3D Sub-Bottom Profiling High resolution 3D imaging of buried targets – GeoChirp 3D

Martin Gutowski, Jérôme Malgorn – Kongsberg Maritime Mark Vardy – National Oceanography Centre, Southampton

Motivation

• Produce a 3D sub-bottom profiler data set – Decimetre resolution – 10s of metre penetration

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• Applications • Marine engineering and site surveys • Marine geology and geophysics • Defence: UXO & MCM • Marine archaeology

• Deliverables

• Industry standard SEGY data set • After acquisition: shotgathers with populated source and

receiver positions • After processing: stacked and (partly) migrated

Background

– Fully sample a reflection wavefield over an area – Produce a 3D data volume rather than 2D sections

• Can be viewed in horizontal, vertical and arbitrary slices independent of acquisition directions

• 3D processing yields better data quality and higher resolution – Standard tool in hydrocarbon exploration – 10s of meter resolution and kilometres penetration

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• 3D seismics

Approach

• surface towed rigid frame housing all components – Absolute position accuracy needs to be in range of wavelength used

• Source – Chirp sub-bottom profiler transducer array (4 transducers)

• Bandwidth 1.5 – 13 kHz • Wavelength < 10 cm

• Receiver – 60 hydrophone groups, – Optimised geometry

• 25 cm separation in x & y • Positioning

– RTK-GPS – MRU

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• Downscaling

Design

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flotation

RTK GPS & heading

tow points

hydrophone sections

25 cm

3.1 m 2 m

Design

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25 cm

25 cm

hydrophone sections

source array

Build

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Deployment

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Deployment

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Data acquisition

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Data coverage

Single channel data QC

Data processing

• Geometry processing – Combine seismic and positioning

• Automated in acquisition system – SEGY output with source and receiver positions in trace headers

• Trace to trace processing

• Filtering • Correlation with source signal (de-chirping) • Deconvolution

• Mid-point binning on 12.5 cm grid OR • 3D pre-stack Kirchhoff migration

– Processing algorithm adopted to high frequency data – Constant velocity

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Data example 1

• Objective: UXO and object detection • Location: HMNB Devonport (HMS Drake - Plymouth Royal Navy Base) • Partner: Royal Navy • Survey: Mark Vardy, Martin Gutowski et al.

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Survey area • A-tidal basin, 8.0 m water depth • Thin veneer sediments • Total area 150 x 200 m • Looking for bedrock protrusions

and buried objects • 20 million traces in 2 days

survey

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Data cube

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Cross section - inline

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Seafloor Bedrock Seafloor and bedrock maps

Seafloor and bedrock

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0 10 m

Timeslice

inline

Water column

sediment

Horizontal section - Time slice

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0 10 m

Timeslices every 0.02 ms TWT = c. 1.55 cm

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0 10 m

Timeslices every 0.02 ms TWT = c. 1.55 cm

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0 10 m

Timeslices every 0.02 ms TWT = c. 1.55 cm

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0 10 m

Timeslices every 0.02 ms TWT = c. 1.55 cm

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0 10 m

Timeslices every 0.02 ms TWT = c. 1.55 cm

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0 10 m

Timeslices every 0.02 ms TWT = c. 1.55 cm

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Sediment thickness and objects

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Object example 1

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Object example 2

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Object example 3

Data example 2

• Objective: Buried wooden shipwreck Grace Dieu Henry V of England’s flagship – built 1418

• Location: River Hamble, UK • Partner: University of Southampton • Survey: Ruth Plets, Martin Gutowski et al.

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Courtesy of Channel 4’s Time Team

Artist’s impression

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Data acquisition

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Data cube

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Inline

3D interpretation

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3D representation of wreck

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Hull reconstruction

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