## 4 Methods
### Inverse Q Filtering and Active Seismic Data Processing
Due to the exponential nature of the amplitude filter, gain control must be included in parametrization of IQF to limit high-frequency noise amplification outside the seismic passband during amplitude compensation. This is achieved through gain capping and high-frequency tapering, as described in the diagram below (Figure 4). Gain capping occurs when the gain reaches the gain limit value specified in the Gain Limit parameter (curve B). Tapering is usually applied between two fixed user-specified frequencies: the High-Frequency Limit and the High-Frequency Cutoff (curves B & C). Alternatively, tapering can also be designed so that it starts when the gain reaches the gain limit value rather than at a fixed frequency (curve A). In this case, the high-frequency limit is not used. Gain limit parameter controls the frequency at which the gain is capped. The amplitude spectrum of the filters is clipped at a user-defined maximum gain level and rolls off smoothly after that point. Limiting the spectrum in this manner has the effect of restricting the amplitude treatment at a particular time to the range of frequencies, over which, the signal is believed to dominate data noise. Gain limiting is required only if amplitude spectra are changed, i.e., the compensation mode is 'Phase and amplitude' or 'Amplitude'. It should not be used as an alternative to noise attenuation, such that we suppress the boosting of higher frequency noise by aggressive use of the low-gain limit. One way of choosing a value for the gain limit is to estimate the frequency at the upper edge of the signal passband, edge (Hz), at a particular time \(T\), and to compute the gain limit from the equation below:  
\[G=\frac{27.29 Tf_{edge}}{\text{Q}_{\text{av}}} \tag{10}\]  
where \(\text{Q}_{\text{av}}=\) the effective average value of Q. The part of the standard requirements of a contemporary relative amplitude preserving processing sequence should be as follows:  
* The processing flow should not include processes that modify the amplitude spectrum of the wavelet based on the seismic data spectrum, e.g., spectral whitening, or wavelet shaping, or data driven gains such as AGC (Automatic Gain Control) or similar. * Random noise should be attenuated, as should coherent noise as much as possible. Noise present in the data will be boosted, and potentially over boosted, by the Q-compensation correction. * Multiples should be attenuated as these have a different travel path and dispersion to the primaries. The simple processing flow, which was used in our case for Q amplitude correction, is shown below (Figure 5). Figure 4: Gain control for amplitude compensation. Illustration of various scenarios: A: tapering is applied from a (variable) frequency controlled by the gain limit value. B: the gain is capped when it reaches the gain limit value and tapered down between two fixed frequencies. C: the gain is tapered down between two fixed frequencies (the gain limit is never reached). To avoid the impact of certain processing procedures on the spectral characteristics of the seismic signal, it is recommended to apply IQF in the phase correction option already at the initial stage of processing (i.e., without scaling and before deconvolution). For the purpose of performing amplitude correction to increase the vertical resolution of seismic data, IQF application is usually implemented on final collections before or after migration in the version of real amplitudes. Collection gathers should be NMO (Normal Moveout) corrected and full static corrections (primary, residuals, trim) should be applied to the traces. In addition, to avoid generating artifacts by the IQF procedure due to the presence of a low-frequency signal (stretch) after the NMO application, it is necessary to apply the correct muting.