Offset-Sampling

Sufficient Near and Far Offset Coverage is Critical for Optimal Subsurface Imaging

Dense near-offset coverage improves shallow imaging resolution and computation of intercept and gradient terms during amplitude-based subsurface characterization studies.

Advantages of Offset Sampling

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Flexible acquisition solutions enable better near and far-offset coverage
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Geometry diagnostics improve planning of near-offset coverage
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Far-offset requirements are commonly dictated by FWI requirements (and may be tested with FWI sensitivity kernels)

Leveraging Longer Offsets for Deeper Geological Imaging

Longer offsets can be beneficial to the imaging of deeper geology by improving the availability of high-angle reflections for amplitude-based reservoir property estimation and for the recovery of deep velocity model updates using diving wave information in Full Waveform Inversion.

The rule of thumb for diving wave FWI is that model updates can be recovered for depths in the range of one-sixth to one-third of the maximum offset recorded (and deeper updates can be recovered for the same offset range using reflection FWI), however, the maximum useful offset for diving waves is determined by the signal-to-noise ratio (SNR). If a representative vertical velocity function is available for the survey area, modeling of the FWI sensitivity kernels (see bottom image) is a more reliable indicator of FWI-based offset requirements.

Wide-tow multi-source solutions with as many as six sources laterally distributed over a distance as large as 350 m enable highly efficient large streamer spreads to be towed, whilst simultaneously acquiring a uniform near-offset distribution for all common midpoint (CMP) sublines.

Offset Sampling - Fig1
Comparison of near offset distribution for two different quad-source configurations. The vertical axis is the source-receiver offset for each CMP subline, displayed for four adjacent sail lines. The horizontal axis is the CMP subline position.
Offset Sampling - Fig2
FWI sensitivity kernel modeled for an offset of 10 km with a representative 1D velocity function.

Acquiring Long Offsets

Long offsets can be acquired in three robust ways, customized to the survey budget and imaging challenges:

  • Long streamer tails: In an efficient single vessel acquisition, two or three of the streamers in a larger spread are up to 10 km long.
  • Extended long offset (ELO): A conventional streamer and multi-source configuration is complemented by a second vessel (either source only or sources and streamers). The frequencies and spatial sampling may be tailored to the subsurface requirements.
  • Combination towed & nodes: New OBN acquisition in an area of existing towed-streamer coverage. The maximum offsets recorded can be as large as logistically reasonable and as large as the signal-to-noise (SNR) of recorded diving wave events allow

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