an overview of seabed surveys (high-resolution geophysical site

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The Hydrographic Society in Scotland

An Overview of Seabed Surveys(High‐resolution Geophysical Site

Surveys)

Anna FulopChief Geoscientist Fugro Survey Limited

22 April 2015

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• Introduction• Site survey - purpose and requirements• Principles, Methods and Tools• AUV Surveys• Site Survey Operations• Interpretation and Presentation

Agenda

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Introduction - The Fugro Group

• Dutch multinational company; headquarter in Leidschendam, NL. • Fugro provides People, Equipment, Expertise and Technology worldwide;

Approximately 12,000 employees in over 60 countries.• Activities: acquire and interpret Earth’s surface and sub-surface data to

support the design, construction, installation, repair and maintenance of infrastructure, on land and at sea.

• 3 Divisions (Geotechnical, Survey and Subsea Services) each containing numerous individual Operating Companies (OpCos).

• Survey division: Oil & Gas industry, Renewables, Mining, etc

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Fugro Survey Division• Meteorology and oceanography,

Weather forecasting

• Offshore structural monitoring

• Marine environmental services

• Geophysical Survey, cable route and hydrographic survey

• Construction survey support• Satellite positioning, monitoring and

mapping

• Aerial mapping, Terrestrial surveying

• Geospacial GIS solutions

IntroductionFugro Survey Limited• High-resolution marine geophysical

site surveys (Oil & Gas industry) -seabed mapping, drilling hazard surveys, regional and deep-water surveys, pipeline route surveys

• Offshore positioning services and construction support - rig moves, pipelayoperations, etc

• Main office in Aberdeen

• Operate worldwide, but predominantly in NW European Continental Shelf (UKCS, NOCS, Dutch and Danish sectors), offshore West Africa

A survey is required for anything that…• is moved from one place to another (e.g. rig

move, site clearance/abandonment)• is laid on the seabed (e.g. pipeline, power/

telecommunication cable)• has foundations in the seabed (e.g.

platforms, buried pipelines, wellheads, manifolds, anchors, offshore wind turbines)

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Site surveys - purpose and requirements

• might affect environmentally sensitive areas

• may be affected by geohazardsor offshore or fishing activity

… goes on or in the seabed

A survey will determine:• Seabed conditions

- Bathymetry: water depth, gradients, relief- Seabed sediment types (clay, sand, gravel, bedrock)- Seabed obstructions (existing infrastructure, wrecks, debris, boulders,

UXO)- Potentially sensitive habitats (e.g. coral, herring spawning grounds,

MDAC)

• Sub-seabed conditions- geology - foundation / anchoring conditions - trenching conditions

• Potential Geohazards- Slope stability- Faulting- Probability of shallow gas

Are these conditions suitable and safe?

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Site surveys - purpose and requirements

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Seabed Conditions - Bathymetry

Iceberg ploughmarks, northern North Sea

Pockmark, central North Sea

Megaripples and spud can depressions, southern North Sea

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Seabed Sediments and Features

Sand with rock outcrop

Pipelines with associated infrastructure; soft clay with numerous pockmarks

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Unexploded Ordnance (UXO)

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Debris (cable / wire)

Abandoned telecom cable near proposed drilling location (not mapped)

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Environmentally Sensitive Habitats (DECC requirements in the North Sea)

Area of possible Coral on SSS data

Seabed photograph of Coral colonies

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Sub-seabed Conditions

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Geohazard Identification – Seabed Stability

Evidence of slope instability

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Geohazard Identification - Faulting

Evidence of faulting on 2DHR seismic data

500m

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Geohazard Identification – Potential Shallow Gas

500m

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What are the risks of not doing a site survey?

West Vanguard Blowout, NOCS (1986)

Punch-through

Basic Principles• Use of geophysical ‘remote sensing’ techniques• Predominantly use acoustic methods

- Range of equipment and methods- Range of acoustic frequencies

• Calibrated by some limited ‘ground truth’ sampling- Geotechnical- Environmental

• Platform: dedicated survey vessels vs vessels of opportunity

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Principles, Methods and Tools

Echo Sounders • Single and multibeam systems, generally hull-mounted

• Water depths measurements

• Topography

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SINGLE BEAM

Side Scan Sonars• High, low, dual frequency towed systems

• Measure seabed reflectivity

• Detection / identification of seabed sediment and obstructions

• Measurement of dimensions (particularly height above seabed)

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Fine SAND

RippledSAND Boulders/

Clay outcrops

RippledSAND

Fine SAND

Side Scan Sonar Data – seabed sediments

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Side Scan Sonar Data – detailed mapping of seabed obstructions

Uncharted WRECKL=33m, W=6m, H=3.3m Acoustic

shadow

Seabed

Area of debris

Surface-laid pipeline

Anchor scarwith pit

Smalldepression

High-resolution Seismic Systems• Used for sub-seabed mapping

• Seismic reflection

• Single-channel systems:- Pingers (generally hull-mounted)- Chirps (towed / hull-mounted)- Boomers / Sparkers (towed)- Single airguns (mini gun and single channel streamer)

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High-resolution Seismic Systems (cont’d)• 2D high-resolution multichannel seismic

- typically 140 cu.in. sleeve airgun source fired at 6.25m / 12.5m intervals

- 48 / 96 channels, 600m / 1200m hydrophone streamer- Digital recording system- Extensive data processing required- Deeper penetration and increased signal / noise ratio

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Summary of Acoustic Methods

Other Tools and Survey Methods• Complement the acoustic geophysical

survey data • Magnetometer, Gradiometers• Geotechnical testing/sampling

- Grab sampler- Gravity / Piston Corer- Vibrocorer- Cone Penetrometer Testing (CPT)- Deep boreholes (>30m bsb)

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Other Tools and Survey Methods (cont’d)• Environmental Surveys

- Underwater camera: sills and video transects- Box coring

• Have become standard part of most seabed surveys

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AUV Surveys – Deep Water Solution

• Traditional survey methods have numerous drawbacks to the acquisition of geophysical data in deep water areas:

- Deployment / recovery difficult in bad weather conditions- Noise / weather effects, vessel motion- Limited penetration and data resolution (hull-mounted systems)- Sensor positioning imitation (acoustic positioning for towed sensors)

• For deep water areas, better resolution and better positioning are needed

• The survey industry’s solution: The Autonomous Underwater Vehicle (AUV)

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Fugro’s “Echo Surveyor IV” AUV

• Kongsberg HUGIN 1000• Semi-autonomous vehicle

independently powered and controlled (cable free).

• Depth rated to 3000m.• Aided Inertial Navigation System

provides extremely accurate positioning.

• Positional accuracy of the AUV is less than 5m, significantly better than towed systems.

Improved bathymetry data quality

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Bathymetry from 3D seismic data

AUV mounted multibeam echo sounder data

Improved sub-bottom profiler data quality

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2DHR Data AUV Chirp Data

Improved seabed imagery quality…

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With option to mosaic images

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Onshore• Vessel PM, main point of contact

between survey vessel and client

• Geoscience Team Leader

Site Survey OperationsOffshore – Full Survey Team• 24 hour operations

• 1 Party Chief

• 2 Online Surveyors (Day/Night)

• 1 Technical Coordinator

• 4 Engineers (Day/Night)

• 1 QC Geophysicist

• 1 Environmental Scientist

• Optional requirements:2nd Geophysicist, Processing surveyor, Seismic processor, Gun mechanic, Geotechnical survey crew

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Data interpretation• Variety of software used for the different datasets

• Large volumes of data handled

• Integration of all available data essential – suitable software required

Final Product – Site Survey Report

• Comprehensive, structured and high-quality survey report

• Often includes integrated geophysical, environmental and geotechnical results

• Detailed figures and charts showing the final interpretation

• Digital deliverable (CAD files, GIS deliverables)

Interpretation and Presentation

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• Geophysical site surveys are essential to ensure the suitability and safety of sites for offshore infrastructure construction

• During a site survey a large amount of information on the seabed and sub-seabed environment is acquired

• A detailed and integrated interpretation is required to turn this information into a high quality site survey report for the end user

• The requirements vary from project to project; techniques, data quality and level of interpretation have to match these requirements

Summary

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Thank you

Any Questions?

an overview of seabed surveys (high-resolution geophysical site

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