2 Academy of Scientific and Innovative Research
(AcSIR), Ghaziabad-201002, India
Abstract
Ground roll is the most common coherent noise observed in the land
seismic data, mainly characterized by low frequency and low velocities.
In this study, we are demonstrating the application of a model-based
surface wave attenuation technique for the efficient removal of ground
roll from 2D seismic data. The method employs inversion and adaptive
subtraction filtering of surface waves. A 1D viscoelastic model is
characterized by considering layer thickness, P & Sv wave velocities,
density, and P & Sv wave quality factors (Qp & Qs) for each layer, to
compute a synthetic seismogram. A linear Radon transform is used to
generate dispersion spectra of input shot gather and synthetic shot
gather generated from the model. The misfit between the dispersion
spectra of input shot gather and synthetic shot gather is given by an
objective function ‘J’,\(J\left(m\right)=\ {|\left|s-o\right||}_{2}\), where ‘s’ is the
dispersion spectrum of the synthetic data generated by the model, ‘o’ is
the dispersion spectrum of the shot gather and \({||.||}_{2}\)is the
L2-norm of the difference between ‘s’ and ‘o’. The main focus is to
minimize the objective function J. The local minima always seems to be
the bottleneck for most of the non-linear inversion problems. To
overcome this constraint, we implemented the Genetic Algorithm (GA)
method in the surface-wave inversion process using the tool available in
a commercial software. GA is a derivative-free search approach towards
global minima, for the solution of the inversion problem. Once an
optimized model is achieved from GA, synthetic data is generated and is
adaptively subtracted from the shot gather. As the 1D model obtained
cannot represent any lateral variations of physical parameters in the
earth, adaptive subtraction and GA are implemented several times to
mitigate this limitation.
The proposed technique is applied to process a 2D shot gather data (Fig.
1-a) accessed from the SEG wiki Open data set collection of oz Yilmaz-40
shots. In this data, ground roll is masking the reflection events in the
shot gather. After implementation of the proposed technique, it is
observed that the ground roll is efficiently attenuated and reflections
became more prominent by restoring the actual amplitudes, as shown in
Fig. (1-b). The dispersion spectrum of both shot gathers is shown in
Fig. 1 (c-d), it is observed that the ground roll present in the
low-frequency zone (0-15 Hz) is attenuated. The results of the study
suggest that the proposed method is quite efficient for surface wave
attenuation.