GNGTS 2014 - Atti del 33° Convegno Nazionale

GNGTS 2014 S essione 3.1 123 Synthetic example. To compare our FDS-RT code and the existing codes on ProMAX according to their abilities on demultiple operation, we used the synthetic data computed on the earth model in Fig. 2a. Fig. 2b shows the data which is characterized by the presence of very strong multiples due to the high velocity basalt layer at 2625 m depth. Muting, NMO correction and spherical divergence correction have been applied to the data by using ProMAX in Fig. 2b. To show the differences of the demultiple results better, the seismic data in Fig. 2 are displayed in the [2000 ms, 8190 ms] time axis window. The synthetic data are made by 162 traces. The offset ranges from 200 m to 4225 m. The time sampling rate is 2 ms. The strong reflection of the water bottom is at 1.5 s. The top basalt reflection is at 3 s. The first multiple of the water bottom at 3 s makes the top basalt reflection indistinguishable. The primaries after 3 s have weak amplitudes. To attenuate the multiples by using Radon transform, we first do a forward Radon transform to the NMO corrected seismic data, then in transform domain we do a muting to the primaries, next we do an inverse Radon transform to the multiples, finally we subtract the multiples from the original NMO corrected data. Muting the multiples in the Radon transform to get directly the primaries is not advisable as we would like to preserve the primaries faithfully. Moreover, as a practical procedure, we can mute the data at times lower than the first observed multiple in the seismogram before the forward Radon transform. On ProMAX the horizontal axis in parabolic Radon transform domain is the moveout Γ instead of the curvature q. The relationship between Γ and q can be described by the equation: Γ = qx 2 (2) Fig. 2 – a) Synthetic Earth model to test the abilities of different Radon transform methods to attenuate multiples. Densities ρ are in g/cm 3, P-wave velocities V p are in m/s . b) Synthetic seismic data derived from the model in (a); muting, NMO correction and spherical divergence correction have been applied to this data by using ProMAX. (c) to (f) show the demultiple results and are sequentially related with: c) our FDS-RT code, d) the FDS-RT code from ProMAX, e) the standard parabolic Radon transform code from ProMAX, f) the standard hyperbolic Radon transform code from ProMAX. To show the differences of the demultiple results better, the seismic data are displayed in the [2000 ms, 8190 ms] time window.