Block diagram of the LFS processing sequence consists of the following stages:
- Preparatory stage;
- Narrowband noise filtering;
- Transient noise filtering;
- Generation of the final Fourier spectra;
- Spectral anomaly parameters estimation;
- Correction for spectral anomaly parameter variations.
The preparatory stage includes assessment of the filed data quality by means of visual analysis of the spectra and dynamic spectrograms of seismic signals. In case of poor data quality, field crew shall carry out reacquisition of the rejected receiver point. Distinguishing features of the desired signal and noise enable to identify the frequency ranges where the desired signal events are unaffected by significant noises. This stage also includes adjustment of automatic filtering techniques for individual features of the microseismic signals in the current survey area.
Fig. 1. Generation of spectra and spectrograms
Narrowband noise filtering is done using the optimization method of quasiharmonic noise filtering with preservation of the background noise level in order to exclude the narrowband components caused by the monochromatic vibrations of the near-surface industrial sources;
Fig. 2. Narrowband noise filtering with preservation of background noise
Transient noise filtering is carried out in order to exclude the snapshot waveforms of the increased energy caused by the influence of the additive surface noise, which significantly distorts the spectrum of the desired background microseismic noise.
Fig. 3. Transient noise filtering
Generation of the final Fourier spectra. Calculation and summation of the Fourier spectra filtered from the noise
Fig. 4. Filtering results
Spectral anomaly parameters estimation (calculation of spectral anomaly’s frequency, anomaly bandwidth, local signal-to-noise ratio) is carried out in an automatic mode using the algorithm of parameterization of the spectral peaks, which was developed based on the optimization of local peaks of the wavelet image of microseismic signal spectrum. In case of the complex spectrum (overlapping of the spectral peaks of several anomalous zones of geologic section), estimation of anomalies’ intensity is carried out “manually”.
Fig. 5. Anomaly parameters estimation
Correction for spectral anomaly parameter variations. Repeated studies of the microseismic wavefield spectra have shown that there are variations of the anomaly parameters which are evident while comparing cumulative spectra recorded, for example, during the day and night. The technology enabling to take into account the changes of anomaly parameters given the limited number of simultaneously observed points is used under the conditions of temporal non-stability and spatial heterogeneity of the microseismic wavefield. To speed-up this stage, the algorithm for automatic calculation of correction coefficients was developed, which performs calculations based on the preliminarily prepared spread-sheet of measurements metadata.
Fig. 6. Correction for spectral anomaly parameter variations