Seismic Imaging in GLOBE Claritas

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 Results Manual 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 Results Manual 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

TRV-434 : structure West East Shooting Direction 27.2km Total Length 3.2km Cable Length Overthrust Tikorangi Limestone Approximate Depth in metres Schematic of TRV-434 taken from previous depth imaging study Note the location and depth of the overthrust relative to the cable length

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 ADMIRE Layer 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 1480m/s 3750m/s 6000m/s Layer-Based Model ADMIRE Model

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