Abstract
Based on seismic interferometry, the virtual source (VS) method is able to produce virtual gathers at buried receiver locations by crosscorrelating the direct-downgoing waves with corresponding reflected-upgoing waves from surface-source gathers. Theoretically, the VS records can improve seismic quality with less negative impact from overburdened complexities. However, shallow complex structures and weathering layers at near surface not only severely distort the wavepaths, but also introduce multiples, surface waves, scattering noise, and interference among different wave modes. These additional seismic responsescontaminate both direct-downgoing and reflected-upgoing wavefields. As a result, the VS gathers experience spurious events and unbalanced illuminations associated with distorted radiation patterns. Conventional stacking operator can produce significant artifacts for sources associated with ineffective-wavepath cancellation. We review three publications and summarize a comprehensive workflow to address these issues using data-driven offset stacking, wavelet-crosscorrelation filtering, and radiation-pattern correction. A data-driven offset stacking theme, with each individual source contribution is weighted by certain quality measures, is applied for available offsets. The wavelet crosscorrelation transforms time-offset data into local time-frequency and local time-frequency-wavenumber domains. Filters are designed for the power-spectrum in each domain. The radiation-pattern correction spatially alters the contaminated direct-wavefields using a zero-phase matched filter, such that the filtered wavefield is consistent with the model-based direct P-wavefields observed at buried receiver locations. Our proposed workflow produces significant improvement as demonstrated in the 13 time-lapse field surveys that included substantial repeatability problems across a 17-month survey gap.
Highlights
Based on seismic interferometry, the virtual source (VS) method is able to produce virtual gathers at buried receiver locations by crosscorrelating the direct-downgoing waves with corresponding reflectedupgoing waves from surface-source gathers
The amplitude radiation-pattern of the VS distorts and creates non-uniform illumination for target reserviors. This leads to degraded image quality where the VS stack has a signal-to-noise ratio (S/N) lower than that obtained by non-VS processing
At the beginning of the methods section, we briefly review the fundamentals of continuous wavelet transformation (CWT) as it underpins the following sections heavily
Summary
The virtual source (VS) method is able to produce virtual gathers at buried receiver locations by crosscorrelating the direct-downgoing waves with corresponding reflectedupgoing waves from surface-source gathers. The VS may help produce images from underneath a complex overburden with without knowledge of the overburden velocities and near-surface changes This workflow crosscorrelates direct-downgoing waves with upgoing (reflected) seismic waves to retrieve the reflection response, and redatums the surface-source records to buried receiver locations[4,5]. We experienced the abovementioned near-surface challenges in this time-lapse monitoring feasibility dataset where the spurious events remain as key issues in the VS processing To address these challenges, we propose a new VS redatuming workflow to improve the processing quality in the steps of traditionalprocessing: offset stacking, crosscorrelation, and unbalanced illumination. The output VS, is expected to possess isotropic radiation patterns and provide balanced illuminations These renovated steps compensate for near-surface complexity and reduce time-lapse noise, and produces redatumed data with fewer artifacts. The output VS records are stacked to produce final image
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