An Overview of Seabed Surveys (High resolution Geophysical Site Surveys)

An Overview of Seabed Surveys

(High‐resolution Geophysical Site 

Surveys)

• Introduction

• Site survey - purpose and requirements

• Principles, Methods and Tools

• AUV Surveys

• Site Survey Operations

• Interpretation and Presentation

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).

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

Introduction

Fugro 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, pipelay operations, etc

• Main office in Aberdeen

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

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)

Site surveys - purpose and requirements

• might affect environmentally sensitive areas

• may be affected by geohazards or offshore or fishing activity

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?

Seabed Conditions - Bathymetry

Iceberg ploughmarks, northern North Sea

Pockmark, central North Sea

Megaripples and spud can depressions, southern North Sea

Seabed Sediments and Features

Sand with rock outcrop

Seabed Sediments and Features

Seabed Sediments and Features

Debris (cable / wire)

Abandoned telecom cable near proposed drilling location (not mapped)

Environmentally Sensitive Habitats (DECC requirements in the North Sea)

Area of possible Coral on SSS data

Geohazard Identification – Seabed Stability

Evidence of slope instability

Geohazard Identification - Faulting

Evidence of faulting on 2DHR seismic data

Geohazard Identification – Potential Shallow Gas

What are the risks of not doing a site survey?

West Vanguard Blowout, NOCS (1986)

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

Echo Sounders

• Single and multibeam systems, generally hull-mounted • Water depths measurements

• Topography

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

Fine SAND Rippled SAND Boulders/ Clay outcrops Rippled SAND Fine SAND

Side Scan Sonar Data – detailed mapping of seabed obstructions Uncharted WRECK L=33m, W=6m, H=3.3m Acoustic shadow Seabed Area of debris Surface-laid pipeline Anchor scar with pit Small depression

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)

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

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)

Other Tools and Survey Methods (cont’d)

• Environmental Surveys

- Underwater camera: sills and video transects - Box coring

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:

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 Bathymetry from 3D seismic data AUV mounted multibeam echo sounder data

Improved sub-bottom profiler data quality

Improved seabed imagery quality…

With option to mosaic images

Onshore

• Vessel PM, main point of contact between survey vessel and client • Geoscience Team Leader

Site Survey Operations

Offshore – 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

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)

• 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

Thank you