globe claritas™ seismic imaging in globe claritas

GLOBE Claritas™

Seismic Imaging in GLOBE Claritas

www.globeclaritas.com

VELSECT : Automatic Velocities

• High Density Velocity Analysis – Picks made at every CDP, approx every 60ms– Optionally constrained by top and base horizons

• Automatic Analysis – Picked on high fidelity semblance spectra – Spectra optimised in pre-processing– Runs efficiently in parallel

• Editing and Smoothing– Geological constraints used for edits– Statistically robust (300,000+ VT pairs)


VELSECT : Stacking ResultsManual Velocities : 2km spacing VELSECT Velocities

• Very similar results – but improved (eg under channels) • 3000km of data picked over a weekend, automatically• Fast, accurate, repeatable and reliable


VELSECT : Interval Velocity ResultsManual Velocities, Dix Inverted VELSECT Velocities, Dix Inverted

• VELSECT results show more detail, better resolution• High velocity limestones resolved to two bands • Tied into wells and show good match with sonic-derived functions• Resolve coals from carbonates through velocity profile• Identify possible overpressure zones • Excellent for curved-angle calculations : AVA and Imaging • VELSECT : Automatically create VELocity SECTions


Solution : Imaging Under Channel, original

Original post stack migration

The channel creates a low velocity zone with steeply dipping sides that defocuses seismic energy.

Imaging is severely disrupted under the channels.

Channels are all across the prospect area.


Solution : Imaging Under Channel, PreSTM

Pre-stack Time Migration using VELSECT velocities

Solution is improved, but not complete.

Ray path bending is not fully accounted for by the preSTM alone, and additional imaging is needed


Solution : Imaging Under Channel, PreSDM

Pre-stack Depth Migration

Complete solution.

Modelled channel and near surface velocities successfully correct for the ray-path bending at the sea-floor, as well as the bright limestone event (approx 1500ms)

Channel shape is unchanged in all cases – but the velocity variation is correctly modelled.


Solution : Imaging Under Channel, Velocity Model

• VELSECT technique employed after preSTM

• VRMS values converted to interval velocities

– test lateral smoothing and use to depth convert

– use smoothing which produces simplest depth image

– secondary smoothing in depth

• Run PreSDM as second imaging phase

• Interpretation free preSDM modelling methodology


West East Shooting Direction

27.2km Total Length

3.2km Cable Length

TRV-434 : structure

Overthrust Tikorangi Limestone

Schematic of TRV-434 taken from previous depth imaging study

Note the location and depth of the overthrust relative to the cable length

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TRV-434 : Original Time Processing

Imaging using a conventional late-1980’s sequence, with DMO and post-stack time migration.

Sub-thrust imaging is poor; shot-receiver ray paths are complex and the simple DMO-Stack-Migration approach cannot resolve the structure. Sub-thrust imaging is confused, with broken, crossing events (circle)


TRV-434 : Modern Time Processing

Imaging using a modern sequence that addresses spatial aliasing and employs two passes of pre-stack time migration

Overall image is much cleaner, and imaging has improved considerably. A layered structure starts to appear, but is still smeared (circle). Pre-stack time migration still assumes the shot-receiver ray-path is symmetrical about the trace midpoint, however.


VELSECT : Raw Velocity Results

RAW VELSECT velocities

• trends can be seen

• data is still noisy

• cannot be used for stacking


VELSECT : Edited Velocities

Edited VELSECT velocities

• around 60% edited out

• still 100,000+ picks

• interval velocity editing

• iterative approach


VELSECT : Smoothed Velocities

Final VELSECT velocities

• spatial frequency filter

• extract low pass component

• 1-2km radius filter

• 10% spatial nyquist limit


VELSECT : Interval Velocities

VELSECT Interval velocities (left) and preSTM data (right). The velocity field shows structure that matches the seismic image, and geological expectations


TRV-434 : ADMIRE Depth Imaging

GNS Science’s Automatic Depth Modelling Iteration via RMS velocity Estimation (ADMIRE) approach creates a grid-based depth model that is ray-traced to produce an image

Imaging is considerably improved with asymmetric ray-paths being managed correctly. Layer structure beneath the overthrust is now imaged sufficiently to resolve faulting, enabling detailed interpretation and analysis.


TRV-434 : Layer Based Imaging Vs ADMIRELayer based pre-stack depth migration approaches use a layered earth model

ASSUMES : layered earth represents the velocity structure accurately

REQUIRES : detailed structural interpretation of each layer with each iteration

- Time consuming, expensive and can result in model-driven solutions

ADMIRE pre-stack depth migration uses a grid based model, created from the data

ALLOWS : velocities to be independent of structure, and extremely complex

REQUIRES : no structural model or interpretation, just careful quality control

- Computer intensive, automatic and data driven

Layer-Based Model ADMIRE Model

1480m/s

6000m/s

3750m/s


TRV-434 : Layer Based Imaging Vs ADMIRE

Conventional Layer-Based Imaging ADMIRE Grid-Based Imaging

Even after a large (13+) number of layer-based model updates the conventional depth imaging approach lacks the clarity and resolution of the ADMIRE image (with 5 model updates)

Where seismic velocities are independent of sub-surface structure the ADMIRE approach produces a more accurate image, with less iterations, and no interpretation

globe claritas™ seismic imaging in globe claritas

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