, since eddies in water yields weak reflected wave, spurious numerical reflections at the lateral PML boundaries must be at least one order smaller than the magnitude of the physical phenomena of interest. We develop a numerical model with three layers, one water layer with an eddy deduced from the observed data in South China Sea and two homogeneous elastic layers. In order to investigate the effect of eddy, seismic wave propagation for water layer without eddy is simulated. The wavefields at t=0.59s are shown in Figs.1a. The difference of depth that seismic wave propagates between having eddy and no-eddy case is 40m. Also, the shot gather is calculated (Fig. 1b). It can be seen that there are reflections in upper and lower part of the eddy. The trace of the signal recorded in a receiver directly above the source is plotted. It is found that the arrival time of the P-wave reflection from the elastic layer for the condition of having eddy is 20ms later than that for no-eddy case. The zero-offset seismic profiling throughout the eddy (Fig.2a) is simulated. In the zoom between 0s and 2.9s (Fig.2b) highly reflective lens-like structures can be observed, and this result coincides with the observations in seismic oceanography. In the zoom between t=2.9s and 3.0s (Fig.2c) the reflections from the first elastic layer can be seen clearly. The maximum arrival-time difference is obtained in the center of the eddy. It is found that the structure of the flat elastic layer is distorted due to the complex reflections from the eddy. The effect of eddies, with various current and radius, on seismic wave propagation are discussed. We obtained that when the intensity (the maximum current speed (m/s) plus radius (km)) of the eddy is less than 1, the effect of the eddy can be neglected. In sum, the eddy plays an important role in seismic wave propagation.