This paper introduces a robust approach for improving seismic data resolution using the spectral inversion algorithm. The proposed method consists of two crucial steps:
(1) Employ the sparsity adaptive hard thresholding pursuit algorithm to solve the norm-constrained optimization problem and obtain accurate reflection coefficients.
(2) Apply shaping regularization for wavelet extraction, followed by wavelet decomposition and high-frequency wavelet replacement, thereby effectively enhancing the overall resolution.
2.2.1. Spectral Inversion Algorithm Based on Compressed Sensing Theory
As shown in the equation (5), we solved the optimization problem to obtain the reflection coefficients.
While threshold algorithms [
47,
48] provide fast computational results for solving sparse constrained problems, they often yield subpar outcomes compared to matching pursuit class algorithms [
49,
50]. The latter has gained significant attention due to its superior practical application results. The orthogonal matching pursuit algorithm, for instance, selects only one atom in each iteration that best matches the residuals. However, this selection process becomes inefficient when dealing with high-dimensional signals. Building upon the IHT algorithm, Foucart [
51] introduced the Hard Thresholding Pursuit (HTP) algorithm. In each iteration, HTP selects the atoms corresponding to the largest K elements in the sparsity coefficients of the signal. The sparsity coefficients of the signal are then obtained by solving a minimization problem. The iterative process of the algorithm can be described as follows:
where
express the support set of
. During the second step, the minimization problem can be solved using either the least squares algorithm or the gradient descent algorithm. However, when the signal's dimensionality is large or when it contains a significant number of non-zero elements, the least squares algorithm becomes computationally challenging due to the calculation of a high-dimensional inverse matrix. To address this issue, we employ the Nesterov accelerated gradient (NAG) algorithm, known for its fast convergence and high efficiency, to solve the minimization problem in Equation (5). The process of reconstructing sparse signals using the HTP algorithm can be summarized as follows:
Input: sensing matrix , measurement vector , sparse level ;
Initialization: sparse coefficient approximation , index set , atomic matrix ;
Loop:
(1) calculate (2) remain the index of the maximum K elements in , and the atomic matrix in this index is ;
(3) solve by the Nesterov accelerate algorithm:
Input: learning rate , attenuation rate Initialization: initial velocity , objective function
If the condition is met, stop iteration; else , return step① continue iteration.
(4) if the condition is met, stop iteration; else n = n +1, return step①continue iteration.
In addition, the sparsity of reflection coefficients in the transform domain is often unknown, and the accuracy of capturing this sparsity directly impacts the reconstruction accuracy. Therefore, it is crucial to develop a reconstruction algorithm that can automatically adapt to the signal's sparsity. In this paper, we propose a sparsity-adaptive hard threshold pursuit algorithm. The algorithm consists of the following steps:
Input:sensing matrix , measurement vector , noise level ;
Initialization: , ,, , solve in the condition of , , ;
Loop:
(1) (2) solve by the Hard Thresholding pursuit algorithm
(3) calculate
(4) if the condition is met, stop iteration; else, , return step (1) continue iteration;
Output: sparse level , sparse coefficient .
To compare the effects of solving the objective function under different norm constraints, we replace the
norm constraint in equation (5) with an
norm constraint of the optimization problem. We present the inversion results under the two norm constraints using a seismic data example, as shown in
Figure 1. The comparison results reveals that the
norm results exhibit a higher level of sparsity and signal-to-noise ratio compared to the
norm constraint. This enables a broader bandwidth seismic reflection coefficient model.
2.2.2. Continuous Wavelet Transform
The Morlet wavelet
can be viewed as a complex sinusoid modulated by a Gaussian envelope (Morlet J, 1982):
Here,
represents the modulation frequency of the Morlet wavelet, which typically needs to be greater than 5.33. The Fourier transform of the Morlet wavelet is:
Subsequently, the optimal scale of the Morlet wavelet is obtained by optimizing the Fourier transform of the seismic trace
and the Fourier transform of the reflection coefficient
. The optimal scale
can be determined by minimizing the objective function in equation (10):
Using the wavelet
with the optimal scale
and the positions of the reflection coefficients, decompose the seismic data to obtain the coefficients of each reflected wavelet. Assuming the positions of the reflection coefficients are
, the coefficients
of each reflected wavelet can be obtained by minimizing the objective function in equation (11):
Finally, replace and reconstruct
using high-frequency Morlet wavelets or other high-frequency wavelets, which can naturally extending low- and high-frequency energy of seismic data.
Applying the aforementioned steps to seismic data processing can significantly improve the resolution of the seismic profile.
Building on the outlined key steps, this paper introduces a robust resolution enhancement technique for spectral inversion by leveraging compressive sensing theory (shown in
Figure 2). To ensure stability during the wavelet calculation process, a shaping regularization wavelet extraction approach is employed. This technique enhances the estimation stability of wavelets, particularly in regions exhibiting low reflection coefficients or a low signal-to-noise ratio in seismic data.