Cold Water Statics

Corrections for water layer variability and associated time shifts.

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Travel-time shifts associated with seasonal water layer variability are often observed in deepwater survey areas. These can negatively impact the 4D differencing between surveys.

PGS estimates water-column velocities independently along each sail line and applies an appropriate static correction to each sail line in each vintage being processed. PGS also typically corrects for residual time shift differences between sail lines. The necessary corrections are generally not trivial to compute.

Prior to cold water statics corrections, a typical 4D processing workflow at PGS includes shot-to-shot residual designature (for repeated shot positions), processing-based source deghosting for both single-sensor and multisensor streamer data, processing-based receiver deghosting for single-sensor data, wavefield separation for multisensor GeoStreamer data, and sea-surface source statics optimization to compensate for variations in sea-surface heights between consecutive shots.

Key Points

  • Removes long-wavelength time shifts between sail lines associated with water layer variability
  • Includes residual corrections
  • Applicable to both towed-streamer and OBN geometries
  • Can be implemented in the image domain to simultaneously use all data for estimating water layer velocity variations

Improving NRMS Differences

NRMS difference can be computed after 4D differencing. The NRMS distribution is significantly improved after water column static corrections.

A New Methodology for Estimating Water-Velocity Changes

We have recently developed an image domain solution that simultaneously uses both datasets to estimate the water-velocity changes between the two surveys and minimize 4D differences in the overburden. The methodology is particularly beneficial for deepwater '4D friendly' acquisition that seeks to repeat all source and receiver positions.

 

4D NRMS difference computed on the 2.5 km offset class after 4D differencing. NRMS differences are pronounced when to water column static corrections are applied (left). The NRMS distribution is significantly improved after water column static corrections (right).

Contact a PGS expert

If you have questions related to our Subsurface Monitoring services please send us an email.