FSC Full Source Characterization

Source signature estimation from recorded measurements

CGG monitors stability of the source signature in the changing sea environment via shot-by-shot far-field signature estimation from near-field recordings using a proprietary inversion method. This delivers improved on board quality control with source stability and status monitoring, It provides improved reliability and confidence in data quality from near real-time QC that is both conventional and BroadSource™ compliant. The derived signatures can be validated by mid-field and far-field measurements. This delivers accurate far-field signatures to processing for better designature and ultimately a better image.

FSC monitoringReal-time far-field reconstruction from near-field hydrophone recordings enables monitoring of shot-to-shot source signature stability.

FSC debub spec

Benefits

The source signature is monitored on a shot-by-shot basis in real time via far-field signature estimation from near-field hydrophone recordings using a proprietary inversion method to deliver:

  • Far field signatures available from the field, either for every shot or one per line
  • More accurate estimates of the bubble and low frequencies from recorded data deliver more stable wavelets, especially at low frequencies. than those from modelled signatures for:
    • Better low frequency phase control
    • More stable inversion
  • Improved on-board quality control with source stability and status monitoring for improved reliability and confidence in data quality. 
  • Avoids issues of inaccurate low frequencies from modelled signatures for designature of broadband data
  • Enables both 1D and 3D directional designature options
  • Accurate full-bandwidth designature compliant with BroadSource
  • Monitoring can be carried out compared to a fixed reference or a sliding window of adjacent shots. For 4D applications the reference may be the shot far-field of the baseline survey (if available)
  • Better 4D repeatability from better source control and stability

FSC stack comparisonsStack comparisons of designature using a modelled signature versus one estimated from NFH.

Causes of signal instability

A source array is typically composed of twenty to forty airguns of different volumes, roped to an alignment of floats and submerged at depths between 6 and 10 m. This flexible configuration is towed by the seismic vessel in a changing sea environment, making the array a complex and dynamic system. Small changes in the relative depths of the different parts of the array can cause significant changes in the way the bubbles interact and therefore the accuracy of a modelled rather than a measured signature. Actual source signals resulting from these coupled systems can vary from the theoretical in both the time (from shot-to-shot) and observation directions. Signal instability, can be caused by gun failures (dropouts and delays) and scattering from the changing rough sea surface. CGG uses various indicators for onboard monitoring of the source far-field signal from shot to shot.

Source monitoring

The far-field source signal of the airgun array (far-field referring to a distance from the array where the signal appears as if it is emitted from a point source) is difficult to measure, but can be estimated from the near-field recordings (hydrophones placed one meter above each airgun). We use a proprietary inversion method which takes the non-spherical pressure propagation into account, offering an improvement on Ziolkowski’s notional method. It is conventional- and BroadSource™- compliant.

Amplitude variations are monitored using the average amplitude and maximum amplitude deviations between the current and a reference within the bandwidth. The reference signature may be fixed or formed from a stack of either all shot signatures from a navigation line, or over a sliding window about the current shot. Large deviations indicate possible gun failures.

Source Directivity

The lateral directivity of the source signal depends on actual array geometry, so directivity monitoring requires an accurate knowledge of each airgun position. The airgun positioning accuracy of the GPS measurements from antennae on the gun floats (2-3m) is insufficient for monitoring the source directivity, so we use inversion of the source array geometry based on joint near-field recordings from alternate starboard and portside sources.

When one source is firing, the near-field recordings from the non-firing source (mid-field recordings) are analyzed to locate the positions of both arrays by joint inversion with the near field hydrophone measurements.  This method allows reconstruction of the actual airgun positions with an accuracy of up to 0.2 m. The radiation pattern of the far-field signal describes its amplitude directivity as a function of frequency and observation direction from the source position. This enables the derivation of an accurate 3D signature for use in 3D designature.

Improved stability of low frequencies

The estimation of the bubbles and low frequencies of the signature obtained from near-field hydrophone recordings, is much more accurate than that obtained from modelled response. This delivers more accurate debubbling and more stable low frequencies after designature, especially 3D designature, and deghosting.

Monitoring of air-gun source signature direct...

Cheikh Niang | Yuan Ni | Julie Svay
©2013 SEG

Air gun source array signature stability moni...

Yuan Ni | Cheikh Niang | Risto Siliqi
©2012 SEG

Shot to shot directional designature using ne...

Gordon Poole | Chris Davison
©2013 SEG
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